JP2015145518A - High strength hot rolled steel sheet and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength hot rolled steel sheet having both high-strength of 980 MPa or more and excellent ductility and to provide a method for producing the same.SOLUTION: There is provided a high strength hot rolled steel sheet which has a component composition containing 0.15 to 0.30 mass% of C, 0.5 to 2.0 mass% of Si, 1.8 to 3.5 mass% of Mn, 0.080 mass% or less of P, 0.03 mass% or less of S, 0.01 to 2.0 mass% of Al, 0.005 mass% or less of N and the balance of Fe and inevitable impurities and contains 8% or more and 20% or less by volume fraction of a ferrite phase and has a metal structure consisting of a martensite phase, a bainite phase and a retained austenite phase and a product (TS×El) of the tensile strength (TS) and the whole elongation (EL) of 28000 MPa% or more.

Description

  The present invention relates to a high-strength hot-rolled steel sheet suitable for use in automobile steel sheets and the like that require high strength and excellent workability, and has a tensile strength of 980 MPa or more and excellent ductility, and a method for producing the same.

  In recent years, in order to reduce carbon dioxide emissions from the viewpoint of protecting the global environment, there has been a strong demand for improving the fuel efficiency of automobiles. In addition, from the viewpoint of ensuring the safety of passengers in the event of a collision, there is a strong demand for improving safety centered on the collision characteristics of the automobile body. In order to satisfy the demands for improving the fuel efficiency and safety of automobiles at the same time, it is effective to increase the strength of the body parts and reduce the plate thickness within a range that does not impair the rigidity. For this reason, the application of high-strength steel sheets to automobile bodies has been actively promoted.

  The weight reduction effect of automobile parts increases as the material steel plate used has higher strength. Therefore, in the automobile industry, there is a trend to use a higher strength steel plate as a steel plate for a vehicle body. For example, for a skeletal structure, the tensile strength (hereinafter sometimes referred to as TS) is 780 MPa or more, and High-strength steel plates of 980 MPa or higher have been used.

  On the other hand, since many automobile parts are formed by press working, the steel plate used as a raw material is required to have not only high strength but also excellent press formability. Generally, since the ductility of a steel sheet decreases as the strength increases, the high-strength steel sheet tends to crack due to insufficient ductility during pressing.

  One of the technologies to deal with this problem is the low alloy TRIP that uses the retained austenite phase to cause transformation induced plasticity (TRIP) (hereinafter sometimes referred to as TRIP) to improve ductility. Steel plates have been developed. This steel sheet achieves high strength and high ductility by the martensitic transformation of the retained austenite phase due to strain accumulated during processing.

  For example, Patent Document 1 includes a cold-rolled steel sheet containing C: 0.07 to 0.12%, Si: 0.5 to 2.0%, and Mn: 1.0 to 2.5% by mass. Disclosed is a technology for obtaining a high-strength steel sheet excellent in formability comprising a structure containing a retained austenite phase having a volume fraction of 3% or more, a ferrite phase, and a bainite phase by annealing in a two-phase coexisting temperature range of 750 to 900 ° C. Has been.

  In Patent Document 2, C is 0.08% or less, Si is 0.5% or less, Mn: 0.5 to 3.0%, P: 0.01% or more, and Al: 0.00% by mass. A hot rolled steel sheet containing 3% or less is treated in an atmosphere containing ammonia at a temperature range of 550 to 800 ° C., N: 0.03 to 2.0% is contained, and the volume ratio of the retained austenite phase is 3 A technique for obtaining a high-strength steel sheet having excellent workability of ˜20% is disclosed.

