JP4161935B2 - Hot-rolled steel sheet and manufacturing method thereof - Google Patents

Description

  The present invention relates to a fine-grain hot-rolled steel sheet excellent in mechanical properties suitable as a material for which ductility and burring workability of automobile undercarriage parts and the like are required, and a method for producing the same.

In recent years, from the viewpoint of global environmental protection, the weight reduction of automobile bodies has been promoted for the purpose of reducing automobile CO ₂ emissions. One of the means for reducing the weight is to reduce the thickness of the vehicle body member. For this reason, the use of high-strength steel sheets is increasing, and the steel sheets are also tending to have higher strength.

  As a high-strength steel sheet for automobiles, in addition to high strength, excellent formability is required for processing into various component shapes. That is, with the movement of the weight reduction of the vehicle body, high strength and high formability are required for the steel plate used for the vehicle body member.

  Although there are various methods for strengthening steel, the refinement of the structure has recently begun to attract attention as a means for increasing the strength that does not require the addition of an alloy element that should be excluded from the viewpoint of environmental protection. However, there is a problem that as the grain refinement proceeds, the yield point becomes extremely high, the elongation decreases, and the workability deteriorates.

  In order to suppress the deterioration of workability accompanying the increase in strength, a technique for refining the structure and controlling the second phase structure in the high-strength steel has been proposed. For example, as a method for simultaneously obtaining high strength and fine structure, there is a method of controlling rolling by including Nb or Ti proposed in Patent Document 1 (Japanese Patent Laid-Open No. 2000-328186). However, since the precipitation-strengthened high-strength steel sheet obtained by this method has a high yield ratio, the spring back after processing such as press molding is large and the shape freezeability is inferior, and the anisotropy of mechanical characteristics is also low. There is a problem of being big.

  As hot-rolled steel sheets excellent in workability by second phase control, those having a mixed structure consisting of a ferrite / martensite structure or a ferrite / bainite structure, or a substantially single-phase structure mainly composed of bainite or ferrite are widely known. .

  As a steel material excellent in both strength and ductility, for example, DP steel (composite structure ferritic / martensitic steel) as described in Patent Document 2 (Japanese Patent Laid-Open No. 55-44551) is known. Steel has micro-voids around the martensite from the beginning of deformation, causing cracks, so there is a problem that the hole expandability is inferior, and it is not suitable for applications that require high hole expandability such as undercarriage parts. .

  Regarding improvement of stretch flangeability and hole expandability, Patent Document 3 (Japanese Patent Laid-Open No. 4-88125) and Patent Document 4 (Japanese Patent Laid-Open No. 3-180426) disclose steel sheets having a structure mainly composed of bainite. Has been. However, since the steel sheet has a structure mainly composed of bainite, the hole expandability is excellent, but the ductility is inferior because there are few soft ferrite phases.

  Patent Document 5 (Japanese Patent Laid-Open No. 2000-109951) discloses a hot-rolled steel sheet that has a bainite single phase and further refines its structure to have both high ductility and high stretch flangeability. Since steel sheets require the addition of at least one of Ti and Nb, an increase in yield strength ratio is inevitable, shape freezeability is inferior, and anisotropy in mechanical properties increases.

JP 2000-328186 A

JP 55-44551 A JP-A-4-88125 Japanese Patent Laid-Open No. 3-180426 JP 2000-109951 A

  An object of the present invention is to provide a hot-rolled steel sheet having a dual phase structure of ferrite and bainite that has both high ductility and stretch flangeability, and a method for producing the same.

  As a result of intensive studies on steel components and production conditions in order to achieve the above-mentioned problems, the present inventors have obtained the following knowledge.

  (A) In a multiphase steel sheet of ferrite and bainite, workability is improved by finely dispersing ferrite grains.

  (B) The fine dispersion of ferrite can suppress the propagation of cracks during processing, improve stretch flangeability, and obtain high ductility by uniformly deforming ferrite grains throughout the entire structure.

  (C) Furthermore, the workability can be further improved by actively utilizing the grain size distribution of ferrite in the plate thickness direction and the texture of the surface layer.

(D) As a manufacturing condition for obtaining the above microstructure without adding Ti, Nb, etc., it is effective to perform ultra-rapid cooling immediately after finish rolling. It is also effective to finish the final rolling pass at a temperature of Ar ₃ or higher and then cool to 640 ° C. or lower within 0.6 seconds.

