RU2459875C1 - Method of producing thick-sheet steel and steel tubes for super strong pipeline - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: proposed steel contains following substances, in wt %: C 0.03-0.06, Si 0.01-0.50, Mn 1.5-2.5, P 0.01 or less, S 0.0030 or less, Nb 0.0001- 0.20, Al 0.0005-0.03, Ti 0.003-0.030, B 0.0003-0.0030, N 0.0010-0.0050, O 0.0050 or less, Fe and unavoidable impurities making the rest. Fused steel is cast into slab to be rolled into thick-sheet steel and water-cooled to preset surface temperature exceeding that of M_s point martensite conversion start. Thick-sheet steel surface is, then, cooled by reiteration of processing whereat heat is recovered one or more times to cool is finally to M_s point or lower. UO-press is used to form tube from sheet. Sheet adjoining edges are arc-welded at flux using welding wire and agglomerated or fused flux to expand the tube.

EFFECT: tensile strength increased to 915 MPa, excellent low-temperature toughness.

10 cl, 3 tbl, 3 dwg

Description

BACKGROUND OF THE INVENTION

FIELD OF THE INVENTION

The present invention relates to a method for producing plate steel for use in an ultrahigh-strength pipeline having a tensile strength (TS) of 915 MPa or higher towards the periphery of the steel pipe, as well as excellent deformability and low temperature impact strength, and to a method for producing a steel pipe for use as an ultra-high-strength pipeline made using this plate steel. In particular, the steel pipe obtained by the production method according to the present invention can be widely used as a pipeline pipe for transporting natural gas and oil.

Priority is claimed for Japanese Patent Application No. 2008-287054 of November 7, 2008, the contents of which are incorporated herein by reference.

Description of the prior art

In recent years, the importance of pipelines as a way of transporting oil and natural gas over long distances has grown. Currently, the American Petroleum Institute's (XI) X65 standard forms the basis for building major pipelines for long distance transportation, and the actual use of X65 pipelines is extremely high. However, there is a need for more durable pipelines to achieve (1) improve transportation efficiency by increasing pressure and (2) improve construction efficiency at the place of use by reducing the outer diameter and weight of the piping. Until now, pipelines of grade up to X120 (with a tensile strength of 915 MPa or more) have been used in practice.

On the other hand, the concept of pipeline construction has changed in recent years. In the past, pipelines were made with constant stress (“stress-based construction”), but recently a design has been recognized in which the zones of ring welds in steel pipes do not crack or the steel pipes themselves do not warp, even if deformation is applied to the pipe pipes ( "strain-based structure"). So far, with regard to high-strength pipelines of grades X80 or higher, the chemical composition or production conditions that can provide low-temperature toughness of the starting materials or toughness in the heat affected zones have been studied. However, in the case of a "strain-based structure", there is a further need for the deformability of the starting materials or the deformability of steel pipes after coating. Without resolving problems with toughness or deformability, it is not possible to produce steel pipes for pipelines X80 or higher having a "strain-based structure". To obtain ultra-high-strength pipelines, production conditions are required that can provide a balance between the strength and low temperature impact strength of the starting materials, the impact strength of the weld metals and heat affected zones (HAZ), the possibility of welding at the place of use, the softening resistance of the joint, and the pipe burst resistance with tensile testing by internal pressure or the like, and which can also produce steel pipes with excellent deformability of the starting materials. As a result, there is a need to develop ultra-high-strength thick pipe pipes of grade X80 or higher, which meet the above steel pipe specifications.

So far, with regard to methods for producing steel pipes for pipelines, the following methods have been proposed, for example, to improve the performance of the above steel pipes. To improve the deformability of steel pipes, in Unexamined Japanese Patent Application, First Publication No. 2004-131799, and Unexamined Japanese Patent Application, First Publication No. 2003-293089, methods are proposed in which steel sheets are slowly cooled in the first stage to 500 ° C. 600 ° C and then, in the second stage, cooled with a higher cooling rate than in the first stage. With these methods, the microstructure of plate steel and steel pipes can be controlled. In addition, in the unexamined Japanese patent application, the first publication No. H11-279700, and in the unexamined Japanese patent application, the first publication No. 2000-178689, in order to improve the longitudinal bending resistance of steel pipes, 16 mm steel sheet is produced by cooling at a constant cooling rate 15 ° C / s or higher.

Patent Reference 1: Unexamined Japanese Patent Application, First Publication No. 2004-131799.

Patent Reference 2: Unexamined Japanese Patent Application, First Publication No. 2003-293089.

Patent Reference 3: Unexamined Japanese Patent Application, First Publication No. H11-279700.

Patent Reference 4: Unexamined Japanese Patent Application, First Publication No. 2000-178689.

However, the methods disclosed in the unexamined Japanese patent application, first publication No. 2004-131799, and in the unexamined Japanese patent application, first publication No. 2003-293089, have strong temperature fluctuations at which water cooling is stopped, and therefore there is a problem that The quality of steel sheets varies greatly. In addition, even the methods disclosed in the unexamined Japanese patent application, first publication No. H11-279700, and in the unexamined Japanese patent application, first publication No. 2000-178689, have large temperature differences at which water cooling is stopped, and therefore, in addition that it is difficult to ensure the deformability of plate steel, there are problems in that the strength of plate steel varies greatly.

