KR100572762B1 - Thin steel sheet for automobile excellent in notch fatigue strength and method for production thereof - Google Patents

Abstract

The present invention provides an automotive thin steel sheet having excellent notch fatigue strength and a method of manufacturing the same, wherein C: 0.01 to 0.3%, Si: 0.01 to 2%, Mn: 0.05 to 3%, P: ≦ 0.1%, and S: ≤0.01%, Al: 0.005 to 1%, the remainder being made of Fe, unavoidable impurities, etc., {100} <011 at an arbitrary depth from the outermost surface to 0.5 mm in the sheet thickness direction. X-ray random intensity in the three directions of the average value of the X-ray random intensity ratio of the>-{223} <110> defense group to {554} <225>, {111} <112>, and {111} <110> and the average value of the ratio is 4 or less, the total reduction ratio in that the thickness of the steel 0.5mm~12mm the notch fatigue strength is excellent automotive steel sheet and the ingredient according to claim at a temperature range of less than Ar ₃ transformation point temperature + 100 ℃ A method for producing the steel sheet, which is subjected to rolling of 25% or more.

Description

THIN STEEL SHEET FOR AUTOMOBILE EXCELLENT IN NOTCH FATIGUE STRENGTH AND METHOD FOR PRODUCTION THEREOF}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin steel sheet for automobiles having excellent notch fatigue strength and a method for manufacturing the same. Particularly, the present invention is suitable for materials such as automobile chassis parts, in which fatigue cracking progresses from stress concentrating parts such as punched parts and welded parts. The present invention relates to a steel sheet for automobiles having excellent notch fatigue strength and a method of manufacturing the same.

Recently, in order to improve fuel efficiency of automobiles, light metals such as Al alloys and high strength steel sheets have been applied to automobile members for the purpose of weight reduction. However, although light metals, such as Al alloy, have the advantage that a specific strength is high, since it is remarkably expensive compared with steel, the application is limited to a special use. Therefore, in order to promote the weight reduction of automobiles in a wider range, the application of inexpensive high strength steel sheets is strongly required.

In response to such demand for high strength, in the fields of cold rolled steel sheets used for white bodies and panels, which account for about one fourth of the weight of the vehicle body, development of steel sheets having strength and deep-drawing properties, steel sheets with hardening hardening, etc. This progressed and contributed to the weight reduction of the vehicle body. However, the current weight reduction target is moving to structural members or chassis members, which occupy about 20% of the weight of the vehicle body, the development of high-strength steel sheet used for such members is urgently needed.

However, since high strength generally deteriorates material characteristics such as formability (processability), how to achieve high strength without deteriorating material characteristics is an important factor in developing high strength steel sheets. In particular, as a characteristic required for structural members and steel plates for chassis members, not only extensibility but also shear, punching, burring, fatigue durability and corrosion resistance are important, and high strength and how to balance these characteristics in a high dimension is important. Do.

For example, a component such as a suspension arm is press-molded after blanking or punching by shearing or punching, and then welded again to form a part depending on the member. In such a component, cracks progress in the sheared section or in the vicinity of the welded portion, which leads to fatigue failure. In other words, the sheared end face or the welded portion becomes a stress converging portion such as a notch, whereby the fatigue crack develops.

On the other hand, in general, the fatigue limit of the material decreases as the notch becomes sharp. However, if the notch becomes sharp to some extent, the phenomenon that the fatigue limit does not fall further occurs. This is because the fatigue limit transitions from the crack generation limit to the crack growth limit. Increasing the material improves the crack initiation limit, but does not improve the crack propagation limit. Therefore, the point where the fatigue limit transitions from the crack initiation limit to the crack propagation limit moves to the side where the notch is sharp. Therefore, even if the material is increased in strength, the fatigue limit caused by the notch becomes remarkable, and the fatigue limit when the notch is sharp cannot enjoy the advantages of high strength. In other words, the higher the strength, the higher the sensitivity to the notch.

At present, steel sheets of 340 to 440 MPa grade are used as the steel sheet for automobile chassis, and the strength level required for these member steel sheets is further increased to 590 to 780 MPa grade. Therefore, in order to meet these demands, it is indispensable to develop a steel sheet that can enjoy the advantages of high strength even in the presence of sharp notches.

There are two ways to improve the fatigue strength when punching or shearing cross sections exist. One is to eliminate sharp notches such as burrs that occur in punching or shearing cross sections, and the other is to increase resistance to crack propagation even if such sharp notches are present.

As an invention belonging to the former, for example, Japanese Patent Laid-Open No. 5-51695 discloses a state in which burr generation is suppressed by reducing the amount of Si added and reducing elongation at break in precipitates of Ti, Nb, and V, and punching or shearing. A technique for improving the fatigue strength at is disclosed. Further, Japanese Patent Laid-Open No. 5-179346 discloses a technique for defining the upper limit of the rolling finish temperature to limit the upper limit of the volume fraction of bainite, and to improve the fatigue strength of punching or shearing as it is. 13033 discloses a technique for improving the fatigue strength in the punching and shearing process by regulating the cooling rate after rolling to suppress the production of martensite.

In addition, Japanese Patent Application Laid-Open No. 8-302446 defines the hardness of the second phase in a composite tissue steel to be 1.3 times or more of ferrite, thereby reducing strain energy during punching or shearing, and improving fatigue strength in a punching or shearing state. The technique to make is disclosed. In addition, Japanese Patent Application Laid-Open No. 9-170048 discloses a technique for defining the length of grain boundary cementite to reduce burrs at the time of punching or shearing, and to improve fatigue strength at the time of punching or shearing. Further, Japanese Patent Laid-Open No. 9-202940 discloses a technique for improving punchability by defining parameters arranged by the amount of Ti, Nb, and Cr added, and improving fatigue strength in the punched state.

On the other hand, as an invention belonging to the latter, Japanese Patent Application Laid-Open No. Hei 6-88161 discloses a technique for reducing the fatigue crack propagation rate by defining an aggregate strength (100) plane parallel to the rolled surface in the surface layer to 1.5 or more. Also, Japanese Patent Application Laid-Open Nos. 8-199286 and 10-147846 disclose the (200) diffraction intensity ratio in the plate thickness direction measured by X-ray as 2.0 to 15.0, and the area ratio of the recovery or recrystallized ferrite is 15 to 40%. By doing so, a technique for reducing the fatigue crack propagation speed is disclosed.

However, it is disclosed in the publications of Japanese Patent Laid-Open Nos. Hei 5-51695, Hei 5-179346, Hei 8-13033, Hei 8-302446, Hei 9-170048 and Hei 9-202940. Techniques for reducing sharp notches, such as burrs that occur in punching or shearing cross sections, can be applied under any conditions because the degree of volley generated varies greatly due to clearance during punching or shearing. It is not a technique, and it cannot be said that it is insufficient as a steel plate excellent in notch fatigue strength.

On the other hand, the technique of controlling the assembly structure disclosed in Japanese Patent Application Laid-Open Nos. 6-88161, 8-199286 and 10-147846 increases the resistance to crack propagation mainly for construction machinery, ships, bridges. This invention is for steels for large structures such as the above, and is not intended for automotive steel sheet like the present invention.

In addition, the above technique mainly controls the rate of crack propagation in the PARIS region, which is referred to in the fracture strength of fatigue cracks, which are advanced from the weld edges. It is insufficient as a technique when it does not exist.

Moreover, the invention which evaluated the notch fatigue characteristic using the test piece shown to FIG. 1 (b) by the planar fatigue test method used for thin steel plates was not found until now.

Summary of the Invention

Accordingly, the present invention is to increase the resistance to crack growth by controlling the aggregate structure of the steel sheet for automobiles, regardless of the fatigue cracks that develop from the notch such as punching or shearing cross section, regardless of the conditions such as the gap during punching or shearing. By improving the technology. That is, an object of the present invention is to provide a manufacturing method capable of manufacturing a steel sheet for automobiles having excellent notch fatigue strength and a steel sheet thereof inexpensively and stably.

MEANS TO SOLVE THE PROBLEM The present inventors earnestly researched in order to achieve the improvement of the notch fatigue strength of the steel sheet for automobiles, keeping in mind the manufacturing process of the steel sheet currently produced on the industrial scale by the manufacturing equipment currently employ | adopted. As a result, the average value of the X-ray random intensity ratios of the {100} <011> to {223} <110> bearing groups of the plate surface at an arbitrary depth from the outermost surface to the plate thickness direction of 0.5 mm was 2 or more, and {554 } <225>, {111} <112>, and {111} <110> mean that the average value of the three-direction X-ray random intensity ratios is 4 or less, and the plate thickness is 0.5 mm or more and 12 mm or less. It was newly discovered that it was effective and came to complete this invention.

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

(1) The average value of the X-ray random intensity ratios of the {100} <011> to {223} <110> bearing groups of the plate surface at an arbitrary depth from the outermost surface to the plate thickness direction of 0.5 mm is 2 or more, and { 554} <225>, {111} <112>, and {111} <110> mean value of the X-ray random intensity ratio of three directions is 4 or less, and the notch fatigue strength characterized by the board thickness being 0.5 mm or more and 12 mm or less. Automotive sheet steel with excellent qualities.

(2) The steel sheet for automobiles having excellent notched fatigue strength according to (1), wherein the microstructure of the steel sheet is a composite structure in which the phase with the largest volume fraction is made of bainite or ferrite and bainite.

(3) The microstructure of the steel sheet is a composite structure containing a volume fraction of 5% or more and 25% or less of retained austenite, and the remaining part is mainly composed of ferrite and bainite. Automotive sheet steel with excellent qualities.

(4) The microstructure of the steel sheet is a composite steel sheet having a notched fatigue strength according to (1), wherein the microstructure of the steel sheet is a composite structure in which a phase having a maximum volume fraction is ferrite and a second phase is martensite.

(5) In mass%, C: 0.01 to 0.3%, Si: 0.01 to 2%, Mn: 0.05 to 3%, P: ≤ 0.1%, S: ≤ 0.01%, Al: 0.005 to 1%, X-ray random strength of the {100} <011>-{223} <110> azimuth group of a plate surface at arbitrary depths from the outermost surface to a plate thickness direction from the outermost surface as a steel plate which consists of Fe and an unavoidable impurity. The average value of the ratios is 2 or more, and the average value of the X-ray random intensity ratios in the three directions of {554} <225>, {111} <112> and {111} <110> is 4 or less, and the plate thickness is 0.5 mm. The steel sheet for automobiles excellent in the notch fatigue strength characterized by more than 12 mm.

