JP2001303175A - Ferritic thin steel sheet excellent in shape freezability and its producing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prepare a ferritic thin steel sheet excellent in shape freezability at the time of performing working essentially consisting of bending. SOLUTION: In this ferritic thin steel sheet, the average value of X-ray random intensity ratios of the 100}<011> to 223}<110> orientation groups in the sheet face is >=3.0, also, the average value of the X-ray random intensity ratios in the three crystal orientations of 554}<225>, 111}<112> and 111}<110> is <=3.5, and further, at least one of the r values in the rolling direction and the direction rectangular thereto is <=0.7.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ferritic thin steel sheet (hereinafter also referred to simply as a steel sheet or a thin steel sheet) having excellent shape freezing property mainly by bending, and is mainly used for automobile parts and the like. .

[0002]

2. Description of the Related Art High-strength steel sheets have been used to reduce the weight of automobile bodies in order to reduce the amount of carbon dioxide gas emitted from automobiles. In addition, in order to ensure the safety of passengers, high-strength steel sheets are increasingly used in automobile bodies in addition to mild steel sheets. Further, in order to further reduce the weight of automobile bodies in the future, new demands for increasing the use strength level of high-strength steel sheets more than ever have been increasing.

However, when bending deformation is applied to a high-strength steel sheet, the shape after processing tends to return from the shape of the processing jig to the shape before processing because of its high strength. The phenomenon of trying to return to the original shape even after processing is called spring back. When this spring back occurs, the desired shape of the processed part cannot be obtained.

[0004] Therefore, in the conventional automobile body, mainly high-strength steel sheets of 440 MPa or less have been used. Despite the need to reduce the body weight of automobile bodies by using high-strength steel sheets of 490 MPa or more, the fact is that there is no high-strength steel sheet with little spring back and good shape freezing properties. It is. Needless to add, it is extremely important to enhance the shape freezing property of a high-strength steel sheet or a mild steel sheet having a pressure of 440 MPa or less after processing, such as an automobile or a home electric appliance, in order to enhance the shape accuracy.

Japanese Patent Application Laid-Open No. Hei 10-72644 discloses an austenitic stainless steel with a small spring back amount, characterized in that the degree of accumulation of {200} texture in a plane parallel to the rolling plane is 1.5 or more. A rolled steel sheet is disclosed. However, there is no description about a technique for reducing the amount of springback of a ferritic steel sheet.

[0006]

When a mild steel sheet or a high strength steel sheet is subjected to bending, large spring back occurs depending on the strength of the steel sheet, and the shape-freezing property of the formed part is poor at present. is there. The present invention drastically solves this problem and provides a ferritic thin steel sheet having excellent shape freezing properties.

[0007]

According to the conventional knowledge, as a measure for suppressing the spring back, it has been considered as important for the moment to lower the yield point of the steel sheet. Then, in order to lower the yield point, a steel sheet having a low tensile strength had to be used. However, this alone improves the bendability of the steel sheet and reduces
It is not a fundamental solution to keep back amount low.

The inventors of the present invention newly focused on the influence of the texture of the steel sheet on the bending workability in order to improve the bending workability and fundamentally solve the occurrence of spring back. The effect was studied and studied in detail. Further, a steel sheet having excellent bending workability has been found.
As a result, {100} <011> to {223} <110
> Orientation group and {554} <225>, {111} <112
>, {111} <110>, and controlling at least one of the r value in the rolling direction and the r value in the direction perpendicular to the rolling direction to be as low as possible. It is clear that the workability is dramatically improved.

The present invention has been made based on the above findings, and the gist thereof is as follows. (1) In ferrite thin steel sheets, at least 1/2
{100} <011> to {223} of the plate surface at the plate thickness
The average value of the X-ray random intensity ratio of the <110> orientation group is 3.
0 or more and {554} <225>, {111} <
112>, and the average value of the X-ray random intensity ratios of the three crystal orientations of {111} <110> is 3.5 or less,
Furthermore, at least one of the r value in the rolling direction and the r value in the direction perpendicular to the rolling direction is 0.7 or less, a ferritic thin steel sheet excellent in shape freezing property.

(2) The ferritic thin steel sheet has a mass% of
And C: 0.0001% or more and 0.05% or less, Si:
0.001% or more and 2.5% or less, Mn: 0.01% or more and 2.5% or less, P: 0.15% or less, S: 0.03
% Or less, Al: 0.01% or more, 2.0% or less, N:
0.01% or less, and O: 0.01% or less,
The ferritic thin steel sheet according to the above (1), which is excellent in shape freezing property, comprising a balance of iron and unavoidable impurities.

