JP2002317246A - Automobile thin steel sheet having excellent notch fatigue resistance and burring workability and production method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an automobile thin steel sheet which has excellent notch fatigue resistance and burring workability, and a production method therefor. SOLUTION: The automobile thin steel sheet having excellent notch fatigue resistance and burring workability consists of steel having a composition containing 0.01 to 0.1% C, <=0.03% S, <=0.005% N and 0.05 to 0.5% Ti, and further containing Ti in a range satisfying Ti-48/12C-48/14N-48/32S>=0%, and the balance Fe with inevitable impurities. The average value of the X-ray random intensity ratios in the 100}<011> to 223}<110> orientation groups in the sheet face in the optional depth to 0.5 mm in the sheet thickness direction from the outermost surface is >=2. Also, the average value of the X-ray random intensity ratios among the three orientations of 554}<225>, 111}<112> and 111}<110> is <=4, and its sheet thickness is 0.5 to 12 mm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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 burring workability, and a method of manufacturing the same. The present invention relates to a thin steel sheet for automobiles having excellent notch fatigue strength and burring workability, which is suitable as a thin steel sheet for automobiles which is a material for undercarriage parts of automobiles or the like in which crack propagation is a problem, and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, the application of light metals such as Al alloys and high-strength steel sheets to automobile members has been promoted for the purpose of weight reduction in order to improve fuel efficiency of automobiles. However, although light metals such as Al alloys have the advantage of high specific strength,
Due to their considerable cost compared to steel, their application is limited to special applications. Therefore, in order to promote weight reduction of automobiles in a wider range, there is a strong demand for the use of inexpensive high-strength steel sheets.

[0003] In response to such demands for high strength, in the field of cold rolled steel sheets used for white bodies and panels that occupy about 1/4 of the body weight, they have both strength and deep drawability. Development of steel sheets and bake-hardening steel sheets has been promoted, which has contributed to weight reduction of vehicle bodies. However, at present, the object of weight reduction is shifting to structural members and undercarriage members occupying about 20% of the vehicle body weight, and there is an urgent need to develop high-strength thin steel sheets used for these members.

However, since high strength generally deteriorates material properties such as formability (workability), how to achieve high strength without deteriorating material properties is the key to the development of high strength steel sheets. Become. In particular, properties required for steel sheets for structural members and suspension members include not only elongation but also shearing and punching workability,
Burring workability, fatigue durability, corrosion resistance, and the like are important, and high strength and how to balance these characteristics with high dimensions are important.

[0005] For example, parts such as a suspension arm
After blanking or punching by shearing or punching, press forming is performed, and some members are further welded into parts. In such parts, cracks often propagate from the sheared end face or the vicinity of the welded portion, resulting in fatigue failure in many cases. That is, the sheared end face or the welded portion becomes a stress concentration portion such as a notch, from which a fatigue crack propagates.

On the other hand, the fatigue limit of a material generally decreases as the notch becomes sharp. However, when the notch is sharpened to some extent, a phenomenon occurs in which the fatigue limit does not decrease any more.
This is because the fatigue limit changes from the crack initiation limit to the crack growth limit. When the strength of the material is increased, the crack initiation limit is improved, but the crack propagation limit is not increased. Therefore, the point at which the fatigue limit transits from the crack initiation limit to the crack propagation limit moves to the sharp side of the notch. Therefore, even if the strength of the material is increased, the fatigue limit is significantly reduced due to the notch, and the advantage of high strength cannot be enjoyed in the fatigue limit when the notch is sharp. That is, the higher the strength, the higher the sensitivity to the notch.

At present, 340-440 MPa grade steel plates are used as the thin steel plates for undercarriage of automobiles, and the required strength level of the steel plates for these members is 590-78.
We are heading for even higher strength toward the 0 MPa class. Therefore, in order to meet these demands, it is indispensable to develop a steel sheet that can enjoy the advantage of high strength even when a sharp notch exists.

[0008] There are roughly two methods for improving the fatigue strength when there is a punched or sheared end face. One is to eliminate sharp notches such as burrs generated on the end face of punching or shearing, and the other is to increase resistance to crack propagation even in the presence of such sharp notches.

The invention belonging to the former is disclosed in, for example,
No. 5,516,955 discloses that the amount of added Si is reduced,
A technique has been disclosed in which the generation of burrs is suppressed by reducing the elongation at break by the precipitates of i, Nb, and V, and the fatigue strength as it is punched or sheared is improved. Also, Japanese Patent Application Laid-Open
In the -179346 publication, the upper limit of the volume fraction of bainite is limited by defining the upper limit of the rolling finishing temperature,
There is disclosed a technique for improving the fatigue strength as it is punched or sheared. Further, Japanese Patent Application Laid-Open No. 8-13033 discloses a technique in which the cooling rate after rolling is regulated and the generation of martensite is suppressed, thereby improving the fatigue strength in punching and shearing.

Japanese Patent Application Laid-Open No. 8-302446 discloses that the hardness of the second phase in a composite structure steel is specified to be at least 1.3 times that of ferrite, and the strain energy during punching and shearing is reduced. There is disclosed a technique for improving the fatigue strength as it is punched or sheared. Further, Japanese Patent Application Laid-Open No. 9-170048 discloses a technique in which the length of grain boundary cementite is specified to reduce burrs at the time of punching or shearing, thereby improving the fatigue strength as it is punched or sheared. . Further, JP-A-9-202
Japanese Patent Application Publication No. 940 discloses a technique for improving the punching property by defining parameters arranged by the added amounts of Ti, Nb, and Cr, and improving the fatigue strength as punched.

On the other hand, the invention belonging to the latter is disclosed in
JP-88161 discloses that the (100) plane strength of the texture parallel to the rolled surface in the surface layer is 1.5 or more,
Techniques for reducing the fatigue crack propagation rate have been disclosed.
Also, JP-A-8-199286 and JP-A-10-1998
No. 147846 discloses that the (200) diffraction intensity ratio in the thickness direction measured by X-rays is set to 2.0 to 15.0, and the area ratio of the recovered or recrystallized ferrite is set to 15 to 40%. A technique for reducing the fatigue crack propagation speed is disclosed.

However, JP-A-5-51695, JP-A-5-179346, JP-A-8-13033, JP-A-8-302446, JP-A-9-170048 and JP-A-9-202940, etc. Disclosed,
The technology to reduce sharp notches such as burrs generated on the punching or shearing end face is a technology that can be applied under any conditions because the degree of burrs that occur varies greatly due to the clearance during punching and shearing. Therefore, it must be said that the steel sheet is not sufficient as a steel sheet having excellent notch fatigue strength.

On the other hand, JP-A-6-88161, JP-A-8-199286 and JP-A-10-1478
The technique disclosed in Japanese Patent Publication No. 46-46, which increases the resistance to crack growth by controlling the texture, is an invention mainly intended for steel for large structures such as construction machines, ships, and bridges. It is not intended for automotive steel sheets. In addition, the above technology mainly controls the crack propagation speed in the PARIS region, which is referred to as the fracture mechanics of a fatigue crack that propagates from the weld toe. The technique is insufficient when there is almost no crack propagation region in the PARIS region.
Further, there has not been found any invention in which the notch fatigue characteristics are evaluated using the test piece shown in FIG. 1 (b) in the flat fatigue test method used for thin steel sheets.

