JP3236339B2 - Manufacturing method of high strength hot rolled steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention has a good balance between strength and ductility, which is useful for use in steel plates for automobiles and the like.
Further, the present invention relates to a method for producing a high-strength hot-rolled steel sheet having a high toughness and a tensile strength of 40 kgf / mm ² or more.

[0002]

2. Description of the Related Art High-strength steel sheets are widely used as steel sheets for automobile inner plates, suspension parts, reinforcing materials and the like in order to reduce the weight of a vehicle body. It is important that such a high-strength steel sheet for automobiles not only has the strength required for ensuring the safety of automobiles but also has excellent workability during forming represented by press working. That is, a steel sheet which is soft before forming and has high strength after processing is required.

[0003] As a steel sheet having such properties, ε
-Cu precipitation hardening type hot-rolled steel sheets are known. For example, Japanese Patent Application Laid-Open No. 2-194146 discloses a method for producing an ε-Cu precipitation hardening hot-rolled steel sheet using a very low carbon steel having a C content of 0.0072 wt% or less. However, the mechanical properties, especially the balance between strength and ductility, are poor, and the toughness is insufficient. Further, when the actual machine is manufactured using ultra-low carbon steel as a raw material, the finishing temperature in hot rolling decreases to near the _Ar3 transformation point. However, there is a large problem that the structure is easily deteriorated such as grensu loss and grain refinement.

[0004]

SUMMARY OF THE INVENTION The present invention advantageously solves the above-mentioned problems. By controlling the steel composition and manufacturing conditions, the present invention has a much higher yield ratio than the conventional one.
% And a method of manufacturing a high-strength hot-rolled steel sheet having a tensile strength of 40 kgf / mm ² or more.

[0005]

In order to satisfy the above object, the microstructure contains almost no low-temperature transformation phase such as pearlite, bainite and martensite. Specifically, the ratio of the low-temperature transformation phase is 5%. % Or less,
It is advantageous to have a ferrite single phase structure. Furthermore, as a result of the inventors' earnest research, even low-carbon steels have
Low-temperature transformation phase can be reduced by forming F (solid solution C and N are completely fixed by carbide forming elements such as Ti and Nb), and dynamic recrystallization progresses during hot rolling by low-temperature slab heating However, as ferrite grains are refined, strength and ductility can be balanced, and low-temperature slab heating increases the strength increase due to ε-Cu precipitation in subsequent heat treatment, even if low carbon steel is used as the material. The inventor of the present invention has newly found that the present invention will become larger, and has completed the present invention.

The gist of the present invention is as follows. (1) C: 0.018 wt% to less than 0.10 wt% , Si: 0.05 Wt% to 1.5 wt%, Mn: 0.05 wt% to less than 2.0 wt%, Cu: 0.5 wt% to 3.0 wt%, Al: 0.001 wt% or more and 0.1 wt% or less, P: 0.05 wt% or less, S: 0.01 wt% or less, N: 0.01 wt% or less, Ti: 0.06 wt% or more and 0.6 wt% or less, and Nb: 0.12 wt% or more
A steel slab containing 1.2 wt% or less of one or two kinds under (Ti + Nb / 2) / C ≧ 4, and the balance being Fe and unavoidable impurities, was heated to a temperature of 1220 ° C. or less . Hot rolling
And finish rolling at a finishing temperature of 800 ° C or more and less than 900 ° C ,
Cooling at a cooling rate of 30 ° C / s or more, and then winding at a temperature range of 650 ° C or less, the ratio of low-temperature transformation phase is 5% or less.
The method for producing a ferrite single-phase structure and a high strength hot rolled steel sheet characterized by a (first invention). (2) C: more than 0.018 wt% to less than 0.10 wt%, Si : 0.05 to 1.5 wt%, Mn: 0.05 to less than 2.0 wt% , Cu: 0.5 to 3.0 wt% , Al: 0.001 wt% or more and 0.1 wt% or less, P: 0.05 wt% or less, S: 0.01 wt% or less and N: 0.01 wt% or less, and Ti: 0.06 wt% or more and 0.6 wt% or less and Nb: 0.12 wt% or more Any one or more of 1.2wt% or less
Species, (Ti + Nb / 2) contained under / C ≧ 4, the balance consists of Fe and unavoidable impurities Steel
The slab is heated to 1220 ° C or lower and then hot rolled
And finish rolling at a finishing temperature of 800 ° C or more and less than 900 ° C,
Cool at a cooling rate of 30 ° C / s or more, and then
Low temperature transformation phase ratio of 5% or less by winding in the temperature range
High-strength heat characterized by a ferrite single-phase structure
Method for producing rolled steel sheet (second invention). (3) The method according to the first or second invention, wherein
Instead of heating the slab to a temperature below 1220 ° C,
High strength heat characterized by hot rolling without
Method for producing rolled steel sheet (third invention).

