JPS6077963A - Austenitic stainless steel for car - Google Patents

Abstract

PURPOSE:To increase fatigue resistance, flexural properties, weather-ability and manufacturing properties by adjusting the content of Mo contained in austenitic stainless steel for car and also limiting the amount of delta ferrite to the value not exceeding a specified value. CONSTITUTION:The austenitic stainless steel used as the material for various cars has the following composition. As the composition, <0.08% C, <1.0% Si, <2.0% Mn, <0.030% S, 6-8% Ni, 16-18% Cr, 0.20-1.00% Mo, 0.05-0.30% N, <0.50% Cu are contained and the amount of delta ferrite shown by an equation I is regulated to the value not exceeding 9.0%. On the other hand, 0.02-0.80% (Nb+ Ta) is incorporated additively into said composition and the amount of delta ferrite shown by an equation II is regulated to the value not exceeding 9.0%. The fatigue resistance is increased by the addition of Mo and the deterioration of delta brittleness and the workability in heat treatment due to the addition of Mo is prevented by controlling the amount of delta ferrite.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to austenitic thread stainless steel for vehicles.
In particular, the present invention relates to an austenitic thread stainless steel for transportation vehicles that has excellent fatigue resistance, bendability, weather resistance, and excellent manufacturability.

Conventionally, carbon steel, light alloys, and the like have been used as materials for transportation vehicles such as railway vehicles, seven-rule vehicles, and convener vehicles. However, light alloys such as carbon steel and aluminum have poor strength and corrosion resistance, and require maintenance such as painting. Recently, stainless steel, which has high strength and excellent corrosion resistance, has overcome the drawbacks of conventional materials and has reduced the plate thickness. It can be made thinner and lighter, achieving high speed and energy saving.
Furthermore, it has come to be used because it has excellent safety advantages.

In particular, the characteristics of metastable austenitic stainless steel, which is mainly used for this purpose, are that it has better formability than ferritic stainless steel, and can easily obtain high strength through cold working, so it is suitable for transportation vehicles. Suitable for materials.

In general, metastable austenitic stainless steels include:
C: 0.15% or less, Si: 1.00% or less, Mn: 2
．． 00% or less, P: 0.040% or less, S: 0.030
% or less, Ni: 6.00-8. O0%, Cr:1
SUS 301 with a composition of 6.00 to 18.00% is known, but in recent years it has hardly been used as a material for transportation vehicles. The reason is as follows.

(b) Stainless steel for transport vehicles must be able to withstand the 1k load of returned green material received while driving from a safety point of view, and at the same time, it is required to have excellent fatigue resistance, and at the same time, it has become particularly important in recent years. In addition to the high speed and low fuel ratio that brings energy savings, there is also a growing demand for light weight. However, conventional 5US301 has a low fatigue resistance limit and is not suitable as a material for transportation vehicles in terms of safety and light weight. That is, materials with a low fatigue resistance limit have to be made thicker from the viewpoint of safety, and their light weight is impaired, resulting in unsatisfactory high speeds and low fuel ratios. Such a low fatigue resistance limit makes it impossible to ensure safety and light weight, which is a serious drawback as a material for transportation vehicles.

(b) Large amounts of various pollutants and corrosive gases are emitted into the atmosphere, such as factory smoke, automobile exhaust gas, and smoke from building heating. Among these air pollutants, sulfur dioxide gas is particularly corrosive, and due to the nature of Japan being an island country, industrial areas are concentrated on the coast, so the combined action of sulfur dioxide gas and chloride ions causes further corrosion. Promotes vomiting.

Under such an environment, materials for transportation vehicles are naturally required to have weather resistance.

However, compared to SUS 304, SUS 301 has N
Since it has low i, Cr and high C, it cannot be said to be a sufficiently weather-resistant material, and it corrodes especially as it is lighter, but conventional 5U
S301 did not have sufficient bendability.

