JP6699711B2 - High-strength steel strip manufacturing method - Google Patents

Description

  The present invention relates to a method for producing a high-strength steel strip, and in particular, a steel strip having excellent formability and a high yield ratio suitable for use as a member used in the industrial field of automobiles, electric equipment and the like. Is to be obtained stably.

  In recent years, improving fuel efficiency of automobiles has become an important issue from the viewpoint of global environment conservation. For this reason, there has been active movement to reduce the thickness of the vehicle body by increasing the strength of the vehicle body material and to reduce the weight of the vehicle body itself.

  However, in general, higher strength of a steel sheet causes lowering of formability. Therefore, if higher strength is aimed at, the formability of the steel sheet lowers, causing problems such as cracking during forming. Therefore, it is not possible to simply reduce the thickness of the steel plate.

  Therefore, development of a material having both high strength and high formability is desired. Further, a steel sheet having a tensile strength (TS) of 980 MPa or more is required to have a property of large impact absorption energy in addition to the high formability. In order to improve the collision absorption energy, it is effective to increase the yield ratio (YR). This is because when the yield ratio is high, the steel sheet can efficiently absorb the collision energy with a low deformation amount.

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

  Further, Patent Document 2 proposes to manufacture a high-strength steel sheet having an excellent balance of strength and ductility by performing heat treatment in a two-phase region of ferrite and austenite using high-Mn steel. .

JP 61-157625 A WO2016/067626A1

  The steel sheet described in Patent Document 1 is manufactured by subjecting a steel sheet containing C, Si and Mn as basic components to austenite, and then performing so-called austempering treatment in which the steel sheet is quenched in the bainite transformation temperature range and kept isothermal. Then, when this austempering treatment is performed, residual austenite is generated due to the concentration of C in 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 markedly deteriorated, and It is difficult to put it into practical use as a steel sheet for use.

  Further, Patent Document 2 forms a retained austenite phase effective for strain-induced transformation by controlling the phase fraction and grain size of ferrite and austenite by annealing in a two-phase region and further controlling the amounts of C and Mn in the austenite phase. This is an epoch-making invention that was successful in doing so. However, when actually attempting to manufacture the steel sheet of Patent Document 2 at the manufacturing site, especially when the hot-rolled sheet coil is heated to a designated temperature using a batch-type heating furnace, the temperature near the center of the coil is unlikely to rise Since the temperature cannot be reached, the target structure cannot be obtained in the inner winding part of the hot-rolled annealed plate coil after batch heating, and there is a problem that the characteristics vary in the longitudinal direction of the coil and the yield deteriorates. It was happening.

  The present invention relates to an improvement of the invention described in Patent Document 2 listed above, and even when the hot-rolled sheet coil is heated using a batch-type heating furnace, TS≧980 MPa, YR over the entire length of the coil. An object of the present invention is to provide a method capable of stably producing a high-strength steel strip having excellent formability of ≧68% and TS×EL≧22000 MPa·%.

Now, the inventors have achieved the above-mentioned problems, excellent in formability, and in order to stably produce a high-strength steel strip having a high yield ratio and tensile strength over the entire length of the coil, a method for producing a steel sheet, particularly a batch. We have earnestly studied the heating conditions in the heating furnace. As a result, the hot-rolled sheet annealing process in the batch type heating furnace was performed at a predetermined temperature of (Ac ₁ transformation point +20°C) or more and (Ac ₁ transformation point +120°C) or less for the outer winding part of the hot-rolled coil at a predetermined temperature of more than 21600s and 129600s It has been found that the intended purpose can be advantageously achieved by carrying out under the conditions of holding for the following time. The present invention has been made based on the above findings.

That is, the gist of the present invention is as follows.
[1]% by mass, C: 0.030% or more and 0.250% or less, Si: 0.01% or more and 3.00% or less, Mn: more than 4.20% and 6.00% or less, P: 0. 001% to 0.100%, S: 0.0001% to 0.0200%, N: 0.0005% to 0.0100%, Al: 0.010% to 2.00% and Ti: A step of heating a steel slab containing 0.005% or more and 0.200% or less and a balance of Fe and inevitable impurities to 1100° C. or more and 1300° C. or less;
A step of hot rolling the steel slab at a finish rolling outlet temperature of 750° C. or higher and 1000° C. or lower to obtain a hot rolled sheet;
Winding the hot rolled sheet at a winding temperature of 300° C. or higher and 750° C. or lower to form a hot rolled sheet coil;
Then, a step of removing the scale by subjecting the hot rolled plate of the hot rolled plate coil to pickling,
The hot rolled sheet coil is heat-treated in a batch-type heating furnace, and a hot rolled sheet annealing step of forming a hot rolled annealed sheet coil,
After that, a step of cold rolling the hot rolled annealed sheet of the hot rolled annealed sheet coil at a rolling reduction of 50% or less to obtain a cold rolled sheet,
Then, the cold-rolled sheet is held at a predetermined temperature of Ac ₁ transformation point or higher and (Ac ₁ transformation point+100° C.) or lower for 20 seconds or more and 900 seconds or less, and then cold-rolled sheet annealing is performed to cool the high-strength steel strip. The process of obtaining
In a method of manufacturing a high strength steel strip having
In the hot-rolled sheet annealing process in the batch-type heating furnace, the outer winding portion of the hot-rolled sheet coil is heated at a predetermined temperature of (Ac ₁ transformation point +20°C) or more and (Ac ₁ transformation point +120°C) or less for more than 21600s and 129600s. Perform under the conditions that hold below,
With respect to the longitudinal direction of the high-strength steel strip, a portion that was an inner winding portion of the hot-rolled annealed plate coil is a first portion, a portion that was an outer winding portion of the hot-rolled annealed sheet coil is a second portion, and the heat The part that was the central part of the annealed plate coil was set as the third part,
The steel structure of the third portion has an area ratio of polygonal ferrite: 15% or more and 55% or less, unrecrystallized ferrite: 8% or more, martensite: 15% or more and 30% or less, and a volume ratio of retained austenite: Contains 12% or more,
And, the area ratio of the polygonal ferrite in the first portion/the area ratio of the polygonal ferrite in the second portion is 1.00 to 1.50, the volume ratio of the retained austenite in the first portion/the second portion. The volume ratio of retained austenite is 0.75 to 1.00,
The third portion has an average crystal grain size of polygonal ferrite: 4.0 μm or less, an average crystal grain size of martensite: 2.0 μm or less, and an average crystal grain size of retained austenite: 2.0 μm or less. ,
And, the average crystal grain size of the polygonal ferrite in the first portion/the average crystal grain size of the polygonal ferrite in the second portion is 1.00 to 1.50,
Further, in the third portion, a value obtained by dividing the Mn amount (mass%) in the retained austenite by the Mn amount (mass%) in the polygonal ferrite is 2.00 or more, a high strength steel. Method of manufacturing obi.

