JP5533729B2 - High-strength hot-rolled steel sheet with excellent local deformability and excellent ductility with less orientation dependency of formability and method for producing the same - Google Patents

Description

  The present invention relates to a hot-rolled steel sheet having excellent local deformability such as bending, stretch flange, burring, and the like, having less orientation dependency of formability and excellent ductility, and a method for producing the same.

  In order to reduce carbon dioxide emissions from automobiles, the weight of automobile bodies is being reduced using high-strength steel sheets. In addition, in order to ensure the safety of passengers, high strength steel plates are often used in automobile bodies in addition to mild steel plates. Furthermore, in order to further reduce the weight of automobile bodies in the future, it is necessary to increase the use strength level of high-strength steel sheets more than before. For example, to use high-strength steel sheets for undercarriage parts, local parts for burring processing The deformability must be improved.

  However, generally, if the strength of the steel plate is increased, the formability is lowered, and the uniform elongation important for drawing or stretch forming is lowered. On the other hand, as disclosed in Document 1, a method is disclosed in which austenite remains in a steel sheet to ensure uniform elongation.

  On the other hand, a metal structure control method for a steel sheet that improves local ductility, represented by bending, hole expanding, and burring, is also disclosed. It is disclosed in Document 2 that reducing the hardness difference is effective for bendability and hole expansion.

  A technique for obtaining an appropriate fraction of ferrite and bainite by controlling the metal structure by cooling control to achieve both ductility and strength, and controlling the precipitates and the transformation structure is also disclosed in Document 3. However, both are methods for improving local deformability that depend on tissue control, and are greatly influenced by the formation of the base structure.

  On the other hand, as a material improvement technique for hot-rolled steel sheets, there is a disclosed technique for improving a material by increasing the amount of reduction in a continuous hot rolling process. This is a so-called crystal grain refinement technology, which reduces the grain size of ferrite, which is the main phase of the product, by transforming ferrite from unrecrystallized austenite by performing large pressure at the lowest possible temperature in the austenite region. This is a technique aimed at increasing the strength and toughness by making the particles finer as in FIG. However, in the manufacturing method described in Document 4, no consideration is given to the improvement of local deformability and ductility that the present invention intends to solve.

: Takahashi, Nippon Steel Technical Report (2003) No.378, p.7 : Kato et al., Steel Research (1984) vol.312, p.41 : K. Sugimoto et al, (2000) Vol.40, p.920 ： Nakayama Steel Works NFG Product Introduction

  As described above, in order to improve the local elongation performance of the high-strength steel sheet, it was mainly to control the structure including inclusions. In the present invention, by controlling the grain size and texture by the rolling process of the hot rolling process, it is excellent in local deformability that can improve the anisotropy of the hot rolled steel sheet, and the orientation dependency of formability. The present invention provides a hot-rolled steel sheet having excellent ductility and a method for producing the same.

  According to the conventional knowledge, as described above, improvement of hole expansibility and bendability has been achieved by inclusion control, refinement of precipitates, homogenization / single phase structure of the structure, and reduction of hardness difference between structures. It was. However, this alone has to limit the main structural configuration, and the anisotropy is extremely large in a high-strength steel sheet to which Nb, Ti, or the like is added. This results in problems such as sacrificing other formability factors and limiting the direction in which the blank before molding is taken, and the application is also limited.

  Therefore, the present inventors have investigated and examined the refinement of the structure and the texture control in the hot rolling process in order to improve the hole expandability and the bending workability. As a result, by controlling the strength of each orientation of a specific crystal orientation group, the local deformability is dramatically improved when the rC value in the direction perpendicular to the rolling direction and the r30 value at 30 ° are balanced with the rolling direction. It is clarified that it improves.

