JP5151227B2 - High strength steel plate and manufacturing method thereof - Google Patents

Description

  TECHNICAL FIELD The present invention relates to a high-strength steel sheet having a tensile strength TS excellent in deep drawability, which is useful for automobile steel sheets and the like and having a tensile strength TS of 340 MPa or more, and a method for producing the same.

In recent years, in order to regulate CO ₂ emissions from the viewpoint of global environmental conservation, improvement in fuel efficiency of automobiles has been demanded. In addition, in order to ensure the safety of passengers in the event of a collision, safety improvements centering on the collision characteristics of automobile bodies are also required. For this reason, the weight reduction and reinforcement of the automobile body are being actively promoted.

  In order to satisfy the weight reduction and strengthening of automobile bodies at the same time, it is said that it is effective to increase the strength of component materials and reduce the plate thickness within a range where rigidity is not a problem. Are actively used in automotive parts. In particular, since the weight reduction effect increases as the strength of the steel sheet used increases, in the automobile industry, for example, a steel sheet having a tensile strength TS of 340 MPa or more is used as an inner / outer panel material. Also, since many automotive parts made of steel sheets are manufactured by press molding, high strength steel sheets with an average r value (hereinafter simply referred to as r value), which is an index of deep drawability, of 1.5 or more are required. Yes.

As a means to increase strength while having a high r value, based on IF (Interstitial Free) steel in which Ti or Nb is added to ultra low carbon steel and solid solution carbon or nitrogen is fixed, Si There is a method of adding solid solution strengthening elements such as Mn and P. For example, Patent Document 1 contains C: 0.002 to 0.015%, Si: 1.2% or less, Mn: 0.04 to 0.8%, P: 0.03 to 0.10%, and Nb C × 3 to (C × 8 + 0.020 )% (Where C represents the content of element C), TS is 340 to 460 MPa, r value is 1.7 or more, elongation El is 36% or more, and non-aging molding A high-tensile cold-rolled steel sheet having excellent properties is disclosed. However, the high-tensile cold-rolled steel sheet described in Patent Document 1 has a problem in that excellent secondary work brittleness resistance cannot always be obtained.
JP-A-56-139654

  An object of the present invention is to provide a high-strength steel sheet having a TS value of 340 MPa or more, which is excellent in deep drawability with an r value of 1.5 or more, and excellent in secondary work brittleness resistance, and a method for producing the same.

  As a result of diligent studies to solve the above problems, the present inventors have controlled the amount of P, Si, Mn, and B and the existence ratio of the high-angle grain boundaries after annealing to the grain boundaries. It was found that segregation of P can be suppressed, and a high-strength steel sheet having a high r value and excellent secondary work brittleness resistance can be obtained.

The present invention has been made based on such findings, and in mass%, C: 0.0005 to 0.04%, Si: 0.01 to 1.0%, Mn: 0.2 to 3%, P: 0.003 to 0.15%, S: 0.015 %: Less than, Al: 0.005-0.5%, N: 0.006% or less, B: 0.0003-0.01%, and Nb: at least one selected from 0.003-0.1% and Ti: 0.003-0.1% A high-strength steel sheet characterized in that the balance has a composition consisting of Fe and inevitable impurities, the average r value is 1.5 or more, and Y defined by the following formula (1) is less than 30 provide.
Y = 1500XP-3 [B] -1.3X ... (1)
However, XP = [P] (1 + 0.1 [Si] +0.2 [Mn]), and X is a crystallographic difference of 50 ° in the high-angle grain boundary where the crystal orientation difference between two grain boundaries is 15 ° or more. The abundance ratio (%) of the above high-angle grain boundaries is represented, and [M] represents the content of element M (mass%, [B] is ppm).

  In the high-strength steel sheet of the present invention, in addition to the above composition, at least one selected from Mo: 0.05 to 0.5%, Cu: 0.05 to 0.5%, and Ni: 0.05 to 0.5% in addition to the above composition. It is preferable to contain.