  [2] The composition of the components is, in mass %, Nb: 0.005% or more and 0.200% or less, B: 0.0003% or more and 0.0050% or less, Ni: 0.005% or more and 1.000. % Or less, Cr: 0.005% or more and 1.000% or less, V: 0.005% or more and 0.500% or less, Mo: 0.005% or more and 1.000% or less, Cu: 0.005% or more 1 0.000% or less, Sn: 0.002% or more and 0.200% or less, Sb: 0.002% or more and 0.200% or less, Ta: 0.001% or more and 0.010% or less, Ca: 0.0005% Or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, and REM: 0.0005% or more and 0.0050% or less, at least one element selected from the above is contained. The method for producing a high-strength steel strip according to [1] above.

  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 MPa·% over the entire length of the coil. Therefore, by applying the high-strength steel strip manufactured according to the present invention to, for example, an automobile structural member, it is possible to improve fuel efficiency by reducing the weight of the vehicle body, and its industrial utility value is extremely large.

(Ingredient composition)
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,% indication concerning the composition of steel and slab means mass %. The balance of the composition of the steel strip is Fe and inevitable impurities.

C: 0.030% or more and 0.250% or less 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 the ductility of steel. If the amount of C is less than 0.030%, it is difficult to secure the desired amount of martensite, and the desired strength cannot be obtained. Further, it is difficult to secure a sufficient amount of retained austenite, and good ductility cannot be obtained. On the other hand, if C is added excessively in excess of 0.250%, the amount of hard martensite becomes excessive, microvoids at the grain boundaries of martensite increase, and cracks occur during bending and hole expanding tests. Propagation is facilitated, and bendability and stretch flangeability deteriorate. Further, excessive addition of C remarkably hardens the welded portion and the heat-affected zone and deteriorates the mechanical characteristics of the welded portion, so that spot weldability, arc weldability and the like deteriorate. From these viewpoints, the amount of C is set to 0.030% or more and 0.250% or less. The range is preferably 0.080% or more and 0.200% or less.

Si: 0.01% or more and 3.00% or less Si improves the work hardening ability of ferrite, and is an element effective in ensuring good ductility. If the Si amount is less than 0.01%, the effect of addition becomes poor, so the lower limit is made 0.01%. On the other hand, excessive addition of Si in excess of 3.00% not only causes embrittlement of steel, but also causes deterioration of surface properties due to generation of red scale and the like. For this reason, the amount of Si is made into 0.01% or more and 3.00% or less. The range is preferably 0.20% or more and 2.00% or less.

Mn: more than 4.20% and 6.00% or less Mn is an extremely important element in the present invention. Mn is an element that stabilizes retained austenite, and is effective in ensuring good ductility. Further, Mn is also an element capable of increasing the strength of steel by solid solution strengthening. Further, due to the Mn concentration in the retained austenite, the ε phase having the hcp structure can be secured at 2% or more, and further, the retained austenite can be secured at a large volume ratio of 12% or more. .. Such an effect is not recognized until the Mn content in steel exceeds 4.20%. On the other hand, excessive addition of Mn in excess of 6.00% causes a cost increase. From such a viewpoint, the amount of Mn is set in the range of more than 4.20% and 6.00% or less. Preferably, it is in the range of 4.80% or more and 6.00% or less.

P: 0.001% or more and 0.100% or less P is an element that is effective for solid solution strengthening and can be added according to desired strength. It is also an element that promotes ferrite transformation and is effective for forming a composite structure of steel sheets. In order to obtain such an effect, the amount of P in a steel plate needs to be 0.001% or more. On the other hand, if the amount of P exceeds 0.100%, the weldability is deteriorated, and when alloying zinc plating, the alloying rate is reduced and the quality of zinc plating is impaired. Therefore, the amount of P is made 0.001% or more and 0.100% or less, preferably 0.005% or more and 0.050% or less.

S: 0.0001% or more and 0.0200% or less S segregates at grain boundaries to embrittle the steel during hot working, and is present as a sulfide to reduce the local deformability 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 due to restrictions in production technology. Therefore, the S amount is set to be 0.0001% or more and 0.0200% or less. It is preferably 0.0001% or more and 0.0100% or less, and more preferably 0.0001% or more and 0.0050% or less.

N: 0.0005% or more and 0.0100% or less N is an element that deteriorates the aging resistance of steel. In particular, when the amount of N exceeds 0.0100%, deterioration of aging resistance becomes remarkable. Therefore, the smaller the amount of N is, the more preferable it is, but the amount of N is set to 0.0005% or more due to the limitation in production technology. Therefore, the N content is set to 0.0005% or more and 0.0100% or less, preferably 0.0010% or more and 0.0070% or less.

Al: 0.010% or more and 2.000% or less Al is an element effective in expanding the two-phase region of ferrite and austenite and reducing the annealing temperature dependency, that is, improving the material stability. Also, Al acts as a deoxidizing agent and is an element effective in maintaining the cleanliness of steel. However, if the amount of Al is less than 0.010%, the effect of addition is poor, so the lower limit is made 0.010%. On the other hand, addition of a large amount of more than 2.000% increases the risk of cracking of the slab during continuous casting and reduces manufacturability. From this point of view, the Al content when added is in the range of 0.010% or more and 2.000% or less. The range is preferably 0.200% or more and 1.200% or less.

Ti: 0.005% or more and 0.200% or less Ti is an important additional element in the present invention. Ti is not only effective for precipitation strengthening of steel, but also secures a desired amount of unrecrystallized ferrite and effectively contributes to increasing the yield ratio of the steel sheet. In addition, by utilizing relatively hard unrecrystallized ferrite, it is possible to reduce the hardness difference from the hard second phase (martensite or retained austenite), which also contributes to the improvement of stretch flangeability. And these effects are acquired by addition of 0.005% or more of Ti. On the other hand, when the Ti content in the steel sheet exceeds 0.200%, the hard martensite content becomes excessive and microvoids at the crystal grain boundaries of martensite increase, causing cracking during bending and hole expanding tests. Propagation is facilitated, and the bendability and stretch flangeability of the steel sheet deteriorate. Therefore, the addition amount of Ti is set to the range of 0.005% or more and 0.200% or less. The range is preferably 0.010% or more and 0.100% or less.

