JPH1143725A - Production of high strength and high workability hot rolled steel sheet excellent in impact resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a hot rolled steel sheet having excellent formability and impact resistance by subjecting a steel slab having a specified compsn. to rolling and cooling under specified temp. conditions and forming the structure of the steel into the one in which secondary phases composed of martensite, acicular ferrite and residual austenite are allowed to contain in the main phases of pro-eutectoid ferrite. SOLUTION: The compsn. of the steel slab is composed of, by mass, 0.05 to 0.40% C, 1.0 to 3.0% Si, 0.6 to 3.0% Mn, 0.2 to 2.0% Cr, and the balance substantial Fe. The slab having this compsn. is heated to 1000 to 1300 deg.C and is subjected to rough rolling. The subsequent finish rolling is finished at 780 to 980 deg.C. Next, it is cooled to form 620 to 780 deg.C, is thereafter subjected to isothermal holding treatment for 1 to 10 sec or slow cooling treatment at <=20 deg.C/sec cooling rate and is subsequently cooled to from 350 to 500 deg.C. Then, it is coiled round a coil and is thereafter cooled to 300 deg.C at 10 to 100 deg.C/hr cooling rate. In this way, the desired steel structure can be obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[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 No. 5-67682 each disclose a so-called high-workability, high-strength hot-rolled steel sheet containing ferrite, bainite and 5% or more retained austenite. A method for producing Transformation Induced Plasticity steel (hereinafter referred to as TRIP steel) is disclosed. 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-mentioned demands and has both excellent moldability and impact resistance. Specifically, the strength-elongation balance (TS × El) is 24000 MPa ·% or more and dynamic n Value is 0.35 or more and the amount of processing and bake hardening (WH
(BH) as high as 100 MPa or more, and an object of the present invention is to propose an advantageous method for producing a high-strength, high-workability hot-rolled steel sheet having excellent impact resistance.

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

[Outside 2] 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. The present invention is based on the above findings.

That is, the present invention comprises: 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 is substantially Fe. Steel slab with the composition of
Heat to 1000 ~ 1300 ℃, after rough rolling, finish rolling 780 ~
Finish at a temperature of 980 ° C, then cool to 620-780 ° C, then keep isothermally for 1-10 seconds or cooling rate: 20
Perform a slow cooling treatment at a temperature of 350 ° C / s or less, then cool to 350 to 500 ° C, wind it up in a coil, and cool to 300 ° C or less at a cooling rate of 10 to 100 ° C / h to perform primary precipitation. A method for producing a high-strength, high-workability hot-rolled steel sheet having excellent impact resistance, characterized by having a steel structure having a second phase composed of martensite, acicular ferrite, and retained austenite in a ferrite main phase. 1 invention).

Further, the present invention comprises: 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 is substantially Fe. Steel slab with the composition of
Heat to 1000 ~ 1300 ℃, after rough rolling, finish rolling 780 ~
Finish at a temperature of 980 ° C, then cool to 620-780 ° C, then keep isothermally for 1-10 seconds or cooling rate: 20
Apply a slow cooling process at ℃ / s or less, then cool to 350-500 ℃, wind it up in a coil, and keep it at isothermal temperature for 2-60 minutes or a slow cooling process at a cooling rate of less than 50 ℃ / h And then at a cooling rate of 50 ° C / h or more by forced cooling.
High strength excellent in impact resistance characterized by having a steel structure having a second phase composed of martensite, acicular ferrite and retained austenite in the main phase of proeutectoid ferrite by cooling to below 00 ° C. This is a method (second invention) for producing a hot-rolled hot-rolled steel sheet.

In the present invention, regarding the composition of the steel slab, at least one element selected from the group consisting of P: 0.01 to 0.2 mass% and Al: 0.01 to 0.3 mass% as an austenite-forming element, in addition to the basic composition described above, Further, at least one selected from the group consisting of 0.005 to 0.25 mass% of Ti and 0.003 to 0.1 mass% of Nb can be contained as a strength improving component.