  The high-strength steel sheet of the present invention is in mass%, C: 0.0005 to 0.04%, Si: 0.01 to 1.0%, Mn: 0.2 to 3%, P: 0.003 to 0.15%, S: 0.015% or less, Al: 0.005 to 0.5%, N: 0.006% or less, B: 0.0003 to 0.01%, and Nb: 0.003 to 0.1% and Ti: at least one selected from 0.003 to 0.1%, the balance being Fe and inevitable Steel slabs composed of impurities are heated to 1000-1300 ° C, hot-rolled at a finishing temperature of 800-950 ° C, and the steel sheet surface at an average cooling rate of 30 ° C / s or more within 0.5 s after hot rolling Winding after cooling until the temperature reaches 400 ° C or lower, raising the temperature so that the steel sheet surface temperature after winding is 550 to 720 ° C, then cold rolling at a rolling rate of 50% or more, 400 to 700 It can be manufactured by a method characterized by heating at a temperature range of ° C. at an average heating rate of 15 ° C./s or less and annealing at an annealing temperature of 820 to 950 ° C.

  In the method of the present invention, in addition to the above composition, at least one selected from Mo: 0.05 to 0.5%, Cu: 0.05 to 0.5%, and Ni: 0.05 to 0.5% is further contained in mass%. It is preferable.

  The present invention makes it possible to stably produce a high-strength steel sheet having a TS of 340 MPa or more, which has an r value of 1.5 or more, excellent deep drawability and excellent secondary work brittleness resistance. By applying the high-strength steel sheet of the present invention to automobile parts, it has become possible to increase the strength of parts that have been difficult to press-form so far, and can sufficiently contribute to collision safety and weight reduction of automobile bodies. . Moreover, the high-strength steel sheet of the present invention is applicable not only to automobile parts but also to home appliance parts and pipe materials.

  Details of the present invention will be described below. The “%” below represents “% by mass” unless otherwise specified.

1) ingredients
C: 0.0005-0.04%
C is effective for increasing the strength. To obtain a TS of 340 MPa or more, the C content is 0.0005% or more. If the C content exceeds 0.04%, the deep drawability deteriorates, so the upper limit of the C content is 0.04%, preferably 0.03%.

Si: 0.01-1.0%
In addition to the effect of solid solution strengthening, Si has the effect of facilitating the ferrite transformation and increasing the C content in the untransformed austenite phase to easily form a composite structure of the ferrite phase and the martensite phase. In order to obtain the above effect, the Si content is 0.01% or more, preferably 0.05% or more. On the other hand, if the Si content exceeds 1.0%, a surface defect called red scale occurs during hot rolling, which deteriorates the surface appearance of the steel sheet. Cause uneven plating. Therefore, the Si content is 1.0% or less, preferably 0.7% or less.

Mn: 0.2-3%
Mn contributes to increasing the r value through refinement of the structure of the hot-rolled steel sheet, and is also effective for increasing the strength by strengthening solid solution and refining. Mn is an element effective for preventing hot cracking due to S. From such a viewpoint, the Mn content is 0.2% or more. In order to obtain TS of 440 MPa or more, the Mn content is preferably 1.2% or more. On the other hand, if the Mn amount exceeds 3%, the r value and weldability are deteriorated, and the secondary work brittleness resistance is lowered by coexistence with P. Therefore, the upper limit of the Mn amount is 3%.

P: 0.003-0.15%
P has a solid solution strengthening effect. However, if the amount of P is less than 0.003%, not only the effect does not appear, but also the dephosphorization cost at the time of steelmaking increases. Therefore, the P content is 0.003% or more, preferably 0.01% or more. On the other hand, if the amount of P exceeds 0.15%, P segregates at the grain boundaries and deteriorates secondary work embrittlement resistance and weldability. In addition, during alloying after hot dip galvanizing, P suppresses the diffusion of Fe at the interface between the plating layer and the steel sheet and degrades the alloying processability. Therefore, alloying at high temperatures is required, and powdering and Plating peeling such as chipping is likely to occur. Therefore, the upper limit of the P content is 0.15%.

