JP2002003993A - High strength steel sheet and high strength galvanized steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength steel sheet having superior press processability at the forming (mainly by bulging) of such as hood, door, fender, side panel, surface roughing resistance after pressing, excellent in surface characteristic and secondary processability. SOLUTION: The high strength steel sheet has a chemical composition consisting of, by mass, 0.0040-0.01% C, <=1.0% Si, 0.1-1.0% Mn, 0.01-0.07% P, <=0.02% S, 0.01-0.1% sol.Al, <=0.004% N, 0.01-0.14% Nb and the balance essentially iron with inevitable impurities and also has >=0.21 n-value at <=10% deformation by the uniaxial tensile test and satisfies the following inequality: YP<=-60×d+770 (wherein, YP represents yield strength [MPa]; and d represents average ferrite grain diameter [μm]).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates mainly to automobile outer panel panels such as hoods, doors, fenders, side panels and the like.
The present invention relates to a high-strength cold-rolled steel sheet or a high-strength galvanized steel sheet which is excellent in forming mainly by overhanging, and has good surface roughness after press forming, resistance to secondary working brittleness, and good surface properties.

[0002]

2. Description of the Related Art In recent years, efforts have been made to increase strength by improving safety, reduce the number of parts by integrating parts, and omit the pressing process. There is a demand for a high-strength steel sheet having heat resistance. Conventionally, the press formability of a steel sheet has been studied from the viewpoint of improving the deep drawing property and the overhang property.

[0003] With respect to the deep drawability of cold rolled steel sheets, many techniques for increasing the r value have been disclosed. For stretch formability, generally, the elongation, the uniform elongation (uniform elongation) and the n value in the high strain region, for example, for a material having a uniform elongation of 20% or more, n of the two-point method of 10% and 20% It has been important to increase the value.

[0004] For example, Japanese Patent Application Laid-Open No. 5-78784 discloses that Mn and Cr are positively added to ultra-low carbon steel to which Ti is added, and Si or
A technique has been proposed in which P is controlled, tensile strength is 343 MPa to 490 MPa, average r value and elongation are good. Thus, the strength can be increased while suppressing the increase in the yield stress, and a high-strength cold-rolled steel sheet having good surface shape and excellent dent resistance can be obtained.

In Japanese Patent Application Laid-Open No. Hei 8-92656, a very low carbon steel sheet is subjected to hot lubrication rolling at Ar _{3 to} 500 ° C., then hot rolled sheet recrystallization treatment, cold rolling, cold rolled sheet recrystallization annealing To increase the r value to 3.0 or more. This technique is to increase the deep drawability by adding Nb and B while greatly reducing C, S and N, and performing cold-rolled sheet recrystallization annealing at 700 to 950 ° C. .

[0006]

SUMMARY OF THE INVENTION However, Japanese Patent Application Laid-Open No. 5-7878
In the technology described in Japanese Patent Publication No. 4 (2004), in the formability, in the case of molding mainly by overhanging a side panel or the like, sufficient formability cannot be obtained only by the r value and elongation, and particularly in the plane strain overhang forming portion due to insufficient strain propagation. There was a problem of breaking at the punch shoulder. Further, since Cr and Mn are added in an amount of 0.9% or more, there is a disadvantage in cost.

[0007] Further, it is described that the steel sheet disclosed in JP-A-8-92656 is subjected to cold-rolled sheet recrystallization annealing at 700 to 950 ° C after cold rolling. The temperature is 880 to 910 ° C., which is a very high temperature recrystallization annealing. Therefore, this steel sheet has a large crystal grain size, and surface properties after press forming, particularly,
Not suitable for strict surface materials such as automobile outer panels. in this way,
Until now, it has been very difficult to achieve both good press formability and good surface properties after pressing.

Further, according to the technology described in this publication, N is set to 0.0
It is specified as low as 020% or less.
~ 5ppm. Reducing N to this level is
It is difficult to implement as an industrial method for producing steel. In addition, most of C and S are 5 ppm or less, which is also difficult in the production of ordinary steel for sheet metal, and requires special hot metal treatment and degassing treatment.

[0009] As in these techniques, increasing the r-value is effective for a part requiring deep drawability accompanied by shrinkage flange deformation. There was a problem that could not be obtained. Further, in this technique, annealing is performed before cold rolling, so that not only energy consumption is large but also productivity is low and cost is disadvantageous.

