JPH1180891A - Hot rolled steel sheet having high strength and high workability and excellent in impact resistance and material homogeneity, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve formability, impact resistance, and material homogeneity and to increase the amounts of work hardening and baking hardening by providing a specific composition containing C, Si. Mn, and Cr, also providing a steel structure in which primary phase is composed of pro-eutectoid ferrite and secondary phase is composed of martensite, etc., and regulating a strength-and- elongation balance and its deviation in a coil to values in specific ranges, respectively. SOLUTION: This steel sheet has a composition consisting of, by mass, 0.05-0.40% C, 1.0-3.0% Si, 0.6-3.0% Mn, 0.2-2.0% Cr, and the balance essemtially Fe and also has a steel structure where primary phase is composed of pro- eutectoid ferrite and secondary phase is composed of martensite, acicular ferrite, and retained austenite. In this steel plate, a strength-and-elongation balance (TS×EI) is regulated to >=26000 MPa.%, and also its deviation (ΔTS×ΔEI) in a coil is regulated to <=100 MPa.%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength and high-workability hot-rolled steel sheet which is suitable for use as a steel sheet for automobiles and has excellent impact resistance and excellent material homogeneity, and a method for producing the same.

[0002]

2. Description of the Related Art With the aim of reducing the weight of automobiles, the demand for high-strength thin steel sheets having excellent formability has become particularly strong. In addition, recently, importance has been placed on the safety of automobiles, and for that purpose, an improvement in impact resistance, which is a measure of safety in a collision, is required. Furthermore, consideration for economic efficiency is also required, and in consideration of such economic efficiency, a hot-rolled steel sheet is more advantageous than a cold-rolled steel sheet.

[0003] Against the background of the above situation, various high-strength hot-rolled steel sheets have been developed. For example, Japanese Patent Publication No. 6-41617, Japanese Patent Publication No. 5-65566, and Japanese Patent Publication
JP-67682 discloses a high-workability, high-strength hot-rolled steel sheet containing a retained austenite: steel containing 5% or more of ferrite, bainite, and a retained austenite structure (hereinafter, referred to as a steel).
(Referred to as TRIP steel). However, although this TRIP steel has high elongation and good formability (TS × El ≧ 24000 MPa ·%), it still has a problem in that it cannot meet the current severe impact resistance. There is also a problem that the work hardening amount (WH) at the time of press molding and the baking hardening amount (BH) at the time of subsequent baking of paint are as low as about 70 MPa. If the amount of work and bake hardening (WH + BH) is low, there is a great disadvantage in terms of guaranteeing the strength after work-paint baking.

On the other hand, as a high-strength hot-rolled steel sheet having excellent impact resistance, as disclosed in Japanese Patent Application Laid-Open No. 9-111396, a so-called Dual Phase steel (hereinafter, referred to as a two-phase structure of ferrite and martensite) is disclosed. DP steel) has been developed. However, although this DP steel is excellent in impact resistance, it cannot be said that elongation is sufficient, and there is a problem in formability.

[0005]

As described above, no hot-rolled steel sheet satisfying both sufficient formability and strict safety has been found so far, and its development has been desired. The present invention advantageously satisfies the above demands and has both excellent formability and impact resistance (specifically, the strength-elongation balance (TS × El) is 26000 MPa ·% or more, and the dynamic n value 0.35 or more) and excellent material homogeneity (ΔTS ×
It is an object of the present invention to propose a high-strength, high-workability hot-rolled steel sheet having a large amount of processing and bake hardening ((WH + BH) ≧ 100 MPa), together with its advantageous production method.

Here, the dynamic n value is newly found by the present inventors as an index of the impact resistance, and by using this dynamic n value, the impact resistance can be more accurately measured than before. Can be evaluated. In other words, in the past, collision safety was considered in relation to strength, and it was considered that the higher the strength, the higher the crash safety. However, the relationship between strength and collision safety is not necessarily unique. It turned out not to be the case. Therefore, as a result of diligent research on this point, it has been found that the collision safety is improved, that is, when the vehicle is deformed at high speed (the strain rate is

[Outside 3] The energy in but increased to 2 × 10 ³ / s), in order to absorb more with the steel sheet, the steel sheet

[Outside 4] It has been clarified that it is effective to increase the n value (hereinafter referred to as dynamic n value) when tensile deformation is performed under the following conditions. Here, an instantaneous n value at an elongation of 10% is defined as a dynamic n value. In addition, it was also found that increasing the dynamic n value is effective for improving strength during high-speed deformation.