  Although the essential components have been described above, other elements described below can be appropriately contained in the present invention.

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. Further, similar to the effect of adding Ti, it secures a desired amount of unrecrystallized ferrite and contributes to a high yield ratio of the steel sheet. In addition, by utilizing the relatively hard unrecrystallized ferrite, the hardness difference from the hard second phase (martensite or retained austenite) can be reduced, which also contributes to the improvement of stretch flangeability. On the other hand, when the amount of Nb exceeds 0.200%, the amount of hard martensite becomes excessive, the number of microvoids at the crystal grain boundaries of martensite increases, and crack propagation during bending test and hole expanding test. It will be easier to proceed. As a result, the bendability and stretch flangeability of the steel sheet deteriorate. It also causes a cost increase. Therefore, when Nb is added, the range is 0.005% or more and 0.200% or less. The range is preferably 0.010% or more and 0.100% or less.

B: 0.0003% or more and 0.0050% or less B has the effect of suppressing the generation and growth of ferrite from the austenite grain boundaries, and enables flexible structure control, so is added as necessary. be able to. The effect of addition is obtained at 0.0003% or more. On the other hand, when the amount of B exceeds 0.0050%, the formability of the steel sheet decreases. Therefore, when B is added, the range is 0.0003% or more and 0.0050% or less. Preferably, it is in the range of 0.0005% or more and 0.0030% or less.

Ni: 0.005% or more and 1.000% or less Ni is an element that stabilizes retained austenite, is effective in ensuring good ductility, and is an element that increases the strength of steel by solid solution strengthening. The effect of addition is obtained at 0.005% or more. On the other hand, when added in excess of 1.000%, the amount of hard martensite becomes excessive, microvoids at the grain boundaries of martensite increase, and crack propagation progresses during bending and hole expanding tests. It will be easier. As a result, the bendability and stretch flangeability of the steel sheet deteriorate. It also causes a cost increase. Therefore, when Ni is added, the range is 0.005% or more and 1.000% or less.

Cr: 0.005% or more and 1.000% or less, V: 0.005% or more and 0.500% or less, Mo: 0.005% or more and 1.000% or less Cr, V and Mo are balances of strength and ductility. Is an element that can be added if necessary because it has the effect of improving The effect of addition is obtained with Cr: 0.005% or more, V: 0.005% or more and Mo: 0.005% or more. On the other hand, when Cr is over 1.000%, V: 0.500%, and Mo: over 1.000%, the amounts of hard martensite become excessive and the grain boundaries of martensite increase. The number of microvoids increases, and crack propagation is facilitated during bending and hole expanding tests. As a result, the bendability and stretch flangeability of the steel sheet deteriorate. It also causes a cost increase. Therefore, when these elements are added, Cr: 0.005% or more and 1.000% or less, V: 0.005% or more and 0.500% or less, and Mo: 0.005% or more and 1.000%, respectively. The range is as follows.

Cu: 0.005% or more and 1.000% or less Cu is an element effective for strengthening steel. The effect of addition is obtained at 0.005% or more. On the other hand, when added in excess of 1.000%, the amount of hard martensite becomes excessive, microvoids at the grain boundaries of martensite increase, and crack propagation progresses during bending and hole expanding tests. It will be easier. As a result, the bendability and stretch flangeability of the steel sheet deteriorate. Therefore, when Cu is added, the range is 0.005% or more and 1.000% or less.

Sn: 0.002% or more and 0.200% or less, Sb: 0.002% or more and 0.200% or less Sn and Sb are desorption of a thickness region of several tens of μm of the steel sheet surface layer caused by nitriding or oxidation of the steel sheet surface. From the viewpoint of suppressing charcoal, it is added as necessary. In this way, by suppressing nitriding and oxidation, it is effective to prevent the amount of martensite on the surface of the steel sheet from decreasing and to secure TS and material stability. In order to obtain this effect, 0.002% or more of each must be added. On the other hand, if over 0.200% is added excessively, toughness is lowered. Therefore, when Sn and Sb are added, the range 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 and contributes to strengthening steel. In addition, by partially forming a solid solution with Nb carbide or Nb carbonitride to form a composite precipitate such as (Nb,Ta)(C,N), coarsening of the precipitate is suppressed, and precipitation strengthening It is considered to have the effect of stabilizing the contribution to the strength improvement of the steel sheet. Here, the effect of adding Ta can be obtained by setting the content of Ta to 0.001% or more. On the other hand, if Ta is added excessively, the effect of addition is saturated and the alloy cost is increased. Therefore, when Ta is added, the range is 0.001% or more and 0.010% or less.

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% or less Ca, Mg and REM have a sulfide shape. It is an element that is effective in forming a sphere and improving the adverse effect of sulfide on the hole expandability (stretch flangeability). In order to obtain this effect, addition of 0.0005% or more is required. On the other hand, excessive addition of each exceeding 0.0050% causes an increase in inclusions and the like, causing surface and internal defects of the steel sheet. Therefore, when Ca, Mg and REM are added, the range is 0.0005% or more and 0.0050% or less.

(Steel structure)
In the present invention, by appropriately controlling the heating conditions in the batch type heating furnace, the conventionally problematic variation in the structure and the characteristics in the longitudinal direction of the coil is improved. Here, in the present specification, with respect to the longitudinal direction of the manufactured high-strength steel strip, the part that was the inner winding part of the hot-rolled annealed plate coil was the first part and the outer winding part of the hot-rolled annealed plate coil. The part is called the second part, and the part that was the central part of the hot-rolled annealed plate coil is called the third part. Here, the "inner winding part of the hot rolled annealed plate coil" refers to a range from the inner end of the hot rolled annealed plate coil to 150 m in the longitudinal direction. The "outer winding portion of the hot rolled annealed plate coil" refers to a range from the outer end of the hot rolled annealed plate coil to 150 m in the longitudinal direction. The "central part of the hot rolled annealed plate coil" refers to a part other than the inner winding part and the outer winding part of the hot rolled annealed plate coil.