[0014]

SUMMARY OF THE INVENTION Accordingly, the present invention is to provide a thin steel sheet for automobiles having excellent burring workability, by eliminating a fatigue crack growing from a notch such as a punched or sheared end face by a clearance at the time of punching or shearing. The present invention relates to a technique for improving the texture by controlling the texture and increasing the resistance to crack propagation regardless of conditions such as the above. That is, an object of the present invention is to provide a thin steel sheet for automobiles having excellent notch fatigue strength and burring workability, and a manufacturing method capable of stably manufacturing the steel sheet at low cost.

[0015]

DISCLOSURE OF THE INVENTION The inventors of the present invention have considered a manufacturing process for thin steel sheets produced on an industrial scale using manufacturing equipment which is usually employed at present, and have taken into consideration the manufacturing process of a thin steel sheet for automobiles having excellent burring workability. Intensive research was conducted to improve the notch fatigue strength of steel sheets. As a result, the average value of the X-ray random intensity ratio of the {100} <011> to {223} <110> orientation group 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 $ 55
4 {225}, {111} <112> and {11
It has been newly added that the average value of the X-ray random intensity ratio in three directions of 1｝ <110> is 4 or less and the plate thickness is 0.5 mm or more and 12 mm or less, which is very effective in improving the notch fatigue strength. Heading, the present invention.

That is, the gist of the present invention is as follows. (1) In mass%, C: 0.01 to 0.1%, S
≦ 0.03%, N ≦ 0.005%, Ti:
0.05-0.5%, and further Ti-48 / 12
C-48 / 14N-48 / 32S is a steel containing Ti in a range satisfying ≧ 0%, the balance being Fe and unavoidable impurities, and an arbitrary depth from the outermost surface to 0.5 mm in the thickness direction. {100} <011> to {22}
The average value of the X-ray random intensity ratio of the 3｝ <110> orientation group is 2 or more, and {554} <225>, {111} <11
Notch fatigue strength and burring, wherein the average value of the X-ray random intensity ratios in the three directions of 2> and {111} <110> is 4 or less, and the plate thickness is 0.5 mm or more and 12 mm or less. Automotive thin steel sheet with excellent properties.

(2) The steel according to (1) further contains Nb = 0.01 to 0.5% by mass%, and the relationship among C, N, S, Ti and Nb is Ti + 48. / 93
Nb-48 / 12C-48 / 14N-48 / 32S ≧ 0
%, Characterized in that it contains Ti and Nb in a range satisfying%, and has excellent notch fatigue strength and burring workability. (3) The steel according to the above (1) or (2) further comprises, in mass%, Si: 0.01 to 2%, Mn: 0.05 to 3%, P ≦ 0.1%, Al: 0.005 to 1%, characterized by being excellent in notch fatigue strength and burring workability, and is a thin steel sheet for automobiles. (4) The steel according to any one of the above (1) to (3) further contains B: 0.0002 to 0.0 in mass%.
A thin steel sheet for automobiles having excellent notch fatigue strength and burring workability, characterized by containing 02%. (5) Notch fatigue, wherein the steel according to any one of (1) to (4) further contains Cu: 0.2 to 1.2% by mass%. Automotive thin steel sheet with excellent strength and burring workability. (6) Notch fatigue, wherein the steel according to any one of the above (1) to (5) further contains Ni: 0.1 to 0.6% by mass%. Automotive thin steel sheet with excellent strength and burring workability. (7) The steel according to any one of (1) to (6), further containing Ca: 0.0005 to 0.5% by mass%.
A thin steel sheet for automobiles having excellent notch fatigue strength and burring workability, characterized by containing one or two kinds of 002% and REM: 0.0005 to 0.02%. (8) The steel according to any one of (1) to (7), further comprising: Mo: 0.05 to 1% by mass%;
V: 0.02 to 0.2%, Cr: 0.01 to 1
%, Zr: one or more of 0.02 to 0.2% of an automotive thin steel sheet having excellent notch fatigue strength and burring workability. (9) An automotive thin steel sheet excellent in notch fatigue strength and burring workability, characterized in that the automotive thin steel sheet according to any one of (1) to (8) is galvanized. steel sheet.

(10) When hot rolling is performed to obtain a thin steel sheet having the component described in any one of (1) to (8) above, a slab having the component is subjected to Ar3 after rough rolling.
An automotive thin film with excellent notch fatigue strength and burring workability, characterized in that finish rolling at a total reduction ratio of 25% or more of the steel sheet thickness is performed in a temperature range of the transformation point temperature + 100 ° C or less, and then cooled and wound. Steel plate manufacturing method. (11) In the hot rolling, lubricating rolling is performed in finish rolling after rough rolling, wherein (10).
2. A method for producing a thin steel sheet for automobiles, which is excellent in notch fatigue strength and burring workability according to item 1. (12) Manufacture of a thin steel sheet for automobiles having excellent notch fatigue strength and burring workability, wherein descaling is performed after rough rolling in the hot rolling according to (10) or (11). Method. (13) In order to obtain a thin steel sheet having the component according to any one of the above (1) to (8), a steel slab having the component is hot-rolled, followed by pickling, and the steel sheet thickness reduction rate is less than 80%. Characterized by having a notch fatigue strength and burring workability, characterized in that after cold rolling, the steel sheet is kept in a temperature range of a recovery temperature or higher and an Ac3 transformation point temperature + 100 ° C or lower for 5 to 150 seconds and subjected to a heat treatment in a cooling step. Manufacturing method of thin steel sheet for automobile. (14) Notch fatigue, in the production method according to any one of (10) to (12), wherein the steel sheet is galvanized by dipping in a galvanizing bath after hot rolling. A method for manufacturing automotive thin steel sheets with excellent strength and burring workability. (15) In the production method according to (13), after the heat treatment, the steel sheet is immersed in a galvanizing bath to galvanize the surface of the steel sheet, and is excellent in notch fatigue strength and burring workability. Manufacturing method of thin steel sheet. (16) A thin steel sheet for automobiles excellent in notch fatigue strength and burring workability according to the above (14) or (15), which is immersed in a galvanizing bath, galvanized and then alloyed. Manufacturing method.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the results of basic research that led to the present invention will be described below. Generally, fatigue cracks occur from the surface. This is not an exception even when a stress concentration portion such as a notch exists. Further, even when a punched or sheared end face is present, it is often observed that a fatigue crack propagates from the steel sheet surface end under a repeated load including a load mode in an out-of-plane bending direction. Therefore, even in such a case, it is clear that increasing the crack propagation resistance to the outermost surface of the steel sheet or to a depth of about several crystal grains is effective for improving the notch fatigue strength. Even if the crack propagation resistance is increased at the center of the thickness, it is difficult to stop the crack already. Therefore, in the present invention, the range of the texture effective for improving the fatigue strength is limited to 0.5 mm in the thickness direction from the outermost surface. It is preferably up to 0.1 mm.

