KR20190063413A - Method for manufacturing high strength steel strip - Google Patents

Abstract

The present invention provides a method capable of stably manufacturing a high strength steel tape, having excellent formability in TS >= 980 MPa, YR >= 68 %, TS × EL >= 22000 MPa·% across the whole length of a coil. When a high strength steel tape is manufactured, a hot rolling plate coil is heated under a condition of maintaining an outer sphere unit of the hot rolling plate coil to exceed 21600 s and be equal to or less than 129600 s at a predetermined temperature equal to or greater than (Ac_1 transformation point + 20 °C) and equal to or less than ((Ac_1 transformation point + 120 °C). Therefore, the method for stably manufacturing a high strength steel tape promotes homogenization of a steel tissue across the whole length of the coil. A first part is an inner sphere unit of the hot rolling annealing plate coil, and a second part is the outer sphere unit of the hot rolling annealing plate coil, in the longitudinal direction of the manufactured high strength steel tape. An area ratio of polygonal ferrite in the first part/ second part is 1.00-1.50. The volume ratio of remaining austenite of the first part/second part is 0.75-1.00. The average crystal grain of polygonal ferrite in the first part/second part is 1.00-1.50.

Description

METHOD FOR MANUFACTURING HIGH STRENGTH STEEL STRIP [0002]

TECHNICAL FIELD The present invention relates to a method of manufacturing a high strength steel strip, and more particularly, to a steel strip excellent in moldability and having a high yield ratio, which is preferable for use as a member used in an industrial field of automobiles, .

Recently, fuel economy improvement of automobile becomes important problem from viewpoint of conservation of global environment. For this reason, there has been an increasing tendency to reduce the thickness of the body material by making the body material stronger and to reduce the weight of the body itself.

However, in general, the higher the strength of the steel sheet leads to the lowering of the formability, the higher the strength of the steel sheet is, the lower the formability of the steel sheet, and the problems such as cracking during molding are caused. Therefore, the steel sheet can not be made thin simply.

Therefore, development of a material that combines high strength and high-strength formation has been desired. Further, a steel sheet having a tensile strength (TS) of 980 MPa or more is required to have a characteristic of being particularly high in collision absorption energy in addition to the formation of this solidification. In order to improve the collision absorbed energy, it is effective to increase the yield ratio YR. If the yield ratio is high, the impact energy can be efficiently absorbed to the steel sheet with a low deformation amount.

For example, Patent Document 1 proposes a high-strength steel sheet having extremely high ductility using a processed organic transformation of retained austenite having a tensile strength of 1000 MPa or more and a total elongation (EL) of 30% or more.

Patent Document 2 proposes to produce a high strength steel sheet excellent in balance between strength and ductility by using high Mn steel and subjecting it to heat treatment in a bimetallic zone of ferrite and austenite.

Japanese Laid-Open Patent Application No. 61-157625 WO2016 / 067626A1

The steel sheet described in Patent Document 1 is produced by austenitizing a steel sheet containing C, Si and Mn as basic components and then performing so-called austemper treatment in which the steel sheet is maintained at an isothermal temperature in the range of the bainite transformation temperature. When this austemper treatment is carried out, the residual austenite is produced by the concentration of C to the austenite. However, in order to obtain a large amount of retained austenite, a large amount of C exceeding 0.3% is required. However, at a C concentration exceeding 0.3%, the spot weldability is remarkably deteriorated and it is difficult to put the steel into an automotive steel sheet .

Patent Document 2 discloses an epoch-making method of successfully forming a retained austenite phase effective for deformation organic transformation by controlling the phase fraction and grain size of ferrite and austenite by bifunctional annealing and further controlling the amounts of C and Mn in the austenite phase Invention. However, when the steel sheet of Patent Document 2 is actually manufactured at the manufacturing site and the hot-rolled sheet coil is heated to the designated temperature using a batch type heating furnace, the temperature in the vicinity of the coil center is difficult to rise, There is a problem that the desired structure can not be obtained in the inner portion of the hot-annealed plate coil after batch heating, and the characteristics are varied in the longitudinal direction of the coil, resulting in a poor yield.

The present invention relates to the improvement of the invention described in the above-mentioned Patent Document 2, and even when the hot-rolled coil is heated by using a batch type heating furnace, TS ≥ 980 MPa, YR ≥ 68% It is an object of the present invention to provide a method capable of stably producing a high strength steel sheet excellent in formability of TS 占 EL? 22000 ㎫ ·%.

However, the inventors of the present invention have found that, in order to stably produce a high-strength steel strip having a high yield ratio and a high tensile strength over the entire length of the coil, The heating conditions at the time of heating are repeated. As a result, 21600 a hot-rolled sheet annealing step in a batch-type heating, from the outer gwonbu (外卷部) of the hot-rolled coil (Ac ₁ transformation point + 20 ℃) above, (Ac ₁ transformation point + 120 ℃) equal to or less than a predetermined temperature s < / RTI > for a time period of not more than 129600 s, the inventors of the present invention have obtained the knowledge that the intended purpose is advantageously achieved. The present invention is based on the above findings.

That is, the structure of the present invention is as follows.

The steel sheet according to any one of the above items [1] to [6], wherein C is 0.030 to 0.250%, Si is 0.01 to 3.00%, Mn is 4.20 to 6.00%, P is 0.001 to 0.100% A steel slab having a composition of N: not less than 0.0005% and not more than 0.0100%, Al: not less than 0.010% and not more than 2.000% and Ti: not less than 0.005% and not more than 0.200%, and the balance of Fe and inevitable impurities, Or more to 1300 占 폚 or less,

Subjecting the steel slab to hot rolling at a finishing rolling output temperature of 750 ° C or higher and 1000 ° C or lower to obtain a hot rolled steel sheet,

Winding the hot-rolled sheet at a coiling temperature of 300 ° C or higher and 750 ° C or lower to obtain a hot-

A step of pickling the hot rolled sheet of the hot rolled coil to remove scale,

A hot-rolled sheet annealing step of subjecting the hot-rolled coil to heat treatment in a batch type furnace to obtain a hot-

Thereafter, the hot-rolled annealing plate of the hot-rolled annealing plate coil is cold-rolled at a reduction ratio of 50% or less to obtain a cold-

Then, the cold-rolled sheet, Ac ₁ transformation point or more, (Ac ₁ transformation point + 100 ℃) and to 20 s at least 900 s or less at a predetermined temperature or less oil, subjected to cold-rolled sheet annealing is cooled and then a step for obtaining a high-strength steel strip The method comprising the steps of:

