JPH05271766A - Manufacture of high strength steel plate excellent in hydrogen induced cracking resistance - Google Patents

Abstract

(57) [Abstract] [Purpose] A method for producing a high strength steel plate (API standard X80 grade steel) having excellent HIC resistance and SSC resistance. [Constitution] In order to improve the center segregation of the continuous casting slab,
A steel in which 0.3% or less of Cr and Mo are added together to a low C-low Mn-Nb-trace Ti steel base is subjected to controlled rolling and accelerated cooling. This has made it possible to achieve high strength and high toughness and excellent HIC resistance and SSC resistance at the same time. [Effect] X80 grade steel with excellent HIC resistance and SSC resistance can now be manufactured, and in addition to the safety of the pipeline, the thinning of the steel pipe has significantly improved the local welding work efficiency.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to hydrogen-induced cracking (HI).
The present invention relates to a method for producing a high-strength steel sheet for pipelines having excellent properties (American Petroleum Institute (API) standard X80 grade or higher strength, thickness 40 mm or less).

[0002]

2. Prior Art Crude oil in cold regions, offshore, and large-diameter line pipes for transporting natural gas require high strength as well as excellent low temperature toughness and field weldability. With the development of sour and inferior resources, excellent resistance to hydrogen induced cracking (HIC) has also been demanded. On the other hand, recently, for the purpose of reducing the amount of steel used by reducing the wall thickness and improving on-site welding construction, American Petroleum Institute (API) standard 5L-X80
The use of ultra-high strength steel pipes (tensile strength of 620 MPa or more) has increased. As a result, the high strength of X80 and excellent HI resistance
The number of cases requiring C-characteristics is gradually increasing.

Conventionally, sour line pipes having excellent HIC resistance are mainly produced by high purity steel, reduction of inclusions, morphology control by addition of Ca of sulfide inclusions, light pressure reduction during continuous casting and accelerated cooling. It has been manufactured by integrating technologies such as improvement of segregation (for example, Japanese Patent Publication No. 63-0.
013695, JP-A-62-112722).
However, high-strength steel pipes such as X80 cannot be manufactured only by conventional techniques. The main reason for this is that in high-strength steel, the amount of alloying elements, especially Mn (usually 1.8 to 2.0% is added in X80) inevitably increases, resulting in segregation in the central segregation zone of the continuous casting slab. This is because a hardened structure is generated and HIC resistance is significantly deteriorated.

[0004]

SUMMARY OF THE INVENTION The present invention provides HIC resistance,
It is intended to provide a method for producing a steel sheet having strength of API standard 5L-X80 class or higher, which is excellent in sulfide stress corrosion corrosion cracking (SSC) resistance.

[0005]

The gist of the present invention is% by weight.
C: 0.02 to 0.08, Si: 0.6 or less, M
n: 1.00 to 1.40, P: 0.010 or less, S:
0.0015 or less, Nb: 0.01 to 0.06, Cr:
0.10 to 0.50, Mo: 0.10 to 0.30, T
i: 0.005-0.025, Al: 0.06 or less, C
a: 0.001-0.005, N: 0.001-0.0
05, O: 0.003 or less, and further V:
0.01-0.10, Ni: 0.05-0.50, C
u: 0.05 to 0.50, one kind or two or more kinds contained, and 0.5 ≦ [Ca] (1-124 [O]) / 1.
A steel having the balance of 25 [S] ≦ 7.0 and consisting of iron and unavoidable impurities as the balance is heated to a temperature range of 1100 ° C. to 1280 ° C., a cumulative rolling reduction of 950 ° C. or less 60% or more, a rolling end temperature 680. After rolling at ℃ ~ 900 ℃, water cooling to 350 ℃ ~ 600 ℃ at a cooling rate of 3 ~ 40 ℃ / second,
Then let it cool down.

