JP2007002276A - High strength steel sheet and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength steel sheet having excellent workability and material stability. <P>SOLUTION: The high strength steel sheet has a steel composition consisting of 0.06 to 0.18% C, 0.005 to 0.5% Si, 2.0 to 3.0% Mn, ≤0.02% P, ≤0.01% S, 0.01 to 0.1% Al, ≤0.01% N, further 0.05%, in total, of either or both of ≤0.2% Ti and ≤0.2% Nb and the balance Fe with impurities and containing, if necessary, one or more kinds selected from the group consisting of ≤1.0% Cr, ≤1.0% Mo and ≤0.003% B and also has a steel structure in which the average grain size of ferrite and the volume fraction of the ferrite are 1 to 3.5μm and ≥40%, respectively, and the product of the volume fraction of the ferrite and the nano-hardness of the ferrite is ≥180%×GPa and the tensile strength is ≥780 MPa. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a high-strength steel plate and a method for producing the same, and for example, to a high-strength steel plate suitable for applications that are used after being pressed or bent, such as a steel plate for automobile bodies.

  In order to reduce the weight of the vehicle body and improve the impact characteristics, the application of high strength steel sheets having a tensile strength of 780 MPa or more to automobile parts is progressing. In general, when the strength is increased, ductility is reduced and workability is deteriorated, and mechanical characteristics of the steel plate are largely changed, so that the accuracy of a part after press forming is lowered. Therefore, there is a demand for a high-strength steel sheet that not only has high strength but also excellent workability and part accuracy after press forming. Further, considering the expansion of application of high-strength steel sheets to automobile parts, it is also important to improve the corrosion resistance.

  Conventionally, as a high-strength steel sheet having excellent workability, a composite structure steel sheet having ferrite as a main phase and a low-temperature transformation phase such as martensite or bainite as a hard second phase has been proposed. For example, in Patent Document 1, C: 0.08 to 0.20% (in this specification, “%” means “mass%” unless otherwise specified), Mn: 1.5 to 3.5% Al: 0.010 to 0.1%, P: 0.010% or less, S: 0.001% or less, Ti: 0.010 to 0.1% and Nb: 0.010 to 0.1% It has a steel composition composed of one or more types, the balance Fe and inevitable impurities, and a composite structure mainly composed of ferrite, and has a tensile strength of 784 MPa or more and a yield ratio of 60% or less, and a balance between tensile strength and elongation. A hot-dip galvanized steel sheet having excellent workability (TS × El) of 17000 to 25000 MPa ·% is disclosed.

  However, a high-strength steel sheet using a hard low-temperature transformation phase is liable to form a crack at the interface between the ferrite and the hard phase, and the bendability and stretch flangeability are not sufficient. In addition, since the strength is secured by the hard phase, when this high-strength steel sheet is produced on a continuous hot-dip galvanizing line with a low cooling rate, the mechanical properties fluctuate significantly due to changes in cooling conditions and alloying conditions after soaking. There is a risk of it.

In order to stabilize the mechanical properties of a steel plate that secures strength by the hard phase, the hardenability of the steel should be increased and the hardness and volume ratio of the hard phase generated after continuous annealing should not be changed even if the continuous annealing conditions change. is important. In view of this, a high-strength steel sheet having improved material stability by increasing the amounts of C and Mn in the steel to enhance hardenability has been proposed. For example, Patent Document 2 discloses that a continuous hot-dip galvanizing line has a soaking temperature, a cooling rate after soaking, and a mechanical strength is small even when the alloying treatment time fluctuates, and a tensile strength excellent in material stability is 784 MPa. A hot-dip galvanized steel sheet having the above is disclosed. In this steel sheet, the C content is increased to 0.13 to 0.3%, the Mn content is increased to 2.5 to 4%, and the soaking temperature in the continuous hot dip galvanizing process is set to Ac _{3 to} 900 ° C. Excellent material stability against changes in manufacturing conditions in the continuous hot dip galvanizing process. However, this hot dip galvanized steel sheet has sufficient strength due to the hard phase and has a high amount of C, so that the weldability is not sufficient. In addition, when the amount of Mn in steel increases, alloying treatment becomes difficult, and the alloying treatment must be performed at a high temperature. However, when the alloying treatment temperature is increased, the hard phase is tempered during the alloying treatment, and the machine Characteristics can vary significantly. However, in this hot dip galvanized steel sheet, no means for suppressing fluctuations in mechanical properties due to an increase in the alloying temperature is taken into consideration, and the material stability is insufficient.

  Therefore, in order to increase the material stability while increasing the strength of the steel sheet, the use of hard phases such as martensite and bainite is suppressed as much as possible, and by the dispersion of fine precipitates such as TiC and NbC and the refinement of crystal grains, We must strengthen the base ferrite.

