JP2002003994A - High strength steel sheet and high strength galvanized steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength steel sheet capable of being inexpensively manufactured, excellent in surface characteristics, in secondary working processability and weldability while keeping both superior bulge formability and surface roughing resistance after pressing. SOLUTION: The high strength steel sheet has a chemical composition consisting of, by mass, 0.0040-0.015% C, <=1.0% Si, 0.7-3.0% Mn, 0.02-0.15% P, <=0.02% S, 0.01-0.1% sol.Al, <=0.004% N, 0.01-0.2% Nb and the balance essentially iron with inevitable impurities, and further, n-value at <=10% deformation by the uniaxial tensile test and average ferrite grain size d[μm] satisfy inequalities n-value >=-0.00029×TS+0.313 and YP<=-60×d+770, respectively (wherein, TS represents tensile strength [MPa]; and YP represents yield strength [MPa]).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength thin steel sheet having a tensile strength of 390 MPa or more, which is mainly used for automobile outer panel panels such as hoods, fenders, side panels, etc., which are mainly formed by overhanging. The present invention also relates to a high-strength cold-rolled steel sheet or a high-strength galvanized steel sheet having excellent surface roughness, surface properties, and secondary work brittleness resistance after press forming.

[0002]

2. Description of the Related Art In recent years, the automobile industry has been actively promoting the strengthening of steel sheets for the purpose of improving safety, the reduction of the number of parts by integrating parts, and the elimination of a pressing process. For steel sheets for use, high-strength steel sheets having extremely high press-formability are required.

[0003] Conventionally, the press formability of cold-rolled steel sheets has been studied mainly from the viewpoints of deep drawability and stretchability. Regarding the deep drawability, the main focus is on increasing the r value.For example, JP-A-5-78784 discloses that while actively adding Mn and Cr to a Ti-added ultra-low carbon steel sheet,
343MPa ~ 490MPa by controlling the amount of P and P
A technique has been proposed in which a good r value and elongation can be obtained with a. According to the description in the gazette, this technique can increase the strength while suppressing the increase in yield stress, and can obtain a high-strength cold-rolled steel sheet having good surface shape and excellent dent resistance.

[0004] Japanese Patent Application Laid-Open No. 8-92656 discloses that a cold rolled and cold rolled sheet is produced by performing a recrystallization treatment on a hot rolled steel sheet which has been hot lubricated and rolled at 3 to 500 ° C. using an ultra-low carbon steel. A technique has been proposed in which a cold-rolled steel sheet having a high r value of 3.0 or more can be obtained by performing recrystallization annealing. This technique is to increase the deep drawability by adding Nb and B while greatly reducing C, S and N, and performing recrystallization annealing after cold rolling at 700 to 950 ° C. It is.

[0005]

SUMMARY OF THE INVENTION However, Japanese Patent Application Laid-Open No. 5-7878
When the cold rolled steel sheet described in JP-A No. 4 and JP-A-8-92656 is applied to a side panel or the like in which overhang forming is performed, at the punch shoulder where plane strain overhang is performed, fracture occurs due to insufficient strain propagation. May occur. With respect to the breakage in such stretch forming, it is impossible to evaluate with the same total elongation and n-value as the conventional soft material due to the increase in the strength of the material, and in fact, no appropriate measures can be taken.

[0006] Further, since a large amount of Cr is added to the steel sheet described in the above publication or annealing is performed on a hot-rolled steel sheet,
There is a problem in cost. Further, it is described that the steel sheet disclosed in Japanese Patent Application Laid-Open No. 8-92656 is to be subjected to cold-rolled sheet recrystallization annealing at 700 to 950 ° C. after cold rolling. 880 ° to 910 °, which is very high temperature annealing for recrystallization annealing. Therefore, this steel plate
The crystal grain size is large, and the surface properties after press molding, particularly the surface roughness, are concerned, and it is not suitable for strict surface materials such as automobile outer panels. As described above, it is difficult in the related art to achieve both the press formability and the good surface properties after pressing.

Further, in the technique described in Japanese Patent Application Laid-Open No. 8-92656, N is specified as low as 0.0020 wt% or less, and most of the steels in Examples are 3 to 5 ppm. N to this level
Is difficult to implement as an industrial method for producing steel. In addition, most of C and S are less than 5 ppm, which is also difficult in the production of ordinary steel for sheet metal, and requires special hot metal treatment and degassing treatment. 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.