(4) In addition to the steel components of the first to third inventions , Ni: 0.1 wt% to 2.0 wt%, Cr: 0.1 wt% to 2.0 wt% and Mo: 0.1 wt% to 2.0 wt% % Or less of any one or more of the following (a fourth invention).

(5) A method for producing a high-strength hot-rolled steel sheet containing B in the range of 0.0003 wt% to 0.010 wt% in addition to the steel components of the first to fourth inventions ( fifth invention).

(6) In addition to the steel components of the first to fifth inventions , one or more of Ca: 0.0005 wt% to 0.01 wt% and REM: 0.001 wt% to 0.02 wt% are contained. ( Sixth invention).

Hereinafter, the results of experiments which became the basis for developing the present invention will be described. C: 0.07 wt%, Si: 0.5 wt%, Mn: 0.6 wt%, Cu: 1.0
wt%, P: 0.01wt%, S: 0.001wt%, Al: 0.045wt
%, O: 0.002 wt%, using a steel slab containing a Ti content varied in the range of 0.05 to 0.3 wt%, and a subsequent hot rolling simulation using a working for master (a hot working simulator). , The sample is heated by a high frequency induction heating method (a cylindrical shape with a bottom diameter of 10mmφ and a height of 10mm)
The heating temperature and the hot deformation resistance during the hot working were investigated on the α particle size using an apparatus capable of performing compression working under specified conditions. For hot working, 50% reduction is performed at 850 ° C after heating at various temperatures.
The simulation was performed under the conditions of cooling at 450 ° C. and winding at 450 ° C.

FIGS. 1 (a) and (b) to (d) show that the Ti content is 0.05%.
The true stress-true strain curve at the time of hot working by the working for master in wt% and the microstructure after working are shown.
Similarly, FIGS. 2A and 2B to 2D show that the Ti content is 0.20 wt%.
5 shows a true stress-true strain curve during hot working and a microstructure after working.

First, as is evident from the microstructure, as the heating temperature decreases, the α grain size becomes finer,
Higher amounts are more pronounced. Next, from the true stress-true strain curve, it can be seen that dynamic recrystallization occurs under the condition where the 0.20 wt% Ti steel having a fine α grain size is heated at 1150 ° C. This is because dynamic recrystallization is likely to occur due to the extremely fine γ grains during heating. That is, it can be seen that static recrystallization is not necessarily required for miniaturization of the α particle size, and miniaturization using dynamic recrystallization is also possible. Note that this phenomenon was observed even when Nb was added alone.
Was also observed in the case of the composite addition of. From the above,
It has been found that a fine-grained structure can be obtained by heating a Ti-based, high-Nb-based and high Ti-Nb-based component composition at a low temperature.

C: 0.02 wt%, Mn: 0.1 wt%, Nb:
0.20wt%, P: 0.007wt%, S: 0.001wt%, Al: 0.03
wt%, O: 0.002 wt%, N: 0.002 wt%,
The steel slab containing 3.5 wt% was changed in the heating temperature range of 1180-1260 ° C and the finishing temperature was 900
The tensile properties of a hot-rolled sheet having a thickness of 2.4 mm obtained by performing hot rolling at a temperature of ° C were investigated.

FIG. 3 shows the relationship between the tensile properties and the slab heating temperature of the steel to which Cu was not added and the steel to which 1.5 wt% was added. As is clear from the figure, 1.5 wt% Cu-added steel has a high TS, and TS-El.
Is good, but if the slab heating temperature exceeds 1220 ° C, the balance of TS-El. Will deteriorate.

FIG. 4 shows the relationship between the elongation after notch tension and the slab heating temperature for 1.5 wt% Cu-added steel. Similar to the balance of TS-El. Shown in FIG. 3, it can be seen that when the slab heating temperature exceeds 1220 ° C., the temperature deteriorates rapidly.