(3) The ferritic thin steel sheet has a mass%
Wherein the composition further contains one or more of Ti: 0.20% or less, Nb: 0.20% or less, and B: 0.007% or less. Ferrite thin steel sheet with excellent shape freezing properties. (4) The ferritic thin steel sheet has a C content of 0.1% by mass.
04% or more, 0.25% or less, Si: 0.001% or more, 2.5% or less, Mn: 0.01% or more, 2.5% or less, P: 0.20% or less, S: 0. 03% or less, Al:
0.01% or more, 2.0% or less, N: 0.01% or less,
And O: 0.01% or less, the balance being iron and unavoidable impurities, the ferritic thin steel sheet having excellent shape freezing property as described in (1) above.

(5) The ferritic thin steel sheet has a mass%
Further, Ti: 0.20% or less, Nb: 0.20% or less, V: 0.20% or less, Cr: 1.5% or less, and
B: The ferritic thin steel sheet having excellent shape freezing property as described in (4) above, containing one or more kinds of 0.007% or less. (6) The ferritic thin steel sheet has a mass%
o (1) to (5), characterized by containing one or more of 1% or less, Cu: 2% or more, Ni: 1% or less, and Sn: 0.2% or less. A ferritic thin steel sheet excellent in shape freezing property according to any one of the above.

(7) The ferritic thin steel sheet according to any one of (1) to (6) above, wherein the ferritic thin steel sheet is plated. (8) In producing the ferritic thin steel sheet excellent in shape freezing property according to any of (1) to (7) above, Ar ₃
The total reduction ratio in the temperature range from the transformation temperature to (Ar ₃ +100) ° C. is controlled to be 25% or more, hot rolling is completed at the Ar ₃ transformation temperature or more, cooled after hot rolling, and cooled.
A method for producing a ferritic thin steel sheet having excellent shape freezing characteristics, wherein the winding is performed at a critical temperature To or lower determined by the mass% of the chemical composition of the steel shown in the following formula (1).

To = −650.4 × C% + B (1) where B = −50.6 × Mneq + 894.3 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% (9) In producing the ferritic steel sheet, A
The total reduction ratio in the temperature range of r ₃ transformation temperature to (Ar ₃ +100) ° C. is 25% or more, and (Ar ₃ +10
0) In hot rolling at a temperature of not more than 0 ° C., at least one pass is rolled so that the friction coefficient becomes 0.2 or less, and the hot rolling is terminated at a temperature not lower than the Ar ₃ transformation temperature. The method for producing a ferritic thin steel sheet having excellent shape freezing property according to (1).

(10) In producing the ferritic thin steel sheet excellent in shape freezing property according to any one of the above (1) to (7), the total of the rolling reductions not more than the Ar ₃ transformation temperature is 25% or more. A method for producing a ferritic thin steel sheet having excellent shape freezing characteristics, wherein the ferrite thin steel sheet is cooled and rolled up, or recovered and recrystallized by additional heat treatment after cooling.

(11) In manufacturing the ferritic thin steel sheet, the total reduction rate below the Ar ₃ transformation temperature is 25.
% Or more and Ar ₃ or less, in at least one pass, rolling is performed so that the friction coefficient becomes 0.2 or less. Excellent method for producing ferritic thin steel sheet. (12) The method for producing a ferritic thin steel sheet having excellent shape freezing properties according to any of (8) to (11) above, wherein the ferritic thin steel sheet obtained by hot rolling is pickled with 80% Cold rolling of less than 600 ° C. to (Ac ₃ +1)
(00) A method for producing a ferritic thin steel sheet having excellent shape freezing properties, wherein the ferritic thin steel sheet is heated to a temperature range of 00 ° C. and cooled.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The contents of the present invention will be described below in detail. {100} <011> of the plate surface at 1/2 plate thickness
To {223} <110> orientation group average X-ray random intensity ratio, and {554} <225>, {111} <1
X of three crystal orientations of 12> and {111} <110>
Average value of line random intensity ratio: These average values are particularly important characteristic values in the present invention. {100} <011> to {22} when X-ray diffraction of the plate surface at the plate thickness center position was performed to determine the intensity ratio of each direction with respect to the random sample.
The average value of the 3｝ <110> orientation group must be 3.0 or more. If this average value is less than 3.0, the shape freezing property will be poor.

