JP4502272B2 - Hot-rolled steel sheet excellent in workability and fatigue characteristics and casting method thereof - Google Patents

Description

  The present invention relates to a hot-rolled steel sheet excellent in workability and fatigue characteristics suitable as a steel sheet for automobile undercarriage parts, and a manufacturing method thereof.

In recent years, the strength of automobile parts has been increased, and the same applies to the suspension parts. With respect to undercarriage parts, there is a demand for improvement in fatigue strength and workability (ductility) as well as higher strength.
Refinement of the structure is effective for improving the strength-ductility balance and fatigue characteristics. For example, in Japanese Patent Laid-Open No. 2000-144316 (Patent Document 1), the main phase of the structure is composed of ferrite, and the average particle diameter of ferrite is 2 to 4 μm, so that the strength-ductility balance (tensile strength TS and (Evaluated as product TS × El with elongation El). However, such a ferrite grain size is not sufficient for improving the fatigue characteristics. JP-A-11-92959 (Patent Document 2) and JP-A-11-152544 (Patent Document 3) describe a strength-ductility balance and a fatigue limit ratio (average diameter of ferrite of 2 μm or less). A hot-rolled steel sheet with good fatigue strength / TS) is described. However, since the grain size is too fine, there is concern about improvement in YP (yield point) and reduction in UE (uniform elongation). In fact, in this steel sheet, YR (yield ratio) is too high, exceeding 0.9. The spring back at the time becomes large, and the workability deteriorates. Thus, even if the particle size is excessively reduced, sufficient improvement in fatigue characteristics cannot be expected.

In this way, sufficient fatigue properties and workability improvement cannot be expected only by controlling the grain size.Thus, by optimizing the surface layer that contributes to fatigue properties and the central structure that contributes to workability, A hot-rolled steel sheet with improved quality has been proposed. For example, in Japanese Patent Application Laid-Open No. 2004-211199 (Patent Document 4), a polygonal ferrite is the main phase, and the average crystal grain size of the polygonal ferrite gradually decreases from the center of the plate thickness toward the surface layer. Hot rolled steel sheets have been proposed. This hot-rolled steel sheet is obtained by gradually refining the polygonal ferrite fraction from the center part of the plate thickness toward the plate thickness surface layer part by applying bending work after hot rolling.
JP 2000-144316 A JP-A-11-92858 Japanese Patent Laid-Open No. 11-152544 Japanese Patent Laid-Open No. 2004-211199

According to the invention described in Patent Document 4, the fatigue characteristics and the strength-ductility balance were improved. However, the grain size of the outermost layer part to be most refined is 0.4 with respect to the grain size of the central part. It can be refined only about twice. In order to improve the workability, as described above, it is desirable to lower YR, and for that purpose, it is desirable to increase the particle diameter D of the central portion to some extent. However, in the invention of this document, the particle diameter d in the outermost layer portion must be about 0.4D, and sufficient fatigue characteristics cannot be obtained. On the contrary, if the particle size of the surface layer portion is made sufficiently fine, the central portion is inevitably too fine, so that YR becomes high and sufficient workability cannot be obtained.
The present invention has been made in view of such a problem, and an object of the present invention is to provide a high-strength hot-rolled steel sheet having both good workability and fatigue characteristics due to low YR and a method for producing the same.

The hot-rolled steel sheet of the present invention has a chemical composition of mass%, C: 0.01 to 0.30%, Si: 0.20 to 3.0%, Mn: 0.5 to 3.0%, Al: The center of the plate thickness including 0.005 to 0.10%, consisting of the balance Fe and unavoidable impurities, and the center of the plate thickness at the center in the width direction is 100 μm in the plate thickness direction and 100 μm in the plate width direction The structure of the part is one of a ferrite single phase structure, a bainite single structure, a ferrite and bainite composite structure, a ferrite and pearlite composite structure, and a ferrite and martensite composite structure, and the orientation difference is 15 in the structure. the average particle size of the effective crystal ferrite phase surrounded by ° or more large angle grain boundaries is 2 to 15 [mu] m, whereas from the plate surface average particle size of the effective crystal surface portion of 100~200μm depth of the central portion Effective crystal The fine particles are 0.2 times or less of the average particle size.

