JPS6289847A - High-strength two-phase stainless steel for rolling stock excellent in young's modulus and corrosion resistance - Google Patents

Abstract

PURPOSE:To improve Young's modulus and corrosion resistance and to prevent deterioration in strength in a weld zone after welding by limiting respective amounts of C, Si, N, Cr, Ni, Cu and Mo and the value of Nibal represented by a specific expression. CONSTITUTION:The titled stainless steel consists of, by weight, <=0.05% C, 1-5% Si, 3-10% Mn, 17-22% Cr, 1-7% Ni, 0.5-3% Cu, 0.5-3% Mo and the balance Fe and has a Nibal value of -12--6. The above Nibal value is represented by -1.1{Cr(%)+1.5XSi(%)+0.5XMb(%)+Mo(%)}+30{C(%)+N (%)}+0.5XMn(%)+0.3XCu(%)+Ni(%)+8.2.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a high-strength duplex stainless steel for vehicles that has a high Young's modulus, excellent corrosion resistance, and does not deteriorate the strength of the welded part after welding.

<Conventional technology and its problems> In recent years, from the viewpoint of saving labor and energy, vehicles have become paintless and
There is a strong demand for weight reduction, and thinning of materials by using high-strength stainless steel materials is being promoted. The use of stable austenitic stainless steel hard materials is becoming common.

However, these metastable austenitic stainless steel hard materials represented by 5US301 are cold-rolled (
Since high strength is obtained by generating deformation-induced martensite (pass-pass rolling), it is undeniable that the Young's modulus deteriorates due to cold rolling, and the amount of deflection of the material also inevitably increases. Therefore, when used as a vehicle material, it is necessary to make the wall thickness thicker than calculated based on the strength value, and the actual situation is that it is not possible to achieve a sufficiently satisfactory weight reduction.

Moreover, these metastable austenitic stainless steels are cold-rolled to achieve the desired strength, so the strength of the welded part after welding is significantly reduced. This necessitated increasing the thickness of the material, and in this respect, it could not be said to be a sufficiently desirable material for reducing the weight of a vehicle.

In this way, SU, which is currently used as a material for vehicles,
S301, SUS304, and 5US201 stainless steel hard materials had major unresolved problems such as a decrease in Young's modulus and deterioration in weld strength.

Means for Solving the Problems> In view of the above-mentioned problems found in current steel materials for vehicles, the present inventors have developed a method for applying cold rolling such as skin pass rolling, which causes a decrease in Young's modulus. As a result of our research to provide a high-strength stainless steel material that has sufficient strength and elongation required for vehicle materials, and also has sufficient corrosion resistance and weld strength, we have found the following (a) to ( The findings shown in f) were obtained. That is, (a) Currently, various stainless steels other than the austenitic stainless steel mentioned above are known, but among these, two-phase stainless steel having two phases, a ferrite phase and an austenite phase, is It has sufficiently high strength and elongation even without rolling, and there is no deterioration in the strength of the welded part, and the strength of the welded part, which is almost the same as that of the base metal, can be easily achieved by welding ordinary stainless steel. .

Figure 1 is a diagram showing the relationship between the structure, tensile strength, and elongation of stainless steel. From Figure 1, it can be seen that two-phase stainless steel consisting of a ferrite phase and an austenite phase has a good balance between strength and elongation. It is clear that the steel has a tensile strength of at least so kgf/mtc', which is sufficient for use as a steel material for vehicles. In addition, the symbol "A" in FIG. 1 represents the austenite phase, and the symbol "M" represents the martensite phase.
The symbol "F" indicates a ferrite phase.

(b) Young's modulus of the above ferritic-austenitic duplex stainless steel is calculated using the formula % × Mn(eA+ 0.3 x Cu(%) + Ni(
] + 8.2, and by adjusting the Ni bat value as small as possible, a sufficiently satisfactory Young's modulus as a steel material for vehicles can be ensured. In addition, hereafter, ``chi'' representing component proportions is expressed as weight %.

Figure 2 is a diagram showing the relationship between the Ni bat value and Young's modulus of ferritic-austenitic duplex stainless steel; : l 8000 kgf/mx2
It is clear that the above can be secured.

(C) However, the elongation of ferritic-austenitic duplex stainless steel is improved as the Ni bat value increases.

Fig. 3 is a diagram showing the relationship between Ni bat value and elongation of ferritic-austenite duplex stainless steel, and it can be seen from Fig. 3 that by adjusting the Ni bat value to -12 or more, elongation: 20 It is clear that it is possible to secure more than %.

(d) Therefore, by adjusting the Ni bat value in consideration of both Young's modulus and elongation, a high-strength duplex stainless steel for vehicles with good Young's modulus and elongation and no deterioration in weld strength can be obtained. thing.