Hot-rolled sheet annealing step in the above batch-type heating, the outer gwonbu of the hot-rolled sheet coil (Ac ₁ transformation point + 20 ℃) above, (Ac ₁ transformation point + 120 ℃) 21600 s than at a predetermined temperature not higher than 129600 s or less And then,

A portion of the hot-rolled annealing plate coil which is the inner portion of the hot-rolled annealing plate coil is referred to as a first portion, a portion of the hot-rolled annealing plate coil which is the outer portion of the hot- As three sites,

The steel structure of the third portion has an area ratio of 15% to 55% of polygonal ferrite, 8% or more of non-recrystallized ferrite, 15% to 30% of martensite, : 12% or more,

The area ratio of the polygonal ferrite of the first portion / the area ratio of the polygonal ferrite of the second portion is 1.00 to 1.50, the volume ratio of the retained austenite of the first portion / the retained austenite of the second portion Is in the range of 0.75 to 1.00,

In addition, the third region preferably has an average grain diameter of 4.0 탆 or less, a mean grain diameter of martensite of 2.0 탆 or less, and an average grain diameter of residual austenite of 2.0 탆 or less of polygonal ferrite,

The average grain diameter of the polygonal ferrite in the first portion / the average grain diameter of the polygonal ferrite in the second portion is 1.00 to 1.50,

The method for producing a high-strength steel cord according to claim 1, wherein a value obtained by dividing the amount (mass%) of Mn in the retained austenite by the amount (mass%) of Mn in the polygonal ferrite is 2.00 or more.

0.005% to 1.000% of Cr, 0.005% to 1.000% of Cr, and 0.005% or more of Cr, V: 0.005 to 0.500%, Mo: 0.005 to 1.000%, Cu: 0.005 to 1.000%, Sn: 0.002 to 0.200%, Sb: 0.002 to 0.200%, Ta: 0.001% 1] or [2], further comprising at least one element selected from the group consisting of Ca: not more than 0.010%, Ca: not more than 0.0005% and not more than 0.0050%, Mg: not more than 0.0005% and not more than 0.0050% Wherein the high-strength steel strip is produced by a method comprising the steps of:

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to stably obtain a high-strength steel strip having excellent formability of TS ≥ 980 MPa, YR ≥ 68% and TS EL ≥ 22000 ㎫ ·% over the entire length of the coil. Therefore, by applying the high-strength steel strip produced by the present invention to, for example, an automotive structural member, the fuel economy can be improved by reducing the weight of the vehicle body, and the industrial utility value is very high.

(Composition of components)

First, the reason why the composition of the steel strip is limited to the above range in the present invention will be described. In addition, the% indication relating to the composition of steel or slab means mass%. The remainder of the composition of the steel strip is Fe and inevitable impurities.

C: not less than 0.030% and not more than 0.250%

C is an element necessary for generating a low temperature transformation phase such as martensite and increasing the strength. It is also an element effective for improving the stability of retained austenite and improving ductility of steel. If the amount of C is less than 0.030%, it is difficult to secure a desired amount of martensite and desired strength can not be obtained. Further, it is difficult to secure a sufficient amount of retained austenite and good ductility can not be obtained. On the other hand, when C is excessively added in excess of 0.250%, the amount of hard martensite becomes excessive, the micro voids in the grain boundary of martensite increase, and propagation of crack propagates during the bending test and hole expansion test And the bendability and stretch flangeability are lowered. Further, the addition of C excessively causes hardening of the welded portion and the heat affected portion and deteriorates the mechanical properties of the welded portion, so that spot weldability, arc weldability and the like are deteriorated. From these viewpoints, the C content is in the range of 0.030% or more and 0.250% or less. , Preferably not less than 0.080% and not more than 0.200%.

Si: not less than 0.01% and not more than 3.00%

Si improves the work hardenability of ferrite and is an element effective for securing good ductility. If the amount of Si is less than 0.01%, the effect of addition becomes insufficient, so the lower limit is set to 0.01%. On the other hand, excessive addition of Si exceeding 3.00% not only causes embrittlement of steel but also causes deterioration of the surface property due to occurrence of red scale or the like. Therefore, the amount of Si is set in the range of 0.01% or more and 3.00% or less. , Preferably not less than 0.20% and not more than 2.00%.

Mn: more than 4.20% 6.00% or less

Mn is a very important element in the present invention. Mn is an element for stabilizing the retained austenite and is effective for ensuring good ductility. Mn is also an element capable of increasing the strength of steel by solid solution strengthening. Further, Mn concentration in the retained austenite can secure 2% or more of the epsilon phase having the hcp structure, and moreover, it is possible to secure a large amount of the retained austenite at a volume ratio of 12% or more. Such an effect is confirmed only when the amount of Mn in the steel exceeds 4.20%. On the other hand, an excessive addition of Mn exceeding 6.00% causes a rise in cost. From this viewpoint, the amount of Mn is set in a range from 4.20% to 6.00%. Preferably, the range is from 4.80% to 6.00%.

P: not less than 0.001% and not more than 0.100%

P is effective for solid solution strengthening and can be added according to the desired strength. It is also an element effective for accelerating the ferrite transformation and for complexing the steel sheet. In order to obtain such an effect, the amount of P in the steel sheet needs to be 0.001% or more. On the other hand, if the P content exceeds 0.100%, the weldability is deteriorated, and in the case of alloying treatment of the zinc plating, the alloying speed is lowered and the quality of the zinc plating is deteriorated. Therefore, the P content is set in a range of 0.001% to 0.100%, preferably 0.005% to 0.050%.

S: 0.0001% or more and 0.0200% or less

S is segregated at the grain boundaries to embrittle steel during hot working and exists as a sulfide to lower the local strain of the steel sheet. Therefore, the S content is 0.0200% or less, preferably 0.0100% or less, and more preferably 0.0050% or less. However, the amount of S is set to 0.0001% or more from the limitations of production technology. Therefore, the amount of S is set in the range of 0.0001% or more and 0.0200% or less. , Preferably not less than 0.0001% and not more than 0.0100%, and more preferably not less than 0.0001% and not more than 0.0050%.

N: 0.0005% or more and 0.0100% or less

N is an element which deteriorates the endurance of steel. Particularly, when the N content exceeds 0.0100%, deterioration of endurance is remarkable. Therefore, the smaller the amount of N, the better, but the amount of N is 0.0005% or more from the viewpoint of production technology. For this reason, the N content is set in the range of 0.0005% to 0.0100%, preferably 0.0010% to 0.0070%.