The present invention will be described in detail below. High strength, excellent low-temperature toughness, excellent weldability along with excellent H resistance
In order to obtain IC property, first of all, it is necessary to limit the chemical composition of steel. Therefore, the amounts of C and Mn were significantly reduced compared to the conventional X80 steel, and Cr and Mo were added in combination as a substitute. The reason for this is to improve the center segregation of the continuous casting (CC) slab and prevent the generation and propagation of HIC.

In high strength steel such as X80 steel, inevitably M
Although the n content becomes high, Mn segregates with P etc. in the central segregation zone of the CC slab, promoting the formation of a hardened structure and resisting HI
Remarkably lowers C property. In order to prevent this, the upper limit of the amount of Mn must be 1.40%. The lower limit of 1.0% of the amount of Mn is the minimum amount for ensuring strength and toughness.

Further, the reduction of the amount of C reduces the central segregation of Mn and P, and also reduces the absolute amount of the hardened structure formed in the center segregated zone, which is effective for making the hardened structure finer. Therefore, the upper limit is limited to 0.08%. The amount of C is reduced by Cr, M
In the steel of the present invention in which o is added in a complex manner, it is essential for improving the low temperature toughness of the base metal and the weld heat affected zone (HAZ) and the field weldability. The lower limit of 0.02% of the amount of C is the minimum amount for ensuring the strength of the base metal / welded portion.

Reducing the amounts of C and Mn has a great effect on improving the center segregation of the CC slab. However, such low C, M
With the n amount, it is not possible to obtain high strength such as X80.
Therefore, as a result of earnest research, the present inventors have invented that the combined addition of Cr and Mo is extremely effective.

It is clear that center segregation of Cr is difficult even in CC slabs, it is effective for high strength of low C-low Mn steel in the controlled rolling / accelerated cooling process, and the low temperature toughness and field weldability are not impaired. It was Also, Mo is low C-
It was found that it has a great effect on the homogenization of the structure of the central segregation zone (fine dispersion of the hardened structure) in the low Mn steel and improves the HIC resistance. In order to obtain an excellent synergistic effect due to the addition of Cr and Mo, Cr and Mo are each 0.1
0% is the minimum requirement. However, if too much is added, H
It is harmful to AZ toughness and field weldability, and the upper limits of Cr and Mo contents should be 0.50% and 0.30%, respectively.

The steel of the present invention contains Nb: 0.0 as an essential element.
1-0.06%, Ti: 0.005-0.025% is contained. Nb contributes to grain refinement and precipitation hardening in controlled rolling, and strengthens steel. Further, the addition of Ti forms fine TiN, and is effective in improving the toughness of the base material and the HAZ toughness by suppressing γ grain coarsening during slab heating and welding. C
When r is added, separation is less likely to occur on the fracture surface such as in the Charpy impact test even in controlled rolled steel, which causes deterioration of low temperature toughness. Therefore, particularly in the present invention steel, addition of Nb and Ti is essential from the viewpoint of ensuring low temperature toughness. I found out. The lower limits of the amounts of Nb and Ti are the minimum amounts for these elements to exert their effects, and the upper limits thereof are HAZ.
This is the limit of the additive amount that does not deteriorate the toughness and field weldability.

Next, the reasons for limiting other elements will be described. If a large amount of Si is added, the field weldability and HAZ toughness deteriorate, so the upper limit was made 0.6%. Only Al and Ti are sufficient for deoxidizing steel, and Si is not necessarily added.

In the steel of the present invention, P and S, which are impurities, are set to 0.010% and 0.0015% or less, respectively, and Ca is added so that 0.5≤ [Ca] (1-124).
[O]) / 1.25 [S] ≦ 7.0. P promotes the center segregation of the CC slab and forms a hardened structure to facilitate the generation and propagation of HIC, so the P content was limited to 0.010% or less.

S forms MnS-based inclusions, and MnS
Is elongated by rolling and becomes a starting point of HIC generation. In order to prevent this, the absolute amount of inclusions must be reduced and the form of sulfide must be controlled to make CaS difficult to control by rolling.