The present invention is configured based on the above-mentioned knowledge, and the main points thereof are as follows.
(1) In mass%,
C: 0.02% or more, 0.5% or less,
Si: 0.001% to 4.0%,
Mn: 0.001% or more and 4.0% or less,
P: 0.001% or more, 0.15% or less,
S: 0.0005% or more, 0.03% or less,
N: 0.0005% or more, 0.01% or less,
O: 0.0005% or more, 0.01% or less,
Al + Si ≦ 4.0% or less,
Furthermore,
Ti: 0.001% or more, 0.2% or less,
Nb: 0.001% or more, 0.2% or less,
V: 0.001% or more, 1.0% or less,
W: 0.001% or more, 1.0% or less,
Cu: 0.001% or more, 2.0% or less,
B: 0.0001% or more, 0.005% or less,
Mo: 0.001% or more, 1.0% or less,
Cr: 0.001% or more, 2.0% or less,
As: 0.0001% or more, 0.50% or less,
Ni: 0.001% or more, 2.0% or less,
Co: 0.0001% or more and 1.0% or less,
Sn: 0.0001% or more, 0.2% or less,
Zr: 0.0001% or more, 0.2% or less,
{100} <011> to the surface of the plate at a thickness of 5/8 to 3/8 at least from the surface of the steel plate. The average value of the X-ray random intensity ratio of the {223} <110> orientation group is less than 4.0, the X-ray random intensity ratio of the crystal orientation of {332} <113> is 5.0 or less, and the rolling direction The r (rC) value in the direction perpendicular to the rolling direction is 0.70 or more, and the r value at 30 ° (r30) in the rolling direction is 1.10 or less. As the steel sheet structure, the retained austenite is 5% or more by area ratio, 30 %, Ferrite is 20% or more, less than 50%, bainite is contained in an amount of 10% or more and less than 60%, and pearlite and martensite are each 20% or less. Less orientation dependency Excellent hot-rolled steel sheet to sex.
(2) Furthermore, the area ratio of grains exceeding 20 μm in the total area is 10% or less, which is excellent in local deformability as described in the above (1), and has little orientation dependency of formability Hot rolled steel sheet with excellent ductility.
(3) Further, in the grains of retained austenite and martensite, the distance to the nearest retained austenite or martensite is L _MA [μm], and the standard deviation of _LMA when measuring 100 or more is 5 or less A hot-rolled steel sheet excellent in local deformability described in (1) or (2) and excellent in ductility with little orientation dependency of formability.
(4) Furthermore,
Mg: 0.0001% or more, 0.010% or less,
REM: 0.0001% or more, 0.1% or less,
Ca: 0.0001% or more, 0.010% or less,
A hot-rolled steel sheet excellent in local deformability according to any one of the above (1) to ( 3 ) and having excellent ductility with little orientation dependency of formability.
(5) In manufacturing the high-strength steel plate according to any one of (1) to ( 4 ) above, after melting into a predetermined steel plate component, casting into a steel ingot or slab, and rough rolling at 1000 ° C. or higher 20% or more reduction in a temperature range of 1200 ° C. or less is performed at least once, the austenite grain size is 200 μm or less, and then the temperature determined by the steel plate component in formula (1) in finish rolling is T1. In the temperature range of T1 + 30 ° C. or more and T1 + 200 ° C. or less, rolling is performed at 30% or more in one pass, and the total reduction ratio in the temperature range is set to 50% or more. Thereafter, T1 or more, T1 + 30 ° C. the total reduction ratio of 30% or less in the temperature range below, further more than 30% of the final rolling after the end of T1 or higher, retention time t from the temperature range of T1 + 200 ° C. or less until the start cooling formula ( In the Suyo meet 2), and terminates the hot rolled at Ar3 transformation temperature or higher, 630 ° C. or higher, cooling at 10 to 100 ° C. / sec to a temperature range of 800 ° C. or less, then 1 second in the temperature range Hold for 20 seconds or less, or cool to a temperature in the range of 550 ° C. or more at a cooling rate of 20 ° C./sec or less after cooling to the temperature range , wind up at 350 to 500 ° C., and the temperature change rate is − A method for producing a hot-rolled steel sheet having excellent ductility, which is excellent in local deformability after being held in a range of 40 ° C./h or more and 40 ° C./h or less for 30 to 300 minutes and then air-cooled, and having less orientation dependency of formability.
T1 (° C.) = 850 + 10 × (C + N) × Mn + 350 × Nb + 250 × Ti + 40 × B + 10 × Cr + 100 × Mo + 100 × V (1)
t ≦ t1 × 2.5 (2)
Here, t1 is represented by Formula (3).
t1 = 0.001 ((Tf−T1) × P1) ² −0.109 ((Tf−T1) × P1) +3.1 (3)
Here, Tf is the temperature after the final reduction of 30% or more, and P1 is the reduction ratio of the final reduction of 30% or more.
(6) The manufacturing method according to ( 5 ) above, wherein cooling is performed such that the cooling temperature change is 40 ° C. or more and 150 ° C. or less at a cooling rate of 50 ° C./sec or more within 1 second from the end of finish rolling. A method for producing a hot-rolled steel sheet having excellent local deformability and excellent ductility with less orientation dependency of formability.
(7) In the manufacturing method according to the above ( 5 ) or ( 6 ), the rolling ratio of the final pass of rolling in a temperature range of T1 + 30 ° C. or higher and T1 + 200 ° C. or lower is 25% or more. A method for producing a hot-rolled steel sheet that is excellent in ductility and has excellent ductility with less orientation dependency of formability.

  According to the present invention, it is possible to obtain a hot-rolled steel sheet having excellent local deformability such as bending, stretch flange, burring, etc., less orientation dependency of its formability, and excellent ductility.