  The gist of the present invention based on the above findings is the hot rolled steel sheet of the following (1) to (4) and the hot rolled steel sheet manufacturing method of (5).

(1) By mass%, C: 0.02-0.25%, Si: 2.0% or less, Mn: 0.3-2.5%, P: 0.2% or less, S: 0.05% or less, Al: 0.005-0.5% and N: 0.01% And the balance is Fe and impurities, the volume ratio is 40 to 95% bainite phase and the balance is ferrite. The ferrite has an average crystal grain size of 1.2 μm or more. A hot-rolled steel sheet that is less than 4 μm and has at least one of the following characteristics (a) and (b).
(a) The ratio (ds / dc) of the average grain size (ds) of ferrite at the position of 1/8 thickness from the surface of the steel plate to the average grain size (dc) of ferrite at the center of the plate thickness is 0.3 to 0.7.
(b) The sum of the polar densities of {110} <111>, {110} <001> and {211} <111> at a position of 1/8 thickness from the surface of the steel plate to the thickness direction has a texture. It must be at least 5 times that of the other and 1.5 times each.

(2) In addition to the above component (1), it contains at least one component selected from the following group 1 components, with the balance being Fe and impurities, 40 to 95% by volume It has a structure containing a bainite phase and the balance being a ferrite phase, the ferrite has an average crystal grain size of 1.2 μm or more and less than 4 μm , and at least one of the above-mentioned features (a) and (b) hot-rolled steel sheet having a.

Group 1 ingredients
Ti: 0.2% or less, Nb: 0.1% or less, V: 1% or less, B: 0.005% or less, Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.0% or less, and Mo: 1.0% or less.

(3) In addition to the above component (1), it contains at least one component selected from the following second group of components, with the balance being Fe and impurities, with a volume ratio of 40 to 95%. A structure containing a bainite phase and the balance being a ferrite phase, wherein the ferrite has an average crystal grain size of 1.2 μm or more and less than 4 μm , and at least one of the features (a) and (b) A hot-rolled steel sheet.

Each component of the second group, or a total of two or more of 0.005% or less of Ca, REM and Mg.

(4) In addition to the component (1) above, the composition contains at least one component selected from the first group of components and at least one component selected from the second group of components, with the balance being Fe and consisting impurities, the remainder contains 40% to 95% of bainite phase by volume has a structure comprising a ferrite phase, the mean crystal grain size of the ferrite is less than 4μm above 1.2 [mu] m, and wherein the A hot-rolled steel sheet having at least one of the characteristics of (a) and (b) .

(5) A method for producing a hot-rolled steel sheet that performs multi-pass hot rolling on a slab or steel sheet having the chemical composition according to any one of (1) to (4) above, The sheet thickness reduction rate in the temperature range of Ar ₃ points or more and “Ar ₃ points + 100 ° C.” or less is 40% or more, the rolling reduction per pass is 10 to 60%, and the rolling temperature in the final rolling pass is Ar ₃ points or more. And then cooled to 640 ° C or lower within 0.6 seconds, the residence time in the temperature range from 600 ° C to 640 ° C or lower is 25 seconds or shorter, and the hot rolling is performed at 600 ° C or lower and 350 ° C or higher. A method of manufacturing a steel sheet.

  In the method (5), it is desirable that the cooling to 640 ° C. or lower is performed at a cooling rate of 400 ° C./second or more, and the cooling is performed to 640 ° C. or lower within 0.4 seconds.

  The hot-rolled steel sheet of the present invention is excellent in properties such as ductility and stretch flangeability, and is a hot-rolled steel sheet suitable as a material that requires ductility and burring workability of automobile undercarriage parts and the like. This steel plate is particularly suitable for automobile bodies and the like that aim to reduce weight. And according to the manufacturing method of this invention, said hot-rolled steel plate can be manufactured reliably.

Hereinafter, the chemical composition and structure of the hot-rolled steel sheet defined in the present invention and the reasons for limiting the manufacturing conditions will be described. In the following description, “mass%” regarding the content is simply referred to as “%”.
Chemical composition of hot-rolled steel sheet C:
C is an important element for securing the strength due to bainite generation, and the finishing temperature of hot rolling can be lowered by stabilizing austenite at a high temperature, thereby promoting the refinement of ferrite crystal grains. . In order to obtain these effects, a content of 0.02% or more is necessary. However, if added excessively, workability deteriorates due to insufficient precipitation of ferrite and precipitation of coarse iron carbide, and weldability also deteriorates. Therefore, the upper limit of the content is 0.25%. A more preferable content is 0.03-0.2%.