[0007]

The details of the invention will be described below. By the way, the present inventors first investigated the relationship between the structure and properties of a conventional TRIP steel in order to achieve the above object. As a result, T
In the RIP steel, it has been indispensable to form a bainite phase in order to obtain a sufficient amount of retained austenite which is advantageous for improving formability. However, this bainite phase causes deterioration of impact resistance. Turned out to be.

[0008] Then, the present inventors have suppressed the formation of such a bainite phase, particularly carbides, that is, the second phase other than the proeutectoid ferrite, which is the main phase, is converted from the conventional bainite + residual austenite into acicular phase. When the structure was changed to a mixed structure of ferrite + martensite + retained austenite, unexpected results were achieved in achieving the intended purpose. Furthermore, when the strain rate in the final stage in the hot finish rolling is increased, the grain size of the main phase ferrite becomes finer, the dispersibility of the second phase becomes uniform, the strength-elongation balance is further improved, and the homogeneity is improved. It was also found that it was advantageously improved. The present invention is based on the above findings.

That is, the gist of the present invention is as follows. 1. C: 0.05 to 0.40 mass%, Si: 1.0 to 3.0 mass%, Mn: 0.6 to 3.0 mass%, Cr: 0.2 to 2.0 mass%, the balance being substantially Fe composition, Precipitated ferrite, the second phase has a steel structure consisting of martensite, acicular ferrite and retained austenite, and has a strength-elongation balance (TS × El) of 26000 MPa ·% or more and its coil deviation (ΔTS × ΔEl) is 100 MPa ・
% High-strength, high-workability hot-rolled steel sheet with excellent impact resistance and material homogeneity, characterized by satisfying the following requirements:

[0010] 2. In the above item 1, the steel composition further comprises at least one selected from the group consisting of P: 0.01 to 0.2 mass% and Al: 0.01 to 0.3 mass%. Excellent high strength and high workability hot rolled steel sheet.

3. In the above 1 or 2, the steel composition is
In addition, the composition contains at least one selected from the group consisting of Ti: 0.005 to 0.25 mass% and Nb: 0.003 to 0.1 mass%. Rolled steel sheet.

[0012] 4. C: 0.05 to 0.40 mass%, Si: 1.0 to 3.0 mass%, Mn: 0.6 to 3.0 mass%, Cr: 0.2 to 2.0 mass%, the balance being a steel slab substantially having a Fe composition.
After heating to 1000-1300 ° C and rough rolling, the final stage rolling strain rate

[Outside 5] And finish rolling at 780 to 980 ° C, and after cooling to 620 to 780 ° C,
Isothermal holding treatment for 10 seconds or cooling rate: Slow cooling treatment at 20 ° C / s or less, then cooling to 350-500 ° C,
After winding on a coil, cool at a cooling rate of 10-100 ° C / h.
A method for producing a high-strength, high-workability hot-rolled steel sheet having excellent impact resistance and material homogeneity, characterized by cooling to a temperature of 00 ° C or lower.

[0013] 5. C: 0.05 to 0.40 mass%, Si: 1.0 to 3.0 mass%, Mn: 0.6 to 3.0 mass%, Cr: 0.2 to 2.0 mass%, the balance being a steel slab substantially having a Fe composition.
After heating to 1000-1300 ° C and rough rolling, the final stage rolling strain rate

[Outside 6] And finish rolling at 780 to 980 ° C, and after cooling to 620 to 780 ° C,
Isothermal holding treatment for 10 seconds or cooling rate: Slow cooling treatment at 20 ° C / s or less, then cooling to 350-500 ° C,
After winding on a coil, perform isothermal holding treatment for 2 to 60 minutes or slow cooling treatment at a cooling rate of less than 50 ° C / h, and then cool to 300 ° C or less at a cooling rate of 50 ° C / h or more by forced cooling. A method for producing a high-strength, high-workability hot-rolled steel sheet having excellent impact resistance and material homogeneity.