The relationship between the average value of the X-ray random intensity ratio of {100} <011> to {223} <110> orientation group and the thickness / minimum bending radius is shown. The relationship between the X-ray random intensity ratio of the {332} <113> orientation group and the thickness / minimum bending radius is shown. The relationship between the r value (rC) in the direction perpendicular to the rolling direction and the sheet thickness / minimum bending radius is shown. The relationship between the r value (r30) of 30 ° in the rolling direction and the sheet thickness / minimum bending radius is shown. The relationship between the ratio of grains having a grain size of 20 μm or more and the thickness / minimum bending radius is shown. The relationship between the rolling frequency | count of 40% or more in rough rolling and the austenite grain size of rough rolling is shown. The relationship between the temporary cooling holding time and ferrite fraction in 630 degreeC or more and 800 degrees C or less after completion | finish of hot rolling is shown. The relationship between a coiling temperature and a retained austenite fraction is shown. The relationship between a coiling temperature and a bainite fraction is shown.

The contents of the present invention will be described in detail below.
Average value of X-ray random intensity ratio of {100} <011> to {223} <110> orientation group on the plate surface at a thickness of 5/8 to 3/8 from the surface, crystal orientation of {332} <113> X-ray random intensity ratio:

  This average value is a particularly important characteristic value in the present invention. {100} <011> to {223} <110> orientation group when X-ray diffraction of a plate surface at a thickness of 5/8 to 3/8 from the surface is performed to determine the intensity ratio of each orientation to a random sample If the average value of is less than 4.0, the thickness / bending radius ≧ 1.5 required for the machining of the most recently required skeletal part is satisfied. Furthermore, when a hole expansibility and a small limit bending characteristic are required, less than 3.0 is desirable. Above 7.0, the anisotropy of the mechanical properties of the steel sheet becomes extremely strong, which improves the local deformability only in one direction, but the material in a direction different from that significantly deteriorates, and the thickness / bending radius ≧ 1. Cannot satisfy 5

  The main orientations included in this orientation group are {100} <011>, {116} <110>, {114} <110>, {113} <110>, {112} <110>, {335} < 110> and {223} <110>.

  The X-ray random intensity ratio in each direction is measured using a device such as X-ray diffraction or EBSD (Electron Back Scattering Diffraction). {110} Using a three-dimensional texture calculated by the vector method based on a pole figure, or a plurality of pole figures (preferably three or more) among {110}, {100}, {211}, {310} pole figures What is necessary is just to obtain | require from the three-dimensional texture calculated by the series expansion method.

  For example, the X-ray random intensity ratio of each crystal orientation in the latter method is (001) [1-10], (116) [1-10], (114) in the φ2 = 45 ° cross section of the three-dimensional texture. ) [1-10], (113) [1-10], (112) [1-10], (335) [1-10], (223) [1-10] may be used as they are.

The average value of {100} <011> to {223} <110> orientation group is an arithmetic average of each of the above-mentioned orientations. When the strengths of all the above directions cannot be obtained, {100} <011>, {116} <110>, {114} <110>, {112} <110>, {223} <110> Alternatively, an arithmetic average of each direction may be substituted.

  For the same reason, the X-ray random intensity ratio of the {332} <113> crystal orientation of the plate surface at a thickness of 5/8 to 3/8 from the surface must be 5.0 or less. Desirably, if it is 3.0 or less, the plate thickness / bending radius ≧ 1.5 required for the processing of the most recently required skeleton parts is satisfied. If this exceeds 5.0, the anisotropy of the mechanical properties of the steel sheet becomes extremely strong, which in turn improves the local deformability only in one direction, but the material in a direction different from that significantly deteriorates, resulting in the thickness / bending. A radius of ≧ 1.5 cannot be satisfied with certainty. On the other hand, it is difficult to realize by the current general continuous hot rolling process, but when it is less than 0.5, there is a concern about deterioration of local deformability.

  A sample subjected to X-ray diffraction is obtained by reducing the thickness of a steel plate from a surface to a predetermined thickness by mechanical polishing or the like, and then removing distortion by chemical polishing or electrolytic polishing, and at the same time, having a thickness of 5/8 to 3/8. What is necessary is just to adjust and measure a sample according to the above-mentioned method so that a suitable surface may become a measurement surface in the range.

  As a matter of course, the above-mentioned limitation of the X-ray intensity is satisfied not only in the vicinity of the plate thickness ½ but also as much as possible, so that the spread performance is further improved. However, the measurement of 3/8 to 5/8 from the surface of the steel sheet can generally represent the material properties of the entire steel sheet, so this shall be specified.

  The crystal orientation represented by {hkl} <uvw> indicates that the normal direction of the plate surface is parallel to <hkl> and the rolling direction is parallel to <uvw>.

R value (rC) in the direction perpendicular to the rolling direction:
The r value of these steel plates is important in the present invention. That is, as a result of intensive studies by the present inventors, it has been found that even if only the X-ray random intensity ratios of various crystal orientations described above are appropriate, good hole expandability and bendability cannot always be obtained. Moreover, it is essential that rC is 0.70 or more simultaneously with the X-ray random intensity ratio.