(6) In mass%, Cu: 0.2 to 2%, B: 0.0002 to 0.002%, Ni: 0.1 to 1%, Ca: 0.0005 to 0.002%, REM: 0.0005 to 0.02%, Ti: 0.05 to 0.5% Nb: 0.01% to 0.5%, Mo: 0.05% to 1%, V: 0.02% to 0.2%, Cr: 0.01% to 1%, Zr: 0.02% to 0.2%. 5) A steel sheet for automobiles having excellent notch fatigue strength of a base material.

(7) The microstructure of the steel sheet includes: 1) bainite or a composite structure of ferrite and bainite having a phase with the largest volume fraction; 2) volume fraction: 5% or more and 25% or less of retained austenite; (5) or (6) characterized in that the remainder is a composite tissue composed mainly of ferrite and bainite, and 3) a composite tissue having a phase with the largest volume fraction as a ferrite and a second phase as martensite. ) Automotive thin steel sheet with excellent notch fatigue strength of base material.

(8) A galvanized steel sheet having excellent notch fatigue strength, which is galvanized to a galvanized steel sheet according to any one of (1) to (7).

(9) Mass%, C: 0.01 to 0.3%, Si: 0.01 to 2%, Mn: 0.05 to 3%, P: ≤ 0.1%, S: ≤ 0.01%, Al: 0.005 to 1%, After the rough rolling of the steel piece consisting of Fe and unavoidable impurities, the hot rolling is carried out to finish rolling with a total reduction ratio of 25% or more of the steel sheet thickness at a temperature range of Ar ₃ transformation point temperature of + 100 ° C or lower. The average value of the X-ray random intensity ratios of the {100} <011>-{223} <110> azimuth groups of the plate surface at arbitrary depths from the outermost surface of the said steel plate to a plate thickness direction to 0.5 mm is 2 or more, and Notch fatigue, characterized in that the average value of the X-ray random intensity ratios in the three directions of {554} <225>, {111} <112> and {111} <110> is 4 or less, and the plate thickness is 0.5 mm or more and 12 mm or less. Method for producing a thin steel sheet for automobiles with excellent strength.

(10) A method for producing a steel sheet for automobiles having excellent notch fatigue strength as described in (9), wherein the sheet is cooled at a cooling rate of 20 ° C / s or more after the finish rolling, and wound at a winding temperature of 450 ° C or more.

(11) After the finish rolling, it is maintained for ₁ to 20 seconds in the temperature range of Ar ₁ transformation point temperature or more and Ar ₃ transformation point temperature or less, thereafter, cooling at a cooling rate of 20 ° C / s or more, and above 350 ° C and less than 450 ° C. The manufacturing method of the steel sheet for automobiles excellent in the notch fatigue strength of the base material of Claim (9) characterized by winding to the winding temperature of the temperature range of (9).

(12) A method for producing an automotive thin steel sheet having excellent notch fatigue strength as described in (9), wherein the coil is wound at a coiling temperature of 350 ° C. or lower after the cooling.

(13) The method for producing a thin steel plate for automobile, which is excellent in the notched fatigue strength according to any one of (9) to (12), wherein in the hot rolling, lubrication rolling is performed.

(14) The method for producing an automotive thin steel sheet having excellent notch fatigue strength according to any one of (9) to (13), wherein in the hot rolling, descaling is performed after the rough rolling is finished.

(15) In mass%, C: 0.01 to 0.3%, Si: 0.01 to 2%, Mn: 0.05 to 3%, P: ≤ 0.1%, S: ≤ 0.01%, Al: 0.005 to 1%, the remainder subjected to Fe and unavoidable couple impurities billet made of a rolled and then, Ar ₃ transformation point temperature + 100 ℃ total rolling reduction of steel sheet thickness in the temperature range of less than the finish rolling of 25% or more, followed by pickling, and further, After cold rolling with a steel sheet thickness reduction rate of less than 80%, the temperature is maintained for 5 to 150 seconds at a recovery temperature not lower than Ac ₃ transformation point temperature + 100 ° C or lower, and recovery or recrystallization annealing of the cooling process is performed, and the outermost surface of the steel sheet The X-ray random intensity ratio of the {100} <011> to {223} <110> bearing groups of the plate surface at any depth up to 0.5 mm in the plate thickness direction is 2 or more, and {554} <225>, Notch fatigue, characterized in that the plate thickness of the average value of the X-ray random intensity ratio of the three directions of {111} <112> and {111} <110> is 4 or less 0.5 mm or more and 12 mm or less Method for producing a thin steel sheet for automobiles with excellent strength.

(16) A notch as described in (15), wherein after the cold rolling, a heat treatment is performed for 5 to 150 seconds at a temperature range of Ac ₁ transformation point temperature or more and Ac ₃ transformation point temperature of + 100 ° C or less and then cooling. Method for producing a thin steel sheet for automobiles with excellent fatigue strength.

(17) After holding at the temperature range for 5 to 150 seconds, cooling to a temperature range above 350 ° C to less than 450 ° C at a cooling rate of 20 ° C / s or more, and then holding for 5 to 600 seconds at the above temperature range. And a heat treatment in a step of cooling to a temperature range of 200 ° C. or less at a cooling rate of 5 ° C./s or more.

(18) The notch fatigue strength of (15) is characterized in that the heat treatment is carried out in the process of cooling to a temperature range of 350 ° C or lower at a cooling rate of 20 ° C / s or more after holding for 5 to 150 seconds in the temperature range. Excellent method for manufacturing automotive thin steel sheet.

(19) In addition to the steel sheet according to any one of (11) to (18), in mass%, Cu: 0.2 to 2%, B: 0.0002 to 0.002%, Ni: 0.1 to 1%, and Ca: 0.0005 to 0.002%, REM: 0.0005 to 0.02%, Ti: 0.05 to 0.5%, Nb: 0.01 to 0.5%, Mo: 0.05 to 1%, V: 0.02 to 0.2%, Cr: 0.01 to 1%, Zr: 0.02 to 0.2 A method for producing a thin steel sheet for automobiles having excellent notch fatigue strength, comprising one kind or two or more kinds of%.

(20) The steel sheet for automobiles having excellent notch fatigue strength according to (10) or (16), wherein the microstructure of the steel sheet is a composite structure in which a phase with the largest volume fraction is bainite or ferrite and bainite. Manufacturing method.

(21) The microstructure of the steel sheet is a composite structure comprising residual austenite having a volume fraction of 5% or more and 25% or less, and the remaining part is mainly composed of ferrite and bainite. Method for manufacturing a thin steel sheet for automobiles having excellent notch fatigue strength.

(22) The microstructure of the steel sheet is an automobile having excellent notch fatigue strength according to (12) or (18), wherein the microstructure of the steel sheet is a composite structure in which the phase with the largest volume fraction is ferrite and the second phase is martensite. Method of manufacturing thin steel sheet.

(23) After the hot rolled steel sheet or the recovery or recrystallization annealing sheet according to any one of (9) to (22) is manufactured, the steel sheet is further immersed in a zinc plating bath to perform zinc plating on the surface of the steel sheet. Method for producing a thin steel sheet for automobiles having excellent notch fatigue strength.

(24) A method for producing a thin steel sheet for automobiles having excellent notch fatigue strength as described in (23), wherein the zinc plating is further performed by alloying.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure explaining the shape of a fatigue test piece, (a) shows a smooth fatigue test piece, (b) shows a notch fatigue test piece.

Figure 2 shows the results of preliminary experiments leading to the present invention the average value of the X-ray random intensity ratio of the {100} <011> to {223} <110> defense group and {554} <225>, {111} <112> and {111 } <110> The figure which shows in the relationship between the average value of the X-ray random intensity ratio of three directions, and notch fatigue strength (time intensity in ¹⁰⁷ times: fatigue limit).

First, the basic research result which arrived at this invention is demonstrated below.

In general, fatigue cracks arise from the surface. This is no exception even when stress concentrations such as notches are present. In addition, even when punching or shearing cross sections exist, fatigue cracks have been observed to develop from the surface end of the steel sheet under repeated loads including load modes in the out-of-plane bending direction. Therefore, even in this case, it is evident that the increase in crack growth resistance up to the depth of the steel sheet outermost surface or the number of grains is effective for improving the notched fatigue strength. Moreover, even if the crack growth resistance is increased in the center of the sheet thickness, it is difficult to rectify the crack already. Therefore, in the present invention, the texture range effective for improving the fatigue strength is limited to 0.5 mm in the sheet thickness direction from the outermost surface. Preferably it is up to 0.1 mm.

Average value and {554} <of X-ray random intensity ratios of {100} <011> to {223} <110> bearing groups of the plate surface at an arbitrary depth from the outermost surface to the plate thickness direction on the notched fatigue strength to 0.5 mm in the plate thickness direction. 225>, {111} <112> and {111} <110>, the influence of the average value of the X-ray random intensity ratios of three directions was investigated. The test materials for this purpose were prepared as follows. In other words, hot-finish the molten cast slab by adjusting the composition to 0.08% C-0.9% Si-1.2% Mn-0.01% P-0.001% S-0.03% A1 so that the plate thickness becomes 3.5mm at any temperature above the Ar ₃ transformation point temperature. After rolling was completed, it was wound up.

The average value and the {554} <of the X-ray random intensity ratios of the {100} <011> to {223} <110> bearing groups of the plate surface at an arbitrary depth of 0.5 mm in the sheet thickness direction from the outermost surface of the steel sheet thus obtained. 225>, in order to average the X-ray random intensity ratios of the three directions of {111} <112> and {111} <110>, the specimen cut out at 30 mm phi from the 1 / 4W or 3 / 4W position of the plate width, Samsan finish grinding was performed to a depth of about 0.05 mm from the surface, and then deformation was removed by chemical polishing or electropolishing.

In addition, 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>. The measurement of the crystal orientation by X-rays followed the method described in pages 274 to 296, for example, "New Edition of Criterion X-ray Diffraction Yoron" (issued in 1986, Gentaro Matsumura, Agne).

In this case, the average value of the X-ray random intensity ratios of the {100} <011> to {223} <110> defense groups is the main bearing included in this defense group, {100} <011>, {116} <110>, { X-ray diffraction intensities of 114} <110>, {113} <110>, {112} <110>, {335} <110>, and {223} <110> based on the {110} pole figure. 3D aggregated tissue calculated by, or 3 calculated by series expansion using multiple pole plots (preferably three or more) among the {110}, {100}, {211}, and {310} pole plots Obtained from the dimensional collective organization.