Patent Document 3 contains C: less than 0.10%, Ti: 0.03-0.10%, Mo: 0.05-0.6%, and fine precipitates having a particle size of less than 10 nm are dispersed. A thin steel sheet having a single-phase ferrite structure and a tensile strength of 550 MPa or more is disclosed. By optimizing the hot rolling conditions and the continuous hot dip galvanizing line soaking conditions, this thin steel sheet secures a tensile strength of 780 MPa or more and exhibits excellent workability with TS × El of 16000 MPa ·% or more. However, this thin steel plate is a hot-rolled steel plate, and a cold rolling process for improving both surface roughness and thickness accuracy is not considered. For this reason, when cold rolling is further performed on this thin steel plate, the recrystallization temperature rises due to containing a large amount of carbonitride-forming elements such as Ti and Nb, and accompanying this, Ac ₃ points or more High-temperature annealing will be performed, and the coarsening of the precipitate will proceed, so that the desired high strength cannot be achieved.

  In Patent Document 4, C: 0.07 to 0.25%, Mn: 1.5 to 2.5%, Nb: 0.10% or less, Ti: 0.3% or less, Si: 0.1% Hereinafter, Cr: 0.1% or less, P: 0.05% or less, Sol. Al: 0.010-0.100%, S: 0.01% or less, N: 0.01% or less, having a steel composition consisting of the balance Fe and inevitable impurities, and having a tensile strength of 441 MPa or more There is disclosed a non-composite high strength high yield ratio hot dip galvanized steel sheet having a yield ratio of 80% or more. This steel sheet exhibits high strength with a tensile strength of 700 MPa or more and a yield ratio of 70% or more by adding Ti and Nb, which are carbonitride-forming elements, and forming a two-phase structure of ferrite and austenite phases during continuous annealing. . However, if the steel to which Ti and Nb are added is annealed at a temperature at which it has a two-phase structure, it becomes a band structure and the mechanical properties deteriorate.

In Patent Document 5, C: 0.03-0.16%, Si: 0.2-2.0%, Mn: 1.0-3.0% and / or Ni: 0.5-3.0 %, Ti: 0.2% or less and / or Nb: 0.2% or less, Al: 0.01 to 0.1%, P: 0.1% or less, S: 0.02% or less, and N: 0 0.005% or less, and C, Si, Mn, Ni, Ti, and Nb are contained in a range that satisfies a specific relational expression, and the balance is a steel material having a composition of Fe and inevitable impurities at 1200 ° C or more. After heating, it is hot-rolled, then cold-rolled, then recrystallized and annealed at A ₃ points or more (A ₃ points + 30 ° C.), then pickled, and then A ₁ which becomes a two-phase structure of ferrite and austenite phase by 5-30 ByoHitoshinetsu above the point _{(a 1} point + 70 ° C.) below the temperature range, the ferrite Volume fraction of 70% or more, the average crystal grain size of the ferrite is excellent in the following ductility 3.5 [mu] m, galvanized steel sheet TS × El exhibit excellent workability and 20000 MPa ·% or more is disclosed. However, in order to produce this steel sheet, annealing is performed in two steps, which is not only problematic in terms of productivity and cost, but if soaking is performed in a temperature range where a two-phase structure is formed, fluctuations in the soaking temperature Mechanical properties change significantly, which is a problem in terms of material stability.

  In Patent Document 6, C: 0.04% to 0.25%, Si: 0.7% or less, Mn: 1.4 to 3.5%, Cr: 0.05 to 1%, P: 0.0. 0.5% or less, S: 0.01% or less, Nb: 0.005 to 0.1%, the balance being substantially composed of Fe, and the average grain size of ferrite and low-temperature transformation phase constituting the composite structure Is a high-strength hot-dip galvanized steel sheet excellent in HAZ softening characteristics. This steel sheet shows a structure in which ferrite and martensite having a particle diameter of about 3 μm are finely dispersed by precipitating fine NbC in a hot-rolled sheet. However, if TiC or NbC is deposited in the hot rolled sheet, it is difficult to cold-roll and not only productivity is lowered, but also sheet thickness accuracy is lowered. Moreover, since precipitates are coarsened in the processes after hot rolling, it is difficult to strengthen the ferrite.

  Thus, in order to ensure the tensile strength of 780 MPa or more even if cold rolling and subsequent annealing are performed, even if it is intended to enhance the strength of the steel sheet by strengthening the ferrite by dispersing fine precipitates or refining crystal grains. It is essential to use a hard phase such as martensite or bainite. In that case, in order to improve workability, the volume ratio and hardness of the ferrite and the hard phase must be controlled.

In Patent Document 7, C: 0.08 to 0.25%, Mn: 0.8 to 3.0%, S: 0.01% or less, Al: 0.01 to 0.1%, N: 0 .001 to 0.010%, the balance is Fe and inevitable impurities, the structure is a low-temperature transformation generation phase or a composite phase of this and ferrite, and the yield ratio is 0.7 or more, after shearing A high-strength steel sheet having excellent stretch flangeability is disclosed. This steel sheet has a structure in which the ratio [Hv (↑) / Hv (s)] of the hardness Hv (s) of the low temperature transformation phase to the hardness Hv (↑) of the ferrite is in the range of 0.3 to 0.6. By doing so, generation of voids can be suppressed, and excellent local ductility is exhibited.
JP-A-4-236671 Japanese Patent Laid-Open No. 5-179402 JP 2002-322539 A JP-A-10-273754 JP 2004-204341 A JP 2002-256386 A JP-A-9-67645

However, Patent Document 7 does not disclose any measures for improving material stability while ensuring excellent local ductility.
An object of the present invention is to provide a high-strength steel sheet having a tensile strength of 780 MPa or more and a method for producing the same. More specifically, an object of the present invention is to provide a high-strength steel sheet excellent in workability and material stability and a method for producing the same. For example, the target value of workability is TS × EL of 10,000 MPa ·% or more. The minimum bending radius is 1.0t or less, and the target value of material stability is that the absolute value of the difference in tensile strength between the central part in the plate width direction and the (1/4) part in the plate width direction is within 10% of the tensile strength. It is preferably within 5%.