  The upper limit of the r value in each direction described above is not particularly defined, but when it is 1.10 or less, better local deformability can be obtained.

R value of 30 ° in the rolling direction (r30):
The r value of this steel sheet is important in the present invention. That is, as a result of intensive studies by the present inventors, it has been found that good local deformability cannot always be obtained even when the X-ray intensities of the various crystal orientations described above are appropriate. As shown in FIG. 4, it is essential for the steel sheet that r30 is 1.10 or less simultaneously with the X-ray intensity.

  Each r value described above is evaluated by a tensile test using a JIS No. 5 tensile test piece. The tensile strain is usually in the range of 5 to 15% in the case of a high-strength steel plate, and may be evaluated in the range of uniform elongation.

  The direction in which the bending process is performed is not particularly limited because it varies depending on the processed part, and the same characteristics can be obtained in any bending direction according to the present invention.

  By the way, it is generally known that there is a correlation between the texture and the r value, but in the present invention, the above-described limitation on the X-ray intensity ratio of the crystal orientation and the limitation on the r value are not synonymous with each other, Good local deformability cannot be obtained unless both limitations are met simultaneously.

  The present invention can be applied to high strength steel sheets in general, and if the above limitation is satisfied, the present invention is not limited to the combination of structures, and the local formability such as bending workability and hole expansibility of high strength thin steel sheets can be achieved. Improve dramatically.

  Further, in order to suppress localization of strain and improve bendability, the area ratio of grains exceeding 20 μm needs to be 10% or less in the total area. If it exceeds this, the bendability deteriorates.

Next, the limiting conditions for the components will be described.
C is essential for securing high strength and securing retained austenite. In order to obtain a sufficient amount of retained austenite, a C amount of 0.02% or more is required. On the other hand, when C is contained excessively, weldability is impaired, so the upper limit of the C content is set to 0.5% or less.

  Si is a deoxidizer and is preferably added in an amount of 0.001% or more. Further, it is an element that stabilizes ferrite during annealing, and suppresses precipitation of cementite after winding, thereby increasing the C concentration of austenite and contributing to securing retained austenite. The higher the Si, the greater the effect. However, if added excessively, the surface properties, paintability, weldability and the like are deteriorated, so the upper limit is made 4.0% or less.

  Mn is an element that stabilizes austenite and improves hardenability. In order to ensure sufficient hardenability, it is necessary to add 0.001% or more of Mn. On the other hand, if Mn is added excessively, ductility is impaired, so the upper limit of the amount of Mn is made 4.0%.

  P is an impurity, and if contained excessively, ductility and weldability are impaired. Therefore, the upper limit of the P content is 0.15% or less. On the other hand, since it is difficult to make P 0.001% or less, this is the lower limit.

  S is an impurity, and if it is contained excessively, MnS stretched by hot rolling is generated, which causes deterioration of moldability such as ductility and hole expansibility. Therefore, the upper limit of the S amount is 0.03% or less. On the other hand, since it is difficult to make S 0.0005% or less, this is the lower limit.

  O is an impurity, and if it is excessively contained, workability is impaired. Therefore, the upper limit of the O amount is 0.01% or less. On the other hand, since it is difficult to make O 0.0005% or less, this is the lower limit.

  Al, like Si, is a deoxidizer and an element that stabilizes ferrite during annealing. However, if the content is excessive, the weldability becomes poor. Therefore, the total content is 4.0% or less in combination with Si.

  N is an impurity. When it exceeds 0.01%, ductility is deteriorated. Therefore, the upper limit of the N amount is 0.01% or less. On the other hand, since it is difficult to make N 0.0005% or less, this is the lower limit.

  Furthermore, when obtaining strength by precipitation strengthening, fine carbonitrides are preferably generated. In order to obtain precipitation strengthening, the addition of Ti, Nb, V, and W is effective, and one or more of these may be contained.

In order to obtain this effect by adding Ti, Nb, V, W and Cu, Ti is 0.001% or more, Nb is 0.001% or more, V is 0.001% or more, and W is 0.001%. As mentioned above, Cu needs to add 0.001% or more. However, even if it is excessively added, the strength increase will be saturated, and in addition, recrystallization after hot rolling makes it difficult to control the crystal orientation. Therefore, Ti is 0.2% or less, and Nb is 0%. 0.2% or less, V 1.0% or less, W 1.0% or less, and Cu 2.0% or less.

  When ensuring the strength by increasing the hardenability of the structure and performing the second phase control, it is effective to add one or more of B, Mo, Cr and As. In order to obtain this effect, it is necessary to add 0.0001% or more of B, 0.001% or more of Mo and Cr, and 0.0001% or more of As. However, excessive addition adversely deteriorates workability, so the upper limit of B is 0.005%, the upper limit of Mo is 1.0%, the upper limit of Cr is 2.0%, and As is 0.50%. .