The purpose of the present invention is to overcome the drawbacks of the conventional stainless steel for vehicles, to make transport vehicles safer and lighter, and to extend their service life by improving fatigue resistance, bendability, and weather resistance. An object of the present invention is to provide a metastable austenitic stainless steel for vehicles which is excellent and has good manufacturability.

This object of the present invention can be effectively achieved by any of the two inventions summarized below.

The gist of the first invention is as follows.

That is, in terms of weight ratio: C: 0.08% or less, Si: 1
．． 0% or less, Mn: 2.0% or less, S: 0.030
% or less, Ni: 6-8%, Cr: 16-18%, M
o: 0.20-1.00%, N: 0.05-0.
30%, C0: 0.50'A or less, and δ ferrite = 2.90 (%Cr-+-1.5x%
Si + 0.5X
%Mn-1-30X (%C + %N) + 0.3x%Cu)
The amount of δ ferrite determined by the relational expression -26,0 is 9.0
% or less, with the remainder consisting of Fe and unavoidable impurities.

The gist of the second invention is as follows.

That is, in addition to the same basic composition as the first invention, Nb
+Ta: 0.02 to 0.80%, and δ ferrite = 2.90 ((%Cr+1.5X%Si+
%Mo+0.5x(%Nb+%Ta))-2,07
(%Ni+0.5x%Mn+30X(%C0%N)+0
．． This is an austenitic stainless steel for vehicles, characterized in that the amount of δ ferrite determined by the relational expression: 3x%Cu)-26,0 is 9.0% or less, and the remainder is composed of Fe and unavoidable impurities. .

That is, the present inventors have conducted repeated research on its fatigue resistance, bendability, weather resistance, etc. with the aim of developing an austenitic stainless steel that overcomes the drawbacks of the conventional austenitic stainless steel for vehicles. By adjusting the content of MO, whose effect on fatigue properties had been neglected, and by adjusting other component elements, we have developed a new material that has excellent fatigue resistance, bendability, and weather resistance, and is even easier to manufacture. We have succeeded in developing a good dual-use metastable austenitic stainless steel rust.

The reason for limiting the components in the present invention will be explained.

C: If the amount of C increases and exceeds 0.08%, the workability and corrosion resistance of the curve after hard rolling will deteriorate, so it was limited to 0.08% or less.

Si: 5i has a great effect on hardening the material, but if it exceeds 1.0%, it forms silicate inclusions and deteriorates fatigue resistance, so it was limited to 10% or less.

Mn: Mn is a strong austenite-forming element and is an effective element for austenitizing the structure after solution treatment, but when it exceeds 2.0%, MnS-based inclusions increase,
In addition, fatigue resistance was limited to 2.0% or less due to deterioration of hair growth.

S: In general, it is preferable that S is low because it can deteriorate weather resistance, but lowering it requires careful selection of raw materials and addition of desulfurization agents, which increases cost. However, in the steel of the present invention, as described below,
Good weather resistance can be obtained by adding Mo, so it should be limited to 0.030% or less like normal austenitic stainless steel.cct Ni: Ni has a strong austenite-forming ability and is a characteristic of austenitic stainless copper. It is an important element, and at least 6
Requires 5A. However, if Ni becomes excessive and exceeds 8%, it will interfere with the transformation into martensite during cold working, making it difficult to obtain high strength, so the content is limited to a range of 6 to 8%.

Cr: If the Cr content is less than 16%, the weather resistance will deteriorate significantly, so the lower limit was set at 16%. On the other hand, if it exceeds 18%, an austenite structure cannot be obtained after solution treatment, so the upper limit was set at 18%, and the range was limited to 16 to 18%.

MO= In order to limit the Mo content, the present inventors conducted the following experiment. That is, using steel whose other components are within the limited range of the present invention, only the MO amount is adjusted from 0.10 to 1.50.
Austenitic stainless steels with varying degrees of % were melted and subjected to fatigue tests, weather resistance tests, and bending tests after cold working. The results are shown in Table 1. In addition, among these tests, the tensile strength after cold rolling was 110
The fatigue test for the first group specimens of IQ'/- was carried out by Sienck plane bending fatigue test, and the stress amplitude was 48 kv/-, and the number of repetitions until breaking pressure was measured. In the cold weather resistance test, the surface condition was compared after being exposed for 240 hours to a corrosive atmosphere containing 1100 pp of sulfur dioxide gas. Next, for the test, the bending angle is 135 degrees, the bending radius is r, and the thickness of the test is t.
Then r-12t'! The specimens were bent and the presence or absence of cracks was compared.

