JP4539447B2 - High strength hot rolled steel sheet and method for producing the same - Google Patents

Description

  The present invention relates to a hot-rolled steel sheet excellent in strength and ductility suitable as a steel sheet for high-strength members used in automobiles, industrial machines, and the like, and a method for producing the same.

  Steel materials used in automobiles and industrial machines are required to improve mechanical properties such as strength, workability, and toughness. Since it is effective to refine the structure as a means for improving the strength, many steel compositions and production methods for obtaining a fine structure have been proposed.

  As a structure refinement method, Nb or Ti precipitation strengthening is used to increase the strength, and a low temperature finish rolling is applied to the recrystallization suppression effect of austenite grains included in Nb and Ti. There is a method of refining ferrite crystal grains by γ → α strain-induced transformation from crystal deformed austenite. However, the steel in which carbides of Nb and Ti are precipitated at a high density may be inferior in ductility and toughness. In addition, when the crystal grains are excessively refined, uniform deformation is difficult in tensile deformation, which causes a problem of reduced ductility and work hardening ability. Therefore, even if high strength is obtained by miniaturization, the use may be limited from the viewpoint of ensuring workability and collision safety.

  From the viewpoint of improving workability, a composite structure reinforced steel (Dual Phase steel) in which a martensite phase is dispersed in a ferrite main phase has been developed. In this steel, the strain in the low strain region is handled by the soft ferrite phase, while the hard martensite phase contributes to the strength in the high strain region. Below, the invention regarding the steel plate which has a composite structure is illustrated.

(1) Patent Document 1 (Japanese Patent Laid-Open No. 6-17203)
What is disclosed in this document is to define the hardness and area ratio of ferrite, to define the area ratio of martensite, bainite and retained austenite that are hard phases, and further to define the shape of grain boundary carbides, It is an invention for obtaining a hot-rolled steel sheet having a high fatigue limit and a high repeated yield stress. The steel sheet contains Si: 1.2 to 2.5% and Mn: 1.5 to 2.3% from the viewpoint of increasing the strength. The average ferrite particle size is in the range of 3.2 to 4.6 μm.

(2) Patent Document 2 (Japanese Patent Laid-Open No. 6-220581)
This document discloses an invention of a steel having a ferrite having a particle size of 5 μm or less as a main phase and a quenched structure as a second phase. In this steel, high BH property (bake hardenability) is obtained by containing 40 ppm or more of solute C. This document discloses a technique for manufacturing a surface-treated original sheet having a ferrite particle diameter of 3 to 4 μm by performing an annealing process on a cold-rolled steel sheet by super rapid heating at 1000 ° C./s.

(3) Patent Document 3 (Japanese Patent Laid-Open No. 7-150294)
The invention disclosed in this document relates to a hot-rolled steel sheet excellent in strength, workability, fatigue characteristics, and low-temperature toughness in which ferrite having an average particle diameter of 5 μm or less is a main phase and martensite is a second phase. For refinement of ferrite and martensite, 0.005% or more of Nb is added, and a structure having an average ferrite particle size of 2.9 to 4.5 μm and an average martensite particle size of 2.8 to 4.5 μm It has gained.

(4) Patent Document 4 (Japanese Patent Laid-Open No. 8-325671)
In the invention disclosed in this document, fine ferrite is the main phase, the average particle size and maximum particle size of martensite, which is the second phase, are specified, and further, the amount of bainite mixed in the second phase is limited. A hot-rolled high-strength steel sheet excellent in durability fatigue has been obtained. This steel sheet contains 0.1 to 1.5% of Si in order to suppress carbide formation, and 0.5 to 2.0% of Mn to ensure hardenability. The manufactured steel sheet has an average ferrite particle size of 3.8 to 6.0 μm, and the second phase has an average particle size of 3.1 to 4.0 μm.

(5) Patent Document 5 (Japanese Patent Laid-Open No. 9-78189)
In the invention of this document, a hot-rolled steel sheet having excellent fatigue properties is defined by specifying the Si and Mn contents and the average ferrite grain size for a multiphase steel sheet having a ferrite main phase space factor of less than 70%. obtain. Specifically, the Si content range is 0.91 to 3.50%, the Mn content range is 0.33 to 2.50%, and the ferrite particle size range is 5 to 15 μm.