  In the present invention, as an index of the variation of the structure, (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/the The volume fraction of retained austenite at two sites and (C) the average crystal grain size of polygonal ferrite at the first site/the average crystal grain size of polygonal ferrite at the second site are adopted, and these are set to predetermined values. Set in the range. By controlling these ratios within the ranges described below, it is possible to improve the characteristic dispersion in the longitudinal direction of the coil, which has been a concern in the past, especially the deterioration of the characteristics in the coil inner winding part of the hot-rolled annealed plate, so that there is no variation over the entire length of the coil. It has excellent characteristics.

Area ratio of polygonal ferrite in the third part (central part): 15% or more and 55% or less In order to secure sufficient ductility, in the present invention, the area ratio of the polygonal ferrite in the third part is 15% or more. There is a need. On the other hand, in order to secure the 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. The area ratio is preferably 20% or more and 50% or less. In the present invention, the area ratio of polygonal ferrite in the first portion (inner winding portion) and the second portion (outer winding portion) as well as the third portion is preferably 15% or more and 55% or less. More preferably 20% or more and 50% or less. The polygonal ferrite in the present invention is a ferrite which is relatively soft and rich in ductility.

Index of organizational variability (A)
Area ratio ratio of polygonal ferrite (inner winding part/outer winding part): 1.00 to 1.50
Then, regarding the area ratio of this polygonal ferrite, by setting the ratio of the first portion (inner winding portion)/the second portion (outer winding portion) to be between 1.00 and 1.50, the characteristic of the entire length of the coil can be improved. It is possible to obtain a steel strip with high yield and no variations. It is preferably in the range of 1.00 to 1.20.

Area ratio of unrecrystallized ferrite in the third portion (central part): 8% or more It is important in the present invention that the area ratio of unrecrystallized ferrite is 8% or more. Here, although the non-recrystallized ferrite is generally effective in increasing the strength of the steel sheet, it is often reduced because it causes a remarkable decrease in ductility of the steel sheet. However, in the present invention, good ductility is ensured by the polygonal ferrite and the retained austenite, and the relatively hard unrecrystallized ferrite is positively utilized, so that, for example, the area ratio exceeds 30%. The desired TS of the steel sheet can be secured without requiring a large amount of martensite, and the amount of the different phase interface between polygonal ferrite and martensite is reduced, so the yield strength (YP) and yield ratio of the steel sheet are reduced. It is possible to increase (YR). In order to obtain the above effects, the area ratio of unrecrystallized ferrite needs to be 8% or more. It is preferably 10% or more. In addition, the non-recrystallized ferrite in the present invention is a ferrite having a crystal orientation difference of less than 15° in a grain, and is harder than the above-mentioned ductile rich polygonal ferrite. In the present invention, the upper limit of the area ratio of unrecrystallized ferrite is not particularly limited, but it is preferably about 20%.

Area ratio of martensite in the third part (central part): 15% or more and 30% or less To achieve a TS of 980 MPa or more, the area ratio of martensite in the third part needs to be 15% or more. On the other hand, in order to secure good ductility, it is necessary to limit the area ratio of martensite in the third portion to 30% or less.

  Further, the area ratio of this martensite is preferably 1.00 to 1.30 in the ratio of the first portion (inner winding portion)/the second portion (outer winding portion), whereby the entire length of the coil is obtained. It is possible to obtain a steel strip with high yield and no variations in properties.

  Here, the area ratio of martensite can be calculated as follows. That is, on behalf of the third part (central part), a sample is taken from the middle position between the front end and the tail end of the steel strip in the rolling direction, and the central part of the strip width in the strip width direction, and the sample is rolled. After polishing a plate thickness cross section (L cross section) parallel to the direction, it was corroded with 3 vol.% Nital, and about the plate thickness 1/4 position (the position corresponding to 1/4 of the plate thickness in the depth direction from the steel plate surface), About 10 visual fields of 50 μm×40 μm are observed with a SEM (scanning electron microscope) at a magnification of 2000 to obtain a tissue image. In this structure image, ferrite (polygonal ferrite and unrecrystallized ferrite) has a gray structure (base structure), and martensite has a white structure, so that they can be distinguished from each other. Based on the obtained tissue image, the area ratio of martensite was calculated for 10 visual fields using Image-Pro manufactured by Media Cybernetics, and the area ratios were averaged.

  The area ratio of polygonal ferrite and unrecrystallized ferrite can be obtained as follows. That is, a sample is taken from a predetermined position described below, and a plate thickness 1/4 position of a plate thickness cross section (L cross section) parallel to the rolling direction of the sample is subjected to the following EBSD observation. Using EBSD (Electron BackScatter Diffraction), low-angle grain boundaries with a crystal orientation difference of 2° to less than 15° and high-angle grain boundaries with a crystal orientation difference of 15° or more are identified. Then, IQ Map is prepared by using ferrite containing low-angle grain boundaries in the grains as unrecrystallized ferrite. Next, after extracting 10 fields of view from the created IQ Map in a field of view of 50 μm×40 μm, the areas of the low-angle grain boundary and the high-angle grain boundary are determined respectively, and the areas of the polygonal ferrite and the unrecrystallized ferrite are respectively determined. The area ratio of polygonal ferrite and unrecrystallized ferrite for 10 fields of view is calculated. Then, the area ratios are averaged to obtain the area ratios of the polygonal ferrite and the unrecrystallized ferrite. When determining the area ratio of polygonal ferrite and unrecrystallized ferrite in the third part (central part), the central part is represented, and in the rolling direction, the intermediate position between the leading end and the tail end of the steel strip, the plate. In the width direction, the sample is taken from the center of the plate width. When determining the area ratio of polygonal ferrite and unrecrystallized ferrite in the first part (inner winding part) and the second part (outer winding part), each part is represented and the rolling direction of the steel strip The sample is taken from a position of 100 m from the end on the side of the first part and a position of 100 m from the end on the side of the second part, and from the center part of the plate width in the plate width direction.

Volume Ratio of Retained Austenite in Third Part (Central Part): 12% or More In the present invention, the volume ratio of retained austenite in the third part needs to be 12% or more in order to secure sufficient ductility. It is preferably at least 14%. In the present invention, the upper limit of the volume fraction of retained austenite in the third portion is not particularly limited, but the amount of unstable retained austenite such as C and Mn having a small effect on improving ductility is unstable and increased. Therefore, it is preferably about 50%.