The present invention provides methods for producing plate steel and steel pipes for ultra-high strength pipelines with a tensile strength of 915 MPa or more (API standard X120 or higher) that have excellent strength, low temperature toughness and deformability of the starting materials and which can be easily welded at the place of use .

The essence of the invention

The inventors have conducted comprehensive studies of the conditions for producing steel sheets and steel pipes to obtain ultra high strength plate steel and steel pipes that have a tensile strength of 915 MPa or higher and excellent low temperature impact strength. As a result, new methods for producing plate steel for ultra-high strength pipelines and steel pipes for ultra-high strength pipelines were invented. The essence of the present invention is as follows:

(1) According to the method of producing plate steel for ultra-high strength pipelines, the method includes: obtaining molten steel containing: C: 0.03-0.06 mass%, Si: 0.01-0.50 mass%, Mn: 1.5-2.5 wt.%, P: 0.01 wt.% Or less, S: 0.0030 wt.% Or less, Nb: 0.0001-0.20 wt.%, Al: 0, 0005-0.03 wt.%, Ti: 0.003-0.030 wt.%, B: 0.0003-0.0030 wt.%, N: 0.0010-0.0050 wt.%, O: 0.0050 wt. .% or less, the rest Fe and inevitable impurities; pouring molten steel into a slab; hot rolling the slab to obtain a steel sheet; conducting water cooling until a predetermined temperature is reached that exceeds the point M _S , and then cooling the surface of the steel plate, repeating the treatment in which heat recovery is carried out one or more times; and conducting a final cooling with water to cool the surface of the steel plate to a temperature of M _S or lower.

Here,

M _s = 545-330 [C] +2 [Al] -14 [Cr] -13 [Cu] -23 [Mn] -5 [Mo] -4 [Nb] -13 [Ni] -7 [Si] + 3 [Ti] +4 [V],

where [C], [Al], [Cr], [Cu], [Mn], [Mo], [Nb], [Ni], [Si], [Ti] and [V] represent the amount (in% ) C, Al, Cr, Cu, Mn, Mo, Nb, Ni, Si, Ti, and V, respectively.

(2) In the method for producing plate steel for ultra high strength pipelines according to (1), the molten steel may further comprise at least one element selected from the group consisting of: Mo: 0.01-1.0 wt.%, Cu : 0.01-1.5 wt.%, Ni: 0.01-5.0 wt.%, Cr: 0.01-1.5 wt.%, V: 0.01-0.10 wt.% W: 0.01-1.0 wt.%, Zr: 0.0001-0.050 wt.% And Ta: 0.0001-0.050 wt.%.

(3) In the method for producing plate steel for ultra-high-strength pipelines according to (1), molten steel may further comprise at least one element selected from the group consisting of: Mg: 0.0001-0.010 wt.%, Ca: 0 , 0001-0.005 wt.%, REM: 0.0001-0.005 wt.%, Y: 0.0001-0.005 wt.%, Hf: 0.0001-0.005 wt.% And Re: 0.0001-0.005 wt. %

(4) In the method for producing plate steel for ultra-high-strength pipelines according to (1), the average cooling rate of the plate surface (in ° C / s) from the start of water cooling to the time when the plate surface reaches the point of martensitic transformation (point M _s ) can be equal to V _C90 or lower.

Here

M _s = 545-330 [C] +2 [Al] -14 [Cr] -13 [Cu] -23 [Mn] -5 [Mo] -4 [Nb] -13 [Ni] -7 [Si] + 3 [Ti] +4 [V]

V _C90 = 10 ^{(2.94-0.75β)}

β = 2.7 [C] +0.4 [Si] + [Mn] +0.45 ([Ni] + [Cu]) + 0.8 [Cr] +2 [Mo],

where [C], [Al], [Cr], [Cu], [Mn], [Mo], [Nb], [Ni], [Si], [Ti] and [V] mean the amount (in%) C, Al, Cr, Cu, Mn, Mo, Nb, Ni, Si, Ti, and V, respectively.

(5) In the method for producing plate steel for ultra high strength pipelines according to (1), the water cooling rate and the final water cooling rate may be V _C90 or higher.

(6) In the method for producing plate steel for ultra-high strength pipelines according to (1) during hot rolling, the reheat temperature of the slab can be 950 ° C or more, and the compression of the slab during rolling outside the recrystallization temperature range can be 3 or more.

(7) In the method for producing plate steel for ultra high strength pipelines according to (1), cooling can be carried out from an initial cooling temperature of 850 ° C or less.

(8) According to the method for producing a steel pipe for ultra-high-strength pipelines, the method includes: shaping the steel sheet for ultra-high-strength pipelines obtained by the production method according to (1) to a pipe shape on a UO press; conducting submerged arc welding on adjacent sections of a steel sheet for ultrahigh-strength pipelines from the outer and inner surfaces using a welding wire and agglomerated or fused flux; and pipe expansion.

(9) In the method for producing a steel pipe for ultra-high-strength pipelines according to (8), the weld can be heat treated after submerged arc welding and before expanding the pipes.

(10) In the method for producing a steel pipe for ultra-high-strength pipelines according to (8), the weld can be heat treated in a temperature range from 200 ° C to 500 ° C.

According to the present invention, it is possible to reduce the strength difference between the steel sheet and the steel pipe and obtain favorable deformability of the steel sheet and the steel pipe before and after deformation aging by hot rolling of plate steel of limited chemical composition and then repeating water cooling and heat recovery to conduct cooling. As a result, pipeline reliability is greatly improved.