Regarding the overhang property, as described in, for example, “Press Processing of Thin Steel Sheet, (Jikkyo Shuppan), p. 161”, it has been considered that the influence of the ductility of the material and the n value is strong.
The ductility of a material is generally evaluated by the total elongation, which is represented by the sum of uniform elongation (uniform elongation) and local elongation. It is known from the plastic theory that uniform elongation is equal to the work hardening coefficient n value. This n-value has been evaluated by the two-point method n-value in the high strain region near uniform elongation. However, as the strength of the material increases, it becomes difficult to obtain the same total elongation and n value as those of the soft material. Further, there have been cases where the effect does not appear even if the n value is increased in the high distortion region.

An object of the present invention is to solve the above-mentioned problems of the conventional cold-rolled steel sheet for press forming, and to provide excellent press formability and post-press forming in overhang forming such as hoods, doors, fenders and side panels. It is an object of the present invention to provide a high-strength thin steel sheet having both surface roughness resistance and excellent surface properties and secondary work brittleness resistance.

[0012]

The above object is achieved by the following invention. In the first invention, the chemical component is mass%
And C: 0.0040 to 0.01%, Si: ≦ 1.0%, Mn: 0.1 to 1.0
%, P: 0.01 to 0.07%, S: ≤ 0.02%, sol. Al: 0.01 to 0.
1%, N: ≦ 0.004%, Nb: 0.01 to 0.14%, the balance being substantially composed of iron, the n value in deformation of 10% or less by a uniaxial tensile test is 0.21 or more, and the following formula: It is a high-strength steel sheet that satisfies (1). YP ≦ -6
0 × d + 770 (1) where YP represents the yield strength [MPa] and d represents the average ferrite grain size [μm].

The present invention has been made while examining in detail the factors governing the formability of parts that are mainly formed by overhanging such as fenders and side panels. In the process, it is understood that in the overmolding mainly, the amount of generated strain is small at the punch bottom contact area, which occupies most of the molded product, and the strain is concentrated near the punch shoulder and die shoulder on the side wall. Was done.

Thus, by increasing the amount of strain generated in a wide range of materials at the punch bottom contact portion, it is possible to alleviate the concentration of strain near the punch shoulder and die shoulder of the side wall portion, which is a risk of fracture. To achieve this, it is effective to improve the n value in the low strain region corresponding to the amount of strain generated at the punch bottom contact part, instead of the n value in the high strain region, which was conventionally used for evaluating the overhang property. I learned. Further, they have found that it is necessary to have low YP and fine crystal grains in order to secure the surface roughness resistance after press working while maintaining excellent stretch formability.

[0015] Therefore, unlike conventional IF steel, C
Nb-IF steel using Nb as a carbonitride forming element in a component system with 40 ppm or more added,
Also, by controlling the microstructure and precipitate morphology of the steel sheet, it was possible to significantly improve the n value in the low strain range and to refine the crystal grains, and found for the first time through studies such as detailed electron microscopic observation. . The present invention has been made as a result of further studies based on such findings, and the features thereof are as follows.

First, the reasons for limiting the component composition range (chemical components) will be described. C: 0.0040% to 0.01% C causes the carbide formed with Nb to affect the strength of the material and the propagation of strain in a low strain region during panel forming, thereby increasing the strength and improving the formability. If the C content is less than 0.0040%, no effect is obtained, and if it exceeds 0.01%, although sufficient strength and sufficient strain propagation in a low strain region can be obtained, ductility decreases,
Moldability deteriorates. Therefore, the amount of C is specified in the range of 0.0040 to 0.01%.

Si: ≦ 1.0% Si is an element effective for securing the strength, but if added in excess of 1.0%, the chemical conversion property and the surface properties are significantly deteriorated. Therefore, the amount of Si is specified to be 1.0% or less.

Mn: 0.1-1.0% Mn is an essential element in steel because it has an effect of preventing hot cracking of the slab by precipitating S in the steel as MnS. 0.1% or more is required. Mn is also an element capable of solid solution strengthening the steel without deteriorating the plating adhesion, but excessive addition exceeding 1.0% causes n in a low strain region due to an excessive increase in yield strength.
It is not preferable because the value is lowered. Therefore, the amount of Mn
It is specified in the range of 0.1 to 1.0%.

