JP4464748B2 - High-strength steel sheet, high-strength hot-dip galvanized steel sheet, high-strength alloyed hot-dip galvanized steel sheet excellent in shape freezing property and stretch flangeability, and methods for producing them - Google Patents

Description

The present invention relates to a high-strength steel sheet, high-strength hot-dip galvanized steel sheet and high-strength alloyed hot-dip galvanized steel sheet excellent in shape freezing property and stretch flangeability, and methods for producing them. It is.

  The steel plate of the present invention contains both a hot rolled steel plate and a cold rolled steel plate. In addition to pure zinc, the hot dip galvanizing may contain Fe, Al, Mg, Cr, Cu and the like.

  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 reduce the weight of automobile bodies in the future, new demands for increasing the strength level of use of high-strength steel sheets are increasing.

  However, when forming a high-strength steel sheet, the shape after processing tends to move away from the shape of the processing jig and return to the direction of the shape before processing because of its high strength. -A wall warp phenomenon in which the flat surface of the side wall becomes a curved surface due to the elastic recovery from the bending back, resulting in a dimensional accuracy defect that the shape of the target processed part cannot be obtained.

  Therefore, high-strength steel sheets of 590 MPa or less have been mainly used in conventional automobile bodies. That is, for a car body, although it is necessary to reduce the weight of the car body using a high-strength steel plate of 590 MPa or more, spring back and wall warpage hardly occur, and dimensional accuracy is good. The fact is that there is no high-strength steel sheet with good shape freezing properties.

  Some of the present inventors have disclosed a high-strength steel sheet excellent in shape freezing property focusing on the texture of the steel sheet in Patent Document 1 and Patent Document 2. Patent Document 3 discloses a hot-rolled steel sheet having an r-value in-plane anisotropy Δr of 0.2 or less as a steel sheet having a good shape freezing property.

  However, this invention is characterized by improving the shape freezing property by lowering the yield ratio, and regarding texture control for the purpose of improving the shape freezing property based on the idea described in the present invention. Is not described.

  On the other hand, Patent Document 4 discloses a steel sheet excellent in formability to which Ti and Mo are added, but it is not a technique for improving shape freezeability.

  As described above, attempts to improve the shape freezing property have been made, but the improvement allowance cannot be said to be sufficient, and a high-strength steel sheet having a better shape freezing property is desired.

WO00 / 066791 JP 2001-64750 A JP 2000-297349 A JP 2002-322539 A

  When processing high-strength steel sheets with a tensile strength of over 590 MPa, depending on the strength of the steel sheets, shape defects such as large spring backs and wall sleds occur, and the shape freezing property of processed molded parts is poor. It is. The present invention fundamentally solves this problem and provides a high-strength steel sheet excellent in ductility, stretch flange formability, and weldability, and a method for producing the same.

  According to the conventional knowledge, it has been considered to be important for the time being to reduce the deformation stress of the steel sheet as a measure for suppressing shape defects such as spring back and wall warp. And in order to make a deformation stress low, the steel plate with low tensile strength had to be used. However, this is not a fundamental solution for keeping the springback amount of high-strength steel sheets low.

  Therefore, in order to improve the bending workability and fundamentally solve the occurrence of springback and wall warpage, the present inventors have newly focused on the influence of the texture of the steel sheet and the strengthening method on the bending workability. The effect was investigated and studied in detail. As a result, a combination of precipitation strengthening, which had been positioned as having high yield strength and inferior shape freezing properties, and a predetermined texture resulted in obtaining a high strength steel sheet with better shape freezing properties than ever before. .

  That is, after increasing the strength of the {100} <011> to {223} <110> orientation group and setting at least one of the r value in the rolling direction and the r value in the direction perpendicular to the rolling direction as low as possible, It is clarified that the shape freezing property is remarkably improved by dispersing fine precipitates.

  The reason why such an effect is obtained is not necessarily clear, but is considered as follows.

  In other words, by using such a texture, basically, since the slip system is limited in the bending-based molding, dynamic recovery is likely to occur, resulting in a low deformation resistance. Fine precipitates have the effect of further limiting the slip system and further improve the shape freezeability.

  In addition, when an inversion load such as bending and bending back occurs, dislocations piled up on fine precipitates act as back stress on the inversion load and lower the deformation resistance. In this case as well, the shape freezing property is improved. . Therefore, the steel sheet of the present invention is effective for various forming methods such as draw bending and foam drawing in addition to foam forming.

  In order to achieve both ductility and stretch flangeability and shape freezing properties, it is also important to make the ferrite phase or bainite phase the maximum phase.

  The present invention is configured based on the above-described knowledge, and the gist thereof is as follows.