Brief Description of the Drawings

1A is a schematic view of the distribution of hardness in a produced steel sheet in the thickness direction.

1B is a schematic view of a temperature distribution in a steel sheet in the thickness direction during cooling.

Figure 2 shows a schematic view of one example of the relationship between the cooling nature of the surface of plate steel and the steel conversion diagram.

DETAILED DESCRIPTION OF THE INVENTION

Below, the content of the present invention will be described in detail.

The present invention relates to ultra-high strength pipelines with a tensile strength (TS) of 915 MPa or higher and with excellent low temperature impact strength. Since ultra-high-strength pipelines of this strength class can withstand approximately twice as high pressure as commercially available X65 pipes, more gas can be transported using the same size as in the past. In the case when the brand X65 is used at elevated pressure, it is necessary to increase the thickness of the pipelines. As a result, the costs of materials, transportation and welding at the place of operation increase, and thus the costs of laying pipelines increase significantly. Therefore, in order to reduce the cost of laying pipelines, ultra-high-strength pipelines with a tensile strength (TS) of 915 MPa or higher and with excellent low temperature impact strength are required. On the other hand, as the strength of the required steel pipes increases, the production of steel pipes quickly becomes more complicated. In particular, when a “strain-based structure” is required, it is necessary to obtain not only a balance between the strength and low temperature toughness of the starting materials and the toughness in the areas of roller welding, but also the desired characteristics, including deformability after strain aging. However, it is very difficult to satisfy all these characteristics.

In pipelines requiring a “strain-based structure”, the strength of the weld metal that connects the piping (strength of the annular weld zones) should be higher than the strength of the starting materials (plate steel or the area corresponding to steel sheets in steel pipes) in the longitudinal direction (direction of the axis of the pipe). In pipelined environments, frozen ground may thaw in the summer and freeze again in the winter. In this case, the pipelines undergo deformation, and cracking begins with the annular zones of welding. In particular, in the case when the strength of the zones of the annular seam is less than the strength of the starting materials (mismatch), cracking is caused by slight deformation. Thus, it is necessary that the strength of the starting material in the longitudinal direction is less than the strength of the zones of the annular seam, thus, the upper tensile strength of the starting materials in the longitudinal direction is set by the strength of the zones of the annular seam. In particular, each brand of pipeline pipes has a certain range of strength, and therefore, for the production of pipelines, the strength of the starting materials is limited to a narrow range near the upper limit. Accordingly, there is a need for stable production of pipeline pipes and for starting materials for pipelines for which the strength difference would be reduced.

To limit the tensile strength of the starting materials of the pipeline to 915 MPa or higher and a narrow range, the inventors conducted a comprehensive study. As a result, it was found that it is extremely important to use steel with low content of added C and B for steel sheets and to optimize the cooling conditions of plate steel during hot rolling. For example, if the amount of C exceeds 0.06%, the hardenability will be too high, so the strength will vary significantly in the center and on the surface of plate steel. As a result, steel sheets with a small amount of added C and B are used. In addition, for example, even when the amount of C is 0.06% or less, but if cooling is carried out without any restrictions on the cooling conditions of the plate, martensite is formed or not formed depending on the method of cooling the surface of plate steel. In this case, since there is a difference in strength between the surface of the steel sheet and the center (center of thickness) of the sheet (inside the steel sheet) in the direction of thickness or a change in strength exists in the part of one steel sheet or between produced steel sheets, it becomes impossible to obtain pipelines having narrow strength range.

The above change in strength will be described using FIG. 1A and FIG. 1B. FIG. 1A shows a schematic view of the hardness distribution of the obtained plate steel in the thickness direction, and FIG. 1B shows a schematic view of the temperature distribution of the plate steel in the thickness direction during cooling. 1A and 1B, a dashed line indicates the center of sheet thickness; the dash-dot line (a) shows the result of simple cooling by water cooling (for example, the cooling conditions indicated by the broken line (i) in FIG. 2); the solid line (b) shows the result of the cooling conditions according to the present invention. As shown by the dot-dash line (a) in FIG. 1A, in the case where cooling is carried out without any restrictions on the cooling conditions of the plate steel surface (simple cooling), a difference in hardness arises between the surface of the steel sheet and the center of the sheet (inside the steel sheet) in the thickness direction. The difference in hardness is explained by the temperature distribution in the steel plate in the thickness direction during such cooling, as shown in FIG. 1B. During water cooling, the surface of the steel sheet comes into contact with water and is thus cooled. However, since the cooling rate is limited by the heat transfer inside the plate, it is more difficult to cool the inside of the plate than the surface of the plate. As a result, microstructures with different hardness are obtained on the surface of plate steel and inside plate steel. Thus, as a result of simple cooling in the obtained steel plate, a hardness distribution is formed due to the temperature distribution in the steel sheet during cooling. This distribution of hardness is not limited by the direction of the thickness and can occur in any part of the steel sheet as a result of heterogeneity, for example, an uneven amount of cooling water or the like. This heterogeneity of the strength of plate steel is a problem, since the heterogeneity causes surface defects, such as wrinkles, cracks or the like, in the production of steel pipes when stress concentration occurs on the surface of the plate steel. In addition, with simple cooling, there are cases where the temperature at which the water cooling of plate steel ceases varies for each batch of products, as a result of which there is a high probability of strength fluctuations among the produced steel sheets.

