JP2015028207A - High-strength hot-rolled steel sheet and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-strength steel sheet which is suppressed in occurrence of peeling in shear and punched end surfaces and improved in surface quality, hole expandability and slag encapsulation properties in arc welding.SOLUTION: A high-strength steel sheet has a composition comprising, by mass%, 0.04-0.06% C, 0.02-0.15% Si, 1.6-2.0% Mn, 0.20-0.50% Al, 0.020-0.05% Nb, 0.10-0.15% Ti, 0.02% or less P, 0.005% or less S, 0.005% or less N and remaining Fe and unavoidable impurities. Taking contents of Mn, Si and Al as [Mn], [Si] and [Al], respectively, the ratio of [Mn]/([Si]+[Al]) is 3.4-6.7. The structure is a ferrite/bentonite composite structure which is based on bentonite and has a ferrite fraction is 5-30%.

Description

  The present invention is a high-strength hot-rolled steel sheet used for automobile undercarriage members and the like, in particular, there is no peeling at the shear end face or the punched end face, and the end face property and hole expandability are excellent. The present invention relates to a high-strength hot-rolled steel sheet having excellent packaging properties and a method for producing the same.

  Conventionally, steel sheets have been increased in strength for the purpose of reducing the weight of automobile components. However, in addition to increasing the strength of steel sheets, hole expansion workability, fatigue durability, and corrosion resistance have been improved. Is required.

  Automobile members have many end faces formed by shearing or punching, but if there is a large amount of damage (peeling), there may be a starting point for fracture or a starting point for fatigue cracks. , Stretch flangeability, fatigue durability, etc. For this reason, it is necessary not to cause damage (peeling) to the end face when processing the automobile member. Furthermore, since an automobile member is also required to have an aesthetic appearance, a good surface property without an Si scale pattern is required.

Further, when an automobile member is assembled by arc welding (MAG welding, CO ₂ welding), slag may be generated and remain on the surface of the weld bead. If the amount of slag produced is large, it may be difficult to form a paint film on the surface of the weld bead, and paint detachment may occur due to slag detachment, which reduces the corrosion resistance of the automobile member.

  Patent Document 1 discloses a high-strength hot-rolled steel sheet that is free from peeling and has excellent surface properties and hole expandability, and a method for producing the same. However, in the high-strength hot-rolled steel sheet disclosed in Patent Document 1, the effect of improving the slag encapsulation is not recognized. Moreover, in the manufacturing method of the high intensity | strength hot-rolled steel plate disclosed by patent document 1, in order to ensure favorable surface property, it is necessary to make slab heating temperature less than 1170 degreeC.

  Usually, the slab heating temperature is about 1200 ° C., and lowering the slab heating temperature for a specific steel type is a limitation of the manufacturing conditions, leading to a decrease in productivity. Moreover, since the temperature in a slab falls in the skid part (furnace floor) of a heating furnace, it is difficult to strictly control a slab heating temperature to a specific temperature range. In the low temperature part in the slab, the solution of Ti or Nb may be insufficient, which affects the material.

  Patent Document 2 discloses a welding wire that reduces the slag generated on the weld bead in order to keep the surface of the weld bead clean and prevent paint detachment due to slag peeling in electrodeposition coating after welding. . However, it cannot be said that this welding wire is effective for all steel plates.

  Furthermore, since the welding wire contains 0.05% or more of S, there is a concern about solidification cracking and embrittlement of the weld metal. In particular, when a high-strength steel plate having a thickness of 780 MPa or more is used for a suspension member (plate thickness of about 2 to 4 mm) having a relatively large thickness among automotive steel plates, the concern about the embrittlement increases. Therefore, there is a demand for a steel sheet having good slag encapsulation even when a general welding wire is used.

International Publication No. 2008/123366 JP 2008-178906 A

  Based on the above prior art, the present invention suppresses the occurrence of "peeling" at the shear end face and the punched end face in high-strength steel sheets, improves surface properties and hole expandability, and slag encapsulation during arc welding. An object of the present invention is to provide a high-strength hot-rolled steel sheet that solves the problem and a method for producing the same.

  The present inventors diligently studied a method for solving the above problems. As a result, (i) Si is reduced and Al is increased by an amount commensurate with Si reduction. (Ii) The ratio of the amount of Mn of the austenite-generating element and the total amount of Si and Al of the ferrite-forming element is It was found that the above-mentioned problems can be solved by adjusting within the range.