The main azimuth included in this azimuth group is $ 10.
0 {011>, {116} <110>, {114} <
110>, {113} <110>, {112} <110
>, {335} <110>, and {223} <110
>. The X-ray random intensity ratio in each of these directions is ｛1
A three-dimensional texture calculated by the vector method based on the 10 pole figure, {110}, {100}, {211},
It may be obtained from a three-dimensional texture calculated by a series expansion method using a plurality of pole figures (preferably three or more) of the {310} pole figures. For example, in the latter method, the X-ray random intensity ratio of each crystal orientation is (001) [1-10], (116) [1-10], (1) in the φ2 = 45 section of the three-dimensional texture.
14) [1-10], (113) [1-10], (112) [1-10], (335) [1-1
0], (223) [1-10].

{100} <011> to {223} <11
The average value of 0> azimuth group is an arithmetic mean of each of the above azimuths. If it is not possible to obtain the intensities of all the above directions, {100} <011>, {116} <110>,
{114} <110>, {112} <110>, $ 22
The arithmetic mean of each direction of 3｝ <110> may be substituted.

Further, the thickness of the plate surface at the half plate thickness is
4} <225>, {111} <112>, and {11}
The average value of the X-ray random intensity ratio of three crystal orientations of 1｝ <110> must be 3.5 or less. If this average value exceeds 3.5, {100} <011> to {22}
Even if the intensity of the 3｝ <110> orientation group is appropriate, it is difficult to obtain good shape freezing properties. {554} <22
5>, {111} <112>, and {111} <11
The X-ray random intensity ratio of 0> may also be obtained from the three-dimensional texture calculated according to the above method. More preferably,
The average value of the X-ray random intensity ratio of the {100} <011> to {223} <110> orientation group is 4.0 or more, and {554}
<225>, {111} <112>, and {111}
The arithmetic mean of the X-ray random intensity ratio of <110> is 2.5
Is less than.

The reason why the X-ray intensity of the crystal orientation described above is important for the shape freezing property during bending is not necessarily clear, but it is related to the slip behavior of the crystal during bending deformation. It is presumed. In the sample to be subjected to X-ray diffraction, the steel sheet is reduced to a predetermined thickness by mechanical polishing or the like, and then the distortion is removed by chemical polishing or electrolytic polishing, etc., and at the same time, the 1/2 plate thickness becomes the measurement surface. To be manufactured. When segregation bands or defects are present in the central thickness layer of the steel sheet and measurement inconvenience occurs, 3/8 to 5/5 of the sheet thickness
In the range of / 8, the sample may be adjusted according to the above-described method so that an appropriate surface is the measurement surface, and the measurement may be performed. As a matter of course, the above-described limitation on the X-ray intensity is satisfied not only in the vicinity of the plate thickness of about 1/2, but also as much as possible, so that the shape freezing property is further improved. In addition,
The crystal orientation represented by {hkl} <uvw> means that the normal direction of the plate surface is parallel to <hkl> and the rolling direction is <uvw>
Indicates that it is parallel.

The r value in the rolling direction (rL) and the r value in the direction perpendicular to the rolling direction (rC): These r values are important in the present invention. That is, as a result of intensive studies by the present inventors, it has been found that even if the X-ray intensities in the various crystal orientations described above are appropriate, good shape freezing properties cannot always be obtained. Simultaneously with the above X-ray intensity, it is essential that at least one of rL and rC is 0.7 or less.
More preferably, it is 0.55 or less. The lower limits of rL and rC need not be particularly defined. The effects of the present invention can be obtained without setting a lower limit. The r value is evaluated by a tensile test using a JIS No. 5 tensile test piece. The tensile strain is usually 15%, but if the uniform elongation is less than 15%, the strain may be evaluated as close to 15% as possible within the uniform elongation range.

The direction in which the bending process is performed is different depending on the processed component, and is not particularly limited. However, the bending process is mainly performed in a direction perpendicular or nearly perpendicular to the direction in which the r value is small. preferable. by the way,
Generally, it is known that there is a correlation between the texture and the r value. However, in the present invention, the limitation on the X-ray intensity ratio of the crystal orientation and the limitation on the r value are not synonymous with each other. If both limits are not met at the same time, good shape-freezing properties cannot be obtained.

The present invention can be applied to all types of thin steel sheets ranging from mild steel sheets having a low tensile strength level to high-strength steel sheets. If the above-mentioned conditions are satisfied, the bending workability of the thin steel sheets is dramatically improved. I do. In other words, the X-ray intensity ratio and the r value are basic material indices related to bending deformation, which exceed the mechanical strength level of a thin steel sheet.

The above rules can be universally applied to a thin steel plate, so that it is basically unnecessary to limit the type of the thin steel plate. However, from a practical point of view, referring to the types of thin steel sheets as application examples of this technology, the types of thin steel sheets range from mild steel sheets to high-strength steel sheets. And, of course, the distinction between the hot-rolled steel sheet and the cold-rolled steel sheet does not matter at all.