To reduce this fluctuation in strength, instead of performing a single cooling, the surface of plate steel was cooled by repeating water cooling and heat recovery, which will be described below, due to which the fluctuations in strength were successfully reduced both in the part of one steel sheet and between produced steel sheets. Heat recovery refers to processing that makes the temperature on the surface of plate steel (low temperature section) higher than immediately after the water cooling stops for a predetermined period of time in order to transfer heat from the internal volume of the steel sheet to the surface of the sheet (heat transfer from the high temperature section to the low temperature section) . With heat recovery, the temperature difference between the internal volume of the steel sheet and the surface of the steel sheet is reduced and the temperature distribution in the sheet becomes uniform. In addition, you can uniformly control the temperature change even for different batches of products. However, according to the present invention, in order to obtain a bainite structure or a mixed structure of bainite and ferrite, it is most important to carry out water cooling on the plate steel surface to a predetermined temperature exceeding the temperature of the onset of martensitic transformation (point M _S ), and then carry out cooling, repeating the treatment, in which heat recovery is carried out at least one or more times. In addition, if the average cooling rate of the plate steel surface during the period from the beginning of water cooling (initial water cooling) to the time when the plate steel surface reaches the temperature of the onset of martensitic transformation (point M _S ), the critical cooling rate (the speed at which a microstructure containing 90% martensitic structure) or lower is obtained, strength fluctuations are further suppressed. In this case, heat recovery can be carried out by controlling the amount of cooling water (for example, reducing the amount of water). In addition, heat recovery can be carried out after the final water cooling. In this case, there are periods when the temperature at the termination of cooling by water exceeds the point M _S.

Below will be described the reasons why the chemical composition of the steel plate (starting material) according to the present invention is limited. Here, the unit "%" refers to "wt.%" Based on the chemical composition according to the present invention.

C is a must as a basic element that improves the strength of the starting material. Therefore, it is necessary to add 0.03% or more C. If an excessive amount of C in excess of 0.06% is added, the weldability or toughness of the steel is impaired. Therefore, the upper limit of the amount of added C is set to 0.06%.

Si is needed as a deoxidizing element in steel production. For deoxidation, it is necessary to add 0.01% or more Si to the steel. However, if more than 0.50% Si is added, the toughness of the steel in the HAZ zones deteriorates. Therefore, the upper limit of the amount of Si added is set to 0.50%.

Mn is a necessary element for ensuring the strength and toughness of the starting material. However, if the amount of Mn exceeds 2.5%, the toughness of the starting material in the HAZ noticeably worsens. Since when the amount of Mn is less than 1.5%, it becomes difficult to ensure the strength of the starting material, the Mn content is set in the range from 1.5% to 2.5%.

P is an element that affects the toughness of steel. If the amount of P exceeds 0.01%, not only the toughness of the starting material is noticeably deteriorated, but also the toughness in HAZ. Therefore, the upper limit of the amount of P is set to 0.01%.

If an excessive amount of S is added in excess of 0.0030%, coarse sulfides are formed. Since coarse sulfides degrade toughness, the upper limit of the amount of S is set to 0.0030%.

Nb is an element that acts on the formation of carbides and nitrides, which improves strength. However, the addition of 0.0001% or less of Nb does not give such an effect. In addition, if more than 0.20% Nb is added, this causes a deterioration in toughness. Therefore, the range of Nb amounts is set in the range from 0.0001% to 0.20%.

Al is added as a deoxidizing material. In the present invention, if more than 0.03% Al is added, no Ti-based oxides are formed. Therefore, the upper limit of the amount of Al is set to 0.03%. In addition, in order to reduce the amount of oxygen in the molten steel, it is necessary to add 0.0005% Al or more. Therefore, the lower limit of the amount of Al is set to 0.0005%.

Ti is an element that exhibits the effect of grinding grain, acts as a deoxidizing material and, in addition, as a nitride-forming element. However, since the addition of large amounts of Ti leads to a significant deterioration in toughness due to the formation of carbides, the upper limit of the amount of Ti should be set at 0.030%. However, in order to obtain desired effects, it is necessary to add 0.003% or more Ti. Therefore, the range of Ti amounts is set in the range from 0.003 to 0.030%.

B is, generally speaking, an element that dissolves in steel, increasing hardenability. In particular, this effect can be obtained by adding 0.0003% or more B. However, since the addition of an excessive amount of B leads to a deterioration in toughness, the upper limit of the amount of B is set to 0.0030%.

N is needed for finely divided TiN to reduce the diameter of the austenitic grains. Since the amount of N of 0.0010% is not enough to grind the grains, the lower limit of the amount of N is set at 0.0010%. In addition, if the amount of N exceeds 0.0050%, the amount of dissolved N increases and the low temperature toughness of the starting material deteriorates, therefore, the upper limit of the amount of N is set to 0.0050%.

If an excessive amount of O in excess of 0.0050% is added, coarse oxides are formed and the toughness of the starting material deteriorates. Therefore, the upper limit of the amount of O is set to 0.0050%.

Steel comprising the above elements and a balance consisting of iron (Fe) and unavoidable impurities is the preferred base chemical composition used for the steel plate and steel pipe of the present invention.