According to the hot rolled steel sheet of the present invention, under a predetermined component, the center of plate thickness, the average particle size of the effective crystal of the ferrite phase which is surrounded by the misorientation 15 ° or more large angle grain boundaries is as 2~15μm Therefore, the strength-ductility balance is excellent, the yield ratio can be suppressed to 0.8 or less, and excellent workability by low YR can be obtained. Further, since the average grain size of the effective crystal in the surface layer part at a depth of 100 to 200 μm from the plate surface is a fine grain of 0.2 times or less than the average grain size of the effective crystal in the center part, Even if the diameter is relatively large, the particle size of the surface layer portion can be sufficiently refined, so that excellent fatigue characteristics can be provided.

  In the hot-rolled steel sheet, as a chemical component, group A (Cu: 0.10 to 2.0%, Ni: 0.10 to 2.0%, Cr: 0.10 to 2.0%, Mo: 0) .10 to 2.0%, B: 0.0005 to 0.0050%) or group B (0.002 to 0.3% each for V, Nb, Ti, Zr, and Hf) alone, or It can be contained in combination.

  In the method for producing a hot-rolled steel sheet according to the present invention, the steel having the chemical component is heated to Ae 3 point or more and 1300 ° C. or less, and then the cumulative rolling amount is 50% or more, and the final rolling temperature is Ae 3 point to (Ae 3 The first stage rolling is performed at a point +200) ° C., and then, at the first time, a reduction at a reduction rate of 20 to 60% is applied at least once in the temperature range of (Ae3 point−300) to (Ae3 point−150) ° C. After performing the step rolling, the steel sheet is cooled to a temperature of 500 ° C. or lower at a cooling rate of 1 ° C./sec or higher and wound up.

According to this manufacturing method, the austenite uniformly refined by the first stage rolling is subjected to a predetermined reduction by the second stage rolling, so that the surface layer portion is instantaneously reheated to the Ae3 point or more by processing heat generation, and then the Ae3 point or less. Since the surface layer portion has a fine ferrite structure, the processing heat generation at the central portion of the plate thickness is small and does not exceed the Ae3 point, so that it can be avoided from miniaturization and an average particle size of 2 to 15 μm can be secured. it can. For this reason, the said hot-rolled steel plate can be easily manufactured with normal hot rolling equipment.

According to the hot-rolled steel sheet of the present invention, the average grain size of the effective crystals of the ferrite phase in the center portion of the plate thickness is 2 to 15 μm, and the average grain size of the effective crystals in the surface layer portion is 0.2 Since it is less than double, excellent workability and fatigue strength can be combined without causing deterioration of workability due to an increase in yield ratio. Moreover, according to the manufacturing method of this invention, the said hot-rolled steel plate can be manufactured easily with normal rolling equipment, and it is excellent in productivity.

First, chemical components of the hot rolled steel sheet of the present invention will be described. Hereinafter, the unit is mass%. C: 0.01 to 0.30%
C is added as a strengthening element. If it is less than 0.01%, the strength is too low. On the other hand, if it exceeds 0.30%, ductility is deteriorated. For this reason, the lower limit of C is 0.01%, preferably 0.03%, while the upper limit is 0.30%, preferably 0.20%.

Si: 0.20 to 3.0%
Si is added as a deoxidizing element and contributes to strength improvement as a solid solution strengthening element of ferrite. If it is less than 0.20%, deoxidation is too small and ductility deteriorates. On the other hand, if it exceeds 3.0%, the ferrite strength becomes too high, the ductility deteriorates, and the YR becomes too high. For this reason, the lower limit of Si is 0.20%, preferably 0.30%, while the upper limit is 3.0%, preferably 2.5%.

Mn: 0.5 to 3.0%
Mn is added as a deoxidizing element and has the effect of lowering the transformation temperature, thus contributing to refinement of the structure and improvement of strength. If it is less than 0.5%, deoxidation becomes insufficient and ductility deteriorates. On the other hand, if it exceeds 3.0%, the strength becomes too high, and the ductility also deteriorates. For this reason, the lower limit of Mn is 0.5%, preferably 0.8%, while the upper limit is 3.0%, preferably 2.5%.

Al: 0.005-0.10%
Al is added as a deoxidizing element. If it is less than 0.005%, deoxidation becomes insufficient and ductility deteriorates. On the other hand, if it exceeds 0.10%, ductility also deteriorates. For this reason, the lower limit of the amount of Al is made 0.005%, and the upper limit is made 0.10%.