S: 0.015% or less
In addition to causing hot cracking, S is present as an inclusion in steel and deteriorates various properties of the steel sheet. Therefore, the S amount needs to be 0.015% or less, but is preferably reduced as much as possible.

Al: 0.005-0.5%
In addition to being useful as a deoxidizing element for steel, Al has the effect of improving the normal temperature aging resistance by precipitating solute N as AlN. Therefore, the Al content is 0.005% or more. On the other hand, if the Al content exceeds 0.5%, alloy costs increase and surface defects are induced, so the Al content is 0.5% or less.

N: 0.006% or less
When N is present in a large amount, the room temperature aging resistance is deteriorated, so that a large amount of Al or Ti needs to be added accordingly. Therefore, the upper limit of the N amount needs to be 0.006%, but is preferably reduced as much as possible.

B: 0.0003-0.01%
B is an element effective for strengthening grain boundaries and improving secondary work brittleness resistance. Therefore, the B amount is 0.0003% or more. However, since the effect is saturated when the amount of B exceeds 0.01%, the amount of B is set to 0.01% or less.

Nb: at least one selected from 0.003-0.1% and Ti: 0.003-0.1%
Nb refines the structure of the hot-rolled steel sheet or precipitates as NbC in the hot-rolled steel sheet, thereby reducing the amount of dissolved C and contributing to a high r value. From such a viewpoint, the Nb amount needs to be 0.003% or more. On the other hand, excessive Nb addition reduces ductility, so the upper limit of Nb content is 0.1%.

  Ti, like Nb, refines the structure of the hot-rolled steel sheet and also precipitates as carbides in the hot-rolled steel sheet, thereby reducing the amount of dissolved C and contributing to a higher r value. However, since the effect of refinement of the hot-rolled steel sheet is larger in Nb, it is preferable to add Ti appropriately to the Nb-added steel. Furthermore, Ti contributes to the improvement of formability such as high r value by forming precipitates with S and N in the high temperature range of hot rolling. From such a viewpoint, the Ti amount needs to be 0.003% or more. On the other hand, excessive addition of Ti reduces ductility as with Nb, so the upper limit of Ti content is 0.1%.

  The balance is Fe and inevitable impurities. Here, unavoidable impurities include 0.01% or less Sb, 0.1% or less Sn, 0.01% or less Zn, 0.1% or less Co, and the like.

  In addition to the above component composition, at least one selected from Mo: 0.05 to 0.5%, Cu: 0.05 to 0.5%, and Ni: 0.05 to 0.5% can be contained for the following reason. That is, Mo, Cu, and Ni are elements that promote high strength by solid solution strengthening like Mn, Si, and P, but have little influence on ductility, r value, and the like. In particular, Mo is an element that contributes to increasing the r value by precipitating and fixing C. In order to obtain these effects, each content is preferably 0.05% or more. However, excessive addition not only saturates these effects, but also increases alloy costs, so each upper limit is preferably made 0.5%.

  In addition, there is no problem even if Ca, REM, etc. are contained within the normal steel composition range. Ca and REM have the effect of controlling the form of sulfide inclusions, thereby preventing the deterioration of the local ductility and the like of the steel sheet. Since such an effect is saturated when the content of at least one selected from Ca and REM exceeds 0.01% in total, it is preferable to make the total 0.01% or less.

2) Average r value of 1.5 or more The steel sheet of the present invention satisfies the above component composition and also satisfies an average r value of 1.5 or more. On the other hand, the average r value is also related to the abundance ratio of high-angle grain boundaries in which two crystal orientation differences sandwiching the grain boundaries described below are 15 ° or more. In order to improve the secondary work brittleness resistance by increasing the existence ratio of the high-angle grain boundaries, it is effective that the <111> direction is oriented perpendicular to the plate surface, which corresponds to a high r value. To do. That is, in order to increase the existence ratio of the high-angle grain boundaries and satisfy Y <30, which will be described later, it is necessary to form grain boundaries having an average r value of 1.5 or more.