  This invention was made | formed based on the said knowledge, and the summary is as follows.

(1) The component composition is mass%,
C: 0.04 to 0.06%,
Si: 0.02 to 0.15%,
Mn: 1.6-2.0%,
Al: 0.20 to 0.50%,
Nb: 0.02 to 0.05%,
Ti: 0.10 to 0.15%,
P: 0.02% or less,
S: 0.005% or less,
N: 0.005% or less,
The balance: Fe and unavoidable impurities, and the contents of Mn, Si, and Al are [Mn], [Si], and [Al], respectively.
[Mn] / ([Si] + [Al]) is 3.4 to 6.7,
A high-strength hot-rolled steel sheet characterized in that the structure is a composite structure of ferrite-bainite mainly composed of bainite and having a ferrite fraction of 5 to 30%.

  (2) The high-strength hot-rolled steel sheet according to (1), wherein the high-strength hot-rolled steel sheet has a tensile strength of 780 MPa or more.

  (3) The hole expansion rate of the high-strength hot-rolled steel sheet measured by the hole expansion test method described in JFS T 1001-1996 is 60% or more, as described in (1) or (2) above High strength hot rolled steel sheet.

  (4) The high-strength hot-rolled steel sheet according to any one of (1) to (3), wherein the high-strength hot-rolled steel sheet has a thickness of 2 to 4 mm.

  (5) In the method for producing a high-strength hot-rolled steel sheet according to any one of (1) to (4), the slab having the component composition according to (1) is heated to a temperature range of 1220 to 1260 ° C. Then, it is subjected to rough rolling, finish rolling is finished in a temperature range of 880 to 950 ° C., and then cooling is started at an average cooling rate of 70 ° C./second or more within 3 seconds, and a temperature range of 660 to 740 ° C. And then held at an average cooling rate of 5 ° C./second or less for 3 to 10 seconds, then cooled to a temperature range of 400 to 550 ° C. at an average cooling rate of 20 ° C./second or more, and then wound. A method for producing a high-strength hot-rolled steel sheet.

  According to the present invention, it is possible to provide a high-strength hot-rolled steel sheet having no occurrence of peeling at a shear end face or a punched end face, excellent in surface properties and hole expandability, and excellent in slag encapsulation, and a method for producing the same. it can.

  Therefore, the high-strength hot-rolled steel sheet of the present invention is a steel sheet that is required for durability and reliability (fatigue / corrosion) and is suitable as a material for steel members manufactured by shearing, punching, hole expansion, or the like. Moreover, since the high strength hot rolled steel sheet of the present invention is superior in slag encapsulation to conventional steel sheets, it is difficult to peel off slag without using a special welding wire, and a highly versatile steel sheet with high corrosion resistance. It is.

It is a figure which shows the relationship between Al (%) and slag encapsulation rate (%). It is a figure which shows the relationship between a ferrite fraction (%) and the change rate (%) of a hole expansion rate. It is a figure which shows the relationship between coiling temperature (CT) (degreeC) and a peeling score (point). It is a figure which shows the external appearance of a weld bead.

The high-strength hot-rolled steel sheet of the present invention (hereinafter sometimes referred to as “the present invention steel sheet”)
Ingredient composition is mass%,
C: 0.04 to 0.06%,
Si: 0.02 to 0.15%,
Mn: 1.6-2.0%,
Al: 0.20 to 0.50%,
Nb: 0.02 to 0.05%,
Ti: 0.10 to 0.15%,
P: 0.02% or less,
S: 0.005% or less,
N: 0.005% or less,
The balance: Fe and unavoidable impurities, and the contents of Mn, Si, and Al are [Mn], [Si], and [Al], respectively.
[Mn] / ([Si] + [Al]) is 3.4 to 6.7, and the structure is a composite structure of ferrite and bainite mainly composed of bainite and having a ferrite fraction of 5 to 30%. It is characterized by.