Next, for the test materials shown as the second group in Table 1 whose tensile strength after cold rolling is 90 kHz/-, the fatigue test was conducted using the Sienck plane bending fatigue test in the same manner as the first group, and the stress The amplitude is measured by the number of repetitions to break at a stress of 34 to 9/-,
The weather resistance test is the same as the first group above, and the bending test is 18
It was bent to 0 degrees rt and the presence or absence of cracks was compared.

It is clear from Table 1 that if the amount of MO is less than 0.20%, fatigue resistance and weather resistance are poor;
It is shown that bending workability deteriorates when the value exceeds 1-. Therefore, in the present invention, the content of MO is limited to a range of 0.20 to 1.00%. As such <0.20-1.00% M
The inclusion of O is a major feature of the present invention.

By the way, the effect of improving fatigue resistance due to Mo inclusion is presumed to be due to the following reason. That is, MO is a solid solution in austenite or martensite produced by cold working, but M.

When atoms form a solid solution, the crystal lattice is greatly distorted, creating large lattice distortions. This lattice strain inhibits the movement of dislocations, which in turn suppresses the generation of fatigue cracks, and even if fatigue cracks occur, their growth and propagation may be hindered due to the lattice strain around the MO atoms. It is thought that this will lead to fracture. Dropped, M
The effect on fatigue life based on the interaction between lattice strain caused by solid solution in IJ flux and dislocations generated during the fatigue process of so-called substitutional atoms such as MO can also be caused by elements other than MO. In the case of Mo, the atomic radius is considered to be appropriate for producing the above effect.

N: N is an effective element for improving strength, but 0.0
If it is less than 5%, there is almost no effect, so the lower limit is set at 0.05%.
And so. If the addition of N increases and exceeds 0.30%, the above effect will be saturated, and it is difficult to add more than 0.30% in normal steel shell work, so the upper limit is set at 0.30%, and 0.30% is added.
．． It was limited to a range of 0.05 to 0.30%.

Cu: Hot workability deteriorates if the Cu content exceeds 0.150%, so it was limited to 0.50% or less.

The above composition is the basic composition of the austenitic stainless steel of the present invention, and in addition, Nb and Ta: 0.02 to
A case including conversion to 0.80 is also effective in achieving the purpose of non-invention. The reasons for the seven limitations are as follows.

Nb and Ta: Nb and Ta are effective for improving the intergranular corrosion resistance of austenitic stainless steel, but their effects ii:
If the total amount of Nb and Ta is less than 0.02%, it is ineffective.
Also, if it exceeds 0.80%, the bending workability deteriorates, so 0.
．． It was limited to a range of 0.02 to 0.80%.

As mentioned above, each component is limited, and especially non-invention is M.

Although we succeeded in providing excellent fatigue resistance by adding 0.20 to 1.00% of MO, the addition of MO tends to cause σ embrittlement due to σ phase precipitation and deteriorates hot workability. Therefore, in order to prevent troubles during hot rolling, in the case of a basic composition without the addition of Nb and Ta, the following (1)
) The amount of δ ferrite determined by the formula was limited to 9.0% or less.

δ Ferrite-290 (%Cr-4-1.5X%Si +%Mo)-2,07 (%Ni +0.5X%Mn-t-3
0x(%C0%N)+0.3X%Cu1-26.0-
(1) In addition to the above basic composition, central VCNb+Ta
: When containing 0.02 to 0.80%,
For the same reason as above, the value δ(F)-l determined by the following formula (2) was limited to 9.0% or less.