  In order to improve local forming ability, Mg, REM, and Ca are important additive elements for detoxifying inclusions. Each lower limit for obtaining this effect was made 0.0001%. On the other hand, excessive addition leads to deterioration of cleanliness, so 0.010% for Mg, 0.1% for REM, and 0.010% for Ca were made the upper limit.

Ni, Co, Sn, and Zr are elements that increase the strength. Addition of 0.001% or more for Ni, 0.0001% or more for Co, 0.0001% or more for Sn, and 0.0001% or more for Zr is effective. It is. However, if too much is added, the moldability is lost, so Ni is 2.0.
%, Co 1.0%, Sn 0.2% and Zr 0.2%.

  Even if the hot-rolled steel sheet of the present invention is surface-treated, the effect of improving the local deformability is not lost. Electroplating, hot dipping, vapor deposition plating, organic film formation, film laminating, organic salt / inorganic salt treatment, non-chromic treatment The effect of the present invention can be obtained by any treatment.

  Next, a method for producing the thin steel sheet of the present invention will be described. In order to achieve excellent local deformability, details of manufacturing conditions for forming a texture having an X-ray random intensity ratio will be described below.

  The production method preceding hot rolling is not particularly limited. That is, various secondary smelting may be performed following the smelting by a blast furnace or an electric furnace, and then the casting may be performed by a method such as a thin slab casting in addition to a normal continuous casting and an ingot method. In the case of continuous casting, after cooling to low temperature once, it may be heated again and then hot rolled, or the cast slab may be continuously hot rolled. Scrap may be used as a raw material.

The high-strength hot-rolled steel sheet having excellent local deformability according to the present invention is obtained when the following requirements are satisfied.
In order to produce a hot-rolled steel sheet having excellent local deformability, it is desirable that the austenite grain size before finish rolling is small, and that the above value is satisfied if it is 200 μm or less. In order to obtain the austenite grain size before finish rolling of 200 μm or less, the rolling is reduced by at least 20% (preferably 40% or more) by rough rolling in the temperature range of 1000 ° C. to 1200 ° C. (preferably 1150 ° C.). It has also been found that a predetermined austenite grain size can be obtained by rolling at a rate one or more times.

  Finer grains can be obtained as the rolling reduction ratio and the number of rolling reductions are increased, and in order to obtain this effect more efficiently, it is desirable to obtain an austenite grain size of 100 μm or less, and for this purpose, 40% It is desirable to perform the above rolling twice or more. However, the reduction exceeding 70% or the rough rolling exceeding 10 times may cause a decrease in temperature or excessive generation of scale. Thus, reducing the austenite grain size before finish rolling is effective in improving local deformability by promoting recrystallization of austenite in subsequent finish rolling.

  This is presumed to be due to the function of the austenite grain boundary after rough rolling (that is, before finish rolling) as one of the recrystallization nuclei during finish rolling. In order to confirm the austenite grain size after rough rolling, it is desirable to cool the plate piece before finishing rolling as much as possible, and the plate piece is cooled at a cooling rate of 10 ° C./s or more. The structure of the cross section is etched to raise the austenite grain boundary and measured with an optical microscope. At this time, 20 fields of view or more are measured by image analysis or a point count method at a magnification of 50 times or more.

Moreover, the average value of the X-ray random intensity ratios of {100} <011> to {223} <110> orientation groups on the plate surface at a thickness of 5/8 to 3/8 from the surface of the steel plate, {332} <113> In order to make the X-ray random intensity ratio of the crystal orientation in the above-mentioned range, the T1 temperature determined by the steel plate component in the finish rolling after the rough rolling
T1 (℃) = 850 + 10 × (C + N) × Mn + 350 × Nb + 250 × Ti + 40 × B + 10 × Cr + 100 × Mo + 100 × V (Formula 1)
Based on the above, processing with a large reduction ratio in the temperature range of T1 + 30 ° C or more and T1 + 200 ° C or less, and processing with a small reduction ratio at T1 or more and less than T1 + 30 ° C, the local deformability of the final product Can be secured. As shown in Tables 2 and 3 below, the large pressure in the temperature range of T1 + 30 ° C. or more and T1 + 200 ° C. or less and the subsequent light pressure at T1 or more and less than T1 + 30 ° C. are 5/8 from the surface of the steel plate. Average value of X-ray random intensity ratio of {100} <011> to {223} <110> orientation group of plate surface at plate thickness of ˜3 / 8, X-ray random intensity of crystal orientation of {332} <113> Control the ratio and dramatically improve the local deformability of the final product. The T1 temperature itself is determined empirically. Based on the T1 temperature, the inventors have empirically found that recrystallization in the austenitic region of each steel is promoted. In order to obtain better local deformability, it is important to accumulate distortion due to large pressure, and 50% or more is essential. Furthermore, it is desirable to take a reduction of 70% or more. On the other hand, taking a reduction ratio exceeding 90% results in securing a temperature and adding an excessive rolling load.