(e) When Mo is added to the ferritic-austenitic duplex stainless steel, the corrosion resistance of the stainless steel
In particular, corrosion resistance against non-oxidizing acids, pitting corrosion resistance, and crevice corrosion resistance are further improved.

(f) Furthermore, when N is added or a combination of N and Nb is added to the ferritic-austenitic dual-phase stainless steel to which MO is added, the strength of the stainless steel is further improved.

This invention has been made based on the above knowledge, and provides a stainless steel material for vehicles with the following characteristics: C: 0.05 or less, Si: more than 1% to 5%, Mn: 3
Ultra-10%, Cr: 17-22%, Ni: 1-7%
, Cu: 0.5-3%, Mo: 0.5-3%, or further N: 0.04-0.20% alone, or Nb: O○5-0.50 %, the remainder consists of Fe and unavoidable impurities, and the Ni bat value -1.1 (cr (%) + 1.5 ×
Si (%) + 0.5X N1) (@ + Mo ((6
) l + 30 (C'f%) 10N (%)) + 〇, 5 ×M
n (%) + 0.3 xcu (n + Ni (%) +
The Ni bat value according to the formula 8.2 is [-12 to -6]
It is characterized by ensuring excellent base material strength, Young's modulus, corrosion resistance, and welded part strength by being constructed of duplex stainless steel.

Next, in the high-strength duplex stainless steel for car trunks of the present invention, the reason why the composition ratio and Ni bat value are numerically limited as described above will be explained in detail.

(1) C The C component has the effect of improving the strength of stainless steel, and from this point of view it is a preferable element, but since a high C content causes deterioration of corrosion resistance, the C content should be 0.
It was set at 0.5% or less.

(iil 5L Si component has the effect of improving the strength of stainless steel as well as its Young's modulus, but if its content is less than 1%, the desired effect cannot be obtained; If Si content exceeds 1%, the steel will harden and the elongation will deteriorate.

0ii) Mn The Mn component has the effect of securing the austenite phase at a relatively low cost, and it is preferable to add a large amount in order to reduce the amount of Ni, which has the same effect. However, if its content is 3% or less, the austenite phase is tends to decrease significantly and become a single ferrite phase, which may lead to deterioration of elongation.
Since Mn content exceeding 10% causes deterioration of Young's modulus, the Mn content is set to a value exceeding 3% and 10% or less.

Qv) Cr Similar to Si, the Cr component has the effect of improving the strength and Young's modulus of steel, but if its content is less than 17%, the desired strength cannot be secured. The Cr content was set at 17 to 22% because if the Cr content exceeds 10%, the elongation deteriorates markedly and σ brittleness easily occurs.

The NiN1 component is extremely effective in securing the austenite phase, and it is necessary to add 1 t or more to ensure the desired elongation, but if it is added in excess of 7 ts, the Young's modulus of the steel will decrease. Since the Ni content deteriorates significantly, the Ni content was set at 1 to 7%.

(v# Cu The Cu component tends to have the effect of significantly improving the crevice corrosion resistance of stainless steel, and also has a great effect of preventing stress corrosion cracking starting from crevice corrosion, but if the content is less than 0.5 - However, the Cu content was set at 0.5 to 3% because the desired effect could not be obtained in the above-mentioned action, and on the other hand, if the Cu content exceeded 3%, the Young's modulus of the steel would be significantly degraded, similar to N1. .

Chapter p M.

The MO acid component has the effect of improving the corrosion resistance of steel, particularly the corrosion resistance to non-oxidizing acids, pitting corrosion resistance, and plow corrosion resistance, but if its content is less than 0.5%, the desired effect is not achieved. On the other hand, MO content of 0.5 to 3 chips tends to precipitate the σ phase and cause embrittlement, and is also disadvantageous in terms of cost. It was determined that

Figure 4 shows the MO content and pitting potential (0.5 M NaC2 at 40°C) in three types of Cr-Ni stainless steel.
FIG. 4 is a graph showing the relationship between MO content (in an aqueous solution), and it can be seen from FIG.

(viiD N N is an unavoidable element that is normally contained in stainless steel at about 0.01%, but since it is a strong austenite-forming element and an interstitial element, it strengthens the austenite phase and increases the It has the effect of improving strength such as 2% proof stress and tensile strength.Therefore, it is an element that is actively added when higher strength is required, but N content of 0.0
If it is less than 4%, the desired effect cannot be obtained; on the other hand,
If the N content exceeds 0.20%, the deterioration of hot workability will be significant, so the N content should be 0.04 = -0.20
%.

Gx) Nb Nt) is added in combination with N to refine the crystal grains of the steel after solution treatment and to form Nt) precipitates.
．． Since it has the effect of improving the tensile strength and tensile strength, it can be added or contained when high strength is required, but if the content is less than 0.051, the above effects will be impaired. The desired effect cannot be obtained, and on the other hand, if the Nb content exceeds 0.50%, the hardening of the steel will become significant and the workability after cold rolling will deteriorate, so the Nb content should be 0.05 to 0.
．． It was set at 50%.