The component systems of the steel sheets described in the above (2) to (6) are very low carbon steel sheets, solute carbon and nitrogen are converted to Ti and N
b, so-called IF (Interstiti
alFree) steel sheet, low carbon steel sheet, high-strength steel sheet reinforced with solid solution, high-strength steel sheet strengthened by precipitation, high-strength steel sheet reinforced by transformation structure such as martensite and bainite,
Further, it includes a high-strength steel plate that utilizes these strengthening mechanisms in combination.

The component system (2) is mainly intended for ultra-low carbon steel sheets, low carbon steel sheets, and solid solution reinforced high strength steel sheets. The component system (3) is mainly composed of an IF steel sheet,
It is intended for precipitation-strengthened high-strength steel sheets. The component system (4) is mainly intended for a solid solution reinforced high strength steel sheet and a transformation structure reinforced high strength steel sheet. Further, the component system (5) relates to a solid solution reinforced high-strength steel sheet and a steel sheet in which a precipitation strengthening mechanism is used in combination with a transformation structure reinforced high-strength steel sheet. Furthermore, the component system (7) relates to a steel sheet obtained by plating the above-mentioned thin steel sheet.

First, the limiting conditions for the component system (2) will be described. The lower limit of C is set to 0.0001% because the lower limit of C in practical steel is used. If C exceeds 0.05%, workability deteriorates, so the upper limit is set to 0.05%. Si is an element effective for increasing the mechanical strength of the steel sheet, but if it exceeds 2.5%, the workability is deteriorated and surface flaws are generated, so the upper limit is 2.5%. . On the other hand, in practical steel, S
It is difficult to make i less than 0.001%.
001% is the lower limit.

Mn is also an effective element for increasing the mechanical strength of the steel sheet, but if it exceeds 2.5%, the workability deteriorates, so the upper limit is 2.5%. On the other hand, in practical steel, it is difficult to make Mn less than 0.01%.
The lower limit is 0.01%. When an element such as Ti that suppresses the occurrence of hot cracking due to S is not sufficiently added other than Mn, Mn / S that satisfies Mn / S ≧ 20 in mass%.
It is desirable to add n amount.

P and S are each 0.15% or less,
And 0.03% or less. This is to prevent deterioration in workability and cracks during hot rolling or cold rolling. Al
Is added at 0.01% or more for deoxidation. Also, Al
Significantly increases the γ → α transformation point,
It is an effective element when directing hot rolling at _three or less points. However, if the content is too large, the workability is reduced and the surface properties are deteriorated. Therefore, the upper limit is set to 2.0%.

N and O are impurities and are not more than 0.01% and 0. 0%, respectively, so as not to deteriorate workability.
01% or less. Ti in the component system of (3),
Since Nb and B improve the material through mechanisms such as fixing of carbon and nitrogen, precipitation strengthening, and fine grain strengthening, 0.005%, 0.001%, and
It is desirable to add 0.0001% or more, but excessive addition deteriorates workability, so the upper limit is
0.20%, 0.20%, and 0.007% were set.

Next, the limiting conditions relating to the component system (4) will be described. The lower limit of C was set to 0.04% because
This is because the lower limit of C in a practical high-strength steel sheet is used. On the other hand, if C exceeds 0.25%, workability and weldability deteriorate, so the upper limit is made 0.25%. S
i is an element effective for increasing the mechanical strength of the steel sheet, but if it exceeds 2.5%, the workability is degraded or surface flaws are generated, so the upper limit is made 2.5%. . On the other hand, in practical steel, it is difficult to make Si less than 0.001%, so the lower limit is made 0.001%.

Mn is also an effective element for increasing the mechanical strength of the steel sheet, but if it exceeds 2.5%, the workability deteriorates, so the upper limit is 2.5%. On the other hand, in practical steel, it is difficult to make Mn less than 0.01%.
The lower limit is 0.01%. When an element such as Ti that suppresses hot cracking due to S is not sufficiently added in addition to Mn, Mn that satisfies Mn / S ≧ 20 in mass%.
It is desirable to add an amount.

P and S are set to 0.20% or less and 0.03% or less, respectively. This is to prevent deterioration in workability and cracks during hot rolling or cold rolling. Al
Is added at 0.01% or more for deoxidation. Also, Al
Significantly increases the γ → α transformation point,
It is an effective element when directing hot rolling at _three or less points. However, if the content is too large, the workability is reduced and the surface properties are deteriorated. Therefore, the upper limit is set to 2.0%.