  Patent Document 3 discloses a TRIP steel sheet using Al. This technique provides a cold-rolled steel sheet and a hot-rolled steel sheet having a fine grain structure and excellent workability while having high strength. Specifically, C: 0.06-0 in mass%. .25 mass%, Si: 0.01 to 2.0 mass%, Mn: 0.5 to 2.0 mass%, Al: 0.01 to 2.0 mass%, and Ti: 0.20 mass% or less, Nb: Containing one or two of 0.10 mass% or less, containing a predetermined amount of (Si + Al) and (Ti + Nb), and containing 70% or more of the ferrite phase and 3% or more of the retained austenite phase by volume% The balance has a steel structure composed of a bainite phase and a martensite phase.

JP 2001-329340 A JP 2002-012948 A JP 2011-149066 A

  However, the relationship between strength and elongation in the steel sheets disclosed in Patent Documents 1 to 3 is, for example, that the steel sheets of Patent Documents 1 and 2 have a tensile strength of 590 to 690 MPa and a total elongation of 31 to 39%. The steel sheet of Reference 3 has a tensile strength (TS) of 676 to 898 MPa and a balance between strength and ductility (TS × El) of only 22,000 to 27,800 MPa ·%, and the effect of reducing the thickness by increasing the strength is achieved. There is a demand for the development of a steel sheet that does not have a sufficient balance between strength and ductility (hereinafter, sometimes referred to as a product of strength and ductility or TS × El), and is excellent in total elongation (El). Furthermore, it is desired from the viewpoint of cost to obtain a highly workable steel sheet by hot rolling without going through a cold rolling process.

  The present invention has been made in view of the above-mentioned problems of the prior art, and the object thereof is not to require a complicated structure or a complicated manufacturing method, and has a high strength of 980 MPa or more and excellent. Another object is to propose a high-strength hot-rolled steel sheet having both ductility and a method for producing the same.

  The inventors have intensively studied to solve the above problems. As a result, by performing finish rolling at a temperature corresponding to the transformation point determined by the component composition of the steel, at a normal rolling temperature, the hard phase has a high abundance ratio. And it discovered that the hot-rolled steel plate excellent in the balance of intensity | strength and ductility was obtained, and came to complete this invention.

The gist of the present invention based on the above findings is as follows.
(1) C: 0.15-0.30 mass%, Si: 0.5-2.0 mass%, Mn: 1.8-3.5 mass%, P: 0.080 mass% or less, S: 0.03 mass% Hereinafter, Al: 0.01 to 2.0 mass% and N: 0.005 mass% or less are contained, the remainder has a component composition consisting of Fe and inevitable impurities, and the volume fraction is 8% or more and 20% or less. It includes a ferrite phase, and further has a metal structure composed of a martensite phase, a bainite phase, and a retained austenite phase, and the product of tensile strength (TS) and total elongation (El) (TS × El) is 28,000 MPa · % High-strength hot-rolled steel sheet.
(2) In addition to the above component composition, the composition further contains one or two selected from Nb: 0.4 mass% or less and Ti: 0.4 mass% or less. High strength hot rolled steel sheet.
(3) above (1) or (2) to a process for producing high strength hot rolled steel sheet according the steel material, finish rolling temperature (A ₃ point -X-85) ° C. or higher (A ₃ points -X + 85) ° C. or less, hot rolled at a total rolling reduction of 25% or more in the final three passes, and retained for 100 seconds or more in a temperature range of 350 ° C. to 450 ° C. .
However, X = 300-135 [Al], ([Al] indicates the Al content (mass%).)
In the present invention, the high strength hot rolled steel sheet is a hot rolled steel sheet having a tensile strength (TS) of 980 MPa or more.

  According to the present invention, a steel plate having high strength and high ductility can be provided without going through a cold rolling-annealing step. Therefore, when the steel plate of the present invention is applied to an automobile body, it becomes possible to apply a high-strength material to a part that has been difficult to press-form conventionally. It becomes possible to improve safety at low cost.

It is explanatory drawing which showed the relationship between the product of intensity | strength and ductility (TSxEl), and finish rolling completion temperature. It is a microstructure when the finish rolling end temperature is changed.