(6) Patent Document 6 (Japanese Patent Laid-Open No. 2000-297350)
The invention of this document improves fatigue resistance, impact resistance, bake hardenability, and room temperature aging resistance by limiting the ferrite grain size and the amount of dissolved nitrogen for a composite structure steel sheet having ferrite as a main phase. Regarding technology. In order to improve the hardenability, Cr, Mo and Ni are added to the steel corresponding to Dual Phase steel.

(7) Patent Document 7 (Japanese Patent Laid-Open No. 2000-63986)
The invention of this document relates to a hot-rolled steel sheet excellent in durability fatigue property having a fine ferrite phase as a main phase and a martensite phase as a second phase. This steel plate may contain Cr for improving hardenability.

(8) Patent Document 8 (Japanese Patent Laid-Open No. 2000-87183)
The invention of this document is an invention directed to hot-rolled steel sheets and cold-rolled steel sheets excellent in warm press formability, and is an invention related to a dual-phase steel sheet whose main phase is a ferrite phase having an average grain size of 3.5 μm or less. is there. From the viewpoint of strengthening the ferrite phase, Si is added and substantially contains 0.5 to 0.8% Si. The average ferrite particle size is 3.0 to 3.5 μm.

(9) Patent Document 9 (Japanese Patent Laid-Open No. 11-61327)
The invention of this document relates to a high-strength dual phase steel sheet having a martensite space factor of 3 to 30% and an excellent martensite particle size of 5 μm or less and excellent in crashworthiness safety and formability. The substantial average ferrite grain size is 3.4 to 4.9 μm.

JP-A-6-17203 JP-A-62-220581 JP-A-7-150294 JP-A-8-325671 JP-A-9-78189 Japanese Unexamined Patent Publication No. 2000-297350 JP 2000-63986 JP JP 2000-87183 A JP, 11-61327, A In all of the above techniques, a hot-rolled steel sheet of C-Mn steel that does not contain microalloy does not realize a fine grain structure with a ferrite grain size of 3.0 μm or less, and improves strength. Needs to increase the amount of reinforcing elements such as Si and Mn. Furthermore, in order to generate a martensite phase as the second phase, it is necessary to add an element with high hardenability. Furthermore, in order to build a fine grain structure with a ferrite grain size of 3.0 μm or less, it is necessary to add a microalloy such as Nb, and any component design is economically disadvantageous.

  An object of the present invention is to reduce the addition amount of Si and Mn as much as possible, and even to a dual phase hot-rolled steel sheet having excellent strength, ductility and toughness even for a C-Mn steel containing no microalloy, and its production It is to provide a method.

From another viewpoint , it is an object of the present invention to further provide a dual phase hot rolled steel sheet having a yield strength of 350 MPa or more, a tensile strength of 590 MPa or more, and a yield ratio of 0.50 to 0.90 and a method for producing the same. .

  The present invention is based on the following new idea.

  (a) Reheat-treat the low-Si-low-Mn steel hot-rolled steel sheet to refine it into a fine dual phase structure.

  (b) Therefore, a thermally stable ultrafine ferrite structure is first created in the hot rolling process. Thereby, even if it re-heats, the coarsening of a ferrite grain is suppressed.

  (c) The purpose of the reheat treatment is to reversely transform the fine-grained ferritic steel sheet to a ferrite + austenite two-phase region or an austenite single-phase region, and then obtain a martensite phase by rapid cooling.

  (d) When the temperature is increased to the two-phase region of ferrite and austenite, the reverse transformation region to austenite undergoes martensitic transformation during the cooling process.

  (e) When the temperature is raised to the austenite single phase region, pro-eutectoid ferrite is generated in the cooling process, and the remaining austenite region undergoes martensitic transformation.

  It is considered that a steel sheet having a fine dual phase structure can be stably produced by the above method. Therefore, first, the chemical composition of the steel sheet was selected as follows. In the following, “%” for the component content means “% by mass”.