At the same time, according to the present invention, it is possible to add at least one element selected from the group consisting of Mo, Cu, Ni, Cr, V, Zr and Ta, as required, as an element that improves strength and toughness.

Mo is an element that improves hardenability and at the same time forms carbides and nitrides, improving strength. To obtain this effect, it is necessary to add 0.01% or more Mo. However, the addition of a large amount of Mo in excess of 1.0% increases the strength of the starting material more than necessary, and also significantly impairs the toughness. Therefore, the Mo content range is set from 0.01% to 1.0%.

Cu is an effective element for increasing strength without compromising toughness. However, an amount of Cu less than 0.01% does not produce such an effect, and if the amount of Cu exceeds 1.5%, cracking in a slab or weld is very possible when heated. Therefore, the amount of Cu is set in the range from 0.01% to 1.5%.

Ni is an effective element for improving toughness and strength. To obtain this effect, it is necessary to add 0.01% Ni or more. However, when more than 5.0% Ni is added, weldability deteriorates. Therefore, the upper limit of the amount of Ni is set at 5.0%.

Cr is an element that improves the strength of steel by precipitation hardening. Therefore, it is necessary to add 0.01% or more Cr. However, if a large amount of Cr is added, the hardenability increases and, consequently, a martensitic structure is formed and the toughness decreases. Therefore, the upper limit of the amount of Cr is set at 1.5%.

V is an element having the effect of the formation of carbides and nitrides, which improves strength. However, the addition of 0.01% or less of V does not produce such an effect. In addition, the addition of more than 0.10% V leads to a deterioration in toughness. Therefore, the range of V content is set from 0.01% to 0.10%.

W is an element that improves hardenability and at the same time forms carbides and nitrides, improving strength. To obtain such effects, it is necessary to add 0.01% or more W. However, the addition of an excessive amount of W in excess of 1.0% increases the strength of the starting material more than necessary, and at the same time the toughness deteriorates significantly. Therefore, the amount of W is set in the range from 0.01% to 1.0%.

Similarly to niobium, Zr and Ta are elements having the effect of forming carbides and nitrides, improving strength. However, the addition of 0.0001% or less does not produce such an effect. In addition, the addition of more than 0.050% Zr or Ta leads to a deterioration in toughness. Therefore, the content of Zr or Ta is set in the range from 0.0001% to 0.050%.

In addition, in the present invention, at least one element selected from the group consisting of Mg, Ca, REM, Y, Hf and Re can be added, depending on need, in order to improve the pinning effect or the resistance to longitudinal cracking due to oxides.

Mg is added mainly as a deoxidizing material. However, if more than 0.010% of magnesium is added, coarse oxides are very likely to occur, thereby impacting the toughness of the starting material and the toughness in HAZ. In addition, with the addition of less than 0.0001% Mg, it is impossible to expect a sufficient degree of intracrystalline transformations and the formation of oxides necessary as fixing particles. Therefore, the addition of Mg is set in the range from 0.0001% to 0.010%.

Ca, REM, Y, Hf and Re form sulfides, which suppress the formation of MnS, which tends to stretch in the rolling direction, and improve the steel characteristics in the thickness direction, especially resistance to the formation of longitudinal cracks. If the amount of any of Ca, REM, Y, Hf and Re is less than 0.0001%, this effect cannot be obtained. Therefore, the lower limit of the amounts of Ca, REM, Y, Hf and Re is set to 0.0001%. On the contrary, if the amount of any of Ca, REM, Y, Hf and Re exceeds 0.0050%, the number of oxides of Ca, REM, Y, Hf and Re increases, and the number of oxides including ultrafine Mg decreases. Therefore, the upper limit of the amounts of Ca, REM, Y, Hf and Re is set to 0.0050%.

Steel containing the above chemical components is prepared as molten steel in a steelmaking process and then cast using a continuous casting method or the like to obtain a slab. The slab is subjected to hot rolling (heating and then rolling the slab) to obtain plate steel. In this case, the slab is heated to a temperature of point A _C3 or higher (reheat temperature) and then rolled to obtain a rolling reduction (compression ratio) of 2 or more in the recrystallization temperature range and 3 or more reduction outside the recrystallization temperature range. As a result, the average grain diameter of the initial austenite in the resulting plate steel becomes 20 μm or less.

The reheat temperature of the slab is preferably 950 ° C or higher. In addition, if the reheat temperature becomes too high, the size of the γ grains increases when heated, so the reheat temperature is preferably 1250 ° C. or lower.

As for drawing in the recrystallization temperature range, if the reduction is less than 2, the recrystallization does not occur sufficiently, therefore, the reduction during rolling is preferably 2 or more.

If the reduction during rolling outside the recrystallization range is 3 or higher, the average diameter of the primary austenite grains in plate steel becomes 20 μm or less. Therefore, the rolling reduction outside the recrystallization range is preferably 3 or higher, more preferably 4 or higher. In this case, it is possible to make the average grain diameter of primary austenite in plate steel equal to 10 μm or less.

As for the temperature at which water cooling begins (the initial water cooling temperature), it is preferable to cool the steel plate from the temperature of the onset of water cooling of 850 ° C or lower. That is, the cooling of plate steel starts from point A _e3 or lower. In this case, ferritic transformation occurs, and the ratio of the yield strength to the tensile strength of plate steel decreases, in accordance with which the deformability of plate steel becomes favorable.