,

,

,

,

,

,

의 강도를 그대로 사용하면 좋다. 다만 {100}<011>∼{223}<110> 방위군의 x선 랜덤 강도비의 평균치란 상기 각 방위의 상가 평균이다. For example, in the latter method, the X-ray random intensity ratio of each crystal orientation is in the three-dimensional aggregate structure? 2 = 45 ° cross section.

,

,

,

,

,

,

It is good to use the strength as it is. However, the average value of the X-ray random intensity ratios of the {100} <011> to {223} <110> defense groups is the average of the malls of the above-mentioned orientations.

If the strengths of all the bearings cannot be obtained, the malls of the respective bearings of {100} <011>, {116} <110>, {114} <110>, {112} <110>, and {223} <110> It may be replaced by an average.

Next, the average value of the X-ray random intensity ratios of the three directions of {554} <225>, {111} <112>, and {111} <110> may be calculated from the three-dimensional aggregate structure calculated in the same manner as described above.

Next, in order to examine the notched fatigue strength of the steel sheet, a fatigue test piece of the shape shown in Fig. 1 (b) was taken from the 1 / 4W or 3 / 4W position of the plate width so as to have a long side and a fatigue test was carried out. It was. At this time, the fatigue test piece shown in Fig. 1 (a) is a smooth test piece for obtaining fatigue strength of a general material, whereas the fatigue test piece shown in Fig. 1 (b) is a notch test piece produced for obtaining notched fatigue strength. However, the fatigue test piece was subjected to Samsan finish grinding to the depth of about 0.05 mm from the outermost surface. The fatigue test used an electrohydraulic servo type fatigue tester, and the test method was according to JIS Z 2273-1978 and JIS Z 2275-1978.

Mean value of X-ray random intensity ratio of {100} <011> to {223} <110> defense groups on notched fatigue strength and 3 of {554} <225>, {111} <112> and {111} <110> The result of having examined the influence of the average value of the X-ray random intensity ratio of an orientation is shown in FIG. Here, ○ and in figure la Fig. 1 (b) fatigue limit (1O time strength at ⁷⁾ the shape resulting from the fatigue test conducted using a test piece shown in fatigue notch, and the notch below the fatigue strength.

Average value of X-ray random intensity ratio of {100} <011> to {223} <110> defense group and X-ray random of three directions of {554} <225>, {111} <112> and {111} <110> There was a strong correlation between the mean value of the strength ratio and the notch fatigue strength, and it was found that the notch fatigue strength was remarkably improved at each average value of 2 or more and 4 or less.

The present inventors have studied these experimental results in detail and found that, in order to improve notch fatigue strength, {100} <011> to {223} <110> of the plate surface at an arbitrary depth from the outermost surface to the plate thickness direction of 0.5 mm. The average value of the X-ray random intensity ratio of the defense group is 2 or more, and the average value of the X-ray random intensity ratio of the three directions of {554} <225>, {111} <112>, and {111} <110> is 4 or less. It was newly recognized as very important.

However, in order to improve not only the notch but also the fatigue crack initiation resistance at the smooth portion, the {100} <011> to {223} <110> bearing groups of the plate surface at an arbitrary depth from the outermost surface to the plate thickness direction of 0.5 mm. It is preferable that the average value of the X-ray random intensity ratios is 4 or more, and the average value of the X-ray random intensity ratios of the three directions of {554} <225>, {111} <112> and {111} <110> is 2.5 or less. .

This mechanism is not necessarily obvious, but is assumed as follows.

In general, the fatigue limit in the presence of a sharp notch is determined by the crack propagation limit, i.e., the magnitude of the crack propagation resistance for rectifying the crack. The fatigue crack propagation is a small plastic deformation repetition at the bottom of the notch or stress concentration, but in the case where the crack length is relatively short and the plastic deformation occurs in the range of grain size, the influence of the crystallographic sliding surface and the sliding direction is not affected. I guess it's big. Therefore, with respect to the crack growth orientation and the crack surface, when there are many crystal ratios having a sliding surface with high crack growth resistance and a sliding direction, the growth of the fatigue crack is suppressed.

Next, the reason for limiting the plate thickness of the steel sheet in the present invention will be described. If the sheet thickness is less than 0.5 mm, small yielding conditions cannot be satisfied regardless of the degree of stress concentration, which leads to a risk of monotonic ductile failure. In addition, since sufficient plastic restraint is required from the viewpoint of crack rectification, it is preferable that the thickness is at least 1.2 mm or more in order to maintain the planar deformation state.

On the other hand, when plate | board thickness exceeds 12 mm, the fall of fatigue strength by plate | board thickness effect (dimension effect) will become remarkable. In addition, when the plate thickness exceeds 8 mm, 8 mm or less is preferable because excessive load load may be applied to the equipment in order to achieve hot or cold rolling conditions for obtaining an aggregate structure effective for improving notched fatigue strength. Therefore, in this invention, the plate | board thickness is limited to 0.5 mm or more and 12 mm or less. Preferably they are 1.2 mm or more and 8 mm or less.

Next, the micro structure of the steel plate in this invention is demonstrated.

In the present invention, it is not necessary to specifically limit the microstructure of the steel sheet for the purpose of improving the notched fatigue strength, and the set of the scope of the present invention in the ferrite, bainite, pearlite, and martensite structures of ordinary steel When the structure (X-ray random intensity ratio of the present invention range) is obtained, the effect of improving the notched fatigue strength of the present invention is obtained, and therefore it is preferable to define the microstructure in accordance with other necessary characteristics. However, a specific microstructure, for example, a residual austenite containing 5% or more by volume and 25% or less by volume, and the remaining part is mainly composed of ferrite, bainite, or a composite structure composed of ferrite, or the largest volume fraction is made of ferrite. This effect can be further enhanced in complex tissues such as martensite mainly having two phases.

In addition, the bainite mentioned here also includes bainitic ferrite and acicular ferrite structure. However, in the case where the crystal structure such as retained austenite is not bcc in the two-phase composite structure, the X-ray random intensity ratio converted to the other tissue volume fraction may be within the scope of the present invention. In addition, since the pearlite containing coarse carbide may be a site of fatigue cracking, and the fatigue strength may be extremely reduced, the volume fraction of pearlite containing coarse carbide is preferably 15% or less. In addition, the volume fraction of pearlite containing coarse carbide is preferably 5% or less in order to secure good fatigue characteristics.

In this case, the volume fraction of ferrite, bainite, pearlite, martensite and retained austenite means that the sample cut out from the 1 / 4W or 3 / 4W position of the steel sheet plate is ground in the rolling direction cross section, and the nital reagent and And / or is defined as the microstructure area fraction at 1 / 4t of plate thickness observed using a reagent disclosed in Japanese Patent Laid-Open No. 5-163590 and observed at a magnification of 200 to 500 times using an optical microscope. . However, since residual austenite may not be easily discriminated by etching with the said reagent, you may calculate a volume fraction by the following method.

That is, austenite can be easily crystallized because ferrite and crystal structures are different. Therefore, the volume fraction of retained austenite can also be obtained experimentally by the X-ray diffraction method. That is, it is a method of simply calculating the volume fraction from the difference of the reflecting surface strength of austenite and ferrite using Mo Kα ray.

Vγ = (2/3) {100 / (0.7 × α (211) / γ (220) +1)}

+ (1/3) {100 / (0.78 × α (211) / γ (311) +1)}

However, α 211, γ 220 and γ 311 are the X-ray reflecting surface intensities of the ferrite (α) austenite (γ), respectively.

In the present invention, in order to provide good burring processability in addition to the improvement of notch fatigue strength, the microstructure is a composite structure in which the maximum volume fraction phase is bainite or ferrite and bainite. However, it is allowed to include inevitable martensite, residual austenite and pearlite. In order to obtain good burring workability (hole expansion value), the volume fraction of hard residual austenite and martensite is preferably less than 5%. In addition, the volume fraction of bainite is preferably 30% or more. In addition, in order to obtain good ductility, the volume fraction of bainite is preferably 70% or less.

In addition, in the present invention, in order to provide good ductility in addition to improving notch fatigue strength, the microstructure includes a retained austenite having a volume fraction of 5% or more and 25% or less, and the remainder is a composite structure mainly consisting of ferrite and bainite. It is done. However, it is allowed to include inevitable martensite and pearlite of less than 5% in total.

In addition, in the present invention, in order to impart a resistance ratio for obtaining good shape freezing property in addition to improving notch fatigue strength, the microstructure is a composite structure in which the maximum volume fraction phase is made of ferrite, and the second phase is mainly martensite. It is done. However, it is allowed to include unavoidable bainite, residual austenite and pearlite of less than 5% in total. In addition, in order to ensure a resistance ratio of 70% or less, the volume fraction of ferrite is preferably 50% or more.

Next, the reason for limitation of the chemical component of this invention is demonstrated.

C is an element necessary to obtain the desired microstructure. However, since the workability will deteriorate when it contains more than 0.3%, since it will be 0.3% or less and also when it contains more than 0.2%, weldability will deteriorate, Preferably it is 0.2% or less. On the other hand, if it is less than 0.01%, the strength is lowered, so it is made 0.01% or more. Further, in order to stably obtain sufficient residual austenite amount for obtaining good ductility, it is preferably 0.05% or more.

Si is effective for increasing strength as a solid solution strengthening element. In order to obtain desired intensity | strength, it is necessary to contain 0.01% or more. However, when it contains more than 2%, workability will deteriorate. Therefore, content of Si is made into 0.01 to 2%.

Mn is a solid solution strengthening element, and is effective for increasing strength. 0.05% or more is required to obtain a desired strength. In addition, when elements other than Ti, such as Ti which suppress the generation | occurrence | production of hot crack by S, are not added sufficiently, it is preferable to add the amount of Mn which becomes Mn / S≥20 by mass%. In addition, Mn is an austenite stabilizing element, and the addition amount for stably obtaining sufficient residual austenite amount for obtaining good ductility is preferably 0.1% or more. On the other hand, when it exceeds 3%, slab cracking occurs, so it is 3% or less.

Since P is an impurity, it is so preferable that it is low, and when it contains more than 0.1%, since it will adversely affect workability and weldability, and will also reduce a fatigue characteristic, you may be 0.1% or less.

Since S is an impurity, a low one is preferable, and if too large, an A-based inclusion deteriorating local ductility or burring workability is produced. However, if S is to be reduced as much as possible, 0.01% or less is an acceptable range.

Although A1 needs to be added 0.005% or more for molten steel deoxidation, since it raises a cost, the upper limit shall be 1.0%. In addition, since the addition of too large a quantity increases the base metal inclusions and worsens the elongation, it is preferably 0.5% or less.