(1) Chemical composition of steel plate C: 0.05% or more and less than 0.20%, Mn: 0.5% or more and less than 1.5%, sol.Al: 0.002% or more and less than 0.05% Si is less than 0.1%, Cr is less than 0.1%, Ti is less than 0.01%, Nb is less than 0.005%, V is less than 0.01%, and N is less than 0.005%. The remainder is composed of Fe and impurities.

  Next, a method for forming a thermally stable ultrafine ferrite structure in the hot rolling process will be described below.

(2) the production method after the austenitizing the steel strip C-Mn steel having the chemical composition of the, upon extending tandem heat, in the final stand or roll stand of the first stage from the final stand, "Ae ₃ -20 Rolling in a temperature range of “° C.” to “Ae ₃ + 20 ° C.”, cooling is started within 0.3 seconds after rolling, and rapidly cooled to a temperature range of 580 to 680 ° C. at a cooling rate of 400 ° C./s or more, Then, it is allowed to cool to room temperature.

  The structure of the hot-rolled steel sheet thus obtained is a ferrite + cementite or ferrite + pearlite structure having an average particle size of 3.0 μm or less. The characteristics of this organization are as follows.

  (1) The shape of ferrite grains is equiaxed, and (2) the dislocation density in ferrite is extremely low. (3) The amount of C solid solution in ferrite is an equilibrium solid solution amount level. (4) 80% or more of the ferrite grain boundaries are large tilt grain boundaries having an orientation difference of 15 ° or more. (5) Spherical cementite is precipitated at the ferrite grain interface and the triple point of the grain boundary.

  Various heat treatments were applied to the hot-rolled steel sheet obtained by the above method, and the thermal stability was investigated. As a result, it has been clarified that the ferrite + cementite structure or ferrite + pearlite structure having the above characteristics is thermally extremely stable, and the ferrite grains are hardly coarsened during the heat treatment process. Further, it was found that if cementite precipitated at the ferrite grain boundary does not dissolve, a pinning effect on the grain growth of the reverse transformed austenite works, and austenite coarsening hardly occurs.

  By increasing the heating rate during re-heat treatment to 10 ° C./s or more, it is clear that the ferrite is hardly coarsened and the cementite is not completely solid solution, so that the coarsening of austenite can be suppressed. It was.

  A structure in which the martensite phase was uniformly dispersed in the fine-grained ferrite structure was obtained by reverse transformation into the ferrite + austenite two-phase region or the austenite region by the above method and then quenching to room temperature. The amount of martensite can be adjusted by the reheat treatment temperature and the cooling rate according to the composition of the steel sheet.

  The tensile properties of the steel sheets thus obtained were investigated. As a result, if the martensite content is less than 10%, the ductility is low and sufficient strength cannot be obtained. On the other hand, if the amount of martensite exceeds 30%, the tensile strength may exceed 1000 MPa, and there is a problem in workability. Therefore, it is necessary to select a steel composition, a hot rolling condition, and a reheat condition so that the martensite amount is 10 to 30%.

  The gist of the present invention based on the above new findings is as follows.

  (1) C: 0.05% or more and less than 0.20%, Mn: 0.5% or more and less than 1.5%, sol.Al: 0.002% or more and less than 0.05%, Si Is less than 0.1%, Cr is less than 0.1%, Ti is less than 0.01%, Nb is less than 0.005%, V is less than 0.01%, and N is less than 0.005%. And the structure at the position where the balance is composed of Fe and impurities and the depth from the steel sheet surface is 1/4 of the plate thickness is 10-30% by volume of martensite with the ferrite phase as the main phase. High strength hot rolling characterized by containing a site phase, wherein the ferrite phase has an average crystal grain size of 1.1 to 3.0 μm, and the martensite phase has an average grain size of 3.0 μm or less. steel sheet.

(2) The high-strength hot-rolled steel sheet according to (1) above , wherein the yield strength is 350 MPa or more, the tensile strength is 590 MPa or more, and the yield ratio is 0.50 to 0.90.