  In order to provide a steel plate having the above characteristics, the present inventors have repeatedly studied from the viewpoints of steel composition, steel structure, and production conditions. As a result, by optimizing both the composition of the steel material and the hot rolling conditions, the precipitation of Ti or Nb in the hot-rolled sheet is suppressed, and fine carbides are precipitated in the processes after the cold rolling to produce ferrite. As a result, the average grain size of ferrite is 1 to 3.5 μm and the volume fraction of ferrite is 40% or more, and the product of the volume fraction of ferrite and the nano hardness of ferrite is 180%. It is possible to obtain a high-strength cold-rolled steel sheet having excellent properties that have not been heretofore, such as a GPa or higher and a tensile strength of 780 MPa or higher, thereby reducing the workability and the strength level. As a result of finding out that a high-strength steel sheet excellent in material stability can be provided, the present invention was completed through further studies.

  In the present invention, the average crystal grain size of ferrite is 1 to 3.5 μm, the volume fraction of ferrite is 40% or more, and the product of the volume fraction of ferrite and the nano hardness of ferrite is 180% · GPa or more. A high strength steel sheet having a steel structure having a tensile strength of 780 MPa or more.

  The high-strength steel sheet according to the present invention includes C: 0.06% to 0.18%, Si: 0.005% to 0.5%, Mn: 2.0% to 3.0%, P : 0.02% or less, S: 0.01% or less, Al: 0.01% or more and 0.1% or less, N: 0.01% or less, Ti: 0.20% or less, and Nb: 0.0. 1 type or 2 types of 20% or less in total, 0.05% or more, if necessary, from the group consisting of Cr: 1.0% or less, Mo: 1.0% or less, and B: 0.003% or less It is exemplified to have a steel composition composed of one or more selected, the balance Fe and impurities.

From another point of view, the present invention provides, for example, a method in which a steel having the above-described steel composition is continuously cast, directly hot-rolled, directly-rolled hot-rolled, or once cooled and then reheated to 1150 ° C. or higher and 1300 ° C. or lower. The steel sheet is rolled between the temperatures of 800 ° C. or higher and 950 ° C. or lower, and the finish rolling is completed within 30 seconds. A steel sheet in which the total amount of Nb is 40% or more of the total amount of Ti and the total amount of Nb in the steel is cold-rolled, and then the temperature of Ac is _{3 to} 900 ° C. by continuous annealing equipment. A method for producing a high-strength steel sheet, characterized by annealing from 650 ° C. to 550 ° C. at an average cooling rate of 5 ° C./second to 200 ° C./second after annealing at a range of 10 seconds to 300 seconds.

  According to the present invention, a high-strength cold-rolled steel sheet having a tensile strength of 780 MPa or more and excellent in workability and material stability can be provided. Therefore, the effect of contributing to weight reduction of automobile body parts and improvement in collision safety is It is remarkable.

  In addition, according to the present invention, since the hot-rolled steel sheet can be softened by suppressing precipitation of TiC and the like in the hot-rolled steel sheet that is a material of the high-strength cold-rolled steel sheet, the consumption of the rolling roll of the cold rolling mill is reduced. It is also possible to improve the productivity of this high-strength cold-rolled steel sheet.

Hereinafter, the best mode for carrying out the high-strength steel sheet and the manufacturing method thereof according to the present invention will be described in detail.
The high-strength hot-rolled steel sheet of the present embodiment has an average crystal grain size of ferrite of 1 μm or more and 3.5 μm or less, a volume ratio of ferrite of 40% or more, and the volume ratio of ferrite and the nano hardness of ferrite. The steel structure has a product of 180% · GPa or more and a tensile strength of 780 MPa or more. The high-strength steel sheet of the present embodiment is based on the technical idea that it has excellent workability and material stability directly by having such a structural feature.

  In the following description, as described above, as a specific mode for securing the characteristics in the structure, as described above, an appropriate amount of alloying elements such as C, Mn, Ti, Nb and the like are included, as well as hot rolling conditions and annealing. Although the aspect which controls both conditions and cooling conditions after annealing is taken as an example, this is only one aspect for ensuring the above-mentioned feature to the last, and if this aspect can ensure this feature, this embodiment Even aspects other than the aspects disclosed in the embodiments are equally applicable. For this reason, the technical idea of the present invention described above is not unduly limited to this aspect exemplified as the best mode.