In the present invention, the ratio of the second phase in the steel structure is preferably set to 3 to 40%.
For the ratio of each phase in the phase, martensite: 10
8080%, retained austenite: 8-30%, acicular ferrite: 5-60%.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be specifically described below. FIG. 1 shows a typical continuous cooling transformation curve diagram (CCT diagram) of a conventional TRIP steel. As shown in the figure, after hot rolling, the conventional TRIP steel is slightly retained in the pro-eutectoid ferrite region to precipitate pro-eutectoid ferrite (also called polygonal ferrite), and at the same time, to form a solid solution in the untransformed austenite phase. After promoting the concentration of carbon to increase the stability of austenite, it was led to a bainite region, and the region was gradually cooled to cause a bainite transformation while leaving a predetermined amount of austenite. However, as described above, the TRIP steel manufactured as described above has excellent strength and workability but does not have sufficient impact resistance.

The inventors have conducted numerous experiments and studies in order to avoid bainite transformation. As a result, (1) when a small amount of Cr is contained as a steel component, the nose of the bainite transformation region in the CCT diagram recedes. As a result, the precipitation of bainite (particularly the precipitation of carbides) is suppressed, and instead needle-like ferrite (also called acicular ferrite) precipitates.
(2) The thus formed second phase consisting of acicular ferrite, retained austenite and martensite is
They have sought to significantly improve the impact resistance without impairing the formability.

FIG. 2 shows a representative CCT diagram for the component system of the present invention. As shown in the figure, by adding a small amount of Cr, the nose of the bainite transformation region recedes, and instead, a needle-like ferrite region appears remarkably. By rapidly cooling, the second phase can have a mixed structure composed of acicular ferrite, retained austenite and martensite, and thus a hot-rolled steel sheet having both excellent formability and impact resistance can be obtained. It was.

Here, the acicular ferrite refers to a ferrite having a major axis of crystal grains of about 10 μm or less, an aspect ratio of 1: 1.5 or more, and a precipitation of cementite of 5% or less. In addition, since the precipitation of cementite is often observed in the bainite of the conventional TRIP steel (10% or more), the acicular ferrite of the present invention and the bainite of the TRIP steel are clearly distinguished.

FIG. 3 (a) shows a second example obtained according to the present invention.
FIG. 3 (b) shows a conventional TR structure.
The phase structure of the second phase of the IP steel is schematically shown. Whereas the second phase of the conventional TRIP steel has a phase structure in which residual austenite is scattered in bainite, the second phase of the present invention is characterized in that acicular ferrite and martensite are layered and the interface thereof is formed. (Martensite side) in a form in which retained austenite is scattered. As described above, it is one of the features of the present invention that the needle-like ferrite is precipitated in the second phase. The needle-like ferrite phase increases TS × El and improves the dynamic n value. It is considered something. According to the findings of the inventors, it has been confirmed that the dynamic n value tends to increase as the interface area ratio between the acicular ferrite and martensite increases.

Further, according to the experiments by the inventors, in the hot rolling step shown in FIG. 2, when the final stage of finish rolling is performed at a strain rate of 250 s ⁻¹ or more, the grain size of the main phase ferrite is reduced. And the dispersibility of the second phase was made uniform, and the strength-elongation balance (TS × El) was further improved, and the homogeneity (ΔTS × ΔEl) was advantageously improved. .

Here, the rolling at a high strain rate as described above is an endless rolling developed by the applicant company, that is, continuously joining the rear end of the preceding sheet bar and the front end of the following sheet bar successively. This has been achieved for the first time by a rolling method of rolling. By the way, in the conventional general finish rolling, even if the rolling speed is increased in order to increase the strain speed, the rolling speed can be increased only up to about 700 to 800 mpm at best, so the obtained strain speed is 200 s ^-1. However, in the endless rolling described above, rolling at a high speed of 1200 mpm is possible.