The hot-rolled steel sheet of the present invention is composed of the balance of Fe and unavoidable impurities in addition to the basic components described above, and further includes group A (Cu: 0.10 to 2.0%, Ni: 0.10 to 2.0%, Cr: 0.10 to 2.0%, Mo: 0.10 to 2.0%, B: 0.0005 to 0.0050%) or B group (V, Nb, Ti, Zr, and Hf, each 0.00. One or more elements can be added to the following components (1), (2) and (3). It is to be noted that the impurity elements P and S are preferably as small as possible because they cause embrittlement of the material, and it is desirable to keep P: 0.03% or less and S: 0.03% or less.
(1) Basic component + one or more elements from group A
(2) Basic component + one or more elements from group B
(3) Component (1) above + one or more elements from Group B

Each element of the group A contributes to refinement of the structure and improvement of strength by delaying the transformation from austenite to a low temperature phase. For Cu, Ni, Cr, and Mo, less than 0.10% and B for less than 0.0005%, the effect is too low, while Cu, Ni, Cr, and Mo exceed 2.0%, and B is less than 0.00%. If it exceeds 0050%, the strength becomes too high and the ductility deteriorates. For this reason, it is good to add in the said range.
On the other hand, each element of group B is a precipitation strengthening element and contributes to the improvement of the strength and fatigue properties of ferrite. When the content is less than 0.002%, the effect is too small. On the other hand, when the content exceeds 0.3%, the strength becomes too high and the ductility deteriorates. For this reason, it is good to add in the said range.

Next, the structure of the hot rolled steel sheet of the present invention will be described.
The hot-rolled steel sheet of the present invention has a large angle with an orientation difference of 15 ° or more in the structure of the sheet thickness center part (the part of 100 μm in the sheet thickness direction and 100 μm in the sheet width direction centered on the sheet thickness center at the center in the width direction). the average particle diameter D of the effective crystal surrounded by a ferrite phase at the grain boundaries is a 2.0～15Myuemu, whereas the surface layer portion of 100~200μm depth from the leaf surface, the average particle diameter d is the center of the effective crystal D ≦ 0.2 × D with respect to the average particle diameter D of the effective crystals of part.

In the present invention, whether it is the central part or the surface layer part of the plate thickness, the problem of the effective crystal grains of the ferrite phase surrounded by the large-angle grain boundaries with an orientation difference of 15 ° or more is the orientation difference. An aggregate of crystal grains of less than 15 ° behaves like a single crystal grain, and the grain boundary does not work as a grain boundary that affects mechanical properties such as YP, TS, El, and fatigue strength. For this reason, when considering the mechanical characteristics, it is effective to observe the effective crystal grains as unit grains. Such handling is also employed, for example, in the toughness evaluation of hot-rolled steel sheets described in JP-A-2001-355040.

In the present invention, the average grain size of the ferrite phase effective crystal at the center of the plate thickness is set to 2 to 15 μm so that the YR does not become too high and the workability is ensured while the good strength-ductility balance is achieved. To get. That is, if it is less than 2 μm, YR becomes too high and workability deteriorates. On the other hand, when it exceeds 15 μm, the strength is remarkably deteriorated and the strength-ductility balance is deteriorated. For this reason, the lower limit of the average particle diameter of the effective crystals is set to 2.0 μm, preferably 2.5 μm, and the upper limit is set to 15 μm, preferably 10 μm. In addition, as long as the average particle diameter of the effective crystal is satisfied, the structure of the central portion is not particularly limited, but in the component system of the present invention, usually a ferrite single phase structure, a bainite single structure, a ferrite + bainite composite structure, It is composed of a composite structure of ferrite and pearlite and a composite structure of ferrite and martensite.

  On the other hand, when the average particle diameter d of the effective crystals in the surface layer portion is d ≦ 0.2D, even when the center portion is adjusted to an appropriate particle size in order to ensure low YR, the surface layer portion is Fatigue characteristics can be improved without being sufficiently miniaturized and impairing workability. In evaluating the mechanical properties of the surface layer of the hot-rolled steel sheet, in the present invention, the surface layer portion is not defined from the plate surface, and a portion of 100 to 200 μm from the plate surface is considered to generate and propagate fatigue cracks. This is because the structure immediately below the surface is not significantly affected by, but is most affected by the structure of the part slightly away from the outermost surface, and the grain size of the effective crystal at this part is important.

  The structure of the surface layer portion is not particularly limited as long as the effective crystal satisfies the condition of d ≦ 0.2D, but in the component system of the present invention, ferrite is the main phase (ferrite phase is 90% or more). The second phase is formed from fine carbide and retained austenite.