  Moreover, the manufacturing method (henceforth "the method of this invention") of the high intensity | strength hot-rolled steel plate of this invention heats the slab of the component composition of this invention steel plate to the temperature range of 1220-1260 degreeC, and then Then, it is subjected to rough rolling, finish rolling is finished at a temperature range of 880 to 950 ° C., and then cooling is started at an average cooling rate of 70 ° C./second or more within 3 seconds, and then cooled to a temperature range of 660 to 740 ° C. And thereafter, held at an average cooling rate of 5 ° C./second or less for 3 to 10 seconds, then cooled to a temperature range of 400 to 550 ° C. at an average cooling rate of 20 ° C. or more and then wound up. .

  First, the reasons for limiting the component composition of the steel sheet of the present invention will be described. Hereinafter,% is mass%.

C: 0.04 to 0.06%
C is an element that increases the strength of the steel sheet and suppresses the occurrence of “peeling” (fracture surface cracks) at the end face of the steel sheet during shearing or punching. C is an element that forms carbides with Nb, Ti, and the like and contributes to improvement in strength.

  If it is less than 0.04%, the effect of addition is not sufficiently exhibited, so the content is made 0.04% or more. Preferably it is 0.045% or more. On the other hand, if it exceeds 0.06%, the carbide that becomes the starting point of the hole expansion crack increases and the hole expandability decreases, so the content is made 0.06% or less. Preferably it is 0.055% or less.

Si: 0.02-0.15%
Si is an element that enhances the strength of the steel sheet by solid solution strengthening. If it is less than 0.02%, the effect of addition is not sufficiently exhibited, so the content is made 0.02% or more. Preferably it is 0.05% or more. On the other hand, if it exceeds 0.15%, Si-based scale remains on the surface of the steel sheet and the surface properties deteriorate, so the content is made 0.15% or less. Preferably it is 0.10% or less.

Mn: 1.6-2.0%
Mn is an element that enhances hardenability and enhances the strength of the steel sheet by solid solution strengthening. If it is less than 1.6%, the effect of addition is not sufficiently exhibited, so the content is made 1.6% or more. Preferably it is 1.7% or more. On the other hand, if it exceeds 2.0%, the toughness and ductility deteriorate, so the content is made 2.0% or less. Preferably it is 1.9% or less.

Al: 0.20 to 0.50%
Al is an element that improves the slag encapsulation of the steel sheet, and is an important element in the steel sheet of the present invention. Al is added at 0.20 to 0.50% under Si: 0.02 to 0.15% and Mn: 1.6 to 2.0%.

  At this time, the ratio of Mn% ([Mn]) and the total% of Si% ([Si]) and Al% ([Al]) ([Si] + [Al]), [Mn] / ([Si] + [Al]) is an important index for obtaining the steel sheet of the present invention. This point will be described later.

  Here, FIG. 1 shows the relationship between Al (%) and slag encapsulation rate (%). The slag encapsulation rate (%) is the ratio of the area of the slag to the surface area of the arc weld bead.

  The slag encapsulation rate (%) decreases to 7% or less when Al is 0.20% or more, and decreases to 5% or less when Al is 0.25% or more. If the slag encapsulation rate exceeds 7%, the paintability deteriorates, so Al is 0.20% or more. Preferably it is 0.25% or more.

  As shown in FIG. 1, when Al is increased, the slag encapsulation rate decreases to 2%. On the other hand, the castability of the slab decreases and the ferrite fraction exceeds 30%. And Preferably it is 0.40% or less.

The reason why the slag encapsulation rate (%) decreases when the Al amount is increased (slag encapsulation is improved) is not clear at present, but Al is an element that is more easily oxidized than Mn or Si. It is presumed that slag components were reduced by reducing typical slag components such as MnO and SiO ₂ .

Nb: 0.02 to 0.05%
Nb is an element that exerts a crystal grain refining effect and contributes to improvement in strength, and forms carbides to fix C and suppress the formation of cementite that is harmful to hole expansibility. Moreover, Nb carbide contributes to the strength improvement of a steel plate (precipitation strengthening). If it is less than 0.02%, the effect of addition is not sufficiently exhibited, so the content is made 0.02% or more. Preferably it is 0.025% or more.

  On the other hand, if it exceeds 0.05%, the effect of addition is saturated and an excessive increase in the solution temperature during slab heating is caused. Preferably it is 0.045% or less.