Here, the reason why the ductility deteriorates sharply at a slab heating temperature of 1220 ° C., as shown in FIGS. 3 and 4, is presumed to be as follows. In other words, when the slab heating temperature is 1220 ° C. or less, refinement of the crystal grains is progressed by the moderately remaining undissolved NbC particles, and the undissolved NbC particles have little effect of quantitatively and dimensionally inhibiting ductility. However, it is considered that when the temperature exceeds 1220 ° C., the effect is increased. The refinement of the crystal grains improves the ductility because the refinement increases the grain boundary area, decomposes the accumulated amount of dislocations at the grain boundaries, and the macroscopic plastic deformation caused by random orientation. It is considered that the effect of reducing anisotropy is caused by working.

This phenomenon was observed both when adding Nb alone and when adding Ti and Nb in combination. From the above, the fine-grained structure obtained by heating at a low temperature a high Ti-based, high Nb-based and high Ti-Nb-based component composition has a good strength-ductility balance and a notch elongation. It was also found that the stretch flangeability was excellent, as represented by

Further, with respect to the above 1.5 wt% Cu-added steel, the as-hot-rolled material was subjected to ε-Cu precipitation treatment at 550 ° C. × 1 h (hereinafter referred to as post-heating treatment), and the tensile properties were examined. did. FIG. 5 shows the relationship between the amount of increase in TS due to the post-heating treatment and the slab heating temperature. As shown in FIG. Large increase in strength due to heat treatment. This is because the higher the slab heating temperature is, the more the matrix is strengthened by the precipitation of NbC, so that the ε-Cu precipitation strengthening that should occur afterwards apparently decreases, while the matrix at the low temperature side is due to the precipitation of NbC. Since no strengthening has been performed, it is considered that the ε-Cu precipitation strengthening that occurred thereafter became apparent.

This phenomenon was observed both when adding Nb alone and when adding Ti and Nb in combination. From the above, the fine-grained structure obtained by heating at a low temperature a high Ti-based, high Nb-based, and high Ti-Nb-based component composition has an increased strength due to ε-Cu precipitation due to post-heating treatment. The amount turned out to be high.

[0020]

Next, the reasons for limiting each component will be described.

C: more than 0.015 wt% and not more than 0.15 wt% C is indispensable as a reinforcing component, and its content is determined in relation to the amount of Ti and / or Nb according to the required strength. In order to obtain a TS of 40 kgf / mm ² or more, which is the target of the present invention, the content must be at least 0.015 wt% or more. However, if the amount of C exceeds 0.15 wt%, the amount of Ti and / or Nb necessary for fixing C becomes too large, and the precipitation strengthening by ε-Cu, which is a main subject of the present invention, is effective. Malfunctions. Therefore, C exceeds 0.015 wt%.
0.15 wt% or less.

Si: 1.5 wt% or less Si has an action of accelerating the precipitation reaction of TiC and NbC in the cooling process after hot rolling, and through this action, it is easy to achieve a low YR characteristic and a ferrite single phase structure. Component.
It also has the effect of dulling the substrate and increasing ductility. However, if the content exceeds 1.5 wt%, the effect is saturated, and a large amount of scale is generated during hot rolling, which deteriorates the surface properties of the steel sheet.

Mn: more than 0.05 wt% and less than 2.0 wt%, Mn is
Basically, it has the effect of suppressing the precipitation reaction rate of TiC and NbC. This is because Mn delays the Ar3 transformation after hot rolling, thereby delaying the formation of α grains in which TiC and NbC are precipitated. By appropriately utilizing this action, the TiC and NbC particles can be refined and high strength can be obtained even with the same amount of C, Ti, and Nb. Therefore, the content exceeding 0.05 wt% is advantageous. . On the other hand, 2.0 wt
%, The precipitation reaction rate of TiC and NbC is remarkably slowed down. As a result, the phase shifts to strengthening by a low-temperature transformation structure such as bainite, and the desired characteristics of the present invention cannot be obtained. Therefore, Mn is set to be more than 0.05 wt% and less than 2.0 wt%.

Cu: not less than 0.5 wt% and not more than 3.0 wt%, and Cu is relatively soft by being kept in a solid solution state without being precipitated at the end of hot rolling in combination with the hot rolling conditions described below, It is a necessary component for achieving high strength by performing aging precipitation treatment after cold working. This requires a content of 0.5 wt% or more, and a content of less than 0.5 wt% cannot ensure the target strength at the final product stage. If you do not need low-temperature heating after molding,
Precipitation may be performed during winding after the completion of hot rolling to increase the strength. On the other hand, when the content exceeds 3.0 wt%, the strength increase due to the aging precipitation treatment reaches a peak and hot brittleness is easily generated during hot rolling. Therefore, the upper limit is 3.0 wt%.