N and O are impurities and are not more than 0.01% and 0.1%, respectively, so as not to deteriorate workability.
01% or less. Ti in the component system of the above (5),
Nb, V, Cr, and B improve the material through mechanisms such as fixation of carbon and nitrogen, precipitation strengthening, structure control, and fine-grain strengthening.
0.001%, 0.001%, 0.01%, and 0.1%.
It is desirable to add 0001% or more. However, excessive addition has no remarkable effect, but rather deteriorates workability and surface properties. Therefore, the upper limits are 0.20% and 0. 0%, respectively.
20%, 0.20%, 1.5%, and 0.007% were set.

In the component system of (6), Mo, Cu,
Since Ni and Sn have the effect of increasing mechanical strength and improving the material, if necessary, each component may have a content of 0.1%.
Although it is desirable to add 001% or more, excessive addition adversely deteriorates processability.
1%, 2%, 1%, and 0.2%. Although not particularly limited in the present invention, an appropriate amount of Ca or Mg may be added for the purpose of deoxidation or morphological control of sulfide.

In the ferrite thin steel sheet (7), the type of plating is not particularly limited. The effects of the present invention can be obtained by any of electroplating, hot-dip plating, vapor deposition plating and the like. Next, a method for producing a ferritic thin steel sheet according to the present invention will be described. The production method prior to hot rolling is not particularly limited. That is, various secondary smelting may be performed following smelting using a blast furnace or an electric furnace, and then casting may be performed by a method such as thin continuous slab casting in addition to ordinary continuous casting or ingot casting. In the case of continuous casting, after once cooling to a low temperature, it may be heated again and then hot-rolled, or the cast slab may be continuously hot-rolled. Scrap may be used as a raw material.

The ferritic thin steel sheet excellent in shape freezing property of the present invention is prepared by casting a steel having the above-mentioned components, and then subjecting the steel sheet to cooling after hot rolling, cooling after hot rolling, or heat treatment after pickling. After hot rolling, cold rolling, pickling and cold rolling, annealing, or while hot-rolled steel sheets or cold-rolled steel sheets are subjected to heat treatment in a hot-dip coating line, these steel sheets are subjected to a separate surface treatment. It is also obtained by applying.

For example, in the manufacturing method of the above (8),
Hot rolling is performed by Ar determined by the mass% of the chemical composition of the steel._Three
When completed above the transformation temperature, the second half of the hot rolling
And Ar _ThreeAbove the transformation temperature (Ar_Three+100)
Rolling with a gauge reduction of 25% or more. This rolling is done
If not, the texture of the rolled austenite
It does not develop sufficiently and no matter what cooling is applied after hot rolling,
The surface of the hot-rolled steel sheet obtained finally
Of specified X-ray intensity level
Each crystal orientation cannot be obtained. Therefore, (Ar_Three+100)
The lower limit of the total rolling reduction at a temperature of not more than ° C was 25%.

(Ar ₃ +100) ° C. or lower The higher the total rolling reduction at the Ar ₃ transformation temperature or higher, the sharper the texture formation is expected. Therefore, the total rolling reduction is preferably 35% or more. Exceeds 97.5%, it is necessary to excessively increase the rigidity of the rolling mill, which causes an economic disadvantage. Therefore, the sum of the reduction rates is preferably
97.5% or less.

Here, in the manufacturing method of the above (9),
(Ar ₃ +100) ° C. or less If the friction coefficient between the hot-rolling roll and the steel sheet at the time of hot rolling at the Ar ₃ transformation temperature or more exceeds 0.2, the {110} plane is added to the sheet surface near the steel sheet surface. Since the main crystal orientation is developed and the shape freezing property is deteriorated, if a better shape freezing property is to be obtained, (Ar
₃ + 100) ° C. or less For at least one pass during hot rolling at an Ar ₃ transformation temperature or more, it is desirable that the friction coefficient between the hot rolling roll and the steel sheet be 0.2 or less.

The lower the coefficient of friction, the more preferable. If better shape freezing property is required,
(Ar ₃ +100) ° C. or less It is desirable that the friction coefficient be 0.15 or less for all passes of hot rolling at or above the Ar ₃ transformation temperature. In order to pass on the austenite texture formed in this way to the final hot-rolled steel sheet, it is necessary to wind it at the To temperature or lower. Therefore, To determined by the mass% of the chemical composition of the steel was set as the upper limit of the winding temperature. This temperature To is thermodynamically defined as the temperature at which austenite and ferrite having the same component as austenite have the same free energy. Considering the influence of components other than C, the temperature To is simply calculated using the following equation (1). Can be calculated. The effect of the other components specified in the present invention on the temperature To is not so large and has been neglected here.