Cu is added as needed because it has an effect of improving fatigue properties in solid solution. However, when less than 0.2%, the effect is small, and even if it contains exceeding 2%, the effect will be saturated. Therefore, content of Cu is made into 0.2 to 2% of range. However, in the case where the coiling temperature is 450 ° C or higher, if it contains more than 1.2%, there is a possibility that it will precipitate after winding and remarkably deteriorate workability, so it is preferable to be 1.2% or less.

Since B has the effect of raising a fatigue limit by adding complex with Cu, it adds as needed. However, if it is less than 0.0002%, the effect is insufficient to obtain the effect, and when it exceeds 0.002%, slab cracking will occur. Therefore, addition of B is made into 0.0002 to 0.002%.

Ni is added as needed in order to prevent hot brittleness by Cu containing. However, if it is less than 0.1%, the effect is small, and even if it adds more than 1%, since the effect is saturated, it is set to 0.1 to 1%.

Ca and REM are elements which are harmless by changing the form of nonmetallic inclusions that are a starting point of destruction or deteriorate workability. However, even if it is added less than 0.0005%, the effect is ineffective, and the effect is saturated even if it is added more than 0.002% for Ca and more than 0.02% for REM, so Ca: 0.0005 to 0.002% and REM: 0.0005 to 0.02% It is desirable to.

In addition, in order to impart strength, one or two or more kinds of precipitation strengthening or solidifying strengthening elements of Ti, Nb, Mo, V, Cr, and Zr may be added. However, the effect cannot be obtained at less than 0.05%, 0.01%, 0.05%, 0.02%, 0.01%, and 0.02%, respectively. Moreover, even if it adds exceeding 0.5%, 0.5%, 1%, 0.2%, 1%, and 0.2%, respectively, the effect is saturated.

Moreover, you may contain Sn, Co, Zn, W, Mg in 1% or less in total in the steel which has these as a main component. However, since Sn may have a flaw at the time of hot rolling, 0.05% or less is preferable.

Next, the reason for limitation of the manufacturing method of this invention is demonstrated in detail below.

According to the present invention, after casting, hot rolling, and then cooling, or after hot rolling, cooling and pickling, after cold rolling, annealing or heat-treating the hot-rolled steel sheet or cold-rolled steel sheet by hot-dip galvanizing line, It is obtained by processing.

In the present invention, the production method preceding the hot rolling is not particularly limited. That is, even after the solvent by a blast furnace, a converter, etc., component adjustment is carried out so that it may become the target component content in various secondary smelting, and also casting by a method, such as thin slab casting, in addition to casting by the normal continuous casting and the ingot method, good. Scrap may be used for the raw material. In the case of the slab obtained by continuous casting, the hot slab may be sent directly to a hot rolling mill as it is, or after cooling to room temperature, it may be hot rolled after reheating in a heating furnace.

Although there is no restriction | limiting in particular about reheating temperature, Since the scale-off amount will become large amount and a yield will fall if it is 1400 degreeC or more, the reheating temperature is preferable less than 1400 degreeC. In addition, since heating below 1000 degreeC significantly inhibits operation efficiency on schedule, as for reheating temperature, 1000 degreeC or more is preferable.

In the hot rolling process, after finishing rough rolling, finish rolling is carried out, but when descaling after rough rolling is completed, the impact pressure P (MPa) x flow rate L (liter / cm ² ) of the high-pressure water on the surface of the steel sheet ≥ It is desirable to meet the condition of 0.0025.

The impact pressure P of the high pressure water on the surface of the steel sheet is described as follows (see "Iron and Steel" 1991, vo1.77, No. 9, p1450).

P (MPa) = 5.64 × P ₀ × V / H ²

only,

P ₀ (MPa): liquid pressure

V (liters / min): Nozzle fluid flow rate

H (cm): distance between steel plate surface and nozzle

The flow rate L is described as follows.

L (liters / cm ² = V / (W × v)

only,

V (liters / min): Nozzle fluid flow rate

W (cm): Width of spray liquid per nozzle touching steel plate surface

v (cm / min): Rate

The upper limit of the impingement pressure P x flow rate L does not need to be specifically determined in order to obtain the effect of the present invention. However, the nozzle fluid amount is preferably 0.02 or less, because increasing the nozzle fluid amount causes the nozzle to wear out. .

Moreover, it is preferable that the maximum height Ry of the steel plate after finishing rolling is 15 micrometers (15 micrometers Ry, 12.5 mm, ln 12.5 mm) or less. This is apparent from the fact that the fatigue strength of the steel sheet in the hot rolled or pickled state correlates with the maximum height Ry of the steel sheet surface, as described, for example, in the Metal Material Fatigue Design Manual, Japanese Materials Society, p. 84. Do. In addition, it is preferable to perform subsequent finish rolling within 5 second in order to prevent a scale generate | occur | producing again after descaling.

In addition, after rough rolling or after descaling thereafter, the sheet bars may be joined and finish-rolled continuously. The jaw bar at this time may be once wound on a coil, stored in a cover having a thermal insulation function if necessary, and rejoined once again to be joined.

The finish rolling is required to be carried out In the Ar ₃ transformation point temperature + total rolling reduction of 25% or more in a rolling temperature range of below 100 ℃ in the second half of the finish rolling when the hot-rolled steel sheet as the final product. At this time, for example, it is the Ar ₃ transformation point temperature is represented by a simple rigid minutes by the following calculating formula relationship. In other words,

Ar ₃ = 910-310 ×% C + 25 ×% Si-80 ×% Mn

If the total reduction ratio is less than 25% in the temperature range of the Ar ₃ transformation point temperature of + 100 ° C. or less, the aggregated structure of the rolled austenite is not sufficiently developed. Therefore, the effect of the present invention is not obtained by any cooling in the future. . In order to obtain a sharper texture, it is preferable to make the total reduction ratio in the temperature range of the Ar ₃ transformation point temperature at + 100 ° C or lower to 35% or more.

The lower limit of the temperature range in which rolling with a total reduction ratio of 25% or more is not particularly limited, but if the temperature is lower than the Ar ₃ transformation point, the processed structure remains in the ferrite precipitated during rolling, so that the ductility decreases and the workability deteriorates. the lower limit of the temperature range for performing rate of 25% or more rolling is more than Ar ₃ transformation point temperature is preferable. However, this shall not apply in the case where the temperature is in even less than Ar ₃ transformation point temperature, the recovery or recrystallization proceeds to some extent by the heat treatment after the processing following the winding or take-up process.

In the present invention, the upper limit of the total reduction ratio in the temperature range of the Ar ₃ transformation point temperature of + 100 ° C. or less is not particularly limited. However, when the total reduction ratio exceeds 97.5%, the rolling load is increased to increase the rigidity of the rolling mill excessively. In addition, since it causes an economic defect, it is preferably 97.5% or less.

At this time, when the friction between the hot rolled roll and the steel sheet during hot rolling in the temperature range of the Ar ₃ transformation point temperature of 100 ° C. or less is large, a crystal orientation mainly composed of {110} planes develops on the plate surface near the steel sheet surface. Since notch fatigue strength deteriorates, lubrication is performed as needed in order to reduce the friction of a hot rolled roll and a steel plate.

In the present invention the upper limit of the coefficient of friction between the hot rolling roll and the steel sheet is not particularly limited, since in a development of the crystal orientation of the plane {110} mainly becomes remarkable, notched fatigue strength deteriorates 0.2 excess, Ar ₃ It is preferable to make the friction coefficient of a hot rolling roll and a steel plate into 0.2 or less with respect to at least 1 pass at the time of hot rolling in the temperature range of transformation point temperature +100 degrees C or less. More preferably, the friction coefficient between the hot rolled roll and the steel sheet is made 0.15 or less for all the passes during hot rolling in the temperature range of the Ar ₃ transformation point temperature of 100 ° C. or less.

At this time, the friction coefficient between the hot rolled roll and the steel sheet is a value calculated by calculation based on the rolling theory from values such as the advance rate, the rolling load, and the rolling torque.

Although not particularly limited with respect to the final pass temperature (FT) of the finish rolling, the final pass temperature (FT) of the finish rolling is preferably Ar ₃ transformation point temperature or more at the end. This is because if the rolling temperature during the hot rolling is less than the Ar ₃ transformation point temperature, the processed structure remains in the ferrite precipitated before or during the rolling and the ductility is lowered and the workability is deteriorated. However, the final pass temperature (FT) of the finish rolling, even less than Ar ₃ transformation point temperature, the same shall not apply if the recovery heat treatment for recrystallization after the next winding process or winding processing.

On the other hand, there is no particular upper limit regarding the upper limit of the finishing temperature. However, when the Ar ₃ transformation point temperature is higher than + 100 ° C, it is virtually impossible to perform rolling with a total reduction ratio of 25% or more in the temperature range below the Ar ₃ transformation point temperature of + 100 ° C. Therefore, the upper limit of the finishing temperature is preferably Ar ₃ transformation point temperature + 100 ° C or lower.

In the present invention, it is not necessary to specifically limit the microstructure of the steel sheet only for the purpose of improving the notched fatigue strength, so that the cooling process after finishing the finish rolling and winding up to a predetermined winding temperature is completed. Is not specifically defined, but is cooled as necessary to wind to a predetermined winding temperature or to control the microstructure. Although the upper limit of a cooling rate is not specifically limited, Since the plate warpage by heat deformation is concerned, it is preferable to set it as 300 degrees C / s or less. If the cooling rate is too fast, the cooling end temperature cannot be controlled, and there is a possibility of overshooting and overcooling to a predetermined winding temperature or less. Therefore, the cooling rate here is preferably 150 ° C / s or less. Moreover, although the minimum of cooling rate is not specifically determined, The air cooling rate at the time of not cooling is 5 degrees C / s or more.

In the present invention, after finishing finish rolling in order to give a composite structure of bainite or ferrite and bainite in which the microstructure volume fraction is the maximum in order to impart good burring workability in addition to improving notch fatigue strength, The process up to winding at a predetermined winding temperature is not specifically determined except for the cooling rate in the meantime, but the temperature from the Ar ₃ transformation point to the Ar ₁ transformation point when aiming at compatibility with ductility without degrading the burring property to such an extent. You may hold | maintain for 1 to 20 second in a station (two-phase station of ferrite and austenite). At this time, the retention is performed in order to promote ferrite transformation in the two-phase region, but since the ferrite transformation is insufficient in the two-phase region in less than l seconds, sufficient ductility is not obtained, and pearlite is generated in excess of 20 seconds. As the microstructure with the largest volume ratio, no bainite or a complex structure of ferrite and bainite can be obtained.