(3) C: 0.05% or more and less than 0.20%, Mn: 0.5% or more and less than 1.5%, sol.Al: 0.002% or more and less than 0.05%, Si Is less than 0.1%, Cr is less than 0.1%, Ti is less than 0.01%, Nb is less than 0.005%, V is less than 0.01%, and N is less than 0.005%. The temperature range of “Ae ₃ point−20 ° C.” to “Ae ₃ point + 20 ° C.” is applied to the steel ingot or steel slab having the composition consisting of Fe and impurities in the balance and the balance consisting of Fe and impurities in the balance. The steel sheet is formed by hot-rolling to complete the rolling at a temperature of 580 to 680 ° C. at an average cooling rate of 400 ° C./s or more. The steel sheet obtained by rapid cooling to the range is cooled to a temperature range of 710 to 880 ° C. at a heating rate of 10 ° C./s or more. Reheated, then high-strength hot-rolled steel sheet manufacturing method, characterized by cooling to room temperature at 200 ° C. / s or more average cooling rate.

  The reasons for limiting the chemical composition, metal structure and production conditions of the steel sheet of the present invention will be described in detail together with the respective actions.

1. Chemical composition of steel plate C: 0.05% or more and less than 0.20% C is an important element that effectively increases the strength of the steel and controls the austenite / ferrite transformation. Therefore, a necessary amount is contained according to the intended ferrite phase rate and steel plate strength. If the content is less than 0.05%, a sufficient amount of martensite phase cannot be obtained, and the intended strength and ductility cannot be ensured. On the other hand, if it is 0.20% or more, the amount of martensite becomes excessive. Therefore, the C content is 0.05% or more and less than 0.20%. Desirably, it is 0.07% or more and 0.15% or less.

Mn: 0.5% or more and less than 1.5% Mn lowers the Ae ₃ transformation point and contributes to refinement of crystal grains. It also enhances hardenability and facilitates the formation of martensite. Furthermore, ferrite is strengthened by the solid solution strengthening action by Mn. If the content is less than 0.5%, the hardenability is insufficient and sufficient strength and ductility cannot be obtained. On the other hand, when the content is 1.5% or more, cementite precipitates in a plate shape along the grain boundary and deteriorates ductility and moldability. Therefore, the Mn content is 0.5% or more and less than 1.5%. Desirably, it is 0.7% or more and 1.0% or less.

sol.Al: 0.002% or more and less than 0.05% Al is added for deoxidation of steel. However, if the sol.Al content is less than 0.002%, the effect is not sufficient. On the other hand, when the sol.Al content is 0.05% or more, not only the effect is saturated but also the ductility is lowered. Therefore, the sol.Al content is 0.002% or more and less than 0.05%. More preferred is 0.01% or more and 0.04% or less.

  The contents of Si, Cr, Ti, Nb, V, and N are regulated as follows.

Si: Less than 0.1% Since Si suppresses the formation of cementite, the pinning effect on ferrite grains and austenite grains is inhibited when the Si content is large. Therefore, the Si content is less than 0.1%. The smaller the Si content, the better.

Cr: Less than 0.1% Cr suppresses spheroidization of cementite and precipitates in a plate shape along the grain boundary, so that ductility and formability are deteriorated. Therefore, the Cr content is less than 0.1%. The smaller the Cr, the better.

Ti: 0.01% or less, Nb: less than 0.005%, V: 0.01% or less Ti, Nb and V combine with C to form fine carbides, adversely affecting ductility and formability. . In particular, this adverse effect becomes significant at Ti exceeding 0.01%, Nb of 0.005% or more, and V exceeding 0.01%. Therefore, it is desirable that the contents of Ti, Nb, and V are as small as possible within the above ranges.

N: less than 0.005% N dominates the austenite / ferrite transformation and strengthens it by dissolving in ferrite. However, when the content is 0.005% or more, coarse nitrides are formed, and ductility and formability are deteriorated. Therefore, the N content is less than 0.005%. Desirable is 0.002 to 0.004%.

  The balance other than the above is Fe and impurities. It is desirable to suppress P and S in the impurities as follows.