(2) Ni bat value As shown in FIGS. 2 and 3, a smaller Ni bat value is preferable for improving Young's modulus, but conversely, a larger Ni bat value is preferable for improving elongation. As a material for vehicles, considering its workability, the Ni bat value [-
Ni bat The value is (-12 to -6
).

Note that S, which is an unavoidable impurity in the ferritic-austenitic duplex stainless steel of the present invention, does not cause any particular inconvenience if it is at the level normally associated with it. The following regulations are highly recommended since the hot workability of steel will be further improved.

Next, the present invention will be specifically explained using Examples and comparing with Comparative Examples.

<Example> First, a 10-square steel ingot was melted in a small experimental high-frequency furnace.
501111 at a heating temperature of 1250℃! 6B from thickness
After hot rolling to a thick thickness, annealing at 1050°C and pickling were performed, the mechanical properties and corrosion resistance were investigated. In addition, the corrosion resistance test was carried out at 25°C [2% FeCl2 + O,
A method was adopted in which the specimen was immersed in an aqueous solution of 1 M HCL for 48 hours and the corrosion state was visually inspected.

The results are also shown in Table 1.

As is clear from the results shown in Table 1, the steel of the present invention is significantly superior in Young's modulus and corrosion resistance to the conventional steel. Separately, the joint strength after welding was also measured, and it was confirmed that the steel of the present invention exhibited an excellent value also in the joint strength.

<Overall Effects> As explained above, according to the present invention, it is possible to exhibit high strength and elongation without performing cold rolling such as skin pass rolling, as well as exhibit excellent Young's modulus and corrosion resistance, making it suitable for vehicles. This brings about extremely useful effects industrially, such as being able to provide duplex stainless steel suitable as a material.

[Brief explanation of drawings]

Figure 1 is a graph showing the relationship between the structure, tensile strength, and elongation of stainless steel. Figure i4 is a graph showing the relationship between Ni bat and Young's modulus of stainless steel. Graph showing the relationship between Ni bat and elongation d. FIG. 4 is a graph showing the relationship between Mo content and pitting potential of stainless steel. Applicant Nippon Stainless Co., Ltd. Agent Tomi 1) Kazuo and one other person Youngji
(gf/mm2) Is it the largest? (gf/mm2) Stretch (”/,) lk Electric if ((V) [V, S, C, E
】Kamibi° (%)

Claims (3)

[Claims] (1) In terms of weight percentage, C: 0.05% or less, Si: more than 1 to 5%, Mn: more than 3 to 10%, Cr: 17 to 22%, Ni: 1 to
7%, Cu: 0.5-3%, Mo: 0.5-3%, and the balance consists of Fe and inevitable impurities, and Nibal value = -1.1 {Cr (%) + 1.5 ×Si(
%)+0.5×Nb(%)+Mo(%)}+30{C(
%)+N(%)}+0.5×Mn(%)+0.3×Cu
(%) + Ni (%) + 8.2 High-strength two-phase for vehicles, characterized by an N1bal value of [-12 to -6] according to the formula, which has excellent Young's modulus and corrosion resistance, and does not deteriorate the strength of welded parts. stainless steel. (2) In terms of weight percentage, C: 0.05% or less, Si: more than 1 to 5%, Mn: more than 3 to 10%, Cr: 17 to 22%, Ni: 1 to
7%, Cu: 0.5-3%, Mo: 0.5-3%, N: 0.04-0.20%, and the remainder consists of Fe and inevitable impurities, and Nibal value = - 1.1 {Cr(%)+1.5×Si(
%)+0.5×Nb(%)+Mo(%)}+30{C(
%)+N(%)}+0.5×Mn(%)+0.3×Cu
(%) + Ni (%) + 8.2 High-strength two-phase for vehicles, characterized by a Nibal value of [-12 to -6] according to the formula, which has excellent Young's modulus and corrosion resistance, and does not deteriorate the strength of welded parts. stainless steel. (3) In terms of weight percentage, C: 0.05% or less, Si: more than 1 to 5%, Mn: more than 3 to 10%, Cr: 17 to 22%, Ni: 1 to
7%, Cu: 0.5-3%, Mo: 0.5-3%, N: 0.04-0.20%, Nb: 0.05-0.50%, and the balance is Fe and It consists of unavoidable impurities, and Nibal value = -1.1 {Cr (%) + 1.5 × Si (
%)+0.5×Nb(%)+Mo(%)}+30{C(
%)+N(%)}+0.5×Mn(%)+0.3×Cu
(%) + Ni (%) + 8.2 High-strength two-phase for vehicles, characterized by a Nibal value of [-12 to -6] according to the formula, which has excellent Young's modulus and corrosion resistance, and does not deteriorate the strength of welded parts. stainless steel.