First, the reasons for limiting the steel composition of the high-strength steel sheet according to the present embodiment will be described.
(C: 0.06% to 0.18%)
C is an element that contributes to improving the strength of the high-strength steel sheet, and is contained at least 0.06% in order to make the tensile strength 780 MPa or more. However, when the C content exceeds 0.18%, the weldability is remarkably deteriorated. For this reason, the amount of C is limited to 0.06% or more and 0.18% or less. From the same viewpoint, the lower limit of the C content is preferably 0.07%, and the upper limit is preferably 0.15%.

(Si: 0.005% to 0.5%)
Si is an element that contributes to improving the strength of the high-strength steel sheet, and is contained in an amount of 0.005% or more. However, when the Si content exceeds 0.5%, the wettability and chemical conversion treatment of the plating deteriorate. For this reason, the amount of Si is limited to 0.005% or more and 0.5% or less. From the same viewpoint, the upper limit of the Si content is desirably 0.1%.

(Mn: 2.0% to 3.0%)
Mn is an element that contributes to improving the strength of the high-strength steel sheet, and is contained at least 2.0% in order to ensure a tensile strength of 780 MPa or more. However, if the Mn content exceeds 3.0%, not only is it difficult to produce ferrite, but the structure becomes non-uniform and workability deteriorates. For this reason, the amount of Mn is limited to 2.0% or more and 3.0% or less. The lower limit of the Mn content is desirably 2.2% in order to ensure a stable tensile strength of 780 MPa or more, specifically, to ensure a tensile strength of 800 MPa or more. The upper limit of the Mn content is desirably 2.8% from the same viewpoint.

(P: 0.02% or less)
P is an unavoidable impurity in the high-strength steel sheet of the present embodiment, and if included excessively, the structure becomes non-uniform and deteriorates workability. For this reason, the P content is limited to 0.02% or less. From the same viewpoint, the upper limit of the P content is desirably 0.015%, and is desirably 0.005% or more in order to suppress an increase in manufacturing cost.

(S: 0.01% or less)
In the high-strength steel sheet of the present embodiment, S exists as a sulfide and becomes a stress concentration source, so that workability deteriorates. For this reason, it is desirable that the S content is as low as possible. However, if it is 0.01% or less, even a high-strength steel sheet with a tensile strength of 780 MPa or more targeted by this embodiment does not adversely affect bendability. For this reason, S content is limited to 0.01% or less. In addition, Preferably it is 0.005% or less.

(Sol.Al: 0.01% or more and 0.1% or less)
Al is an element added for deoxidation of steel, and effectively acts to improve the cleanliness of the steel. In order to remove silicate inclusions and improve processability, sol. Al content of 0.01% or more. However, sol. If the Al content exceeds 0.1%, the oxide inclusions increase, so the surface properties and workability deteriorate. For this reason, sol. The amount of Al is limited to 0.01% or more and 0.1% or less. From the same viewpoint, sol. The upper limit of the amount of Al is preferably 0.06%, and the lower limit is preferably 0.02%.

(N: 0.01% or less)
N is an unavoidable impurity in the high-strength steel sheet according to the present embodiment, and if it is excessively contained, coarse nitrides are precipitated and workability is deteriorated. For this reason, it is desirable to reduce the N content as much as possible. However, if it is 0.01% or less, even a high-strength material as intended in the present invention does not adversely affect the workability. For this reason, N content is limited to 0.01% or less. Note that the upper limit of the N content is preferably 0.005%, and more preferably 0.003%.

(Ti: 0.20% or less, Nb: 0.20% or less, Ti + Nb: 0.05% or more)
Ti and Nb are both important elements in the present invention, and contain one or two of them. By containing one or two of Ti and Nb, a carbide is formed, and ferrite is strengthened by precipitation strengthening and crystal grain refinement. Thereby, the tensile strength is 780 MPa or more without deteriorating material stability. High strength can be achieved. By adding a small amount of one or two of Ti and Nb, the yield strength, tensile strength or elongation is remarkably increased. Therefore, in order to reduce fluctuations in mechanical properties, the total content of Ti and Nb is 0.05% or more. However, if either Ti or Nb is contained in excess of 0.20%, precipitates in the steel are coarsened and workability deteriorates. For this reason, each content of Ti and Nb is limited to 0.20% or less. From the same viewpoint, the upper limit of the content of each of Ti and Nb is preferably 0.15%, and the lower limit is preferably 0.05%.

(Cr: 1.0% or less, Mo: 1.0% or less, B: 0.003% or less)
In the high-strength steel sheet of the present embodiment, Cr, Mo, and B are all optional addition elements that are added as necessary, and effectively act on the strengthening of ferrite. It may be contained in combination. However, if the Cr and Mo contents exceed 1.0%, and the B content exceeds 0.003%, ferrite is difficult to be generated and workability deteriorates. For this reason, the Cr and Mo contents are limited to 1.0% or less, and the B content is limited to 0.003% or less. In order to surely obtain the above effect, Cr and Mo are each preferably contained in an amount of 0.1% or more, and B is preferably contained in an amount of 0.0005% or more.

  The balance other than the components described above is Fe and inevitable impurities. As unavoidable impurities, O: 0.006% or less, Cu: 0.05% or less, Ni: 0.05% or less, and Ca: 0.0003% or less are allowed.