δ ferrite = 2.90 ((%Cr+1.5x%S
1+%Mo-1-0,5X(%Nb+%Ta))-2,
07(%Ni+0.5x%Mn+3ox(%C0%N)
+0.3X%Cu)-26,0-(2) In this way, the invention limits the main components, especially adds a limited amount of MO, and in order to prevent σ brittleness due to MO addition, each δ determined by equations (1) and (2)
The amount of ferrite is limited to 9.0% or less.
It was possible to obtain an austenitic stainless steel for vehicles that has excellent fatigue resistance, weather resistance, bendability, and good manufacturability.

Inventive steels Ax 1 to 4 as shown in Example Table 2 and comparative steels 5 to 8, which have similar compositions but contain the limited sequence components of the present invention, were used as 1.
The limited column components and the amount of δ ferrite of the present invention in the comparative steels in Table 2, which were melted in a 00 kf high frequency small melting furnace, are underlined.

The above melted one sample material ingot was hot rolled into a hot rolled plate with a thickness of 4 mm, the hot rolled plate was annealed and then cold rolled, and the cold rolled plate was further annealed and hard rolled to give tensile strength. -g1i0ky
A first group sample material with a tensile strength of /- and a first group sample material with a tensile strength of 90/- were obtained.

The presence or absence of split rt during hot working of each of the above specimen materials and the hard L (fatigue resistance, respectively, for each specimen material after 2 rollings)
The results of measuring the weatherability of the corns are shown in Table 2.

In addition, in Table 2, the tensile strength bag after cold) Jll is 11
The first group test material of 0kg/πd and the second group of 90g/-
The test methods for fatigue resistance, weather resistance, and bending properties of group test materials are as follows for Group 1 and Group 2, respectively, in Table 1.
The test method is the same as the test method for the sample in the case of a group.

As is clear from Table 2, the steel of the present invention has significantly better fatigue resistance, weather resistance, and bendability than the comparative steels, has no cracking during hot working, and has excellent manufacturability. It shows that there is.

As is clear from the above examples, the austenitic nutless steel for vehicles according to the present invention limits the main components, further adds a limited amount of (Nb+Ta) as necessary, and in particular contains 0.20 to 1 % of MO, which has not been considered in the past. By limiting the amount of δ ferrite, troubles during hot working due to σ brittleness caused by Mo addition can be prevented, and the product has excellent fatigue resistance, weather resistance, and bendability, and has good manufacturability. We were able to obtain austenitic stainless steel for vehicles, and we were able to achieve the effect of ensuring its safety as a material for transportation vehicles such as railway vehicles that will be used in the future for high-speed fossilization and lightweight bag bricks.

Claims (2)

[Claims] (1) Weight ratio: C: 0.08% or less, Si: 1.
0% or less, Mn: 2.0% or less, S: 0.030%
Below, Ni: 6-s5A, Cr: 16-18%, Mo
: 0.20-1.00%, N: 0.05-0.3
0%, Cu: 0.50% or less, and δ ferrite = 2.90 (%Cr + 1.5x%Si 10%
MO) -2,07(%Ni+0.5X%Mn+30X(
%C0%N)+0.3X%CLI)-26,0 The amount of δ ferrite is 9.0% or less,
An austenitic stainless steel for vehicles, characterized in that the remainder consists of MFe and unavoidable impurities. (2) Weight ratio C: 0.08% or less, Si: 1.0
% or less, Mn: 2.0% or less, S: 0.030% or less, Ni: 6-8%, Cr: 16-18%, Mo:
0.20-1.00%, N: 0.05-0.30%
, Cu: 0.50% or less, further contains Nb + Ta: 0.02 to 0.80%, and δ ferrite = 2.90 ((%Cr + 1.5X%S
i10%MO+0.5x(%Nb+%Ta))-2,07
(%Ni+0.5xXMn+30X(%C0%N)+0
．． 3x%Cu)-26,0 The constant amount of δ ferrite is 9.0% or less, and the remainder consists of Fe and unavoidable impurities.
Austenitic stainless steel for vehicles.