P: 0.01 to 0.07% P is an element effective for strengthening steel, and this effect appears when added at 0.01% or more. However, if P is added in excess of 0.07%, the alloying treatment at the time of zinc plating is deteriorated, resulting in poor plating adhesion and poor panel appearance due to undulation. Therefore, the P content is defined in the range of 0.01 to 0.07%.

S: ≦ 0.02% S is present in steel as MnS, and if contained excessively, ductility is deteriorated and press formability is reduced. Practically, the S content at which no inconvenience occurs in moldability is 0.02% or less. Therefore,
S content is regulated to 0.02% or less.

Sol. Al: 0.01 to 0.1% Al is added in an amount of 0.01% or more in order to precipitate N in steel as AIN and not to leave solid solution C. If the sol.Al is less than 0.01%, the above effect is not sufficient, and if the addition exceeds 0.1%, the solute Al causes a decrease in ductility.
Restrict to 1% range.

N: ≦ 0.004% N precipitates as AlN and is rendered harmless. However, even when sol.Al is in the lower limit amount, to precipitate all N as AlN, 0.004%
It must be: Therefore, the amount of N is regulated to 0.02% or less.

Nb: 0.01 to 0.14% Nb combines with C to form fine carbides, which affects the strength of the material and the propagation of strain in a low strain range during panel forming, and improves the formability and surface distortion resistance. Let it. However, if it is less than 0.01%, there is no effect, and if it exceeds 0.14%, the yield strength increases,
Sufficient strain propagation in a low strain range cannot be obtained, ductility decreases, and formability deteriorates. Therefore, the amount of Nb is specified in the range of 0.01 to 0.14%.

Next, as a feature of the present invention, by increasing the strain propagation in the low strain region of the material, the amount of strain generated in the material in contact with the punch bottom in a wide range is increased, and the stretch formability is improved. Here, as the low strain range,
As a result of the study of the above-mentioned factors controlling the formability, it was found that the strain should be set to a region of 10% or less.
Therefore, in the present invention, a necessary value from the viewpoint of formability was determined as an n value in a region where the nominal strain of uniaxial tension was 10% or less. As a result, the n value was set to 0.21 or more, and the stretch formability was significantly improved. As the n value in the deformation of 10% or less, the n value of the two-point method of 1% and 10% of the nominal strain may be used.

Further, the steel of the present invention is intended for a material having a strict surface such as an automobile outer panel, and it is necessary to secure excellent surface properties even after severe press molding. Therefore, various conditions for ensuring high stretch formability and preventing roughening after pressing were examined. In the process, they have found that it is necessary to reduce the crystal grain size according to the yield stress. The results of the study are summarized in the above equation (1), and by reducing the crystal grain size so as to satisfy this equation, we succeeded in preventing roughening after pressing. As described above, in the present invention, the yield strength YP [MPa] and the average ferrite grain size d
[Μm] is controlled so as to satisfy Expression (1).

According to a second aspect of the present invention, there is provided the high-strength steel sheet according to the first aspect, wherein the chemical components are replaced by mass%
C: 0.0040-0.01%, Si: ≦ 1.0%, Mn: 0.1-1.0%,
P: 0.01 to 0.07%, S: ≤ 0.02%, sol. Al: 0.01 to 0.1
%, N: ≦ 0.004%, Nb: 0.01 to 0.14%, and 0.05% or less of Ti, and the balance is substantially composed of iron.

In the present invention, the structure of the hot-rolled sheet is refined by further adding Ti to the chemical component of the first invention. Ti forms carbonitrides and refines the structure of the hot-rolled sheet to improve formability. However, when Ti is added in excess of 0.05 wt%, the precipitates become coarse, and a sufficient effect cannot be obtained. Therefore, the amount of Ti is specified to be 0.05% or less.

According to a third aspect, in the high-strength steel sheet according to the first aspect, the chemical composition is replaced by mass% by mass instead of the description.
C: 0.0040-0.01%, Si: ≦ 1.0%, Mn: 0.1-1.0%,
P: 0.01 to 0.07%, S: ≤ 0.02%, sol. Al: 0.01 to 0.1
%, N: ≦ 0.004%, Nb: 0.01 to 0.14%, B: 0.002% or less, with the balance being substantially composed of iron.