In addition, in order to facilitate the ferrite transformation, the temperature range of 1 to 20 seconds is preferably not less than Ar ₁ transformation point and not more than 800 ° C. In addition, the residence time for 1 to 20 seconds is preferably set to 1 to 10 seconds in order not to reduce the productivity extremely. In addition, in order to satisfy these conditions, it is necessary to quickly reach the temperature range at a cooling rate of 20 ° C / s or more after finishing rolling.

Although the upper limit of a cooling rate is not specifically determined, 300 degrees C / s or less is a reasonable cooling rate on cooling facility capability. If the cooling rate is too fast, the cooling end temperature cannot be controlled, and there is a possibility of overshooting and overcooling up to the Ar ₁ transformation point or less, and the ductility improvement effect is lost. Is preferred.

Next, the cooling is performed at a cooling rate of 20 ° C./s or more from the temperature range to the winding temperature CT, but bainite containing pearlite or carbide is produced at a cooling rate of 20 ° C./s or less, and the desired volume is achieved. As the microstructure of the maximum rate, bainite or a complex structure of ferrite and bainite is not obtained. The upper limit of the cooling rate up to the coiling temperature is not particularly determined, and the effect of the present invention can be obtained. However, the sheet warping due to heat deformation is concerned, and therefore it is preferably 300 ° C / s or less.

In addition, in the present invention, in order to impart good ductility in addition to improving notch fatigue strength, the microstructure includes a retained austenite having a volume fraction of 5% or more and 25% or less, and the remaining part is mainly composed of ferrite and bainite. to the tissue, a process after the end of finish rolling, first, Ar ₃ transformation point temperature is 1-20 seconds residence in the temperature region (ferrite and austenite two sangyeok knight) to Ar ₁ transformation point temperature from. At this time, the retention is carried out in order to promote ferrite transformation in the two-phase region, but in less than one second, sufficient ductility is not obtained because the ferrite transformation is insufficient in the two-phase region. A microstructure containing residual austenite having a volume fraction of 5% or more and 25% or less, mainly consisting of ferrite and bainite, is not obtained.

In addition, since the temperature range in which 1 to 20 seconds is retained facilitates ferrite transformation, the Ar ₁ transformation point temperature is preferably 800 ° C. or more. In addition, the residence time for 1 to 20 seconds is preferably set to 1 to 10 seconds in order not to reduce the productivity extremely. In addition, in order to satisfy these conditions, it is necessary to reach the said temperature range rapidly at the cooling rate of 20 degree-C / s or more after finishing rolling types. Although the upper limit of a cooling rate is not specifically determined, 300 degrees C / s or less is a reasonable cooling rate on cooling facility capability. If the cooling rate is too fast, the cooling end temperature cannot be controlled, and there is a possibility of overshooting and overcooling to an Ar ₁ transformation point temperature or less. Therefore, the cooling rate at this time is preferably 150 ° C / s or less.

Next, from the temperature range to the coiling temperature CT, the cooling is performed at a cooling rate of 20 ° C / s or more, but at a cooling rate of less than 20 ° C / s, bainite containing pearlite or carbide is formed, and sufficient residual austenite is formed. It cannot be obtained, and the microstructure which contains residual austenite of 5% or more and 25% or less of the target volume fraction, and a remainder mainly consists of ferrite and bainite is not obtained. Although the upper limit of the cooling rate to a coiling temperature is not specifically determined, the effect of this invention can be acquired, but it is preferable to set it as 300 degrees C / s or less since it is concerned about the plate curvature by heat deformation.

Further, in the present invention, in order to impart a resistance ratio for obtaining good shape freezing property in addition to improving notch fatigue strength, a composite structure in which the microstructure volume fraction maximum phase is made of ferrite and the second phase is mainly martensite. In order to achieve this, the process after finishing rolling is first left for ₁ to 20 seconds in the temperature range (two phase regions of ferrite and austenite) from the Ar ₃ transformation point temperature to the Ar ₁ transformation point temperature. At this time, the retention is carried out in order to promote ferrite transformation in the two-phase region, but in less than one second, the ferrite transformation is insufficient in the two-phase region, and sufficient ductility is not obtained. The composite structure whose volume fraction maximum phase is made into ferrite and a 2nd phase mainly uses martensite is not obtained.

In addition, since the temperature range in which 1 to 20 seconds is retained facilitates ferrite transformation, the Ar ₁ transformation point temperature is preferably 800 ° C. or more. In addition, the residence time for 1 to 20 seconds is preferably set to 1 to 10 seconds in order not to reduce the productivity extremely. In addition, in order to satisfy these conditions, it is necessary to reach the temperature range rapidly at the cooling rate of 20 degrees C / s or more after finishing rolling. Although the upper limit of a cooling rate is not specifically determined, 300 degrees C / s or less is a reasonable cooling rate on cooling facility capability. If the cooling rate is too fast, the cooling end temperature cannot be controlled, and there is a possibility of overshooting and overcooling to below the Ar ₁ transformation point temperature. Therefore, the cooling rate at this time is preferably reduced by l50 ° C / s.

Next, from the temperature range to the coiling temperature CT, the cooling is performed at a cooling rate of 20 ° C / s or more, but at a cooling rate of less than 20 ° C / s, pearlite or bainite is formed, and sufficient martensite is not obtained. The microstructure which makes ferrite to be the phase with the largest volume fraction and makes martensite the 2nd phase is not obtained.

Although the upper limit of the cooling rate to a coiling temperature is not specifically determined, the effect of this invention can be acquired, but it is preferable that it is 300 degrees C / s or less since it is concerned about the plate curvature by heat deformation.

In the present invention, it is not necessary to specifically limit the microstructure of the steel sheet only for the purpose of improving the notched fatigue strength, but the upper limit of the coiling temperature is not particularly limited, but the Ar ₃ transformation point temperature is not more than + 100 ° C. In order to pass the aggregate structure of austenite obtained by rolling with a total reduction ratio of 25% or more, it is preferable to wind up to the coiling temperature T ₀ or less shown below. However, T ₀ does not have to be below room temperature. This T ₀ is a temperature thermodynamically defined as the temperature at which ferrite of the same component as austenite and austenite has the same free energy, and considering the influence of components other than C, the following equation can be easily calculated.

T ₀ = -650.4 ×% C + B

At this time, B is determined as follows.

 B = -50.6 × Mneq + 894.3

In addition, Mneq is determined by the mass% of the containing element shown below at this time.

 Mneq =% Mn + 0.24 ×% Ni + 0.13 ×% Si + 0.38 ×

     % Mo + 0.55 ×% Cr + 0.16% Cu-0.50 ×

% Al-0.45 ×% Co + 0.90 ×% V

In addition, negligible due to the influence of the weight% of the components other than the one specified by the present invention on T ₀ is not so large, in this case.

In addition, since the lower limit of the coiling temperature does not need to specifically limit the microstructure of the steel sheet only for the purpose of improving its notched fatigue strength, there is no need to specifically limit it. Since it becomes a concern, 50 degreeC or more is preferable.

In the present invention, in order to give good burring processability in addition to improving notch fatigue strength, in order to make the microstructure volume fraction maximum phase into bainite, or a composite structure of ferrite and bainite, the burring property is lowered at a winding temperature of less than 450 ° C. The amount of residual austenite or martensite that is considered to be harmful may be generated, and the winding temperature is 450 because it is not possible to obtain bainite, which is a microstructure with the largest desired volume ratio, or a composite structure composed of ferrite and bainite. It is limited to ℃ or more.

In addition, although the cooling rate after winding is not specifically limited, When Cu is added 1.2% or more, since Cu precipitates after winding and worsens workability, there exists a possibility that the molten state Cu which is effective for improving fatigue characteristics may disappear. As for the cooling rate after winding, it is preferable to make up to 200 degreeC to 30 degreeC / s.

In addition, in the present invention, in order to impart good ductility in addition to the improvement of notch fatigue strength, the microstructure comprises a retained austenite having a volume fraction of 5% or more and 25% or less, and the remaining part is mainly composed of ferrite and bainite. In order to achieve this, bainite containing carbide is produced at a coiling temperature of 450 ° C. or higher, and sufficient residual austenite is not obtained, and residual austenite having a target volume fraction of 5% or more and 25% or less is contained, and the remainder is mainly ferrite. Since no microstructure composed of bainite is obtained, the winding temperature is limited to less than 450 ° C. In addition, when the coiling temperature is 350 ° C. or less, martensite is produced in a large amount, and sufficient retained austenite is not obtained, and the retained austenite having a target volume fraction of 5% or more and 25% or less is contained, and the remaining part is mainly ferrite and bay. Since no microstructure consisting of knight is obtained, the winding temperature is limited to more than 350 ° C.

In addition, although the cooling rate after winding is not specifically limited, When Cu is added 1% or more, since Cu precipitates after winding and worsens workability, and there exists a possibility that Cu of the solid solution state effective for improving a fatigue characteristic may disappear. As for the cooling rate after winding up, it is preferable to make up to 200 degreeC to 30 degreeC / s or more.

In addition, in the present invention, in order to impart a resistance ratio for obtaining good shape freezing property in addition to improving notch fatigue strength, a composite structure in which the microstructure volume fraction maximum phase is made of ferrite, and the second phase is mainly martensite. To achieve this, bainite is formed when the coiling temperature is higher than 350 ° C., sufficient martensite is not obtained, and the microstructure of the target ferrite as the maximum volume fraction and the martensite as the second phase is not obtained. Therefore, winding temperature is limited to 350 degrees C or less. The lower limit of the coiling temperature does not need to be particularly limited. However, if the coil is in a wet state for a long time, the appearance defects due to rust may be concerned, and therefore, 50 ° C. or more is preferable.

After the hot rolling process, pickling may be carried out as necessary, followed by cold rolling of a skin pass having a reduction ratio of 10% or lower or a rolling reduction of about 40% in-line or offline.

Next, although the case where it is set as a final product as a cold rolled steel sheet is demonstrated, the finish rolling conditions in hot are not specifically limited. However, it is preferred in order to obtain a better notched fatigue strength than the total reduction ratio in the temperature range of less than Ar ₃ transformation point temperature + 100 ℃ 25%. The final path temperature FT of the finish rolling may be terminated below the Ar ₃ transformation point temperature. In this case, however, since the processing structure remains strong in the ferrite precipitated before or during the rolling, the final rolling temperature FT may be used for subsequent winding or heat treatment. Recovery and recrystallization are preferred.