3) Y (= 1500XP-3 [B] -1.3X) <30
A high-angle grain boundary with two crystal orientation differences of 15 ° or more across the grain boundary is rotated by 60 ° around the <111> axis, and the atomic arrangement at the grain boundary is regular compared to the random grain boundary, Low grain boundary energy. Here, in the case of grain boundary segregation such as P, there is a high-angle grain boundary having a crystal orientation difference of 50 ° or more among two high-angle grain boundaries having a crystal orientation difference of 15 ° or more sandwiching the grain boundary. The ratio X is important. The larger the value, the better the secondary workability.

  XP (= [P] (1 + 0.1 [Si] +0.2 [Mn])) is a regression equation obtained by the present inventors etc. from the relationship between secondary work resistance and the amounts of P, Si, and Mn. Yes, it corresponds to P equivalent, but as this value increases, the secondary work resistance decreases.

  Further, as mentioned above, B strengthens the grain boundaries and improves the secondary work brittleness resistance. Therefore, the amount [B] is preferably as large as possible within the scope of the present invention.

  Figure 1 shows the relationship between these parameters and the brittle transition temperature. If Y = 1500XP-3 [B] -1.3X is less than 30, the brittle transition temperature is greatly reduced to -40 ° C or less. It can be seen that the secondary work brittleness resistance is excellent. If Y is 10 or less, the brittle transition temperature is −80 ° C. or less, and better secondary work brittleness resistance can be obtained. Here, the brittle transition temperature was determined as follows. After blanking a 65 mm diameter disk, a 33 mm diameter steel ball was used to make a conical cup, and the ear was cut off to a height of 27 mm to make a test cup. After the test cup was cooled to a predetermined temperature, a 5 kg weight was dropped from a height of 80 cm onto the test cup, and the transition temperature was determined based on whether or not the cup was cracked. Then, the test was performed three times, and the lowest temperature at which cracking did not occur three times was defined as the brittle transition temperature.

  In addition, the steel plate of this invention can be obtained as a cold-rolled steel plate so that it may mention later, and can also be obtained as a plated steel plate which has a plating layer on the surface of a cold-rolled steel plate. That is, the steel plate in the present invention includes a cold-rolled steel plate and a plated steel plate having a plating layer on the surface of the cold-rolled steel plate.

4) Manufacturing method Slab heating temperature: 1000-1300 ℃
In the production method of the present invention, first, a steel slab having the above composition (hereinafter simply referred to as slab) needs to be heated prior to hot rolling. At this time, the slab heating temperature needs to be 1000 ° C. or higher in order to ensure the finishing temperature described later. In addition, if the slab heating temperature exceeds 1300 ° C, not only precipitates such as TiN and sulfides are formed during heating, but also coarsening of γ grains, an increase in thermal energy costs, and an increase in scale loss. The slab heating temperature must be 1300 ° C or lower.

  At this time, the slab to be used is preferably manufactured by a continuous casting method to prevent macro segregation of components, but may be manufactured by an ingot-making method. The continuous casting method may be a thin slab casting method. Moreover, in order to hot-roll the slab, in addition to the conventional method in which the slab is once cooled to room temperature and then reheated and rolled, a method in which the slab is charged in a heating furnace without being cooled to room temperature and rolled, etc. The energy saving process can also be applied.

Hot rolling finishing temperature FT: 800 ~ 950 ℃
The heated slab is subjected to hot rolling consisting of rough rolling and finish rolling. The slab is made into a sheet bar by rough rolling, but the conditions for the rough rolling are not particularly defined, and may be performed according to a conventional method. When the slab heating temperature is lowered, it is preferable to use a sheet bar heater to heat the sheet bar from the viewpoint of preventing troubles during rolling.

  The sheet bar is made into a hot-rolled steel sheet by finish rolling. At this time, FT shall be 800-950 degreeC. This is because when FT is less than 800 ° C, the rolling load becomes large, and depending on the component, ferrite region rolling occurs, the structure after hot rolling becomes coarse, and excellent deep drawability cannot be obtained after annealing, exceeding 950 ° C. This is because the structure of the hot-rolled steel sheet becomes coarse, and not only excellent deep drawability cannot be obtained after annealing, but also induces surface defects such as scale defects.