According to the present invention, B is added to the above-mentioned chemical components to improve the resistance to secondary working embrittlement. As described above, B strengthens the crystal grain boundaries, but when added in excess of 0.002 wt%, the formability is significantly impaired. Therefore, the upper limit of the amount of B is set to 0.002%.

According to a fourth aspect of the present invention, in the first aspect, the chemical components are expressed by mass% as C: 0.0040 to 0.02%, Si: 1.0% or less, Mn: 0.7 to 3.0%, P: 0.02 to 0.15%, S: 0.02% or less, Al: 0.01 to 0.1%, N: 0.004% or less, Nb: 0.2% or less, Ti: 0.05% or less, B: 0.002% or less, the balance substantially consisting of iron and unavoidable impurities. It is a high-strength thin steel sheet characterized by the following.

In the present invention, Ti and B are further added to the first invention in order to improve the formability and the resistance to secondary working brittleness. As a result, Ti forms carbonitrides and refines the structure of the hot-rolled sheet to improve formability, and B strengthens grain boundaries and improves secondary work brittleness resistance. However,
When Ti is added in excess of 0.05%, the precipitate becomes coarse,
If B is added in excess of 0.002%, the formability is greatly reduced. Therefore, the upper limit of Ti is set to 0.05% and the upper limit of B is set to 0.002%.

According to a fifth aspect of the present invention, there is provided the high-strength thin steel sheet according to any one of the first to fourth aspects of the present invention, further comprising:
It is a high-strength thin steel sheet containing one or more of Ni: 1.0% or less and Cu: 1.0% or less.

According to the present invention, a steel sheet is further strengthened by adding at least one of Cr, Mo, Ni and Cu to the chemical components of the above-mentioned invention. Hereinafter, the reasons for limiting each element will be described.

Cr: 1.0% or less Cr is added to increase the strength, but if added in excess of 1.0%, the formability is reduced. Therefore, the upper limit of Cr content
Defined as 1.0%.

Mo: 1.0% or less Mo is an element effective for securing the strength, but if added in excess of 1.0%, recrystallization in the γ region (austenite region) during hot rolling is delayed, and the rolling load is reduced. increase. Therefore,
The upper limit of the amount of Mo is specified as 1.0%.

Ni: 1.0% or less Ni is added. If Ni is added in excess of 1.0%, the transformation point is greatly reduced, and a low-temperature transformation phase tends to appear during hot rolling. Therefore, the upper limit of the amount of Ni is defined as 1.0%.

Cu: 1.0% or less Cu is effective as a solid solution strengthening element, but if added in excess of 1.0%, a low melting point phase is formed during hot rolling and surface defects are likely to occur. Therefore, the Cu content is specified to be 1.0% or less. It is desirable that Cu be added together with Ni.

The sixth invention is a high-strength galvanized steel sheet excellent in stretch formability and surface roughening resistance, characterized in that a zinc-based plating film is provided on the steel sheet surface of the first to fifth inventions. .

The present invention imparts corrosion resistance to the steel sheet by further applying a zinc-based plating film to the surface of the steel sheet of the above-described invention. Here, the plating method is not particularly limited, and hot-dip galvanizing, electroplating, and other various plating methods can be used.

In these means, "the balance is substantially iron" means that the substance containing other trace elements including unavoidable impurities is used in the present invention unless the function and effect of the present invention are lost. Is included in the range.

[0041]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In practicing the present invention, the chemical components may be adjusted as described above. However, for some of the chemical components, the respective characteristics are improved by the following. Can be.

As for C, in order to properly control the morphology and dispersion state of the precipitates and to bring out more excellent moldability and more preferable overall performance, the amount of C added is 0.0050 to 0.0080.
%, More preferably in the range of 0.0050 to 0.0074%.

The content of Si is desirably regulated to 0.7% or less in order to improve surface properties and plating adhesion.

With respect to Nb, in order to further improve the n value in the low strain range, it is desirable that the Nb content be Nb> 0.035%. In order to further improve the formability and the overall performance,
It is desirable that Nb ≧ 0.08%. However, in consideration of cost and the like, it is preferable to set the upper limit to Nb ≦ 0.14%.