Al: 0.001 wt% or more and 0.1 wt% or less, Al is
It is a necessary component to improve the cleanliness of steel as a deoxidizing agent. It must be contained in an amount of 0.001 wt% or more.
If it exceeds, the effect is saturated, and the amount of alumina generated increases, thereby deteriorating the surface quality of the steel sheet. Therefore, the content of Al is set to 0.001 wt% or more and 0.1 wt% or less.

P: 0.05 wt% or less, P induces internal defects such as center segregation, so that it is suppressed to 0.05 wt% or less.

S: 0.01 wt% or less N: 0.01 wt% or less S and N combine with Ti to form nitrides, not only deteriorating workability, but also consume Ti due to the reaction and precipitate TiC. In order to reduce the amount of effective Ti acting on the steel, the content of each is suppressed to 0.01 wt% or less.

Ti: 0.06 wt% or more and 0.6 wt% or less and Nb:
One or two kinds of 0.12 wt% or more and 1.2 wt% or less are contained under (Ti + Nb / 2) / C ≧ 4 Ti and / or Nb are indispensable together with C as a strengthening component, and have a microstructure. It is an indispensable component for utilizing the effect of forming a ferrite single structure. To exhibit these effects, at least Ti: 0.06 wt% and Nb: 0.12 wt%
Is required. On the other hand, Ti: 0.06 wt% and Nb: 1.
If the content exceeds 2 wt%, it adversely affects the mechanical properties and significantly deteriorates the weldability, so that Ti: 0.06 wt% and Nb: 1.
The upper limit is 2 wt%. Furthermore, in order to make the microstructure a ferrite single structure, the content ratio with C is (Ti + Nb / 2) /
It is important that C is 4 or more.

In the second invention, one or more of Ni, Cr and Mo are added to the above-described component composition.
It is contained at 0.1 wt% or more and 2.0 wt% or less. Ni, Cr and Mo have the effect of _{retarding the Ar3} transformation, TiC and
It is effective to use NbC in an appropriate amount range in measuring the miniaturization of NbC. Therefore, it is necessary to contain 0.1 wt% or more,
If the content exceeds 2.0 wt%, precipitation of TiC and NbC is significantly delayed, and a low-temperature transformation phase such as pearlite or bainite is mixed in the microstructure, so that the ferrite single phase structure intended in the present invention cannot be obtained. 2.0 wt% is the upper limit. Note that Ni can also be expected to have an effect of preventing hot brittleness due to Cu, and can relax the upper limit of a slab heating temperature described later.

In the third invention, in addition to the above component composition, B is contained in the range of 0.0003 wt% to 0.010 wt%. B is a component that can lower the Ar3 transformation point when contained in a trace amount, and can make TiC and NbC finer without forming a low-temperature transformation phase. Further, it has an effect of improving the resistance to secondary working brittleness. To achieve these effects, 0.0003
A content of at least wt% is necessary, but a content of more than 0.010 wt% is likely to cause slab cracking, which is not preferable in operation. Therefore, the content of B is set to 0.0003 wt% or more and 0.010 wt% or less.

In the fourth invention, in addition to the above component composition, Ca: 0.0005 wt% to 0.01 wt% and REM: 0.00
Contains 1 or more of 1 wt% or more and 0.02 wt% or less.
Ca and REM are components that control the morphology of MnS and improve workability and toughness.
M: 0.001 wt% or more must be contained. On the other hand, Ca: 0.
01 wt% REM: If the content exceeds 0.02 wt%, Ca and REM themselves become large inclusions and deteriorate ductility.

Next, the reason why the hot rolling conditions of the present invention are limited will be described. The hot rolling is performed immediately after the slab casting (so-called CC-direct rolling), or when heated, the temperature range is 1220 ° C. or less. When performing CC-direct rolling, heat retention or some heating may be performed. Also, when heating, if the heating temperature exceeds 1220 ° C, austenite will rapidly coarsen, resulting in insufficient grain refinement of the finally obtained structure. The transformation phase is mixed, the strength-ductility balance and the notch elongation, etc., the stretch flangeability is significantly deteriorated, the amount of precipitation strengthening by ε-Cu is reduced, and further, the adverse effect of Cu barge The temperature is set to 1220 ° C. or less because many troubles such as an increase in temperature are caused. On the other hand, the lower limit of the heating temperature is at least the temperature range of the austenite single phase, and there is no particular limitation as long as the target finishing temperature can be secured.