P: 0.02% or less, S: 0.005% or less Since P and S are impurity elements that adversely affect workability, it is desirable to keep them low. It is preferable to set the allowable upper limit values to 0.02% and 0.005%, respectively.

2. Metal structure The reason why the ferrite average crystal grain size, the martensite content, and the martensite average particle size are limited as described above will be described. The mechanical properties are governed by the steel structure at a position where the depth from the steel plate surface is 1/4 of the plate thickness. Therefore, in this invention, the metal structure in the position where the depth from a steel plate surface is 1/4 of plate | board thickness is prescribed | regulated.

(1) Ferrite average crystal grain size: 1.1 to 3.0 μm
When ferrite is refined to have an average particle size of 3.0 μm or less, strength and ductility are improved comprehensively, and desired high strength and ductility can be obtained even if the amount of alloy elements is minimized. However, if the average particle size is reduced to be less than 1.1 μm, the ductility is lowered, so the lower limit is 1.1 μm.

(2) Martensite content: 10-30% by volume
If the martensite content is less than 10%, the ductility is low, and sufficient strength and ductility cannot be obtained. On the other hand, if the amount of martensite exceeds 30%, the tensile strength may exceed 1000 MPa, and there is a problem in workability. Therefore, the martensite space factor is 10% or more and 30% or less.

(3) Martensite average particle size: 3.0 μm or less If the average particle size of martensite is refined to 3.0 μm or less, strength and ductility are improved overall, and it is desirable even if the amount of alloying elements is minimized. High strength and ductility can be obtained. There is no restriction | limiting in particular in the minimum of an average particle diameter.

  The high-strength hot-rolled steel sheet according to the present invention contains ferrite as a main phase and contains a martensite phase of 10 to 30% by volume, and is substantially a two-phase structure of ferrite and martensite. However, even if a total of 5% or less of bainite, cementite, carbide or the like is mixed, the effect of the present invention is not impaired.

3. Mechanical properties The reason why the mechanical properties (tensile properties) are limited as described above will be described.

  The strength of the material, which was conventionally 440 MPa class, is made a high strength material exceeding 590 MPa class due to the synergistic effect of fine graining and double phase. The yield strength is 350 MPa or more, the tensile strength is 590 MPa or more, and the yield ratio is 0.50 to 0.90. More preferable proof stress is 450 MPa or more, and more preferable tensile strength is 650 MPa or more. For the yield ratio, the lower limit was set to 0.50 to ensure rigidity. More desirable is 0.5 to 0.8. Most desirable is 0.6 to 0.7.

4). Manufacturing Method Next, the reason for limiting the manufacturing conditions will be described.

  By reheating, the hot-rolled base material capable of generating a dual phase structure whose main phase is ferrite having an average crystal grain size of 1.1 to 3.0 μm is ferrite + with an average grain size of 3.0 μm or less. It is preferable that the cementite is a cementite or ferrite + pearlite structure and satisfies the following conditions.

(a) The shape of the ferrite grains is equiaxed,
(b) the dislocation density in the ferrite is very low,
(c) The amount of C solid solution in ferrite is at the level of equilibrium solid solution,
(d) 80% or more of the ferrite grain boundaries are large tilt grain boundaries having an orientation difference of 15 ° or more;
(e) Spherical cementite is precipitated at the ferrite grain interface and the triple point of the grain boundary.
In order to obtain the above structure, it is necessary to define the following hot rolling conditions.

(1) Rolling completion temperature: “Ae ₃ −20 ° C.” to “Ae ₃ + 20 ° C.”
When the rolling completion temperature is lower than “Ae ₃ -20 ° C.”, a structure with a high dislocation density is formed, and abnormal grain growth occurs during subsequent reheating. On the other hand, when the rolling completion temperature is higher than “Ae ₃ + 20 ° C.”, the ferrite average crystal grain size exceeds 3.0 μm. Therefore, the rolling completion temperature range is “Ae ₃ −20 ° C.” to “Ae ₃ + 20 ° C.”.