The reason why Nb improves the n value in a low distortion region is as follows.
Although not always clear, a detailed microscopic observation using an electron microscope reveals that when the amounts of Nb and C are properly controlled, a large amount of NbC precipitates in the crystal grains and no precipitates exist near the grain boundaries. A depletion zone (hereinafter PFZ) is formed,
Since the precipitates are depleted in the PFZ, the strength is lower than that in the grains, the plastic deformation can be performed at a low stress level, and a high n value can be obtained in a low strain range. To this end, it is effective to control the atomic equivalent ratio of Nb and C to an appropriate value,
As a result of the study, it has been found that it is more preferable to limit the Nb / C (atomic equivalent ratio) to a range of 1.3 to 2.5 for improving the n value.

When Ti is added, the content is preferably less than 0.02% from the viewpoint of the surface properties of the hot-dip galvanizing, and is preferably 0.005% or more in order to obtain a necessary grain-reducing effect.

Regarding B, as described above, the steel of the present invention exhibits excellent secondary work brittleness resistance even without the addition of B. Therefore, when B is added, a decrease in formability is suppressed as much as possible.
It is preferable to control the amount of B added to the range of 0.0001 to 0.001%.

As for the manufacturing method, a hot-rolled steel sheet is manufactured from the steel whose composition has been adjusted as described above, and then cold-rolled and annealed to obtain a cold-rolled steel sheet. Further, if necessary, the surface can be galvanized to obtain a galvanized steel sheet. The manufacturing method may be as described below.

For example, heating may be performed by a bar heater during hot rolling for the purpose of ensuring the finishing rolling temperature during the production of thin materials. Further, from the viewpoints of descaling by pickling and stability of the material, the hot-rolled steel sheet is preferably set to a winding temperature of 680 ° C. or lower. In addition, the lower limit of the winding temperature is 600 ° C when subjected to continuous annealing, and 5 ° C when subjected to box annealing.
Preferably, the temperature is 40 ° C.

In the descaling of the surface of the hot-rolled steel sheet, it is preferable to sufficiently remove not only the primary scale but also the secondary scale generated during hot rolling in order to impart excellent suitability for the outer plate. When the hot-rolled steel sheet is descaled and then cold-rolled, the cold-rolling ratio is preferably set to 50% or more in order to impart the deep drawability required for the outer plate.

In the case where the cold-rolled steel sheet is annealed by continuous annealing, the annealing temperature is preferably set to 780 to 880 ° C. On the other hand, when performing annealing by box annealing, since box annealing has a long soaking time, a uniform recrystallized structure can be obtained at an annealing temperature of 680 ° C or higher, but the upper limit of the annealing temperature is 750 ° C. Is preferred. The annealed cold-rolled steel sheet can be subjected to galvanizing by hot-dip galvanizing or electroplating. Further, an organic film treatment may be performed after plating.

The tensile properties and composition of the steel sheet of the present invention will be described in detail. FIG. 1 is a diagram showing an example of an equivalent strain distribution in the vicinity of a fracture danger site for a front fender model molded product on an actual part scale. Fig. 2 shows the outline of this molded product. As shown in FIG. 1, the fracture-prone portion is the side wall portion, and the generated strain rises to about 0.3, but the generated strain at the punch bottom is 0.10 or less.

Thus, by increasing the strain propagation of the material in the low strain range, the amount of strain generated in the material in contact with the punch bottom is increased over a wide range, and the stretch formability is improved. It is known from plastic deformation theory that the strain propagation increases with the increase in work hardening (n value) of the material.

Therefore, in order to increase strain propagation in a low strain region of 10% or less, it is necessary to increase the n value in deformation of 10% or less. Here, the nominal strain of uniaxial tension 1
% And 10%, the n value of the two-point method is set to 0.21 or more, thereby significantly improving stretch formability. In order to further improve the overhang property, it is preferable that the n value of the two-point method of 1% and 10% of nominal strain be 0.214 or more. The uniaxial tension is based on JIS No. 5 test.