First, the basic technical idea of the present invention will be described.
The present invention uses a steel with an optimized component composition as will be described later, and the final rolling finish temperature is (A ₃ point−X−85) ° C. or higher and (A3 point−X + 85) ° C. or lower, and the final rolling of the last three passes. Hot rolling at a rate of 25% or more, and then retaining it in a temperature range of 350 ° C. to 450 ° C. for 100 seconds or more, so that a soft ferrite phase and a residual austenite phase (hereinafter referred to as “residual γ phase”) A metal structure composed of a hard second phase (a martensite phase, a bainite phase, and a retained austenite phase) is also formed. By having such a metal structure, a high-strength hot-rolled steel sheet having a balance of strength and ductility (TS × El) superior to that of conventional steel can be obtained. Where X is a function of the amount of Al added,
X = 300−135 [Al]. However, [Al] indicates the Al content (mass%).

A ₃ point is an equilibrium transformation point in the austenite-ferrite transformation, and can be calculated by thermodynamic calculation software such as thermo-Calc. In actual rolling, the austenite phase is transformed to the ferrite phase at a temperature lower than the equilibrium transformation point. Since the austenite phase before transformation is stabilized by processing, rolling in the austenite phase region is effective for obtaining a stable retained austenite phase. In addition, by performing rolling in the austenite phase region at a lower temperature, even in steel types that are usually difficult to transform into the ferrite phase and the martensite phase fraction becomes extremely high, the ferrite phase fraction is increased and the product of strength and ductility is increased. It is thought that this relationship can be improved.

FIG. 1 shows a steel slab containing C: 0.24 mass%, Mn: 2.30 mass%, Si: 1.51%, Al: 0.02 mass% (steel A in Examples described later), and C: 0. Steel slab containing .24 mass%, Mn: 2.25 mass%, Si: 0.50, Al: 1.02 mass%, hot rolling by changing the finish rolling finishing temperature The sample was retained for 180 seconds in a temperature range of 350 ° C. to 450 ° C. and wound up. FIG. 1 shows the influence of the finish rolling temperature on the product (TS × El) of the strength and ductility of the hot-rolled sheet. Note that the rolling rate of the final final three passes was 35%, and the finished plate thickness was 3.5 mm. FIG. 2 shows the microstructure when the finishing rolling finish temperature is changed. The equilibrium transformation temperature A ₃ is 808 ° C. for steel A and 941 ° C. for steel C.

From FIG. 1, in rolling at a finish rolling finishing temperature of 805 ° C., steel A has only a hard phase, TS is as low as 24,582 MPa, and the product of strength and ductility is comparable to that of conventional steel (see the examples described later). No. 1 in Table 2). When the finish rolling end temperature is lowered, the product of strength and ductility is improved, and when the finish rolling end temperature is 502 ° C., the product of strength and ductility is as high as 30,058 MPa ·% (described later). No. 6 in Table 2 of Examples). The finish rolling finish temperature is within the range of (A ₃ point -X-85) ° C. or higher and (A ₃ point -X + 85) ° C. or lower, and the product of strength and ductility has a value of 28,000 MPa ·% or higher. I understand.

  On the other hand, when the finish rolling finish temperature is 800 ° C., the product of strength and ductility is as high as 30,870 MPa ·% for Steel C (No. 11 in Table 2 of Examples described later). . When the finish rolling end temperature decreases, the product of strength and ductility decreases, and when the finish rolling end temperature is 512 ° C., the product of strength and ductility is a low value of 26,581 MPa ·% (in the examples described later). No. 16 in Table 2). In the range where the finish rolling finish temperature is (A3 point−X−85) ° C. or more and (A3 point−X + 85) ° C. or less, the product of strength and ductility is 28,000 MPa ·% or more.

  FIG. 2 shows an example of the microstructure when the finishing rolling finishing temperature is changed in Steel A and Steel C. In steel A, when the finish rolling finish temperature is 805 ° C., there are many hard phases, and the product of strength and ductility is low as described above. Further, in steel C, many ferrite phases that have been stretched and processed are observed under the finish rolling conditions at a finish rolling end temperature of 512 ° C. and a low temperature side, and as described above, the product of strength and ductility is low.