(2) Time to start cooling after rolling: within 0.3 seconds In the production method of the present invention, it is important to start cooling as soon as possible after rolling. When the time until the start of cooling exceeds 0.3 seconds after rolling, the rolling strain is recovered, and a fine grain structure having an average ferrite grain size of 3.0 μm or less cannot be obtained. In addition, cementite tends to aggregate and become coarse. The shorter the time from the start of rolling to the start of cooling, the better.

(3) Cooling rate after rolling: 400 ° C./s or more This is rapid cooling with an average cooling rate of 400 ° C./s or more. If the cooling rate is lower than this, a sufficient fine grain structure cannot be obtained, and the average ferrite grain size exceeds 3.0 μm. The faster the cooling rate, the better, so there is no particular upper limit.

(4) Rapid cooling stop temperature: 580-680 ° C
In order to transform ferrite from processed austenite, it is necessary to set a quenching stop temperature. If this stop temperature is lower than 580 ° C., the ferrite transformation does not proceed sufficiently, the remainder undergoes bainite transformation, or cementite precipitates in a plate shape at the grain interface, resulting in a thermally stable structure. On the other hand, if the quenching stop temperature exceeds 680 ° C., a sufficient fine grain structure cannot be obtained, and the ferrite average crystal grain size exceeds 3.0 μm. Therefore, the quenching stop temperature is set to a range of 580 to 680 ° C. The rapid cooling stop time is preferably 0.2 seconds or longer. The cooling conditions after the rapid cooling stop are not particularly limited, but it is preferable to cool to 400 ° C. or lower or to cool by air.

(5) Reheating conditions
(5) -1. Rate of temperature rise: 10 ° C./s or more When the rate of temperature rise is less than 10 ° C./s, ferrite coarsens during the temperature rise, and solid solution of cementite is promoted and the pinning effect disappears. Therefore, the temperature rising rate is set to 10 ° C./s or more. The higher the rate of temperature rise, the better. Therefore, there is no particular upper limit. In addition, since the regulation of the temperature rising rate is for suppressing solid solution of cementite, the temperature rising rate is an average temperature rising rate in a temperature range from 300 ° C. to a predetermined heating temperature.

(5) -2. Reheating temperature: 710-880 ° C
Heat to austenite + ferrite two-phase region or austenite single-phase region. If the heating temperature is less than 710 ° C., it does not reversely transform to austenite, and no martensite phase is generated as the second phase. On the other hand, if the heating temperature exceeds 880 ° C., the martensite content becomes excessive, and the desired processability cannot be obtained. Therefore, the range of heating temperature shall be 710-880 degreeC. Although it does not specifically limit about holding time, 2 to 10 second is desirable.

(5) -3. Cooling conditions after reheating: cooling to room temperature at an average cooling rate of 200 ° C./s or higher In order to obtain martensite by quenching of reverse transformed austenite by reheating, quenching is performed at an average cooling rate of 200 ° C./s or higher. When the average cooling rate is lower than 200 ° C./s, a martensite phase is not generated. Since a cooling rate is so high that it is preferable, there is no upper limit in particular.

  A slab composed of steel types 1 to 9 having the chemical composition shown in Table 1 (slab size: 80 mm width × 100 mm length × 35 mm thickness) is heated under the conditions shown in Table 2, and after hot rolling, water-cooled, A 1.2 mm hot-rolled steel sheet was obtained.

  The obtained steel sheet was reheated under the conditions shown in Table 2. A JIS No. 5 tensile test piece was collected from the obtained steel sheet and examined for mechanical properties. In addition, four plate materials each having a width of 10 mm × 50 mm were stacked, and after a 2 mmV notch process, a Charpy impact test was performed to measure impact absorption energy at room temperature. The results are shown in Table 3.

As shown in Table 3, the steel sheet produced according to the technique of the present invention does not contain microalloys such as Ti, V, and Nb, but the ferrite average grain size after reheating and the martensite phase The average particle size is suppressed to 3.0 μm or less. Moreover, the space factor of the martensite phase is in the range of 10 to 30% by the predetermined reheating treatment, the yield strength by the tensile test is 390 MPa or more, the tensile strength is 590 MPa or more, and the yield ratio is 0.5 to 0.9. is there. Furthermore, the tensile breaking elongation (total elongation) is 20% or more, and the ductility is good. Furthermore, the impact absorption energy by the Charpy impact test is 200 J / mm ² or more, and the toughness is also good.

