JP3132728B2 - Ferritic stainless steel with excellent formability - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a ferritic stainless steel excellent in formability. In particular, the present invention provides a material for press forming which is used as a steel strip or a steel sheet to be processed into a durable consumable material such as kitchen equipment and automobile parts by press forming. The present invention relates to a ferritic stainless steel that has excellent primary workability and secondary workability after primary work, has excellent surface properties, and has improved ridging properties.

[Conventional technology]

Ferritic stainless steels, such as SUS430, have characteristics such as less time cracking after forming than austenitic stainless steels, low sensitivity to stress corrosion cracking, and relatively low cost without containing expensive nickel. Therefore, it is widely used mainly in durable consumer materials.

However, on the other hand, ferritic stainless steel is generally inferior to austenitic stainless steel in material properties such as corrosion resistance, weldability, press formability, etc., and thus has limited applications in certain aspects. In particular, press formability (hereinafter simply referred to as formability) is one of the important material properties because stainless steel sheet is often processed into a final product by press forming. For example, SUS430, a typical ferritic stainless steel, is 16.00 to 18.00% Cr, 0.12%
It is a steel containing the following C, 0.75% or less of Si, 1.00% or less of Mn, 0.040% or less of P, and 0.030% or less of S, but the formability is not always sufficient.

Therefore, many researches and developments have been made to improve the formability of ferritic stainless steel.
One of the results is the combination of carbon and nitrogen reduction with the addition of relatively large amounts of carbonitride forming elements such as Ti and Nb. Examples thereof include, for example, JP-B-51-29694, JP-B-51-35369, and JP-A-51-14811.
JP-A-51-14812, JP-A-52-31919, and JP-A-58-61258. Also, JI
SG4305: SUS430LX 16.00-18.00% Cr, 0.03%
The following C, 0.75% or less Si, 1.00% or less Mn, 0.040% or less P, 0.030% or less S, and 0.10-1.00% Ti or Nb
Is specified.

In addition, Al for the purpose of improving formability and ridging properties
Addition (for example, JP-B-51-44888), B addition (JP-B-4
4-736), B-Ti composite addition (for example, JP-B-47-4786)
And Japanese Patent Publication No. 51-8733).

[Problems to be solved by the invention]

As described above, in the case of SUS430, the formability is not always sufficient, and the value which is an index of deep drawability is generally as low as about 1, and the ductility is not sufficient. For this reason, although light press forming is possible, it cannot be put to practical use when the degree of processing is large or the shape is complicated.

On the other hand, in ferritic stainless steels with reduced carbon and nitrogen, as represented by SUS430LX, and with the addition of a relatively large amount of carbonitride forming elements such as Ti and Nb, the value as an index of deep drawability improved considerably. And the ductility has also been improved. However, on the other hand, compared with SUS430, there are many problems such as poor manufacturability and surface properties, and poor secondary workability after deep drawing. For example, in terms of manufacturability, SUS430LX and SUS444 steels are so-called ferritic single-phase steels that do not form an austenite phase even at high temperatures. In order to avoid this, special measures must be taken to prevent the temperature of the slab from dropping, and measures such as changing the cooling method and working method are required. For this reason, the manufacturability is inferior, which causes an increase in raw material costs and an increase in manufacturing costs. In particular, when a relatively large amount of Ti is added, a surface flaw called titanium streak is likely to occur due to the nonmetallic inclusions TiN or Ti (CN) generated, and the surface properties are poor. Not always good. In addition, SUS430LX-based steel is subject to brittle cracks (generally referred to as “vertical cracks”) parallel to the drawing direction during secondary processing after severe deep drawing as primary processing in press forming. Often), which can be problematic. This vertical cracking is a kind of work embrittlement caused by a decrease in toughness due to deep drawing as a primary work.
Also, the lower the temperature, the more easily it occurs. For this reason, SUS430LX-based steel has excellent deep drawability as a primary process,
Since the secondary workability is inferior, the workability is practically limited.

On the other hand, in the ferritic stainless steel to which Al is added,
Not only is the formability not sufficiently improved, but Al ₂ O ₃ adheres and accumulates on the inner wall of the immersion nozzle during continuous casting, so that nozzle clogging is likely to occur, and large oxides mainly composed of Al ₂ O ₃ are formed on the steel sheet surface. Manufacturability is not good because the non-metallic inclusions are exposed and surface defects are likely to occur.

Also, the addition of B and the ferritic stainless steel to which a small amount of Ti is added in addition to B do not sufficiently improve the formability.