As in these prior arts, increasing the r value is effective for a part requiring deep drawability accompanied by shrinkage flange deformation.
There was a problem that sufficient moldability could not be obtained. Regarding the overhang property, for example, “Press processing of thin steel sheet, (Jikkyo Shuppan),
As described in “p.161”, it has been important to increase the total elongation and the n value.

However, the total elongation of a material 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.

An object of the present invention is to solve the above-mentioned problems of the conventional cold-rolled steel sheet for press forming, to be able to be manufactured at low cost, and to have both excellent stretch formability and resistance to surface roughness after pressing. Further, it is an object of the present invention to provide a high-strength thin steel sheet having excellent surface properties, secondary work brittleness resistance and weldability, particularly a high-strength thin steel sheet having a tensile strength of 390 MPa or more.

[0011]

The above object is achieved by the following invention. In the first invention, the chemical component is mass%
And C: 0.0040 to 0.015%, Si: 1.0% or less, Mn: 0.7 to 3.
0%, P: 0.02 to 0.15%, S: ≤ 0.02%, sol. Al: 0.01 to
0.1%, N: ≤ 0.004%, Nb: 0.01 to 0.2%, the balance is substantially composed of iron, and the n value and the ferrite average particle size d [μm] at a deformation of 10% or less by a uniaxial tensile test are shown. ,
A high-strength thin steel sheet characterized by satisfying the following expressions (1) and (2). n value ≧ −0.00029 × TS + 0.313 (1) YP ≦ −60 × d + 770 (2) where TS represents tensile strength [MPa] and YP represents yield strength [MPa].

The present invention has been made while examining in detail the factors governing the moldability, taking as an example a front fender in which the overhang is mainly formed. In the process, it was found that in the overhang forming main body, the generated strain was small at the punch bottom contact portion, and the strain was concentrated near the punch shoulder and the die shoulder on the side wall portion.

Thus, by slightly increasing the amount of strain generated in the steel sheet in contact with the punch bottom over a wide range, strain concentration on the side wall portion near the punch shoulder and the die shoulder can be reduced. Therefore, instead of the n value in the high strain region, which has been conventionally used for evaluation of the overhang property, the n in the low strain region corresponding to the amount of strain generated at the punch bottom contact portion.
It was found that improving the value is effective. As a result of the study, it was found that the lower limit of the n value had to be determined according to the TS, and the above equation (1) was obtained. 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, in the case of strict surface materials such as automobile outer panels, it is necessary to ensure excellent surface properties even after severe press molding. It has been found that crystal grains need to be refined in order to ensure high stretch formability and prevent roughening of the surface after press molding. As a result of the study, it was found that the ferrite average grain size d needed to be determined according to YP, and the above-mentioned formula (2) was obtained.

Next, the reasons for limiting the chemical components of the present invention will be described. C: 0.0040 to 0.015% (mass%, the same applies hereinafter) C forms a carbide with Nb, affects the material strength and the work hardening in the low strain range during panel forming, and increases the strength and improves the formability. . If the C content is less than 0.0040%, no effect is obtained, and if it exceeds 0.015%, a high n value in the strength and low strain range can be obtained, but ductility is reduced. Therefore, the amount of C is specified in the range of 0.0040 to 0.015%.

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

Mn: 0.7-3.0% Mn is an effective element for precipitating S in steel as MnS to prevent hot cracking of the slab and strengthen the steel without deteriorating plating adhesion. . S is precipitated as MnS,
0.7% or more is required to ensure strength. Mn 3.0
%, The moldability deteriorates. Therefore, the Mn content is specified in the range of 0.7 to 3.0%.

P: 0.02-0.15% P is an element effective for strengthening steel, and this effect appears when added at 0.02% or more. However, if P is added in excess of 0.15%, the alloying processability of galvanizing is deteriorated. Therefore, the P content is defined in the range of 0.02 to 0.15%.

S: ≦ 0.02% S is present in steel as MnS, and if contained in excess of 0.02%, the ductility is degraded. Therefore, the amount of S is regulated to 0.02% or less.

Sol. Al: 0.01-0.1% Al is required to be 0.01% or more in order to precipitate N in steel as AIN and not to leave solute N. When sol.Al is added in excess of 0.1%, ductility is reduced due to solid solution Al. Therefore, the amount of sol.Al is restricted to the range of 0.01 to 0.1%.

N: ≦ 0.004% N precipitates as AlN and is rendered harmless. However, even when the above sol.
Must be 4% or less. Therefore, the N content is specified to be 0.004% or less.