To = −650.4 × C% + B (1) where B is the mass% of the chemical composition of the steel according to the following formula:
It is a value determined by B = -50.6 × Mneq + 894.3 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% Also, if the hot rolling is below Ar ₃ transformation temperature,
The ferrite generated before processing is processed to form a strong rolled texture.

As described in the above (10), in order to make such a texture finally a texture advantageous for shape freezing property, the ferrite processed at a high temperature is wound up during cooling, or It is necessary to recover and recrystallize by once cooling and then heating again. Ar
_If the total draft under the transformation temperature is less than 25%, winding at a temperature higher than the recrystallization temperature, or recovery and recrystallization by cooling and reheating, the above (1) Since each crystal orientation at the prescribed X-ray intensity level cannot be obtained, 25% was set as the lower limit of the total rolling reduction below the Ar ₃ transformation temperature. 35% is a more desirable lower limit.

Here, as described in the above (11),
The coefficient of friction between the hot rolling roll and the steel sheet during hot rolling is 0.
If it exceeds 2, a crystal orientation mainly composed of {110} planes develops in the sheet surface near the steel sheet surface, and the shape freezing property is deteriorated. For at least one pass in the hot rolling of Ar ₃ or less, the coefficient of friction between the roll and the steel sheet is preferably 0.2 or less. The friction coefficient is preferably as low as possible, and particularly when severe shape freezing property is required, it is preferable that the friction coefficient be 0.15 or less for all hot rolling passes of Ar ₃ or less.

In the hot rolling, the sheet bars may be joined after the rough rolling, and the finish rolling may be continuously performed. At this time, the coarse bar may be temporarily wound into a coil shape, and if necessary, stored in a cover having a heat retaining function, and may be rewound again before joining. The hot-rolled steel sheet may be subjected to skin pass rolling as necessary. It goes without saying that skin pass rolling has the effect of preventing stretcher strain generated at the time of working and correcting the shape.

When the hot-rolled steel sheet (or the heat-treated hot-rolled steel sheet) thus obtained is cold-rolled and annealed to obtain a final thin steel sheet, the cold-rolled steel sheet is less than 80% cold rolled. Is applied. When the total rolling reduction of the cold rolling is 80% or more,
The {111} plane and the {554} plane component in the X-ray diffraction integrated plane intensity ratio of the crystal plane parallel to the plate plane, which is a general cold rolling-recrystallization texture, are high, which is a feature of the present invention. Since the requirement for the crystal orientation specified in the ferrite thin steel sheet of (1) is not satisfied, the upper limit of the rolling reduction in cold rolling was set to 80%. In order to enhance the shape freezing property, it is desirable to limit the cold rolling reduction to 70% or less. There is no particular need to set the lower limit of the cold rolling reduction. Although the effects of the present invention can be obtained without setting the lower limit, the content is preferably set to 3% or more in order to control the strength of the crystal orientation in an appropriate range.

When annealing a cold-rolled steel sheet cold-worked in such a rolling reduction range, if the annealing temperature is lower than 600 ° C., the work structure remains and the formability is remarkably deteriorated. Is set to 600 ° C. On the other hand, when the annealing temperature is excessively high, the texture of ferrite generated by recrystallization is transformed into austenite, and then randomized by austenite grain growth, and the texture of ferrite finally obtained is also randomized. In particular, when the annealing temperature exceeds (Ac ₃ +100) ° C., such a tendency becomes remarkable. Therefore, the annealing temperature is (Ac ₃
(+100) ° C. or lower. The cold-rolled steel sheet may be subjected to skin pass rolling as necessary.

The structure obtained by the present invention is mainly composed of ferrite. However, the metal structure other than ferrite may contain compounds such as pearlite, bainite, martensite, austenite and carbonitride. . In particular, the crystal structures of martensite and bainite
Since the crystal structure is the same or similar to that of ferrite, these structures may be mainly used instead of ferrite.

The steel sheet according to the present invention can be applied not only to bending but also to composite forming mainly based on bending, such as bending, overhang, drawing, and the like.

[0051]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The technical contents of the present invention will be described with reference to embodiments of the present invention. As an example, a description will be given of the results of studies using steels A to W having the component compositions shown in Table 1. These steels are reheated after casting, as they are, or once cooled to room temperature,
Heated to a temperature range of 900C to 1300C,
Hot-rolled, finally 1.4mm, 3.0m
m or a hot-rolled steel sheet having a thickness of 8.0 mm. 3.0
The hot-rolled steel sheets having a thickness of 8.0 mm and 8.0 mm were formed into cold-rolled steel sheets having a thickness of 1.4 mm by cold rolling, and then subjected to annealing in a continuous annealing step. For these 1.4 mm thick test specimens, "Press Forming Difficulty Handbook" supervised by Seita Yoshida
A 90-degree bending test is performed in accordance with the U-bending test method described on pages 417 to 418 of Nikkan Kogyo Shimbun, 1987, and the value obtained by subtracting 90 degrees from the opening angle (spring back amount) is obtained. The shape freezing property was evaluated. The bending was performed so that a polygonal line was perpendicular to the direction in which the r value was low. Tables 2 and 3 (continuation of Table 2) show the manufacturing conditions for each steel plate (test piece).