The total rolling reduction rate of the cold rolling following pickling is to be less than 80%. This is because when the total reduction ratio of cold rolling is 80% or more, the intensity ratio of the X-ray diffraction integrated plane of the {111} plane or {554} plane of the crystal plane parallel to the plate plane, which is a general cold-rolled recrystallized texture, becomes high. Also preferably, it is 70% or less. Although the minimum of cold rolling rate can obtain the effect of this invention, even if it does not specifically determine, it is preferable to set it as 3% or more in order to control the intensity | strength of a crystal orientation in an appropriate range.

The heat treatment of the cold rolled steel sheet is based on a continuous annealing process. First, the exemplary 5-150 seconds in a temperature range of less than Ac ₃ transformation point temperature + 100 ℃.

If the upper limit of the heat treatment temperature is higher than Ac ₃ transformation point temperature + 100 ° C, the ferrite produced by recrystallization transforms into austenite, and the texture of the austenite grain grows is randomized. since discarding the screen, the upper limit to the heat treatment at a temperature Ac ₃ transformation point temperature + 100 ℃ below.

At this time, the Ac ₁ transformation temperature and the Ac ₃ transformation temperature are, for example, the relationship with the steel components according to the calculation formula described on page 273 of Leslie Steel Materials (published in 1985, Kumai Hiroshi-Noda Tatsuhiko Station, Maruzen Corporation). Represented by

On the other hand, the lower limit of the heat treatment temperature does not need to specifically limit the microstructure of the steel sheet for the purpose of improving the notch fatigue strength, but if it is above the recovery temperature, it is fine. In order to remarkably deteriorate, the lower limit of the heat treatment temperature is at least the recovery temperature. In addition, the retention time in this temperature range is insufficient to completely re-employ cementite in less than 5 seconds, while the effect is not only saturated but also lowers the productivity even after heat treatment for more than 150 seconds. 5 to 150 seconds.

Although it does not specifically limit about subsequent cooling conditions, In order to control micro structure, you may hold and cool to the following cooling or arbitrary temperatures as needed.

In the present invention, in order to impart good burring processability in addition to improving notch fatigue strength, the lower limit temperature of the heat treatment temperature is Ac in order to make the microstructure volume fraction maximum phase into bainite or a composite structure of ferrite and bainite. _{1 It is} more than transformation point temperature. When this lower limit temperature is less than Ac ₁ transformation point temperature, the bainite of the target volume fraction maximum phase or the composite structure of ferrite and bainite is not obtained. At this time, if the server ringseong direct the compatibility of and the degree without deteriorating ductility is less in order to increase the volume fraction of the ferrite, the temperature range for more than Ac ₁ side taejeom temperature Ac ₃ transformation temperature (the ferrite and austenite We assume temperature range of two phases). In addition, in order to obtain a better ringseong member to increase the volume fraction of bainite, at least Ac ₃ transformation temperature Ac ₃ transformation point temperature range of temperatures below + 100 ℃ is preferred.

Next, the cooling step is not specifically defined in the present invention, but in the case where the heat treatment temperature is Ac ₁ transformation point temperature or more and Ac ₃ transformation point temperature or less, the temperature below 350 ° C. or more and the T ₀ temperature or less at a cooling rate of 20 ° C./s or more. It is preferable to cool to the station. This is because at a cooling rate of less than 20 ° C./s, there is a possibility of being caught in a nose of bainite or pearlite transformation containing a large amount of carbide. Also, the end temperature of cooling is considered to be detrimental to burring at 350 ° C. or lower. Since a large amount of sites may be formed, and bainite, which is a microstructure of the largest desired volume ratio, or a composite structure composed of ferrite and bainite is not obtained, more than 350 ° C. is preferable. T ₀ or less is preferable in order to pass the aggregate obtained by the process.

Finally, the cooling rate up to the end temperature of the cooling process may generate a large amount of martensite, which is considered to be detrimental to burring properties during cooling, at 20 ° C / s or more, and bainite, which is a microstructure having a maximum volume ratio of the target, Or since the composite structure which consists of ferrite and bainite may not be obtained, it is preferable to set it as less than 20 degree-C / s. In addition, since the aging property may deteriorate when the end temperature of a cooling process exceeds 200 degreeC, it is preferable to set it as 200 degrees C or less. In addition, the lower limit is preferably 50 ° C. or more, because when the coil is wetted with water for a long time when cooling by water cooling or spraying, the appearance defect due to rust is concerned.

On the other hand, if the heat treatment temperature is preferably cooled to a temperature not higher than the Ac ₃ transformation point to 200 ℃ temperature exceeds Ac ₃ transformation point temperature + 100 ℃ or less is more than 20 ℃ / s cooling rate. This is because at 20 ° C / s or more, there is a possibility of being caught in a nose of bainite or pearlite transformation containing a large amount of carbide. In addition, since the aging property may deteriorate when cooling end temperature exceeds 200 degreeC, 200 degrees C or less is preferable. When the lower limit is cooled by water cooling or spraying, if the coil is wet with water for a long time, the appearance defect due to rust may be feared, so 50 ° C. or more is preferable.

In addition, in the present invention, in order to impart good ductility in addition to improving notch fatigue strength, the microstructure comprises a retained austenite having a volume fraction of 5% or more and 25% or less, and the remaining part is mainly composed of ferrite and bainite. to the tissue it is carried 5-150 seconds in a temperature range of less than Ac ₁ transformation point temperature above Ac ₃ transformation point temperature + 100 ℃ as described above. At this time, if the temperature becomes too low even in the temperature range, if cementite is precipitated in the hot rolling step, it takes too long for cementite to be re-used. If the temperature becomes too high, the volume fraction of austenite becomes excessively large, Since C concentration falls and it is easy to catch the nose of bainite or pearlite transformation containing a large amount of carbide, it is preferable to heat to 780 degreeC or more and 850 degrees C or less. If the cooling rate after holding | maintenance is less than 20 degree-C / s, it may become caught in the bainite or pearlite transformation nose containing a large amount of carbides, and it is set as the cooling rate of 20 degree-C / s or more.

Next, a process of promoting bainite transformation to stabilize a necessary amount of retained austenite is described. When the cooling end temperature is 450 ° C or higher, the retained austenite is decomposed into bainite or pearlite containing a large amount of carbides. For example, a microstructure containing residual austenite having a desired volume fraction of 5% or more and 25% or less and mainly consisting of ferrite and bainite cannot be obtained. In addition, below 350 ° C, martensite may be produced in a large amount, and sufficient residual austenite cannot be obtained, and thus, residual austenite having a target volume fraction of 5% or more and 25% or less is contained, and the remaining part is mainly ferrite and bainite. Since the microstructure which consists of these is not obtained, it cools to the temperature range exceeding 350 degreeC.

In addition, the holding time in the temperature range is insufficient for bainite transformation for stabilizing residual austenite in less than 5 seconds, and there is a risk of martensite transformation at the end of cooling followed by unstable residual austenite. A microstructure comprising a retained austenite having a fraction of 5% or more and 25% or less, mainly consisting of ferrite and bainite, is not obtained. In addition, the bainite transformation is excessively promoted in excess of 600 seconds, and a stable amount of retained austenite of the required amount cannot be obtained, and the retained austenite having a target volume fraction of 5% or more and 25% or less is contained, and the remainder is mainly ferrite, No microstructure consisting of bainite is obtained. Therefore, the holding time in that temperature range is made into 5 second or more and 600 second or less.

Finally, the cooling rate until the end of the cooling is likely to promote the bainite transformation too much during cooling at less than 5 ° C / s, and stable residual austenite of the required amount cannot be obtained. Thus, the desired volume fraction is 5% or more to 25%. Since the microstructure which consists of the following residual austenite and consists of a remainder mainly consisting of ferrite, bainite, etc. may not be obtained, it shall be 5 degrees C / s or more.

In addition, since the aging property may deteriorate when cooling end temperature exceeds 200 degreeC, it shall be 200 degrees C or less. The lower limit of the cooling end temperature is not particularly limited. However, when cooling by water cooling or spraying, if the coil is wet with water for a long time, the appearance defect due to rust may be concerned, and therefore 50 ° C. or more is preferable.

Further, in the present invention, in order to impart a resistance ratio for obtaining good shape freezing property in addition to improving notch fatigue strength, a composite structure in which the microstructure volume fraction maximum phase is made of ferrite and the second phase is mainly martensite. to, the carried 5-150 seconds in a temperature range of less than Ac ₁ transformation point temperature Ac ₃ transformation point temperature + 100 ℃ as described above. At this time, if the temperature becomes too low even in the temperature range, if cementite is precipitated in the hot-rolled sheet stage, it takes too long for cementite to be re-used. If the temperature becomes too high, the volume fraction of austenite becomes excessively large, Since the C concentration in a nitrate falls and it is easy to be caught by the nose of the bainite or pearlite transformation containing a large amount of carbide, it is preferable to heat to 780 degreeC or more and 850 degrees C or less.

If the cooling rate after holding | maintenance is less than 20 degree-C / s, there exists a possibility that it may be caught by the nose of the bainite or pearlite transformation containing a large amount of carbides, and it shall be 20 degree-C / s or more cooling rate. If the cooling finish temperature exceeds 350 ° C, the microstructure using the target ferrite as the maximum volume fraction phase and martensite as the second phase is not obtained, and thus is cooled to a temperature range of 350 ° C or lower. The lower limit of the end temperature of the cooling step is not particularly limited, but when cooling by water cooling or spraying, if the coil is wet with water for a long time, appearance defects due to rust may be concerned, and therefore 50 ° C. or more is preferable.

Moreover, you may perform skin pass rolling after that as needed. In order to perform zinc plating on the hot rolled steel sheet after pickling or the cold rolled steel sheet after completion | finish of recrystallization annealing, it may be immersed in a zinc plating bath and may be alloyed as needed.

Example

Example 1

In the following, the present invention will be described in detail by the first embodiment.

The steel of A-L which has the chemical component shown in Table 1 was melted by the converter, it was reheated after continuous casting, it was wound up after setting to 1.2-5.5 mm of sheet thickness by the finish rolling following rough rolling. However, the indication about the chemical composition in a table | surface is mass%.

Next, Table 2 shows the details of the manufacturing conditions. At this time, "SRT" is a slab heating temperature, "FT" is the final pass finish rolling temperature, the "rolling ratio" represents the sum of the rolling reduction in the temperature range of less than Ar ₃ transformation point temperature + 100 ℃. However, when rolling is performed next in a cold rolling process, since it does not have such a restriction | limiting, it was set as "-". In addition, "lubricant", was characterized by the presence or absence of lubricant from the temperature range of less than Ar ₃ transformation point temperature + 100 ℃.