  Steel C has a finish rolling end temperature of around 800 ° C. and a very high product of strength and ductility. In the vicinity of 800 ° C., the temperature is about 140 ° C. lower than the equilibrium transformation point. On the other hand, steel A has a finish rolling finish temperature of around 500 ° C., and a very high product of strength and ductility is obtained. In the vicinity of 500 ° C., the temperature is about 300 ° C. lower than the equilibrium transformation point. From these results, the finish rolling finish temperature at which the product of strength and ductility is very high varies depending on the Al content, and the finish rolling finish temperature and the equilibrium transformation point are higher for steels with lower Al content. It can be seen that the difference is increasing. This is presumably because Al is very effective in stabilizing the ferrite phase. It can be seen that it is necessary to determine an appropriate finish rolling end temperature depending on the amount of Al added.

As described above, the strength and ductility of the steel material having the component composition described later within the scope of the present invention is performed at a finish rolling end temperature of (A ₃ point−X−85) ° C. or higher and (A ₃ point−X + 85) ° C. or lower. It can be seen that it is possible to produce a hot-rolled steel sheet having an excellent product.

  As described above, the present invention is based on completely new knowledge that has not existed in the past.

  Next, the component composition that the high-strength steel sheet of the present invention should have will be described.

C: 0.15-0.30 mass%
C is an element necessary for distributing to the γ phase, lowering the Ms point, and forming a retained austenite phase. To obtain such an effect, addition of 0.15 mass% or more is necessary. However, if C exceeds 0.30 mass%, the strength of the welded portion is increased and cracking is likely to occur, such being undesirable. Therefore, C is in the range of 0.15 to 0.30 mass%. Preferably it is the range of 0.18-0.25 mass%.

Si: 0.5-2.0 mass%
Si is an excellent strengthening element that is excellent in the solid solution strengthening ability of steel and has a small decrease in ductility. Moreover, it is an element which accelerates | stimulates formation of a retained austenite phase like Al mentioned later. Furthermore, Si suppresses the formation of carbides in the cooling process after finish rolling and promotes the concentration of carbon in the austenite phase, thereby making it easy to form residual γ. In order to obtain these effects, Si is added by 0.5 mass% or more. However, if Si is added in excess of 2.0 mass%, the hot-rolled sheet is hardened and the cold rollability is lowered, and the surface quality such as chemical conversion treatment is deteriorated. Therefore, the Si content is set to 2.0 mass% or less. Preferably it is the range of 0.5-1.5 mass%.

Mn: 1.8 to 3.5 mass%
Mn not only contributes to increasing the strength of steel, but also has the effect of stabilizing the austenite phase. Therefore, Mn needs to be added by 1.8 mass% or more. However, if it exceeds 3.5 mass%, even if rolling is performed in accordance with the transformation point in the low temperature range of the austenite phase, a sufficient ferrite phase cannot be obtained, and the product of strength and ductility decreases. Therefore, Mn is in the range of 1.8 to 3.5 mass%. Preferably it is the range of 1.8-2.8 mass%.

P: 0.080 mass% or less P is an element that is excellent in solid solution strengthening ability and has a small decrease in elongation. However, addition exceeding 0.080 mass% is not preferable because it lowers the strength of the welded joint and inhibits alloying during hot dip galvanization. Therefore, P is set to 0.080 mass% or less. Preferably it is 0.030 mass% or less. More preferably, it is 0.015 mass% or less.

S: 0.03 mass% or less S is a harmful element that causes a decrease in hot ductility and a decrease in surface quality, and it is desirable to reduce it as much as possible. Therefore, S is limited to 0.03 mass% or less. Preferably it is 0.01 mass% or less. More preferably, it is 0.003 mass% or less.