Ti: 0.10 to 0.15%
Ti, like Nb, is an element that forms carbides and fixes C, and suppresses the generation of cementite that is harmful to hole expansibility. Moreover, Ti carbide contributes to the strength improvement of a steel plate (precipitation strengthening). If it is less than 0.10%, the effect of addition is not sufficiently exhibited, so the content is made 0.10% or more. Preferably it is 0.12% or more.

  On the other hand, if it exceeds 0.15%, the effect of addition is saturated, and an excessive increase in the solution temperature during slab heating is caused, so the content is made 0.15% or less. Preferably it is 0.145% or less.

P: 0.02% or less P is an impurity element unavoidably present in steel. If it exceeds 0.02%, it segregates at the grain boundaries and inhibits toughness, hole expansibility, weldability, etc., so it is made 0.02% or less. Preferably it is 0.015% or less. P is preferably as small as possible, but reducing it to 0.001% or less leads to an increase in production cost, so 0.001% is a practical lower limit.

S: 0.005% or less S is an impurity element unavoidably present in steel. If it exceeds 0.005%, cracks occur during hot rolling, or A-based inclusions are generated and the hole expandability is inhibited, so the content is made 0.005% or less. Preferably it is 0.0035% or less. The smaller the S, the better, but a reduction to 0.001% or less leads to an increase in manufacturing cost, so 0.001% is a practical lower limit.

N: 0.005% or less N is an impurity that is inevitably mixed during the refining of steel, and is an element that forms a nitride by combining with Ti, Nb, or the like. If it exceeds 0.005%, the nitride precipitates at a relatively high temperature and becomes coarse and becomes the starting point of hole expansion cracking, so the content is made 0.005% or less. Preferably it is 0.0035% or less.

  N is preferably as small as possible, but reducing it to 0.001% or less leads to an increase in manufacturing cost, so 0.001% is a practical lower limit.

  The steel sheet of the present invention may contain Cu, Ni, Cr, Mo, V, Ca, and REM (rare earth element) as necessary within the range not impairing the properties of the steel sheet of the present invention.

[Mn] / ([Si] + [Al]): 3.4 to 6.7
Since Mn is an austenite forming element and Si and Al are ferrite forming elements, [Mn] / ([Si] + [Al]) is an important index for securing a ferrite fraction of 5 to 30%. is there.

  If [Mn] / ([Si] + [Al]) is less than 3.4, the ferrite fraction exceeds 30%, and the required hole expansion rate cannot be ensured. On the other hand, if it exceeds 6.7%, the ferrite fraction becomes less than 5%, the elongation is lowered, and the fraction of precipitation strengthened ferrite with respect to the main structure is lowered, so that the strength is lowered.

  Next, the structure of the steel sheet of the present invention will be described.

  The structure of the steel sheet of the present invention is a ferrite-bainite composite structure having a ferrite fraction of 5 to 30% and the balance being bainite. FIG. 2 shows the relationship between the ferrite fraction (%) and the rate of change (%) of the hole expansion rate. The change rate (%) of the hole expansion rate indicates the change rate of the hole expansion rate when the ferrite fraction increases or decreases with reference to the hole expansion rate when the ferrite fraction is 30%.

  As shown in FIG. 2, when the ferrite fraction exceeds 30%, the rate of change (%) in the hole expansion rate decreases (hole expansion property decreases). This is presumably because if the ferrite fraction exceeds 30%, the probability of occurrence of voids due to the hardness difference between ferrite and bainite increases during hole expansion. Therefore, the ferrite fraction is 30% or less. Preferably it is 25% or less.

  When the ferrite fraction is low, the hole expandability is good, but the elongation is lowered and the strength is lowered, so the ferrite fraction is made 5% or more. Preferably it is 10% or more.

  The ferrite fraction was calculated by the following method. That is, the L cross section of the steel sheet (the cross section in the rolling direction and parallel to the plate thickness direction) is polished using colloidal silica, and then the KAM value (Kernel Average Misorientation) is calculated using the EBSD method. The following area ratio was defined as the ferrite fraction. At this time, the measurement range was 250 μm in the rolling direction and 150 μm in the plate thickness direction centering on a quarter depth of the plate thickness, and the measurement pitch was 0.5 μm. Further, the KAM value was a value in the third proximity.

  According to the composition and structure of the steel sheet of the present invention, the steel sheet of the present invention can have a tensile strength of 780 MPa or more.