Al: 0.010% or more and 2.000% or less

Al is an element effective for increasing the bimodal of ferrite and austenite and reducing annealing temperature dependency, that is, improving material stability. In addition, Al acts as a deoxidizing agent and is an element effective for maintaining the cleanliness of the steel. However, if the amount of Al is less than 0.010%, the effect of addition is insufficient, so the lower limit is set to 0.010%. On the other hand, the addition of a large amount exceeding 2.000% increases the risk of cracking of the steel strip during continuous casting, thereby lowering the composition. From this viewpoint, the amount of Al in the case of addition is set in a range of 0.010% or more and 2.000% or less. Preferably, it is in the range of 0.200% or more and 1.200% or less.

Ti: not less than 0.005% and not more than 0.200%

Ti is an important additive element in the present invention. Ti is not only effective for precipitation strengthening of steel but also secures a desired amount of undrawn recrystallized ferrite and effectively contributes to high yielding of the steel sheet. Further, by utilizing a relatively hard non-recrystallized ferrite, it is possible to reduce the difference in hardness between the hard second phase (martensite or retained austenite) and also to improve the stretch flangeability. These effects are obtained by adding 0.005% or more of Ti. On the other hand, when the amount of Ti in the steel sheet exceeds 0.200%, the amount of hard martensite becomes excessive, the micro voids in the grain boundary of martensite increase, and propagation of cracks tends to proceed during the bending test and hole expanding test , The bending property and the stretch flangeability of the steel sheet are lowered. Therefore, the addition amount of Ti is set in the range of 0.005% or more and 0.200% or less. It is preferably in the range of 0.010% or more and 0.100% or less.

The essential components have been described above, but in the present invention, the following elements can be appropriately contained in addition to them.

Nb: 0.005% or more and 0.200% or less

Nb is effective for precipitation strengthening of steel, and its addition effect is obtained at 0.005% or more. In addition, like the effect of Ti addition, the desired amount of non-recrystallized ferrite is ensured, contributing to the high yielding of the steel sheet. Further, by utilizing a relatively hard non-recrystallized ferrite, it is possible to reduce the difference in hardness between the hard second phase (martensite or retained austenite) and also to improve the stretch flangeability. On the other hand, when the amount of Nb exceeds 0.200%, the amount of hard martensite becomes excessive, micro voids in the grain boundary of martensite increase, and crack propagation tends to proceed during bending test and hole expanding test. As a result, the bendability and stretch flangeability of the steel sheet are lowered. In addition, the cost may be increased. Therefore, in the case of adding Nb, the content is set in a range of 0.005% or more and 0.200% or less. It is preferably in the range of 0.010% or more and 0.100% or less.

B: not less than 0.0003% and not more than 0.0050%

B has an effect of suppressing the formation and growth of ferrite from the austenitic grain boundaries, and enables to control the structure on a timely basis, so that B can be added as needed. The addition effect is obtained at 0.0003% or more. On the other hand, if the B content exceeds 0.0050%, the formability of the steel sheet is lowered. Therefore, in the case of adding B, it is set in the range of 0.0003% or more and 0.0050% or less. , Preferably not less than 0.0005% and not more than 0.0030%.

Ni: 0.005% or more and 1.000% or less

Ni is an element that stabilizes retained austenite, is effective in securing good ductility, and is an element that increases the strength of steel by solid solution strengthening. The addition effect is obtained at 0.005% or more. On the other hand, when it is added in an amount exceeding 1.000%, the amount of hard martensite becomes excessive, micro voids in the grain boundary of martensite increase, and propagation of cracks tends to proceed during the bending test and hole expanding test. As a result, the bendability and stretch flangeability of the steel sheet are lowered. In addition, the cost may be increased. Therefore, in the case of adding Ni, the Ni content is set in the range of 0.005% to 1.000%.

0.005 to 1.000%, V: 0.005 to 0.500%, Mo: 0.005 to 1.000%

Cr, V, and Mo have an effect of improving the balance between strength and ductility, and are elements that can be added as needed. The addition effect is obtained at not less than 0.005% of Cr, not less than 0.005% of V, and not less than 0.005% of Mo. On the other hand, if it is excessively added in excess of 1.000% of Cr, 0.500% of V and 1.000% of Mo, the amount of hard martensite becomes excessive, micro voids in grain boundaries of martensite are increased, The propagation of cracks is likely to proceed during the hole expansion test. As a result, the bendability and stretch flangeability of the steel sheet are lowered. In addition, the cost may be increased. Therefore, when these elements are added, the content of Cr is preferably 0.005 to 1.000%, V is 0.005 to 0.500%, and Mo is 0.005 to 1.000%.

Cu: 0.005% or more and 1.000% or less

Cu is an effective element for strengthening steel. The addition effect is obtained at 0.005% or more. On the other hand, when it is added in an amount exceeding 1.000%, the amount of hard martensite becomes excessive, micro voids in the grain boundary of martensite increase, and propagation of cracks tends to proceed during the bending test and hole expanding test. As a result, the bendability and stretch flangeability of the steel sheet are lowered. Therefore, in the case of adding Cu, the content is set in the range of 0.005% to 1.000%.

Sn: not less than 0.002% and not more than 0.200%, Sb: not less than 0.002% and not more than 0.200%

Sn and Sb are added as needed in view of suppressing decarburization in the thickness region of several tens of micrometers of the surface layer of the steel sheet caused by nitriding or oxidation of the surface of the steel sheet. Thus, by suppressing nitrification and oxidation, it is effective to prevent the amount of martensite on the surface of the steel sheet from decreasing, and to secure the TS and the material stability. In order to obtain this effect, 0.002% or more of addition is required. On the other hand, when it is added in an excess amount exceeding 0.200%, the toughness is lowered. Therefore, when Sn and Sb are added, the content is 0.002% or more and 0.200% or less, respectively.

Ta: 0.001% or more and 0.010% or less

Ta, like Ti and Nb, produces alloy carbides and alloy carbonitrides, contributing to the strengthening of steel. In addition, it is partially solved in Nb carbide or Nb carbonitride to generate coarse precipitates such as (Nb, Ta) (C, N) to suppress the coarsening of the precipitates and contribute to the improvement of the strength of the steel sheet by precipitation strengthening It is thought that there is stabilizing effect. Here, the addition effect of Ta is obtained by setting the content of Ta to 0.001% or more. On the other hand, if Ta is added excessively, the addition effect becomes saturated and the alloy cost also increases. Therefore, in the case of adding Ta, the content is set in the range of 0.001% to 0.010%.

Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, REM: 0.0005% or more and 0.0050%

Ca, Mg, and REM are effective elements for making the shape of the sulfide spherical and improving the adverse effect of the sulfide on the hole expandability (stretched flange formability). In order to obtain this effect, 0.0005% or more of addition is required. On the other hand, an excessive addition of more than 0.0050% causes an increase in inclusions and the like, causing surface and internal defects of the steel sheet and the like. Therefore, when Ca, Mg, and REM are added, they are each set within a range of 0.0005% or more and 0.0050% or less.

(Steel structure)

In the present invention, by suitably controlling the heating conditions in the batch type heating furnace, deviation of the structure and characteristics over the length direction of the coil, which has been a problem in the past, is improved. Here, in the present specification, with regard to the longitudinal direction of the high-strength steel strip to be manufactured, the portion of the hot-rolled annealing plate coil that was the inner portion was referred to as the first portion, the portion that was the outer- The portion that was in the middle portion is called the third portion. Here, the "inner portion of the hot-rolled annealing plate coil" refers to a range from the inner end of the hot-rolled annealing plate coil to 150 m in the longitudinal direction. "Outer portion of hot-rolled annealing plate coil" refers to a range from the outer end of the hot-rolled annealing plate coil to 150 m in the longitudinal direction. The "central portion of the hot-rolled annealing plate coil" refers to a portion other than the inner and outer portions of the hot-rolled annealing plate coil.

In the present invention, as the index of the deviation of the structure, there are (A) the area ratio of the polygonal ferrite in the first portion / the area ratio of the polygonal ferrite in the second portion, (B) the volume ratio of the retained austenite in the first portion / (C) the average grain diameter of the polygonal ferrite in the first portion / the average grain diameter of the polygonal ferrite in the second portion are adopted, and these are set to a predetermined range . By controlling these ratios in the range described below, it is possible to improve the deviation in characteristics in the length direction of the coil, which is conventionally concerned, in particular, deterioration of characteristics in the hot-rolled annealing plate coil inner portion, Excellent properties that do not exist.

Area ratio of polygonal ferrite at the third portion (central portion): 15% or more and 55% or less

In order to secure sufficient ductility, in the present invention, it is necessary to set the area ratio of the polygonal ferrite in the third portion to 15% or more. On the other hand, in order to secure a strength of 980 MPa or more, it is necessary to suppress the area ratio of polygonal ferrite in the third portion to 55% or less. Preferably, the area ratio is in the range of 20% or more and 50% or less. In the present invention, the area ratio of the polygonal ferrite in the first portion (inner portion) and the second portion (outer portion) is preferably not less than 15% and not more than 55%, more preferably not less than 20% and not more than 50% % Or less. In addition, the polygonal ferrite in the present invention refers to a relatively soft and ductile ferrite.

Indicators of Organizational Deviations (A)

Area ratio ratio of polygonal ferrite (internal / external part): 1.00 to 1.50

The ratio of the first portion (inner portion) / the second portion (outer portion) to the area ratio of the polygonal ferrite is set to be between 1.00 and 1.50, You can get a high strength. And preferably in the range of 1.00 to 1.20.

Area ratio of non-recrystallized ferrite in the third portion (central portion): 8% or more

It is important for the present invention that the area ratio of the non-recrystallized ferrite is 8% or more. Here, the non-recrystallized ferrite is generally effective for increasing the strength of the steel sheet, but it causes a remarkable decrease in ductility of the steel sheet, so that it is often reduced. However, in the present invention, by using polygonal ferrite and retained austenite to secure good ductility and positively utilizing relatively hard non-recrystallized ferrite, a large amount of martensite exceeding 30% (YP) and yield ratio (YP) of the steel sheet can be ensured without securing the required tensile strength of the steel sheet, securing the TS of the steel sheet required, and further reducing the interfacial amount of polygonal ferrite and martensite, (YR) can be increased. In order to obtain the above effect, it is necessary to set the area ratio of the non-recrystallized ferrite to 8% or more. Preferably, it is 10% or more. In addition, the non-recrystallized ferrite in the present invention means a ferrite containing deformation less than 15 degrees in crystal orientation in its mouth, which is harder than the above-mentioned soft polygonal ferrite. In the present invention, the upper limit of the area ratio of the non-recrystallized ferrite is not particularly limited, but is preferably about 20%.

Area ratio of martensite at the third portion (center portion): 15% or more and 30% or less

In order to achieve a TS of 980 MPa or more, it is necessary to set the area ratio of martensite at the third portion to 15% or more. On the other hand, in order to secure good ductility, it is necessary to limit the area ratio of the martensite at the third portion to 30% or less.

The area ratio of the martensite is preferably 1.00 to 1.30 in terms of the ratio of the first portion (inner portion) / the second portion (outer portion), so that there is no variation in characteristics over the entire length of the coil , A high-yielding steel strip can be obtained.

Here, the calculation of the area ratio of the martensite can be carried out as follows. That is, as for the third portion (middle portion), regarding the rolling direction, a sample is taken from the central portion of the width of the plate, with respect to the intermediate position between the leading end and the tail end of the steel strip and the plate width direction, (L section) was polished and then corroded with 3 vol.% Or more of deviation. The plate thickness was measured at 1/4 position (the ratio of the thickness of the steel sheet Position) is observed with a scanning electron microscope (SEM) at a magnification of 2000 times by using a field of view of 50 占 퐉 占 40 占 퐉 at about 10 fields, and a tissue image is obtained. In this tissue image, ferrites (polygonal ferrite and non-recrystallized ferrite) show a gray texture (underlying texture), and martensite shows a whitish texture, so that both can be identified. On the basis of the obtained tissue image, the area ratio of martensite is calculated by using Image-Pro manufactured by Media Cybernetics Co., and the area ratio thereof is averaged.

The area ratio of the polygonal ferrite and the non-recrystallized ferrite can be determined as follows. That is, a sample is taken from a predetermined position to be described later, and a plate thickness 1/4 position of a plate thickness section (L section) parallel to the rolling direction of the sample is provided for the following EBSD observation. By using Electron Back Scattering Diffraction (EBSD), a low angle grain boundary with a crystal orientation difference of 2 DEG to less than 15 DEG and a diagonal grain boundary with a crystal orientation difference of 15 DEG or more are identified. Then, an IQ map is formed by using ferrite containing a low grain boundary as a non-recrystallized ferrite. Next, the area of the polygonal ferrite and the non-recrystallized ferrite were respectively calculated by extracting 10 fields of view from the prepared IQ Map in the field of 50 탆 40 탆, and then calculating the areas of the low angle grain boundary and the diagonal grain boundary respectively. Minute area of the polygonal ferrite and the non-recrystallized ferrite. Then, the area ratios of the polygonal ferrite and the non-recrystallized ferrite are obtained by averaging their area ratios. When calculating the area ratio of the polygonal ferrite and the non-recrystallized ferrite in the third portion (middle portion), the center portion is represented by the center portion, the intermediate portion between the tip end portion and the tip end portion of the steel strip in the rolling direction, . When calculating the area ratio of the polygonal ferrite and the non-recrystallized ferrite in the first portion (inner portion) and the second portion (outer portion), it is preferable that, in the rolling direction, And 100 m from the end on the second site side, and from the center of the plate width for the plate width direction.