[Outside 7] The rolling in the is achieved stably.

In the present invention, the ratio of the second phase in the steel structure is preferably 3 to 40%. The reason is that if the phase ratio is less than 3%, sufficient impact resistance cannot be obtained, while if it exceeds 40%, the elongation and hence the strength-elongation balance are reduced. A more desirable ratio is 10 to 30%. In the present invention, the phase ratio was calculated by polishing a steel sample, etching with a 2% nitric acid + ethyl alcohol solution, and performing image analysis on a micrograph.

Next, the ratio of each phase in the second phase is as follows: martensite: 10 to 80% (preferably 30 to 60%).
%), Retained austenite: 8 to 30% (preferably 10 to 10%)
20%), acicular ferrite: 5-60% (preferably 20-50)
%). The reason is that if the martensite ratio is less than 10%, sufficient impact resistance cannot be obtained, while if it exceeds 80%, the elongation and hence the strength-elongation balance are reduced. If the ratio of retained austenite is less than 8%, sufficient elongation cannot be obtained, while 30%
%, The impact resistance deteriorates. Further, if the proportion of acicular ferrite is less than 5%, good impact resistance cannot be obtained, while if it exceeds 60%, elongation is reduced.

The proportion of each phase in the entire steel structure is preferably about 5 to 15% for martensite and acicular ferrite, respectively, and about 2 to 10% for retained austenite. Further, in the present invention, the steel structure does not always consist of a mixed phase of proeutectoid ferrite as a main phase and martensite, acicular ferrite and retained austenite as a second phase. Although some precipitation may occur, even if such a third phase is mixed, there is no problem in characteristics as long as the ratio is 10% or less of the entire second phase.

Next, the reason why the composition of the steel sheet is limited to the above range in the present invention will be described. C: 0.05 to 0.40 mass% C is an element that not only effectively contributes to the strengthening of steel but is also useful in obtaining retained austenite. However, if the content is less than 0.05 mass%, the effect is poor,
On the other hand, if it exceeds 0.40 mass%, the ductility decreases, so the C content is limited to the range of 0.05 to 0.40 mass%.

Si: 1.0 to 3.0 mass% Si is an element indispensable for the formation of retained austenite. For this purpose, at least 1.0 mass% must be added. In addition, Si content is limited to the range of 1.0 to 3.0 mass%, because not only does this cause deterioration of the scale properties, but also causes a problem in surface quality.

Mn: 0.6 to 3.0 mass% Mn is not only useful as a strengthening element for steel, but also useful for obtaining retained austenite. However, if the content is less than 0.6 mass%, the effect is poor, while if it exceeds 3.0 mass%, the ductility is reduced.
Limited to the range of 0.6 to 3.0 mass%.

Cr: 0.2 to 2.0 mass% This addition of Cr is one of the features of the present invention. As described above, the addition of Cr causes the second phase to become acicular ferrite. For this purpose, it is necessary to add 0.2 mass% or more. However, if it exceeds 2.0 mass%, coarse Cr carbides are formed, ductility is inhibited, and the strength-elongation balance and the dynamic n value deteriorate. , Cr content is 0.2-2.0 mass
%. Preferably it is 0.3 to 1.8 mass%.

FIGS. 4 and 5 show the results of investigations on the relationship between the Cr content and the strength-elongation balance and dynamic n value, respectively. As is apparent from FIGS.
TS × El ≧ 260 in the range of 0.2 mass% or more and 2.0 mass% or less
Excellent workability and impact resistance of 00 (MPa ·%) and dynamic n value ≧ 0.35 are obtained.

Although the basic components have been described above, in the present invention, P or Al as an austenite-forming element and Ti or Nb as a strength improving component can be appropriately contained in the following ranges. P: 0.01 to 0.2 mass% P is useful as a retained austenite-forming element, but if the content is less than 0.01 mass%, the effect of its addition is poor, while if it exceeds 0.2 mass%, the secondary workability deteriorates. Therefore, when it is added, it is desirable to set it in the range of 0.01 to 0.2 mass%.