In order to prevent surface roughness after pressing, in order to obtain more excellent surface properties in the present invention, the condition of yield strength YP [MPa] and ferrite average particle size d [μm] are expressed by the following equation (1). More preferably, the following equation (1 ′) is satisfied. YP ≦ -60 × d + 750 (1 ')

[0056]

EXAMPLES (Example 1) The following tests were performed using steels having the chemical components shown in Table 1. After smelting steel of steel No. 1 ~ 10,
A slab was manufactured by continuous casting. 1200 ℃ this slab
After heating, finishing temperature 880 ~ 940 ℃, winding temperature 540 ~ 560 ℃
(For box annealing), hot-rolled at 600 to 660 ° C (for continuous annealing, continuous annealing + hot-dip galvanizing) to form a hot-rolled steel sheet with a thickness of 2.8 mm, and after pickling, 50-85% cold-rolled gave.

[0057]

【table 1】

Thereafter, continuous annealing (annealing temperature 800 to 860)
C), box annealing (annealing temperature 680 ° C to 740 ° C), and continuous annealing + hot-dip galvanizing (annealing temperature 800 to 860 ° C). For continuous annealing + hot dip galvanizing, 460 after annealing
The hot-dip galvanizing treatment was performed at ℃, and the alloying treatment of the plating layer was immediately performed at 500 ° C. in an in-line alloying treatment furnace. The steel sheet after annealing or annealing and hot dip galvanizing was subjected to temper rolling at a reduction of 0.7%.

The mechanical properties and grain size of these steel sheets were measured. The tensile test was performed using a JIS No. 5 tensile test piece sampled from the L direction. In addition, the front fender was press-formed with the above-mentioned steel sheet, and the break limit cushion force was investigated, and the occurrence of rough skin after the press molding was investigated.

Further, the secondary working brittle transition temperature was measured. Here, a blank with a diameter of 105 mm was punched from a steel plate and deep-drawn into a cup shape as the primary processing (drawing ratio 2.
1) The ears were cut so that the cup height was 35 mm.
Next, after making the obtained cup sample a constant temperature in various refrigerants (such as ethyl alcohol), a process of expanding the cup end portion with a conical punch as a secondary process is added,
The temperature at which the fracture mode transitioned from ductile to brittle was measured and defined as the secondary working embrittlement transition temperature. Table 2 shows the test results.

[0061]

[Table 2]

The steel sheets Nos. 1 to 8 of the present invention had high breaking limit cushion caps of 65 tons or more, and exhibited excellent overhanging properties.
On the other hand, the comparative materials No. 9 to 12 had a small n value in the low strain region, and
Breakage occurred with a low cushion force of on or less. In Comparative materials Nos. 9 to 12, the crystal grain size was large, and roughening was observed after press molding.

Furthermore, the inventive examples Nos. 1 to 8 have fine grains and a structure in which the precipitate morphology is optimally controlled.
All show extremely excellent secondary work brittleness resistance. In addition, it was confirmed that the steel of the present invention had good tailored blanking properties and fatigue properties in addition to excellent formability, and that the galvanized material had very good surface properties. All of them have been proved to have extremely excellent overall performance especially as a steel plate for an automobile outer panel.

Example 2 FIG. 3 shows steel numbers N shown in Table 2 above.
A model forming test was performed on o.3 material (Example of the present invention) and No. 10 material (Comparative Example). In the test, the strain distribution in the vicinity of the risk of fracture when the front fender model shown in FIG. The test results are shown in FIG.

In the example of the present invention (N0.3 material, ● mark in the figure), the amount of strain generated at the bottom of the punch was larger than that in the comparative example (No. 10 material, ○ mark in the figure), and the strain on the side wall was larger. The occurrence is suppressed. From this, it is clear that the steel sheet of the present invention is advantageous for breaking.

[0066]

According to the present invention, Nb-containing a predetermined amount of C and Nb is added.
By controlling the microstructure and precipitate morphology of the steel sheet using IF steel, the n value in the low strain range can be significantly improved, and the crystal grains can be refined. As a result, the steel sheet of the present invention has excellent formability in forming mainly overhanging automobile fenders, side panels and the like, and also has excellent surface properties, resistance to secondary working brittleness, and resistance to surface roughness after press forming. Therefore, it is particularly effective as a steel plate for automobile outer panels.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of an equivalent strain distribution in the vicinity of a fracture-prone portion in a molded product of a front fender model on an actual part scale.