Index of organizational variation (B)
Volume ratio of retained austenite (inner winding/outer winding): 0.75 to 1.00
Furthermore, regarding the volume ratio of retained austenite, it is important to control the ratio of the first part (inner winding part)/the second part (outer winding part) to be between 0.75 and 1.00. It is possible to obtain a steel strip with high yield and no variation in characteristics over the entire length of the coil.

  Here, the volume ratio of retained austenite is such that a sample taken from a predetermined position described below is polished to a ¼ surface in the plate thickness direction (a surface corresponding to ¼ of the plate thickness in the depth direction from the surface of the steel plate). It is determined by measuring the intensity of the diffracted X-ray of the 1/4 surface of the plate thickness. MoKα rays are used as the incident X-rays, and the {110}, {200}, and {211} of ferrite having the integrated intensity of the peaks of {111}, {200}, {220}, and {311} planes of retained austenite. The intensity ratios of all 12 combinations with respect to the integrated intensity of the peak of the surface are obtained, and the average value thereof is taken as the volume ratio of the retained austenite. When determining the volume ratio of retained austenite in the third part (central part), the central part is represented, the middle position between the leading end and the tail end of the steel strip in the rolling direction, and the strip width in the strip width direction. Take the sample from the center. When determining the volume ratio of the retained austenite in the first portion (inner winding portion) and the second portion (outer winding portion), the end of the steel strip on the first portion side in the rolling direction is represented on behalf of each portion. The sample is taken from a position of 100 m from the end on the side of the section and the second portion, and from the center of the plate width in the plate width direction.

Average crystal grain size of polygonal ferrite in the third portion (central part): 4.0 μm or less Finer crystal grains of polygonal ferrite contribute to improvement of YP and TS. Therefore, in order to secure high YP and high YR and a desired TS, it is necessary to set the average crystal grain size of the polygonal ferrite in the third portion to 4.0 μm or less. The thickness is preferably 3.0 μm or less. In the present invention, the lower limit of the average crystal grain size of the polygonal ferrite in the third portion is not particularly limited, but industrially it is preferably about 0.2 μm. In the present invention, the average crystal grain size of the polygonal ferrite in the first portion (inner winding portion) and the second portion (outer winding portion) as well as the third portion is preferably 4.0 μm or less. , 3.0 μm or less, more preferably about 0.2 μm or more.

Index of organizational variation (C)
Average crystal grain size ratio of polygonal ferrite (inner winding part/outer winding part): 1.00 to 1.50
Regarding the average crystal grain size of polygonal ferrite, the ratio of the first part (inner winding part)/the second part (outer winding part) needs to be controlled between 1.00 and 1.50. As a result, it is possible to obtain a steel strip with a high yield and no variation in characteristics over the entire length of the coil.

Average grain size of martensite in the third portion (central portion): 2.0 μm or less Finer grain size of martensite contributes to improvement of bendability and stretch-flangeability (hole expandability). Therefore, in order to secure high bendability and high stretch flangeability (high hole expandability), it is necessary to suppress the average crystal grain size of martensite in the third portion to 2.0 μm or less. It is preferably 1.5 μm or less. In the present invention, the lower limit of the average crystal grain size of martensite in the third portion is not particularly limited, but industrially it is preferably about 0.05 μm.

Average crystal grain size of retained austenite in the third portion (central part): 2.0 μm or less Refining the crystal grains of retained austenite contributes to improvement of ductility and bendability and stretch flangeability (hole expandability). .. Therefore, in order to secure good ductility, bendability, and stretch flangeability (hole expandability), the average crystal grain size of the retained austenite in the third portion needs to be 2.0 μm or less. It is preferably 1.5 μm or less. In the present invention, the lower limit of the average crystal grain size of retained austenite in the third portion is not particularly limited, but industrially it is preferably about 0.05 μm.

  The average crystal grain size of polygonal ferrite, martensite, and retained austenite can be determined as follows. That is, a sample is taken from a predetermined position described below, and a plate thickness 1/4 position of a plate thickness cross section (L cross section) parallel to the rolling direction of the sample is subjected to the following observation. Using the above-mentioned Image-Pro, the area of each of the polygonal ferrite grains, the martensite grains and the retained austenite grains was calculated in one visual field of 50 μm×40 μm, the equivalent circle diameter was calculated, and The average particle size is calculated. Martensite and retained austenite are identified by Phase Map of EBSD. In the present invention, when the average crystal grain size is obtained, the average grain size is 0.01 μm or more. The reason is that the particle size of less than 0.01 μm does not affect the present invention. When obtaining the average crystal grain size of each phase in the third part (central part), the central part is represented, in the rolling direction, at the intermediate position between the front end and the tail end of the steel strip, and in the strip width direction. The sample is taken from the center of the plate width. When determining the average crystal grain size of polygonal ferrite in the first portion (inner winding portion) and the second portion (outer winding portion), the first portion of the steel strip in the rolling direction is represented on behalf of each portion. The sample is sampled from the end on the side and the end on the side of the second portion, respectively, at a position of 100 m from the center of the plate width in the plate width direction.

A value obtained by dividing the Mn amount (mass %) in the retained austenite in the third part (central part) by the Mn amount (mass %) in the polygonal ferrite: 2.00 or more. In the third part, Mn in the retained austenite. It is extremely important in the present invention that the value obtained by dividing the amount (mass %) by the Mn amount (mass %) in the polygonal ferrite is 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 Mn content (mass %) in the retained austenite by the Mn content (mass %) in the polygonal ferrite is not limited, but from the viewpoint of ensuring stretch flangeability, 16 It is preferably about 0.0.

  The Mn content (mass %) in the retained austenite and the Mn content (mass %) in the polygonal ferrite can be determined as follows. That is, on behalf of the third portion (central part), a sample is taken from the middle position between the front end and the tail end of the steel strip in the rolling direction and from the central part of the strip width in the strip width direction, and the sample is taken from EPMA (Electron Probe Micro Analyzer; electron probe microanalyzer) is used to quantify the distribution state of Mn in each phase of the cross section in the rolling direction at the sheet thickness 1/4 position. Next, the Mn amounts of 30 retained austenite grains and 30 ferrite grains are analyzed. Then, the Mn amount obtained from the result of the analysis can be averaged.