The finishing temperature of the hot rolling is 800 ° C. or higher. That is, if the temperature is lower than 800 ° C., a work structure is formed after hot rolling, the yield ratio increases, and the strength-ductility balance deteriorates. Therefore, the finishing temperature is set to 800 ° C. or higher.

The cooling rate after hot rolling is 30 ° C./s or more. That is, if the temperature is lower than 30 ° C./s, the structure obtained finally is insufficiently refined. It should be noted that rapid cooling beyond necessity is preferably performed at a rate of 200 ° C./s or less, since the final structure may be burned and workability may be deteriorated.

Further, the winding temperature is 650 ° C. or less.
That is, if the film is wound in a temperature range exceeding 650 ° C., the structure obtained finally is insufficiently refined. Further , in the case where the hot-rolled sheet is formed, and when it is necessary to be soft before forming, the winding temperature is set at 480 to keep Cu in a solid solution state as described above.
It should be below ° C. However, unnecessarily low-temperature winding is preferably performed at a temperature of 250 ° C. or higher, since the final structure is burned and the workability is deteriorated.

When the film is wound at 480 ° C. or less, ε−
The low-temperature heating conditions for precipitating Cu are not particularly limited, but it is preferable to maintain the temperature in a temperature range of 500 to 650 ° C. for at least 30 minutes.

[0037]

EXAMPLES Steel having various component compositions shown in Tables 1 to 3 was taken out of a converter, turned into a slab by continuous casting, and immediately or heated, and then subjected to hot working under the conditions shown in Table 4. Rolled.

[0038]

[Table 1]

[0039]

[Table 2]

[0040]

[Table 3]

[0041]

[Table 4]

[0042]

[Table 5]

[0043]

[Table 6]

[0044]

[Table 7]

With respect to the hot-rolled sheet thus obtained, mechanical properties, strength increase after post-heating treatment (550 ° C. × 1 h), 0 ° C.
Absorption energy, ductile-brittle transition temperature at
_v T _rs, and the results of surface quality was investigated of the steel plate, it was also shown in Table 4-7.

The notch elongation was performed using test pieces having the shape and dimensions shown in FIG. The hole expansion rate is
Punched hole of 36mmφ (D ₀ ) diameter, 150 × 150
The central portion of mm of the test piece according to the method shown in FIG. 7, the push-up tip in a spherical punch with a radius 50 mm, and measuring the diameter D ₁ of the when the micro-cracks occurred, was calculated from the following equation. Hole expansion rate (%) = (D ₁ −D ₀ ) / D ₀ × 100

As is clear from Tables 4 to 7, all of the conforming examples according to the present invention have a tensile strength of 40 kgf / mm ² or more, a strength-ductility balance, a notch elongation, a hole expansion ratio, and a steel sheet surface. It had good quality, high strength increase after low-temperature heating and high Charpy absorbed energy, and low transition temperature. Furthermore, these conforming examples also showed good results in the strength of the spot welds separately investigated. Sample No. 46 and
47 is the case where ε-Cu is precipitated during the winding after hot rolling, for the case where low-temperature heating after forming is not required, and shows good values for both YR and TS-El. ing. Here, in all of these compatible examples, the low-temperature transformation phase in the structure was 5% or less.

On the other hand, in the comparative example, since the C content of the sample No. 41 was below the lower limit of the applicable range, the strength did not reach 40 kgf / mm ² , and further, the finishing temperature at the top and end of the hot-rolled coil was reduced. It was out of range. Sample No. 42 is C
Since the amount exceeded the upper limit of the conformity range, the strength of the spot weld was greatly deteriorated. Samples Nos. 3 and 10 had a finishing temperature of less than 800 ° C., had a worked structure after hot rolling, and had significantly reduced mechanical properties. Therefore, other properties were not investigated.

[0049]

According to the present invention, the conventional so-called ε
Compared to -Cu precipitation strengthening hot-rolled steel sheet, tensile properties, stretch flangeability after hot rolling excellent in toughness, etc., and intensity increase amount after post low-temperature heat treatment is large, tensile strength 40 kgf / mm ²
The hot-rolled steel sheet described above is obtained, and a steel sheet suitable for an inner plate of an automobile, a suspension component, a strength material, and the like can be provided.