Al: 0.01-2.0 mass%
Al, like Si, is an element that forms a residual austenite phase and at the same time is an element necessary as a deoxidizer. In order to obtain the latter effect, addition of 0.01 mass% or more is necessary. However, if the addition exceeds 2.0 mass%, the transformation temperature becomes high and rolling in the low temperature region of the austenite phase becomes difficult. Furthermore, surface defects due to inclusions are caused, and alloy costs are increased. Therefore, Al is added in the range of 0.01 to 2.0 mass%. Preferably it is the range of 0.02-1.0 mass%.

N: 0.005 mass% or less N is a harmful element that causes aging of steel, and it is desirable to reduce it as much as possible. However, 0.005 mass% or less is acceptable. Preferably it is 0.003 mass% or less.

  Components other than the above are Fe and inevitable impurities. However, the following elements can be appropriately added as necessary within the range not impairing the effects of the present invention.

  The high-strength steel sheet of the present invention may further contain one or two selected from Nb: 0.4 mass% or less and Ti: 0.4 mass% or less in addition to the above component composition.

  Nb and Ti both combine with C and N to form fine precipitates and contribute to precipitation strengthening. In addition, it has an effect of refining crystal grains and an effect of preventing coarsening of crystal grains during annealing. In order to acquire the said effect, it is preferable to contain Nb and Ti individually or in total 0.01 mass% or more. However, when each element is added in excess of 0.4 mass%, not only the ductility is lowered, but the alloy cost may be increased. Therefore, it is preferable to add Nb and Ti in a range of 0.4 mass% or less.

  Next, the steel structure and strength characteristics of the high-strength steel sheet of the present invention will be described.

In order to have a product of high strength and ductility, the steel sheet of the present invention has a ferrite phase of 8% or more and 20% or less in volume fraction in addition to satisfying the above component composition. is necessary. When the amount of C or Mn increases, when rolling under normal rolling conditions, the fraction of the hard phase becomes high, the fraction of the soft ferrite phase becomes less than 8%, and the product of strength and ductility decreases. . If it exceeds 20%, the product of high strength and ductility intended in the present invention cannot be obtained.
The remaining structure other than the ferrite phase described above is composed of a martensite phase, a bainite phase, and a retained austenite phase. As a balance other than the above-mentioned phase, a pearlite phase inevitably generated may be recognized. If the total of such unavoidably generated phases is less than 3% in volume fraction, The effect of the invention is not affected.

  The volume fraction of the ferrite phase is, for example, a point count method or an image obtained by polishing a steel sheet cross section, corroding with a nital solution, etc. to reveal a steel structure, and then taking a structure photograph taken with an optical microscope or the like. It can be obtained by analysis processing or the like.

  Further, the volume fraction of the austenite phase can be obtained by an X-ray diffraction method, a saturation magnetization method or the like.

  The high-strength steel sheet of the present invention having the above component composition and steel structure has a tensile strength TS of 980 MPa or more and is defined by the product of the tensile strength TS and the total elongation El (the product of strength and ductility). It has excellent properties with a strength-ductility balance of 28,000 MPa ·% or more.

  Next, the manufacturing method of the high strength steel plate of this invention is demonstrated.

The high-strength steel sheet of the present invention is a steel slab (steel material) adjusted to a composition suitable for the present invention described above, and a hot rolling process in which the steel slab is hot-rolled to form a hot-rolled sheet. Can be manufactured. In particular, the desired steel structure and mechanical properties can be imparted by performing the finish rolling process under the conditions specified in the present invention. As needed, you may pass through the cold rolling process which rolls an above-described hot-rolled sheet to make the cold-rolled sheet of final thickness, and the process of recrystallizing the cold-rolled sheet as needed.
<Steel slab (steel material)>
The steel slab used in the production of the high-strength steel sheet of the present invention is produced by melting the steel having the above composition using a converter or the like and then producing it by a continuous casting method or an ingot-bundling method. From the viewpoint of preventing this, it is preferable to use a continuous casting method. Further, a thin slab casting method may be used.
<Hot rolling process>
When hot rolling the steel slab, it is necessary to bring the steel slab to a predetermined temperature in order to ensure the finish temperature of the hot finish rolling. In addition to the conventional method in which the steel slab is once cooled to room temperature and then reheated, the steel slab is charged into a heating furnace with a warm piece without being cooled to room temperature, and hot rolled after mild reheating. An energy saving process such as a direct rolling method in which a steel slab is formed by a single charging method, continuous casting, or the like, and then immediately hot-rolled in a hot piece state may be used.