Next, the suitable manufacturing method of the said hot-rolled steel plate is demonstrated with reference to the heat processing diagram of FIG.
As shown in FIG. 1, this manufacturing method includes a heating step of heating and holding a steel slab (slab) of the above chemical composition at a temperature 1 of Ae 3 or more and 1300 ° C. or less, and then the cumulative reduction amount is 50% or more. And the first rolling step for performing the first stage rolling 2 with the final rolling temperature of Ae3 point to (Ae3 point + 200) ° C., and (Ae3 point−300) to (Ae3 point−150) following the first stage rolling. ) In the temperature range of ° C., it has a second stage rolling process in which the second stage rolling is performed by adding at least one reduction at a reduction rate of 20 to 60%. A winding step 5 for cooling and winding at a cooling rate 4 of sec or more is provided.

  The heating step is a step of heating the steel slab to an austenite single phase. An austenite single phase cannot be obtained if the heating and holding temperature is less than the Ae 3 point, while if it exceeds 1300 ° C., the austenite becomes too coarse and the final structure is not sufficiently refined. The holding time is preferably 10 min or longer.

  By the first stage rolling, the austenite is uniformly refined before the second stage rolling. If the final rolling temperature is less than Ae3, the transformation starts before the austenite is sufficiently refined, so that the final structure is mixed. On the other hand, when it exceeds (Ae3 point + 200) ° C., austenite becomes coarse and good mechanical properties cannot be obtained. Further, even if the cumulative reduction amount is less than 50%, austenite is not sufficiently refined and consequently the final structure is not refined.

  By the second stage rolling, the surface layer portion is instantaneously generated by processing heat generation by adding a suitable amount of reduction in a moderately low temperature range from Ae3 point, that is, in a state in which the transformation from austenite to low temperature phase is over a certain level. Reheating to Ae3 point or higher (6 in FIG. 1) and then rapidly cooling to Ae3 or lower makes only the surface layer part a fine ferrite structure. Since the processing heat generation at the center is small, it is cooled as it is without exceeding the Ae3 point, and a structure having an appropriately large particle size is obtained. When the rolling temperature is less than (Ae3 point−300) ° C., the temperature becomes too low, and even when a predetermined reduction is applied, the temperature of the surface layer portion does not exceed the Ae3 point and a fine structure cannot be obtained. On the other hand, at (Ae3 point−150) ° C. or higher, since the transformation is applied at a stage where the transformation has not sufficiently progressed, the structure is not refined. In addition, if the amount of reduction in one pass is less than 20%, it is difficult to refine the structure in the above temperature range. Further, when a reduction exceeding 60% is applied by rolling in one pass, the processing heat generation becomes large up to the central part, and not only the surface layer part but the whole becomes finer and YR rises. For this reason, the amount of reduction is stopped to 60% or less.

  The cooling rate after the second stage rolling is such that the microstructure obtained by rolling is not coarsened and is cooled to 500 ° C. at a rate of 1 ° C./sec or higher, preferably 5 ° C./sec or higher. If the cooling stop temperature exceeds 500 ° C. or the cooling rate is less than 1 ° C./sec, the structure may be coarsened. Then, it winds up at the temperature below 500 degreeC.

The above production method can be carried out with normal hot rolling equipment and is excellent in productivity. However, the production method of the hot rolled steel sheet of the present invention is not limited to this method. Instead of rolling, it can also be produced by rapidly heating and rapidly cooling the steel sheet surface using a high-frequency heating device.
Next, the hot-rolled steel sheet and the method for producing the same according to the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the examples.

Steel types shown in Table 1 were melted by vacuum melting and cast to obtain steel pieces. The Ae3 point in the table was calculated using calculation software (Thermo-Calc manufactured by Thermo-Calc Sotware AB).
The steel slab was subjected to partial rolling with a small rolling mill to produce thick plates having the thicknesses shown in Table 2. A 200 mm x 120 mm flat slab was cut out from the thick plate, heated and held at the heating temperature and heating time shown in Table 2, and then the first stage rolling shown in the same table except for some processes (No. a, k) And the second stage rolling was performed. In Table 2, “rolling temperature” of the second stage rolling means the rolling start temperature. In the first stage rolling, multi-pass rolling was performed with the reduction amount of one pass being 30 to 60%. The final thickness was 3 mm.
After the second stage rolling, it is cooled to 500 ° C. (excluding process No. g in Table 2) at the cooling rate shown in the same table, and 500 ° C. (process No. g is 400 ° C.) to simulate winding. ) For 30 minutes, and then cooled in the furnace.
In addition, according to Patent Document 4, the process No. k is bent and bent back so that the first stage rolling (hot rolling) is followed by cooling to 800 ° C. at 50 ° C./sec and the strain at the surface layer becomes 0.5. After processing, it is rapidly cooled to 450 ° C. at 50 ° C./sec, held at 450 ° C. for 30 minutes to simulate winding, and then cooled in the furnace.

