JPS5928621B2 - Duplex stainless steel with excellent hot workability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a duplex stainless steel containing 10 to 75% of ferrite and the remainder being austenite, which has excellent hot workability.

Duplex stainless steel is corrosion resistant (especially stress corrosion cracking resistant)
, has been widely used in recent years due to its excellent weld cracking properties. However, since duplex stainless steel contains ferrite and austenite phases with different mechanical properties, cracks can occur at the interface between the ferrite and austenite phases during hot working such as blooming and hot rolling. It is a well-known fact that as the ferrite phase increases, workability deteriorates significantly. Therefore, duplex stainless steel has a very poor yield in the blooming and hot rolling processes. Furthermore, even if the yield of forged blooming is improved, it is still extremely disadvantageous in terms of workability and economy. The present invention relates to a duplex stainless steel that improves the yield by reducing the occurrence of cracks in the blooming and hot rolling processes. SiO, 01-3
%, Mn 15% or less, Cr 15-35%, Ni 10-3
0 ratio, N: The main component is 0.01-004%, Nb■
2% or less, Mo■ 6% or less, Ti: 1% or less, Cu
: Contains one or more types of 3 or below, and further A6,
0501-6% of one or more of Ga and In
2 with excellent hot workability, characterized by containing P: 0.08% or less, S: 0003% or less, the remainder consisting of Fe and unavoidable impurities, and having a ferrite content of 10 to 75 degrees. The phase is in stainless steel. S and P, which are conventionally contained as impurities in stainless steel, are 0.006 to 0.02010 and 001 to 0, respectively, unless a special melting or refining method is applied to them or the raw materials are carefully examined.
It is normal for the content to be about 0.3%.

Even if these amounts of impurities P and S are contained, there is almost no adverse effect on hot workability in the case of ferritic stainless steel, and in many cases it is not so great even in the case of austenitic stainless steel. However, in duplex stainless steel, even if P and S are contained in this amount, the hot workability is significantly deteriorated.

S can be reduced by special melting methods, refining methods, etc. It is difficult to reduce P by refining during steel melting, and low-P raw materials are usually used. However, the use of low P raw materials is extremely disadvantageous industrially and economically since the raw materials themselves are very expensive and the products become very expensive. The inventor believes that P is O,
Based on the idea that it is sufficient to fix P even if it contains more than O1%, we added various elements and examined the hot workability test results of duplex stainless steel. They discovered that the negative effect of P on hot workability was eliminated.

That is, group IIIB elements, group VB elements, GaAs, InSb
Although it is well known that m-v group compound semiconductors can be made, the present inventors have discovered that even in steel, MB group elements have a strong interaction with VB group elements. At this time, it is not enough to simply remove the negative effects of P; it is also necessary to remove the negative effects of S at the same time. When we observe the structure of cracks in duplex stainless steel, we find that ferrite crystal grains and austenite crystal grains coexist, and cracks during hot working occur from the two-phase interface between these austenite grains and ferrite grains. is propagating.

The reason why workability improves when P is fixed with IIIB group elements such as AA, Ga, and In is because solid solution P is reduced.
This is because solid solution P at grain boundaries is also reduced, and the bonding strength of austenite grain boundaries, ferrite grain boundaries, and grain boundaries between ferrite grains and austenite grains is each significantly increased.

Another reason is the difference in hot deformability between the ferrite and austenite phases due to the difference in solid solubility of P between the two phases.

That is, if a large amount of P is present in solid solution, more P is dissolved in the ferrite grains than in the austenite grains, making the ferrite grains extremely hard and significantly deteriorating the deformability of the ferrite phase at high temperatures. Therefore, the interface between ferrite grains and austenite grains becomes more susceptible to cracking as the solid solution P increases. Furthermore, the reason why hot workability is improved by reducing S is that, as in the case of P, by reducing S, grain boundary S
This is because the bond strength of grain boundaries is significantly increased.

In order to lower the S content in steel, it can be lowered by special melting or refining methods. Of course, the hot workability of the duplex steel can be improved by appropriately controlling the crystal grain shape and distribution of the second phase according to the quantitative ratio of the two phases with different deformability that constitute the duplex steel. However, the ferrite grains containing a large amount of P in solid solution are very strong, and it is extremely difficult to change the shape distribution even by heat processing, and the degree of effect is small. Therefore, in order to improve hot workability, it is essential to simultaneously lower the amounts of solid solution P and S. Next, the reasons for limiting the chemical composition in the present invention will be explained.

C is an element necessary to increase strength, and has a minimum of 000
5% or more is required.

Also, if too much is added, carbide will precipitate at grain boundaries and deteriorate hot workability, so the upper limit should be set at 0.2%.
shall be. The preferred range is 0.01-008%.
Si is added for deoxidation during steel manufacturing, and for this purpose 0.01% or more is required. Si also increases the oxidation resistance during use at high temperatures, but if the amount exceeds 3%, workability and weldability are inhibited, so the upper limit of Si is set at 3%. M
n is added to deoxidize and prevent hot embrittlement. However, it has a strong effect of stabilizing austenite and can be used as an alternative element to the expensive Ni.

However, if too much Mn is present, the oxidation resistance deteriorates, so the amount is limited to 15% or less. Although 15% or more of Cr is necessary to improve oxidation resistance, too much Cr tends to cause sigma embrittlement, so it is set to 35% or less.

Ni has the effect of preventing sigma embrittlement, carburization, and nitridation, but if too much Ni is added, the effect will almost reach saturation and the material cost will increase, so it was set to lθ ~ 30%.

Nb is a strong carbide-forming element like Ta, and C weight%
Intergranular corrosion resistance is improved by adding about 10 times as much.