  In addition, it is preferable to make the heating temperature in the case of reheating a steel slab into the range of 1100-1300 degreeC. If it is less than 1100 degreeC, the rolling load in hot rolling will increase and there exists a possibility of causing a rolling trouble. On the other hand, if it exceeds 1300 ° C., it causes an increase in heating cost and an increase in scale loss, which is not preferable.

  The steel slab reheated to the above temperature or the steel slab immediately after continuous casting is then roughly rolled into a sheet bar and then finish-rolled to obtain a hot-rolled sheet having a predetermined thickness.

  The conditions for the rough rolling are not particularly limited, and may be performed according to a conventional method. Moreover, you may apply the sheet bar heater which can heat a sheet bar by induction heating etc. from a viewpoint of making the reheating temperature of a slab low, or preventing the trouble at the time of hot rolling.

The finish rolling needs to be performed at a finish rolling finishing temperature of (A ₃ point−X−85) ° C. or higher and (A ₃ point−X + 85) ° C. or lower and the total rolling reduction of the final three passes is 25% or higher. here,
X = 300-135 [Al] (However, [Al] indicates the Al content (mass%).)
It is.

As described in FIG. 1, the product of strength and ductility is improved by performing finish rolling in a temperature range corresponding to the transformation point (A ₃ ) determined by the components. It has been found that Al particularly has an effect of stabilizing the ferrite phase, and it is necessary to determine an appropriate finish rolling end temperature depending on the amount of Al added. That is, using X, which is a function of the amount of Al that satisfies X = 300−135 [Al], when rolling below (A ₃ point−X−85) ° C., ferrite transformation occurs before rolling, and the steel expands and becomes ductile. A poor ferrite phase is formed, and the strength ductility balance is lowered. On the other hand, if it exceeds (A ₃ points -X + 85) ° C., the ability to generate a ferrite phase due to rolling strain becomes insufficient, the martensite phase fraction increases, and the product of strength and ductility decreases with hardening. In this case, the amount of rolling strain is also important. In particular, if the total rolling reduction of the final three passes is less than 25%, the amount of accumulated rolling strain is small, and the ability to generate a ferrite phase becomes insufficient. The rate increases and the product of strength and ductility decreases with hardening. Therefore, the total reduction ratio of the final three passes is 25% or more. A more preferable range of the total rolling reduction of the last three passes is 30% or more. The upper limit of the total rolling reduction of the final three passes is not particularly defined, but is 50% or less due to the rolling load restriction of the rolling mill.

  In addition, since the said finish rolling makes the shape of a steel plate uniform and the material homogenize, it is good also considering a part or all pass as lubrication rolling. The coefficient of friction during lubrication rolling is preferably in the range of 0.10 to 0.25.

  In addition, the hot rolling may be performed by applying a continuous rolling process in which the preceding sheet bar and the succeeding sheet bar are joined and finish-rolled continuously from the viewpoint of improvement in material uniformity and operational stability. Good.

  Hot-rolled hot-rolled sheet is wound into a coil shape, but it retains the temperature range of 350-450 ° C for 100 seconds or longer during cooling of this steel sheet, and stabilizes the austenite phase, resulting in high strength and ductility. It is possible to sufficiently secure a retained austenite phase (excellent in balance of strength and ductility). The winding temperature (CT) is not particularly limited as long as the above retention can be ensured.

  Thereafter, the hot-rolled sheet is appropriately pickled and descaled as necessary to obtain a final product sheet.