  Moreover, according to the component composition and structure of the steel sheet of the present invention, the steel sheet of the present invention is excellent in hole expansibility. The hole expansion rate, which is an index of the hole expansion property, can be calculated according to the hole expansion test method described in, for example, Japan Iron and Steel Federation Standard JFS T 1001-1996, but usually the test condition range shown in the above standard is wide. , Varies depending on the difference.

  In the steel sheet of the present invention, the hole expansion rate is preferably 60% or more because the hole expansion rate is 60% or more as measured by the Japan Iron and Steel Federation standard. When the hole expansion rate is less than 60%, the hole expansion process required for an automobile undercarriage member or the like to which the steel sheet of the present invention is applied cannot be endured and cracks.

  Although the plate | board thickness of this invention steel plate is not specifically limited, As a general purpose steel plate, 2-4 mm is preferable.

  The method of the present invention will be described.

  In the method of the present invention, the slab having the component composition of the steel plate of the present invention is heated to a temperature range of 1220 to 1260 ° C. and then subjected to rough rolling, and finish rolling is finished in a temperature range of 880 to 950 ° C. Cooling is started at an average cooling rate of 70 ° C. or more and cooled to a temperature range of 660 to 740 ° C., and thereafter held at an average cooling rate of 5 ° C. or less for 3 to 10 seconds. It winds up, after cooling to the temperature range of 550 degreeC.

Slab heating temperature (SRT): 1220 to 1260 ° C
Prior to hot rolling, it is necessary to sufficiently heat the slab in order to form a solution of Ti carbide and Nb carbide in the steel. When the slab heating temperature is less than 1220 ° C., the heating becomes insufficient, and Ti carbide and Nb carbide are not sufficiently filled. As a result, since Ti and Nb necessary for precipitation strengthening are insufficient, the precipitation strengthening ability is reduced and the strength is insufficient.

  On the other hand, when the slab heating temperature exceeds 1260 ° C., a large amount of scale is generated on the slab surface, the yield decreases, and the surface properties after rolling deteriorate. Therefore, the heating temperature of the slab is set to 1220 to 1260 ° C. Preferably it is 1230-1250 degreeC.

Finishing temperature (FT: ° C.): 880-950 ° C.
The slab heated to 1220 to 1260 ° C is subjected to rough rolling, and finish rolling is performed at 880 to 950 ° C. These hot rollings themselves may be ordinary hot rolling, but if the finishing temperature is less than 880 ° C., the rolling load increases and the load on the rolling mill increases, and the texture of the steel sheet develops, resulting in a difference in structure. Since the directivity increases and the hole expansion rate decreases, the finishing temperature is set to 880 ° C. or higher. Preferably it is 900 degreeC or more.

  On the other hand, when the finishing temperature exceeds 950 ° C., austenite (γ) grains grow by the start of cooling after the end of rolling, and the crystal grains become coarse. Therefore, there is a possibility that the steel plate strength is lowered. Therefore, the finishing temperature is 950 ° C. or lower. Preferably it is 940 degrees C or less.

Pre-cooling step to 660-740 ° C. Cooling is started at an average cooling rate of 70 ° C./second or more within 3 seconds after finishing rolling, and then cooled to the pre-cooling stop temperature of 660-740 ° C. This process is called a pre-cooling process. In the high temperature range after finish rolling, γ grains are likely to be coarsened and scales are likely to be generated. Therefore, it is preferable to start cooling quickly, and it is preferable that the average cooling rate is also high. Therefore, it is preferable to start the cooling within 2 seconds after finishing the finish rolling.

Intermediate cooling step with low cooling rate Next, the temperature is maintained for 3 to 10 seconds at an average cooling rate of 5 ° C./second or less from the preceding cooling stop temperature (660 to 740 ° C.). This process is called an intermediate cooling process. By holding in this temperature range for several seconds, ferrite which is precipitation strengthened by Ti and Nb carbides is generated, and a necessary ferrite fraction (5 to 30%) can be secured. This holding time is called intermediate cooling time. In addition, precipitation strengthening ferrite contributes to the reduction of the hardness difference with the bainite which is the main structure, improves the hole expansion rate, and contributes to securing the strength of the steel sheet.