Si:
It is effective to add Si to improve the strength by suppressing the reduction of the ductility of the steel, but excessive addition raises the Ar ₃ point and not only makes ferrite grain refinement difficult, In order to deteriorate the surface properties and weldability of the steel plate, the upper limit is set to 2.0%. Since Si does not need to be positively added, the lower limit of the content may be an impurity level. However, if it is desired to obtain the above effect, it is desirable to contain 0.01% or more.

Mn:
Mn is an important element for securing strength due to the formation of a bainite structure, and also stabilizes austenite at high temperatures, so that the finishing temperature of hot rolling can be lowered and the ferrite crystal grains can be refined. Promote. In order to obtain this effect, a content of 0.3% or more is required. However, excessive addition deteriorates stretch flangeability due to a large amount of nonmetallic inclusions, so the upper limit of the content is set to 2.5%. A more preferable content is 0.5 to 2.0%.

P:
P acts as a solid solution strengthening element, so it may be added to increase the strength. On the other hand, since it is also an element that easily segregates, excessive addition causes deterioration of workability due to grain boundary segregation. Therefore, the upper limit of the content is 0.2%. More preferred is 0.1% or less, and further more preferred is 0.05% or less. In addition, since it is not necessary to add P actively, the minimum of content may be an impurity level. Generally, about 0.005% is inevitably mixed in the steelmaking stage. However, in order to obtain the effect of increasing the strength described above, it is preferable to contain 0.01% or more.

S:
Since S is an impurity element that forms sulfide inclusions and reduces workability, it is preferable to reduce the content thereof as much as possible. The allowable upper limit of the content is 0.05%, but 0.008% or less is preferable to further ensure excellent workability, and 0.003% or less is more preferable.

Al:
Al is necessary as a deoxidizing element for steel and must be contained in an amount of 0.005% or more. On the other hand, excessive addition not only raises the Ar ₃ point and makes ferrite grain refinement difficult, but also degrades weldability, so the upper limit of the content is 0.5%.

N:
N is an impurity element that reduces the workability of steel, and its content is preferably suppressed to 0.01% or less. More preferred is 0.006% or less.

  One of the hot-rolled steel sheets of the present invention is composed of Fe and impurities in addition to the above components. The other includes at least one of the first group of components. The components belonging to the first group are all components that contribute to improving the strength of the hot-rolled steel sheet.

Ti:
Ti precipitates as Ti (C, N) and increases the strength of the steel. Moreover, this precipitate suppresses the coarsening of austenite and ferrite, and promotes refinement of crystal grains. If these effects are desired, they may be added. However, if added excessively, a large amount of coarse Ti (C, N) is generated during heating before hot rolling, causing deterioration of stretch flangeability and increasing in-plane anisotropy. %. Preferable is 0.15% or less. In addition, in order to acquire said effect, it is good to contain 0.005% or more.

Nb:
Nb precipitates as Nb (C, N), increases the strength of the steel, and this precipitate suppresses the coarsening of austenite and ferrite and promotes the refinement of crystal grains. If these effects are desired, they may be added. However, if added excessively, a large amount of coarse Nb (C, N) is generated during heating before hot rolling, resulting in deterioration of stretch flangeability and in-plane anisotropy. Therefore, the upper limit of the content is 0.1%. Preferable is 0.06% or less. In addition, in order to acquire said effect, it is good to contain 0.005% or more.

V:
V increases the strength of the steel by precipitation strengthening, and this precipitate suppresses the coarsening of ferrite and promotes the refinement of crystal grains. If these effects are desired, they may be added. However, for the same reason as Ti and Nb, ductility and workability are hindered, so the upper limit of the content is made 1%. Preferred is 0.5% or less, and more preferred is 0.3% or less. In addition, in order to acquire said effect, it is good to make it contain 0.01% or more.

B:
B is an element that contributes to high strength by improving hardenability, so it may be added, but adding too much not only saturates its effect, but also reduces ductility, so the content of The upper limit is 0.005%. In addition, in order to acquire said effect, it is good to contain 0.0005% or more.