Volume ratio of retained austenite at the third portion (central portion): 12% or more

In the present invention, in order to ensure sufficient ductility, it is necessary to set the volume percentage of the retained austenite in the third portion to 12% or more. And preferably at least 14%. In the present invention, the upper limit of the volume percentage of retained austenite in the third portion is not particularly limited, but it is preferable that the retained austenite such as C or Mn, , It is preferable to set it to about 50%.

Indicator of tissue deviation (B)

Volume ratio ratio of retained austenite (inner volume / outer volume): 0.75 to 1.00

It is important to control the ratio of the first portion (inner portion) / the second portion (outer portion) to the range between 0.75 and 1.00 with respect to the volume percentage of retained austenite, A high-yielding steel strip can be obtained.

The volume percentage of retained austenite is obtained by polishing a sample taken from a predetermined position to be described later from a quarter of the plate thickness direction (a surface corresponding to 1/4 of the plate thickness from the steel plate surface in the depth direction) Ray diffraction intensity is obtained by measuring the diffracted X-ray intensity of the plate thickness 1/4 surface. 110}, {200}, and {211} of the integrated intensities of the peaks of the {111}, {200}, {220}, and {311} planes of the retained austenite are used for the incident X- The intensity ratio of the combinations of all twelve to the integrated intensity of the peak of the surface is obtained, and the average value thereof is defined as the volume percentage of the retained austenite. When calculating the volume ratio of the retained austenite at the third portion (central portion), the sample is sampled from the central portion of the plate width with respect to the middle portion, the intermediate position between the leading end portion and the not-end portion of the steel in the rolling direction, and the plate width direction. When calculating the volume ratio of the retained austenite in the first portion (inner portion) and the second portion (outer portion), it is preferable that the volume ratio of the austenite And the sample is taken from the center of the plate width with respect to the plate width direction.

Average grain diameter of polygonal ferrite at the third portion (central portion): 4.0 m or less

Minuteness of crystal grains of polygonal ferrite contributes to improvement of YP and TS. Therefore, in order to secure a high YP and a high YR and a desired TS, it is necessary to set the average crystal grain diameter of the polygonal ferrite in the third region to 4.0 m or less. Preferably, it is 3.0 m or less. Further, in the present invention, the lower limit of the average grain diameter of the polygonal ferrite in the third region is not particularly limited, but it is preferably about 0.2 mu m industrially. In the present invention, not only the third portion but also the average crystal grain hardness of the polygonal ferrite of the first portion (inner portion) and the second portion (outer portion) is preferably 4.0 m or less, more preferably 3.0 m or less , And more preferably about 0.2 m or more.

Indicator of organization deviation (C)

Average crystal grain ratio of polygonal ferrite (internal / external part): 1.00 to 1.50

The ratio of the first portion (inner portion) / the second portion (outer portion) to the average grain size of the polygonal ferrite should be controlled within the range of 1.00 to 1.50, It is possible to obtain a high-yielding steel strip with no deviation of thickness.

Average grain diameter of martensite at the third portion (central portion): not more than 2.0 占 퐉

Minuteness of the grain size of the martensite contributes to improvement of the bending property and the stretch flangeability (hole expandability). Therefore, in order to ensure high bending property and high stretch flangeability (high hole expandability), it is necessary to suppress the average crystal grain size of the martensite at the third portion to 2.0 m or less. Preferably 1.5 m or less. Further, in the present invention, the lower limit of the mean grain diameter of the martensite at the third portion is not particularly limited, but it is preferably about 0.05 mu m or so on an industrial scale.

Average grain diameter of retained austenite at the third portion (central portion): not more than 2.0 占 퐉

Minuteness of the crystal grains of the retained austenite contributes to improvement of ductility and improvement of bending property and elongation flangeability (hole expandability). Therefore, in order to secure good ductility, bending property and stretch flangeability (hole expandability), it is necessary to set the average crystal grain diameter of the retained austenite at the third portion to be 2.0 占 퐉 or less. Preferably 1.5 m or less. In the present invention, the lower limit of the average grain diameter of the retained austenite in the third region is not particularly limited, but it is preferably about 0.05 mu m or so on the industrial scale.

The average grain diameter of polygonal ferrite, martensite and retained austenite can be determined as follows. That is, a sample is taken from a predetermined position to be described later, and a plate thickness 1/4 position of the plate thickness section (L section) parallel to the rolling direction of the sample is provided for the following observation. Using the above-described Image-Pro, the area of each of the polygonal ferrite lips, the martensite triples, and the residual austenite lips was determined in a field of view of 50 占 퐉 占 40 占 퐉, the circle equivalent diameter was calculated, The particle size of each particle is averaged. In addition, martensite and retained austenite are identified as Phase Map of EBSD. In the present invention, when the average crystal grain size is obtained, the grain size of 0.01 탆 or more is measured. Less than 0.01 탆 does not affect the present invention. The average grain diameter of each phase of the third portion (central portion) is obtained by calculating the center of gravity of the sample from the central portion of the plate width in terms of the center portion, the intermediate position between the leading end portion and the tip end portion of the steel in the rolling direction, Collecting. When calculating the average grain diameter of the polygonal ferrite in the first portion (inner portion) and the second portion (outer portion), it is preferable that, in the rolling direction, The sample is sampled at a position of 100 m from the end on the site side and from the center of the plate width for the plate width direction.

A value obtained by dividing the amount (mass%) of Mn in the retained austenite by the amount (mass%) of Mn in the polygonal ferrite at the third portion (central portion): 2.00 or more

It is very important in the present invention to set the value obtained by dividing the amount (mass%) of Mn in the retained austenite by the amount (mass%) of Mn in the polygonal ferrite to the value of 2.00 or more. This is because, in order to secure good ductility, it is necessary to increase the amount of stable retained austenite in which Mn is concentrated. In the present invention, the upper limit of the value obtained by dividing the amount (mass%) of Mn in the retained austenite by the amount (mass%) of Mn in the polygonal ferrite is not limited, but from the viewpoint of securing the stretch flangeability, .