Nb: 0.01 to 0.2% Nb is an important element of the present invention, and reduces the amount of solute C by forming NbC and improves the n value in a low strain region by an appropriate amount of solute Nb. Equation (1) described above is surely satisfied. However, if the Nb content is less than 0.01%, there is no effect, and if the Nb content exceeds 0.2%, the yield strength increases and n
This leads to lower values and lower ductility. Therefore, the Nb amount is 0.01 to 0.2
%.

According to a second invention, in the high-strength thin steel sheet according to the first invention, the chemical composition is replaced by mass% instead of the description.
C: 0.0040 to 0.015%, Si: 1.0% or less, Mn: 0.7 to 3.0
%, P: 0.02-0.15%, S: ≦ 0.02%, sol.Al: 0.01-0.
A high-strength steel sheet containing 1%, N: ≤ 0.004%, Nb: 0.01 to 0.2%, and Ti: 0.05% or less, with the balance substantially consisting 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 thin steel sheet according to the first aspect, the chemical composition is replaced by mass% by mass%.
C: 0.0040 to 0.015%, Si: 1.0% or less, Mn: 0.7 to 3.0
%, P: 0.02-0.15%, S: ≦ 0.02%, sol.Al: 0.01-0.
A high-strength thin steel sheet containing 1%, N: ≤ 0.004%, Nb: 0.01 to 0.2%, and B: 0.002% or less, with the balance substantially consisting 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, in the high-strength thin steel sheet according to the first aspect, the chemical components are replaced by mass% by mass%.
C: 0.0040 to 0.015%, Si: 1.0% or less, Mn: 0.7 to 3.0
%, P: 0.02-0.15%, S: ≦ 0.02%, sol.Al: 0.01-0.
1%, N: ≦ 0.004%, Nb: 0.01-0.2%, Ti: 0.05% or less, B: 0.002% or less, the balance being substantially iron It is.

According to the present invention, a combination of Ti and B is 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 sheets according to the first to fourth aspects of the present invention, wherein, in addition to the chemical components, further, in mass%, Cr: 1.0% or less, Mo: 1.0% or less, 1.0
% Or less, and Cu: 1.0% or less.

In the present invention, one or more of Cr, Mo, Ni and Cu are further added to the chemical components of the above-mentioned invention to make the steel sheet higher in strength. 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 as a solid solution strengthening element. However, 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. However, 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.

A sixth invention is a high-strength galvanized steel sheet according to the first to fifth invention, wherein a zinc-based plating film is provided on the surface of the steel sheet.

According to the present invention, a corrosion resistance is imparted to the steel sheet by further applying a zinc-based plating film to the surface of the steel sheet according to 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.

[0038]

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 precipitate, and to obtain 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 the surface properties and plating adhesion.

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

Although the reason why Nb improves the n value in a low strain range is not always clear, the following findings were obtained when the structure was observed in detail using an electron microscope. If the amounts of Nb and C are properly controlled, a large amount of NbC precipitates in the crystal grains,
Precipitate depletion zone where no precipitate exists near the grain boundary (hereinafter PF)
Z) is formed, and since the precipitates are depleted in this PFZ, the strength is lower than that in the grains, it is possible to plastically deform at a low stress level, and a high n value is obtained in the low strain region It is inferred. For this purpose, it is effective to control the atomic equivalence ratio of Nb and C to an appropriate value. As a result of intensive studies, in order to obtain such a desirable precipitation form in the present invention, Nb / C ( It has been found that regulating the atomic equivalent ratio) to a range of 1.3 to 2.5 is more preferable for improving the n value.

Thus, the high-strength cold-rolled steel sheet of the present invention
Since a special element such as Cr is not added in a large amount and can be manufactured by a normal process as described later, it is inexpensive. Further, the steel of the present invention is excellent in terms of weldability and resistance to secondary working embrittlement since the crystal grains are refined by NbC precipitation.

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.

As for B, as described above, the steel of the present invention exhibits excellent secondary work brittleness resistance even without B addition. Therefore, when B is added, it is desirable to add B to minimize the decrease in formability.
It is preferable to control the addition amount in the range of 0.0001 to 0.001%.

As a manufacturing method, after smelting the steel whose composition has been adjusted as described above, a slab is formed by continuous casting.
The slab is reheated or hot-rolled directly to produce a hot-rolled steel sheet. An ordinary cold rolled steel sheet manufacturing process in which the hot rolled steel sheet is pickled, then cold rolled and then annealed can be applied.

Further, if necessary, a zinc-based plating such as electrogalvanizing or hot-dip galvanizing may be applied to the surface, and the same effect as in the case of a cold-rolled steel sheet can be obtained in terms of press formability. Examples of the zinc-based plating include pure zinc plating, alloyed zinc plating, zinc-Ni alloy plating, and the like, and an organic coating treatment may be further performed after the plating. It can be a steel plate.