Next, the reason for limiting the steel structure of the high-strength steel plate according to the present embodiment will be described.
The high-strength steel sheet of the present embodiment having the steel composition described above has an average crystal grain size of ferrite of 1 μm or more and 3.5 μm or less, and a volume ratio of ferrite evaluated by an area ratio of 40% or more. The product of the volume ratio of ferrite and the nano hardness of ferrite is 180% · GPa or more, and further has a steel structure having a tensile strength of 780 MPa or more.

  If the average crystal grain size of the ferrite exceeds 3.5 μm, cracks are likely to occur at the interface between the ferrite and the hard phase, not only the workability is deteriorated but also the tensile strength is difficult to be increased to 780 MPa or more. Become. On the other hand, if the average crystal grain size of ferrite is less than 1 μm, uniform deformation becomes difficult and workability deteriorates. For this reason, the average crystal grain size of ferrite of the high-strength steel plate of this embodiment is limited to 1 μm or more and 3.5 μm or less. The upper limit of the average crystal grain size of ferrite is desirably 3.2 μm in order to ensure stable tensile strength of 780 MPa or more, specifically, to ensure tensile strength of 800 MPa or more. From the same viewpoint, the lower limit of the average crystal grain size of ferrite is desirably 1.2 μm.

  Further, if the volume fraction of ferrite is less than 40%, non-uniform deformation is promoted and not only the workability is deteriorated, but also the contribution of the hard phase relating to the strength of the steel sheet is increased, and the material stability is also deteriorated. If the ferrite volume fraction is 40% or more, a tensile strength of 780 MPa or more can be realized while suppressing deterioration of workability. For this reason, the volume fraction of ferrite of the high-strength steel plate of the present embodiment is limited to 40% or more. From the same viewpoint, the lower limit of the volume fraction of ferrite is desirably 50%.

  Furthermore, the greatest new feature on the structure of the high-strength steel sheet according to the present embodiment is that the product of the volume fraction of ferrite and the nano hardness of ferrite is 180% · GPa or more, which is higher than the range that does not exist before this application. This is the point.

  As is well known, the hardness of ferrite can be accurately measured by measuring the nano hardness by AFM nanoindentation. If the product of the volume fraction of ferrite and the nano hardness of the ferrite is less than 180% · GPa, and the tensile strength of this steel sheet is 780 MPa or more, the contribution of the hard phase becomes too large with respect to the strength of the steel sheet, and the material stability Deteriorates. By setting the product of the volume fraction of ferrite and the nano-hardness of ferrite to 180% · GPa or more, it becomes possible to achieve a tensile strength of 780 MPa or more while suppressing deterioration of material stability.

The high-strength steel plate of the present embodiment has the steel composition and steel structure described above. Next, the reason for limitation of the manufacturing method of the high strength steel plate of this Embodiment is demonstrated.
In the present embodiment, it is desirable that the molten steel having the steel composition described above is melted by a conventional method such as a converter or an electric furnace, and a steel material such as a slab is formed by a continuous casting method. Instead of the continuous casting method, a steel material may be used by an ingot casting method, a thin slab casting method, or the like.

  Hot rolling is performed by means commonly used for this steel material to obtain a hot rolled steel sheet. This hot rolling is a direct feed rolling in which the cast steel material is rolled after being charged in a heating furnace without being cooled to room temperature and then heated immediately, or directly after a slight heat retention. The rolling may be performed by either rolling the steel material once and then reheating it and then rolling it.

(Reheating temperature of steel material: 1150 ° C or higher and 1300 ° C or lower)
When steel material is cooled and then reheated before rolling, workability is not deteriorated, and ferrite produced after continuous annealing is effectively strengthened to achieve both material stability and tensile strength of 780 MPa or more. Therefore, it is effective to re-dissolve TiC or NbC during heating. For this reason, it is effective to heat the steel material to 1150 ° C. or higher. However, heating the steel material to over 1300 ° C. not only saturates the effect but also increases the scale loss. For this reason, it is desirable that the reheating temperature of the steel material be 1150 ° C. or higher and 1300 ° C. or lower.

(Finish rolling finish temperature: 800 ° C or higher and 950 ° C or lower)
In the present embodiment, hot rolling is performed on a steel material, but the finish rolling finish temperature of hot rolling is limited to 800 ° C. or more and 950 ° C. or less. When the finish rolling finish temperature is less than 800 ° C., not only the deformation resistance during rolling becomes large and the productivity is deteriorated, but also the structure becomes a non-uniform band structure and the workability after cooling annealing deteriorates. On the other hand, when the finish rolling finish temperature exceeds 950 ° C., during the subsequent cooling, most of Ti or Nb in the steel precipitates as carbides inside the hot-rolled steel sheet, making subsequent cold rolling difficult. In addition, the carbide coarsens during the continuous annealing, and it becomes difficult to improve the material stability after the cold rolling annealing and to ensure the tensile strength of 780 MPa or more, and the workability deteriorates.

For this reason, in this Embodiment, the finish rolling finish temperature of hot rolling is limited to 800 degreeC or more and 950 degrees C or less.
(Cooling conditions after finishing rolling: Winding starts within 30 seconds after finishing rolling)
In the present embodiment, winding is started within 30 seconds after finishing rolling. If it takes more than 30 seconds to start winding, most of Ti or Nb in the steel precipitates as carbides in the hot-rolled sheet, and subsequent cold rolling becomes difficult, and the carbides are being continuously annealed. Thus, it is difficult to increase the material stability after cold rolling annealing and to ensure the tensile strength of 780 MPa or more.