The effect of notch on the plate surface at an arbitrary depth from the outermost surface to 0.5 mm in the sheet thickness direction on the notch fatigue strength
The average of the X-ray random intensity ratios of the 00｝ <011> to {223} <110> orientation groups and {554} <225
>, {111} <112> and {111} <110>
The effect of the average value of the X-ray random intensity ratio in three directions was investigated. The test material for that was prepared as follows. That is, 0.03% C-0.3% Si-0.5% Mn-
0.001% S-0.11% Ti-0.03% Nb-
The cast slab which had been adjusted to 0.001% N and melted was finished after hot finish rolling so that the plate thickness became 3.5 mm at any temperature higher than the Ar3 transformation temperature. .

X-rays of the {100} <011> to {223} <110> orientation group of the plate surface at an arbitrary depth from the outermost surface of the obtained steel plate to 0.5 mm in the plate thickness direction. Average value of random intensity ratio and {554} <22
5>, {111} <112> and {111} <110
30 mm from the 1/4 W or 3/4 W position of the plate width to obtain the average value of the X-ray random intensity ratio in the three directions
The sample cut into φ was ground to a depth of about 0.05 mm from the outermost layer, and then subjected to three-step finish grinding, and then the strain was removed by chemical polishing or electrolytic polishing.

Note that the crystal orientation represented by {hkl} <uvw> indicates that the normal direction of the sheet surface is parallel to <hkl> and the rolling direction is parallel to <uvw>. X
The measurement of the crystal orientation by X-rays is described in, for example, “New Edition of Curity X-ray Diffraction” (issued in 1986, Agne Corporation)
296 pages. Where {100} <
The average value of the X-ray random intensity ratios of the group of orientations 011> to {223} <110> means the main orientations included in this orientation group,
{100} <011>, {116} <110>, {11
4 {<110>, {113} <110>, {112} <
110>, {335} <110> and {223} <1
10>, the three-dimensional texture calculated by the vector method based on the {110} pole figure, or {11}
It was obtained from a three-dimensional texture calculated by a series expansion method using a plurality of pole figures (preferably three or more) among the pole maps of {0}, {100}, {211}, and {310}.

For example, in the latter method, the X-ray random intensity ratio of each of the above crystal orientations is represented by φ2 = 4 in the three-dimensional texture.
(001) [1-10], (116) in 5 ° section
[1-10], (114) [1-10], (113)
[1-10], (112) [1-10], (335)
[1-10], (223) The intensity of [1-10] may be used as it is. However, {100} <011>-$ 22
The average value of the X-ray random intensity ratio of the 3｝ <110> azimuth group is an arithmetic mean of each azimuth described above.

When it is not possible to obtain the intensity in all the above directions, {100} <011>, {116} <11
0>, {114} <110>, {112} <110>,
The arithmetic mean of each direction of {223} <110> may be substituted. Next, {554} <225>, {111} <112
> And {111} <110> may be obtained from the three-dimensional texture calculated in the same manner as the above method.

Next, in order to investigate the notch fatigue strength of the steel sheet, the shape shown in FIG. 1 (b) was set so that the rolling direction became the longer side from the 1 / 4W or 3 / 4W position of the sheet width. Fatigue test pieces were collected and subjected to a fatigue test. Here, FIG.
While the fatigue test piece described in (a) is a smooth test piece for obtaining the fatigue strength of a general material, the fatigue test piece described in FIG. 1 (b) is manufactured to obtain notch fatigue strength. This is a cut-out test piece. However, the fatigue test piece was subjected to three-side finish grinding to a depth of about 0.05 mm from the outermost layer. The fatigue test uses an electro-hydraulic servo type fatigue test machine,
The test method is JISZ 2273-1978 and JIS
Z 2275-1978.

The effect of {100} <0 on notch fatigue strength
11> to {223} <110> average value of the X-ray random intensity ratio of the orientation group, and {554} <225>, {11}
FIG. 2 shows the results of investigating the influence of the average value of the X-ray random intensity ratio in three directions of 1｝ <112> and {111} <110>. Here, the numerals in the ○ a diagram 1 (b) to show the shape of the notch fatigue test piece obtained from fatigue tests conducted with fatigue limit (time strength at 10 ⁷ times), following notch fatigue Strength.

{100} <011> to {223} <11
0> the average value of the X-ray random intensity ratio of the orientation group and $ 5
54 {<225>, {111} <112> and {11
There is a strong correlation between the average value of the X-ray random intensity ratios in three directions of 1｝ <110> and the notch fatigue strength. When the average value is 2 or more and 4 or less, the notch fatigue strength is significantly improved. It was shown to be.

The present inventors have examined these experimental results in detail, and as a result, in order to improve the notch fatigue strength, the plate surface at an arbitrary depth from the outermost surface to 0.5 mm in the plate thickness direction was examined. {100} <011> to {223} <11
0> The average value of the X-ray random intensity ratio of the group of orientations is 2 or more, and the average value of the X-ray random intensity ratios of the three orientations {554} <225>, {111} <112>, and {111} <110> It is newly found that it is very important that the value is 4 or less.

However, in order to improve not only the notch but also the smooth fatigue crack initiation resistance, the thickness of the sheet surface at an arbitrary depth from the outermost surface to the sheet thickness direction to 0.5 mm is increased.
The average value of the X-ray random intensity ratios of the 0 <011> to {223} <110> orientation groups is 4 or more, and {554} <22
5>, {111} <112> and {111} <110
It is desirable that the average value of the X-ray random intensity ratio in the three directions of> is 2.5 or less.

Although this mechanism is not always clear, it is presumed as follows. Generally, the fatigue limit in the presence of a sharp notch is determined by the crack growth limit, that is, the magnitude of the crack growth resistance for stopping the crack. Fatigue crack growth is a repetition of small-scale plastic deformation at the notch bottom or stress concentration point.However, when the crack length is relatively short and the plastic deformation occurs within the size range of crystal grains, It is presumed that the influence of the crystallographic slip plane and slip direction is large. Therefore, if the ratio of the slip surface having a high crack growth resistance to the crack growth direction and the crack surface and the proportion of the crystal having the slip direction are large, the growth of the fatigue crack is suppressed.

Next, the reason for limiting the thickness of the steel sheet in the present invention will be described. If the plate thickness is less than 0.5 mm, the small-scale yield condition cannot be satisfied regardless of the degree of stress concentration, and there is a risk of causing monotonic ductile fracture. In addition, since sufficient plastic restraint is required from the viewpoint of crack arrest, it is desirable that the thickness be at least 1.2 mm or more in order to maintain a plane strain state. On the other hand, if the plate thickness exceeds 12 mm, the fatigue strength is significantly reduced due to the plate thickness effect (size effect). When the sheet thickness is more than 8 mm, in order to achieve hot or cold rolling conditions for obtaining a texture effective for improving notch fatigue strength,
Since there is a possibility that excessive load may be applied to the equipment, 8
mm or less is desirable. Therefore, in the present invention, the plate thickness is limited to 0.5 mm or more and 12 mm or less. Desirably, it is 1.2 mm or more and 8 mm or less.