In addition, if the "take-up" refers to a coiling temperature (CT) is less than T ₀ In the case of "○", more than T ₀ has been to "×." However, in the case of a cold rolled steel sheet, since it does not need to specifically limit as a manufacturing condition, it was called "-".

Next, pickling, cold rolling, and annealing were performed for some of them after hot rolling. The plate thickness is 0.7-2.3 mm. If this time, the "rolling rate" means the total cold reduction "Time" is included in the temperature range of the annealing time, hereinafter, "annealing" refers to more than the annealing temperature of the recovery temperature of Ar ₃ transformation point temperature + 100 ℃ "○", out It was set as "x" if there was any. In addition, about the steel L, after rough rolling, descaling was performed on the conditions of 2.7 MPa of impact pressures and 0.001 liter / cm <^2> of flow rates. On the other hand, zinc plating was performed about steel G and steel F-5 in the said steel plate.

In this way, the tensile test of the hot-rolled sheet thus obtained was carried out in accordance with JIS Z 2241 test method by first processing the test material into test piece No. 5 of JIS Z 2201. Table 2 shows the yield strength (σ Y), tensile strength (σ B), and elongation at break (El).

Further, the triacid finish grinding is performed to a depth of about 0.05 mm from the outermost surface of the specimen cut into 30 mm phi from the 1/4 W or 3/4 W position of the plate width, and then deformed by chemical polishing or electropolishing, and then produced, The X-ray diffraction intensity was measured in accordance with the method described in pages 274 to 296 of "New Edition of Curity X-ray Diffraction Yoron" (I986, Gentaro Matsumura Station, Agne Co., Ltd.).

In this case, the average value of the X-ray random intensity ratios of the {100} <011> to {223} <110> defense groups is the main bearing included in this defense group, {100} <011>, {116} <110>, { X-ray diffraction intensities of 114} <110>, {113} <110>, {112} <110>, {335} <110>, and {223} <110> were determined using the vector method based on the {110} pole figure. Three-dimensional aggregates computed by using or a series of three-dimensional calculations using a plurality of pole plots (preferably three or more) among the {110}, {100}, {211}, and {310} pole plots Collected tissues were obtained.

,

,

,

,

,

,

의 강도를 그대로 사용하면 좋다. 다만 {1001<011>∼{223}<110> 방위군의 X선 랜덤 강도비의 평균치란, 상기 각 방위의 상가평균이다.For example, in the latter method, the X-ray random intensity ratio of each crystal orientation is in the φ2 = 45 ° cross section of the three-dimensional texture.

,

,

,

,

,

,

It is good to use the strength as it is. However, the average value of the X-ray random intensity ratios of the {1001 <011> to {223} <110> defense groups is the average of the malles of the respective orientations.

If the strengths of all the bearings cannot be obtained, the malls of the respective bearings of {100} <011>, {116} <110>, {114} <110>, {112} <110>, and {223} <110> It may be replaced by an average.

Next, the average value of the three-direction X-ray random intensity ratios of {554} <225>, {111} <112>, and {111} <110> may be obtained from the three-dimensional aggregated tissue calculated in the same manner as described above.

In Table 2, the "strength ratio 1" in the X-ray random intensity ratio is the average value of the X-ray random intensity ratios in the {100} <011> to {223} <110> defense groups, and the "strength ratio 2" is {554} <225>, {111} <112>, and {111} <110> are average values of the X-ray random intensity ratios of three directions.

Next, in order to examine the notched fatigue strength of the steel sheet, a fatigue test piece of the shape shown in Fig. 1 (b) was taken from the 1 / 4W or 3 / 4W position of the plate width so as to have a long side and a fatigue test was carried out. It was. However, the fatigue test piece was subjected to Samsan finish grinding to the depth of about 0.05 mm from the outermost layer. The fatigue test used an electrohydraulic servo type fatigue tester, and the test method was according to JIS Z 2273-1978 and JIS Z 2275-1978. Table 2 shows the notch fatigue limit (σWK) and the notch fatigue limit ratio (σWK / σB).

According to the present invention are 11 steels of steels A, E, F-1, F-2, F-5, G, H, I, J, K, L, containing a predetermined amount of steel components, The average value of the X-ray random intensity ratios of the {100} <011> to {223} <110> bearing groups of the plate surface at an arbitrary depth up to 0.5 mm in the plate thickness direction was 2 or more, and {554} <225>, Thin steel sheet for automobiles having excellent notch fatigue strength, characterized in that the average value of the X-ray random intensity ratios of the three directions of {111} <112> and {111} <110> is 4 or less and the plate thickness is 0.5 mm or more and 12 mm or less. Is obtained and, therefore, the fatigue limit ratio of the conventional steel evaluated by the description method of the present invention exceeds 0.2 to 0.3.

Steel other than the above is outside the scope of the present invention for the following reasons.

That is, since steel B content of C is out of the range of this invention, sufficient intensity (σB) was not obtained. Since steel C content of P is out of the range of this invention, sufficient notch fatigue strength ((sigma) WK / (sigma) B) was not obtained. Since steel D content of S is out of the range of this invention, sufficient elongation El was not obtained. Steel F-3 has a notch fatigue strength (? WK / σB) because the aggregate reduction ratio in the temperature range of the Ar ₃ transformation point temperature of + 100 ° C. or less is outside the range of the present invention, and thus, the aggregate structure for the purpose of the present invention is not obtained. ) Was not obtained.

Since steel F-4 has a finish rolling end temperature (FT) outside the range of the present invention, and the winding temperature is also outside the range of the present invention, an aggregate structure for the purpose of the present invention cannot be obtained, and sufficient notch fatigue strength (? WK) is obtained. / σB) was not obtained. Since steel F-6 had a cold rolling ratio outside the range of this invention, the aggregate structure of this invention was not obtained and sufficient notch fatigue strength ((sigma) WK / (sigma) B) was not obtained. Since the annealing temperature of steel F-7 was out of the range of this invention, the aggregate structure for which this invention aims was not obtained, and sufficient notch fatigue strength ((sigma) WK / (sigma) B) was not obtained. Since the annealing time of steel F-8 was out of the range of this invention, the aggregate structure of this invention was not obtained and sufficient notch fatigue strength ((sigma) WK / (sigma) B) was not obtained.

Example 2

Next, Example 2 demonstrates this invention further in detail.

The two steels of G and H having the chemical components shown in Table 1 were reheated to the heating temperature shown in Table 3, and after winding to a sheet thickness of 1.5 to 5.5 mm by finish rolling followed by rough rolling. As shown in Table 3, some of the descaling rings were carried out after rough rolling under conditions of an impingement pressure of 2.7 MPa and a flow rate of 0.001 liter / cm ² .

Table 3 shows the details of the manufacturing conditions. At this time, "SRT" is a slab heating temperature, "FT" is the final pass finish rolling temperature, the "rolling ratio" represents the sum of the rolling reduction in the temperature range of less than Ar ₃ transformation point temperature + 100 ℃. However, when rolling by a cold rolling process later, since there is no such a restriction | limiting, it was set as "-". In "lubrication" it was characterized by the presence of lubrication in a temperature range of less than Ar ₃ transformation point temperature + 100 ℃. "CT" represents the winding temperature. However, in the case of a cold rolled steel sheet, since it does not need to specifically limit as a manufacturing condition, it was set as "-". Next, some of them were subjected to hot rolling, pickling, cold rolling, and heat treatment. The plate thickness is 0.7 to 2.3 mm. The "cold rolling rate" is the total cold rolling rate, "ST" is a heat treatment temperature, and "Time" is a heat treatment time. In addition, some of the said steel sheets were galvanized.

The tensile test of the hot rolled sheet and cold rolled sheet thus obtained was carried out in the same manner as above.

Table 4 shows the yield strength (σ Y), tensile strength (σ B), elongation at break (El) and yield ratio (YR), and strength-ductility balance (σ B × El). On the other hand, the burring workability (hole expandability) was evaluated according to the hole expansion test method described in Japanese Steel Federation Standard JFST 1001-1996. Table 4 shows the hole expansion ratio λ.

Table 4 also shows the microstructure. Here, the other is a structure other than pearlite and / or ferrite, bainite, residual austenite, and martensite shown individually in Table 4. In the microstructure of the steel sheet, the volume fraction of ferrite, bainite, retained austenite, pearlite, and martensite means that the sample cut out from the 1 / 4W or 3 / 4W position of the sheet width of the steel sheet is polished in the rolling direction cross section, and the nital reagent And a microstructure area fraction at l / 4t of plate thickness observed using a reagent disclosed in Japanese Patent Application Laid-Open No. Hei 5-163590 and observed at a magnification of 200 to 500 times using an optical microscope.

On the other hand, austenite can be easily crystallized because of its different crystal structure from ferrite. Therefore, the volume fraction of retained austenite can also be obtained experimentally by the X-ray diffraction method. That is, it is a method of simply calculating the volume fraction using the following equation from the difference between the reflecting surface intensities of austenite and ferrite using Mo Kα rays.

Vγ = (2/3) {100 / (0.7 × α (211) / γ (220) +1)}

+ (1/3) {100 / (0.78 × α (211) / γ (311) +1)}

Are the X-ray reflective surface intensities of the ferrite (α) austenite (γ), respectively. Since the volume fraction of the retained austenite can be almost matched by any method of optical microscopy and X-ray diffraction, any measurement value is used.

In addition, the X-ray diffraction intensity was measured and the fatigue test was conducted in the same manner as described above.

The fatigue test was conducted in the same manner as above. Table 4 shows the notch fatigue limit (σWK) and the notch fatigue limit ratio (σWK / σB).

According to the present invention is the ninth class of steel g-1, g-2, g-3, g-5, g-6, g-7, h-1, h-2, h-3, and a predetermined amount of steel. Component, and the average value of the X-ray random intensity ratios of the {100} <011> to {223} <110> azimuth groups of the plate surface at an arbitrary depth from the outermost surface to 0.5 mm in the plate thickness direction is 2 or more, and { 554} <225>, {111} <112>, and {111} <110> have an average value of the X-ray random intensity ratios in three directions of 4 or less, and a plate thickness of 0.5 mm or more and 12 mm or less, and the volume fraction maximum. Phase contains bainite, or a composite structure of ferrite and bainite, or a volume fraction of 5% or more and 25% or less of retained austenite, the remainder being mainly composed of ferrite, bainite, or A thin steel sheet for automobiles having excellent notched fatigue strength is obtained, wherein the phase is made of ferrite, and the second phase is mainly composed of martensite. Standing, significant difference is recognized with respect to a conventional Fatigue limit ratio 20-30% assessed by the method of the present invention.