Therefore, the amount of S is set to 0.0015% or less (desirably 0.0010% or less), and Ca is set to 0.001% to 0.001%.
0.005% was added. In order to sufficiently control the morphology of sulfide by Ca, ESSP = [Ca] (1-124
[O]) / 1.25 [S] ≧ 0.5. Moreover, ES
If the SP is too large, Ca-based inclusions increase and become a starting point of HIC generation, so the upper limit was set to 7.0.

In relation to the above, the amount of O is limited to 0.003% or less. This is to reduce the oxide inclusions that are the origin of HIC and to control the morphology of sulfide with a small amount of Ca.

Al is an element contained in steel as a deoxidizing element, but deoxidizing can be performed with Ti or Si, and it is not always necessary to add it. When the amount of Al is 0.06% or more, Al-based nonmetallic inclusions increase and impair the cleanliness of steel, so the upper limit was made 0.06%.

N forms TiN and improves the base metal and HAZ toughness by suppressing coarsening of γ grains during slab reheating and welding. The minimum amount required for this is 0.001%. However, if it is too much, HA will be caused by slab surface defects and solid solution N
Since it causes deterioration of Z toughness, its upper limit is 0.005%.
It is necessary to keep below.

Next, the reason for adding V, Ni and Cu will be described. The main purpose of adding these elements to the basic composition is to improve the properties such as strength and toughness without impairing the excellent characteristics of the steel of the present invention. Therefore, the amount of addition is limited by itself.

V has almost the same effect as Nb, and is effective in improving the low temperature toughness by increasing the microstructure, increasing the hardenability, and increasing the strength by precipitation hardening. The minimum amount for exerting the effect is 0.01%. However, if the addition amount is too large, on-site weldability and HAZ toughness are deteriorated, so the upper limit was made 0.10%.

Ni improves both strength and toughness without adversely affecting the on-site weldability and HAZ toughness, and is also effective in preventing hot cracking when Cu is added. The minimum amount for exerting the effect is 0.05%. However, if it exceeds 0.5%, it is not preferable from the economical point of view.
The upper limit was 0.5%.

Cu is also effective in corrosion resistance and HIC resistance. The minimum amount for exerting the effect is 0.05%. However, if it exceeds 0.5%, Cu-
Cracks occur, making manufacturing difficult. Therefore, the upper limit is set to 0.5%.

In order to improve the low temperature toughness of the base metal in the Cr-added steel as described above, the manufacturing method must be further appropriate. For this reason, steel (slab) reheating, rolling,
It is necessary to limit the cooling conditions. First, set the reheating temperature to 11
It is limited to the range of 00 to 1280 ° C. Reheating temperature is Nb
In order to form a solid solution with the precipitate and to secure the rolling end temperature, the temperature must be 1100 ° C or higher (desirably 11
50-1200 ° C). However, the reheating heat temperature is 12
At 80 ° C. or higher, the γ grains are significantly coarsened and cannot be completely refined even by rolling, so that excellent low temperature toughness cannot be obtained. Therefore, the reheating temperature is set to 1280 ° C or lower.

Furthermore, the cumulative rolling reduction below 950 ° C. is 60%.
As described above, the rolling end temperature must be 680 to 900 ° C. This is to improve the low temperature toughness by stretching the γ grains refined by the recrystallization region rolling by the low temperature rolling to thoroughly refine the ferrite grain size. If the cumulative reduction amount is less than 60%, the elongation of the γ structure is insufficient and fine ferrite grains cannot be obtained. The rolling end temperature is 900 ° C.
With the above, fine ferrite grains cannot be achieved even if the cumulative reduction amount is 60% or more. However, if the rolling end temperature is too low, the rolling will be excessive (γ + α) two-phase region rolling and the low temperature toughness will be deteriorated. Therefore, the lower limit of the rolling end temperature was set to 680 ° C.