(1) In mass%,
C: 0.02% or more, 0.15% or less,
Si: more than 0.5%, 1.6% or less,
Mn: 0.01% or more, 3.0% or less,
P: 0.02% or less,
S: 0.01% or less,
Al: 2.0% or less,
N: 0.01% or less,
O: 0.01% or less ,
Ti: 0.054% or more, 0.4% or less ,
B: 0.0002% or more and 0.0070% or less,
Nb: 0.4% or less, Mo: 1.0% or less of 1 type or 2 types ,
The balance consists of iron and unavoidable impurities, with ferrite or bainite as the maximum phase in area ratio, and X-ray random strength of {001} <110> to {223} <110> orientation groups on the plate surface at 1/2 plate thickness The average value of the ratio is 6.0 or more, and among these orientation groups, the X-ray random intensity ratio of either or both of the {112} <110> orientation and the {001} <110> orientation is 8 In addition, at least one of the r value in the rolling direction and the r value in the direction perpendicular to the rolling direction is 0.8 or less and the number of compound particles having a diameter of 15 nm or less is the total number of compound particles. A high-strength steel sheet excellent in shape freezeability and stretch flangeability, characterized by being 60% or more of the number.

(3) High strength excellent in shape freezing property and stretch flange formability as described in (1) or (2) above, further comprising B: 0.0002 to 0.0070% by mass% steel sheet.

(2) In the above (1), the composition further contains one or more of V: 0.4% or less, Zr: 0.2% or less, and Cr: 2% or less. High-strength steel sheet with excellent shape freezing and stretch flange formability.

(3) By mass%, it further contains Rem: 0.001% or more and 0.1% or less, in the shape freezing property and stretch flange formability described in the above (1) or (2) Excellent high-strength steel sheet.

(4) the (1) to a high-strength galvanized steel sheet having excellent shape fixability and stretch flangeability which is characterized by having a surface galvanized high strength steel sheet according to any one of (3) .

(5) High strength alloying excellent in shape freezing property and stretch flange formability, characterized by having alloyed hot dip galvanizing on the surface of the high strength steel plate according to any one of (1) to (3) . Hot dip galvanized steel sheet.

(6) the (1) to (3) a method for producing a high strength steel sheet excellent in shape fixability and stretch flangeability according to any one of any one of (1) to (3) When hot-rolling a steel slab having the composition described in 1. above, it is heated to 1150 ° C. or higher, hot-rolled so that the total rolling reduction at less than 880 ° C. is 30% or more, and heated at less than 880 ° C. The shape freezing property and stretch flange forming are characterized in that the hot rolling is finished and the temperature from the hot rolling finish temperature to 700 ° C. or lower is cooled at an average cooling rate of 30 ° C./s or more and then wound at 580 to 680 ° C. A method for producing high-strength steel sheets with excellent properties.

(7) the (1) to (3) a method for producing a high strength steel sheet excellent in shape fixability and stretch flangeability according to any one of any one of (1) to (3) When hot-rolling a steel slab having the composition described in 1. above, it is heated to 1150 ° C. or higher, hot-rolled so that the total rolling reduction at less than 880 ° C. is 30% or more, and heated at less than 880 ° C. Finished hot rolling, cooled at an average cooling rate of 15 ° C./s or more, air-cooled in the range of 650 to 720 ° C. for 5 s or more, and then wound up at 660 ° C. or less. For producing high-strength steel sheets with excellent resistance.

(8) The high- strength steel sheet according to (6) or (7) is cold-rolled at a rolling reduction of less than 60%, and then heated to a temperature range of 500 to 800 ° C. and cooled. A method for producing high-strength steel sheets with excellent shape freezing properties and stretch flangeability.

(9) A temperature range of 500 to 800 ° C. of a steel plate produced by the method for producing a high- strength steel plate according to (6) or (7) , or a steel plate cold-rolled with a reduction rate of less than 60%. A method for producing a high-strength hot-dip galvanized steel sheet having excellent shape freezing property and stretch flange formability, characterized by heating to galvanizing and then hot-dip galvanizing.

(10) High-strength galvannealed alloy with excellent shape freezing property and stretch flange formability, characterized by being alloyed in the range of 450 to 600 ° C. following hot dip galvanizing described in (9 ) above Manufacturing method of plated steel sheet.

  According to the present invention, by simultaneously controlling the precipitate and texture of the thin steel plate, the shape freezing property is remarkably improved, and a thin steel plate having excellent hole expansibility can be provided. In particular, a high-strength steel plate can be used even for parts that have conventionally been difficult to apply a high-strength steel plate due to the problem of shape defects. If a high-strength steel sheet having a small amount of spring back and wall warpage and having excellent shape freezing property and hole expandability can be applied, it is possible to further reduce the weight of the automobile body.

  The contents of the present invention will be described in detail below.

Average value of X-ray random intensity ratio of {001} <110> to {223} <110> orientation group on the plate surface at 1/2 plate thickness:
This is an important characteristic value in the present invention. {001} <110> to {223} <when X-ray diffraction of the plate surface at the plate thickness center position is performed, normalized using a random sample, and the pole density (random intensity ratio) in each direction is obtained. The average value of 110> direction group must be 6.0 or more. If this is less than 6.0, the shape freezing property becomes poor.