  Here, in the microstructure of the present invention, in addition to the above-mentioned polygonal ferrite, martensite, etc., it is usually recognized in steel sheets such as granular ferrite, acicular ferrite, bainitic ferrite, tempered martensite, pearlite and cementite. There are carbides (excluding cementite in pearlite). As long as the area ratio of these structures is 10% or less, the effect of the present invention is not impaired even if they are included.

  Further, in the present invention, it is preferable that the ε phase having the hcp structure is contained in an area ratio of 2% or more. Here, there is a risk of embrittlement in steel containing a large amount of ε phase having an hcp structure. However, when the ε phase having an appropriate amount of hcp structure is finely dispersed in the grain boundaries and in the grains of ferrite and unrecrystallized ferrite as in the present invention, excellent balance is achieved while ensuring good strength and ductility. Shows vibration performance.

  The ε-phase having the hcp structure, martensite, and retained austenite can be identified by using Phase Map of EBSD. Further, in the present invention, the upper limit of the area ratio of the ε phase is not limited, but there is a risk of embrittlement, so it is preferably about 35%. By satisfying the above requirements, it is possible to cause the work-induced transformation (TRIP) phenomenon, which is the main factor for improving the ductility, to be intermittently expressed until the end of the working of the steel sheet, and to achieve the so-called stable formation of retained austenite. You can

(Manufacturing conditions)
The method for producing a high-strength steel strip according to the present invention comprises a step of heating a steel slab having the above composition, a step of hot rolling the steel slab to obtain a hot rolled sheet, and rolling the hot rolled sheet. Step of forming a hot rolled sheet coil, then, a step of removing the scale by pickling the hot rolled sheet of the hot rolled sheet coil, heat treatment of the hot rolled sheet coil in a batch heating furnace, heat A hot-rolled sheet annealing step of forming a rolled annealed sheet coil, then a step of cold-rolling the hot-rolled annealed sheet of the hot-rolled annealed sheet coil, and then a cold-rolled sheet to the cold-rolled sheet And annealing to obtain a high-strength steel strip. Hereinafter, the manufacturing conditions and the reasons for the limitation will be described.

Heating temperature of steel slab: 1100° C. or more and 1300° C. or less Precipitates that are present in the heating stage of the steel slab (or simply called slab) are present as coarse precipitates in the steel sheet that is finally obtained, Does not contribute. Therefore, the Ti and Nb-based precipitates deposited during casting need to be redissolved. Here, if the heating temperature of the steel slab is less than 1100°C, not only is it difficult to form a sufficient solid solution of carbides, but there is also a problem that the risk of troubles during hot rolling increases due to an increase in rolling load. Therefore, the heating temperature of the steel slab needs to be 1100° C. or higher. Also, from the viewpoint of scaling off defects such as bubbles and segregation in the slab surface layer and reducing cracks and irregularities on the steel plate surface to achieve a smooth steel plate surface, the heating temperature of the steel slab must be 1100° C. or higher. is there. On the other hand, when the heating temperature of the steel slab exceeds 1300° C., the scale loss increases as the amount of oxidation increases. Therefore, the heating temperature of the steel slab needs to be 1300°C or lower. Therefore, the heating temperature of the slab needs to be 1100°C or higher and 1300°C or lower. Preferably, it is in the range of 1150°C or higher and 1250°C or lower.

  The steel slab is preferably manufactured by a continuous casting method in order to prevent macro segregation, but it is also possible to manufacture it by an ingot making method or a thin slab casting method. Further, in the present invention, after the steel slab is manufactured, it is possible to use a conventional method of once cooling to room temperature and then heating again. Furthermore, in the present invention, an energy-saving process such as direct-feed rolling or direct rolling in which a hot piece is charged into a heating furnace as it is without cooling to room temperature, or a small amount of heat is retained and immediately rolled is also applied without any problem. be able to. The steel slab is formed into a sheet bar by rough rolling under normal conditions, but if the heating temperature is lowered, a bar heater or the like is used before finish rolling in order to prevent problems during hot rolling. It is preferred to use to further heat the sheet bar.

Finish rolling end temperature of hot rolling: 750° C. or higher and 1000° C. or lower The steel slab after heating is hot rolled by rough rolling and finish rolling to be a hot rolled sheet. At this time, when the finishing temperature exceeds 1000° C., the production amount of oxide (scale) sharply increases, the interface between the base iron and the oxide becomes rough, and the steel sheet after pickling and cold rolling is applied. The surface quality of the product tends to deteriorate. In addition, if some of the hot rolled scale remains after pickling, the ductility and stretch flangeability of the steel sheet are adversely affected. Furthermore, the crystal grain size may become excessively large, and the surface of the pressed product may be roughened during processing. On the other hand, when the finishing temperature is less than 750° C., the rolling load increases, and the rolling reduction in the unrecrystallized state of austenite increases. As a result, an abnormal texture develops in the steel sheet, the in-plane anisotropy in the final product becomes remarkable, and not only the uniformity of the material (material stability) is impaired, but also the ductility of the steel sheet itself decreases. .. Therefore, in the present invention, it is necessary to set the finish rolling outlet temperature of hot rolling to 750°C or higher and 1000°C or lower. The temperature is preferably 800° C. or higher and 950° C. or lower.

Winding temperature: 300° C. or higher and 750° C. or lower The winding temperature is the average value of the winding temperature of the entire length of the hot rolling coil after hot rolling. If the winding temperature after hot rolling exceeds 750° C., the crystal grain size of ferrite in the structure of the hot rolled sheet becomes large, and it becomes difficult to secure the desired strength. On the other hand, when the winding temperature after hot rolling is less than 300° C., the strength of the hot-rolled sheet increases, the rolling load in cold rolling increases, and the sheet shape becomes defective. descend. Therefore, the winding temperature after hot rolling needs to be 300° C. or higher and 750° C. or lower. It is preferably in the range of 400°C or higher and 650°C or lower.

  In the present invention, rough rolling plates may be joined to each other and continuously finish-rolled during hot rolling. Further, the rough rolled plate may be once wound. Furthermore, in order to reduce the rolling load during hot rolling, part or all of the finish rolling may be lubrication rolling. Performing the lubrication rolling is also effective from the viewpoint of making the shape of the steel sheet uniform and making the material uniform. The coefficient of friction during lubrication rolling is preferably 0.10 or more and 0.25 or less.