As regards cooling methods, it is most important to cool the surface of the steel plate by repeating water cooling and heat recovery until the surface of the steel plate reaches the temperature of the onset of martensitic transformation. With this cooling method, the aforementioned non-uniformity of strength of plate steel can be suppressed. In addition, if the average cooling rate (° C / s) of the plate steel surface during the period from the beginning of water cooling (initial water cooling) to the time when the plate steel surface reaches the temperature of the onset of martensitic transformation (point M _S ) is set to the critical cooling rate V _C90 (° C / s) (the rate at which it is possible to obtain a microstructure 90% consisting of a martensitic structure) or lower, strength fluctuations are further suppressed. Meanwhile, the following formulas (1), (2) and (3) are formulas for calculating the points M _S and V _C90 .

M _s = 545-330 [C] +2 [Al] -14 [Cr] -13 [Cu] -23 [Mn] -5 [Mo] -4 [Nb] -13 [Ni] -7 [Si] + 3 [Ti] +4 [V] (1)

V _C90 = 10 ^{(2.94-0.75β)} (2)

β = 2.7 [C] +0.4 [Si] + [Mn] +0.45 ([Ni] + [Cu]) + 0.8 [Cr] +2 [Mo] (3)

Here [C], [Al], [Cr], [Cu], [Mn], [Mo], [Nb], [Ni], [Si], [Ti] and [V] in formulas (1) - (3) indicate the amount (in%) of C, Al, Cr, Cu, Mn, Mo, Nb, Ni, Si, Ti, and V, respectively.

At the same time, the surface temperature of plate steel is measured from the center of the steel sheet in the width direction.

Heat recovery according to the present invention will now be described. Heat recovery in the present invention relates to an operation in which, during cooling of a steel sheet, first the surface of the sheet is cooled by water cooling to a predetermined temperature exceeding the point M _S , then water cooling is stopped for a while, thereby increasing the surface temperature of the steel plate compared to temperature immediately after water cooling. Thus, the surface of the steel plate is cooled by water cooling to a predetermined temperature that is higher than the point M _S , and then processing is repeated in which heat recovery is carried out one or more times. After this, the last water cooling (final water cooling) is carried out in order to cool the surface of the steel plate to the temperature of the point M _S or lower. After the final cooling with water, another heat recovery can be carried out. In the case where heat recovery is carried out, the final cooling temperature means the temperature after the last heat recovery. Here, in order to prevent a difference in strength in plate steel, the number of heat recovery cycles of the steel sheet before final water cooling is preferably two or more. In addition, in order to provide performance, the water cooling rate and the final water cooling rate are preferably equal to V _C90 or higher. The cooling device used in the present invention has several places (called zones) where nozzles are assembled that are capable of monitoring to make the water flow the same. For example, in the present invention, zones are classified into water cooling zones where water cooling is to be carried out and heat recovery zones where water cooling is not carried out. That is, when water cooling is carried out in the first zone (water cooling zone) and water cooling is not carried out in the second zone (heat recovery zone), the surface temperature of plate steel at the outlet of the second zone becomes higher than at the outlet of the first zone. In addition, if water cooling is carried out in the third zone (water cooling zone), the surface temperature of the steel plate decreases. Essentially, when water cooling zones and heat recovery zones are repeated, the surface temperature of the steel sheet becomes lower. Zones where water cooling is not carried out (heat recovery zones) can be arbitrarily determined taking into account cooling parameters or similar plate steel. Finally, the plate steel surface is cooled to a point temperature M _S or lower in the last water cooling zone.

Below with reference to figure 2 will be described in detail the reasons why the cooling is carried out in the above conditions. Figure 2 shows an example of the relationship between the cooling pattern of the surface of plate steel and the steel conversion diagram. The dashed line (i) in FIG. 2 shows a cooling pattern for the case where the steel plate is cooled at a cooling rate of V _C90 . With this type of cooling, approximately 90% of the plate becomes martensitic. As shown by the dotted line (ii) in FIG. 2, in the case where the average cooling rate of the plate steel surface is greater than the cooling rate V _C90 , almost the entire surface of the steel sheet acquires a martensitic structure. Thus, even if heat recovery is carried out on the surface of the plate, there are cases where the toughness on the surface of the plate is markedly deteriorated and surface defects such as surface cracks or the like occur in the manufacture of steel pipes on the plate surface. On the other hand, as shown by the solid lines (iii) and (iv) in FIG. 2, in the case where the average cooling rate of the plate steel surface is less than the cooling rate V _C90 , the steel plate acquires a bainite structure or a mixed structure of bainite and ferrite according to the present invention. In addition, when conducting heat recovery on the surface of plate steel, the microstructure of plate steel becomes uniform and it is therefore possible to produce sheet steel with small fluctuations in strength.

As for the cooling cessation temperature, if the last water cooling (final water cooling) ends at 200 ° C or lower, defects that are believed to be caused by hydrogen occur in the middle of the plate thickness. Therefore, the lower limit of the cooling termination temperature is preferably set to 200 ° C.

Next, a method for producing pipe pipes by a bending process (UO press) using sheet steel for ultra-high-strength pipelines produced by the above production method will be described. After the manufacture of plate steel with a thickness of 12 to 25 mm, the steel sheet is shaped into a pipe using a UO press (C-press, U-press and O-press). Then the edges of the steel sheet, which is given the shape of a pipe, are butt-welded and tack welded. For tack welding, use arc welding with a consumable electrode or arc welding with a metal electrode in an inert gas environment. After tack weld welding, submerged arc welding is carried out on adjacent sections of the steel sheet, which is shaped like a pipe, from the outer and inner surfaces. For submerged arc welding, a welding wire and sinter or fused flux are used. Finally, a pipe connection is made to obtain a steel pipe.