  Furthermore, in order to promote uniform recrystallization by releasing accumulated strain, it is necessary to minimize the amount of processing in the temperature range from T1 to less than T1 + 30 ° C after a large pressure at T1 + 30 ° C or more and T1 + 200 ° C or less. Thus, the rolling reduction at T1 or more and less than T1 + 30 ° C. is set to 30% or less, and the rolling reduction is preferably 10% or more from the plate shape. However, when the local deformability is more important, the rolling reduction is preferably 0%. In addition, if the rolling reduction at T1 or more and less than T1 + 30 ° C. is large, recrystallized austenite grains expand, and if the retention time is short, recrystallization does not proceed sufficiently and local deformability deteriorates. That is, in the manufacturing conditions of the present invention, a method for improving the local deformability such as hole expandability and bendability by controlling the texture of hot rolled products by recrystallizing austenite uniformly and finely in finish rolling. It is.

  When rolling is performed at a temperature lower than the temperature range defined above and a large reduction ratio is obtained, the austenite texture develops, and the plate surface of the finally obtained hot-rolled steel sheet is described in (1). Each crystal orientation at a predetermined X-ray intensity level cannot be obtained. On the other hand, when rolling is performed at a temperature higher than the above-mentioned temperature range or a small reduction ratio is taken, coarse grains and mixed grains are formed, and the area ratio of crystal grains exceeding 20 μm increases. Whether the rolling specified above is performed or not can be determined by the actual result or calculation from the rolling load, sheet thickness measurement, etc., and the temperature can be measured if there is an inter-stand thermometer, or the line It can be obtained by a calculation simulation considering processing heat generation from the speed and rolling reduction, or both.

When hot rolling is finished at Ar ₃ or less, austenite and ferrite become two-phase rolling, and accumulation in {100} <011> to {223} <110> orientation groups becomes strong, and as a result, local deformability is increased. Deteriorates significantly.

In addition, cooling after rolling down at the final rolling stand of rolling in a temperature range of T1 + 30 ° C. or higher and T1 + 200 ° C. greatly affects the austenite grain size. If the austenite grains are coarsened after the final rolling in the above range for 0.05 seconds or more and within 5 seconds, the austenite grains are coarsened, so that the thickness / bending radius ≧ 2.0 cannot be satisfied. Furthermore, it is desirable that the time (t) from the final pass under the reduction at TL ° C. in the temperature range of T1 + 30 ° C. or higher and less than T1 + 200 ° C. to be 0.05 second or more and 5 seconds or less. If it is 0.05 seconds or less, hot rolling recrystallization does not occur sufficiently and the anisotropy increases, and if it is 5 seconds or more, austenite grains become coarse and strength and elongation decrease. Furthermore, the time until the start of cooling needs to satisfy the formula (2) with respect to the final rolling reduction temperature Tf and the final rolling ratio P1 of 30% or more. Here, t 1 is a numerical value that can be obtained by Expression (3).
t ≦ t1 × 2.5 Formula (2)
t 1 = 0.001 ((Tf−T1) × P1) ² −0.109 ((Tf−T1) × P1) +3.1 Formula (3)
If it is lower than this, sufficient recrystallization cannot be obtained and the anisotropy becomes high.

  After finish rolling, it is cooled to a temperature of 630 ° C. or higher and 800 ° C. or lower, which is in the vicinity of the nose of the pro-eutectoid ferrite region, and is held for 1 second or longer and 20 seconds or shorter. By slowly cooling at ℃ / sec or less, ferrite as a main phase can be easily obtained, and crystal grains can be refined by cooling to 630 ° C. or more and 800 ° C. or less. In the case of isothermal holding treatment, if the holding time exceeds 20 seconds, the ferrite fraction becomes too high and the strength decreases. On the other hand, if the holding time is less than 1 second, the amount of ferrite produced is insufficient. Further, if the temperature at which the slow cooling is stopped is below 550 ° C., pearlite transformation may occur.

  It cools and winds up to the temperature in the range of 350-500 degreeC, It is made to hold | maintain for 30 to 300 minutes as the temperature change rate of the coil wound up is -40 degreeC / h or more and 40 degrees C / h or less. If the coiling temperature exceeds 500 ° C., the bainite transformation proceeds excessively, and if it falls below 350 ° C., the bainite transformation is excessively suppressed, the stabilization of residual austenite due to C concentration is insufficient, and martensite during air cooling Transformation occurs and a sufficient amount of retained austenite cannot be obtained. Further, if the holding time at 350 ° C. to 500 ° C. is less than 30 minutes, the progress of the bainite transformation is not sufficient and the retained austenite fraction becomes insufficient. On the other hand, if it exceeds 300 minutes, precipitation and coarsening of cementite will proceed, and the desired retained austenite fraction cannot be obtained. Furthermore, when the temperature change rate of the coil is outside the range of −40 ° C./h to 40 ° C./h, and the temperature changes suddenly, the material variation in the coil becomes significant.