After rolling, it is essential to accelerate the steel sheet. Accelerated cooling is effective in improving the microstructure including the central segregation zone, and makes it possible to increase strength and HIC resistance without impairing toughness. As a condition for accelerated cooling, immediately after rolling, immediately after cooling, a cooling rate of 3 to 40 ° C / sec and a temperature of 350 ° C to 600 ° C
It must be cooled to the following temperature and then air cooled.
If the cooling rate is too slow or the cooling stop temperature is too high, the effect of accelerated cooling cannot be obtained sufficiently and an appropriate microstructure cannot be obtained. On the other hand, if the cooling rate is too high or the stopping temperature is too low, a hardened structure is formed, resulting in low temperature toughness and HI resistance.
C property is significantly deteriorated.

It should be noted that the characteristics of the present invention will not be impaired if the steel is reheated at a temperature below the Ac ₁ point for the purpose of tempering, dehydrogenation, etc. after the steel is manufactured. The CC slab may be hot-charged and rolled in a heating furnace for the purpose of energy saving.

The present invention is most preferably applied to thick plate mills, but can also be applied to hot coils. Further, the steel sheet produced by this method is excellent in low temperature toughness and field weldability, and is therefore suitable not only for pipelines in cold regions but also for pressure vessels.

[0028]

EXAMPLES Steel sheets (thickness: 12 to 30 mm) of various steel components were manufactured in a converter-continuous casting-thick plate process, and their strength, low temperature toughness, HAZ toughness and HIC resistance were investigated. Examples are shown in Table 1.

[0029]

[Table 1]

[0030]

[Table 2]

The steel sheets manufactured according to the method of the invention (steel of the invention) all have good properties. On the other hand, the comparative steels not according to the present invention have strength, low temperature toughness, HAZ toughness,
Either of the HIC resistance is inferior.

In Comparative Steels 7 to 14, Steels 7, 8 and 9 are inferior in both HAZ toughness and HIC resistance because the amounts of C, Mn, Mo or Cr are too high. Comparative Steel 10 has a large amount of S and a small ESSP,
The HIC resistance is poor. The comparative steels 11 to 14 have the same composition as the steel of the present invention, but are inferior in base material strength, toughness or HIC resistance because the manufacturing conditions are not appropriate. Steel 11 has a too slow cooling rate after rolling, steel 12 has a low slab reheating temperature, steel 13 has a high water cooling stop temperature, and steel 14 has a small cumulative reduction of 950 ° C. or less.

[0033]

Industrial Applicability According to the present invention, it becomes possible to mass-produce ultra-high strength X80 steel excellent in HIC resistance at low cost. As a result, the on-site welding work efficiency and the safety of the pipeline were significantly improved.

Claims (1)

[Claims] 1. By weight%, C: 0.02 to 0.08, Si: 0.6 or less, Mn: 1.00 to 1.40, P: 0.010 or less, S: 0.0015 or less, Nb: 0.01-
0.06, Cr: 0.10 to 0.50, Mo: 0.10
0.30, Ti: 0.005-0.025, Al: 0.06 or less, Ca: 0.001-0.005, N: 0.001-
0.005, O: 0.003 or less, and if necessary, V: 0.01-0.10, Ni: 0.05-
0.50, Cu: 0.05 to 0.50, containing 1 or 2 or more and 0.5 ≦ [Ca]
A steel having a balance of iron and unavoidable impurities that satisfies (1-124 [O]) / 1.25 [S] ≦ 7.0 is 110
Heating to a temperature range of 0 ° C to 1280 ° C, a cumulative rolling reduction of 950 ° C or less 60% or more, a rolling end temperature 680 ° C to 90 ° C.
After rolling at 0 ° C, cooling rate is 3 to 40 ° C / sec.
A method for producing a high-strength steel sheet excellent in hydrogen-induced cracking resistance, which comprises water cooling to 50 ° C. to 600 ° C. and then cooling.