JPH0570683B2 - - Google Patents

Description

[Detailed description of the invention]

<Industrial Application Field> The present invention relates to a method for hot forging high-Si-containing duplex stainless steel that exhibits excellent corrosion resistance even in a high-temperature, high-concentration nitric acid environment. <Background Art> Conventionally, a 25% Cr-20% Ni-based material (URANUS56: trade name) has been used as a material used in a high-temperature nitric acid environment such as in nitric acid production equipment. However, in medium to high concentration nitric acid solutions, and even in nitric acid solutions where Cr ⁶⁺ ions are present, the conventional 25%Cr-20%Ni material cannot be said to have sufficient nitric acid resistance. . Therefore, recently, in such a highly oxidizing environment, 17%Cr-14%Ni-4%Si with increased Si has been developed.
Materials based on 8% Cr-20% Ni-6% Si have also been proposed. In addition, each of the above materials also contains Cr6 ⁺ , which is an oxidizing agent.
Due to insufficient corrosion resistance in hot concentrated nitric acid environment in the presence of ions, the amount of ferrite was reduced to 30 to 70% by volume.
A high Si-containing 25% Cr-20% Ni duplex stainless steel was also proposed (Japanese Patent Application Laid-open No. 33342/1983). However, although these high-Si content steels certainly showed excellent corrosion resistance in oxidizing environments, on the other hand, they had poor hot workability due to their high Si content, making hot forging extremely difficult. There was a problem with this. By the way, it is generally believed that the higher the temperature of duplex stainless steel, the more the amount of ferrite increases and the hot workability improves, and thus it has been heated to a temperature range of around 1250°C and subjected to hot forging. However, high-Si duplex stainless steel did not exhibit sufficient workability even when heated extremely carefully to the above temperature. In addition, ``If duplex stainless steel is heated to the ferrite single phase range, wrinkle defects will occur, so it is better to process it at a high temperature that does not reach the ferrite single phase range (Japanese Patent Application Laid-Open No. 55-89427 ``Method for reducing S and P contents (Japanese Patent Application Laid-open No. 138420/1983)'' as a means to improve the hot workability of duplex stainless steel, and ``Method for reducing N content''. A method of adding/containing Ca, Mg, Y, Ce, La, etc. to reduce the amount of carbon dioxide (Japanese Unexamined Patent Publication No. 127823/1983) was also proposed. However, all of these methods are aimed at duplex stainless steel with a relatively low Si content, and cannot exhibit sufficient effects on duplex stainless steel with high Si content. Although there is no ferrite single-phase region in the high-temperature region, there is a problem in that many defects occur when high-temperature heating and hot forging are applied to steels with a large amount of ferrite. <Means for solving the problem> From the above-mentioned viewpoint, the present inventors have achieved a method that does not cause wrinkles, cracks, etc., and can ensure productivity that is industrially satisfactory. We have discovered a hot forging method for high-Si content duplex stainless steel, and have carried out various experiments and research to provide stable and low-cost structural members that exhibit excellent corrosion resistance even in hot concentrated nitric acid environments in the presence of oxidizing agents. As a result of repeated efforts, the knowledge shown below was obtained. That is, (a) As mentioned earlier, it has been conventionally believed that the higher the temperature of duplex stainless steel, the higher the amount of ferrite, which improves hot workability. This idea does not apply, and if the heating temperature during forging exceeds 1200℃, cracks during forging will increase rapidly, and especially when the heating temperature is around 1300℃, the ductility will become zero, effectively (b) The reason why the hot workability of high-Si duplex stainless steel deteriorates rapidly in the high-temperature range mentioned above is that ingots of such materials have a high Si content. Because of this, the segregation of Si is large, and the melting point of the segregated part is unexpectedly low. (c) However, when a specific amount of Ca is added to such high-Si duplex stainless steel, the corrosion resistance in an oxidizing acid environment is affected. ductility is maintained at high temperatures without any adverse effects, and ductility does not decrease until the temperature exceeds 1220°C; (d) Furthermore, in the Ca-added high-Si duplex stainless steel, Contrary to common sense, 1100 ~
Even through hot forging at a heating temperature as low as 1220℃, the forging work can be completed without any cracks or other defects, allowing us to stably produce highly satisfactory forged products (slabs, etc.). What can be done. This invention was made based on the above findings, and contains C: 0.02% or less (hereinafter, % representing component proportions is expressed as weight %), Si: 2 to 5%, Mn: 0.1 to 2.0%, Cr Duplex stainless steel containing: 20 to 30%, Ni: 5 to 20%, P: 0.02% or less, S: 0.006% or less, N: 0.05 to 0.30%, O: 0.005% or less, and the balance is substantially Fe. 0.001 to 0.020% Ca is added to the steel, and the heating temperature is 1110 to 1220℃ during hot forging.