Cu:
Cu may be added because it is an effective element for solid solution strengthening and precipitation strengthening, but adding too much will not only saturate the effect but also increase the cost, so 1.0% is made the upper limit of the content . In addition, in order to acquire said effect, it is good to make it contain 0.01% or more.

Ni:
Ni is an effective element for improving the hardenability and obtaining a desired structure. Further, when Cu is added, there is an effect of preventing grain boundary embrittlement of the slab. When these effects are desired, they may be added. The content is preferably in the range of Ni ≧ Cu / 2, but excessive addition increases the cost, so 1.0% is made the upper limit. The lower limit of the desirable content when not combined with Cu is 0.01%.

Cr:
Cr may be added because it is an effective element for improving the hardenability and obtaining the desired structure, but the effect will be saturated even if added excessively, and the production cost of the steel sheet will be increased. Therefore, the upper limit of the content is set to 1.0%. In addition, in order to acquire said effect, containing 0.01% or more is desirable.

Mo:
Mo may also be added because it is an effective element for improving the hardenability to obtain a desired structure, but if added too much, the effect is saturated, and the manufacturing cost of the steel sheet also increases. Therefore, the upper limit is set to 1.0%. Preferred is 0.5% or less, and more preferred is 0.3% or less. In addition, in order to acquire said effect, containing 0.005% or more is desirable.

  Still another one of the hot-rolled steel sheets of the present invention contains at least one of the above-mentioned second group of components. All of the components belonging to the second group are components that contribute to the improvement of workability of the hot-rolled steel sheet. In addition, you may use together at least 1 sort (s) of the 2nd group component and at least 1 sort (s) of the said 1st group component.

Ca, Mg, REM:
Since these elements have the effect of improving the workability of steel by controlling the form of MnS, they may be added to obtain this effect. However, when the content of these elements increases, the effect tends to saturate, and since both are expensive elements, the upper limit is set to 0.005% for the single content or the total content. In order to obtain the effect of improving the workability described above, it is desirable to contain 0.0002% or more alone or in total. REM is a general term for rare earth elements and includes Y and Sc.

2. Structure of hot-rolled steel sheet From the viewpoint of improving stretch flangeability of a hot-rolled steel sheet, it is preferable that the metal structure is homogeneous. In order to combine high strength and high stretch flangeability, a higher bainite phase fraction is preferable, but bainite, which is a hard phase, has poor ductility and high ductility cannot be obtained. Therefore, from the viewpoint of improving ductility, it is preferable to increase the volume fraction of ferrite, but due to the increase in the interface between ferrite and bainite, the probability that a crack will occur at the initial stage of processing increases and stretch flangeability deteriorates. Therefore, the optimum amount of bainite phase is determined from the viewpoint of balance between ductility and stretch flangeability.

  When the volume fraction of bainite is less than 40%, high strength cannot be obtained and the stretch flangeability is inferior, and when it exceeds 95%, the ductility is inferior. The volume fraction of bainite is preferably 50 to 95%.

  Further, particularly important in the hot-rolled steel sheet of the present invention is the refinement of ferrite. Due to the fine dispersion of the ferrite grains, the deformation of the ferrite phase becomes uniform over the entire thickness, so that the ductility is improved. Further, even when a crack is generated at the interface between the ferrite phase and the bainite phase during the hole expanding process, the stretch flangeability is improved by the effect of delaying the crack propagation.

  In order to obtain these effects, it is necessary to control the average grain size of ferrite in the entire steel sheet from 1.2 μm to 4 μm. If it is less than 1.2 μm, the inherent ductility of the ferrite grains cannot be exhibited, and the yield strength ratio (yield strength / maximum strength) becomes high, leading to ductility deterioration. On the other hand, if it exceeds 4 μm, the effect of delaying crack propagation at the time of hole expansion processing cannot be obtained, and stretch flangeability is not improved. Preferred is 1.5 to 3.0 μm.

  Further, the refinement of the structure on the surface of the steel sheet, which is the starting point of the crack, has the effect of delaying the crack generation itself, and the refinement of the surface layer of the steel sheet is effective in improving the stretch flangeability. However, excessive refinement increases the yield strength ratio of the steel sheet surface layer, leading to ductility deterioration, and the ductility of the steel sheet itself deteriorates. Therefore, it is preferable that the range of the average ferrite grain size (ds) at the thickness position of 1/8 in the thickness direction is the ratio to the average ferrite grain size (dc) at the center of the thickness, that is, “ds / dc”. ”Is in a range satisfying 0.3 ≦ ds / dc ≦ 0.7.