  After finishing rolling, it is necessary to perform cooling at a cooling rate of 50 ° C./sec or more within 1 second so that the change in cooling temperature is 40 ° C. or more and 150 ° C. or less. If the time until cooling is longer than 1 second, the recrystallized austenite grains are held at a high temperature, so that the grains grow and the strength and ductility are lowered. If the temperature change is less than 40 ° C., recrystallized austenite grains will grow. On the other hand, if it exceeds 150 ° C, there is a risk of overshooting below the Ar3 transformation point temperature. In this case, even if the transformation is from recrystallized austenite, as a result of sharpening of the variant selection, a texture is also formed and isotropicity is lowered. As a result, the orientation dependency of workability increases. If the cooling rate in this cooling is less than 50 ° C./sec, recrystallized austenite grains will grow. On the other hand, the upper limit of the cooling rate is not particularly defined, but 200 ° C./sec or less is considered appropriate from the viewpoint of the plate shape.

  Furthermore, in order to further improve the homogeneity of the hot-rolled sheet and increase the elongation and the local ductility, the rolling rate of the final pass is 25 in the temperature range of T1 + 30 ° C. or higher and T1 + 200 ° C. or lower. % Or more. However, if higher workability is required, the final two passes must be 25% or more.

Next, the microstructure of the steel sheet of the present invention will be described.
The microstructure of the steel sheet of the present invention comprises ferrite and bainite, tempered martensite, retained austenite, pearlite, and martensite.

  Ferrite and bainite concentrate C in retained austenite and are essential for improving ductility by the TRIP effect. It is possible to change the fraction of ferrite and bainite depending on the strength level targeted for development, but obtaining excellent ductility by setting ferrite to 20% or more and less than 50% or bainite to 10% or more and less than 60%. Can do.

  Residual austenite is a structure that enhances ductility, particularly uniform elongation, by transformation-induced plasticity, and the area ratio is required to be 5% or more. Moreover, since it transforms into martensite by processing, it contributes to the improvement of strength. The higher the area of retained austenite, the better. However, in order to ensure retained austenite with an area ratio of more than 30%, it is necessary to increase the amount of C and Si, which impairs weldability and surface properties. Therefore, the upper limit of the area ratio of retained austenite is set to 30% or less.

  Further, each of pearlite and martensite may be contained by 20%. Further, the particle size is such that the area ratio occupied by grains exceeding 20 μm is 10% or less in the entire structure. As the number of large grains increases, the tensile strength decreases and the local deformability also decreases. Therefore, it is preferable to make it as fine as possible.

In order to improve local deformability such as bending, stretch flange, and burring, it is preferable that hard structures such as retained austenite and martensite are dispersed. Therefore, the distance to the nearest retained austenite or martensite is L _MA [μm] among the grains of retained austenite and martensite, and the standard deviation of _LMA when 100 or more are measured is 5 or less.
[Example]

The technical contents of the present invention will be described with reference to examples of the present invention.
As examples, the results of studies using steels satisfying the components of claims of the present invention from a to j having the composition shown in Table 1 and comparative steels a to d will be described. After casting, these steels are reheated as they are or once cooled to room temperature, heated to a temperature range of 900 ° C. to 1300 ° C., and then hot-rolled under the conditions shown in Table 2 to 2 to 5 mm. A thick hot-rolled steel sheet was used.

  Tables 2 and 3 show the production conditions, the structure formation, and the mechanical properties. As indices of local deformability, the hole expansion rate of the final product and the critical bending radius by 60 ° V-bending were used. In the bending test, C direction bending and 45 ° direction bending were performed, and the ratio was used as an index of orientation dependency of formability. The tensile test and the bending test were compliant with JIS Z 2241 and Z2248 (V-block 90 ° bending test), and the hole expansion test was compliant with the ironwork standard JFS T1001. The X-ray random intensity ratio was measured at a 0.5 μm pitch in the 5/8 to 3/8 region of the cross section parallel to the rolling direction using the aforementioned EBSD. Further, the r value in each direction was measured by the method described above.

  As shown in Table 2, it can be seen that only those using a steel sheet satisfying the requirements of the present application can have both excellent hole expansibility and low bendability and molding anisotropy. Furthermore, it can be seen that those in the desired production condition range exhibit a superior hole expansion rate, bendability, and anisotropy of formability.