  The hot-rolled sheet of the hot-rolled sheet coil manufactured through these steps is subjected to pickling. Since pickling can remove oxides on the surface of the steel sheet, it is important for ensuring good chemical conversion treatment and plating quality of the high-strength steel sheet as the final product. Moreover, the pickling may be performed once or may be performed in multiple times.

Hot-rolled sheet annealing: The outer winding part of the hot-rolled sheet coil is kept at a predetermined temperature of (Ac ₁ transformation point +20°C) or higher (Ac ₁ transformation point +120°C) or lower for more than 21600s and 129600s or less. Is annealed by using a batch type heating furnace. At that time, the outer winding part of the hot-rolled sheet coil is at a predetermined temperature of (Ac ₁ transformation point +20°C) or more (Ac ₁ transformation point +120°C) or less for more than 21600s. Holding for 129600 seconds or less is extremely important in the present invention. When the annealing temperature of hot-rolled sheet annealing is less than (Ac ₁ transformation point +20°C) or (Ac ₁ transformation point +120°C), or when the holding time is less than 21600 s, both are particularly the tip of the steel strip. In the portion, the concentration of Mn in austenite does not proceed, and it becomes difficult to secure a sufficient volume ratio of retained austenite after the final annealing, and the ductility decreases. On the other hand, if it is maintained for more than 129600 s, not only the concentration of Mn in austenite is saturated and the structure becomes coarse, but also the cost increases. Therefore, in the annealing of the hot-rolled sheet coil, the outer wound portion of the hot-rolled sheet coil is held at a predetermined temperature of (Ac ₁ transformation point +20°C) or more (Ac ₁ transformation point +120°C) or less for a time of more than 21600 seconds and 129600 seconds or less. I shall. The holding time is preferably 25000 s or more.

  After the above heat treatment, it is cooled to room temperature. The cooling method and cooling rate at that time are not particularly limited, but furnace cooling or air cooling in batch annealing is preferable.

Cold rolling reduction: 50% or less In the cold rolling of the present invention, the reduction is 50% or less. If cold rolling is performed at a rolling reduction of more than 50%, coarse polygonal ferrite is generated during heat treatment. As a result, a soft phase is obtained in the steel sheet, and the strength-ductility balance decreases. In addition, the bendability and stretch flangeability (hole expandability) of the steel sheet are also reduced. The rolling reduction in cold rolling is preferably 30% or more.

Cold-rolled sheet annealing: Hold the cold-rolled sheet at a predetermined temperature of Ac ₁ transformation point or higher and (Ac ₁ transformation point +100° C.) or lower for 20 to 900 seconds. When annealing the cold-rolled sheet, the cold-rolled sheet has Ac ₁ transformation point or higher. , (Ac ₁ transformation point+100° C.) or less, it is extremely important in the present invention to hold for 20 to 900 seconds. When the annealing temperature of the cold-rolled sheet is below the Ac ₁ transformation point or above (Ac ₁ transformation point +100°C), and when the holding time is less than 20 s, the enrichment of Mn in the austenite progresses. However, it becomes difficult to secure a sufficient volume ratio of retained austenite, and the ductility decreases. On the other hand, when it is held for more than 900 s, the area ratio of unrecrystallized ferrite decreases, the amount of heterophase interface between ferrite and the hard second phase (martensite and retained austenite) increases, and YP decreases. , YR also decreases.

Steel having the composition shown in Table 1 and the balance of Fe and inevitable impurities was melted in a converter and made into a slab by a continuous casting method. The obtained slab was processed 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 galvanized. As a hot-dip galvanizing bath, in a hot-dip galvanized steel strip (GI), a zinc bath containing Al: 0.19 mass% was used, and in a hot-dip galvanized steel strip (GA), Al: 0.14 mass%. A zinc bath containing was used. In both cases, the bath temperature was 465° C., and the coating weight was 45 g/m ² per side (double-sided plating). Further, in GA, the Fe concentration in the plating layer was adjusted to 9% by mass or more and 12% by mass or less.

  The results of examining the microstructure and the average crystal grain size of each phase in the third part (central part) of the steel strip thus obtained are shown in Table 3, and the first part (inner winding part) and the second part (outer part) Table 4 shows the results of examining the microstructure in the winding part) and the average crystal grain size of each phase. Further, Table 5 shows the results of an examination of the tensile properties and hole expandability of the steel strip in the third portion (central portion). In addition, Table 5 also shows the results of examining the surface properties and the productivity (passability) of the steel strip. In addition, Table 6 shows the results of examining the tensile properties and hole expandability of the steel strip in the first portion (inner winding portion) and the second portion (outer winding portion).

The Ac ₁ transformation point was determined using the following formula.
Ac ₁ transformation point (℃)
=751-16x(%C)+11x(%Si)-28x(%Mn)-5.5x(%Cu)-16x(%Ni)+13x(%Cr)+3.4x(% Mo)
Here, (%C), (%Si), (%Mn), (%Ni), (%Cu), (%Cr) and (%Mo) are the contents of each element in the steel (mass%). ).

  The tensile test is performed according to JIS Z 2241 (2011) using a JIS No. 5 test piece in which a sample is taken so that the tensile direction is perpendicular to the rolling direction of the steel sheet, and YP, YR, TS and EL was measured. Note that YR is a value obtained by dividing YP by TS and expressing it as a percentage. In the present invention, YR≧68% and TS×EL≧22000 MPa·%. Further, TS:980 MPa class EL≧26%, TS:1180 MPa class EL≧22%, TS:1470 MPa class. Each of EL≧18% was judged to be good. In this example, 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, and TS: 1470 MPa class is a TS of 1470 MPa. It is a steel plate having a pressure of not less than 1760 MPa.

The hole expandability was measured according to JIS Z 2256 (2010). After cutting each of the obtained steel plates into 100 mm×100 mm, a hole having a diameter of 10 mm was punched out with a clearance of 12%±1%. Then, with a die having an inner diameter of 75 mm and a wrinkle holding force of 9 tons (88.26 kN), the 60° conical punch was pushed into the hole to measure the hole diameter at the crack initiation limit. Further, the limit hole expansion rate λ (%) was obtained from the following formula, and the hole expandability was evaluated from the value of the limit hole expansion rate.
Limit hole expansion rate λ(%)={(D _f −D ₀ )/D ₀ }×100
However, D _f is a hole diameter (mm) when a crack is generated, and D ₀ is an initial hole diameter (mm). In the present invention, it was determined that λ≧20% in the TS:980 MPa class, λ≧15% in the TS:1180 MPa class, and λ≧10% in the TS:1470 MPa class, respectively.