  On the other hand, the sum of the polar densities of {110} <111>, {110} <001> and {211} <111> at a thickness of 1/8 from the surface in the thickness direction has no texture. It is also preferable that the ratio is 1.5 times or more and each is 1.5 times or more. This is because workability can be improved when these orientations are developed. The reason why the workability is improved is not clear, but is presumed as follows. That is, in order for the grain boundary to function as a grain boundary, the grain boundary is preferably a grain boundary (large tilt grain boundary) having an angular difference of 15 degrees or more with adjacent grains. It is considered that the ratio of the boundary increases and the fine graining effect is efficiently exhibited. More preferably, the sum of the extreme densities is 6 times or more that of no texture.

  The structure of the hot-rolled steel sheet of the present invention is composed of 40% to 95% bainite by volume and the balance being ferrite.

  The volume fraction of bainite in the present invention is an arithmetic average value measured by a so-called mesh method for each of five or more visual fields at thickness positions of 1/8, 1/4 and 1/2 in the thickness direction. . The average grain size of ferrite is the sphere equivalent to the arithmetic average value obtained by measuring any five or more visual fields at 1/8, 1/4 and 1/2 thickness positions in the plate thickness direction by the cutting method. The average grain size of ferrite of the entire steel sheet is an arithmetic average value of ferrite grain diameters at the three plate thickness positions.

3. Manufacturing method In the manufacturing method of the hot-rolled steel sheet of the present invention, a steel slab or steel sheet having the chemical composition described so far is used as a raw material. These materials are slabs obtained by continuous casting or casting / splitting, steel plates obtained by strip casting, etc., and if necessary, hot working or cold working. If they are cold materials, they are reheated to a temperature of Ac ₃ point or higher and rolled.

The heating temperature before hot rolling is not particularly limited as long as it is a temperature of Ac ₃ point or higher. Generally, it is 900-1350 degreeC. In the case of a steel type in which precipitates such as TiC and NbC do not need to be sufficiently dissolved in austenite, it is preferable to reheat to a relatively low temperature (1100 ° C. or lower) within this range. This is because the initial austenite crystal grains are refined, and the final ferrite and bainite grains are easily refined.

Rolling is performed in the austenite temperature range using a lever mill or a tandem mill. From the viewpoint of industrial productivity, it is preferable to use a tandem mill for at least the last several stages. Rolling temperature, the Ar ₃ point or more temperature in order to transformation to ferrite from austenite after rolling. The temperature at which the rolling is finished is better as it is closer to the Ar ₃ point. This is because the effect of accumulating processing strain introduced into austenite by rolling increases, and the refinement of the structure is promoted.

The rolling reduction needs to be 40% or more in terms of the rolling reduction (sheet thickness reduction rate) in the temperature range from the Ar ₃ point to the “Ar ₃ point + 100 ° C.” in order to obtain the effect of fine graining. More preferred is 60% or more. Rolling does not have to be performed in one pass, and may be rolling in a plurality of consecutive passes. The rolling reduction per pass is in the range of 10 to 60%. Increasing the rolling reduction per pass increases the accumulation of strain in austenite, and refinement of ferrite and bainite generated by transformation progresses. Therefore, it is preferable. However, if the rolling reduction per pass is too large, the structural difference in the sheet thickness direction of the steel sheet becomes significant, leading to deterioration of workability. Therefore, the preferable rolling reduction per pass is 15 to 50%.

  After the rolling is completed, the working strain introduced into the austenite is transformed into a fine phase structure of ferrite and bainite from austenite as a driving force without releasing it. The conditions are as follows.

  First, it cools within 0.6 second to the temperature of 640 degrees C or less after completion | finish of rolling, and the residence time in the temperature range exceeding 600 degreeC to 640 degreeC shall be 20 seconds or less.

  The time required for cooling to 640 ° C. or less after the end of rolling is within 0.6 seconds as described above, but more preferably within 0.4 seconds. This cooling is preferably water cooling, and the cooling rate is preferably 400 ° C./second or more.