For this reason, in this Embodiment, it is limited to starting winding within 30 seconds after completion | finish of finish rolling. From the same viewpoint, it is desirable to start winding within 25 seconds.
(Taking-up temperature: 500 ° C to 700 ° C)
In the present embodiment, the winding temperature is set to 500 ° C. or higher and 700 ° C. or lower. When the coiling temperature is less than 500 ° C., hard bainite and martensite are generated, and subsequent cold rolling becomes difficult. On the other hand, when the coiling temperature exceeds 700 ° C., the carbides become coarse, and it becomes difficult to improve the material stability after cold rolling annealing and to ensure the tensile strength of 780 MPa or more.

  For this reason, in this Embodiment, coiling temperature is limited to 500 degreeC or more and 700 degrees C or less. From the same viewpoint, the lower limit of the coiling temperature is desirably 530 ° C., and the upper limit is desirably 680 ° C.

(The total amount of solute Ti and solute Nb in the hot-rolled steel sheet is 40% or more of the total amount of total Ti and total Nb in the steel)
In the present embodiment, by passing through the hot rolling step and the winding step described above, the total amount of solute Ti and solute Nb in the rolled hot-rolled steel sheet is calculated as the total amount of Ti in the steel and the total amount of solute Nb. 40% or more of the total amount of Nb. If the total amount of solute Ti and solute Nb in the hot-rolled steel sheet is less than 40% of the total amount of total Ti and total Nb in the steel, not only the subsequent cold rolling becomes difficult, It becomes difficult to reinforce the ferrite formed after the cold rolling annealing because the carbide coarsens during continuous annealing, and it becomes difficult to improve both the material stability and the tensile strength of 780 MPa or more.

  As described above, in this embodiment, the precipitation of Ti or Nb in the hot-rolled steel sheet is suppressed by adjusting the components of the steel material and optimizing the hot rolling conditions. Carbide can be precipitated and ferrite can be effectively strengthened.

  Thus, the hot-rolled steel sheet in which the total amount of solute Ti and solute Nb is less than 40% of the total amount of total Ti and total Nb in the steel is pickled according to a conventional method. After that, cold rolling is performed to obtain a cold rolled steel sheet. The rolling reduction in the cold rolling is not particularly limited, but is desirably 30% or more in order to make the austenite grain size during annealing finer and further improve the material stability.

(Annealing conditions of cold-rolled steel sheet: Ac _{3 to} 900 ° C. for 10 to 300 seconds)
In the present embodiment, cold rolling and annealing are performed on a hot-rolled steel sheet in which the total amount of solute Ti and solute Nb is less than 40% of the total amount of total Ti and total Nb in the steel. However, the cold-rolled steel sheet is annealed continuously and is heated until the temperature at which the cold-rolled steel sheet has an austenite single-phase structure becomes Ac ₃ points or higher, and the temperature ranges from Ac ₃ points to 900 ° C. for 10 seconds to 300 seconds. Annealing for less than a second. Once the cold-rolled steel sheet has an austenite single-phase structure, a cold-rolled annealed sheet having a uniform and fine structure is obtained.

It is less than the annealing temperature Ac _3, not only the workability becomes worked structure remained band-like tissue is significantly deteriorated, material stability variation of mechanical properties is increased to deteriorate. On the other hand, if the annealing temperature exceeds 900 ° C., the carbides become coarse and it becomes difficult to ensure a tensile strength of 780 MPa or more after cold rolling annealing.

  Further, if the annealing time is less than 10 seconds, segregation of substitutional elements such as Mn remains, the structure of the cold-rolled annealing plate becomes non-uniform, and the workability deteriorates. On the other hand, if the annealing time exceeds 300 seconds, the carbides become coarse, and the ferrite after annealing becomes coarse and the workability deteriorates.

Therefore, in this embodiment to limit the annealing conditions of the cold-rolled steel sheet, Ac ₃ above 900 ° C. and 300 seconds or less 10 seconds or more in a temperature range of below.
(Cooling conditions for cold-rolled steel sheet: average cooling rate from 650 ° C. to 550 ° C. is 5 ° C./second or more and 200 ° C./second or less)
The cold-rolled steel sheet is cooled from 650 ° C. to 550 ° C. at an average cooling rate of 5 ° C./second or more and 200 ° C./second or less after annealing. When the average cooling rate is less than 5 ° C./second, not only the crystal grains become coarse, but it becomes difficult to ensure a strength of 780 MPa or more. On the other hand, if the average cooling rate is more than 200 ° C./second, it is difficult to produce a ferrite phase and the workability deteriorates. When better material stability is required, the cooling rate is preferably 5 ° C./second or more and 80 ° C./second or less.

  In addition, by making the austenite grain size before cooling 4 μm or less, the amount of ferrite produced is stabilized and the material stability is increased. Therefore, when better material stability is required, the austenite grain size before cooling is increased. Is preferably 4 μm or less.

