JP3477113B2 - High-purity ferritic stainless steel sheet with excellent secondary work brittleness after deep drawing - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-purity ferritic stainless steel sheet excellent in secondary work brittleness resistance after deep drawing.

[0002]

2. Description of the Related Art Ferrite-based stainless steel sheets are widely used in kitchen appliances, home appliances, etc. because they are cheaper and have better rust resistance than austenitic stainless steel sheets. In recent years, due to improvements in decarburization and denitrification technology in the steelmaking process, high-purity ferritic stainless steel sheets with further improved workability and rust resistance have been developed, and are processed into complex shapes for a wider range of applications. The opportunity to be used is increasing.

The high purity of a ferritic stainless steel sheet by decarburization and denitrification increases elongation and r-value, so that deep drawability is remarkably improved. However, in the secondary processing in which the pipe expansion processing is further performed thereafter, the formability is low, and there is a defect that the so-called secondary processing brittleness is inferior such as easy occurrence of intergranular cracks.

Regarding the improvement of the secondary working brittleness of a high-purity ferritic stainless steel sheet, for example, Japanese Patent Publication No. 2-7391.
JP-A-8-296000 and JP-A-8-296000 disclose techniques for adding Ti and B. In addition, JP-A-8-2084
Japanese Patent Publication No. 3 proposes a technique for adding Ti and a small amount of Nb and B.

However, even in a high-purity ferritic stainless steel sheet in which these elements are controlled, secondary work cracks often occur depending on the balance of other elements, and further improvement has been desired. Further, since B acts as an interstitial solid solution element in steel similarly to C and N, there is a problem that elongation and r-value decrease due to excessive addition for ensuring secondary work embrittlement resistance. Occurs.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to improve secondary work embrittlement resistance after deep drawing in a high-purity ferritic stainless steel sheet without lowering its excellent ductility and deep drawability. To provide a method.

[0007]

Means for Solving the Problems The present inventors have
As a result of earnest studies to achieve the goal, high-purity ferrite
P is the main cause of secondary work embrittlement in system stainless steel sheets.
Segregation of Mg, and to improve it, the balance of addition of Mg
Found that the effect of
It was That is, the present invention is based on the above findings.
The main points are as follows. (1) In wt%, C: 0.010% or less, Si: 0.5% or less, Mn: 1.0% or less, P:0.018 ~
0.04%, S: 0.01% or less, Cr: 9 to 25%, Ti + Nb: 0.6% or less, and (Ti + Nb) /
(C + N) is 7.0 or more, Al: 0.002-0.05%, Ca: 0.0020%
Less than, Mg: 0.0050% or less, N: 0.01% or less
under, Contains and needs moreToDepending on, Mo: 3% or less, Cu: 1% or less, Ni: 1% or less, B:0.0006-
0.0030% 1 or 2 or more of, and the balance is substantially Fe
And satisfy Mg (%) ≧ 0.05 × P (%).
Excellent secondary processing brittleness resistance after deep drawing
High-purity ferritic stainless steel sheet.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors conducted the following experiment in order to investigate the influence of P on the secondary work embrittlement. Due to lab dissolution, C: 0.005%, Si:
0.20%, Mn: 0.44%, S: 0.002%, C
r: 17.4%, Ti + Nb: 0.40%, Al: 0.
034%, Ca: 0.0005%, N: 0.009%, P is 0.01%, 0.03%, 0.05% 3
0% Mg (no addition), 0.0010%,
0.0020%, 0.0040%, 0.0060% of 5
Test slabs were prepared with almost the same adjustments as above. After heating this test slab to 1200 ° C., it was hot-rolled into a hot-rolled sheet having a thickness of 2.5 mm. This hot-rolled sheet was subjected to annealing pickling-cold rolling-annealing pickling to obtain a cold-rolled sheet having a plate thickness of 0.5 mm.

The secondary work brittleness resistance of the test piece obtained by the above method was evaluated by the following method. That is, a cylindrical cup was produced as a primary processed product by deep drawing with a drawing ratio of 2.1. The cups are then -10
After holding at a temperature of ~ 30 ° C, impact on the end of the cup 3
% Pipe expansion strain was applied, and the presence or absence of brittle cracking on the side wall of the cup was examined. The number of repetitions was three. As for the evaluation, ⊚ indicates no cracking at a test temperature of 0 ° C or higher, and ○ indicates no cracking at 15 ° C or higher,
The ● mark indicates that there is cracking at 15 ° C or higher. The results are shown in Table 1.
And shown in FIG. In the Mg-free test piece, the secondary work embrittlement resistance tended to deteriorate as P increased. In the test piece to which Mg was added, the secondary work embrittlement resistance was improved as Mg was increased, but with the material having P of 0.05, good results were not obtained even if the amount of Mg was increased. From the above, in order to improve the secondary processing brittleness resistance, P and M
It was found that the amount of g should be controlled within the range of the rectangle in FIG.

