JPS592726B2 - Method for producing thermosetting high-strength cold-rolled steel sheet for deep drawing - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a high tensile strength cold rolled steel sheet having thermosetting properties.

High-strength cold-rolled steel sheets for processing are used, for example, as materials for automobile bodies, and by improving the γ value, deterioration in deep drawability is prevented.

Furthermore, thermosetting properties are often required to increase the plastic deformation resistance of the product.

A method for imparting thermosetting properties to cold-rolled steel sheets is described, for example, in JP-A-54
It is disclosed in Japanese Patent Application Laid-open No. 107415, Japanese Patent Application Laid-open No. 107419-1980, and Japanese Patent Application Laid-Open No. 107420-1984.

These inventions mainly relate to mild steel plates,
This suggests adding P (phosphorus) to improve the yield point, or suggests over-aging treatment conditions to improve thermosetting properties.

As described above, it has been well known that P is added to obtain high-strength steel, but the synergistic effect obtained by adding P and N (nitrogen) in combination has not been known. It wasn't.

The present inventors have discovered that by adding a combination of P and N as steel components, the crystal grains of the steel material are expanded and the deep drawability is increased.

Therefore, the object of the invention is to obtain a tensile strength of 35-45 k
The object of the present invention is to obtain a method for producing a high-strength cold-rolled steel sheet that has a tensile strength of g1 mvt class, excellent deep drawability, and also has thermosetting properties.

The characteristics of the present invention are that a composite of P and N is contained as a steel component in order to impart deep drawability, and that H in C (carbon) is o, o2 to 0.0 in order to impart thermosetting properties. 1
5% and perform batch annealing in the α-10γ phase region.

The method of the present invention includes C2 0.020-0.15%, Si:
0.20% or less, Mn: 0.25-1.00%, P
: 0.020 to 0.150%, S: 0.030% or less,
5olkt: 0.020-0.120%, N: 0.0
050 to 0.0160%, the remainder being substantially Fe and unavoidable impurities. A slab obtained by producing steel by a steel ingot method or a continuous casting method is heated by a conventional method, and the finishing outlet temperature is adjusted to Ar3 transformation. or higher, and the coiling temperature is 680°C or lower, the winding diameter is pickled, and cold rolling is performed, and then the cold rolled steel sheet is batch annealed in the temperature range of A1 to A3 transformation point. This is a characteristic feature.

Next, we will discuss the reasons for limiting the ingredients.

The reason for setting C to 0.020% and 0.15% is that 0.020% is the lower limit that can be melted without vacuum degassing, and the upper limit is 0.020%.
．． The reason for setting the content to 15% or less is to prevent reduction in drawability (γ value) due to excessive C during recrystallization.

Si is effective in stabilizing 5otkl and increasing the strength level as a high-strength steel, but if it exceeds 0.20%, surface flaws due to scale generated during hot rolling and surface coloring defects due to annealing will occur, so Si is .20% or less.

Mn is effective in increasing strength, but if it is less than 0.25%, there is a risk of red-hot embrittlement, while if it exceeds 1.00%, it reduces drawability (γ value) and at the same time reduces the amount of solid solute carbon after annealing. This is not preferable because it reduces the thermosetting property.

Therefore, Mn was set to 0.25% to 1.00%.

P is necessary to increase the strength level and the γ value, and at the same time increases the solid solution carbon after annealing and increases thermosetting properties, so a large content is desirable.

However, excessive addition may cause secondary work embrittlement after deep drawing, so the upper limit was set at 0.150%.

S forms sulfide-based inclusions, and a low S content is desirable in order to reduce molding problems caused by these sulfide-based inclusions.
The upper limit was set at 0.030%.

5otkl is required to be 0.020% or more in order to form AtN and make the crystal grains after annealing into elongated grains to obtain a high γ value.

On the other hand, if it exceeds 0.120%, it becomes difficult to obtain expanded grains again, so the content was set at 0.120% or less.

N forms elongated grains through the formation of AtN and improves the γ value, so normally 0.0020 to 0.0050% is required, but in the case of P-added steel, it is further required to be 0.0050 to 0.005%.
It has been found that by incorporating a large amount of N in a complex manner, it is effective to obtain a high γ value equivalent to or higher than that of ordinary A4 killed steel, even though it is a high strength steel, and the preferable range is 0.0050 to ゛0. .0160%.

The reason why the annealing temperature was set to the A3 transformation point is to form large cementite as the α+γ phase region shown by the shaded area in Fig. 1. This is because most of the carbon in the steel is made into large lumpy cementite. By increasing the distance between cementites, the solid solution carbon in the ferrite can be precipitated during the cooling process.
This is to allow solid solution carbon to remain in the ferrite after annealing, thereby imparting thermosetting properties as shown in FIG.

Patch annealing is used for patch annealing in order to precipitate AtN by heat retention and obtain a high γ value, patch annealing may be performed and either tight coil or loose coil annealing may be used.

The cooling rate is 200~b on average between 600~250℃ means are not required.

This is because by limiting the contents of C, P, and Mn, enough solid solution carbon remains to have thermosetting properties at a cooling rate within this range.

Next, examples of the present invention will be described.

Heating a slab of ingredients shown in Table 1 to 1150°C or higher,
Hot rolling is carried out at a finish exit temperature of A7's transformation point or higher,
After winding at a winding temperature of 680° C. or less, pickling and cold rolling were performed, annealing was performed in a patch type annealing furnace at a temperature range of A1 to A3 transformation points, and normal skin pass rolling was performed.

Samples were taken from each of these steel plates and tested for various properties.

The results of this test are shown in Table 1. The test results of the conventional method are also shown for comparison.

As is clear from Table 1, according to the method of the present invention, P,
Tensile strength is 36-471y/mm due to composite H content
Despite its high value, it also has excellent deep drawability (γ value 1
．． 6 or more), thermosetting properties of 4 kg/- or more could be imparted by roughly controlling the C content and annealing conditions.

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[Brief explanation of drawings]

FIG. 1 is a phase diagram of the Fe-C system, and FIG. 2 is a graph of experimental data showing the relationship between annealing temperature and hardening amount after box annealing.

Claims (1)

[Claims] IC: 0.026-0.15%, Si: 0.20%
Below, Mn: 0.25-1.00%, P: 0.
020-0.150%, S: 0.030% or less, 5ot
At: 0.020~0.120%, N: 0.0050~
A method for producing a high-strength cold-rolled steel sheet for deep drawing with thermosetting properties, characterized by batch annealing a cold-rolled steel sheet consisting of 0.0160% Fe and unavoidable impurities in the temperature range of A1 to A3 transformation points. .