In the present invention, the ratio of the second phase in the steel structure is preferably set to 3 to 40%, and the ratio of each phase in the second phase is preferably set to 10 to 80% of martensite. , Retained austenite: 8-30%, acicular ferrite: 5-60%.

[0013]

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 the formation of bainite. 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 is increased. It recedes and suppresses the precipitation of bainite (especially the precipitation of carbides) and instead precipitates acicular ferrite (also called acicular ferrite). (2) Acicular ferrite, residual austenite thus formed It has been clarified that the second phase comprising martensite can significantly improve impact resistance without impairing 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.

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.

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.

It is preferable that the ratio of each phase in the entire steel structure is approximately 5 to 15% for martensite and acicular ferrite, and approximately 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, in the present invention, the reason why the composition of the steel sheet is limited to the above range 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, and therefore requires addition of at least 1.0 mass%, but addition exceeding 3.0 mass% decreases ductility. 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 poses 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: 0.2-2.0 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 examining the relationship between the Cr content and the strength-elongation balance and dynamic n value, respectively. As is apparent from FIGS.
TS × El ≧ 240 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 its 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 0.1 mass% Both Ti and Nb are effective in improving the strength by making the ferrite, which is the main phase, finer. It can be added accordingly. 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, the production method of the present invention will be specifically described. In the present invention, in short, a mixed structure consisting of martensite, acicular ferrite, and retained austenite may be formed as the second phase.
The cooling may be performed along the cooling curve shown in FIG. First, slab heating is performed prior to hot rolling.
The heating temperature must be 1000-1300 ° C. The reason is that if the slab heating temperature is less than 1000 ° C, the surface quality of the steel sheet deteriorates remarkably, while if it exceeds 1300 ° C, the crystal grains of the steel become coarse, leading to deterioration of the homogeneity and ductility of the material. is there. 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.

Next, finish rolling is carried out after hot rough rolling, and the finishing 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, after cooling to the vicinity of the nose of the proeutectoid ferrite region at 620 to 780 ° C, the temperature is maintained at this temperature for 1 to 10 seconds or gradually cooled at a speed of 20 ° C / s or less to obtain 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.

Then, the 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 can be obtained in which the second phase composed of acicular ferrite, martensite and retained austenite is present in the proeutectoid ferrite main phase.

[0035]

【Example】

Example 1 A steel slab containing 0.15 mass% of C, 1.50 mass% of Si, 1.0 mass% of Mn and 0.20 mass% of Cr, and the balance being substantially Fe, was heated to 1200 ° C. After rough rolling, according to pattern X and pattern Y shown in FIGS. 6 (a) and 6 (b),
After finish rolling at 880 ° C, it is cooled near the nose of the pro-eutectoid ferrite region at 750 ° C, and then subjected to isothermal holding or slow cooling under the conditions shown in Table 1, and then cooled to the needle-like ferrite region at 500 ° C. Then, in this region, a slow cooling treatment or an isothermal holding (slow cooling) -forced cooling treatment was performed under the conditions shown in Table 1, followed by cooling to room temperature. Tensile test specimens were cut out from the obtained hot-rolled sheet, and the strain rates of the test specimens were as follows:
A tensile test was conducted under the conditions of 2 × 10 ^-2 / s, yield strength (YS),
Tensile strength (TS) and elongation (El) were determined. In addition, Hopkinson pressure bar test materials (Materials and Process vol.9
(1996) P.1108-1111), strain rate: 2 × 10 ³ /
The tensile test is performed under the condition of s, and the moment n when the elongation is 10%
The value (dynamic n value) was determined. Furthermore, the amount of work hardening (WH) during press molding and subsequent baking (170
C)), the bake hardening amount (BH) was also measured.
In addition, WH and BH were determined from FIG. 7 using a tensile tester having a strain rate of 2 × 10 ⁻² / s. Table 1 summarizes the results of a study on the steel structure, TS × El balance, dynamic n-value, WH + BH and YR of each hot-rolled steel sheet.