  In the central part in the width direction of the hot-rolled steel sheet manufactured as described above, the center part of the plate thickness (100 μm in the plate thickness direction and the region of 100 μm in the width direction), the surface layer having a depth of 100 to 200 μm from the plate surface Part (a region of 100 μm in the width direction) was subjected to three visual field measurements to determine the average grain size of effective crystals of the ferrite phase. For some samples (Nos. 1, 2 and 30), the particle size distribution in the thickness direction from the plate surface was examined. These measurement results are shown in Table 3 and FIG. For the surface layer portion, the area ratio of the ferrite phase was also obtained. In the structure of Table 3, F is polygonal ferrite, B is bainite, P is pearlite, and θ is cementite.

  The method for measuring the average particle size of the effective crystals is as follows. The point where the orientation difference between ferrite and ferrite is 15 ° or more is mapped on the phase map as an effective grain boundary, and the area of the ferrite phase surrounded by the effective grain boundary is image software (Image-Pro made by Micromedia) ), The equivalent circle diameter was determined from the area of each grain, and the average value was taken as the average grain diameter. As the observation apparatus, a Schottky field emission scanning electron microscope (FE-SEM) (Philips XL30S-FEG) and an EBSP system (Techsemura Laboratories OIM system (ver. 4.0)) are used with a step interval of 0.25 μm. Measurement was performed under measurement conditions. An example (sample No. 2) is shown in FIG. Further, the area ratio of the ferrite phase was obtained by dividing the area of the ferrite phase obtained in the measurement of the average particle diameter by the area of the measurement region.

  Furthermore, the mechanical property was investigated by the tensile test and the fatigue test using the hot-rolled steel plate of each sample. In the tensile test, the front and back surfaces of the steel plate were ground by 0.05 mm, then processed into a No. 5 test piece described in JISZ2201, and carried out according to JISZ2241. Further, in the fatigue test, the front and back surfaces of the steel sheet were ground by 0.05 mm, and then the fatigue limit (FL) was measured according to the plane bending fatigue test described in JISZ2275. From the viewpoint of molding processability, it can be evaluated that YR is 0.8 or less. Further, it can be evaluated that the strength-ductility balance is 15000 MPa% or more in terms of TS × EL value and the fatigue limit ratio (FL / TS) is 0.6 or more. These measurement results are also shown in Table 4.

  From Table 3 and Table 4, Sample No. 1 (Comparative Example), which was rolled corresponding to the conventional hot rolling conditions, has a ratio (d / D) of the average grain size of effective crystals in the center portion and the surface layer portion. Since it inevitably increases, the fatigue limit ratio is high, and sufficient fatigue characteristics are not obtained. On the other hand, the hot-rolled steel sheets of Sample Nos. 2 to 17, 23 and 24 corresponding to the examples have a YR of 0.8 or less, a strength-ductility balance of 15000 MPa or more, and a fatigue limit ratio of 0. It can be seen that it has 6 or more and has excellent workability and fatigue characteristics.

