JPS6126756A - Dead soft steel sheet having high suitability to chemical conversion treatment - Google Patents

Abstract

PURPOSE:To obtain a dead soft steel sheet having high suitability to chemical conversion treatment by incorporating a specified amount of Nb into a dead soft steel sheet contg. a specified amount or below of Ti basing on the total amount of C and N. CONSTITUTION:The composition of a dead soft steel is composed of, by weight, <=0.01% C, <=0.01% N, <=0.15% Si, <=0.6% Mn, <=0.1% Al, >=4X(C+N)% Ti, 1/ 6XTi-31/4XC% Nb and the balance Fe with inevitable impurities. The amounts of C, N, Si and Mn are restricted as mentioned above so as to ensure press workability, and Al is added by only an amount necessary for deoxidation.

Description

[Detailed Description of the Invention] (Industrial Application Field) The technical content described in this specification regarding improving the chemical conversion treatment properties of ultra-low carbon steel sheets containing carbonitrides is a technology that allows chemical conversion treatment to be performed without impairing the material properties of the steel sheets. The aim is to provide benefits by expanding the uses of the steel plate.

(Conventional technology) Ti-added ultra-low carbon steel sheet has a low YP due to its material properties.

It has excellent properties such as high ductility (Special Publication No. 44-1806
However, when this is to be used, for example, as an outer panel for an automobile, there is a problem in that the chemical conversion treatment property is poor, and the parts where it can be used are limited.

This is mainly due to the Ti oxide film covering the steel sheet surface, which is caused by the fact that a large amount of Ti in the steel sheet stays at high temperature for a long time after hot rolling and/or During recrystallization annealing after cold rolling, Ti is concentrated on the surface of the steel sheet and formed into an oxide film by oxygen in the air, which has the adverse effect of further promoting the concentration.

As a way to prevent this drawback, Mn and S are added to the surface.

Means for concentrating P etc. (for example, JP-A-59-74259
However, if the amount of addition of all three elements is increased, the quality of the material deteriorates, and the efforts to achieve ultra-low carbon and high material properties by adding Ti are wasted. There is.

(Problems to be Solved by the Invention) It is an object of the present invention to provide a steel that has good chemical conversion treatment properties without causing the above-mentioned material deterioration.

(Means for solving problems) This invention has the following conditions: C≦0.0t wt%, N≧0.01 wt%, Si≦0.15 wt%, Mn≦o,e wt%, A°C≦0.10 wt% , T1≧4. Including OX ((C) + (N)), and more than ("l/6)X (Ti), it is (31/4
) x (C) It is an ultra-low carbon thin steel sheet having good chemical conversion treatability, which is characterized by containing Nb or less and having a composition of unavoidable impurities and Fe.

This invention was inspired by a phenomenon discovered while investigating the materials and chemical conversion treatability of Nb, Nb + Ti, and 7i-added steel.

That is, steel with the chemical composition shown in Table 1, that is, steel containing a specific amount of Ti or more with respect to the carbon and nitrogen contents, was melted in a laboratory, and then bloomed and rolled.
Hot-rolled and then cold-rolled steel was electrolytically degreased and then heated at 830℃ for 20 minutes.
After continuous annealing for seconds, the material properties and phosphate treatability were investigated.

The phosphorus M salt treatment property was evaluated by the pinhole area ratio (expressed in PHE) when a steel plate was degreased, washed with water, and treated with phosphate, and the pinhole test described below was performed.

In the pinhole test, a filter paper soaked with a reagent that reacts with iron ions to form a color is brought into close contact with the test surface to detect areas where phosphoric acid crystals remain on the surface of the steel plate, and image analysis is performed to determine the pinhole area ratio. It was quantified as PHE.

It has been separately confirmed that when PHE≦3.0, good curing treatment is exhibited.

Figure 1 shows the relationship between Ti and Nb contents and PHE.Firstly, steels with many Ti-containing steels have high PHE but Nb
As the content increases, the PHE decreases, and for any Ti-containing steel, the Nb content is one of the Ti-containing steels.
It has been found that when the value is /6 or more, the PHE becomes 3 or less and good chemical conversion treatment properties are exhibited.