Al: 0.01 to 0.3 mass% Al is also useful as a retained austenite forming element, like P, but if the content is less than 0.01 mass%, the effect of its addition is poor. If the amount exceeds the above range, the ductility is reduced. Therefore, when added, the content is preferably in the range of 0.01 to 0.3 mass%.

Ti: 0.005 to 0.25 mass%, Nb: 0.003 to 0.2%
Since both 1 mass% Ti and Nb effectively contribute to the improvement of the strength by making ferrite as the main phase finer, they can be added as necessary. In particular, when Ti is included, the nose of the acicular ferrite shifts to a short time side, and the acicular ferrite is sufficiently precipitated even at the coil end where the cooling rate is faster than the coil middle part,
There is also an effect of improving the yield. However, if the content is too small, the effect of the addition is poor. On the other hand, excessive addition causes a decrease in ductility. Therefore, it is preferable that each content is in the above range.

Next, a method for producing the steel according to the present invention will be described. In the present invention, in short, it is sufficient to form a mixed structure composed of martensite, acicular ferrite, and retained austenite as the second phase. Therefore, cooling may be performed along the cooling curve shown in FIG. . First, slab heating is performed prior to hot rolling, and the heating temperature must be 1000 to 1300 ° C. I mean,
If the slab heating temperature is less than 1000 ° C., the surface quality of the steel sheet is significantly deteriorated, while if it exceeds 1300 ° C., the crystal grains of the steel are coarsened, resulting in deterioration of the homogeneity and ductility of the material. The heating time is not particularly limited, but if it is too long, the crystal grains become coarse, so that it is preferable to set the heating time to about 60 minutes or less.

Then, after the hot rough rolling, the finish rolling is performed, and the finish rolling temperature at this time needs to be 780 to 980 ° C. That is, if the finish rolling end temperature is less than 780 ° C, a work structure remains in the steel and the ductility deteriorates,
On the other hand, if the temperature exceeds 980 ° C, the structure becomes coarse and the formability is reduced due to the delay of ferrite transformation.

Then, the final stage rolling in the finish rolling is

[Outside 8] In this case, the absolute value of the strength-elongation balance and the homogeneity thereof are improved. Here, the lower limit of the strain rate is limited to 250 s ^-1, and at a strain rate lower than this value, the strength-elongation balance improvement and homogeneity improvement as expected in the present invention cannot be realized. Because. More preferred strain rate is 3
00 s ^-1 or more. The rolling at such a high strain rate is performed by the endless rolling described above, and the rolling speed is 1000 m.
It can be achieved by carrying out under the condition of not less than pm and the rolling reduction: not less than 25%.

Then, after cooling to the vicinity of the nose of the pro-eutectoid ferrite region at 620 to 780 ° C., the temperature is maintained at this temperature for 1 to 10 seconds or gradually cooled at a rate of 20 ° C./s or less to form the main phase. Precipitates certain pro-eutectoid ferrite and promotes the concentration of solid solution carbon in the austenite phase. The above temperature range of 620 to 780 ° C is a temperature range in which the ferrite transformation proceeds most smoothly, so that a desired amount of pro-eutectoid ferrite can be obtained by a short holding treatment or slow cooling treatment for about 1 to 10 seconds. it can. In the case of the slow cooling treatment, if the cooling stop temperature is lower than 600 ° C., pearlite transformation may occur. Therefore, the cooling stop temperature is preferably set to 600 ° C. or higher.