  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 (pole density) in each of these directions is obtained by using a plurality of pole figures (preferably three or more) among the pole figures of {110}, {100}, {211}, and {310}. What is necessary is just to obtain | require from the three-dimensional texture calculated by the series expansion method.

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

  The average value of {001} <011> to {223} <110> azimuth group is an arithmetic average of the above azimuths. When the strengths of all the above directions cannot be obtained, {100} <110>, {116} <110>, {114} <110>, {112} <110>, {223} <110> Alternatively, an arithmetic average of each direction may be substituted.

  Among these orientation groups, the {001} <110> and {112} <110> orientations are extremely effective orientations for reducing wall warpage. Therefore, among these orientation groups, {001} It is preferable that the X-ray random intensity ratio in the <110> or {112} <110> orientation is maximum and 8.0 or more because the shape freezing property is further improved.

  Furthermore, the average value of the X-ray random intensity ratio of the three crystal orientations of {554} <225>, {111} <112> and {111} <110> on the plate surface at 1/2 plate thickness is 3.5 or less. It is preferable that If this exceeds 3.5, it will be difficult to obtain good shape freezing properties even if the strength of the {100} <011> to {223} <110> orientation groups is appropriate.

  The X-ray random intensity ratio of {554} <225>, {111} <112>, and {111} <110> may be obtained from the three-dimensional texture calculated according to the above method.

  More preferably, the average value of the X-ray random intensity ratios in the {001} <011> to {223} <110> orientation groups is 8.0 or more, and the {001} <011> or {112} <110> orientation X The linear random intensity ratio is 10.0 or more, and the arithmetic average value of the X-ray random intensity ratios of {554} <225>, {111} <112> and {111} <110> is less than 2.5.

  The sample to be subjected to X-ray diffraction is thinned to a predetermined plate thickness by mechanical polishing or the like, and then the distortion is removed by chemical polishing or electrolytic polishing, and at the same time, the 1/2 plate thickness becomes the measurement surface. To make.

  When there is a segregation zone or a defect in the thickness center layer of the steel sheet, which causes inconvenience in measurement, the above-described surface is set so that an appropriate surface becomes the measurement surface in the range of 3/8 to 5/8 of the plate thickness. The sample may be adjusted according to the above method.

  As a matter of course, the above-described 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 shape freezing property is further improved. 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 (rL) in the rolling direction and r value (rC) in the direction perpendicular to the rolling direction:
Important in the present invention. That is, as a result of intensive studies by the present inventors, the shape density of the above-mentioned various crystal orientations is appropriate and at the same time, at least one of rL and rC is 0.8 or less. Turned out to be essential to obtain. More preferably, it is 0.6 or less.

  The lower limit of rL and rC is not particularly defined, and the effect of the present invention can be obtained. The r value is evaluated by a tensile test using a JIS No. 5 tensile test piece. The tensile strain is usually 15%, but if the uniform elongation is less than 15%, the strain may be evaluated as close to 15% as possible within the range of uniform elongation.

  The direction in which the bending process is performed differs depending on the processed part, and is not particularly limited. However, it is preferable that the bending process is mainly performed in a direction perpendicular to or close to the perpendicular to the direction in which the r value is small.

  Incidentally, it is generally known that there is a correlation between the texture and the r value. However, in the present invention, the above-mentioned limitation on the polar density of the crystal orientation and the limitation on the r value are not synonymous with each other. Unless the above limitations are satisfied at the same time, good shape freezing property cannot be obtained.

Organization:
From the viewpoints of ductility, stretch flangeability, and shape freezing, the structure has a ferrite or bainite phase as the maximum phase. However, when the textures of ferrite and bainite are compared, the texture of {001} <011> to {223} <110> orientation, which is advantageous for shape freezing, easily develops in the bainite portion.

  The reason for this is not clear, but it is considered that the bainite structure easily inherits the austenite texture that is formed during hot rolling and has an excellent shape freezing property. Therefore, it is more desirable that the bainite space factor is large.

  From this point of view, the area ratio of bainite is desirably more than 50%. The area ratio of ferrite or bainite is determined from an average value obtained by observing the central part of the plate thickness with an optical microscope at 100 to 1000 times at least 5 views. If a segregation layer or the like is present in the center of the plate thickness, observe it while avoiding it. The plate thickness center portion indicates a plate thickness of 3/8 to 5/8.

Compound particles:
This is very important in the present invention. That is, as described above, the presence of many fine compound particles makes the texture effect more remarkable. Moreover, it acts advantageously also from a viewpoint of ductility and stretch flange formability.

  The number of compound particles having a diameter of 15 nm or less needs to occupy 60% or more of the total number of compound particles. If the diameter exceeds 15 nm, such an effect is small, so this is the upper limit. Preferably it is 10 nm or less, More preferably, it is 5 nm or less.