Claims (7)

% By mass
C: 0.02% or more, 0.5% or less,
Si: 0.001% to 4.0%,
Mn: 0.001% or more and 4.0% or less,
P: 0.001% or more, 0.15% or less,
S: 0.0005% or more, 0.03% or less,
N: 0.0005% or more, 0.01% or less,
O: 0.0005% or more, 0.01% or less,
Al + Si ≦ 4.0% or less,
Furthermore,
Ti: 0.001% or more, 0.2% or less,
Nb: 0.001% or more, 0.2% or less,
V: 0.001% or more, 1.0% or less,
W: 0.001% or more, 1.0% or less,
Cu: 0.001% or more, 2.0% or less,
B: 0.0001% or more, 0.005% or less,
Mo: 0.001% or more, 1.0% or less,
Cr: 0.001% or more, 2.0% or less,
As: 0.0001% or more, 0.50% or less,
Ni: 0.001% or more, 2.0% or less,
Co: 0.0001% or more and 1.0% or less,
Sn: 0.0001% or more, 0.2% or less,
Zr: 0.0001% or more, 0.2% or less,
{100} <011> to the surface of the plate at a thickness of 5/8 to 3/8 at least from the surface of the steel plate. The average value of the X-ray random intensity ratio of the {223} <110> orientation group is less than 4.0, the X-ray random intensity ratio of the crystal orientation of {332} <113> is 5.0 or less, and the rolling direction The r (rC) value in the direction perpendicular to the rolling direction is 0.70 or more, and the r value at 30 ° (r30) in the rolling direction is 1.10 or less. %, Ferrite is 20% or more, less than 50%, bainite is contained in an amount of 10% or more and less than 60%, and pearlite and martensite are each 20% or less. Less orientation dependency Excellent hot-rolled steel sheet to sex.   Furthermore, it is excellent in the local deformability of Claim 1 characterized by the area ratio which the particle | grains exceeding 20 micrometers occupy among the total area, and it was excellent in ductility with little orientation dependence of a moldability Hot rolled steel sheet. Further, the grain of the residual austenite and martensite, the distance to the closest retained austenite or martensite and L _{MA [μm],} the standard deviation of the L _MA when measured more than 100 is 5 or less claim 1 or A hot-rolled steel sheet having excellent local deformability described in 2 and excellent ductility with less orientation dependency of formability. Furthermore,
Mg: 0.0001% or more, 0.010% or less,
REM: 0.0001% or more, 0.1% or less,
Ca: 0.0001% or more, 0.010% or less,
Excellent local deformability according to any one of claims 1 to 3, containing one or more of,
Hot-rolled steel sheet with excellent ductility and less orientation dependency of formability. In producing the high-strength steel sheet according to any one of claims 1 to 4 , after melting into a predetermined steel plate component, casting into a steel ingot or slab, rough rolling is performed at 1000 ° C or more and 1200 ° C or less. T1 + 30 ° C. or more, assuming that the reduction of 20% or more is performed at least once in the temperature range, the austenite grain size is 200 μm or less, and then the temperature determined by the steel sheet component in formula (1) in finish rolling is T1. In the temperature range of T1 + 200 ° C. or less, rolling is performed at 30% or more in one pass, and the total rolling reduction in the temperature range is set to 50% or more, and then the temperature range of T1 or more and less than T1 + 30 ° C. and the total reduction ratio of 30% or less in an additional 30% or more of the final rolling after the end of T1 or more, satisfy the residence time t is the formula to the start cooling from the temperature range of T1 + 200 ° C. or less (2) In the Suyo, exit the hot rolled at Ar3 transformation temperature or higher, 630 ° C. or higher, the temperature range of 800 ° C. or less and cooled at 10 to 100 ° C. / sec, then 20 seconds or less for more than one second in the temperature range Hold or cool to the temperature range, cool to a temperature in the range of 550 ° C. or higher at a cooling rate of 20 ° C./sec or less, take up at 350 to 500 ° C., and the temperature change rate is −40 ° C./h As described above, a method for producing a hot-rolled steel sheet having excellent ductility, which is excellent in local deformability after being held in a range of 40 ° C./h or less for 30 to 300 minutes and then air-cooled and has less orientation dependency of formability.
T1 (° C.) = 850 + 10 × (C + N) × Mn + 350 × Nb + 250 × Ti + 40 × B + 10 × Cr + 100 × Mo + 100 × V (1)
t ≦ t1 × 2.5 (2)
Here, t1 is represented by Formula (3).
t1 = 0.001 ((Tf−T1) × P1) ² −0.109 ((Tf−T1) × P1) +3.1 (3)
Here, Tf is the temperature after the final reduction of 30% or more, and P1 is the reduction ratio of the final reduction of 30% or more. 6. The method according to claim 5 , wherein the cooling is performed so that the change in cooling temperature is 40 ° C. or more and 150 ° C. or less at a cooling rate of 50 ° C./sec or more within 1 second from the end of finish rolling. A method for producing a hot-rolled steel sheet having excellent ductility and excellent ductility with less orientation dependency of formability. The manufacturing method according to claim 5 or 6 , wherein the rolling rate of the final pass of rolling in a temperature range of T1 + 30 ° C or higher and T1 + 200 ° C or lower is 25% or more, and has excellent local deformability and has formability. A method for producing a hot-rolled steel sheet having excellent ductility with little orientation dependency.

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