Next, the microstructure of the steel sheet according to the present invention will be described. The microstructure of the steel sheet is preferably a ferrite single phase in order to ensure excellent burring workability (hole expanding property). However, it is allowed to partially contain bainite as necessary. In order to ensure good burring workability, the volume fraction of bainite should be 10%.
The following is desirable. However, inevitable martensite,
It is allowed to contain residual austenite and pearlite. Furthermore, when the crystal structure such as retained austenite includes a material that is not bcc, the X-ray random intensity ratio converted by the volume fraction of other structures may be within the range of the present invention, and the effect of the present invention is not affected. can get.
The ferrite referred to here includes bainitic ferrite and ashular ferrite structures.

In order to ensure good fatigue characteristics, the volume fraction of pearlite containing coarse carbides is desirably 5% or less. Good burring (hole-expanding)
In order to ensure the following, the combined volume fraction of retained austenite and martensite is preferably less than 5%. Where ferrite, bainite, retained austenite,
What is the volume fraction of pearlite and martensite? A sample cut from a 1 / 4W or 3 / 4W position of the steel sheet width is polished into a section in the rolling direction, etched using a nital reagent, and 200 to 500 times using an optical microscope. Is defined as the area fraction of the microstructure observed at a magnification of 1 / 4t of the plate thickness.

Next, the reasons for limiting the chemical components of the present invention will be described. If the content of C exceeds 0.1%, the workability and the weldability deteriorate, so the content of C is set to 0.1% or less. If the content is less than 0.01%, the strength is reduced.
% Or more.

If S is too large, it causes cracks during hot rolling. Therefore, S should be reduced as much as possible.
Below is an acceptable range.

N forms a precipitate with Ti and Nb at a higher temperature than C, and Ti and N are effective for fixing C.
b is reduced. Therefore, it should be reduced as much as possible,
If it is 0.005% or less, it is within an acceptable range.

[0037] Ti is one of the most important elements in the present invention. That is, Ti contributes to an increase in the strength of the steel sheet by precipitation strengthening. However, if the content is less than 0.05%, this effect is insufficient, and if the content exceeds 0.5%, the effect is not only saturated but also causes an increase in alloy cost. Therefore Ti
Is 0.05% or more and 0.5% or less.

Furthermore, in order to precipitate and fix C, which causes carbide such as cementite, which deteriorates the burring workability, and to contribute to the improvement of the burring workability, Ti-48 is used.
It is necessary to satisfy the condition of / 12C-48 / 14N-48 / 32S ≧ 0%. Here, S and N form precipitates with Ti in a relatively high temperature range than C, so that Ti ≧
To secure 48 / 12C, it is inevitable to use Ti-48
It is necessary to satisfy the condition of / 12C-48 / 14N-48 / 32S ≧ 0%.

In addition to the above components, the following components can be contained as required. Nb contributes to an increase in the strength of the steel sheet by precipitation strengthening like Ti. Further, there is also an effect of improving the burring processability by making the crystal grains fine. However, if less than 0.01%, this effect is insufficient,
If the content exceeds 0.5%, not only the effect is saturated, but also the alloy cost is increased. Therefore, the content of Nb is 0.01
% Or more and 0.5% or less.

Further, in order to precipitate and fix C, which causes carbide such as cementite, which deteriorates burring workability, and to contribute to improvement of burring workability, Ti + 48 is required.
/ 93Nb-48 / 12C-48 / 14N-48 / 32
It is necessary to satisfy the condition of S ≧ 0%. Where Nb
Forms carbides at relatively lower temperatures than Ti, so i
To secure + 48 / 93Nb ≧ 48 / 12C,
Inevitably Ti + 48 / 93Nb-48 / 12C-48 /
It is necessary to satisfy the condition of 14N-48 / 32S ≧ 0%.

Si is effective for increasing the strength as a solid solution strengthening element. In order to obtain a desired strength, the content needs to be 0.01% or more. However, if the content exceeds 2%, the workability deteriorates. Therefore, the content of Si is 0.01% or more,
% Or less.

Mn is effective in increasing the strength as a solid solution strengthening element. To obtain the desired strength, 0.05% or more is required. When an element such as Ti that suppresses the occurrence of hot cracking due to S is not sufficiently added in addition to Mn, it is desirable to add an Mn amount that satisfies Mn / S ≧ 20 by mass%. On the other hand, if added over 3%, slab cracks occur, so the content is set to 3% or less.

P is an impurity and is preferably as low as possible.
If the content exceeds 0.1%, the workability and the weldability are adversely affected and the fatigue characteristics are also reduced.

Al needs to be added in an amount of 0.005% or more for deoxidizing molten steel. However, the cost is increased, so the upper limit is set to 1%. Further, if added in an excessively large amount, nonmetallic inclusions increase and elongation deteriorates. Therefore, the content is desirably 0.5% or less.

B has an effect of increasing the fatigue limit by suppressing grain boundary embrittlement due to P, which is considered to be caused by a decrease in the amount of solute C, and is added as necessary. However, if it is less than 0.0002%, it is insufficient to obtain the effect, and if it exceeds 0.002%, slab cracking occurs. Therefore, the addition of B is 0.0002% or more and 0.002% or more.
% Or less.

Since Cu has the effect of improving fatigue characteristics in a solid solution state, it is added as necessary. However, if the content is less than 0.2%, the effect is small, and if the content exceeds 1.2%, it may precipitate during winding and significantly deteriorate workability. Therefore, the content of Cu is set in the range of 0.2 to 1.2%.

Ni is added as necessary to prevent hot brittleness due to the inclusion of Cu. However, if the content is less than 0.1%, the effect is small, and if the content exceeds 0.6%, the effect is saturated. Therefore, the content is set to 0.1 to 0.6%.

Ca and REM are elements that become the starting point of destruction or change the form of nonmetallic inclusions that degrade workability and render them harmless. However, even if added less than 0.0005%, there is no effect, and if Ca is added, 0.002%
If the content of REM is more than 0.02%, the effect is saturated even if it is added more than 0.02%.
M: It is desirable to add 0.0005 to 0.02%.

Further, in order to impart strength, one or more of Mo, V, Cr and Zr precipitation strengthening or solid solution strengthening elements may be added as necessary. However,
0.05%, 0.02%, 0.01%, 0.0
If it is less than 2%, the effect cannot be obtained. The effect is saturated even if it exceeds 1%, 0.2%, 1% and 0.2%, respectively. It should be noted that steel containing these as main components may contain Sn, Co, Zn, W, and Mg in a total amount of 1% or less. However, since Sn may cause flaws during hot rolling, 0.05% or less is desirable.