[Means to solve the problem]

In order to solve various problems of the SUS430 steel and the above-mentioned ferritic stainless steel in which the formability has been improved, the present inventors have intended to minimize the decrease in manufacturability and the increase in product cost and to improve surface properties and surface properties in product characteristics. In order to improve the formability of ferritic stainless steel without deteriorating the secondary workability, a systematic study was conducted from the aspect of chemical composition. As a result, basic knowledge that a ferritic stainless steel that achieves the above objectives can be obtained by adding a small amount of Ti and / or a small amount of Zr in addition to a small amount of B and V without particularly reducing C. I got

The present invention has been completed on the basis of this finding, and the gist of the present invention is that, in weight%, C: more than 0.030% to 0.080% or less, Si: 0.75% or less, Mn: 2.00% or less, P; 0.040% Below, S; 0.010% or less, Cr; 14.00 to 20.00%, Ni; 0.60% or less, N; 0.030% or less, V; 0.03 to 0.20%, B; 0.0010 to 0.0300%, Al; 0.02% or less. , And one or two of Ti; 0.01 to 0.30% and Zr; 0.01 to 0.30% in the range of (Ti + Zr) / (C + N) = 0.5 to 4.0, with the balance being iron and unavoidable impurities Ferritic stainless steel with excellent formability.

[Detailed Description of the Invention]

The reasons for limiting the ranges of the respective component amounts of the steel of the present invention are as follows.

C is an element that increases the strength after cold rolling annealing, and if it is too high, the ductility is reduced. Further, in order to obtain a predetermined (Ti + Zr) / (C + N) ratio described later, it is preferable that the C amount is lower because the addition amount of Ti and / or Zr can be reduced. Therefore, the upper limit is 0.080%. On the other hand, C
If it is less than%, it takes a long time to decarburize at the steel making stage, which leads to an increase in cost. In addition, the ridging characteristics are degraded, so the content should be more than 0.030%.

Si is an effective element for deoxidation, but if its content is too high, the material hardens and ductility is reduced, so the content is set to 0.75% or less.

Mn is an element that improves hot workability and toughness of a weld. In addition, its solid solution strengthening ability for ferrite phase is smaller than C and Si, and its adverse effect on ductility is small. However, 2.00
%, The improvement effect is saturated.
2.00% or less.

P is an element that lowers the toughness, and is preferably set to 0.040% or less from the viewpoint of secondary workability.

S is preferably set to 0.010% or less because S has a bad influence on hot workability and corrosion resistance, particularly rust resistance.

The lower limit of Cr of 14.00% is the minimum necessary for ensuring the corrosion resistance of ferritic stainless steel. On the other hand, if the content exceeds 20.00%, the steel containing more than 0.030% C, such as the steel of the present invention, has a remarkable decrease in toughness and formability. For this reason, the Cr content is set to 14.00 to 20.00%.

Ni, like Mn, improves the toughness of the weld, but hardens the material and reduces ductility, so the upper limit is 0.60%, which is normally allowed for ferritic stainless steels.

As with C, the lower the N, the lower the content of Ti and / or Zr to obtain a predetermined (Ti + Zr) / (C + N) ratio. Also, from the viewpoint of preventing the deterioration of the surface properties due to the nitride with Ti or Zr, the N content is preferably as low as possible. Therefore, the upper limit of the N content is 0.030%, and more preferably 0.020% or less.

The addition of V and B is an important point of the present invention. By adding V and B in combination with a small amount of Ti and / or a small amount of Zr to be described later, the synergistic effect of these elements is used as an index of deep drawability. Certain r values are significantly improved. Both the effects of V and B are exhibited by adding a small amount, but at least 0.03% or more of V and 0.0010% or more of B are required. On the other hand, even if a large amount of both V and B are added, the above effect is saturated and the material is hardened.
The upper limit is 0.20% for B, and 0.0300% for B. In addition, as demonstrated in Examples described later, Ti or Z
There is almost no effect of increasing r value only by adding V alone, adding B alone, or adding V-B composite, and adding r in addition to V and B by adding Ti and / or Zr in combination Is important.

Al is an effective element for deoxidation during steelmaking. In particular, it is effective as a deoxidizing agent added immediately before adding Ti or Zr to reduce the oxygen concentration in the molten steel and then add Ti or Zr to improve and stabilize the yield. However, Al not only increases surface defects and causes deterioration of surface properties, but also has an adverse effect on secondary workability. Therefore, excessive addition beyond the purpose of deoxidation is not preferable. Therefore, although Al is not added as an alloy component, it may slightly remain in steel, so 0.02% is allowed as the upper limit.