Subsequently, a desired amount of needle-like ferrite is precipitated by cooling to a needle-like ferrite region of 350 to 500 ° C., and gradually cooling this region, or maintaining isothermal or slow cooling and then forcibly cooling. In the case of the slow cooling treatment (a in FIG. 2), if the cooling rate is less than 10 ° C./h, there is a high possibility that bainite transformation occurs, while if it exceeds 100 ° C./h, it becomes difficult to obtain a desired amount of acicular ferrite. The cooling rate for slow cooling was limited to the range of 10 to 100 ° C / h. Also,
In the case of forced cooling after isothermal holding or slow cooling (b in FIG. 2), first, the holding or slow cooling time is 2 to 60 minutes. This is because if the holding or cooling time is less than 2 minutes, a sufficient amount of acicular ferrite cannot be obtained, while if it exceeds 60 minutes, bainite transformation may occur.
Further, the reason why the cooling rate in the slow cooling was set to less than 50 ° C / h is that if this rate was too high, sufficient needle-like ferrite could not be obtained. The reason for using / h or more is to prevent bainite transformation.

Next, the temperature is cooled to 300 ° C. or less by the above-mentioned slow cooling process or isothermal holding (slow cooling) -forced cooling process.
The following is also to avoid the possibility that bainite transformation occurs.

By the above series of treatments, a desired steel structure and high strength can be obtained in which the second phase consisting of acicular ferrite, martensite and retained austenite is homogeneously present in the fine pro-eutectoid ferrite main phase. A high-strength and high-workability hot-rolled steel sheet having an elongation balance (TS × El) and low (ΔTS × ΔEl) and excellent in material uniformity can be obtained.

[0040]

【Example】

Example 1 Steel slabs having various component compositions shown in Table 1 were heated to 1200 ° C., rough-rolled, and then finished with hot finishing rolling at a finishing temperature of 820 ° C., as shown in Table 2, The strain rate at the stage was varied. Then, cool at a rate of 120 ° C / s to 700 ° C, slowly cool at a rate of 10 ° C / s for 10 seconds, cool to a rate of 120 ° C / s to 450 ° C, wind the coil, Hold for 8 minutes after taking, then 100 ℃ / h
At room temperature. Tensile test pieces were cut out from the obtained hot-rolled sheet, and a tensile test was performed on the test pieces under the conditions of a strain rate of 2 × 10 ⁻² / s, and the yield strength (Y
S), tensile strength (TS) and elongation (El) were determined. In addition, Hopkinson pressure bar test materials (material and process vo
l.9 (1996) P.1108-1111), strain rate: 2 ×
A tensile test was performed under the condition of 10 ³ / s, and an instantaneous n value (dynamic n value) when the elongation was 10% was determined. Further, the work hardening amount (WH) at the time of press molding and the bake hardening amount (BH) at the time of subsequent baking (170 ° C.) were also measured. WH and BH are the strain rates: 2 × 10 ^-2 / s
Was determined according to FIG. Steel structure, TS × El balance, ΔTS × ΔEl, WH + of each obtained hot rolled steel sheet
Table 2 also shows the results of the investigation on BH and dynamic n value.

[0041]

[Table 1]

[0042]

[Table 2]

As shown in Table 2, according to the present invention, any of the second phases which formed a mixed structure of martensite, acicular ferrite and retained austenite and improved the dispersibility of the second phase were used. , TS × El ≧ 26000 M
Not only excellent strength-elongation balance and impact resistance properties such as Pa ·% and dynamic n value ≧ 0.35, but also WH + BH ≧ 100 MPa
, ΔTS × ΔEl ≦ 100 MPa ·%, excellent workability, bake hardenability and material homogeneity are obtained.

Example 2 A steel slab having the composition shown in Table 1 for steel type A was prepared as shown in FIG.
Table 3 shows patterns X and Y shown in (a) and (b).
To give a hot-rolled steel sheet. Steel structure, TS × El balance, ΔTS × ΔEl, WH + B of each obtained hot rolled steel sheet
Table 4 shows the results of examining the H and the dynamic n value.

[0045]

[Table 3]

[0046]

[Table 4]

In each of the hot-rolled steel sheets obtained according to the present invention, the second phase forming a mixed structure of martensite, acicular ferrite and retained austenite is uniformly dispersed in the proeutectoid ferrite main phase. As a result, TS × El ≧ 26000
MPa ·%, dynamic n value ≧ 0.35, WH + BH ≧ 100 MPa, Δ
Excellent characteristic values of TS × ΔEl ≦ 100 MPa ·% are obtained.