  In the present embodiment, in order to suppress the formation of martensite that degrades the material stability, after cooling to a cooling stop temperature of 300 ° C. or more and 550 ° C. or less at the cooling rate as described above, 300 ° C. or more and 600 ° C. It is preferable to hold for 30 seconds to 600 seconds in the following temperature range.

  In addition, the high-strength steel plate of this Embodiment is good also as a hot-dip galvanized steel plate by forming the hot-dip galvanized coating film on the surface, when the outstanding corrosion resistance is calculated | required. Here, the hot dip galvanizing is hot dip plating mainly composed of Zn and Zn, and includes those containing alloy elements such as Al and Cr in addition to Zn. The high-strength steel plate of the present embodiment subjected to hot dip galvanizing may be left as it is or may be alloyed after plating. In addition, when performing hot dip galvanizing, Si content is desirably 0.3% or less, and more desirably 0.1% or less, in order not to deteriorate the plating property and alloying processability. .

  Thus, according to the present embodiment, by adjusting the composition of the steel material and optimizing the annealing conditions after hot rolling and cold rolling, the average crystal grain size of ferrite is 1 to 3.5 μm. A cold rolled steel sheet having a steel structure in which the volume fraction of ferrite is 40% or more, the product of the volume fraction of ferrite and the nano hardness of the ferrite is 180% · GPa or more, and the tensile strength is 780 MPa or more. A high-strength cold-rolled steel sheet having excellent workability and material stability, specifically, TS × EL is 10000 MPa ·% or more, minimum bending radius is 1.0 t or less, According to the evaluation method described later, a high strength cold-rolled steel sheet having material stability in which the variation in tensile strength is within 10%, preferably within 5% of the tensile strength can be provided.

Furthermore, the present invention will be described more specifically with reference to examples.
Molten steel having the composition shown in Table 1 was made into a 1000 mm wide slab by continuous casting. These slabs were hot-rolled steel sheets having a thickness of 2.4 mm under the conditions shown in Table 2-1, and after pickling, cold rolling was performed at the rolling reduction shown in Table 2-2 to obtain cold-rolled steel sheets. Thereafter, the cold-rolled steel sheet was heated, annealed and cooled after annealing under the conditions shown in Table 2-2 by continuous annealing, and a skin pass with a rolling rate of 0.2% was performed.

When continuous annealing was a hot dip galvanizing line, the steel plate temperature in the plating bath was 460 ° C. Measure the solid solution Ti amount and solid solution Nb amount of hot-rolled steel sheets manufactured under the conditions shown in Table 2-1 and the _three- point Ac of cold-rolled steel sheets and the austenite grain size during continuous annealing. At the same time, the obtained cold-rolled annealed plate was subjected to structure observation, nano hardness measurement, tensile test and bending test. Moreover, about the hot dip galvanized steel plate, plating property and alloying processability were evaluated. The test method is shown below.

(experimental method)
(Ac ₃ point measurement)
Test pieces were collected from each cold-rolled steel sheet, and the Ac ₃ points of each cold-rolled steel sheet were measured by analyzing the change in expansion coefficient when heated from room temperature to 1000 ° C. at 10 ° C./s.

(Measurement of solid solution Ti amount and solid solution Nb amount of hot-rolled sheet)
A specimen was taken from the center in the width direction at the position of the center in the longitudinal direction of each hot-rolled steel sheet, and the amount of solute Ti and solute Nb in the hot-rolled sheet was measured by chemical analysis of the electrolytic extraction residue.

(Measurement of austenite grain size during annealing)
Samples were taken from various cold-rolled plates, heated from room temperature to 500 ° C. at 10 ° C./s, heated to the annealing temperature shown in Table 2 at 3 ° C./s, and heat treatment at the annealing temperature and annealing time shown in Table 2. A test piece was prepared by water cooling later. The cross section in the rolling direction and the structure of the cross section perpendicular to the rolling direction were photographed with an optical microscope, the prior austenite grain size was measured in accordance with JISG0552, and this was used as the austenite grain size.

(Tissue observation)
Test specimens were taken from each cold-rolled annealed plate, and the cross section in the rolling direction of the test specimen and the structure of the cross section perpendicular to the rolling direction were photographed with an optical microscope or an electron microscope, and ferrite grains were observed in accordance with JIS G 0552. The diameter was measured. The volume fraction of ferrite was measured by image analysis.

(Nano hardness measurement)
The nano hardness of the ferrite was measured using the AFM nano-indentation, the indentation load was 1000 μN, and the 10 nano hardness values extracted from the ferrite phase were measured from the steel sheet surface, and the average was obtained. Values were used.

(Tensile test)
JIS No. 5 tensile test specimens were sampled from the direction perpendicular to the rolling direction of various cold-rolled annealed plates and examined for tensile properties (tensile strength TS, elongation El). The specimens were sampled in two places, the center in the width direction at the position of the center in the longitudinal direction of the obtained steel sheet and the 1/4 part in the width direction 250 mm from the end, and the average of both was taken as the tensile property value.