Ti and Zr are components necessary for improving the r value and improving the ductility by softening, and the r value is further improved by adding V and B in combination. However, the addition of large amounts of Ti and Zr increases the production cost and deteriorates the surface properties, so both are added at 0.30%.
Is the upper limit. Further, with respect to Ti and Zr, regulation of their absolute amounts alone is not sufficient, and it is important to define them in correlation with the (C + N) amount. (Ti + Zr) / (C +
If the N) ratio is less than 0.5, the effect of improving the r value and softening is not sufficient, and if it exceeds 4.0, there is a possibility of forming a ferritic single-phase steel that does not generate austenite even at high temperatures in the metallographic structure. Not only is it not preferable in terms of manufacturability, for example, but also the deterioration of secondary workability after deep drawing is concerned. Therefore (Ti + Zr) /
The (C + N) ratio is in the range of 0.5 to 4.0.

 Hereinafter, the present invention will be further described with reference to examples.

〔Example〕

A steel having the chemical composition shown in Table 1 was melted and hot-rolled into a hot-rolled sheet having a thickness of 3.6 mm. After the hot-rolled sheet annealing, a cold-rolled annealed sheet having a thickness of 0.7 mm was produced by cold rolling twice (3.6 mm → 1.8 mm → 0.7 mm) with intermediate annealing and finish annealing. The mechanical properties, r-value and longitudinal cracking resistance were investigated for these, and the results are shown in Table 2.

The longitudinal cracking resistance was as shown in Fig. 1 by using a deep drawn cup with a draw ratio of 3.1 and an outer diameter of 27 mm using a step-drawn cold-rolled blunt sheet, and dropping the ears to set the cup height to 42 mm as shown in Fig. 1. A drop weight test was conducted at a test temperature of -10 ° C in which a weight 2 (500 g) was dropped from a height of 1 m on the side, and the presence or absence of brittle fracture on the cup side wall was determined.

As can be seen from Table 2, the steels of the present invention all have good elongation and values, and are excellent in formability. In addition, it has excellent vertical cracking resistance and is practically at a level that does not cause any problem.

On the other hand, the comparative steel No.10 (SUS430) does not have sufficient elongation, has poor value, and is inferior in formability.

Comparative steel No. 11 (SUS430LX) has a high value but is inferior in vertical crack resistance, and the practical forming limit is restricted.

Comparative steel No. 12 has V alone (no B, Ti, Zr added)
Steel No. 13 was a steel with B alone (no V, Ti, Zr added) steel, and Comparative Steel No. 14 was a steel with V and B added but no Ti and Zr added.
Comparative steel No. 15 added B and Ti, but did not contain V,
Comparative No. 16 is a steel in which B and Zr are added, but no V is added, and the elongation and value are not sufficient even with the steels with V and B alone and the composite steels with B, Ti and Zr. Therefore, it is understood that the composite addition of Ti and Zr in addition to V and B as in the steel of the present invention greatly contributes to the improvement of the formability.

In addition, the comparative steel No. 17 is more than A used as a deoxidizer.
This is a steel to which l is added, but the value is not always sufficient, and the resistance to longitudinal cracking is poor.

In the above embodiment, an example was given in which a cold-rolled annealed sheet was manufactured by a hot-rolled sheet annealing-twice cold-rolling / double annealing method. However, regardless of the presence or absence of hot-rolled sheet annealing, no intermediate annealing was performed. Even if it is manufactured by one-time cold rolling and one-time annealing, a steel excellent in formability can be obtained similarly.

〔The invention's effect〕

As described above in detail, according to the present invention, a ferritic stainless steel having excellent formability can be obtained without impairing the productivity.
It can be provided at low cost and can greatly contribute to expanding the applications of ferritic stainless steel.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view showing a relationship between a test cup and a weight in a dropping weight test performed as an evaluation test of longitudinal cracking resistance. 1 ... test cup, 2 ... weight.

 ──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Hiroshi Fujimoto 4976 Tomita, Odai, Shinnanyo-shi, Yamaguchi Pref. JP-A-62-30856 (JP, A) JP-A-51-62112 (JP, A)

Claims (1)

(57) [Claims] 1. In weight%, C: more than 0.030% to 0.080%, Si; 0.75% or less, Mn: 2.00% or less, P; 0.040% or less, S; 0.010% or less, Cr; 14.00 to 20.00%, Ni; 0.60% or less, N; 0.03% or less, V; 0.03 to 0.20%, B; 0.0010 to 0.0300%, Al; 0.02% or less, and Ti; 0.01 to 0.30%, Zr; 0.01 to 0.30% A ferritic stainless steel containing one or two of (Ti + Zr) / (C + N) in the range of 0.5 to 4.0, with the balance being iron and unavoidable impurities.