Next, the reasons for limiting the production method of the present invention will be described in detail below. The present invention, after casting, heat treatment after cooling and pickling after cold rolling or hot rolling after hot rolling or hot rolling, or while hot-rolled steel sheet or cold-rolled steel sheet is subjected to heat treatment in a hot-dip plating line, Can also be obtained by subjecting these steel sheets to a separate surface treatment.

In the present invention, the production method prior to hot rolling is not particularly limited. In other words, following smelting in a blast furnace or electric furnace, the components are adjusted in the various secondary refining processes so that the desired component content is obtained, and then normal continuous casting, casting by ingot method, thin slab casting, etc. The casting method may be used. Scrap may be used as a raw material. In the case of a slab obtained by continuous casting, the slab may be directly sent to a hot rolling mill as it is, or may be cooled to room temperature and then re-heated in a heating furnace and then hot-rolled.

The reheating temperature is not particularly limited,
If the temperature is 1400 ° C. or more, the scale-off amount becomes large and the yield decreases, so the reheating temperature is desirably less than 1400 ° C. Further, since the heating at a temperature lower than 1000 ° C. significantly impairs the operation efficiency according to the schedule, the reheating temperature is 1000
C or higher is desirable. Further, heating at a temperature lower than 1100 ° C. not only causes the precipitates containing Ti and / or Nb to be re-dissolved in the slab but coarsens and loses the precipitation strengthening ability, but also has the desired size and distribution of Ti and And / or the precipitate containing Nb is not deposited,
The reheating temperature is desirably 1100 ° C. or higher.

In the hot rolling step, finish rolling is performed after rough rolling is completed. When descaling is performed after rough rolling is completed, collision pressure P (MPa) of high-pressure water on the steel sheet surface × flow rate L (liter / liter) cm ² ) ≧ 0.0025. The collision pressure P of the high-pressure water on the steel plate surface is described as follows ("Iron and Steel", 1991, vol. 7).
7, no. 9, p. 1450). P (MPa) = 5.64 × P O × V / H 2 However, P _O (MPa): liquid pressure V (liters / min): nozzle flow liquid quantity H (cm): the steel sheet surface and the distance between the nozzle

The flow rate L is described as follows. L (liter / cm ² ) = V / (W × v), where V (liter / min): Nozzle flow amount W (cm): Width of jet liquid per nozzle hitting steel sheet surface v (cm / min): Passing speed The upper limit of the collision pressure P × the flow rate L does not need to be particularly determined in order to obtain the effects of the present invention. However, increasing the flow rate of the nozzle causes inconveniences such as intense wear of the nozzle.
It is desirable to set it to 0.02 or less.

Further, the maximum height R of the steel sheet after the finish rolling is performed.
y is 15 μm (15 μm Ry, 12.5 mm, ln1
2.5 mm) or less. This is because the fatigue strength of a hot-rolled or pickled steel sheet has a correlation with the maximum height Ry of the steel sheet surface, as described in, for example, “Handbook for Designing Fatigue of Metallic Materials” edited by The Society of Materials Science, Japan, page 84. It is clear from Further, the subsequent finish rolling is desirably performed within 5 seconds in order to prevent scale from being formed again after descaling.

Further, after rough rolling or subsequent descaling, the sheet bars may be joined and continuously subjected to finish rolling. At that time, the coarse bar may be temporarily wound in a coil shape, stored in a cover having a heat retaining function, if necessary, and re-wound before joining.

In the finish rolling, when a final product is formed as a hot-rolled steel sheet, it is necessary to perform rolling in the latter half of the finish rolling at a total reduction rate of 25% or more in a temperature range of Ar3 transformation point temperature + 100 ° C. or less. Here, the Ar3 transformation point temperature is simply shown in relation to the steel composition by the following calculation formula, for example. That is, Ar3 = 910-310 *% C + 25 *% Si-80 *
% Mn

If the total rolling reduction in the temperature range of the Ar3 transformation point temperature + 100 ° C. or less is less than 25%, the texture of the rolled austenite is not sufficiently developed. However, the effect of the present invention cannot be obtained. To obtain a sharper texture, Ar3
The total rolling reduction in the temperature range below the transformation point temperature + 100 ° C is 3
It is desirable to set it to 5% or more.

The lower limit of the temperature range in which rolling at a total draft of 25% or more is not particularly limited. However, if the temperature is lower than the Ar3 transformation point temperature, a work structure remains in ferrite precipitated during rolling and ductility decreases. And the workability deteriorates.
The lower limit of the temperature range in which rolling at a total draft of 25% or more is Ar
3 Transformation point temperature or higher is desirable. In the present invention, the upper limit of the total reduction in the temperature range of not more than the Ar3 transformation point temperature + 100 ° C. is not particularly limited. However, when the total reduction exceeds 97.5%, the rolling load increases and the rigidity of the rolling mill becomes excessive. In order to bring about an economic disadvantage, the content is desirably 97.5% or less.

Here, when the friction between the hot rolling roll and the steel sheet during hot rolling in the temperature range of the Ar3 transformation point temperature + 100 ° C. or less is large, the sheet surface near the steel sheet surface has a thickness of ｛1.
Since the crystal orientation mainly consisting of the 10 ° plane develops and the notch fatigue strength deteriorates, lubrication is applied as necessary to reduce the friction between the hot rolling roll and the steel sheet.

In the present invention, the upper limit of the friction coefficient between the hot-rolled roll and the steel sheet is not particularly limited, but if it exceeds 0.2, the development of the crystal orientation mainly with the {110} plane becomes remarkable and the notch fatigue strength Deteriorates, so that the Ar3 transformation point temperature +1
For at least one pass during hot rolling in a temperature range of 00 ° C. or less, it is desirable that the friction coefficient between the hot rolling roll and the steel sheet be 0.2 or less. More preferably,
The friction coefficient between the hot rolling roll and the steel sheet is set to 0.15 or less for all passes during hot rolling in the temperature range of the Ar3 transformation point temperature + 100 ° C or less. Here, the coefficient of friction between the hot rolling roll and the steel sheet is a value obtained by calculation based on a rolling theory from values such as an advanced ratio, a rolling load, and a rolling torque.

The final pass temperature (FT) of the finish rolling is not particularly limited.
T) is desirably terminated at the Ar3 transformation point temperature or higher. This is because if the rolling temperature during the hot rolling is lower than the Ar3 transformation point temperature, the processed structure remains in the ferrite precipitated before or during the rolling and the ductility is reduced, thereby deteriorating the workability. is there. On the other hand, there is no particular upper limit on the finishing temperature, but the Ar3 transformation point temperature +10
If the temperature exceeds 0 ° C., it is practically impossible to perform rolling at a total draft of 25% or more in the temperature range of the Ar 3 transformation point temperature + 100 ° C. or less. Therefore, the upper limit of the finishing temperature is desirably the Ar 3 transformation point temperature + 100 ° C. or less. .