  On the other hand, even if the steel composition range is within the range of the present invention, the steel whose average particle size and martensite space factor do not fall within the scope of the present invention is poor in strength, ductility, or toughness. It is. Specifically, it is as follows.

  Test number C: Since the time until rolling starts after rolling is long, the average grain size of ferrite grains and martensite grains exceeds 3.0 μm. Therefore, the yield strength is low.

  Test number D: The finish rolling temperature is high, and the average grain size of ferrite grains and martensite grains exceeds 3.0 μm. Therefore, the yield strength is low.

  Test number E: The water cooling rate after rolling is low, and the average particle diameter of ferrite grains and martensite grains exceeds 3.0 μm. Therefore, the yield strength is low.

  Test number F: The rate of temperature increase during reheating is low, and the average particle size of ferrite grains and martensite grains exceeds 3.0 μm. Therefore, the yield strength is low.

  Test number G: The average cooling rate after reheating is small, and no martensite phase is generated. Therefore, the tensile strength is low.

  Moreover, the composition of steel types 6, 7, 8, and 9 is outside the range defined by the present invention. Therefore, a desired particle size or martensite space factor cannot be obtained, and any of strength, ductility, and toughness is poor. Specifically, it is as follows.

  In the test number L, the contents of C and Mn are low, and martensite is not generated. Therefore, the tensile strength is low. In test number M, a fine grain structure can be obtained by adding Ti, but the yield ratio is high and the ductility is poor. In test number N, a fine grain structure can be obtained by adding Ti and Nb, but the yield ratio is high and the ductility is poor. In test number O, a fine grain structure can be obtained by adding Cr, Ti, Nb and V, but the yield ratio is high and the ductility is poor.

According to the present invention, it is possible to stably provide a high-strength hot-rolled steel sheet that is excellent in strength, ductility, and toughness. This hot-rolled steel sheet can be applied to industrial equipment members such as automobile undercarriage parts to further improve the performance and life of those products, and has an extremely significant effect on the industry.

Claims (3)

  In mass%, C: 0.05% or more and less than 0.20%, Mn: 0.5% or more and less than 1.5%, sol.Al: 0.002% or more and less than 0.05%, Si is less than 0.1%, Cr is less than 0.1%, Ti is less than 0.01%, Nb is less than 0.005%, V is less than 0.01%, and N is less than 0.005%. The balance is Fe and impurities, and the structure at a position where the depth from the steel sheet surface is 1/4 of the plate thickness is 10-30% by volume with the ferrite phase as the main phase. A high-strength heat comprising a martensite phase, wherein the ferrite phase has an average crystal grain size of 1.1 to 3.0 μm, and the martensite phase has an average grain size of 3.0 μm or less. Rolled steel sheet. The high strength hot-rolled steel sheet according to claim 1, wherein the yield strength is 350 MPa or more, the tensile strength is 590 MPa or more, and the yield ratio is 0.50 to 0.90. In mass%, C: 0.05% or more and less than 0.20%, Mn: 0.5% or more and less than 1.5%, sol.Al: 0.002% or more and less than 0.05%, Si is less than 0.1%, Cr is less than 0.1%, Ti is less than 0.01%, Nb is less than 0.005%, V is less than 0.01%, and N is less than 0.005%. The steel ingot or steel slab having a composition composed of Fe and impurities in the balance is subjected to hot rolling to complete rolling in a temperature range of “Ae ₃ point−20 ° C.” to “Ae ₃ point + 20 ° C.” No steel plate, cooling of the steel plate was started within 0.3 seconds after completion of hot rolling, rapidly cooled to a temperature range of 580 to 680 ° C. at an average cooling rate of 400 ° C./s or more, and obtained by cooling. The steel sheet is reheated to a temperature range of 710 to 880 ° C. at a temperature increase rate of 10 ° C./s or more, and then 200 ° C./s or more. A method for producing a high-strength hot-rolled steel sheet, characterized by cooling to room temperature at an average cooling rate of.