  The diameter indicates the longest diameter of the particle. That is, when the observed particle is circular, the diameter is represented, when the particle is elliptical, the major axis is represented, and when the observed particle is quadrilateral, the length of the diagonal line is represented.

  Such fine particles are mainly composed of Ti-based carbides, nitrides, and carbonitrides, but may be oxides, sulfides, or compounds other than Ti.

  All the compound particles need not be 15 nm or less, and the total number of compound particles is 60% or more, more preferably 85% or more. If this is less than 60%, the effect on the shape freezing property is limited even if the texture is suitably developed.

  As a sample for observing the compound particles, a thin film collected from the range of 3/8 to 5/8 of the plate thickness was used, and 100,000 times with a transmission electron microscope (preferably an electrolytic emission electron gun). With the above magnification, an area of 5 μm or more is observed.

  Select a field of view at random, and observe at least 5 fields of view. At this time, the fine compound existing in the crystal grain boundary is difficult to observe, so it is not an object, and only the inside of the crystal grain is an object. In order to avoid overlooking the compound, it is preferable to use a defocus method in which the focus is intentionally shifted.

  Obviously, depending on the thickness of the thin film, the number of compounds observed is different even when the same area is observed, but here, the size of the fine compounds and the abundance ratio of them are considered. Therefore, it is preferable that the thickness is sufficiently thick as long as it can be clearly observed with a transmission electron microscope.

  The number of particles observed under the above conditions is the total number of particles here. The density of the fine compound particles is not particularly limited, but is 10 or more per square μm as observed with the electron microscope. Preferably it is 100 or more.

  Further, when the occupation ratio of cementitious carbide such as cementite at the grain boundary exceeds 0.1 or the maximum particle diameter of the iron carbide exceeds 1 μm, these iron carbides are connected at the grain boundary, and the stretch flangeability is remarkably deteriorated. . Therefore, it is preferable that the occupation ratio of the iron carbide in the grain boundary is 0.1 or less and the maximum particle diameter of the iron carbide is 1 μm or less. Since the iron carbide occupancy and the maximum particle size are preferably as small as possible, the lower limit is not particularly defined.

  The grain boundary occupancy (-) by iron carbide is the ratio d / of the sum d of the total length L of grain boundaries in a certain region and the length of grain boundaries occupied by iron carbide in a cross-sectional sample of iron material. L is given.

  In the measurement, L and d may be directly obtained by image processing in an optical microscope observation photograph having a magnification of 200 times or more. As a simpler method, the number N of intersections between the n straight lines drawn on the photograph and the grain boundary, and the number M when iron carbide exists at the position of the intersections among the N intersections. You may obtain | require by M / N using.

  Sufficient accuracy can be ensured by setting the number N of straight lines to be 3 or more. Moreover, sufficient accuracy can be ensured by selecting the magnification of the photograph so that the number of intersections of the single straight line and the grain boundary is 10 or more.

  Next, the limiting conditions of the component range will be described. In addition,% means the mass%.

  The reason why the lower limit of C is set to 0.02% is that when the lower limit is not reached, it is difficult to obtain a tensile strength of 590 MPa or more, and a fine compound cannot be obtained sufficiently. On the other hand, if it exceeds 0.15%, stretch flange formability deteriorates, so the upper limit is set to 0.15%.

  Si is important in the present invention. That is, it was newly found that the compound becomes finer by adding more than 0.5% of Si, and as a result, the shape freezing property is improved. Therefore, the lower limit is made over 0.5%.

  On the other hand, if it exceeds 1.6%, the workability deteriorates or problems such as surface defects and plating properties occur, so 1.6% is made the upper limit. More than 0.5% to 1.2% is a more preferable range. Moreover, when performing hot dip galvanization, it is preferable to add more than 0.5% to less than 0.8%.

  Mn is also an element effective for increasing the mechanical strength of the steel sheet. However, if it exceeds 3.0%, ductility and stretch flange formability deteriorate, so 3.0% is made the upper limit. On the other hand, in practical steel, if Mn is less than 0.01%, the cost is high and there is no merit in material, so 0.01% is made the lower limit.

  P and S are 0.02% or less and 0.01% or less, respectively. This is to prevent weldability deterioration and cracking during hot rolling or cold rolling to ensure weldability.

  Al improves ductility through deoxidation and promotion of ferrite formation. On the other hand, if it is too much, workability and weldability deteriorate and surface properties become poor, so the upper limit is made 2.0%.

  N and O are useful for obtaining a fine compound. However, if too much is added, a coarse compound is increased and the shape freezing property is deteriorated, so the content is made 0.01% or less and 0.01% or less, respectively.