  The reason for prescribing the time (dwell time) until cooling to a temperature of 640 ° C or lower is that if the cooling is stopped at a temperature of 640 ° C or higher, or if the elapsed time until reaching a temperature of 640 ° C or lower becomes longer, If the structure is coarsened due to the release of strain introduced by processing or changes in the strain's existence form, the structure becomes coarse, or the cooling is stopped at the temperature at which ferrite transformation occurs most actively, the ferrite This is because the bainite volume fraction becomes insufficient because of the rapid precipitation in a very short time.

  Although the temperature range below 640 ° C. is outside the temperature range where the ferrite transformation is most active, the temperature range above 600 ° C. is still the temperature range where the ferrite transformation proceeds. During cooling in this temperature range, ferrite precipitates uniformly and finely. However, in a material with high hardenability, the precipitation of ferrite may be insufficient and the bainite phase may exceed 95%, so after reaching 640 ° C, the cooling is temporarily stopped or the cooling rate is decreased, It is better to extend the residence time in the temperature range below 640 ℃ and over 600 ℃. However, if the residence time exceeds 25 seconds, deterioration of "strength-elongation balance" and "strength-elongation flange balance" occurs due to excessive increase in ferrite volume fraction, coarsening of ferrite grains, pearlite precipitation, etc. Therefore, it is preferably within 25 seconds.

  The scooping temperature is preferably 350 ° C. or higher and 600 ° C. or lower. Below 350 ° C, martensite is formed, so both ductility and stretch flangeability deteriorate. When wound at a temperature exceeding 600 ° C, both strength and stretch flangeability deteriorate due to the coarsening of ferrite grains and the coarsening of cementite in the structure. A preferable winding temperature is 400 to 550 ° C.

Steel having the chemical composition shown in Table 1 was melted and made 30 mm thick by hot forging. Then, after reheating to a temperature range of 1100 to 1300 ° C., rolling was performed with a small tandem mill for testing to finish a sheet thickness of 2 mm. In all rolling, the finishing temperature of rolling was higher than the Ar ₃ point of each steel type. The thickness of the obtained steel sheet was observed with a scanning electron microscope to examine the structure. Regarding mechanical properties, tensile properties and stretch flangeability were investigated by the following methods.

  Tensile properties are measured with JIS No. 5 tensile test piece. Stretch flangeability is 100 mm in length and width, and a 10 mm diameter punch hole (clearance: 15%) is punched in the center. This hole was expanded with a conical punch with a tip angle of 60 °, and the critical hole expansion rate calculated from the hole diameter when a crack penetrated the edge of the hole was evaluated.

Table 2 shows the manufacturing conditions, and Table 3 shows the results of the investigation of the structure and mechanical properties. It should be noted that the "ds / dc", {110} <111>, {110} <001> and {211} <111> pole densities in Table 3 and their sum are 1 in the thickness direction from the surface of the steel plate. It is a magnification with respect to a material having no texture at a position of / 8 thickness . Each of the examples of the present invention has an excellent balance of tensile strength (TS) and elongation (EL) (TS × EL) and balance of tensile strength (TS) and hole expansion rate (HER) (TS × HER). Yes. On the other hand, all of the comparative examples in which at least one of the chemical composition, the bainite volume fraction, and the ferrite particle size is outside the range defined by the present invention are inferior in mechanical properties.

Since the hot-rolled steel sheet of the present invention is excellent in properties such as ductility and stretch flangeability, it is suitable as a material requiring ductility and burring workability of automobile undercarriage parts, etc., and reducing the weight and cost of an automobile body. Contribute to Moreover, according to the manufacturing method of this invention, said hot-rolled steel plate can be manufactured reliably.

Claims (6)