FIG. 2 is a diagram showing an outline of a molded product of a front fender model on a real part scale.

FIG. 3 is a diagram showing a strain distribution in the vicinity of a fracture risk part when molded into a front fender model.

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[Procedure amendment]

[Submission date] August 31, 2001 (2001.8.3)
1)

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0022

[Correction method] Change

[Correction contents]

N: ≦ 0.004% N precipitates as AlN and is rendered harmless. However, even when sol.Al is in the lower limit amount, to precipitate all N as AlN, 0.004%
It must be: Therefore, the N content is specified to be 0.004 % or less.

[Procedure amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0036

[Correction method] Change

[Correction contents]

Ni: 1.0% or less Ni is added in order to make the steel sheet higher in strength as described above. However, if it exceeds 1.0%, the transformation point is greatly reduced, and the low-temperature transformation phase during hot rolling is reduced. It becomes easy to appear. Therefore, the upper limit of the amount of Ni is defined as 1.0%.

[Procedure amendment 3]

[Document name to be amended] Statement

[Correction target item name] 0057

[Correction method] Change

[Correction contents]

[0057]

[Table 1]

[Procedure amendment 4]

[Document name to be amended] Statement

[Correction target item name] 0064

[Correction method] Change

[Correction contents]

(Example 2) A model forming test was carried out on steel No. 3 (inventive example) and No. 10 (comparative example) shown in Table 2 above. In the test, the strain distribution in the vicinity of the risk of fracture when the front fender model shown in FIG. Figure of test result
See Figure 3.

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (Reference) C23C 2/40 C23C 2/40 (72) Inventor Toshiaki Obube 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Steel Pipe F term (reference) 4K027 AA02 AA05 AA23 AB05 AB28 AB42 AC12 AC73 AC87

Claims (6)

[Claims] Claims: 1. The chemical component is mass%, C: 0.0040 to 0.01.
%, Si: ≦ 1.0%, Mn: 0.1-1.0%, P: 0.01-0.07%,
S: ≦ 0.02%, sol.Al: 0.01-0.1%, N: ≦ 0.004%, N
b: containing 0.01 to 0.14%, the balance substantially consisting of iron,
N value of 0.2 at deformation of 10% or less by uniaxial tensile test
A high-strength steel sheet that is at least 1 and satisfies the following expression. YP ≦ −60 × d + 770 where YP represents the yield strength [MPa] and d represents the average ferrite grain size [μm]. 2. The high-strength steel sheet according to claim 1, wherein the chemical component is replaced by mass% and C: 0.0040 to 0.0.
1%, Si: ≤1.0%, Mn: 0.1-1.0%, P: 0.01-0.07
%, S: ≦ 0.02%, sol.Al: 0.01 to 0.1%, N: ≦ 0.004
%, Nb: 0.01 to 0.14%, and 0.05% or less of Ti, and the balance substantially consists of iron. 3. The high-strength steel sheet according to claim 1,
Instead of the description of the chemical components, in terms of mass%, C: 0.0040-
0.01%, Si: ≤1.0%, Mn: 0.1-1.0%, P: 0.01-0.07
%, S: ≦ 0.02%, sol.Al: 0.01 to 0.1%, N: ≦ 0.004
%, Nb: 0.01 to 0.14%, B: 0.002% or less, with the balance substantially consisting of iron. 4. The high-strength steel sheet according to claim 1,
Instead of the description of the chemical components, in terms of mass%, C: 0.0040-
0.01%, Si: ≤1.0%, Mn: 0.1-1.0%, P: 0.01-0.07
%, S: ≦ 0.02%, sol.Al: 0.01 to 0.1%, N: ≦ 0.004
%, Nb: 0.01 to 0.14%, Ti: 0.05% or less, B: 0.002% or less, and the balance is substantially composed of iron. 5. The high-strength thin steel sheet according to claim 1, further comprising, in addition to the described chemical components, ma
High-strength steel sheet characterized by containing one or more of ss%, Cr: 1.0% or less, Mo: 1.0% or less, Ni: 1.0% or less, Cu: 1.0% or less . 6. A high-strength galvanized steel sheet, wherein a zinc-based plating film is provided on the surface of the steel sheet according to claim 1.