  Ti is an important element in the present invention. Ti precipitates finely as carbides, nitrides, carbonitrides or compounds such as sulfides, carbon sulfides, carbonitride sulfides, and is effective in increasing strength, while reducing iron carbides including cementite. Therefore, shape freezing property and stretch flangeability are improved.

Further, Ti has an effect of developing a texture preferable for the shape freezing property described above. Therefore, 0.05% or more is added depending on the desired strength. However, even if added excessively, there is no remarkable effect. Rather, the workability and surface properties are deteriorated, so 0.4% was made the upper limit. In the present invention, the lower limit of Ti is set to 0.054% based on the values of examples described later.

  Nb and Mo, like Ti, are finely precipitated as carbides, nitrides, carbonitrides or sulfides, carbonitrides, carbonitrides, and the like, and are effective in increasing strength, and iron such as cementite. Since the carbides are reduced, the shape freezing property and stretch flangeability are improved. It also has the effect of developing a texture that is favorable for shape freezing.

  Therefore, Nb is added at 0.4% or less and Mo is added at 1.0% or less depending on the desired strength. However, even if added excessively, there is no remarkable effect, but rather the workability and surface properties are deteriorated, so the upper limits of Nb and Mo were set to 0.4% and 1.0%, respectively. Nb and Mo may exist in the Ti-based compound.

  B improves the shape-freezing property by contributing to the refinement of the compound, and is added as necessary. If the content is less than 0.0002%, the effect is small. On the other hand, addition of a large amount deteriorates the ductility of the steel sheet, so the content is made 0.0070% or less.

  V, Zr, W, Cr, Cu, Ni, and Sn increase the mechanical strength and have the effect of improving the material such as hydrogen embrittlement resistance, workability, and shape freezing properties. Add. However, since excessive addition adversely degrades workability, the upper limits of V, Zr, W, Cr, Cu, Ni, and Sn are set to 0.4%, 0.2%, and 0.3%, respectively. 2%, 2%, 2%, and 0.2%.

  Ca, Mg, and Rem form a fine compound by controlling the form of sulfide, and improve shape freezing property and stretch flange formability. Therefore, if necessary, 0.0005% or more, It is desirable to add 0.0001% or more and 0.001% or more. Even if added excessively, there is no remarkable effect and the cost is high, so the upper limits of Ca, Mg, and Rem were set to 0.01%, 0.03%, and 0.1%, respectively.

  Although not particularly limited in the present invention, O is useful for forming fine compound particles, so 0.0005% or more may be added.

  The chemical component of the hot dip galvanized film is not particularly limited, and may contain Al, Mg, Cr, Ni, Cu, Mn, Fe, etc. in addition to zinc (Zn). In the case of alloyed hot dip galvanizing, an alloy phase of zinc (Zn) and iron (Fe) is mainly used, but other compounds may be contained.

  In addition to the above hot dip galvanizing and alloying hot dip galvanizing, various platings and coatings may be applied to the surface of the steel sheet of the present invention. The type of plating is not particularly limited, and the effects of the present invention can be obtained by any of electroplating, hot dipping, vapor deposition plating and the like.

  Next, a manufacturing method will be described.

  The production method preceding hot rolling is not particularly limited. That is, various secondary smelting may be performed following smelting in a blast furnace, converter, electric furnace, etc., and then cast by a method such as thin slab casting in addition to normal continuous casting and casting by an ingot method. In the case of continuous casting, after cooling to a 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 steel sheet excellent in shape freezing property of the present invention, after casting the steel of the above component composition, as it is cooled after hot rolling, heat treatment after hot rolling, cooling after cold rolling, pickling and annealing after cold rolling, Alternatively, it can be obtained by plating a hot-rolled steel sheet or a cold-rolled steel sheet, or by subjecting these steel sheets to a separate surface treatment while being heat-treated in a hot dipping line.

  The heating temperature for hot rolling is 1150 ° C. or higher. When the heating temperature is less than 1150 ° C., the Ti and Nb compounds are not sufficiently re-dissolved, so that not only a sufficient tensile strength cannot be obtained, but also the effect of sharpening the texture is reduced, and the coarse compound Hole expandability deteriorates.

  The upper limit is not particularly limited, but even if the heating temperature exceeds 1350 ° C., the effect is saturated, and the disadvantage is large in terms of cost and equipment, so 1350 ° C. is a practical upper limit.

  In order to achieve the X-ray intensity ratio level of each crystal orientation defined in the invention of (1) above, a total of 30% or more rolling is performed at less than 880 ° C. If this rolling is not performed, the texture of the rolled austenite does not develop sufficiently, and therefore, whatever the cooling is applied to the plate surface of the hot-rolled steel sheet finally obtained, The pole density specified in the invention of (1) cannot be obtained. Therefore, the lower limit of the total rolling reduction at less than 880 ° C. is set to 30%.

  As the total rolling reduction is higher, sharper texture formation is expected. Therefore, it is preferable to set it to 40% or more. However, if the total rolling reduction exceeds 97.5%, it is necessary to excessively increase the rigidity of the rolling mill. There is an economic demerit, so 97.5% or less is desirable.