In mass%, C: 0.02 to 0.25%, Si: 2.0% or less, Mn: 0.3 to 2.5%, P: 0.2% or less, S: 0.05% or less, Al: 0.005 to 0.5% and N: 0.01% or less And the balance is composed of Fe and impurities, has a volume ratio of 40 to 95% bainite phase and the balance is composed of a ferrite phase, and the average crystal grain size of the ferrite is 1.2 μm or more and less than 4 μm. A hot-rolled steel sheet having at least one of the following characteristics (a) and (b):
(a) The ratio (ds / dc) of the average grain size (ds) of ferrite at the position of 1/8 thickness from the surface of the steel plate to the average grain size (dc) of ferrite at the center of the plate thickness is 0.3 to 0.7.
(b) The sum of the polar densities of {110} <111>, {110} <001> and {211} <111> at a position of 1/8 thickness from the surface of the steel plate to the thickness direction has a texture. It must be at least 5 times that of the other and 1.5 times each. % By mass, C: 0.02-0.25%, Si: 2.0% or less, Mn: 0.3-2.5%, P: 0.2% or less, S: 0.05% or less, Al: 0.005-0.5% and N: 0.01% or less, and Ti: 0.2% or less, Nb: 0.1% or less, V: 1% or less, B: 0.005% or less, Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.0% or less, and Mo: 1.0% or less It contains one or more selected components, the balance is composed of Fe and impurities, has a volume ratio of 40 to 95% bainite phase and the balance is composed of a ferrite phase, and the average crystal grain size of the ferrite is A hot-rolled steel sheet that is 1.2 μm or more and less than 4 μm and has at least one of the following characteristics (a) and (b).
(a) The ratio (ds / dc) of the average grain size (ds) of ferrite at the position of 1/8 thickness from the surface of the steel plate to the average grain size (dc) of ferrite at the center of the plate thickness is 0.3 to 0.7.
(b) The sum of the polar densities of {110} <111>, {110} <001> and {211} <111> at a position of 1/8 thickness from the surface of the steel plate to the thickness direction has a texture. It must be at least 5 times that of the other and 1.5 times each. % By mass, C: 0.02-0.25%, Si: 2.0% or less, Mn: 0.3-2.5%, P: 0.2% or less, S: 0.05% or less, Al: 0.005-0.5% and N: 0.01% or less, and Containing one or more of Ca, REM and Mg in a total amount of 0.005% or less, the balance being Fe and impurities, the volume fraction of 40% to 95% bainite phase and the balance being ferrite phase A hot-rolled steel sheet having an average crystal grain size of 1.2 μm or more and less than 4 μm and having at least one of the following characteristics (a) and (b):
(a) The ratio (ds / dc) of the average grain size (ds) of ferrite at the position of 1/8 thickness from the surface of the steel plate to the average grain size (dc) of ferrite at the center of the plate thickness is 0.3 to 0.7.
(b) The sum of the polar densities of {110} <111>, {110} <001> and {211} <111> at a position of 1/8 thickness from the surface of the steel plate to the thickness direction has a texture. It must be at least 5 times that of the other and 1.5 times each. % By mass, C: 0.02-0.25%, Si: 2.0% or less, Mn: 0.3-2.5%, P: 0.2% or less, S: 0.05% or less, Al: 0.005-0.5% and N: 0.01% or less, and Ti: 0.2% or less, Nb: 0.1% or less, V: 1% or less, B: 0.005% or less, Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.0% or less, and Mo: 1.0% or less One or more selected, further one or more of Ca, REM, and Mg contain 0.005% or less in total, the balance consists of Fe and impurities, and contains a bainite phase of 40 to 95% by volume. A hot-rolled steel sheet having a structure in which the balance is composed of a ferrite phase, the ferrite having an average crystal grain size of 1.2 μm or more and less than 4 μm , and having at least one of the following characteristics (a) and (b):
(a) The ratio (ds / dc) of the average grain size (ds) of ferrite at the position of 1/8 thickness from the surface of the steel plate to the average grain size (dc) of ferrite at the center of the plate thickness is 0.3 to 0.7.
(b) The sum of the polar densities of {110} <111>, {110} <001> and {211} <111> at a position of 1/8 thickness from the surface of the steel plate to the thickness direction has a texture. It must be at least 5 times that of the other and 1.5 times each. A method for producing a hot-rolled steel sheet in which multi-pass hot rolling is performed on a slab or steel sheet having the chemical composition according to any one of claims 1 to 4, wherein the hot rolling is performed at an Ar point of ₃ or more. The thickness reduction rate in the temperature range below “Ar ₃ point + 100 ° C.” is 40% or more, the rolling reduction per pass is 10 to 60%, the rolling temperature in the final rolling pass is Ar ₃ point or more, and then 640 ° C. A method for producing a hot-rolled steel sheet, which is cooled within 0.6 seconds until the following, wherein the residence time in the temperature range from 600 ° C. to 640 ° C. is 25 seconds or less, and winding is performed at 600 ° C. or less and 350 ° C. or more. The method for producing a hot-rolled steel sheet according to claim 5 , wherein cooling to 640 ° C or lower is performed at a cooling rate of 400 ° C / second or more, and cooling to 640 ° C or lower within 0.4 seconds.