In the present invention, there is no particular limitation on the process after finishing the finish rolling until winding at a predetermined winding temperature (CT). However, when aiming at compatibility with ductility without significantly deteriorating the burring property. Is from the Ar3 transformation point to Ar1
It may be retained for 1 to 20 seconds in a temperature range up to the transformation point (two-phase range of ferrite and austenite). The retention is performed in order to promote ferrite transformation in the two-phase region. However, if it is less than 1 second, sufficient ductility cannot be obtained due to insufficient ferrite transformation in the two-phase region.
And / or the size of the precipitate containing Nb is coarsened,
There is a possibility that it does not contribute to the increase in strength due to precipitation strengthening.

The temperature range in which the stagnation is carried out for 1 to 20 seconds is set to Ar1 to facilitate the ferrite transformation.
The temperature is preferably from the transformation point to 860 ° C. In addition, 1-20
The residence time per second is desirably 1 to 10 seconds in order not to significantly reduce the productivity. 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.

The upper limit of the cooling rate is not particularly defined, but 300 ° C./s or less is a reasonable cooling rate in view of the capacity of the cooling equipment.
Further, if the cooling rate is too high, the cooling end temperature cannot be controlled, and overshooting may result in overcooling to the Ar1 transformation point or lower, and the effect of improving ductility is lost. The speed is desirably 150 ° C./s or less.

Next, from the temperature range, a predetermined winding temperature (C
It cools down to T), but the cooling rate does not need to be particularly determined in order to obtain the effect of the present invention. However, if the cooling rate is too low, the size of the precipitate containing Ti and / or Nb may become coarse and may not contribute to the increase in strength due to precipitation strengthening. Therefore, the lower limit of the cooling rate is 20 ° C./s.
The above is desirable. In addition, the effect of the present invention can be obtained without particularly setting the upper limit of the cooling rate up to the winding temperature.
C./s or less is desirable.

In the present invention, the winding temperature (CT) is not particularly defined, but the upper limit is the Ar3 transformation point temperature + 10.
In order to inherit the texture of austenite obtained by rolling at a total reduction ratio of 25% or more in a temperature range of 0 ° C. or less, it is desirable to wind at a winding temperature T _O or less shown below. The T _O is the temperature at which ferrite austenite and austenite same components are thermodynamically defined as the temperature with the same free energy, taking into consideration the influence of components other than C, simplified by using the following formula Can be calculated. T _O = −650.4 ×% C + B Here, B is determined as follows. B = −50.6 × Mneq + 894.3 Here, Mneq is the mass% of the contained element shown below.
Determined by Mneq =% Mn + 0.24 ×% Ni + 0.13 ×% S
i + 0.38 ×% Mo + 0.55 ×% Cr + 0.16 ×
% Cu-0.50x% Al-0.45x% Co + 0.9
0 ×% V Note that the influence of the mass% of the components other than those specified in the present invention on T _O is not so large and can be ignored here.

On the other hand, the lower limit of the winding temperature (CT) is 350
If the temperature is lower than 0 ° C, precipitates containing sufficient Ti and / or Nb are not generated, and there is a possibility that solid solution C may remain in the steel to lower the workability. Further, the cooling rate after winding is not particularly limited, but when Cu is added in an amount of 1% or more, if the winding temperature (CT) is higher than 450 ° C., Cu precipitates after winding to deteriorate workability. Not only that, there is a possibility that Cu in the solid solution state effective for improving the fatigue properties may be lost, so the winding temperature (CT)
Is higher than 450 ° C., the cooling rate after winding is preferably 30 ° C./s or more up to 200 ° C. After the hot rolling step, pickling may be performed, if necessary, followed by in-line or off-line skin pass with a rolling reduction of 10% or less or cold rolling to a rolling reduction of about 40%.

Next, there is a case where a final product is formed as a cold-rolled steel sheet, but the hot finish rolling conditions are not particularly limited.
However, in order to obtain better notch fatigue strength,
It is desirable that the total rolling reduction in the temperature range of the Ar3 transformation point temperature + 100 ° C or less is 25% or more. Although the final pass temperature (FT) of the finish rolling may be completed below the Ar3 transformation point temperature, in this case, a strong work structure remains in the ferrite precipitated before or during the rolling, so It is desirable to recover and recrystallize by removing or heating.

The total rolling reduction of the cold rolling after the subsequent pickling was 80.
%. This is because the total reduction of cold rolling is 80%
With the above, the X-ray diffraction integral plane intensity ratio of the {111} plane and the {554} plane of the crystal plane parallel to the sheet plane, which is a general cold-rolled recrystallization texture, is increased. Further, it is desirably 70% or less. The effect of the present invention can be obtained without any particular lower limit of the cold rolling reduction, but is preferably 3% or more in order to control the strength of the crystal orientation in an appropriate range.

The heat treatment of the cold-rolled steel sheet is based on a continuous annealing process. First, the recovery temperature or more Ac
3 Perform for 5 to 150 seconds in the temperature range of transformation point + 100 ° C or lower. Here, the Ac3 transformation point temperature is, for example, "Leslie Iron and Steel Science" (issued in 1985, Maruzen Co., Ltd.) 273.
It is shown in relation to steel components by the calculation formula described on page.

If the heat treatment temperature (ST) is lower than the recovery temperature, the processed structure remains and significantly deteriorates the ductility. Therefore, the heat treatment temperature (ST) is higher than the recovery temperature. In order to obtain better ductility, the temperature is desirably higher than the recrystallization temperature.
Further, the heat treatment temperature (ST) is changed to Ac3 transformation point temperature +100.
If the temperature exceeds ℃, the ferrite generated by recrystallization is transformed into austenite, the texture by austenite grain growth is randomized, and the finally obtained texture is also randomized. Ac3 transformation point temperature + 100 ° C or lower. On the other hand, if the holding time (Time) in this temperature range is less than 5 seconds, the carbonitride of Ti and Nb is insufficient to completely re-dissolve solid solution. On the other hand, even if the heat treatment is performed for more than 150 seconds, The retention time (Time) is set to 5 to 150 seconds because the effect is not only saturated but also lowers productivity.

[0073] Although subsequent no particular limitation on the cooling conditions, at 20 ° C. / s or more cooling rate 350 ° C. ultra wherein T _O
It is desirable to cool to a temperature range below the temperature. This is because if the cooling rate is less than 20 ° C./s, the size of the precipitate containing Ti and / or Nb may become coarse and may not contribute to the increase in strength due to precipitation strengthening.

When the cooling end temperature is lower than 350 ° C., precipitates containing sufficient Ti and / or Nb are not generated, and there is a possibility that solid solution C remains in the steel to lower the workability. Ultra is desirable. If the ending temperature of the cooling step exceeds 200 ° C., the aging property may be deteriorated. In addition, the lower limit is preferably 50 ° C. or higher, since when the coil is in a state of being wet for a long time in the case of cooling with water or mist, the appearance of the coil is likely to be poor due to rust. Thereafter, skin pass rolling may be performed as necessary. In order to apply galvanization to the hot-rolled steel sheet after pickling or to the cold-rolled steel sheet after the above-mentioned heat treatment step, the steel sheet may be immersed in a galvanizing bath and subjected to an alloying treatment as necessary.