  Moreover, in order to reduce the rolling load at the time of hot rolling, lubrication rolling can also be performed between some or all passes of finish rolling. The coefficient of friction during lubrication rolling is preferably in the range of 0.10 to 0.25. Furthermore, from the viewpoint of operational stability of hot rolling, it is preferable to apply a continuous rolling process in which sheet bars are joined and finish-rolled continuously.

Cooling conditions after hot rolling: Cooling until the steel sheet surface temperature reaches 400 ° C or lower at an average cooling rate of 30 ° C / s or higher within 0.5 s after rolling.In the present invention, the work structure before completion of recrystallization is refined. However, it is necessary to start cooling within 0.5 s after rolling. Moreover, it is necessary to cool at an average cooling rate of 30 ° C./s or more so that the structure rolled in an unrecrystallized state in the γ region does not recover. Furthermore, to increase the existence ratio of high-angle grain boundaries whose crystal orientation difference is 50 ° or more after annealing by promoting the formation and growth of Nb and Ti carbides when heated to 550-720 ° C as shown below In addition, the cooling needs to be performed until the steel sheet surface temperature is 400 ° C. or lower.

Steel sheet surface temperature after winding: 550-720 ° C
As described above, the hot-rolled steel sheet cooled until the steel sheet surface temperature becomes 400 ° C. or lower is wound to form a steel sheet coil. Here, in order to increase the r value by refining the microstructure of the hot-rolled steel sheet and precipitating NbC and TiC, it is necessary to raise the temperature so that the steel sheet surface temperature after winding is 550 to 720 ° C. The temperature increase may be achieved by recuperation or transformation heat generation of the steel plate itself, or may be achieved by heating from the outside. In other words, once the steel sheet surface temperature is lowered to 400 ° C. or lower, the cooling is stopped, so that heat is recovered by the amount of heat of the steel sheet itself, or transformed from austenite to a ferrite structure to generate heat, and the steel sheet surface is raised. Can be warm. If the steel sheet surface temperature cannot be raised to 550 to 720 ° C. even by such recuperation or transformation heat generation, the steel sheet coil may be further heated. Examples of the heating means include a method of covering a steel plate coil with a heat retaining cover and heating the cover. In addition, in the state wound by the steel plate coil, since the temperature of the surface and inside of a steel plate is homogenized, the steel plate surface temperature after winding is equivalent to the temperature inside a steel plate, and calculates | requires as a steel plate coil temperature. be able to.

  If the surface temperature of the steel sheet after the temperature rise is less than 550 ° C, the precipitation of carbide becomes insufficient. On the other hand, if it exceeds 720 ° C, the ferrite grain size becomes coarse and the r value decreases, so the steel plate after winding The surface temperature needs to be raised to 550 to 720 ° C, but preferably raised to 600 to 680 ° C.

Rolling ratio: 50% or more The hot-rolled steel sheet wound up is pickled for scale removal, and then cold-rolled into a cold-rolled steel sheet. At this time, if the rolling rate of cold rolling is less than 50%, {111} recrystallized texture does not develop, and it becomes difficult to obtain excellent deep drawability, so 50% or more, preferably 60% or more It is necessary to. On the other hand, in the present invention, in the range up to 90%, the r value increases as the rolling rate increases, but when the rolling rate exceeds 90%, not only the effect is saturated, but also the load on the roll during rolling increases. Therefore, the upper limit of the rolling rate is preferably 90%.

Heating conditions for annealing: Heating the temperature range of 400-700 ° C at an average heating rate of 15 ° C / s or less.Then, the cold-rolled steel sheet is annealed, but the existence ratio of high-angle grain boundaries with a crystal orientation difference of 50 ° or more In order to improve the above, it is necessary to control the carbide and recrystallization texture of NbC and TiC which control the grain growth property precipitated during the winding described above. For this purpose, the heating rate from 400 to 700 ° C, which is the initial stage of recrystallization from the recovery phase, is important. The heating rate in this temperature range is set to 15 ° C / s or less to increase the existence ratio of high-angle grain boundaries There is a need.