When the hot rolling end temperature is 880 ° C. or higher, the entire texture is randomized, and the shape freezing property deteriorates. Therefore, the upper limit is less than 880 ° C. Furthermore, less than 850 degreeC is more preferable. Although the lower limit is not particularly specified, the rolling load is likely to fluctuate below the Ar ₃ transformation temperature and the plate thickness accuracy is deteriorated. Therefore, it is preferable to finish at the Ar ₃ transformation point or higher.

When the friction coefficient between the hot rolling roll and the steel plate at the time of hot rolling at less than 880 ° C. exceeds 0.2, the crystal orientation mainly consisting of {110} plane is present on the plate surface near the steel plate surface. Since it develops and shape freezeability deteriorates, when aiming for better shape freezeability, at least one pass during hot rolling at an Ar ₃ transformation temperature or higher (Ar ₃ +100) ° C. or lower is hot rolled. It is desirable that the coefficient of friction between the roll and the steel plate be 0.2 or less.

The lower the coefficient of friction, the better. The lower limit is not defined, but if a better shape freezing property is required, all passes of hot rolling above the Ar ₃ transformation temperature and below (Ar ₃ +100) ° C. It is desirable that the coefficient of friction be 0.15 or less. The method for measuring the friction coefficient is not particularly defined, but it is desirable to obtain it from the advanced rate and the rolling load, as is generally well known.

  After the hot rolling is finished, it is cooled according to one of the following two methods and then wound up.

  The first method will be described. By cooling from the hot rolling end temperature to 700 ° C. or less at an average cooling rate of 30 ° C./s or more, a sharp texture is formed, and at the same time, formation of a fine compound during winding is promoted. If it is less than 30 ° C./s, such an effect becomes small, so 30 ° C./s is made the lower limit.

  The coiling temperature is set to a range of 580 to 680 ° C. for the same reason. When the temperature exceeds 680 ° C., a coarse compound appears, and the balance between shape freezing property and stretch flange formability and tensile strength deteriorates. On the other hand, if it is less than 580 ° C., it is difficult to obtain a fine compound sufficiently, so 580 ° C. is the lower limit.

  Next, the second method will be described. The average cooling rate from the hot rolling end temperature to the subsequent air cooling start temperature is set to 15 ° C./s or more. If it is less than 15 degreeC / s, the quantity of a coarse compound will increase and shape freezing property and stretch flange formability will deteriorate. Subsequently, it is air-cooled for 5 seconds or more within a temperature range of 650 to 720 ° C. This has the effect of promoting the formation of a texture that is favorable for the production of fine compounds and the shape freezing property.

  Thereafter, the coiling temperature is 660 ° C. or lower. When it exceeds 660 degreeC, a coarse precipitate will produce | generate and shape freezing property will deteriorate. The lower limit is not limited, but if it is less than room temperature, the cost will increase and there will be no special effect.

  In hot rolling, a sheet bar may be joined after rough rolling, and finish rolling may be performed continuously. At that time, the coarse bar may be wound once in a coil shape, stored in a cover having a heat retaining function, if necessary, and rewound again before joining.

  The hot-rolled steel sheet may be subjected to skin pass rolling as necessary. Needless to say, the skin pass rolling has the effect of preventing stretcher strain generated during processing and shape correction.

  When the hot-rolled steel sheet thus obtained is cold-rolled and annealed to obtain a final thin steel sheet, the total rolling reduction of the cold-rolling is set to less than 60%. If it is 60% or more, the {111} plane and {554} plane components become high in the X-ray diffraction integral plane intensity ratio of the crystal plane parallel to the plate surface, which is a general cold rolling-recrystallization texture. This is because the crystal orientation defined in the invention of (1), which is a feature of the invention, is not satisfied.

  In order to enhance the shape freezing property, it is desirable to limit the cold reduction rate to 40% or less. The lower limit of the cold rolling rate is not particularly defined, and the effects of the present invention can be obtained. However, in order to control the strength of the crystal orientation within an appropriate range, it is preferably 3% or more.

  When annealing a cold-rolled steel sheet that has been cold worked in such a range, if the annealing temperature is less than 500 ° C, the processed structure remains and the formability deteriorates significantly, so the lower limit of the annealing temperature is 500 ° C. And On the other hand, when the annealing temperature exceeds 800 ° C., TiC and NbC are coarsened and the hole expansibility deteriorates, and the shape freezing property also decreases. Accordingly, the annealing temperature is set to 500 to 800 ° C.

  The cold-rolled steel sheet may be subjected to skin pass rolling as necessary.