In the method of producing a steel pipe for ultra-high-strength pipelines according to the present invention, it is preferable to conduct heat treatment of the weld (roller welding zone) after conducting submerged arc welding on the inner and outer surfaces and before connecting the pipes. In addition, with regard to the heat treatment conditions of the steel pipe, it is preferable to conduct heat treatment of the weld at a temperature of from 200 ° C to 500 ° C. With this heat treatment, it is possible to reduce the fraction of the mixed structure of austenite and martensite (MA), which is formed in the weld (weld metal) and which is harmful to toughness. If the weld is heated to a temperature of 200 ° C-500 ° C, the coarse MA structure formed along the grain boundaries of the initial austenite decomposes into finely dispersed cementite. However, if the weld is subjected to heat treatment at temperatures below 200 ° C, the coarse MA structure does not decompose into cementite. Therefore, the lower limit of the weld heat treatment temperature is 200 ° C. In addition, if the weld is subjected to heat treatment at temperatures above 500 ° C, the toughness of the weld deteriorates. Thus, the upper limit of the weld heat treatment temperature is 500 ° C.

Examples

Next will be described examples according to the present invention.

After heating the slabs with a thickness of 240 mm having the chemical composition shown in Table 1 to 1000 ° C-1210 ° C, hot rolling was performed in the range of recrystallization temperatures of 950 ° C or higher, until the thickness of the slabs (intermediate thickness) reaches 70-100 mm . In addition, hot rolling was carried out outside the range of recrystallization temperatures in the range from 880 ° C to 750 ° C, until the thickness of the slabs (sheet thickness) becomes 12-25 mm. Then, cooling of the steel plate (initial water cooling) was started at a temperature of 650 ° C to 795 ° C, and water cooling was continued to a predetermined temperature that is higher than the point M _S , then the treatment for heat recovery was repeated at least once or more, most cooling. After that, cooling (final cooling) was stopped at a temperature of from 300 ° C to 470 ° C. Moreover, table 1 for information also shows the electrode equivalent of C _eq and the index of sensitivity of the weld to cracking P _cm.

In order to evaluate the yield strength and tensile strength of each steel sheet obtained, full-layer test samples were taken from each steel sheet based on the API 5L standard, and tensile strength at room temperature was tested. As for the direction of sampling, full-layer samples were taken so that the longitudinal directions of the full-layer samples corresponded to the direction of the width of the steel sheets. In addition, full-layer samples were taken at a distance of 1 m from the front end and the rear end of the steel sheet in the longitudinal direction of the steel sheet. Two full-layer samples were taken on both sides in the middle of the thickness of the steel sheet in each of these positions.

Further, after forming the steel sheets with a UO press, welding was carried out on adjacent sections of the steel sheet with a tack weld by arc welding in a protective gas atmosphere of CO ₂ . After that, roller welding was carried out by submerged arc welding on adjacent sections of steel sheets from the external and internal surfaces, using a welding wire and fused flux to obtain steel pipes. The average heat input during roller welding was set in the range from 2.0 kJ / mm to 4.0 kJ / mm. At the same time, heat treatment was carried out at a temperature of 250 ° C-450 ° C in the area of roller welding of part of steel pipes. Table 2 shows the conditions for obtaining steel sheets and steel pipes.

To evaluate the yield strength and tensile strength of each steel pipe produced, an API test sample was taken from each steel pipe and tensile strength tests were performed. Regarding the direction of sampling, API test samples were taken so that the longitudinal direction of the samples corresponded to the direction of the axis of the steel pipes. In addition, two API test pieces were taken on both sides in a 1/4 cycle position from each zone of the roller weld of the steel pipe on a surface cut off perpendicular to the pipe axis. In addition, for the sake of information, in order to evaluate the deformability after deformation aging, steel pipes were heat treated at 210 ° C (holding for 5 minutes and then cooling in air) and two API test samples were taken in the same places as above, and then conducted tensile tests. The tensile test is based on API 2000. In addition, Charpy tests at -30 ° C and DWT tests (falling load tensile test) were performed to evaluate the impact strength of steel pipes. Charpy tests and DWT tests are also based on the API 2000 standard. Charpy tests and DWT tests were taken in 1/2 cycle positions from the roll welding zone of the steel pipe on a surface cut perpendicular to the pipe axis so that the longitudinal direction of the samples corresponded to the circumferential directions of steel pipes. Two DWT test specimens were taken from each steel pipe, and three Charpy test specimens were taken at the center of thickness of each steel pipe.

In addition, toughness at HAZ was evaluated for each steel pipe produced. Samples for evaluating toughness in HAZ were taken from the heat affected zone (HAZ) near the zone of roller welding in a steel pipe and an incision was formed in the position FL + 1 mm (at a distance of 1 mm from the boundary between the HAZ and the zone of roller welding towards the HAZ). Three test samples were taken from each steel pipe. All samples were evaluated by Charpy test at -30 ° C.

Table 3 shows the test results. At the same time, Table 3 shows for information not only the tensile strength, but also the yield strength and the ratio of the yield strength to tensile strength.