The manufacturing method may be described below. For example, as for hot rolling conditions, finish rolling is performed in a temperature range from the Ar ₃ transformation point to 960 ° C. in view of surface quality and material uniformity. Further, the hot-rolled steel sheet is preferably wound at 680 ° C. or lower from the viewpoint of descaling by pickling and stability of the material. Further, the winding temperature after hot rolling is preferably 600 ° C. or higher when performing continuous annealing (CAL or CGL) after cold rolling, and 540 ° C. or higher when performing box annealing (BAF). In addition, in order to secure the hot rolling finishing temperature at the time of manufacturing a thin material, the rough bar can be heated by a bar heater during hot rolling.

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.

The annealing temperature is preferably in the range of 780 to 880 ° C. when the cold-rolled steel sheet is annealed by continuous annealing, and 680 ° C. to 750 ° C. when the annealing is performed by box annealing.

Here, the tensile properties specified in the steel sheet of the present invention,
The component composition 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. From FIG. 1, it can be seen that the amount of generated strain near the punch shoulder and die shoulder on the side wall portion is large and rises to about 0.3, but the generated strain on the punch bottom is small at 0.1 or less.

Thus, if the amount of strain generated in the steel sheet in contact with the punch bottom is slightly increased over a wide range, the concentration of strain on the side wall portion near the punch shoulder and die shoulder can be relaxed, and breakage at this portion can be prevented. become. To do so, set the n value in the low strain region of 10% or less to TS
For [MPa], it has been found for the first time that the structure should be controlled so as to satisfy the above expression (1). Here, as the n value, an n value calculated by a two-point method of 1% and 10% of the nominal strain of uniaxial tension is used.

In order to prevent surface roughness after pressing, in order to obtain more excellent surface properties in the present invention, the condition (2) for the yield strength YP [MPa] and the average ferrite particle size d [μm] is defined as follows: More preferably, the following equation (2 ′) is satisfied. YP ≦ -120 × d + 1240 (2 ')

[0054]

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 ~ 13,
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)
After pickling, 50-85% cold rolling was performed.

[0055]

【table 1】

Then, continuous annealing (annealing temperature 800 to 840)
C), box annealing (annealing temperature 680 ° C to 750 ° C), and continuous annealing + hot dip galvanizing (annealing temperature 800 to 840 ° 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 + hot-dip galvanizing was subjected to temper rolling at a rolling reduction of 0.7%.

The mechanical properties and grain size of these steel sheets were investigated. In addition, the front fender was press-formed with the above-mentioned steel sheet, and the breaking limit cushion force was investigated. In addition, the presence or absence of occurrence of rough skin after press molding was evaluated.

Further, the secondary working brittle transition temperature was measured. Here, a blank with a diameter of 100 mm was punched from a steel plate and deep-drawn into a cup shape as the primary processing (drawing ratio 2.
0), ear trimming was applied to make the cup height 30 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.

[0059]

[Table 2]

The steel sheets Nos. 1 to 6 of the present invention had high breaking limit cushion caps of 65 tons or more, and exhibited excellent overhang properties.
On the other hand, the comparative materials No. 9 and 10 have the n value in the conventional 10 to 20% strain range.
Although it showed a high value of 0.23 or more, the n value in the 1 to 10% strain range was 0.
Since it was smaller than 18, it broke with a low cushion cap of 50 tons or less. In addition, comparative materials No. 10, 11, 13 to 15
(Steel Nos. 8, 9, 11 to 13) have an excessively large amount of Ti (and a large amount of Si in steel No. 8), and thus have extremely poor surface properties.

The steel of the present invention has a vertical crack transition temperature of -65 ° C. or lower at any level, and shows very good secondary work brittleness resistance. Further, since the steel of the present invention had fine crystal grains, no rough surface occurred after press forming. Further, it was confirmed that the steel of the present invention was excellent in surface quality after hot-dip plating, workability of a welded portion, and fatigue properties.

Steel No. 3 material shown in Table 2 above (Example of the present invention)
And a No. 10 material (comparative example) were subjected to a model forming test. In the test, the strain distribution in the vicinity of the risk of fracture when the front fender model of FIG. 2 was formed under the condition of 40 tons of cushion cushion was measured. The test results are shown in FIG.

In the example of the present invention (N0.3 material, symbol ● 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, symbol ○ 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.

[0064]

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 rough surface resistance after press forming, and furthermore, secondary work brittleness resistance, surface properties, Also has excellent weldability and very good overall properties.