  If the pre-cooling stop temperature is less than 660 ° C., it becomes difficult to generate ferrite even if the intermediate cooling time is increased. Therefore, the pre-stage cooling stop temperature is set to 660 ° C. or higher. Preferably it is 680 degreeC or more. On the other hand, if the pre-cooling stop temperature exceeds 740 ° C., it is difficult to produce ferrite, and the interval between precipitates becomes wide and coarse, thereby reducing the precipitation strengthening ability. Therefore, the pre-stage cooling stop temperature is set to 740 ° C. or lower. Preferably it is 730 degrees C or less.

Subsequent cooling step and winding step Subsequently, the steel sheet is cooled to a temperature range of 400 to 550 ° C. at an average cooling rate of 20 ° C./second or more, and then the steel plate is wound up in the same temperature range. This process until winding is referred to as a post-stage cooling process. The reason why the coiling temperature (CT: ° C.) is set to 400 to 550 ° C. is to secure the steel sheet strength by ensuring the ferrite fraction required in the intermediate cooling step and then making the balance a bainite structure. Therefore, it is necessary to cool quickly to the coiling temperature. The average cooling rate at this time does not need to be as high as the cooling rate in the previous cooling step, but is set to 20 ° C./second or more in order to suppress the formation of ferrite during cooling.

  The required time from the finish rolling to the winding process is determined from the rolling speed and the runout table length. Therefore, in order to ensure the pre-cooling stop temperature and the intermediate cooling time, the average cooling rate in the pre-cooling process and the post-cooling process may be appropriately adjusted within a range that does not fall below the lower limit. Therefore, there is no particular upper limit on the average cooling rate. In addition, after cooling to the coiling temperature range in the subsequent cooling step, if there is a distance to the coiler, it may be cooled at a low cooling rate, or may be idle without cooling.

  In the production method described above, any numerical index is an important index for providing the steel sheet of the present invention. Among them, slab heating temperature, finishing temperature, pre-cooling stop temperature, intermediate cooling time, and Winding temperature is the most important indicator.

  FIG. 3 shows the relationship between the coiling temperature (° C.) and the peeling score (point). The peeling score is a score given to the presence state of “peeling” in the inner periphery of the punched hole. Specifically, the steel plate is punched out by the same method as the above-mentioned hole expansion test method, and the inner periphery is divided into 12 sections, that is, virtually divided at 30 ° intervals. For punching, the die diameter was selected so that the punch diameter was 10 mm and the ratio C / t of clearance (C) to plate thickness (t) was within the range of 12 to 14%.

  When the “peel” is small and exists within a range of 30 ° or less per section, one point is given for each “peel”. Further, when it exists in a range exceeding 30 °, the number of existing sections (minimum 2, maximum 12) is multiplied by 2 points. The peeling score is the total score. Therefore, the highest score of the peeling score is 12 sections × 2 points = 24 points. In addition, the peeling score was an average score of three samples.

  If the peeling score is 4 or less, the peeling score can be set to “0” by adjusting the punching conditions. Therefore, the winding temperature is 400 to 550 ° C. However, in a region where the coiling temperature is low, cooling of the steel plate becomes unstable, and temperature controllability is reduced. Therefore, it is preferable to wind in a temperature range of 430 to 530 ° C. from the viewpoint of securing a stable winding temperature, reducing “peeling”, and securing required strength.

  Next, examples of the present invention will be described. The conditions in the examples are one example of conditions used for confirming the feasibility and effects of the present invention, and the present invention is based on this one example of conditions. It is not limited. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

(Example)
Steel No. 1 having the component composition shown in Table 1. A to P steel is melted in a converter and continuously cast, and then the slab is reheated, subjected to rough rolling, and subsequently rolled to a sheet thickness of 2.3 to 3.6 mm. After cooling with a run-out table, it was wound up to produce a hot-rolled steel sheet. Table 2 shows the manufacturing conditions for the hot-rolled steel sheet. In the component composition shown in Table 1, the balance is Fe and inevitable impurities.

  In Table 2, “slab heating temperature” is the temperature at which the slab is heated in a heating furnace before being subjected to hot rolling, and “finishing temperature” is the temperature at which the finish rolling process is completed. "Cooling end temperature" is the average temperature at the end of the previous stage cooling process, "Intermediate cooling time" is the time to hold at the low cooling rate in the intermediate cooling process, "Winding temperature" is in the winding process It is the temperature which winds with a coiler.