[0075]

The present invention will be further described below with reference to examples. The steels of A to L having the chemical components shown in Table 1 were melted in a converter, continuously cast, reheated at the heating temperature shown in Table 2, and subjected to finish rolling following rough rolling to 1.2 to 1.2. After the thickness was 5.5 mm, the film was wound. However, the indication of the chemical composition in the table is% by mass. In addition, as shown in Table 2, some lubricated rolling was performed. After rough rolling, steel L was subjected to descaling under the conditions of a collision pressure of 2.7 MPa and a flow rate of 0.001 liter / cm ² . Further, as shown in Table 2, for a part after the hot rolling step,
Pickling, cold rolling and heat treatment were performed. The board thickness is 0.7 to 2.3m
m. On the other hand, among the above steel plates, steel G and steel A-6 were galvanized.

Table 2 shows details of the manufacturing conditions. here,
"SRT" is the slab heating temperature, "FT" is the final pass finish rolling temperature, and "rolling ratio" is the Ar3 transformation point temperature +100.
Shows the total rolling reduction in the temperature range below ° C. However, when rolling is performed later in the cold rolling process, such a limitation is not applied, so "-" is used. "Lubrication" indicates the presence or absence of lubrication in the temperature range of the Ar3 transformation point temperature + 100 ° C or lower. Further, “CT” indicates a winding temperature. However, in the case of a cold-rolled steel sheet, it is not necessary to particularly limit the manufacturing conditions, and thus “−” is used. Next, “cold rolling reduction” is a total cold rolling reduction, “ST” is a heat treatment temperature, and “Time” is a heat treatment time.

The tensile test of the hot-rolled sheet obtained as described above was performed by first processing the test material into a No. 5 test piece described in JIS Z2201, and following the test method described in JIS Z2241. Table 2 shows the yield strength (σ _Y ) and tensile strength (σ
_B ) and breaking elongation (El). On the other hand, regarding the burring workability (hole expanding property), Japan Iron and Steel Federation Standard JFST
The evaluation was performed according to the hole expansion test method described in 1001-1996. Table 2 shows the hole expansion ratio (λ). Here, the volume fractions of ferrite, bainite, retained austenite, pearlite, and martensite are as follows. A sample cut from a 1 / 4W or 3 / 4W position of the steel sheet width is polished and etched into a cross section in the rolling direction, and an optical microscope is used. It is defined as the area fraction of the microstructure at 1 / 4t of the plate thickness observed at a magnification of 200 to 500 times.

Further, 1/4 W or 3/4 W of the plate width
0.1 mm from the outermost layer of the specimen cut to 30 mmφ from the position.
Grinding of the three hills finish to a depth of about 05mm, then removing the strain by chemical polishing or electrolytic polishing, and producing it, "The New Version of Carity X-ray Diffraction" (published in 1986,
X-ray diffraction intensity was measured according to the method described on pages 274 to 296 of Agne Corporation.

Here, {100} <011> to {223}
The average value of the X-ray random intensity ratio of the <110> orientation group is
Main orientation included in this orientation group, {100} <011
>, {116} <110>, {114} <110>,
{113} <110>, {112} <110>, $ 33
5D <110> and {223} <110> are three-dimensional textures calculated by vector method based on {110} pole figure, or {110}, {100},
It was obtained from a three-dimensional texture calculated by a series expansion method using a plurality (preferably three or more) of the {211} and {310} pole figures.

For example, in the latter method, the X-ray random intensity ratio of each of the above crystal orientations is expressed as φ2 = 4 in the three-dimensional texture.
(001) [1-10], (116) in 5 ° section
[1-10], (114) [1-10], (113)
[1-10], (112) [1-10], (335)
[1-10], (223) The intensity of [1-10] may be used as it is. However, {100} <011>-$ 22
The average value of the X-ray random intensity ratio of the 3｝ <110> azimuth group is an arithmetic mean of each azimuth described above. If the intensity cannot be obtained in all the above directions, {100} <01
1>, {116} <110>, {114} <110>,
The arithmetic mean of each orientation of {112} <110> and {223} <110> may be used instead. Next, {554} <225
>, {111} <112> and {111} <110>
The average value of the X-ray random intensity ratios in the three directions may be obtained from the three-dimensional texture calculated in the same manner as in the above method.

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

Next, in order to investigate the notch fatigue strength of the steel sheet, the shape shown in FIG. 1 (b) was set so that the rolling direction became the longer side from the 1 / 4W or 3 / 4W position of the sheet width. Fatigue test pieces were collected and subjected to a fatigue test. However, the fatigue test piece was subjected to three-side finish grinding to a depth of about 0.05 mm from the outermost layer. The fatigue test uses an electrohydraulic servo-type fatigue tester, and the test method is JIS Z 2273 (197).
8) and JIS Z2275 (1978).
Table 2 shows the notch fatigue limit (σ _Wk ) and the notch fatigue limit ratio (σ _Wk / σ _B ).

According to the present invention, steels A-1, A-3,
A-4, A-6, A-8, C, E, G, H, I, J, L
, Containing a predetermined amount of a steel component, and {100} <011> to {223} <110> orientation of the plate surface at an arbitrary depth from the outermost surface to 0.5 mm in the plate thickness direction. The average value of the X-ray random intensity ratio of the group is 2 or more, and $ 55
4 {225}, {111} <112> and {11
An automobile having excellent notch fatigue strength and burring workability, characterized in that the average value of the X-ray random intensity ratio in three directions of 1｝ <110> is 4 or less and the plate thickness is 0.5 mm or more and 12 mm or less. Steel sheet is obtained,
A significant difference is observed for the fatigue limit ratio of the conventional steel evaluated by the method described in the present invention of 20 to 30%.

Steels other than those described above were used for the following reasons.
Out of the range of light. That is, steel A-2 is finished at the finish rolling.
Temperature (FT) and Ar3 transformation point temperature + 100 ° C or less
The total rolling reduction in the temperature range is outside the range of claim 10 of the present application.
Therefore, the target texture described in claim 1 can be obtained.
Not enough, notch fatigue strength (σ_Wk/ Σ_B) Got
Not in. Steel A-5 has a heat treatment temperature (ST) of the present invention.
13. The target collection according to claim 1, which is out of the range of 13.
No joint structure was obtained and sufficient notch fatigue strength (σ_Wk/ Σ
_B) Is not obtained. Steel A-7 has a cold rolling reduction claimed in the present application.
Since it is out of the range of item 13, it is the object of claim 1.
No texture was obtained and sufficient notch fatigue strength (σ_Wk/
σ_B) Is not obtained.