[0010]

[Table 1]

It is assumed that the secondary work embrittlement of a high-purity ferritic stainless steel sheet is improved by reducing P and adding Mg. Conventional steels have relatively high C and N, and these elements segregate as interstitial types at the crystal grain boundaries, contributing to the stabilization of the grain boundary structure. But C,
In high-purity steel with low N, P, which is a substitutional element, segregates at the grain boundaries in place of these elements, so that the electronic structure of the grain boundaries is changed, and the grain boundary bonding force is reduced, resulting in embrittlement. It is supposed to be. It is considered that the addition of Mg to this has the effect of forming a compound with P and reducing the amount of P segregated at the grain boundaries. Furthermore, Mg produced by adding Mg
It is considered that the oxide containing P has the effect of suppressing the segregation of P at the grain boundaries.

Next, the reasons for limiting the various elements of the present invention will be described in detail. C is limited for the purpose of ensuring workability and corrosion resistance. In particular, the lower the better for improving the deep drawing workability, the longer the decarburizing time in the steelmaking process and the higher the cost. The upper limit was set to 0.010% as a condition for maintaining high elongation and high r value.

The amount of Si is limited. Si is an element effective for deoxidation and is recommended for some applications because it improves high temperature oxidation resistance, but it is limited in the processing applications used in the present invention product, so the range is 0.5%. Below. It is more preferable if it is 0.25% or less.

The amount of Mn is limited. Mn is an element useful for eliminating the damage of the deoxidizer and S, but if it is added excessively, cold workability is deteriorated. The upper limit of the range is 1.0%.

The amount of P is limited. When P is high, not only the secondary work brittleness resistance is significantly deteriorated, but also hot workability is deteriorated and surface defects frequently occur. Therefore, the upper limit is 0.04%.
And The lower limit is 0.018% in relation to Mg.

The amount of S is limited. Since S is a harmful element that segregates at the crystal grain boundaries and promotes grain boundary embrittlement at high temperatures, its upper limit was made 0.01%. Preferably 0.0
It is set to 06% or less.

Cr is added in a required amount. Cr is a basic element that imparts corrosion resistance to the product of the present invention. If the amount is less than 9%, the corrosion resistance is insufficient, and if it exceeds 25%, the cold workability is deteriorated and the manufacturability is deteriorated. Therefore, Cr is added in the range of 9 to 25%.

Ti and Nb are added in required amounts. Ti
And Nb are elements that fix C and N, improve corrosion resistance and stabilize the metal structure, but in order to exert their effect, the value of (Ti + Nb) / (C + N) is 7.0 or more. Must be However, excessive addition impairs the surface properties of the slab and reduces the manufacturability.
The upper limit is limited to 0.6% by the sum of + Nb.

A necessary amount of Al is added. Al is an element effective as a deoxidizing agent, but if its amount is less than 0.002%, it has no effect, and if it exceeds 0.05%, it causes disadvantages such as deterioration of bending workability. Therefore, Al is 0.002-
Limited to 0.05%.

The amount of Ca is limited. Ca is an element effective in suppressing nozzle clogging during casting,
It is selectively added according to the amount of Ti. However, if added excessively, Ca-based inclusions are increased, and corrosion resistance and manufacturability are reduced, so the range is limited to 0.0020% or less. Preferably, it is 0.0010% or less.

Like C, N is limited for the purpose of ensuring workability and corrosion resistance. A lower value is preferable for improving the deep drawing workability, but it causes an increase in cost, so the upper limit is 0.01%.
Limited to:

Mg is an important element that has the greatest influence on the improvement of secondary work embrittlement in the steel of the present invention, and is added in a necessary amount in relation to P. The lower limit is 0.05 x P (%), and if it is less than this, improvement in secondary work embrittlement resistance cannot be exhibited. On the other hand, if the amount of Mg is increased more than necessary, the effect on the secondary work embrittlement is saturated and the cost becomes high. In addition, the cleanliness of the steel decreases, the inclusions increase, and the ductility decreases, which is not preferable. For the above reasons, the upper limit is set to 0.0050%.

Mo is added as necessary to improve the corrosion resistance. Mo has the effect of suppressing the progress of pitting corrosion peculiar to stainless steel sheets. However, the addition of more than 3% saturates the effect and raises the cost, so the range is limited to 3% or less.

