JPS61276951A - Cold rolled steel sheet for press working having superior suitability to chemical conversion treatment - Google Patents

Abstract

PURPOSE:To obtain a cold rolled steel sheet for press working having improved suitability to chemical conversion treatment as well as high press formability by adding a very small amount of Ca to a steel having a very low C content and by further adding Ti or Nb. CONSTITUTION:This cold rolled steel sheet contains, by weight, <=0.050% C, <=1.0% Si, <=1.0% Mn, <=0.15% P, 0.010% S, 0.010-0.100% Al, <=0.0050% N, 0.010-0.050% Ti and/or 0.004-0.30% Nb and 0.0009-0.0025% Ca. A steel contg. said components refined as usual in a converter or an electric furnace is formed into a slab by ingot making-cogging or continuous casing, and the slab is hot rolled, cold rolled and annealed to manufacture the cold rolled steel sheet. This steel sheet has superior suitability to chemical conversion treatment as well as superior deep drawability and aging resistance.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention relates to a cold rolled steel sheet for press working that has excellent chemical conversion properties, and in particular, to a cold rolled steel sheet for press forming that has excellent chemical conversion properties, and in particular to a Ti+Nb-added ultra-low C network that has excellent press formability. This is a major improvement.

Now, steel sheets for automobiles are required to have good press formability, especially deep drawability and aging resistance.

On the other hand, it is also required to have excellent chemical conversion treatment properties as a pre-painting treatment. This is because this type of steel sheet is subjected to chemical conversion treatments such as zinc phosphate treatment and chromate treatment prior to painting, and the adhesion and rust resistance of the paint film strongly depend on such chemical conversion treatment film. Therefore, it is no exaggeration to say that the quality of chemical conversion treatment determines the quality of corrosion resistance of a steel sheet.

(Prior art) By the way, as an automotive copper sheet with excellent deep drawability and aging resistance, so-called Ti and Nb-added ultra-low C steel is produced by adding Ti and Nb to ultra-low C steel to fix C and N in the steel. Generally, ultra-low C steel has high cleanliness to begin with, so
Compared to conventional low C-AA killed steels, there is a problem in that zinc phosphate and chromate films are less likely to be formed during chemical conversion treatment, and this tendency is particularly severe when Ti and Nb are added.

As a solution to this problem, Japanese Patent Laid-Open No. 58-52456 proposed a method of increasing the amount of Mn and S in the steel, but with this method, workability deteriorates as the amount of Mn and S increases. There remained a problem.

(Problems to be Solved by the Invention) As described above, hitherto, no steel plate has been found that is excellent in both steel plate material and chemical conversion treatment properties, and the development of such a steel plate has been desired.

The present invention advantageously meets the above requirements, and
The purpose of the present invention is to propose a cold-rolled steel sheet for press working that takes advantage of the excellent press formability of Nb-added ultra-low C steel and effectively improves chemical conversion treatment properties.

(Means for solving the problem) In order to solve the above problem by adjusting the composition of steel, the inventors tried adding various elements.
The addition of Ca, which has not traditionally been applied to this type of steel sheet as an element that tends to cause rust, is effective.In other words, by adding a small amount of Ca, the activity of the chemical conversion treatment liquid on the surface of the steel sheet increases. As a result, we obtained the knowledge that chemical conversion treatment properties were significantly improved.

This invention is based on the above knowledge.

That is, this invention provides C: 0.0050wtχ or less (hereinafter expressed in %), St: 1.0% or less, Mn
: 1.0! Below, P: 0.15% or less, S:
Contains 0.010χ, ^ρ: 0.010 to 0.100χ and N: 0.0050% or less, and Ti:
0.010-0.050χ and Nb: 0.004
-0.030χ, further contains Ca: 0.0009 to 0.0025χ, and the remainder is substantially Fe, and has excellent chemical conversion properties. This is a cold-rolled steel plate for processing.

This invention will be explained in detail below.

The reason why the composition of the steel sheet is limited to the above range in this invention is as follows.

C: 0.0050% or less In order to improve the lower value and ductility, which are most important for steel sheets for processing, the smaller the C content, the better. Therefore, the C content is preferably 0.0035% or less than 0.0050%. limited to a range. As C increases, large amounts of Ti and Nb are required to fix it as carbides, but the resulting precipitation strengthening of TiC and NbC deteriorates workability.

Si: 1.0% or less, Mn: 1.0% or less Si
Both Mn and Mn effectively contribute to improving the strength of the steel sheet, but excessive addition causes deterioration of weldability, so it was decided to add each in a range of 1.0% or less.

p: o, t 5% or less Like Si and Mn, P also effectively contributes to increasing the strength of steel sheets, but if the content exceeds 0.15χ, it will also cause deterioration of weldability, so 0.15% Limited to the following range.

s: 0.010% or less If a large amount of S is contained, it will not be fixed by Mn or Ti, causing cracks to occur during hot rolling, so the S content was limited to a range of 0.010% or less. .

Aj! : 0.010~0.100χ ^l fixes 0 in the steel and increases Ti and N by bonding with 0.
Although it is an element useful for avoiding a decrease in the effective amount of b,
If it is less than 0.010χ, the effect of the addition is poor;
Since the effect reaches saturation even if it is added in excess of 10x, it is set in the range of 0.010 to 0.100x.