  Table 3 shows various properties of the obtained hot-rolled steel sheet.

  The “microstructure” is a microstructure at a depth of a quarter of the plate thickness in the L cross section of the steel plate. “Tensile strength” is the tensile strength using a JIS No. 5 test piece taken from the rolling direction and the orthogonal direction (C direction). “Yield strength” is the yield point or 0.2% yield strength. The “hole expansion rate” is a hole expansion rate obtained by the hole expansion test method described in JFS T 1001-1996.

  Of the “peeling evaluation”, the “peeling score” is a score given by the above-described evaluation method, and the “peeling judgment” is OK when the peeling score is 4 or less, and NG when it is more than 4 It is a judgment. “Surface property” indicates the result of visual confirmation of the presence or absence of Si scale defects. The case where there was no scale defect was determined as OK, and the case where there was a scale defect was determined as NG.

  “Winding temperature stability” was OK when the winding temperature was 400 ° C. or higher, and NG when the temperature was less than 400 ° C.

  “Slag encapsulation rate” indicates the slag encapsulation rate of the surface of the weld bead produced by the welding method described later. “Slag encapsulation rate determination” is OK when the slag encapsulation rate is 7% or less. The determination is NG when the value exceeds 7%.

In order to evaluate the slag encapsulation rate, pulse MAG welding employing “Ar + 20% CO ₂ ” as a shielding gas was performed using a digital pulse welding power source. The welding mode was bead-on-plate welding with the torch angle being vertically downward, and the welding speed was 0.8 m / min.

  The welding conditions were a tip-base material distance of 15 mm, a wire feed speed of 4 m / min, and a current and voltage of about 120 A and about 22 V, respectively. As the welding wire, a solid wire having a diameter of 1.2 mm conforming to the JIS Z3312 YGW15 standard was used.

  As shown in Tables 1 to 3, Invention Examples (Production Nos. 1, 2, 4, 5, 7, 8, 10, 11, 13, 14, 16, in which the component composition and production conditions are within the scope of the present invention) 17, 19, 20, 22, 23, 25, 26, 28, 29, 31, and 32), the ferrite fraction is 5 to 30%, the tensile strength (TS) is 780 MPa or more, and the hole expansion ratio is More than 60%, peeling determination, surface properties, slag encapsulation, and winding temperature stability are all OK.

  Production No. In the comparative examples of 3, 6, 9, 12, 15, 18, 21, 24, 27, 30, and 33, the component composition is within the scope of the present invention (see the steel No. in the table). ) And the manufacturing method is outside the scope of the present invention.

  Production No. In the comparative example 3, the slab heating temperature exceeds the range of the present invention, and the surface properties are deteriorated.

  Production No. In the comparative example of 6, the finishing temperature exceeds the range of the present invention and the strength is insufficient.

  Production No. In Comparative Example 9, the pre-stage cooling stop temperature is below the range of the present invention, and the intermediate cooling time is above the range of the present invention. Therefore, the ferrite fraction is insufficient, the elongation is low, and the strength is insufficient. Although an increase in the intermediate cooling time contributes to an increase in the ferrite fraction, it is necessary to select an appropriate pre-stage cooling stop temperature.

  Production No. In the twelve comparative examples, the intermediate cooling time exceeds the range of the present invention. Therefore, the ferrite fraction is excessive and the hole expansion rate is insufficient.

  Production No. In the 15 comparative examples, the coiling temperature exceeds the range of the present invention, a lot of “peeling” occurs, and the determination is NG.

  Production No. In 18 comparative examples, the slab heating temperature is below the range of the present invention, and the strength is insufficient.

  Production No. In 21 comparative examples, the coiling temperature is below the range of the present invention. At this temperature, the cooling of the steel sheet is unstable, the temperature controllability is low, and stable production cannot be achieved.

  Production No. In 24 comparative examples, the finishing temperature is below the range of the present invention, and the hole expansion rate is insufficient. Moreover, a low finishing temperature causes an increase in the rolling load, and the load on the rolling mill increases.

  Production No. In the 27 comparative examples, the intermediate cooling time is below the range of the present invention. Therefore, the ferrite fraction is insufficient, the elongation is low, and the strength is insufficient.