Steel other than the above is outside the scope of the present invention for the following reasons.

That is, since steel g-4 has a total rolling reduction in the temperature range of the finish rolling finish temperature (FT) and the Ar ₃ transformation point temperature of + 100 ° C. or less, the texture of the present invention is obtained. Not enough fatigue strength (σWK / σB) was not obtained. Since steel g-8 has a cold rolling rate outside the scope of the present invention, a notched fatigue strength (? WK /? B) was not obtained without obtaining an aggregate structure for the purpose of the present invention. Steel h-4 is because the finish rolling end temperature (FT) and the Ar ₃ transformation point temperature + 100 ℃ et total rolling reduction in the range of the present invention in a temperature range of or less, the texture of the object of the present invention not be obtained. Sufficient notch fatigue strength (? WK /? B) was not obtained.

[Effects of the Invention]

As described above, the present invention relates to a thin steel plate for automobile having excellent notch fatigue strength and a method of manufacturing the same, and the progress of fatigue cracking in stress concentration parts such as punched parts and welded parts is problem by using such thin plate. This is an invention of high industrial value because it can be expected that the notch fatigue strength, which is one of the important characteristics in a member requiring durability, such as an automobile chassis part, will be greatly improved.

Claims (24)

The average value of the X-ray random intensity ratios of the {100} <011> to {223} <110> bearing groups of the plate surface at an arbitrary depth from the outermost surface to the plate thickness direction of 0.5 mm was 2 or more, and the {554} < 225>, {111} <112> and {111} <110> are excellent in notch fatigue strength, characterized in that the average value of the three-direction X-ray random intensity ratio is 4 or less, and the plate thickness is 0.5 mm or more and 12 mm or less. For sheet steel. The method of claim 1, The microstructure of the steel sheet is a composite steel sheet having a maximum volume fraction as bainite or a composite structure having a maximum volume fraction as ferrite and bainite. The method of claim 1, The microstructure of the steel sheet, the automotive steel sheet having excellent notched fatigue strength, characterized in that the volume fraction: 5% or more and 25% or less residual austenite and the remainder is a composite structure mainly consisting of ferrite and bainite. The method of claim 1, The microstructure of the steel sheet is a steel sheet for automobiles having excellent notched fatigue strength, characterized in that the composite having a maximum volume fraction as ferrite and a second phase as martensite. In mass%, C: 0.01 to 0.3%, Si: 0.01 to 2%, Mn: 0.05 to 3%, P: ≤ 0.1%, S: ≤ 0.01%, Al: 0.005 to 1%, and the remaining portion A steel sheet composed of Fe and unavoidable impurities, the average value of the X-ray random intensity ratios of the {100} <011> to {223} <110> orientation groups of the plate surface at an arbitrary depth from the outermost surface to the plate thickness direction of 0.5 mm. Is 2 or more, and the average value of the X-ray random intensity ratios in the three directions of {554} <225>, {111} <112>, and {111} <110> is 4 or less, and the plate | board thickness is 0.5 mm or more and 12 mm or less The steel sheet for automobiles which is excellent in notch fatigue strength, It characterized by being. The method of claim 5, In mass%, Cu: 0.2-2%, B: 0.0002-0.002%, Ni: 0.1-1%, Ca: 0.0005-0.002%, REM: 0.0005-0.02%, Ti: 0.05-0.5%, Nb: 0.01 It is excellent in notch fatigue strength characterized by including 1 type (s) or 2 or more types of -0.5%, Mo: 0.05-1%, V: 0.02-0.2%, Cr: 0.01-1%, Zr: 0.02-0.2%. Steel sheet for automobiles. The method according to claim 5 or 6, The microstructure of the steel sheet may include 1) a composite structure having a maximum volume fraction of bainite or a composite structure having a maximum volume fraction of ferrite and bainite, and 2) a volume fraction: 5% or more and 25% or less 3) A composite tissue containing residual austenite of which the remainder is mainly composed of ferrite and bainite, and 3) one of the complex tissues having the largest volume fraction as ferrite and the second phase as martensite. Automotive steel sheet with excellent notch fatigue strength. The steel sheet for automobiles of any one of Claims 1-6 is galvanized, The automotive steel sheet excellent in the notch fatigue strength. In mass%, C: 0.01 to 0.3%, Si: 0.01 to 2%, Mn: 0.05 to 3%, P: ≤ 0.1%, S: ≤ 0.01%, Al: 0.005 to 1%, and the remaining portion After rough-rolling a steel piece made of Fe and unavoidable impurities, when hot rolling, a finish rolling of 25% or more of the total reduction ratio of the steel sheet thickness is performed at a temperature range of Ar ₃ transformation point temperature of + 100 ° C. or lower, and the steel sheet The average value of the X-ray random intensity ratios of the {100} <011> to {223} <110> azimuth groups of the plate surface at an arbitrary depth from the outermost surface to 0.5 mm in the plate thickness direction was 2 or more, and the {554} <225>, {111} <112> and {111} <110> are excellent in notch fatigue strength, characterized in that the average value of the three-direction X-ray random intensity ratios is 4 or less, and the plate thickness is 0.5 mm or more and 12 mm or less. Method for manufacturing thin steel sheet for automobiles. The method of claim 9, After said finishing rolling, it cools at the cooling rate of 20 degreeC / s or more, and winds it up at the winding temperature of 450 degreeC or more, The manufacturing method of the steel sheet for automobiles excellent in the fatigue strength of the notch characterized by the above-mentioned. The method of claim 9, After the finish rolling, the temperature is maintained for 1 to 20 seconds in the temperature range of Ar ₁ transformation point or more and Ar ₃ transformation point temperature or less, thereafter, cooling at a cooling rate of 20 ° C./s or more, and the temperature range of more than 350 ° C. and less than 450 ° C. The manufacturing method of the steel sheet for automobile excellent in the fatigue strength of the notch characterized by winding up at the winding temperature of the. The method of claim 9, After cooling, the manufacturing method of the automotive steel sheet excellent in notch fatigue strength characterized by winding up to the winding temperature of 350 degrees C or less. The method according to any one of claims 9 to 12, A method for producing a thin steel sheet for automobiles having excellent notch fatigue strength, characterized in that lubrication rolling is performed in the hot rolling. The method according to any one of claims 9 to 12, A method for manufacturing a thin steel sheet for automobiles having excellent notch fatigue strength, characterized in that descaling is performed after finishing rough rolling in the hot rolling. In mass%, C: 0.01 to 0.3%, Si: 0.01 to 2%, Mn: 0.05 to 3%, P: ≤ 0.1%, S: ≤ 0.01%, Al: 0.005 to 1%, and the remaining portion Fe And roughly rolling the steel pieces made of unavoidable impurities, followed by finish rolling of 25% or more of the total reduction ratio of the steel sheet thickness at a temperature range of the Ar ₃ transformation point temperature of + 100 ° C. or lower, followed by pickling and further reduction of the steel sheet thickness reduction ratio. After cold rolling of less than 80%, the temperature is maintained for 5 to 150 seconds at a recovery temperature not lower than Ac ₃ transformation point temperature + 100 ° C. or lower, and recovery or recrystallization annealing of the cooling process is performed, and the sheet thickness from the outermost surface of the steel sheet. The average value of the X-ray random intensity ratios of the {100} <011> to {223} <110> bearing groups of the plate surface at an arbitrary depth up to 0.5 mm in the direction is 2 or more, and {554} <225>, {111 } <112> and {111} <110> furnace characterized in that the plate thickness of the average value of the three-direction X-ray random intensity ratio is 4 or less 0.5mm or more and 12mm or less The method of the high fatigue strength steel sheet for automobiles. The method of claim 15, After cold rolling, the automotive foil having excellent notch fatigue strength, which is maintained at a temperature range of Ac ₁ transformation point temperature or more and Ac ₃ transformation point temperature of + 100 ° C. or less for 5 to 150 seconds, and then subjected to heat treatment in a cooling process. Method of manufacturing steel sheet. The method of claim 15, After holding at the said temperature range for 5 to 150 second, it cools to the temperature range exceeding 350 degreeC and less than 450 degreeC at the cooling rate of 20 degree-C / s or more, and then hold | maintaining for 5 to 600 second at the said temperature range further, and 5 degreeC A method for producing a thin steel sheet for automobiles having excellent notch fatigue strength, characterized in that the heat treatment is performed at a cooling rate of not less than / s to a temperature range of 200 ° C or less. The method of claim 15, After maintaining for 5 to 150 seconds in the temperature range, the method of manufacturing a thin steel sheet for automobiles having excellent notch fatigue strength, characterized in that the heat treatment in the step of cooling to a temperature range of 350 ℃ or less at a cooling rate of 20 ℃ / s or more. . The steel sheet according to any one of claims 11, 12 or 15 to 18, in addition, in mass%, Cu: 0.2 to 2%, B: 0.0002 to 0.002%, Ni: 0.1 to 1% Ca: 0.0005 to 0.002%, REM: 0.0005 to 0.02%, Ti: 0.05 to 0.5%, Nb: 0.01 to 0.5%, Mo: 0.05 to 1%, V: 0.02 to 0.2%, Cr: 0.01 to 1%, A method for producing a steel sheet for automobiles having excellent notch fatigue strength, comprising one or two or more of Zr: 0.02 to 0.2%. The method according to claim 10 or 16, The microstructure of the steel sheet is a composite steel sheet having a maximum volume fraction of bainite or a composite structure having a maximum volume fraction of ferrite and bainite. Manufacturing method. The method according to claim 11 or 17, The microstructure of the steel sheet, the volume fraction: 5% or more, 25% or less containing austenite, and the remainder is a composite steel sheet having excellent notch fatigue strength, characterized in that the composite structure mainly consisting of ferrite, bainite Way. The method according to claim 12 or 18, wherein The microstructure of the steel sheet is a method for producing a thin steel sheet for automobiles having excellent notched fatigue strength, wherein the microstructure of the steel sheet is a composite structure in which a phase having a maximum volume fraction is ferrite and a second phase is martensite. After manufacturing the hot rolled steel sheet or the recovery or recrystallization annealing plate according to any one of claims 9 to 12 or 15 to 18, additionally, the steel sheet is immersed in a zinc plating bath to perform zinc plating on the surface of the steel sheet. Method for producing a thin steel sheet for automobiles having excellent notch fatigue strength, characterized in that. The method of claim 23, After galvanizing, an additional method for producing a thin steel sheet for automobiles having excellent notch fatigue strength, characterized in that the alloying treatment.

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