[Brief description of the drawings]

1A is a graph showing a true stress-true strain curve during hot working, and FIGS. 1B to 1D are photographs showing a metal structure after working.

2A is a graph showing a true stress-true strain curve during hot working, and FIGS. 2B to 2D are photographs showing a metal structure after working.

FIG. 3 is a graph showing a relationship between a slab heating temperature and a tensile property.

FIG. 4 is a graph showing a relationship between a slab heating temperature and a notch elongation.

FIG. 5 is a graph showing a relationship between a slab heating temperature and an increase in strength after a low-temperature heat treatment.

FIG. 6 is a schematic view showing a notch elongation test piece.

FIG. 7 is a schematic view showing a method of measuring a hole expansion ratio.

Continuation of the front page (56) References JP-A-4-107217 (JP, A) JP-A-4-198422 (JP, A) JP-A-4-110418 (JP, A) JP-A-2-205625 (JP) , A) JP-A-2-38523 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 8/02-8/04 C21D 9/46-9/48

Claims (6)

(57) [Claims] C: 0.018 wt% to less than 0.10 wt% , Si: 0.05 wt% to 1.5 wt%, Mn: 0.05 wt% to less than 2.0 wt%, Cu: 0.5 wt% to 3.0 wt%, Al: 0.001 wt% or more and 0.1 wt% or less, P: 0.05 wt% or less, S: 0.01 wt% or less and N: 0.01 wt% or less, and Ti: 0.06 wt% or more and 0.6 wt% or less and Nb: 0.12 wt% %that's all
A steel slab containing 1.2 wt% or less of one or two kinds under (Ti + Nb / 2) / C ≧ 4, and the balance being Fe and unavoidable impurities, was heated to a temperature of 1220 ° C. or less . Hot rolling
And finish rolling at a finishing temperature of 800 ° C or more and less than 900 ° C ,
Cooling at a cooling rate of 30 ° C / s or more, and then winding at a temperature range of 650 ° C or less, the ratio of low-temperature transformation phase is 5% or less.
Method for producing a ferrite single-phase structure and high-strength hot-rolled steel sheet you wherein ever. (2) C: more than 0.018 wt% and less than 0.10 wt%, Si: 0.05 Wt% or more and 1.5 wt% or less, Mn: 0.05 wt% or more and less than 2.0 wt%, Cu: 0.5 wt% or more and 3.0 wt% or less, Al: 0.001 wt% or more and 0.1 wt% or less, P: 0.05 wt% or less, S: 0.01 wt% or less and N: contains 0.01 wt% or less, and Ti: 0.06 wt% or more and 0.6 wt% or less and Nb: 0.12 wt% or more
Any one or two of 1.2 wt% or less, (Ti + Nb / 2) / C ≧ 4 Steel consisting of Fe and inevitable impurities
The slab is 1220 ° C or less Hot rolling after heating to temperature
And finish rolling at a finishing temperature of 800 ° C or more and less than 900 ° C,
Cool at a cooling rate of 30 ° C / s or more, and then
Low temperature transformation phase ratio of 5% or less by winding in the temperature range
High-strength heat characterized by a ferrite single-phase structure
Manufacturing method of rolled steel sheet. (3) The method according to claim 1 or 2,
Instead of heating the steel slab to a temperature below 1220 ° C,
High rolling characterized by hot rolling without heat treatment
Manufacturing method of high strength hot rolled steel sheet. 4. The composition according to claim 1 , further comprising: Ni: 0.001% to 0.1% by weight, Cr: 0.001% to 0.1% by weight, and Mo: 0.001% to 0.1% by weight. Claims 1-3 containing any one or two or more kinds.
The method for producing a high-strength hot-rolled steel sheet according to any one of the above. In addition to 5. The composition of claims 1 to 4, B: high strength according to claim 1 containing less 0.0003% or more 0.010 wt%
Manufacturing method of hot rolled steel sheet. 6. The composition according to claim 1 , further comprising: 0.0005% by weight or more and 0.01% by weight or less of Ca and 0.001% by weight of REM.
6. The composition according to claim 1, which contains at least one kind of at least 0.02% by weight.
2. The method for producing a high-strength hot-rolled steel sheet according to claim 1. "