  Production No. In 30 comparative examples, the pre-stage cooling stop temperature exceeds the range of the present invention, the ferrite fraction is insufficient, the elongation is low, and the strength is insufficient.

  Production No. In Comparative Example 33, the pre-cooling stop temperature is below the range of the present invention, the ferrite fraction is insufficient, the elongation is low, and the strength is insufficient.

  Production No. The comparative examples 34 to 38 are comparative examples using steel whose component composition is outside the scope of the present invention (see steel Nos. L to P in Table 1).

  Production No. In Comparative Example 34, the amount of Si exceeds the range of the present invention, and the surface properties are deteriorated. Production No. In the comparative example of 35, the value of the formula (1) is below the range of the present invention, the ferrite fraction is large, and the hole expansion rate is insufficient. Production No. In the comparative example of 36, the amount of Al is below the range of the present invention, and the slag encapsulation is deteriorated.

  Production No. In the comparative example of 37, the amount of C exceeds the range of the present invention, and the hole expansion rate is insufficient. Production No. In the comparative example of 38, the value of the formula (1) exceeds the range of the present invention, the ferrite fraction is insufficient, the elongation is inferior to other examples, and the strength is insufficient.

  As described above, according to the comparison between the inventive example and the comparative example, the present invention has high tensile strength, and has good hole expandability, “peeling” of the punched end face, surface property of the steel sheet, and slag encapsulation during arc welding. It is clear that an excellent high strength hot rolled steel sheet can be provided.

  FIG. 4 shows the appearance of the weld bead. In the weld bead shown in FIG. 4, the slag encapsulation rate was 21%.

  As described above, according to the present invention, there is no occurrence of peeling at the shear end face or the punched end face, the surface property and the hole expandability are excellent, and the high-strength hot-rolled steel sheet excellent in slag encapsulation and a method for producing the same Can be provided.

  Therefore, the high-strength hot-rolled steel sheet of the present invention is a steel sheet that requires durability and reliability (fatigue / corrosion), and is suitable as a material for a member manufactured by punching or hole expansion. In addition, the high-strength hot-rolled steel sheet of the present invention is a versatile steel sheet that does not require a special welding wire because the slag encapsulation is superior to conventional steel sheets. Therefore, this invention has a high applicability in steel plate manufacture and utilization industry.

Claims (5)

Ingredient composition is mass%,
C: 0.04 to 0.06%,
Si: 0.02 to 0.15%,
Mn: 1.6-2.0%,
Al: 0.20 to 0.50%,
Nb: 0.02 to 0.05%,
Ti: 0.10 to 0.15%,
P: 0.02% or less,
S: 0.005% or less,
N: 0.005% or less,
The balance: Fe and unavoidable impurities, and the contents of Mn, Si, and Al are [Mn], [Si], and [Al], respectively.
[Mn] / ([Si] + [Al]) is 3.4 to 6.7,
A high-strength hot-rolled steel sheet characterized in that the structure is a composite structure of ferrite-bainite mainly composed of bainite and having a ferrite fraction of 5 to 30%.   The high-strength hot-rolled steel sheet according to claim 1, wherein the high-strength hot-rolled steel sheet has a tensile strength of 780 MPa or more.   The high-strength hot-rolled steel sheet according to claim 1 or 2, wherein a hole-expansion rate of the high-strength hot-rolled steel sheet measured by the hole-expansion test method described in JFS T 1001-1996 is 60% or more.   The high-strength hot-rolled steel sheet according to any one of claims 1 to 3, wherein a thickness of the high-strength hot-rolled steel sheet is 2 to 4 mm.   In the manufacturing method of the high intensity | strength hot-rolled steel plate of any one of Claims 1-4, the slab of the component composition of Claim 1 is heated to the temperature range of 1220-1260 degreeC, and is then rough-rolled. And finish-finishing in a temperature range of 880 to 950 ° C., then start cooling at an average cooling rate of 70 ° C./second or more within 3 seconds, and then cool to a temperature range of 660 to 740 ° C. High strength, characterized by holding for 3 to 10 seconds at an average cooling rate of 5 ° C / second or less, then cooling to a temperature range of 400 to 550 ° C at an average cooling rate of 20 ° C / second or more, and then winding up A method for producing a hot-rolled steel sheet.