[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] Claim 4

[Correction method] Change

[Correction contents]

[Procedure amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0043

[Correction method] Change

[Correction contents]

Thus, the high-strength cold-rolled steel sheet of the present invention
Since a special element such as Cr is not added in a large amount and can be manufactured by a normal process as described later, it is inexpensive. Further, the steel of the present invention is excellent in weldability and secondary work brittleness resistance since the crystal grains are refined by NbC precipitation.

[Procedure amendment 3]

[Document name to be amended] Statement

[Correction target item name] 0047

[Correction method] Change

[Correction contents]

Further, if necessary, a zinc-based plating such as electrogalvanizing or hot-dip galvanizing may be applied to the surface, and the same effect as in the case of a cold-rolled steel sheet can be obtained in terms of press formability. Examples of the zinc-based plating include pure zinc plating, alloyed zinc plating, zinc-Ni alloy plating, and the like, and an organic coating treatment may be further performed after the plating.

[Procedure amendment 4]

[Document name to be amended] Statement

[Correction target item name] 0053

[Correction method] Change

[Correction contents]

In order to prevent surface roughness after pressing, in order to obtain excellent surface properties in the present invention, the yield strength YP
It is necessary to satisfy the expression (2) of the conditions regarding [MPa] and the average ferrite particle diameter d [μm] .

[Procedure amendment 5]

[Document name to be amended] Statement

[Correction target item name] 0055

[Correction method] Change

[Correction contents]

[0055]

[Table 1]

[Procedure amendment 6]

[Document name to be amended] Statement

[Correction target item name] 0060

[Correction method] Change

[Correction contents]

The steel sheets Nos. 1 to 6 of the present invention had high breaking limit cushion caps of 65 tons or more, and exhibited excellent overhang properties.
On the other hand, the comparative materials No. 9 and 10 have the n value in the conventional 10 to 20% strain range.
Although it showed a high value of 0.23 or more, the n value in the 1 to 10% strain range was 0.
Since it was smaller than 18, it broke with a low cushion cap of 50 tons or less. In addition, comparative materials No. 10, 11, 13 to 15
(Steel Nos. 8, 9, 11 to 13) have remarkably poor surface properties due to an excessive amount of Ti.

Claims (6)

[Claims] Claims: 1. The chemical component is mass%, C: 0.0040 to 0.01.
5%, Si: 1.0% or less, Mn: 0.7 to 3.0%, P: 0.02 to 0.15
%, S: ≦ 0.02%, sol.Al: 0.01 to 0.1%, N: ≦ 0.004
%, Nb: 0.01 to 0.2%, the balance being substantially composed of iron. The n value and the average ferrite grain size d [μm] at a deformation of 10% or less by a uniaxial tensile test are expressed by the following formula (1). A high-strength steel sheet satisfying (2). n value ≧ −0.00029 × TS + 0.313 (1) YP ≦ −60 × d + 770 (2) where TS represents tensile strength [MPa] and YP represents yield strength [MPa]. 2. The high-strength steel sheet according to claim 1,
Instead of the description of the chemical components, in terms of mass%, C: 0.0040-
0.015%, Si: 1.0% or less, Mn: 0.7 to 3.0%, P: 0.02 to
0.15%, S: ≦ 0.02%, sol.Al: 0.01-0.1%, N: ≦ 0.0
A high-strength steel sheet comprising: 04%, Nb: 0.01 to 0.2%, Ti: 0.05% or less, and the balance substantially consisting of iron and unavoidable impurities. 3. The high-strength thin steel sheet according to claim 1,
Instead of the description of the chemical components, in terms of mass%, C: 0.0040-
0.015%, Si: 1.0% or less, Mn: 0.7 to 3.0%, P: 0.02 to
0.15%, S: ≦ 0.02%, sol.Al: 0.01-0.1%, N: ≦ 0.0
A high-strength thin steel sheet containing 04%, Nb: 0.01 to 0.2%, and B: 0.002% or less, with the balance being substantially composed 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-
0.015%, Si: 1.0% or less, Mn: 0.7 to 3.0%, P: 0.02 to
0.15%, S: ≦ 0.02%, sol.Al: 0.01-0.1%, N: ≦ 0.0
04%, Nb: 0.01-0.2%, Ti: 0.05% or less, B: 0.002%
A high-strength thin steel sheet comprising the following, with the balance substantially consisting of iron. A high-strength thin steel sheet excellent in stretch formability and surface roughening resistance characterized by containing B in an amount of 0.002% or less. 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 of the high-strength thin steel sheet according to claim 1.