The amount (mass%) of Mn in the retained austenite and the amount (mass%) of Mn in the polygonal ferrite can be obtained as follows. For the rolling direction, a sample is taken from the central portion of the width of the steel plate in the middle position between the leading and trailing ends of the steel strip and the width direction of the steel plate, and the sample is subjected to EPMA (Electron Probe Micro Analyzer; Probe microanalyzer), and the distribution state of Mn relative to each phase in the rolling direction section at the plate thickness 1/4 position is quantified. Then, the amount of Mn of 30 remaining austenite grains and 30 ferrite grains is analyzed. And the Mn amount obtained from the analysis result can be averaged.

In the microstructure of the present invention, a steel plate such as granulafelite, needle ferrite, bainitic ferrite, tempered martensiticite, pearlite and cementite is usually observed in addition to polygonal ferrite or martensite as described above. (Excluding cementite in pearlite). The effect of the present invention is not impaired even if these tissues are contained in an area ratio of 10% or less.

In the present invention, it is preferable that the? -Phase having the hcp structure is contained in an area ratio of 2% or more. Here, there is a risk of embrittlement in a steel containing a large amount of an e phase having an hcp structure. However, as in the present invention, when the ε-phase having an appropriate amount of the hcp structure is finely dispersed in the grain boundaries and the mouths of the ferrite and the non-recrystallized ferrite, a good balance of strength and ductility is ensured and excellent vibration damping performance is exhibited.

In addition, the ε phase with hcp structure, martensite and retained austenite can be identified using the phase map of EBSD. In the present invention, the upper limit of the area ratio of the epsilon phase is not limited, but there is a risk of embrittlement, so it is preferable that the upper limit is about 35%. By satisfying the above-described requirements, it is possible to intermittently develop the processed organic transformation (TRIP) phenomenon, which is a main cause of improvement in ductility, to the end of machining of the steel sheet, and so-called stable formation of retained austenite can be achieved.

(Manufacturing conditions)

A method of manufacturing a high strength steel strip according to the present invention includes the steps of heating a steel slab having the above composition, hot rolled steel slab to obtain a hot rolled steel sheet, winding the hot rolled steel sheet to form a hot rolled coil, A step of pickling the hot rolled sheet of the hot rolled coil to remove the scale; a hot rolled sheet annealing step of subjecting the hot rolled coil to heat treatment in a batch type furnace to obtain a hot rolled annealed sheet coil; A step of cold-rolling the hot-rolled annealing plate of the hot-rolled annealing plate coil to obtain a cold-rolled sheet, and thereafter performing cold-rolled sheet annealing on the cold-rolled sheet to obtain a high- Hereinafter, the manufacturing conditions and the reason for the limitation will be described.

Heating temperature of steel slab: 1100 ℃ ~ 1300 ℃

The precipitate present in the heating step of the steel slab (or simply referred to as the slab) exists as a coarse precipitate in the finally obtained steel sheet, and does not contribute to the strength. For this reason, the Ti and Nb-based precipitates precipitated at the time of casting need to be redissolved. Here, when the heating temperature of the steel slab is less than 1100 ° C, sufficient solubility of the carbide is difficult, and the risk of occurrence of trouble during hot rolling due to an increase in rolling load is increased. Therefore, the heating temperature of the steel slab needs to be 1100 DEG C or higher. Also, from the viewpoint of scaling off defects such as bubbles and segregation in the surface layer of the slab and reducing cracks and unevenness on the surface of the steel sheet to achieve a smooth steel sheet surface, the heating temperature of the steel slab needs to be not lower than 1100 占 폚. On the other hand, when the heating temperature of the steel slab exceeds 1300 DEG C, the scale loss increases with the increase of the oxidation amount. Therefore, the heating temperature of the steel slab needs to be 1300 DEG C or less. Therefore, it is necessary to set the heating temperature of the slab to 1100 DEG C or more and 1300 DEG C or less. It is preferably in the range of 1150 DEG C to 1250 DEG C or less.

The steel slab is preferably produced by the continuous casting method to prevent macro segregation, but it is also possible to produce the steel slab by the roughing method or the thin slab casting method. In the present invention, the conventional method may be used in which after the steel slab is manufactured, the steel slab is once cooled to room temperature, and then heated again. Further, in the present invention, an energy saving process such as direct rolling or direct rolling in which the steel sheet is rolled immediately after being charged into a heating furnace without cooling to room temperature or after a little boiling heat is applied can be applied without any problem. Further, the steel slab is formed into a sheet bar by rough rolling under ordinary conditions. When the heating temperature is lowered, from the viewpoint of preventing troubles during hot rolling, It is preferred to further heat the bars.

Finishing of hot rolling Rolling out temperature: 750 ℃ or more and 1000 ℃ or less

The steel slab after heating is hot-rolled by rough rolling and finish rolling to form a hot-rolled sheet. At this time, if the finishing temperature exceeds 1000 캜, the amount of oxide (scale) to be produced increases sharply, the interface between the steel and the oxide becomes rough, and the surface quality of the steel sheet after the pickling and cold rolling is deteriorated have. In addition, if the remaining hot rolled steel sheet remains after the pickling of the hot rolled steel sheet, the ductility and stretch flangeability of the steel sheet are adversely affected. Furthermore, the grain size of the crystal grains becomes excessively large, which may cause surface roughness of the pressed product at the time of processing. On the other hand, when the finishing temperature is less than 750 캜, the rolling load is increased and the reduction rate of the austenite in the non-recrystallized state is increased. As a result, an abnormal texture is developed in the steel sheet, the in-plane anisotropy in the final product becomes remarkable, not only the uniformity (material stability) of the material is deteriorated, but also the ductility of the steel sheet itself is deteriorated. Therefore, in the present invention, it is necessary to set the temperature at the finish rolling-out side of hot rolling to 750 ° C or higher and 1000 ° C or lower. And preferably in the range of 800 DEG C or more and 950 DEG C or less.

Coiling temperature: 300 ° C or more and 750 ° C or less

The coiling temperature is an average value of the coiling temperatures of the entire length of the hot rolling coils after hot rolling. If the coiling temperature after hot rolling exceeds 750 캜, the crystal grain size of the ferrite in the hot rolled steel sheet becomes large, and it becomes difficult to secure the desired strength. On the other hand, when the coiling temperature after hot rolling is less than 300 占 폚, the hot rolled sheet strength is increased, the rolling load during cold rolling is increased, or the plate shape is defective. Therefore, the coiling temperature after hot rolling is required to be not less than 300 ° C and not more than 750 ° C. And preferably in the range of 400 DEG C or more and 650 DEG C or less.

