JPS61261460A - Ferritic stainless steel sheet having excellent secondary operation characteristic after deep drawing - Google Patents

Abstract

PURPOSE:To provide a ferritic stainless steel sheet which obviates the brittle fracture referred to as a longitudinal crack in the stage of secondary operation after deep drawing by decreasing the content of C in particular to an extremely slight content and adding B to said steel. CONSTITUTION:The titled ferritc stainless steel sheet contains, by wt%, 0.002-0.030% C, <=1.0% Si, <=1.0% Mn, 10.0-20.0% Cr, <=0.02% N, 0.002-0.150% Al in which >=6 Ti/(C+N) and 0.0003-0.0050% B and consisting of the balance Fe and inevitable impurities. The steel having the above- mentioned component constitution is melted in an ordinary steel making furnace such as electric furnace or converter and is made into a slab by a continuous casting method, etc. The slab is subjected to an ordinary process for production including hot rolling-annealing and pickling-cold rolling-annealing and pickling to form the steel sheet. The steel sheet has the excellent secondary operation characteristic and the mechanical properties as excellent as 36-38% elongation and is provided with the excellent deep drawing characteristic (see Table).

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a ferritic stainless steel sheet that does not cause brittle fractures such as vertical cracks during secondary processing after deep drawing.

[Conventional technology]

In general, ferritic stainless steel sheets are widely used in kitchen equipment and home appliances, but especially in water-containing tanks, they are formed through a harsh primary press process, and then undergo secondary processing, such as stretching by expanding the circumference. In many cases, the final molded product is made by adjusting the shape. Therefore, there is a strong demand for ferritic stainless steel sheets to have excellent deep drawability and secondary workability that can withstand harsh primary press working and subsequent secondary working.

On the other hand, ferritic stainless steel sheets, which are said to be used for conventional deep drawing, can withstand normal deep drawing with a drawing ratio of about 1. Due to these problems, ultra-low carbon TI-added ferritic stainless steel sheets were developed as ferritic stainless steel sheets for ultra-deep drawing. -1177
The technology is disclosed in Japanese Patent Publication No. 57-5578-7, which discloses a ferritic stainless steel sheet in which the r value is improved by adding Bi.

[Problem that the invention seeks to solve]

The ultra-low carbon TI-added ferritic stainless steel sheet disclosed in Japanese Patent Publication No. 54-11770 has a drawing ratio of 2.
．． Although it can withstand very severe press working of 3 or more, when secondary processing is performed after such severe press processing, such as stretching by widening the circumference, brittle fractures (called secondary processing cracks) may occur. Cracks) were occasionally observed, and improvements were needed to meet the current demand for ferritic stainless steel sheets that have excellent deep drawability and secondary workability.

Also, B disclosed in Japanese Patent Publication No. 57-55787
Added ferritic stainless steel sheets have a relatively high C1t-1 to adjust their strength, so they may not be suitable for severe deep drawing.

The present invention is a ferritic stainless steel that can be subjected to severe deep drawing with a drawing ratio exceeding 2.1, and that can be subjected to secondary processing such as stretching by widening the circumference after such processing. The purpose is to provide steel plates.

[Means for solving problems]

The present inventor investigated ways to improve secondary workability after performing severe deep drawing, and as a result, achieved this by reducing carbon to an extremely low level and adding B. Summary of the Invention teeth.

C: 0.002-0.030%, St: 1.0
% or less, Mn: 1.0S or less, Cr: 10.0~
20.0%, N: 0.02% or less, )L: 0.0
02~0.150chi, T1: 0.02~0.70%
i/(C+N)〉6, B: 0.0003-0.
This is a ferritic stainless steel sheet for deep drawing with excellent secondary workability, containing 0.050% and the remainder consisting of iron and unavoidable impurities.The reasons for limiting the chemical composition of the steel of the present invention will be described below.

When C is 0.030% or less, the elongation and 7 value (Lankford value) are significantly improved, and severe deep drawing processing with a drawing ratio exceeding 2.1, which is the object of the present invention, becomes possible. The lower the C content, the more preferable it is, but it is difficult to reduce it to less than 0.002% in terms of melting. Therefore, C content'i0.0
It was limited to 0.0296% to 0.030%.

Since excessive inclusion of St deteriorates cold workability, the upper limit was set at t-1.0%. Preferably it is 0.15% or less.

Mn is necessary as a deoxidizer and to prevent grain boundary embrittlement due to segregation of S to grain boundaries, but too much Mn deteriorates the cold workability of the steel sheet, so the upper limit was set at 1, OS. The higher the O Cr, the better the corrosion resistance, but 20.0% f
If it exceeds 10.t, the deep drawability of the gold will deteriorate significantly. If the temperature is less than 0, excellent corrosion resistance will not be exhibited, so the range t-
The content was set at 10.0 to 20.0%.