Steels No. 1-22 relate to the examples according to the present invention. As can be seen from table 3, these sheet steels and steel pipes have a tensile strength corresponding to grade X120 or higher, and the heterogeneity of the strength of sheet steels and steel pipes is reduced to 60 MPa or lower. In addition, steel pipes have a Charpy energy of 200 J or more and the proportion of the viscous component in the DWT test is 85% or more, and the absorbed energy in the Charpy tests in the heat affected zone (impact strength in HAZ) exceeds 50 J. Thus, steel the pipes in the examples according to the present invention have a high impact strength. Steels No. 23-35 relate to comparative examples that do not satisfy the conditions of receipt according to the present invention. Thus, steel No. 23 has a lower C content than the range according to the present invention, and therefore it exhibits an insufficient tensile strength. In steels No. 24-29, at least one element of the basic chemical components and selective elements added to the steel is contained in an amount exceeding the range of the present invention, and therefore they exhibit insufficient toughness in HAZ. On the other hand, steels No. 30-35 were cooled without heat recovery on the surface of the steel sheet and therefore they exhibit large changes in strength, of the order of 100 MPa or higher, in steel sheets and steel pipes.

Industrial applicability

It is possible to provide a method for producing plate steel and steel pipes for ultra-high-strength pipelines that have excellent strength, low temperature toughness and deformability of the starting materials, are easily welded at the place of use and have a tensile strength of 915 MPa or more (API standard X120 or higher).

Claims (10)

1. The method of producing plate steel for pipes of ultra-high strength pipelines, including the preparation of molten steel containing, wt.%:
FROM 0.03-0.06 Si 0.01-0.50 Mn 1.5-2.5 P 0.01 or less S 0.0030 or less Nb 0.0001-0.20 Al 0.0005-0.03 Ti 0.003-0.030 AT 0.0003-0.0030 N 0.0010-0.0050 ABOUT 0.0050 or less Fe and unavoidable impurities rest,
pouring molten steel into a slab, hot rolling a slab to produce plate steel, conducting water cooling until the surface reaches a predetermined temperature above the onset of the martensitic transformation of point M _S , then cooling the surface of plate steel by repeating the treatment in which heat is recovered one or more times, and carrying out the final water cooling of the plate steel surface to a point temperature M _S or lower, wherein
M _s = 545-330 [C] +2 [Al] -14 [Cr] -13 [Cu] -23 [Mn] -5 [Mo] -4 [Nb] -13 [Ni] -7 [Si] + 3 [Ti] +4 [V], where
[C], [Al], [Cr], [Cu], [Mn], [Mo], [Nb], [Ni], [Si], [Ti] and [V] mean the amount, wt.%, Cr, Cu, Mn, Mo, Nb, Ni, Si, Ti, and V, respectively. 2. The method according to claim 1, in which the molten steel further includes at least one element selected from the group comprising, wt.%:
Mo 0.01-1.0 Cu 0.01-1.5 Ni 0.01-5.0 Cr 0.01-1.5 V 0.01-0.10 W 0.01-1.0 Zr 0.0001-0.050 That 0.0001-0.050 3. The method according to claim 1, in which the molten steel further comprises at least one element selected from the group comprising, wt.%:
Mg 0.0001-0.010 Sa 0.0001-0.005 Rem 0.0001-0.005 Y 0.0001-0.005 Hf 0.0001-0.005 Re 0.0001-0.005. 4. The method according to claim 1, in which the average cooling rate, ° C / s, for the period from the initial water cooling to the time when the surface of the steel plate reaches the temperature of the point M _s , is equal to V _C90 or lower, while
M _s = 545-330 [C] +2 [Al] -14 [Cr] -13 [Cu] -23 [Mn] -5 [Mo] -4 [Nb] -13 [Ni] -7 [Si] + 3 [Ti] +4 [V],
V _C90 = 10 ^{(2.94-0.75β)} ,
β = 2.7 [C] +0.4 [Si] + [Mn] +0.45 ([Ni] + [Cu]) + 0.8 [Cr] +2 [Mo], where
[C], [Al], [Cr], [Cu], [Mn], [Mo], [Nb], [Ni], [Si], [Ti] and [V] mean the amount, wt.%, C, Al, Cr, Cu, Mn, Mo, Nb, Ni, Si, Ti, and V, respectively. 5. The method according to claim 1, in which the speed of water cooling and final cooling with water is equal to V _C90 or more. 6. The method according to claim 1, in which, during hot rolling, the reheat temperature of the slab is 950 ° C. or higher, and the slab is drawn out of the recrystallization temperature range of 3 or more. 7. The method according to claim 1, in which the cooling is carried out from the temperature of the beginning of cooling of 850 ° C or lower. 8. A method of obtaining a steel pipe for ultra-high-strength pipelines, including giving plate steel for ultra-high-strength pipelines, obtained by the method according to claim 1, the shape of the pipe using a UO press, conducting submerged arc welding on adjacent sections of steel plate for ultra-high-strength pipelines from the external and internal surfaces using welding wire and agglomerated or fused flux and pipe expansion. 9. The method according to claim 8, in which the weld is subjected to heat treatment after conducting arc welding submerged arc and before expanding the pipes. 10. The method of claim 8, in which the heat treatment of the weld is carried out in the temperature range from 200 ° C to 500 ° C.

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