Further, in the present invention, the hot rolled sheet may be subjected to continuous rolling by bonding the rough rolled plates to each other at the time of hot rolling. Further, the rough rolling plate may be once wound. In order to reduce the rolling load during hot rolling, a part or all of the finish rolling may be lubricated and rolled. Performing lubrication rolling is effective also from the viewpoints of uniformity of the steel sheet shape and uniformity of materials. The friction coefficient at the time of lubrication rolling is preferably 0.10 or more and 0.25 or less.

The hot rolled sheet of the hot rolled coil manufactured through such a process is pickled. Pickling is important for the good chemical treatment of the high-strength steel sheet of the final product and for securing the quality of the plating, since oxides on the surface of the steel sheet can be removed. The pickling may be performed once, or may be performed in a plurality of circuits.

Hot-rolled sheet annealing: Maintains the outer edge of the hot-rolled coil at 216 ° C or higher and 129600 ° C or less at a predetermined temperature (Ac ₁ transformation point + 20 ° C) or higher (Ac ₁ transformation point + 120 ° C)

In the present invention, for carrying out the annealing of hot-rolled steel coil using a batch-type heating, then, the outer gwonbu of hot-rolled sheet coil (Ac ₁ transformation point + 20 ℃) over (Ac ₁ transformation point + 120 ℃) given below It is very important for the present invention to maintain the time at 21600 s or higher and 129600 s or lower at the temperature. When the annealing temperature of the hot-rolled sheet annealing is less than (Ac ₁ transformation point + 20 ° C), (Ac ₁ transformation point + 120 ° C), and the holding time is less than 21600 s, The concentration of Mn in the steel sheet does not progress, and it becomes difficult to secure a sufficient retained austenite volume ratio after the final annealing, and the ductility is lowered. On the other hand, if it is kept over 129600 s, the concentration of Mn in the austenite is saturated and the structure is not only coarsened but also the cost is increased. Thus, the annealing of hot-rolled sheet coil, the outer gwonbu of hot-rolled sheet coil (Ac ₁ transformation point + 20 ℃) over (Ac ₁ transformation point + 120 ℃) at a predetermined temperature or less, by holding time of less than 21600 s greater than 129600 s do. The holding time is preferably 25000 s or more.

After the above-mentioned heat treatment, it is cooled to room temperature. The cooling method and the cooling rate at that time are not particularly specified, but it is preferable that the cooling method and the cooling rate in the batch annealing are set to be either liquefied or air-cooled.

Reduction rate of cold rolling: 50% or less

In the cold rolling of the present invention, the reduction rate is 50% or less. When cold rolling is performed at a reduction ratio exceeding 50%, coarse polygonal ferrite is produced at the time of heat treatment. As a result, a soft phase is obtained in the steel sheet, and the strength-ductility balance is lowered. In addition, the bendability and stretch flangeability (hole expandability) of the steel sheet also deteriorate. The rolling reduction in cold rolling is preferably 30% or more.

Cold-rolled sheet annealing cold-rolled sheet to Ac ₁ transformation point or more, (Ac ₁ transformation point + 100 ℃) at a predetermined temperature or less, maintained between 20 ~ 900 s

It is very important for the present invention to maintain the cold-rolled sheet at a temperature range of Ac ₁ transformation point or more and (Ac ₁ transformation point + 100 ° C) or less for 20 to 900 s at the time of annealing the cold-rolled sheet. When the annealing temperature of the cold-rolled sheet is less than the Ac ₁ transformation point (Ac ₁ transformation point + 100 ° C) and the holding time is less than 20 s, the concentration of Mn in the austenite does not progress, It becomes difficult to secure the volume ratio of the knit, and the ductility is lowered. On the other hand, in the case of holding for more than 900 s, the area ratio of the non-recrystallized ferrite is lowered, and the abnormal interface amount between ferrite and the hard second phase (martensite and retained austenite) increases and YP decreases, YR also decreases.

(Example)

A steel having the composition shown in Table 1 and the balance consisting of Fe and inevitable impurities was melted in a converter, and was made into a slab by a continuous casting method. The obtained slab was treated under various conditions shown in Table 2 to obtain a cold-rolled steel strip (CR). Further, a part of the cold-rolled steel sheet was further subjected to a zinc plating treatment. As the hot-dip galvanizing bath, a zinc bath containing 0.19% by mass of Al was used in the hot-dip galvanized steel (GI), and a zinc bath containing 0.14% by mass of Al was used in the galvannealed hot-dip galvanized steel strip (GA). All the baths were set at 465 DEG C, and the amount of plating adhered was 45 g / m < 2 > (double-side plating) per one side. In the GA, the Fe concentration in the plated layer was adjusted to 9 mass% or more and 12 mass% or less.

The results of investigation of microstructure and average grain diameter of each phase in the third region (center portion) of the thus obtained steel strip are shown in Table 3, and in Table 3, in the first region (inner portion) and the second region And the average crystal grain size of each phase are shown in Table 4. < tb > < TABLE > Table 5 shows the tensile properties and hole expandability of the steel strip at the third portion (central portion). Table 5 also shows the results of investigation of the surface properties and productivity of steel strips (throughput). Table 6 shows the tensile properties and hole expandability of the steel strips in the first portion (inner portion) and the second portion (outer portion).

The Ac ₁ transformation point was obtained by using the following equation.

Ac ₁ transformation point (° C)

= 751-16 x% C + 11 x Si% -28 x% Mn -5.5 x Cu x 16 x Ni x + 13 x%

Here, (% C), (% Si), (% Mn), (% Ni), (% Cu), (% Cr) and (% Mo) are the contents (mass%) of each element in the steel.

The tensile test was carried out in accordance with JIS Z 2241 (2011) using a JIS No. 5 specimen in which the tensile direction was the direction perpendicular to the rolling direction of the steel sheet, and YP, YR, TS and EL were measured Respectively. YR is a value obtained by dividing YP by TS and expressed as a percentage. In the present invention, EL ≥ 26%, TS: 1180 MPa, EL ≥ 22%, TS: 1470 MPa, where YR ≥ 68% and TS EL ≥ 22000 ㎫ ·% , And EL ≥ 18% in the class. TS: 980 MPa class is a steel sheet having a TS of 980 MPa or more and less than 1180 MPa, TS: 1180 MPa class is a steel sheet having a TS of 1180 MPa or more and less than 1470 MPa, TS: 1470 MPa class Is a steel sheet having a TS of 1470 MPa or more and less than 1760 MPa.