(Function) Ti preferentially combines with S, N, and C in steel to form precipitates, but at the same time it concentrates on the surface of the steel sheet and forms an oxide film, which is already known to deteriorate chemical conversion properties. I touched it. Therefore, if the amount of Ti added is sufficiently small, there is no problem with the chemical conversion processability, regardless of the material, but if the amount of Ti increases, the chemical conversion processability deteriorates.

In order to obtain a non-aging steel plate, the amount of Ti added must be at least four times < (C) + (N)).

On the other hand, as the amount of Nb added increases, chemical conversion treatment properties improve;
An increase in bm first suppresses the surface concentration of Ti, and furthermore, unless Ti is added in sufficient excess relative to C, N, and Sl in the steel, Ti cannot completely fix carbon, so Nb It is presumed that the NbC-based precipitates formed on the surface of the steel sheet provide phosphate crystal nuclei, thereby improving chemical conversion treatment properties.

It is known that if the solid solute carbon is less than (i, o05vt% (hereinafter simply expressed as %)), even if it extends in the steel, it will be accommodated in the grain boundaries, so it can become a completely non-aging steel sheet. There is.

However, if too much Nb is contained relative to the C content, the chemical conversion properties will deteriorate, so it must be limited to 31/4 times the C content or less.

In this invention, the C content was limited to 0.01% or less and the N content was limited to 0.01% or less in order to ensure press workability in terms of the material, and for the same reason, the Mn content was also limited to 0.01% or less.
And for the same reason, the 3i content is 0.6%, respectively.
It is limited to a range of 0.15% or less. Further, octaβ can be added in a necessary amount for deoxidation, but it may be 0.1% or less. The steel plate of this invention is manufactured through the steps of continuous casting, hot rolling, cold rolling, and annealing according to conventional methods.

In this invention, unavoidable substances are P and S, which are allowed at 0.15% or less and 0.02% or less, respectively.

(Example) Cold rolled steel sheets having the chemical composition shown in Table 2 were manufactured by box annealing and continuous annealing.

Its chemical conversion treatability (PHE) and material (AI: aging index,
kg/l11m2) is shown in Table 3, and the chemical conversion treatment here was adjusted to a total acidity of 14.3 and a free acidity of 0.5 at 55°C using BT3112 manufactured by Nippon Parriki Rising Co., Ltd.
Sprayed with Subu for 120 seconds. Phosphate treatment properties are as described above.
It was evaluated by HE.

1. 0AL: Continuous annealing 830℃, 20 seconds soaking B at
ch: Box annealing 690℃, 5 hours soaking 2, 7.5
% Increased stress due to aging treatment at 100℃ for 30 minutes after pre-strain (
kg/II1m2) First, when the Ti content is less than 4 times ((C) + (N)) (NO05 steel), AI>0, and the steel plate does not become a so-called non-aging steel plate. Even if high-grade ultra-low carbon steel is used, which has been melted so that both carbon and nitrogen are below O and oi%, its use will be limited if non-aging properties cannot be ensured.Therefore, Ti needs to be at least 4 times as much as C+N. shall be.

On the other hand, as Til increases, its chemical conversion properties deteriorate, but N
By adding b to 1/6 or more of Ti, chemical conversion treatment properties are improved. If Nb is less than 1/6 of [T1] (
No, 6 steel), and if it exceeds 31/4 of (C) (
No. 7 steel) all have a P l-I E of more than 3 and are poor in chemical conversion treatment properties.

In addition, steel with C>0.01% or Neo, 01% can be made non-aging by adding enough Ti, but in this case, the yield strength is high and the elongation, that is, ductility is poor, so adding a large amount of Ti is necessary. There is no point in adding a large amount of Nb to improve the chemical conversion treatment properties, which leads to greater deterioration of the material (increased yield point, elongation (effects of the invention)). According to this method, a remarkable improvement in chemical conversion treatability is realized without deterioration of the excellent material properties of the T1-added ultra-low carbon steel sheet.

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between Ti and Nb contents and PI-IE.

Claims (1)

[Claims] 1. C≦0.01wt%, N≦0.01wt%, Si≦0.15wt%, Mn≦0.6wt%, Al≦0.10wt%, Ti≧4.0×( [C] + [N]), and also (1/6) × [Ti] or more and (31
/4) x [C] An ultra-low carbon thin steel sheet having good chemical conversion treatability, characterized by containing Nb or less and having a composition of unavoidable impurities and Fe.