  In addition, when performing the said annealing process or plating process, you may perform temper rolling or a leveler process for the purpose of shape correction, surface roughness adjustment, mechanical property improvement, etc. after those processes. In this case, the elongation in temper rolling and leveler processing is preferably in the range of 0.2 to 3.0% in total. If the content is less than 0.2%, the intended purpose such as shape correction cannot be achieved. On the other hand, if the content exceeds 3.0%, a significant reduction in ductility is caused.

  The steel having the composition shown in Table 1 is melted to form a steel slab, and then the steel slab is heated to 1250 ° C. and subjected to rough rolling. The finish rolling finish temperature shown in Table 2 and the final three passes are summed up. The steel sheet was finish-rolled at a reduction ratio of 350 ° C. and cooled at a residence time of 350 ° C. to 450 ° C. as shown in Table 2 to obtain a hot-rolled coil and a hot-rolled sheet having a thickness of 2.6 mm.

  The hot-rolled sheet thus obtained was examined for steel sheet structure and tensile properties in the following manner.

<Steel structure>
Ferrite phase: A test piece is taken from the above hot rolled sheet, a plate thickness section (L section) parallel to the rolling direction is polished, corroded with a nital solution, and a steel structure is revealed. The steel structure was imaged at 400 times, image processing was performed to determine the area ratio of the white portion, and the area ratio was defined as the volume fraction of the ferrite phase.

<Tensile properties>
A JIS No. 5 test piece with the direction perpendicular to the rolling direction (C direction) as the tensile direction was taken from the hot rolled sheet, and subjected to a tensile test at a crosshead speed of 10 mm / min in accordance with JIS Z2241, The thickness (TS) and the total elongation (El) were measured, and the strength-ductility balance represented by the product of strength and ductility (TS × El) was determined. The measurement results are also shown in Table 2.

  From Tables 1 and 2, the steel sheets of the inventive examples that conform to the present invention have excellent tensile strength TS of 980 MPa or more and a product of strength and ductility (TS × El) of 28,000 MPa ·% or more. It can be seen that it has a balance of ductility.

  On the other hand, the steel plate of the comparative example manufactured under conditions outside the scope of the present invention has a tensile strength TS of 980 MPa or more, but the total elongation El is extremely low, and as a result, the product of strength and ductility (TS X El) is less than 28,000 MPa ·%.

  The steel sheet of the present invention has an excellent strength-ductility balance of TS of 980 MPa or more and a product of strength and ductility (TS × El) of 28,000 MPa ·% or more. It can also be used as a pipe material that requires similar characteristics.

Claims (3)

C: 0.15-0.30 mass%, Si: 0.5-2.0 mass%, Mn: 1.8-3.5 mass%, P: 0.080 mass% or less, S: 0.03 mass% or less, Al : 0.01 to 2.0 mass%, N: 0.005 mass% or less, with the balance being composed of Fe and inevitable impurities,
Including a ferrite phase having a volume fraction of 8% or more and 20% or less, and further having a metal structure composed of a martensite phase, a bainite phase, and a retained austenite phase,
A high-strength hot-rolled steel sheet having a product (TS × El) of tensile strength (TS) and total elongation (El) of 28,000 MPa ·% or more.   2. The high strength according to claim 1, further comprising one or two selected from Nb: 0.4 mass% or less and Ti: 0.4 mass% or less in addition to the component composition. Hot rolled steel sheet. A method for producing a high-strength hot-rolled steel sheet according to claim 1 or 2,
The steel material is hot-rolled at a finish rolling finish temperature of (A ₃ points -X-85) ° C. or higher (A ₃ points -X + 85) ° C. or lower and the total rolling reduction of the final three passes is 25% or higher
A method for producing a high-strength hot-rolled steel sheet, which is retained for 100 seconds or more in a temperature range of 350 ° C to 450 ° C.
However, X = 300-135 [Al], ([Al] indicates the Al content (mass%).)