  As the evaluation of the steel strip, the stripability during hot rolling and cold rolling, the surface properties of the final annealed sheet, and the productivity were evaluated.

  The stripability during hot rolling and cold rolling was judged to be poor when the rolling load increased and the risk of troubles during rolling increased. The evaluation results are shown in Table 5.

  The surface quality of the final annealed plate was judged to be poor when the defects such as bubbles and segregation in the slab surface layer could not be scaled off and cracks and irregularities on the surface of the steel sheet increased, and a smooth steel sheet surface could not be obtained. The surface texture of the final annealed sheet is such that when the amount of oxide (scale) is rapidly increased, the interface between the base iron and the oxide is roughened, and the surface quality after pickling and cold rolling deteriorates. It was also judged to be defective if a part of the hot-rolled scale remained after washing. The evaluation results are shown in Table 5.

  Productivity is when (1) defective shape of the hot-rolled sheet occurs, (2) when the shape of the hot-rolled sheet needs to be corrected to proceed to the next step, and (3) when the holding time of the annealing treatment is long. , Etc. Lead time cost was evaluated. Then, when any of (1) to (3) is not satisfied, "good" is determined, and when any of (1) to (3) is satisfied, "bad" is determined. The measurement results are also shown in Table 5.

  From the above results, it is understood that according to the present invention, a high-strength steel strip having excellent formability of TS≧980 MPa, YR≧68%, TS×EL≧22000 MPa·% is obtained over the entire length of the coil. On the other hand, in the comparative example, the characteristics of at least one of YR, TS, EL, and λ were inferior, or the characteristics were deteriorated at the coil tip or tail end.

According to the present invention, even if the hot-rolled sheet coil is heated using a batch-type heating furnace, TS≧980 MPa, YR≧68%, and TS×EL≧22000 MPa·% over the entire length of the coil. High strength steel strip with excellent formability can be stably manufactured. Therefore, by applying the high-strength steel strip of the present invention to, for example, an automobile structural member, it is possible to improve fuel efficiency by reducing the weight of a vehicle body, and its industrial utility value is extremely large.

Claims (2)

% By mass, C: 0.030% or more and 0.250% or less, Si: 0.01% or more and 3.00% or less, Mn: more than 4.20% and 6.00% or less, P: 0.001% or more 0.100% or less, S: 0.0001% or more and 0.0200% or less, N: 0.0005% or more and 0.0100% or less, Al: 0.010% or more and 2.000% or less, and Ti: 0.005. % Or more and 0.200% or less, and heating the steel slab having a composition of the balance Fe and unavoidable impurities to 1100° C. or more and 1300° C. or less,
A step of hot rolling the steel slab at a finish rolling outlet temperature of 750° C. or higher and 1000° C. or lower to obtain a hot rolled sheet;
Winding the hot rolled sheet at a winding temperature of 300° C. or higher and 750° C. or lower to form a hot rolled sheet coil;
Then, a step of removing the scale by subjecting the hot rolled plate of the hot rolled plate coil to pickling,
A heat treatment of the hot rolled sheet coil in a batch type heating furnace, and a hot rolled sheet annealing step of forming a hot rolled annealed sheet coil,
After that, a step of cold rolling the hot rolled annealed sheet of the hot rolled annealed sheet coil at a rolling reduction of 50% or less to obtain a cold rolled sheet,
After that, the cold-rolled sheet is held at a predetermined temperature of Ac ₁ transformation point or higher and (Ac ₁ transformation point+100° C.) or lower for 20 seconds or more and 900 seconds or less, and then cold-rolled sheet annealing is performed to cool the high-strength steel strip. The process of obtaining
In a method of manufacturing a high strength steel strip having
The hot-rolled sheet annealing step is performed under the condition that the outer winding portion of the hot-rolled sheet coil is kept at a predetermined temperature of (Ac ₁ transformation point +20°C) or higher and (Ac ₁ transformation point +120°C) or lower for more than 21600s and 129600s or less. ,
With respect to the longitudinal direction of the high-strength steel strip, a portion that was an inner winding portion of the hot-rolled annealed plate coil is a first portion, a portion that was an outer winding portion of the hot-rolled annealed sheet coil is a second portion, the heat The part that was the central part of the annealed plate coil was set as the third part,
The steel structure of the third portion has an area ratio of polygonal ferrite: 15% or more and 55% or less, unrecrystallized ferrite: 8% or more, martensite: 15% or more and 30% or less, and a volume ratio of retained austenite: Contains 12% or more,
And, the area ratio of the polygonal ferrite in the first portion/the area ratio of the polygonal ferrite in the second portion is 1.00 to 1.50, the volume ratio of the retained austenite in the first portion/the second portion. The volume ratio of retained austenite is 0.75 to 1.00,
The third portion has an average crystal grain size of polygonal ferrite: 4.0 μm or less, an average crystal grain size of martensite: 2.0 μm or less, and an average crystal grain size of retained austenite: 2.0 μm or less. ,
And, the average crystal grain size of the polygonal ferrite of the first portion/the average crystal grain size of the polygonal ferrite of the second portion is 1.00 to 1.50,
Further, in the third portion, a value obtained by dividing the Mn amount (mass%) in the retained austenite by the Mn amount (mass%) in the polygonal ferrite is 2.00 or more, a high strength steel. Method of manufacturing obi. The composition of the components is, in mass %, Nb: 0.005% or more and 0.200% or less, B: 0.0003% or more and 0.0050% or less, Ni: 0.005% or more and 1.000% or less, Cr: 0.005% to 1.000%, V: 0.005% to 0.500%, Mo: 0.005% to 1.000%, Cu: 0.005% to 1.000% Hereinafter, Sn: 0.002% or more and 0.200% or less, Sb: 0.002% or more and 0.200% or less, Ta: 0.001% or more and 0.010% or less, Ca: 0.0005% or more and 0. 2. At least one element selected from the group consisting of 0050% or less, Mg: 0.0005% or more and 0.0050% or less, and REM: 0.0005% or more and 0.0050% or less. The method for producing a high-strength steel strip according to.