N: 0.0050% or less Similar to C, N causes lower value and deterioration of ductility when contained in a large amount, so it was limited to a range of 0.0050% or less.

Ti: 0.010~0.050χ, Nb: 0
．． Both 004 to 0.030χT+ and Nb are useful elements that fix C and N that are dissolved in steel and have a negative effect on deep drawability and ductility, thereby improving workability and aging resistance. However, Ti and Nb are each 0.
010χ.

If it is less than 0.004χ, it will be difficult to sufficiently fix C and N, and satisfactory workability will not be obtained; on the other hand, if it is 0.050χ.

If it exceeds 0.030χ, carbonitrides of Ti and Nb will increase, resulting in deterioration of workability, so Ti and Nb are added within the ranges shown below.

Ca: 0.0009 to 0.0025[chi]Ca is particularly important in this invention and is an essential element for improving chemical conversion treatment properties. However, the content is 0.0009
If it is less than 0.002%, the effect of the addition is poor;
If added in a large amount exceeding 5%, the effect not only reaches saturation, but also increases the amount of sulfide-based inclusions on the slab surface, which may even impair the appearance of the steel plate surface. Therefore, the Ca content should be reduced to 0. It was limited to a range of .0009 to 0.0025%.

Next, there are no particular regulations regarding the manufacturing method of this invented steel.
After forming a slab by the blooming method or continuous casting method, it may be subjected to normal hot rolling or cold rolling, and then annealing treatment.

Note that if the steel plate according to the present invention is left in the atmosphere for a long time, spot rust may occur due to 11□0 and CO□ in the atmosphere, so if there is some time before chemical conversion treatment is applied after manufacturing, It is recommended to apply anti-rust oil.

(Function) Although the reason why chemical conversion treatability is improved by adding Ca to steel according to the present invention has not yet been clearly elucidated, the Ca-based fine compounds formed by Ca addition can be applied to the surface of the steel sheet. This is thought to be due to the fact that it is concentrated and electrically less noble than the base iron, so it dissolves preferentially during the chemical conversion treatment and promotes the formation of the chemical conversion treatment film.

(Example) Various steels (floors 1 to 14) having the compositions shown in Table 1 were melted in a converter, and after 1711 degassing treatment, slabs were formed by continuous casting. Then heating temperature: 1200℃,
Finishing temperature = 910°C 3. Winding temperature: After hot rolling at 600°C to obtain a 2.8 mm thick hot-rolled plate, it was pickled and then cold rolled at a rolling reduction of near 5%. to obtain a cold-rolled plate with a thickness of 0.7 mm, which was then subjected to continuous annealing at 795°C for 40 seconds, and then temper rolled at a rolling reduction of 0.6χ.

Table 2 shows the results of examining the mechanical properties and chemical conversion treatability of each of the cold-rolled steel sheets thus obtained.

Regarding chemical conversion treatment properties, the pinhole area ratio (hereinafter simply referred to as PIIB) was determined when a pinhole test was performed on a cold-rolled sheet and a steel sheet that had been degreased, washed with water, and then subjected to phosphate treatment using a dip method or a spray method. It was evaluated by

The pinhole test is a method for detecting areas where phosphate crystals are not attached after phosphate treatment.A filter paper soaked with potassium ferricyanide solution is placed on the test surface to detect areas where phosphate crystals are not attached. Each time the iron ions eluted from the sample are reacted, a color develops, and this is image analyzed to calculate the percentage of discolored area.If PIIE≦3, it can be said that the phosphate treatment property is good.

As is clear from the same table, floors 3 and 4, in which the C and N amounts in the steel exceeded the upper limits of the present invention, both had low lower values and were inferior in AI properties.

Further, lk5.10, in which the amounts of Ti and Nb are each less than the lower limit of the present invention, also has a low lower value and is inferior in AI properties.

Furthermore, m6 and m12, in which the amounts of Ti and Nb each exceed the upper limit of the present invention, have low lower values and poor chemical conversion treatment properties.

Furthermore, floor 8.1 in which the amount of Ca is less than the lower limit of this invention
All of No. 3 have poor chemical conversion treatment properties.

On the other hand, No.
Nos. 1.2, 7, 9.11 and 14 all exhibit excellent mechanical properties, AI values and chemical conversion treatability.

(Effects of the Invention) Thus, according to the present invention, it is possible to easily obtain a cold rolled steel sheet that is excellent not only in deep drawability and aging resistance but also in chemical conversion treatment properties.

Claims (1)

[Claims] 1. C: 0.0050wt% or less, Si: 1.0wt% or less, Mn: 1.0wt% or less, P: 0.15wt% or less, S: 0.010wt%, Al: 0. 010-0.100wt% and N: 0.0
050 wt% or less, and Ti: 0.010 to 0.050 wt% and Nb: 0.050 wt% or less.
0.004 to 0.030 wt%, further contains Ca: 0.0009 to 0.0025 wt%, and the remainder is substantially Fe. Excellent cold-rolled steel sheet for press working.