Annealing temperature: 820 ~ 950 ℃
After controlling the heating rate from the recovery stage to the initial stage of recrystallization, it is necessary to anneal at 820 to 950 ° C. This is because if the annealing temperature is less than 820 ° C, the grain growth after recrystallization is insufficient, and the existence ratio of high-angle grain boundaries with a crystal orientation difference of 50 ° or more cannot be sufficiently increased. This is because the annealing in the γ single phase results in a decrease in the r value and a decrease in the existence ratio of high-angle grain boundaries having a crystal orientation difference of 50 ° or more.

  The cold-rolled steel sheet after annealing may be subjected to a plating process such as an electroplating process or a hot dipping process to form a plating layer on the surface of the steel sheet. In addition to pure zinc, for example, zinc-based plating in which zinc is the main component and alloying elements are added, or pure Al, Al-based plating in which Al is the main component and alloying elements can be applied. .

  For example, when performing hot dip galvanizing treatment often used for automobile steel plates as plating treatment, the annealing is performed in a continuous hot dip galvanizing line, and after immersion, immersed in a hot dip galvanizing bath, A hot dip galvanized layer may be formed. Further, alloying treatment may be performed to obtain an alloyed hot-dip galvanized steel sheet.

  The cold-rolled steel sheet thus manufactured, and further the plated steel sheet subjected to the plating treatment, can be subjected to temper rolling or leveler processing for the purpose of shape correction, surface roughness adjustment, and the like. The elongation of the temper rolling or leveler processing is preferably in the range of 0.2 to 15%. If it is less than 0.2%, the purpose of shape correction and surface roughness adjustment cannot be achieved, and if it exceeds 15%, the ductility is remarkably lowered. More preferably, it is 2% or less. It has been confirmed that there is no significant difference in the effect between temper rolling and leveler processing, although the processing formats are different. These temper rolling and leveler processing are effective even after the plating treatment.

  Steels A to L having the compositions shown in Table 1 were melted in a converter and made into slabs by a continuous casting method. These slabs were heated to 1250 ° C., roughly rolled into sheet bars, and then hot-rolled steel sheets under the hot-rolling conditions shown in Table 2. The steel plate coil temperatures shown in Table 2 were achieved by heating a steel plate coil wound after hot rolling with a heat insulating cover covered. These hot-rolled steel sheets are pickled and cold-rolled at a rolling rate of 65% to form cold-rolled steel sheets (thickness: 1.2 mm), and subsequently subjected to annealing under the annealing conditions shown in Table 2 in a continuous annealing line. A sample of steel plates Nos. 1 to 26 was prepared by temper rolling at a rate of 0.5%. Steel plate No. 2 is a hot dip galvanized steel plate that has been subjected to continuous annealing in a continuous hot dip galvanizing line, and subsequently hot dip galvanized (plating bath temperature: 480 ° C.). And about the obtained sample, the measurement of the tensile characteristic value, r value, secondary work brittleness resistance, and the existence ratio of the high-angle grain boundary having a crystal orientation difference of 50 ° or more were performed and evaluated by the following methods.

  Tensile properties: JIS No. 5 tensile test specimen is taken from the sample at 90 ° direction with respect to the rolling direction, tensile test is performed at a crosshead speed of 10 mm / min in accordance with the provisions of JIS Z 2241, yield strength YS, TS, The elongation El was obtained.

r value: JIS No. 5 tensile test specimen taken from the sample in the rolling direction, 45 ° direction to the rolling direction, and 90 ° direction to the rolling direction, and the width of each test piece when 10% uniaxial tensile strain was applied. Strain and plate thickness strain were measured, and an average r value (average plastic strain ratio) was calculated from the following formula in accordance with the provisions of JIS Z 2254 to evaluate deep drawability.
r value = (r ₀ + 2r ₄₅ + r ₉₀ ) / 4
Here, r _0, r _{45, and} r ₉₀ are plastic strain ratios measured with test pieces taken from 0 °, 45 °, and 90 ° directions with respect to the rolling direction, respectively.