Steel B does not have sufficient strength (σ _B ) because the content of C is outside the scope of claim 1 of the present application.
Since the steel D has a Ti content outside the scope of claim 1 of the present application, sufficient strength (σ _B ) and fatigue limit ratio (σ _Wk /
σ _B ) has not been obtained. In steel F, the content of C is out of the range defined in claim 1 of the present application, so that a sufficient hole expansion ratio (λ) is not obtained. Steel I has a sufficient hole expansion ratio (λ) and elongation (E) since the content of S is outside the scope of claim 1 of the present application.
l) is not obtained. Since the steel K has an N content outside the scope of claim 1 of the present application, a sufficient hole expansion ratio (λ) and elongation (El) are not obtained.

[0086]

[Table 1]

[0087]

[Table 2]

[0088]

As described in detail above, the present invention relates to a thin steel sheet for automobiles having excellent notch fatigue strength and burring workability, and a method for producing the same. Notch fatigue strength, which is one of the important properties of parts requiring durability, such as automobile undercarriage parts, where the growth of fatigue cracks from stress concentrated parts such as welds and welds becomes a problem. This is an invention having high industrial value because a great improvement can be expected.

[Brief description of the drawings]

FIG. 1A is a diagram illustrating the shape of a smooth fatigue test piece, and FIG. 1B is a diagram illustrating the shape of a notched fatigue test piece.

FIG. 2 shows the results of preliminary experiments leading to the present invention, {100} <
011>-{223} <110> average value of the X-ray random intensity ratio of the orientation group, and {554} <225>, {11}
1} <112> and {111} <110> mean values of X-ray random intensity ratios in three directions and notch fatigue strength (10
It is a figure which shows in the relationship of time intensity | strength in ^seven times: fatigue limit).

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C22C 38/14 C22C 38/14 38/58 38/58 C23C 2/06 C23C 2/06 (72) Inventor Naoki Yoshinaga 20-1 Shintomi, Futtsu-shi Nippon Steel Corporation Technology Development Division 1 Oita Nishinosu, Nippon Steel Corporation Oita Works F-term (reference) 4K027 AA05 AA23 AB02 AB42 AC73 4K037 EA01 EA02 EA05 EA09 EA11 EA13 EA15 EA16 EA17 EA18 EA19 EA20 EA23 EA27 EA25 EA25 EB08 EB09 FA02 FA03 FB00 FB01 FB03 FC04 FC07 FE01 FE02 FE03 FG00 FH01 FJ05 JA06

Claims (16)

[Claims] (1) In terms of mass%, C: 0.01 to 0.1%, S ≦ 0.03%, N ≦ 0.005%, Ti: 0.05 to 0.5%, The relationship between the mass% of C, N, S and Ti is Ti-48 / 12C-48 / 14N-48 / 32S ≧ 0
%, And the balance is made of Fe and unavoidable impurities.
{100} <01 of the plate surface at an arbitrary depth up to 5 mm
The average value of the X-ray random intensity ratio of the 1> to {223} <110> orientation groups is 2 or more, and {554} <225>, {1}
Notch fatigue strength, wherein an average value of X-ray random intensity ratios in three directions of 11｝ <112> and {111｝ <110> is 4 or less, and a plate thickness is 0.5 mm or more and 12 mm or less. Automotive thin steel sheet with excellent burring workability. 2. The steel according to claim 1, further comprising, in mass%, Nb: 0.01 to 0.5%, and the relationship of C, N, S, Ti and Nb in mass% is further defined. , Ti + 48 / 93Nb-48 / 12C-48 / 14N-
A thin steel sheet for automobiles having excellent notch fatigue strength and burring workability, characterized by containing Ti and Nb in a range satisfying 48 / 32S ≧ 0%. 3. The steel according to claim 1, further comprising, in mass%, Si: 0.01 to 2%, Mn: 0.05 to 3%, P ≦ 0.1%, Al: 0. A thin steel sheet for automobiles having excellent notch fatigue strength and burring workability, comprising 0.005 to 1%. 4. The steel according to claim 1, further comprising: B: 0.0002 to 0.1% by mass%.
A thin steel sheet for automobiles having excellent notch fatigue strength and burring workability, characterized by containing 002%. 5. The notch fatigue strength, wherein the steel according to claim 1 further contains, in mass%, Cu: 0.2 to 1.2%. Steel sheet for automobiles with excellent burring workability. 6. The notch fatigue strength, wherein the steel according to claim 1 further contains, by mass%, Ni: 0.1 to 0.6%. Steel sheet for automobiles with excellent burring workability. 7. The steel according to claim 1, further comprising, by mass%, Ca: 0.0005 to 0.002%, and REM: 0.0005 to 0.02%. Or a thin steel sheet for automobiles having excellent notch fatigue strength and burring workability, characterized by containing two types. 8. The steel according to claim 1, further comprising: Mo: 0.05 to 1%, V: 0.02 to 0.2%, Cr: 0 in mass%. A thin steel sheet for automobiles having excellent notch fatigue strength and burring workability, characterized by containing one or more of 0.11% to 1% and Zr: 0.02% to 0.2%. 9. An automotive thin steel sheet having excellent notch fatigue strength and burring workability, characterized in that the automotive thin steel sheet according to any one of claims 1 to 8 is galvanized. . 10. A hot rolling process for obtaining a thin steel sheet having the component according to any one of claims 1 to 8, wherein after the slab having the component is roughly rolled, the Ar3 transformation point temperature + 100 ° C. A method for producing a thin steel sheet for automobiles having excellent notch fatigue strength and burring workability, wherein the steel sheet is subjected to finish rolling at a total reduction ratio of 25% or more of the steel sheet thickness in the following temperature range, and then cooled and wound. 11. The manufacturing of a thin steel sheet for automobiles having excellent notch fatigue strength and burring workability according to claim 10, wherein lubricating rolling is performed in finish rolling after rough rolling in the hot rolling. Method. 12. A method for producing a thin steel sheet for automobiles having excellent notch fatigue strength and burring workability, wherein descaling is performed after rough rolling in hot rolling according to claim 10 or 11. . 13. The method according to claim 1, wherein:
In order to obtain a thin steel sheet having the components described in the above item, a slab having the components is hot-rolled, followed by pickling, and cold-rolled to a steel sheet thickness reduction of less than 80%, and then at the recovery temperature or higher and the Ac3 transformation point temperature +10.
A method for producing a thin steel sheet for an automobile having excellent notch fatigue strength and burring workability, wherein the heat treatment is performed in a step of cooling at a temperature range of 0 ° C. or lower for 5 to 150 seconds and cooling. 14. The notch fatigue strength according to any one of claims 10 to 12, wherein the steel sheet surface is galvanized by dipping in a galvanizing bath after hot rolling. Of thin steel sheets for automobiles with excellent burring workability. 15. The method according to claim 13, wherein:
A method for producing a thin steel sheet for automobiles having excellent notch fatigue strength and burring workability, wherein the steel sheet is galvanized by dipping in a galvanizing bath after the heat treatment. 16. The thin steel sheet for automobiles having excellent notch fatigue strength and burring workability according to claim 14 or 15, characterized in that the steel sheet is immersed in a galvanizing bath, galvanized and then alloyed. Production method.