(Bending test)
JIS No. 3 bending test specimens having a longitudinal direction perpendicular to the rolling direction as a longitudinal direction were collected from various cold-rolled annealed plates, and the bendability was investigated by a V-block method in accordance with JIS Z 2248. The specimen collection is the same as in the tensile test. At that time, a push fitting with an apex angle of 90 ° was pushed so that the burr was inside. The correctness after the test was visually checked, and the minimum radius of the metal fitting that was not cracked after the test was divided by the plate thickness and normalized to calculate the minimum bend radius. In addition, a press fitting having a radius of 2 mm, 1 mm, 0.5 mm, and 0 mm was used.

(Evaluation of material stability)
Material stability was evaluated by the absolute value of the difference in tensile strength between the center in the width direction and 1/4 part in the width direction.
(Evaluation of plating properties and alloying properties)
Samples with no plating and no defects were good, samples without plating and some defects were slightly good, samples with many non-plating and defects were considered defective. Samples with no alloying unevenness were good, samples with slight alloying unevenness were slightly good, and samples with remarkable alloying unevenness were considered defective.

  These results are shown in Table 3. Steel plate No. of the example of the present invention. 1, 3, 5, 7, 8, 10 to 16, 19 to 21, 26 to 29, 31 to 35, 38, 39, and 42 to 45 all have an average crystal grain size of ferrite of 1 μm to 3.5 μm. The volume ratio of ferrite is 40% or more, the product of the volume ratio of ferrite and the nano hardness of ferrite is 180% · GPa or more, and the tensile strength is 780 MPa or more. It is a high-strength steel sheet that has non-existing properties and is excellent in workability and material stability.

  In contrast, the steel plate No. No. 2 shows that the manufacturing conditions deviate from the conditions specified by the present invention, and not only the stability of the material cannot be improved and the tensile strength of 780 MPa or more cannot be ensured. is there.

Steel plate No. In No. 4, the manufacturing conditions deviate from the conditions defined by the present invention, and not only the predetermined strength cannot be secured, but also the workability is unsatisfactory.
Steel plate No. As for 6, 9, and 30, all the manufacturing conditions deviate from the conditions defined by the present invention, and it is impossible to improve both material stability and secure a tensile strength of 780 MPa or more.

Steel plate No. In both Nos. 25 and 41, the manufacturing conditions deviate from the conditions specified by the present invention, and not only the workability is unsatisfactory but also the material stability is unsatisfactory.
Steel plate No. In No. 17, the chemical composition deviates from the conditions specified by the present invention, and not only the workability is unsatisfactory but also the material stability is unsatisfactory.

Steel plate No. 18 and 23 both have compositions that deviate from the steel composition defined by the present invention, and a predetermined strength cannot be ensured.
Steel plate No. In No. 24, the composition deviates from the steel composition defined by the present invention, and the wettability and alloying processability are poor.

Steel plate No. In both Nos. 22 and 36, the composition deviates from the steel composition defined by the present invention, and the workability is poor.
Furthermore, steel plate No. No. 37 has a composition deviating from the range defined by the present invention, and the material stability is poor.

Claims (5)

The average crystal grain size of ferrite is 1 to 3.5 μm, the volume fraction of ferrite is 40% or more, the product of the volume fraction of ferrite and the nano hardness of ferrite is 180% · GPa or more, and the tensile strength A high-strength steel sheet characterized by having a steel structure of 780 MPa or more. In mass%, C: 0.06-0.18%, Si: 0.005-0.5%, Mn: 2.0-3.0%, P: 0.02% or less, S: 0.01 % Or less, Al: 0.01 to 0.1%, N: 0.01% or less, Ti: 0.20% or less, and Nb: 0.20% or less in total of 0.2%. The high-strength steel sheet according to claim 1, which has a steel composition consisting of 05% or more, the balance Fe and impurities. Furthermore, claim 1 or claim 2 containing one or more selected from the group consisting of Cr: 1.0% or less, Mo: 1.0% or less, and B: 0.003% or less in mass%. Item 3. A high-strength steel sheet according to Item 2. It has the steel composition of Claim 2 or Claim 3, and the sum total of the amount of solid solution Ti and solid solution Nb in steel is 40% or more of the sum total of the total Ti amount and total Nb amount in steel. The steel sheet was cold-rolled and then annealed for 10 to 300 seconds in a temperature range of Ac _{3 to} 900 ° C. with a continuous annealing facility, and then cooled at an average cooling rate of 5 to 200 ° C./second from 650 ° C. to 550 ° C. A method for producing a high-strength steel sheet characterized by comprising: The steel having the steel composition according to claim 2 or claim 3 is continuously cast, directly hot rolled, directly fed hot rolled, or once cooled and then reheated to 1150 to 1300 ° C and hot-rolled to 800 ° C. By completing the finish rolling at ˜950 ° C. and winding up in the temperature range of 500 to 700 ° C. within 30 seconds after the completion of finish rolling, the total amount of solute Ti and solute Nb in the steel is After the steel sheet is 40% or more of the total of the total Ti amount and the total Nb amount, the steel sheet is cold-rolled, and then annealed for 10 to 300 seconds in a temperature range of Ac _{3 to} 900 ° C. by continuous annealing equipment. A method for producing a high-strength steel sheet, characterized by cooling from 650 ° C. to 550 ° C. at an average cooling rate of 5 to 200 ° C./second.