(Ta shows half the effect of Nb). In addition, Nb forms fine carbides and nitrides and preferentially precipitates on dislocations, improving high-temperature strength and creep properties. However, if added in excess, hot workability deteriorates, so the upper limit is set at 2. M
When O is added in an amount of 0.5% or more, it exhibits an extremely excellent corrosion resistance effect against non-oxidizing acids. Furthermore, MO strengthens the matrix and increases high temperature strength and creep rupture strength. However, alloying in excess of 6% causes abnormal oxidation, resulting in significant deterioration of high temperature oxidation resistance. Furthermore, since the aforementioned effect also reaches saturation, its upper limit is set at 6%. Ti exhibits strong deoxidation, denitrification, and desulfurization effects.

It is also a strong ferrite-forming element, with a C weight ratio of 4 to 4.
6 times more Ti prevents intergranular corrosion. Pitting corrosion resistance deteriorates significantly when Ti is present as precipitates or inclusions, so the upper limit is set at 1%. Cu is uniformly dissolved as a solid solution in austenite up to about 3%, strengthens the matrix, and increases corrosion resistance against non-oxidizing acids.

It also exhibits resistance to carburizing, nitriding, and oxidation. However, if it exceeds 3%, hot workability deteriorates, so the upper limit is set at 3%. N is normally contained in stainless steel (,.0
Although it is contained at about 1%, it is a strong austenite-forming element and is used as a substitute for Ni. When added in the atmosphere, the upper limit is 0.4%, and this amount of N significantly increases the deformation resistance and makes hot working extremely difficult, so the upper limit is set at 0.4%. S is an impurity element contained in steel, but it is an element that needs to be removed in order to improve hot workability, and hot workability can only be improved if S is reduced to 0.03% or less.

P is an impurity element contained in steel and is usually 0.03%.
The effectiveness of group IIIB is demonstrated in 008% of the following cases.
That's it.

Therefore, the upper limit is set to 0.08. Al(Ga,In
(including group IIIB elements such as P) is an element necessary to fix P and improve hot workability, and its effect is not recognized at less than 0%, and the effect reaches saturation at about 2%. is 6
The effect remains the same up to 6%, so the upper limit is set at 6%.

The preferred range is 0.006 to 5.5%. The reason for limiting the amount of ferrite to 10 to 75% is that normally produced materials usually contain 2 to 3% of ferrite phase in order to keep the added alloy amount to the lower limit of the specification for economic efficiency. The ferrite phase was intentionally increased outside this range, and the lower limit was set at 10%, which causes poor hot workability.

Since it is said that the workability is usually extremely poor in the range of 10 to 75%, the upper limit is set at 75%.

Next, examples of the present invention will be described.

Example Samples with the compositions shown in Table 1 were melted in vacuum and test pieces were prepared from them, and subjected to Greeble test (Takafuchi high speed tensile test) and small block test piece (120 (shoe) x (SO) (M)
Hot workability was investigated by a continuous 8-pass rolling test using Xl9O(FJ).

The amount of ferrite in Table 1 is measured using a ferrite meter, observing the microstructure photographs of 30 fields of view, then measuring the ferrite area ratio, and converting it to a volume ratio.The average amount of ferrite measured using the ferrite meter is 1%. The calculated value agreed with the difference in degree. Figure 1 is /F;. This is a part of a photograph of the structure of No. 1 steel. The amount of ferrite calculated from the microstructure photograph was 12.1%, and the amount of ferrite measured with a ferrite meter was 11.3%. Figure 2, Figure 3,
Figures 4 and 5 show S and A by Greeble test.
This study investigated the influence of group IIIB elements such as l on hot workability.

Bian 1 to 59 steels are comparison steels containing a large amount of P and S, or a large amount of either one of them, and 510 to 15
The steel is the steel of the present invention. Steels of the present invention (510, 511, 512, 51) with low S and fixed P with IB group elements such as 8l
3, Boiled 14 and 415 steels) have higher ductility (
It can be seen that the aperture ratio) has significantly improved. It has been empirically found that the reduction ratio of 510, 511, 12, K13, 514 and 515 steels in the Greeble test is suitable for blooming.

Therefore, the steel of the present invention, in which S is low and P is fixed with a group III8 element such as Al, can be subjected to blooming rolling. The comparative steel has a narrow blooming rolling control range, and in practice, it is difficult to perform blooming rolling on an industrial scale. Reference diagram is 41 steel, AI
A small block material cut from O steel was passed through 8 consecutive passes (12
0m→20 layers) It is a photographic diagram showing the appearance of edge cracks in the rolled material after rolling.

As expected from the Greeble test, no edge cracking was observed in the &10 steel, but significant edge cracking was observed in the &1 steel. As explained above, the steel of the present invention has markedly improved hot workability, and therefore can be subjected to split rolling, which has been difficult in the past, and the effect is extremely large.

[Brief explanation of the drawing]

FIG. 1 is a photograph of the structure of flat steel, and FIGS. 2, 3, 4, and 5 are diagrams showing the results of the Greeble test (high-temperature, high-speed tensile test).

Claims (1)

[Claims] 1 C: 0.005-0.2%, Si: 0.01 by weight
~3%, Mn: 15% or less, Cr: 15-35%, Ni
:10~30%, N:0.1~0.4% as the main component,
Nb: 2% or less, Mo: 6% or less, Ti: 1% or less, C
u: 3% or less of one or more types plus Al, Ga
, contains 0.01 to 6% of one or more of In, P: 0.08% or less, S: 0.003% or less, and the balance consists of Fe and unavoidable impurities, and the amount of ferrite is A duplex stainless steel with excellent hot workability characterized by having a content of 10 to 75%.