  After hot rolling and after cold rolling with a rolling reduction of less than 60%, hot dip galvanizing may be performed. At this time, the maximum temperature reached is 500 to 800 ° C. If it is less than 500 ° C., the intrusion plate temperature when immersed in the hot dip galvanizing bath is much lower than the hot dip zinc bath temperature, and it becomes difficult to keep zinc in a molten state. Further, if it exceeds 800 ° C., the compound particles become coarse and the shape freezing property and the hole expansibility decrease, so 800 ° C. is the upper limit.

  After hot dip galvanization, an alloying treatment may be performed as necessary. The alloying treatment is a heat treatment for reacting Zn with Fe of a steel plate to form a compound of Fe and Zn.

  If the alloying treatment temperature is less than 450 ° C, the time required for the reaction becomes excessive, so 450 ° C is set as the lower limit. On the other hand, if it exceeds 600 ° C., the alloying reaction proceeds too much and the alloy layer becomes brittle, and troubles such as powdering are induced by the forming process. A preferred range is 480-560 ° C.

  Note that the steel sheet according to the present invention can be applied not only to bending, but also to composite forming mainly composed of bending, such as bending, overhanging and drawing.

Example 1
The technical contents of the present invention will be described with reference to examples of the present invention.

  First, the results of studies using steels A to K having the component compositions shown in Table 1 will be described. These steels were heated to 1250 ° C. after casting, and then hot-rolled under the conditions shown in Table 2 to finally form hot rolled steel sheets having a thickness of 1.4 mm. After pickling, 0.5% skin pass was applied.

  A test piece having a width of 50 mm and a length of 270 mm was prepared from this plate, and a hat bending test was performed using a die having a punch width of 78 mm, a punch shoulder R5, and a die shoulder R5.

  The test piece subjected to the bending test was measured for the shape of the central part of the plate width with a three-dimensional shape measuring device, and as shown in FIG. 1, the tangent line of point (a) and point (b) and point (c) The average value on the left and right of the value obtained by subtracting 90 ° from the angle of the intersection of the tangent line and point (d) is averaged on the left and right, and the inverse of the curvature between point (c) and point (e) The shape freezing property was evaluated using the value obtained by subtracting the punch width from the wall warp amount and the length between the left and right points (e) as the dimensional accuracy.

  The bending direction was referred to as L bending when the perpendicular direction to the bending ridge line was parallel to the rolling direction, and C bending when the bending ridge line was parallel to the rolling direction.

  By the way, as shown in FIGS. 2 and 3, the amount of spring back and the amount of wall warp also change depending on the BHF (wrinkle pressing force). The effect of the present invention does not change even if the evaluation is performed with any BHF, but when pressing actual parts with an actual machine, a very high BHF cannot be applied. The hat bending test was performed.

  The X-ray measurement was performed by adjusting a sample parallel to the plate surface at a position of 7/16 thickness as a representative value of the steel plate.

In the hole expansion test, a punched hole with a diameter of 10 mm is processed in the center of a test piece with a side of 100 mm, the initial hole is expanded with a conical punch with an apex angle of 60 °, and the hole diameter when the crack penetrates the steel plate. Evaluation was made with a hole expansion ratio λ (the following formula) for an initial hole diameter of 10 mm.
λ = {(d−10) / 10} × 100 (%)

  As a sample for observing the compound particles, a thin film collected from a range of 3/8 to 5/8 of the plate thickness was used, and a transmission electron microscope equipped with an electrolytic emission electron gun was used at 150,000 times and 10 square μm or more. Observe the area. At this time, the visual field was selected at random, and 10 visual fields or more were selected. The number of particles observed was taken as the total number of particles. However, the compound particles on the grain boundaries were ignored. The ratio of the number of particles having a diameter of 15 nm or less with respect to the total number of particles was determined.

  Table 3 shows the tensile properties, X-ray random strength ratio, spring-back amount, wall warp amount, and hole expansion rate measured with a JIS No. 5 test piece. In addition, as for the structure | tissue of the steel plate which satisfy | fills the conditions of this invention, all were ferrite or bainite.

  It can be seen that the example of the present invention has a smaller amount of spring back and wall warp than those outside the invention, resulting in improved dimensional accuracy. The cases of the present invention have good stretch flangeability in any case. That is, it is possible to obtain a steel sheet with good shape freezing property and high stretch flangeability only after satisfying the conditions of the components limited in the present invention, the polar density of each crystal orientation, the r value, and the compound particles.

(Example 2)
B-1, C-1 and I-1 shown in Example 1 were heated to 700 ° C. and 850 ° C., held for 40 s, cooled to 460 ° C. at a rate of 10 ° C./s, and then molten Zn It was immersed in a plating bath (Al content = 0.09%) and further subjected to alloying treatment at 530 ° C. for 40 s. The skin pass rolling rate was 0.5%.

  As is apparent from Table 4, when the heating temperature was 700 ° C., which is the range of the present invention, better characteristics were shown compared to the case of 850 ° C.