[0036]

[Table 1]

As shown in Table 1, according to the present invention, any of those having a mixed structure of martensite, acicular ferrite and retained austenite as the second phase had excellent TS × El ≧ 24000 MPa ·%. Strength-elongation balance, excellent impact resistance with dynamic n value ≧ 0.35 and WH + B
Excellent processing and bake hardening properties of H ≧ 100 MPa are obtained.

Example 2 A steel slab having the composition shown in Table 2 was treated according to the pattern X and the pattern Y shown in FIG. 6 to obtain a hot-rolled steel sheet. Table 3 shows the results obtained by examining the steel structure, TS × El balance, dynamic n-value, WH + BH and YR of the obtained hot-rolled steel sheet.
Shown in

[0039]

[Table 2]

[0040]

[Table 3]

Each of the steel sheets obtained according to the present invention comprises:
As the second phase, a mixed structure of martensite, acicular ferrite and retained austenite is formed. As a result, TS × El ≧ 24000 MPa ·%, dynamic n value ≧ 0.35, WH +
Excellent strength-elongation balance of BH ≧ 100 MPa, impact resistance and workability and bake hardenability are obtained.

[0042]

Thus, according to the present invention, by forming the main phase as proeutectoid ferrite and the second phase as a mixed structure of martensite, acicular ferrite and retained austenite, excellent formability and impact resistance can be obtained. 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 is a schematic diagram of a cooling pattern according to the present invention.

FIG. 7: Work hardening amount (WH) and bake hardening amount (BH)
FIG.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Tetsuo Shimizu 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Inside the Technical Research Institute of Kawasaki Steel Co., Ltd. (72) Inventor Osamu Furukuni 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba (72) Inventor Shusaku Takagi 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba

Claims (4)

[Claims] (1) C: 0.05 to 0.40 mass%, Si: 1.0 to
A steel slab containing 3.0 mass%, Mn: 0.6-3.0 mass%, Cr: 0.2-2.0 mass%, and the balance substantially consisting of Fe,
Heat to 1000 ~ 1300 ℃, after rough rolling, finish rolling 780 ~
Finish at a temperature of 980 ° C, then cool to 620-780 ° C, then keep isothermally for 1-10 seconds or cooling rate: 20
Perform a slow cooling treatment at a temperature of 350 ° C / s or less, then cool to 350 to 500 ° C, wind it up in a coil, and cool to 300 ° C or less at a cooling rate of 10 to 100 ° C / h to perform primary precipitation. A method for producing a high-strength, high-workability hot-rolled steel sheet having excellent impact resistance, characterized by having a steel structure having a second phase consisting of martensite, acicular ferrite, and retained austenite in a ferrite main phase. 2. C: 0.05 to 0.40 mass%, Si: 1.0 to
A steel slab containing 3.0 mass%, Mn: 0.6-3.0 mass%, Cr: 0.2-2.0 mass%, and the balance substantially consisting of Fe,
Heat to 1000 ~ 1300 ℃, after rough rolling, finish rolling 780 ~
Finish at a temperature of 980 ° C, then cool to 620-780 ° C, then keep isothermally for 1-10 seconds or cooling rate: 20
Apply a slow cooling process at ℃ / s or less, then cool to 350-500 ℃, wind it up in a coil, and keep it at isothermal temperature for 2-60 minutes or a slow cooling process at a cooling rate of less than 50 ℃ / h And then at a cooling rate of 50 ° C / h or more by forced cooling.
High strength excellent in impact resistance characterized by having a steel structure having a second phase composed of martensite, acicular ferrite and retained austenite in the main phase of proeutectoid ferrite by cooling to below 00 ° C. A method for manufacturing hot-rolled steel sheets with high workability. 3. The steel slab composition according to claim 1, wherein the composition of the steel slab further contains at least one selected from the group consisting of P: 0.01 to 0.2 mass% and Al: 0.01 to 0.3 mass%. Of high strength and high workability hot rolled steel sheet with excellent impact resistance. 4. The steel slab composition according to claim 1, wherein the steel slab composition further comprises at least one selected from Ti: 0.005 to 0.25 mass% and Nb: 0.003 to 0.1 mass%. A method for manufacturing a high-strength, high-workability hot-rolled steel sheet with excellent impact resistance.