Tables 2 and 3 simultaneously show whether or not the manufacturing conditions for each steel sheet are within the scope of the present invention.

[0053]

[Table 1]

[0054]

[Table 2]

[0055]

[Table 3]

“Hot rolling temperature 1” means that when hot rolling is completed at a temperature not lower than the Ar ₃ transformation temperature, the temperature of Ar ₃ + 100 ° C.
_The case where the total reduction rate at _three temperatures or more was 25% or more was evaluated as “○”, and the case where the total reduction was less than 25% was evaluated as “x”. “Hot rolling temperature 2” is a case where hot rolling is lower than the Ar ₃ transformation temperature,
The case where the total reduction ratio at the temperature of r ₃ or less was 25% or more was “○”, and the case where the total reduction was less than 25% was “X”. In any of the above cases, when the friction coefficient for at least one pass or more is 0.2 or less in each temperature range, “○” is displayed in the “lubrication” column, and the friction coefficient for all passes exceeds 0.2. In the case of, "△" was used. The winding after the hot rolling was all performed at a temperature equal to or lower than the To temperature determined by the above equation (1). When such a hot-rolled steel sheet is cold-rolled to a thickness of 1.4 mm, the cold-rolling reduction ratio is 80.
% Or more, “cold rolling reduction” is set to “×” and “80”
% If less than "%". Also, if the annealing temperature is 60
When the temperature was 0 ° C. or more and (Ac ₃ +100) ° C. or less, “annealing temperature” was indicated by “○”, and in other cases, “×” was indicated.
Items that are not relevant as manufacturing conditions are marked "-". Skin pass rolling was performed on both the hot-rolled steel sheet and the cold-rolled steel sheet in a range of 0.5 to 1.5%.

The measurement by X-ray was performed by adjusting a sample parallel to the plate surface at a position of 7/16 of the plate thickness as a representative value of the steel plate. Tables 4 and 5 (continuation of Table 4) show the mechanical property values and the amount of spring back of the hot-rolled steel sheet and the cold-rolled steel sheet having a thickness of 1.4 mm manufactured by the above method. In Tables 4 and 5, in all steel types except for the steel type L, examples of the numbers “−2” and “−3” of each steel type relate to the present invention. These are "-1" outside the invention.
And the amount of spring back is smaller than those of numbers "-4". That is, in the ferritic thin steel sheet, the good shape freezing property of the thin steel sheet is achieved only when the X-ray random intensity ratio and the r value of each crystal orientation defined in the present invention are obtained.

The mechanism by which the X-ray random intensity ratio and r value of each crystal orientation are important for shape freezing is not always clear at present. Probably, during bending deformation, the slip deformation progresses easily, and as a result,
It is understood that the amount of spring back during bending deformation is small.

[0059]

[Table 4]

[0060]

[Table 5]

[0061]

It has been described in detail that, when the texture and r value of a ferritic thin steel sheet are controlled, the bending workability is remarkably improved. According to the present invention, a thin steel plate having a small amount of spring back and excellent in shape freezing property mainly by bending can be provided. In particular, even for parts where it was difficult to apply high-strength steel sheets due to the problem of shape defects in the past,
High strength steel sheets can be used. The use of high-strength steel sheets is absolutely necessary in order to promote weight reduction of automobiles. If a high-strength steel sheet having a small amount of spring back and excellent shape freezing properties can be applied to the weight reduction of an automobile, the weight reduction of an automobile body can be further promoted. Therefore, the present invention is an industrially extremely valuable invention.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI theme coat ゛ (Reference) C21D 9/46 C21D 9/46 H C22C 38/06 C22C 38/06 38/58 38/58 (72) Invention Natsuko Sugiura 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation Technology Development Division (72) Inventor Toru Yoshida 20-1 Shintomi, Futtsu-shi, Chiba Prefecture Nippon Steel Corporation Technology Development Division F-term 4K037 EA01 EA02 EA04 EA05 EA11 EA13 EA15 EA16 EA17 EA18 EA19 EA20 EA22 EA23 EA25 EA27 EA28 EA31 EA32 EB02 EB06 EB09 EB11 FB03 FC02 FC03 FC07 FE01 FE02 FE03 FJ04 JA05