The hole expandability was measured in accordance with JIS Z 2256 (2010). Each steel sheet thus obtained was cut into 100 mm x 100 mm, and then a hole having a diameter of 10 mm was punched out with a clearance of 12% 1%. Subsequently, a punch of a cone of 60 ° was press-fitted into the hole and the pore diameter at the crack generation limit was measured in a state where the press force was suppressed to 9 ton (88.26 kN) by using a die having an inner diameter of 75 mm. Further, from the following expression, the limit hole expanding rate (?) (%) Was obtained, and the hole expandability was evaluated from the value of the limit hole expanding rate.

(%) = {(D _f -D ₀ ) / D ₀ } × 100

D _f is the hole diameter (mm) at the time of cracking, and D ₀ is the initial hole diameter (mm). Further, in the present invention, it was judged that the case of λ: ≥ 20% for TS: 980 MPa, the case of λ: ≥ 15% for TS: 1180 MPa, and the case of λ ≥ 10% for TS: 1470 MPa.

As evaluation of the steel strip, the ducting properties during hot rolling and cold rolling, the surface properties of the final annealing plate, and productivity were evaluated.

The ductability of hot rolling and cold rolling was determined to be poor when the risk of occurrence of trouble during rolling was increased due to an increase in rolling load. The evaluation results are shown in Table 5.

The surface properties of the final annealing plate were judged to be defective when it was impossible to scale off defects such as bubbles and segregation in the slab surface layer and cracks and irregularities on the surface of the steel sheet were increased and a smooth steel sheet surface could not be obtained. The surface properties of the final annealing plate are such that when the amount of oxide (scale) is sharply increased, the interface between the base metal and the oxide is roughened, the surface quality after pickling or cold rolling is deteriorated or the hot rolling scale is removed after pickling It was judged to be defective even if there were some remaining residues. The evaluation results are shown in Table 5.

(2) the time required to correct the shape of the hot-rolled sheet in order to proceed to the next step, and (3) the time required to maintain the annealing process is long. Respectively. It was judged that the case corresponding to neither of (1) to (3) was "good" and the case corresponding to any of (1) to (3) was "bad". The measurement results are shown in Table 5.

From the above results, it can be seen that a high strength steel strip excellent in formability of TS ≥ 980 MPa, YR ≥ 68%, and TS EL ≥ 22000 ㎫ ·% is obtained over the entire length of the coil according to the present invention. On the other hand, in the comparative example, at least one of the characteristics of YR, TS, EL, and? Is inferior, or deterioration of the characteristics at the coil end or the end portion is shown.

According to the present invention, even when the heating of the hot-rolled coil is carried out using a batch-type heating furnace, it is possible to obtain a high- A high-strength steel strip excellent in moldability can be stably produced. Therefore, by applying the high-strength steel strip of the present invention to, for example, an automotive structural member, the fuel economy can be improved by reducing the weight of the vehicle body, and the industrial utility value is very high.

Claims (2)

C: not less than 0.030% but not more than 0.250%, Si: not less than 0.01% and not more than 3.00%, Mn: not less than 4.20% : Not less than 0.0005% and not more than 0.0100%, Al: not less than 0.010% and not more than 2.000%, and Ti: not less than 0.005% and not more than 0.200%, and the balance of Fe and inevitable impurities, Or less,
Subjecting the steel slab to hot rolling at a finishing rolling output temperature of 750 ° C or higher and 1000 ° C or lower to obtain a hot rolled steel sheet,
Winding the hot-rolled sheet at a coiling temperature of 300 ° C or higher and 750 ° C or lower to obtain a hot-
A step of pickling the hot rolled sheet of the hot rolled coil to remove scale,
A hot-rolled sheet annealing step of subjecting the hot-rolled coil to heat treatment in a batch type furnace to obtain a hot-
Thereafter, the hot-rolled annealing plate of the hot-rolled annealing plate coil is cold-rolled at a reduction ratio of 50% or less to obtain a cold-
Then, the cold-rolled sheet, Ac ₁ transformation point or more, (Ac ₁ transformation point + 100 ℃) and to 20 s at least 900 s or less at a predetermined temperature or less oil, subjected to cold-rolled sheet annealing is cooled and then a step for obtaining a high-strength steel strip The method comprising the steps of:
The hot-rolled sheet annealing process, and subjected to other gwonbu of the hot-rolled sheet coil (Ac ₁ transformation point + 20 ℃) or more, 21 600 at a predetermined temperature not higher than (Ac ₁ transformation point + 120 ℃) in the condition of maintaining s exceeds more than 129600 s ,
A portion of the hot-rolled annealing plate coil which is the inner portion of the hot-rolled annealing plate coil is referred to as a first portion, a portion of the hot-rolled annealing plate coil which is the outer portion of the hot- As three sites,
The steel structure of the third portion has an area ratio of 15% to 55% of polygonal ferrite, 8% or more of non-recrystallized ferrite, 15% to 30% of martensite, : 12% or more,
The area ratio of the polygonal ferrite of the first portion / the area ratio of the polygonal ferrite of the second portion is 1.00 to 1.50, the volume ratio of the retained austenite of the first portion / the retained austenite of the second portion Is in the range of 0.75 to 1.00,
In addition, the third region preferably has an average grain diameter of 4.0 탆 or less, a mean grain diameter of martensite of 2.0 탆 or less, and an average grain diameter of residual austenite of 2.0 탆 or less of polygonal ferrite,
The average grain diameter of the polygonal ferrite in the first portion / the average grain diameter of the polygonal ferrite in the second portion is 1.00 to 1.50,
The method for producing a high-strength steel cord according to claim 1, wherein a value obtained by dividing the amount (mass%) of Mn in the retained austenite by the amount (mass%) of Mn in the polygonal ferrite is 2.00 or more. The method according to claim 1,
0.005% or more and 1.000% or less of Cr, 0.005% or more and 1.000% or less of V, 0.005% or less of V, 0.005% or more and 0.0050% or less of Ni, 0.002% or more and 0.200% or less of Sb, 0.001% or more and 0.010% or less of Ta, 0.001% or more and 0.50% , Ca: not less than 0.0005% and not more than 0.0050%, Mg: not less than 0.0005% and not more than 0.0050%, and REM: not less than 0.0005% and not more than 0.0050%.