0.002% or more of AL is required as a deoxidizing agent to avoid surface scratches on the steel plate due to the formation of TiO2, but if it is too large, scratches caused by red color will become a problem, so the upper limit should be set at 0.150%.
◎ If N increases, cold workability deteriorates and N @
Since a large amount of Ti is required to fix it as TiN, which is accompanied by an increase in manufacturing costs, the upper limit was set at t-0.02.

If TI is limited to C and Ni as mentioned above, and if it is 5t (S<0.01%), which is normally contained in the current steel manufacturing method, TI is 0.02t to ensure deep drawability. above and TI/(C+N)>6 is required. but.

If it exceeds 0.70*e, the effect will be saturated and the manufacturing cost will increase. Preferably it is in the range of 0.2 to 0.6%.

B is the most important component element for improving secondary workability in the steel of the present invention, and its effect is exhibited at 0.0003% or more. However, if it is too large, the deep drawing properties deteriorate and the slab cracks, so the upper limit was set at 0.0050%. It is preferably 0.0004 to 0.0015%.

[Effect] The effect of B addition on improving secondary processability is speculated below.

The reason that conventional ultra-low carbon TI-added ferritic stainless steel sheets have low grain boundary strength and are prone to grain boundary embrittlement is the C in the steel.
It is thought that this is because C is completely fixed as Tic, the grain boundary strengthening effect of C disappears, and P, which is contained in the steel instead of C, segregates to the grain boundaries and significantly reduces the grain boundary strength. Therefore, although the effect of the addition of B on improving brittle fracture is still unclear, B is an element that is atomically very similar to C, and due to grain boundary segregation, It is thought that it has a faster diffusion rate in steel than P, which lowers the boundary strength, and easily segregates to the grain boundaries, increasing the strength of the grain boundaries like C. S, which is also contained as an unavoidable impurity, is also an element that tends to cause grain boundary embrittlement, so the content should be kept as low as possible, preferably 0.010% or less. As a method for further reducing grain boundary segregation of S, it is also effective to add Ca or RKM to form precipitates of the Ca-S-0 series or the REM-8-0 series.

Note that O is fixed as an oxide by At6 or MnK, but if it is contained in a large amount, workability will deteriorate due to the oxide, so the content should be as low as possible, preferably <0.010% or less. .

Steel with the above composition is melted in a normal steelmaking furnace such as an electric furnace or a converter, and then made into steel slabs by continuous casting or normal ingot making, followed by hot rolling, annealing and pickling ( O manufactured through the normal manufacturing process of cold rolling, annealing and pickling, and if necessary, repeating cold rolling, annealing and pickling, etc. [Example] Table 1 The composition steel of the present invention steel and conventional steel as shown in Fig. 1 was melted in a converter, made into steel billets by a normal casting method, and then hot-rolled into hot-rolled coils with a plate thickness of 6.0 dew. . Next, the hot rolled sheet was annealed and pickled, and then cold rolled and annealed and pickled twice to obtain a cold rolled steel strip with a thickness of 0.8 m.

Table 2 shows the evaluation results of mechanical properties and secondary workability. Secondary workability is determined by blanking a stainless steel plate into a disc shape, producing primary processed products with different drawing ratios through two-step cylindrical drawing processing, and then heating the drawn parts to 20°C. After holding the cup, an impact load was applied to the cup head using a drop weight test to determine whether brittle cracks had occurred on the side wall of the cup.
Evaluation was made by repeating the test. ○ marks in the table indicate no secondary processing cracks.・
The mark indicates a crack in the secondary processing, and the mark X indicates that deep drawing processing could not be performed.

The results in Table 2 show that the steel sheets of the present invention are extremely excellent in secondary workability. In addition, the mechanical properties of the steel sheet of the present invention are excellent, with an elongation of 36 to 38%, and a T of 1.5.
In addition to the above, it is equivalent to the conventional ultra-low carbon T1-added ferritic stainless steel sheet, and also has excellent deep drawing properties.

Claims (1)

[Claims] C: 0.002 to 0.030%, Si: 1.0% or less,
Mn: 1.0% or less, Cr: 10.0 to 20.0%, N
: 0.02% or less, Al: 0.002 to 0.150%,
Ti: 0.02 to 0.70%, Ti/(C+N)≧6,
A ferritic stainless steel sheet containing 0.0003 to 0.0050% of B, with the remainder consisting of iron and inevitable impurities, and has excellent secondary workability after deep drawing.