  Secondary processing brittleness resistance: After blanking on a 65 mm diameter disc, a conical cup was prepared using a 33 mm diameter steel ball, and the ear was cut off to a height of 27 mm to prepare a test cup. After the test cup was cooled to a predetermined temperature, a 5 kg weight was dropped from a height of 80 cm onto the test cup, and the transition temperature was determined based on whether or not the cup was cracked. Then, the test was performed three times, and the lowest temperature at which cracking did not occur three times was regarded as the brittle transition temperature, and the secondary work brittleness resistance was evaluated.

  The existence ratio of high-angle grain boundaries with a crystal orientation difference of 50 ° or more X: EBSD (Electron Back-Scatter Diffraction) is used to measure three fields of view of 500 μm × 500 μm and analyze two crystal orientation differences across the grain boundary The ratio of grain boundaries having a crystal orientation difference of 50 ° or more out of the grain boundaries having a crystal orientation difference of 15 ° or more was calculated.

  The results are shown in Table 2. In the examples of the present invention, TS is 340 MPa or more, r value is 1.5 or more, brittle transition temperature is -40 ° C. or less, and it is understood that secondary work brittleness resistance is excellent.

It is a figure which shows the relationship between Y and a brittle transition temperature.

Claims (4)

In mass%, C: 0.0005 to 0.04%, Si: 0.01 to 1.0%, Mn: 0.2 to 3%, P: 0.003 to 0.15%, S: 0.015% or less, Al: 0.005 to 0.5%, N: 0.006% or less B: 0.0003 to 0.01%, and Nb: 0.003 to 0.1% and Ti: at least one selected from 0.003 to 0.1%, and the balance has a composition consisting of Fe and inevitable impurities, A high-strength steel sheet having an average r value of 1.5 or more and Y defined by the following formula (1) being less than 30;
Y = 1500XP-3 [B] -1.3X ... (1)
Here, XP = [P] (1 + 0.1 [Si] +0.2 [Mn]), and X is a high-angle grain boundary between two crystal orientation differences of 15 ° or more sandwiching the grain boundary, the crystal orientation difference is 50 It represents the abundance (%) of high-angle grain boundaries at or above °, and [M] represents the content of element M (mass%, [B] is ppm).   In addition to the above composition, the composition further comprises at least one selected from Mo: 0.05 to 0.5%, Cu: 0.05 to 0.5% and Ni: 0.05 to 0.5% by mass%. Item 2. The high-strength steel sheet according to Item 1.   In mass%, C: 0.0005 to 0.04%, Si: 0.01 to 1.0%, Mn: 0.2 to 3%, P: 0.003 to 0.15%, S: 0.015% or less, Al: 0.005 to 0.5%, N: 0.006% or less B: 0.0003 to 0.01%, and Nb: 0.003 to 0.1% and Ti: at least one selected from 0.003 to 0.1%, and a steel slab having a composition consisting of Fe and inevitable impurities , Heated to 1000-1300 ° C, hot-rolled at a finishing temperature of 800-950 ° C, and after hot rolling until the steel sheet surface temperature reached 400 ° C or less at an average cooling rate of 30 ° C / s or more within 0.5 s Winding after cooling, heating up so that the steel sheet surface temperature after winding is 550 to 720 ° C, then cold rolling at a rolling rate of 50% or more, and the temperature range of 400 to 700 ° C is 15 ° C / A method for producing a high-strength steel sheet, characterized by heating at an average heating rate of s or less and annealing at an annealing temperature of 820 to 950 ° C.   In addition to the above composition, the steel slab further contains at least one selected from Mo: 0.05 to 0.5%, Cu: 0.05 to 0.5%, and Ni: 0.05 to 0.5% by mass%. 4. The method for producing a high-strength steel sheet according to claim 3, wherein:

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