  On the other hand, the sample No. 18 of the comparative example had a C amount, and No. 20 had too much Si amount and the strength was too high, so the TS × EL balance was lowered. In No. 21, since the amount of Mn was too high, YR was too high, and the TS × EL balance was also lowered. In No. 22, the components are appropriate, but the cumulative reduction amount of the first stage rolling is too small, so that the average particle size at the center becomes coarse and the TS × EL balance is lowered. In No. 25, the rolling temperature of the second stage rolling was too high, and the transformation did not proceed sufficiently before rolling, so that the surface layer portion could not be refined using the processing heat generated during rolling. And the average particle size ratio of the surface layer portion could not be reduced, and therefore a sufficiently high fatigue limit ratio could not be obtained. On the other hand, in No. 26, the rolling temperature of the second stage rolling is too low, and even if the processing heat generated during rolling is used, the temperature of the surface layer does not rise to the temperature causing reverse transformation, and the structure cannot be refined. The average particle size ratio between the center portion and the surface layer portion could not be reduced. As a result, the fatigue limit ratio was sufficiently reduced. In No. 27, because the amount of reduction in the second stage rolling was small, the heat generation amount in the surface layer portion was small, the temperature did not rise to the temperature at which reverse transformation occurred, and the microstructure of the surface layer portion could not be refined. The average particle size ratio between the central part and the surface layer part could not be reduced, and the fatigue limit ratio was lowered. On the other hand, in No. 28, since the cooling rate after hot rolling was small, the fine structure of the surface layer portion was coarsened during cooling, and the particle size of the surface layer portion was increased. As a result, the average particle size ratio between the center portion and the surface layer portion was increased. The fatigue limit ratio could not be sufficiently increased. In No. 29, since the rolling temperature of the first stage rolling was low, the structure of the central part was refined, and as a result, YR increased. Further, No. 30 is obtained by performing bending / unbending processing instead of the second stage rolling and making the crystal grains have an inclined structure so that the surface becomes fine. If the fine particle of 45 μm is used, the central part also becomes a fine particle of 0.8 μm, and the central part could not be made sufficiently large, so that the YR is as high as 0.95 and sufficient workability is ensured. I couldn't.

  FIG. 4 is a diagram in which the relationship between the average grain size of the effective crystals in the central portion and YR is arranged using sample Nos. 1 to 30. YR is remarkably higher than 2 μm and below. I understand that. On the other hand, FIG. 5 is a graph showing the relationship between the average particle diameter ratio d / D, YR, and fatigue limit ratio using Sample Nos. 1 to 30, where YR ≦ 0.8 and fatigue limit ratio ≧ 0. It is shown that d / D must be 0.2 or less to satisfy.

The temperature-processing schematic diagram of the manufacturing method concerning embodiment is shown.

It is a particle size distribution map which shows the relationship between the plate | board thickness direction depth from the plate surface in an Example, and the average particle diameter of an effective crystal. The effective grain boundary map figure which shows the observation result of the effective crystal grain of the ferrite phase in an Example is shown. It is a graph which shows the relationship between the average particle diameter of the effective crystal of the center part in an Example, and YR. It is a graph which shows the ratio d / D of the average particle diameter d of the effective crystal of the surface layer part with respect to the average particle diameter D of the effective crystal of the center part in an Example, YR, and a fatigue limit ratio.

Claims (4)

Chemical component is mass% C: 0.01-0.30%, Si: 0.20-3.0%, Mn: 0.5-3.0%, Al: 0.005-0.10% Comprising the balance Fe and unavoidable impurities,
In the central portion in the width direction, the structure of the central portion of the plate thickness that is 100 μm in the plate thickness direction and 100 μm in the plate width direction centered on the plate thickness center is a ferrite single phase structure, a bainite single structure, a composite structure of ferrite and bainite. The average grain size of the effective crystals of the ferrite phase , which is one of a composite structure of ferrite and pearlite, and a composite structure of ferrite and martensite and surrounded by a large-angle grain boundary with an orientation difference of 15 ° or more in the structure. 2 to 15 μm, and the average grain size of the effective crystals in the surface layer part at a depth of 100 to 200 μm from the surface of the plate is 0.2 or less times the average grain size of the effective crystals in the central part. Hot rolled steel sheet with excellent workability and fatigue characteristics.   Chemical components are further Cu: 0.10 to 2.0%, Ni: 0.10 to 2.0%, Cr: 0.10 to 2.0%, Mo: 0.10 to 2.0%, B : The hot-rolled steel sheet according to claim 1, comprising one or more of 0.0005 to 0.0050%.   Chemical components are further V: 0.002-0.3%, Nb: 0.002-0.3%, Ti: 0.002-0.3%, Zr: 0.002-0.3%, Hf : The hot-rolled steel sheet according to claim 1 or 2, comprising one or more of 0.002 to 0.3%. After heating the steel having the chemical composition according to any one of claims 1 to 3 to Ae 3 points or more and 1300 ° C. or less, the cumulative reduction amount is set to 50% or more, and the final rolling temperature is set to Ae 3 points to (Ae 3 points). 2nd stage of performing first stage rolling to +200) ° C. and subsequently applying at least one reduction to a reduction rate of 20 to 60% in the temperature range of (Ae3 point−300) to (Ae3 point−150) ° C. A method for producing a hot-rolled steel sheet that is excellent in workability and fatigue properties, after rolling and cooling to 500 ° C. or less at a cooling rate of 1 ° C./sec or more.

Images