Cu is added if necessary. Adding Cu in a small amount has the effect of suppressing general corrosion in many corrosive environments. However, if it exceeds 1%, the effect is saturated, so the range is limited to 1% or less.

Ni is added as needed. Addition of a small amount of Ni improves rust resistance. However, if it exceeds 1%, the effect is saturated, so the range is limited to 1% or less.

B is added if necessary. When B is added in a small amount, it strengthens the crystal grain boundary and improves the secondary work embrittlement resistance. However, if added excessively, the workability is lowered like C and N, so the range is 0.0006 to 0.0030%.
It is limited to.

In the manufacturing process of the steel sheet of the present invention, the steel having the above-mentioned composition is melted in an electric furnace, a converter, etc., the composition is adjusted in a secondary refining furnace, and then CC slab is formed by a continuous casting method. Then, it is manufactured through a normal manufacturing process in which hot rolling-annealing pickling (annealing omitted depending on the situation) -cold rolling-annealing pickling, and further cold rolling-annealing pickling are repeated as necessary. To be done.

[0028]

EXAMPLES Steels of the present invention and comparative steels having the compositions shown in Table 2 were melted in a converter-secondary refining furnace to form CC cast pieces, and then 1
After heating to 130 to 1200 ° C., the plate thickness is 3. by hot rolling.
It was a 0 mm hot rolled sheet. The hot rolled sheet was subjected to annealing pickling-cold rolling-annealing pickling to obtain a cold rolled steel sheet having a sheet thickness of 0.6 mm.

[0029]

[Table 2]

Using the steel sheet obtained by the above method as a test material, the secondary work embrittlement resistance was evaluated by the following method. That is, a cylindrical cup was produced as a primary processed product by deep drawing with a drawing ratio of 2.1. Then, after holding these cups at a temperature of −10 to 30 ° C., a pipe expansion strain of 3% was given to the cup end portion by impact, and the presence or absence of brittle cracking on the side wall portion of the cup was examined. The number of repetitions was three.

In the evaluation of secondary working brittleness resistance in Table 3, ⊚ indicates no cracking at a test temperature of 0 ° C. or more, ○ mark indicates no cracking at 15 ° C. or more, and ● mark indicates cracking at 15 ° C. or more. Table 2,
From Table 3, it can be seen that the steel sheet of the present invention exhibits extremely excellent characteristics with respect to resistance to secondary work brittleness after deep drawing.

[0032]

[Table 3]

[0033]

As described above, according to the present invention, the secondary processing brittleness resistance, which has been a problem in the conventional high-purity ferritic stainless steel sheet, is greatly improved, and the ferrite for high-deep drawing can withstand severe processing. System stainless steel sheet can be provided. By using the product of the present invention, it is possible to easily draw an article having an unprecedented complicated shape in a wide range of application fields such as kitchens, home appliances, automobiles, etc., and its economic and social effects are extremely large.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between P and Mg with respect to secondary work embrittlement susceptibility of a high-purity ferritic stainless steel sheet.

Continuation of the front page (56) Reference JP-A-10-324956 (JP, A) JP-A-11-92877 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38 / 00-38/60

Claims (5)

(57) [Claims] 1. By weight%, C: 0.010% or less, Si: 0.5% or less, Mn: 1.0% or less, P: 0.018 to 0.04% , S: 0.01% Below, Cr: 9 to 25%, Ti + Nb: 0.6% or less, and (Ti + Nb) / (C + N) is 7.0 or more, Al: 0.002 to 0.05%, Ca: 0.0020% or less , Mg: 0.0050% or less, N: 0.01% or less, the balance substantially consisting of Fe, and Mg
(%) ≧ 0.05 × P (%), which is a high-purity ferritic stainless steel sheet excellent in secondary processing brittleness resistance after deep drawing. 2. The high-purity ferritic stainless steel sheet excellent in secondary work embrittlement resistance after deep drawing according to claim 1, characterized by further containing Mo: 3% or less by weight. 3. A high-purity ferritic stainless steel sheet excellent in secondary work embrittlement resistance after deep drawing according to claim 1 or 2, further containing Cu: 1% or less by weight. 4. A high-purity ferrite system excellent in secondary embrittlement resistance after deep-drawing according to claim 1, 2 or 3, characterized in that it further contains Ni: 1% or less by weight. Stainless steel plate. 5. In addition, in% by weight, B:0.0006-
0.0030%ToClaim 1 containing,
Excellent in secondary processing brittleness resistance after deep drawing as described in 2, 3 or 4.
High purity ferritic stainless steel sheet.