Claims (12)

[Claims] 1. A ferrite thin steel sheet having a {100} <011>-{100}
The average value of the X-ray random intensity ratio of the {223} <110> orientation group is 3.0 or more, and {554} <225>;
The average value of the X-ray random intensity ratio of the three crystal orientations of {111} <112> and {111} <110> is 3.5.
A ferritic thin steel sheet having excellent shape freezing properties, wherein at least one of an r value in a rolling direction and an r value in a direction perpendicular to the rolling direction is 0.7 or less. 2. The ferritic thin steel sheet has a mass percentage of C: 0.0001% or more and 0.05% or less, Si: 0.001% or more and 2.5% or less, Mn: 0.01%. 2.5% or less, P: 0.15% or less, S: 0.03% or less, Al: 0.01% or more, 2.0% or less, N: 0.01% or less, and O: The ferritic thin steel sheet excellent in shape freezing property according to claim 1, comprising 0.01% or less, and the balance being iron and unavoidable impurities. 3. The steel sheet according to claim 2, wherein
The shape freezing according to claim 2, further comprising one or more of Ti: 0.20% or less, Nb: 0.20% or less, and B: 0.007% or less. Ferritic steel sheet with excellent resistance. 4. The ferritic thin steel sheet is expressed by mass%, C: 0.04% or more, 0.25% or less, Si: 0.001% or more, 2.5% or less, Mn: 0.01%. 2.5% or less, P: 0.20% or less, S: 0.03% or less, Al: 0.01% or more, 2.0% or less, N: 0.01% or less, and O: The ferritic thin steel sheet excellent in shape freezing property according to claim 1, comprising 0.01% or less, and the balance being iron and unavoidable impurities. 5. The ferritic thin steel sheet according to claim 1, wherein
Further, one or more of Ti: 0.20% or less, Nb: 0.20% or less, V: 0.20% or less, Cr: 1.5% or less, and B: 0.007% or less The ferritic thin steel sheet excellent in shape freezing property according to claim 4, comprising: 6. The ferritic thin steel sheet according to claim 1, wherein
Further, one or more of Mo: 1% or less, Cu: 2% or less, Ni: 1% or less, and Sn: 0.2% or less are contained. The ferritic thin steel sheet according to any one of the above, which is excellent in shape freezing property. 7. The ferritic thin steel sheet according to claim 1, wherein the ferritic thin steel sheet is plated. 8. In producing the ferritic thin steel sheet excellent in shape freezing property according to any one of claims 1 to 7, reduction in a temperature range from an Ar ₃ transformation temperature to (Ar ₃ +100) ° C. The hot rolling is terminated at a temperature higher than the Ar ₃ transformation temperature, cooled after hot rolling, and the mass% of the steel chemical component represented by the following formula (1) is controlled.
Characterized by winding at a critical temperature To or less determined by
A method for producing ferritic thin steel sheets with excellent shape freezing properties. To = -650.4 × C% + B… (1) where B = -50.6 × Mneq + 894.3 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% 9. When manufacturing the ferritic thin steel sheet, the total reduction ratio in the temperature range of Ar ₃ transformation temperature to (Ar ₃ +100) ° C. is 25% or more, and (Ar
₃ +100) In hot rolling at a temperature of at most
Rolled above the pass so that the coefficient of friction is 0.2 or less,
Characterized by terminating the hot rolling at Ar ₃ transformation temperature or more, the manufacturing method of ferritic steel sheet excellent in shape fixability according to claim 8. 10. In producing the ferritic thin steel sheet excellent in shape freezing property according to any one of claims 1 to 7, a total reduction ratio at an Ar ₃ transformation temperature or lower is 25% or more, and thereafter, A method for producing a ferritic thin steel sheet having excellent shape freezing properties, wherein the ferrite thin steel sheet is cooled and wound up, or recovered and recrystallized by additional heat treatment after cooling. 11. When producing the ferritic thin steel sheet, the coefficient of friction of at least one pass or more in hot rolling at an Ar ₃ transformation temperature of not less than 25% and Ar ₃ or less is not less than 25%. The method for producing a ferritic thin steel sheet having excellent shape freezing properties according to claim 10, wherein the ferrite-based thin steel sheet is rolled so as to be 0.2 or less. 12. The method for producing a ferritic thin steel sheet having excellent shape freezing properties according to any one of claims 8 to 11, wherein the ferritic thin steel sheet obtained by hot rolling is pickled,
Cold rolling of less than 80% is performed, and then 600 ° C. to (A
A method for producing a ferritic thin steel sheet having excellent shape freezing properties, comprising heating to a temperature range of c ₃ +100) ° C. and cooling.