[0048]

Thus, according to the present invention, the main phase is pro-eutectoid ferrite, the second phase is a mixed structure of martensite, acicular ferrite and retained austenite, and the dispersibility of the second phase is improved. Accordingly, a hot-rolled steel sheet having not only excellent formability and impact resistance but also excellent material homogeneity can be obtained.

[Brief description of the drawings]

FIG. 1 is a typical continuous cooling transformation diagram (CCT diagram) of a conventional TRIP steel.

FIG. 2 is a typical continuous cooling transformation curve (CCT diagram) in the component system of the present invention.

FIG. 3 is a schematic diagram showing (a) a characteristic phase structure of a second phase obtained according to the present invention and (b) a phase structure of a second phase of a conventional TRIP steel.

FIG. 4 is a graph showing the relationship between Cr content and strength-elongation balance.

FIG. 5 is a graph showing a relationship between a Cr amount and a dynamic n value.

FIG. 6: Work hardening amount (WH) and bake hardening amount (BH)
FIG.

FIG. 7 is a schematic diagram of a cooling pattern according to the present invention.

Claims (5)

[Claims] 1. C: 0.05 to 0.40 mass%, Si: 1.0 to 3.0 mass%, Mn: 0.6 to 3.0 mass%, Cr: 0.2 to 2.0 mass%, the balance being substantially Fe composition. The main phase is proeutectoid ferrite, the second phase has a steel structure consisting of martensite, acicular ferrite and retained austenite, and its strength-elongation balance (TS × El) is 26000 MPa ·% or more and its coil Internal deviation (ΔTS × ΔEl) is 100 MPa ・
% High-strength, high-workability hot-rolled steel sheet with excellent impact resistance and material homogeneity, characterized by satisfying the following requirements: 2. The impact resistance according to claim 1, wherein the steel composition further comprises at least one selected from the group consisting of P: 0.01 to 0.2 mass% and Al: 0.01 to 0.3 mass%. High strength, high workability hot rolled steel sheet with excellent properties and material homogeneity. 3. The steel composition according to claim 1, wherein the steel composition is
In addition, the composition contains at least one selected from the group consisting of Ti: 0.005 to 0.25 mass% and Nb: 0.003 to 0.1 mass%. Rolled steel sheet. 4. C: 0.05 to 0.40 mass%, Si: 1.0 to 3.0 mass%, Mn: 0.6 to 3.0 mass%, Cr: 0.2 to 2.0 mass%, and the balance substantially becomes Fe composition. Steel slab,
After heating to 1000-1300 ℃ and rough rolling, the final stage rolling strain rate And finish rolling at 780 to 980 ° C, and after cooling to 620 to 780 ° C,
Isothermal holding treatment for 10 seconds or cooling rate: Slow cooling treatment at 20 ° C / s or less, then cooling to 350-500 ° C,
After winding on a coil, cool at a cooling rate of 10-100 ° C / h.
A method for producing a high-strength, high-workability hot-rolled steel sheet having excellent impact resistance and material homogeneity, characterized by cooling to a temperature of 00 ° C or lower. 5. C: 0.05 to 0.40 mass%, Si: 1.0 to 3.0 mass%, Mn: 0.6 to 3.0 mass%, Cr: 0.2 to 2.0 mass%, and the balance substantially becomes Fe composition. Steel slab,
After heating to 1000-1300 ℃ and rough rolling, the final stage rolling strain rate [2] And finish rolling at 780 to 980 ° C, and after cooling to 620 to 780 ° C,
Isothermal holding treatment for 10 seconds or cooling rate: Slow cooling treatment at 20 ° C / s or less, then cooling to 350-500 ° C,
After winding on a coil, perform isothermal holding treatment for 2 to 60 minutes or slow cooling treatment at a cooling rate of less than 50 ° C / h, and then cool to 300 ° C or less at a cooling rate of 50 ° C / h or more by forced cooling. A method for producing a high-strength, high-workability hot-rolled steel sheet having excellent impact resistance and material homogeneity.