(Example 3)
The hot rolled sheets of B-1, C-1 and I-1 shown in Example 1 were cold-rolled at a reduction rate of 30%, heated to 700 ° C. and 850 ° C., held for 60 s, and then 5 ° C. After being cooled for 20 s at / s and held at 350 ° C. for 250 s, it was cooled to room temperature and a skin pass was applied at 0.3%.

  As is apparent from Table 5, when the heating temperature was set to 700 ° C., which is the range of the present invention, better characteristics were shown compared to the case of 850 ° C.

  According to the present invention, as described above, it is possible to further promote weight reduction of the automobile body. Therefore, the present invention is an invention having very high industrial applicability.

It is a figure which shows the cross section of the test piece used for the hat bending test. It is a figure which shows the relationship between springback amount and BHF (wrinkle pressing force). It is a figure which shows the relationship between wall curvature amount and BHF (wrinkle pressing force).

Claims (10)

% By mass
C: 0.02% or more, 0.15% or less,
Si: more than 0.5%, 1.6% or less,
Mn: 0.01% or more, 3.0% or less,
P: 0.02% or less,
S: 0.01% or less,
Al: 2.0% or less,
N: 0.01% or less,
O: 0.01% or less ,
Ti: 0.054% or more, 0.4% or less ,
B: 0.0002% or more and 0.0070% or less,
Nb: 0.4% or less, Mo: 1.0% or less of 1 type or 2 types ,
The balance consists of iron and unavoidable impurities, with ferrite or bainite as the maximum phase in area ratio, and X-ray random strength of {001} <110> to {223} <110> orientation groups on the plate surface at 1/2 plate thickness The average value of the ratio is 6.0 or more, and among these orientation groups, the X-ray random intensity ratio of either or both of the {112} <110> orientation and the {001} <110> orientation is 8 In addition, at least one of the r value in the rolling direction and the r value in the direction perpendicular to the rolling direction is 0.8 or less and the number of compound particles having a diameter of 15 nm or less is the total number of compound particles. A high-strength steel sheet excellent in shape freezeability and stretch flangeability, characterized by being 60% or more of the number. The shape freezing according to claim 1, further comprising one or more of V: 0.4% or less, Zr: 0.2% or less, and Cr: 2% or less in mass%. High-strength steel sheet with excellent stretchability and stretch flangeability. The high-strength steel sheet excellent in shape freezing property and stretch flange formability according to claim 1 or 2 , further comprising Rem: 0.001% to 0.1% by mass. A high-strength hot-dip galvanized steel sheet excellent in shape freezing property and stretch flange formability, comprising hot-dip galvanizing on the surface of the high-strength steel sheet according to any one of claims 1 to 3 . A high-strength galvannealed steel sheet excellent in shape freezing property and stretch flange formability, characterized in that the surface of the high-strength steel sheet according to any one of claims 1 to 3 is alloyed galvanized. . A claim 1-3 or a method of manufacturing a high strength steel sheet excellent in shape fixability and stretch flangeability according to one of, the component composition according to any one of claims 1 to 3 When hot-rolling a steel slab made of the above, it is heated to 1150 ° C. or higher, hot-rolled so that the total reduction ratio at less than 880 ° C. is 30% or more, and hot rolling is finished at less than 880 ° C. After cooling from the hot rolling end temperature to 700 ° C. or less at an average cooling rate of 30 ° C./s or more, it is wound at 580 to 680 ° C. and has excellent shape freezing property and stretch flange formability. A method for producing a strength steel plate. A claim 1-3 or a method of manufacturing a high strength steel sheet excellent in shape fixability and stretch flangeability according to one of, the component composition according to any one of claims 1 to 3 When hot-rolling a steel slab made of the above, it is heated to 1150 ° C. or higher, hot-rolled so that the total reduction ratio at less than 880 ° C. is 30% or more, and hot rolling is finished at less than 880 ° C. Cooling at an average cooling rate of 15 ° C / s or more, air cooling in the range of 650 to 720 ° C for 5s or more, and then winding up at 660 ° C or less. High strength with excellent shape freezing property and stretch flange formability A method of manufacturing a steel sheet. The shape-freezing property and stretch flange, wherein the high-strength steel sheet according to claim 6 or 7 is cold-rolled at a rolling reduction of less than 60%, and then heated to a temperature range of 500 to 800 ° C and cooled. A method for producing a high-strength steel sheet with excellent formability. A steel plate produced by the method for producing a high-strength steel plate according to claim 6 or 7 , or a steel plate cold-rolled with a reduction rate of less than 60%, is heated to a temperature range of 500 to 800 ° C, and then A method for producing a high-strength hot-dip galvanized steel sheet excellent in shape freezeability and stretch flangeability, characterized by hot dip galvanizing. 10. Production of a high strength galvannealed steel sheet excellent in shape freezing property and stretch flangeability, characterized by being alloyed in the range of 450 to 600 ° C. following the hot dip galvanizing of claim 9. Method.

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