By performing forging in the temperature range of It is characterized by the fact that forged products can be manufactured stably and reliably. Next, the reason why the component ratio of stainless steel and the hot forging heating temperature are limited as described above in the method of the present invention will be explained. (A) Components of stainless steel C C is an element that promotes sensitization and improves the intergranular corrosion resistance of steel, so it is desirable to reduce it as much as possible. Since the above disadvantages can be reduced to a practically acceptable level, C.
The content was set at 0.02% or less. Si The Si component has the effect of improving the corrosion resistance of stainless steel in a nitric acid solution environment containing Cr ⁶⁺ ions, but if its content is less than 2%, the desired effect cannot be obtained; Corrosion resistance in a solution containing only nitric acid deteriorates as the amount of Si increases, and the practical allowable limit is 5%, so the Si content should be between 2 and 2%.
It was set at 5%. However, it is preferable to ensure a content of 2.5% or more. Mn Mn is an effective element as a deoxidizing agent for steel, and for this purpose a content of 0.1% or more is required, but if it is contained in excess of 2.0%, it will have a negative effect on hot workability. The content was set at 0.1-2.0%. Cr The Cr component has the effect of improving the corrosion resistance of high-Si steel in a nitric acid environment (in order to satisfy the corrosion resistance in a nitric acid environment, Si
It is necessary to increase the amount of Cr as well as the amount),
If the content is less than 20%, the desired effect cannot be obtained, while if the content exceeds 30%, it will cause deterioration of workability and increase in cost. Therefore, the Cr content is set at 20 to 30%. Ta. Ni Ni is an austenite-forming element, and a content of 5% or more is required to realize duplex stainless steel and ensure the desired corrosion resistance, but containing more than 20% has the effect Since this would lead to a noticeable increase in costs, the Ni content was set at 5 to 20%. PP It is desirable to reduce P as much as possible from the viewpoint of intergranular corrosion resistance, but it is practically acceptable at 0.02%.
The content range was determined as follows. SS is also an impurity element that needs to be reduced as much as possible from the viewpoint of intergranular corrosion resistance and hot workability, but its content range was set at 0.006% or less, which is practically acceptable. N N is also useful as an austenite-forming component like the Ni component, but if its content is less than 0.05%, its effect is insufficient and the desired corrosion resistance cannot be secured; on the other hand, if its content exceeds 0.30% Since it is actually extremely difficult to produce steel with a N content of 0.05
It was set at ~0.30%. O O is an element that should be reduced as much as possible from the viewpoint of hot workability, but the content range was set at 0.005% or less, which is practically acceptable. Ca As explained earlier, the Ca component has the effect of improving the hot workability of high-Si duplex stainless steel and expanding the temperature range under hot forging, but the amount added is 0.001. If it is less than 0.020%, the desired effect cannot be obtained.
Since Ca content in excess of 20% causes deterioration of weldability, the amount of Ca added was set at 0.001% to 0.020%. (B) Hot forging heating temperature If the heating temperature during hot forging exceeds 1220℃, high-temperature ductility decreases due to Si segregation, making it impossible to secure a high-temperature ductility reduction value of 80% or more. It becomes impossible to perform stable and sound forging, and on the other hand, if the temperature falls below 1100℃, the deformation resistance during forging increases, causing work difficulties.
The heating temperature during hot forging was set at 1100 to 1220°C. Next, the present invention will be specifically explained with reference to Examples. <Example> First, stainless steel having the composition shown in Table 1 was melted in a high frequency vacuum furnace to produce ingots each weighing 150 kg. Next, a tensile test piece having a parallel portion with a diameter of 7 mm and a length of 70 mm was taken from the center of the obtained ingot and subjected to a high temperature tensile test. The high-temperature tensile test was conducted at a tensile speed of 300 mm/min after holding the test piece at each temperature shown in Table 2 for 10 minutes. The results are also shown in Table 2. As is clear from the results shown in Table 2, a product that satisfies the conditions specified in this invention can secure a "high-temperature ductility reduction value of 80% or more" which allows for sufficiently satisfactory hot forging. On the other hand, it can be seen that those that do not satisfy the conditions specified in the present invention exhibit only low high-temperature ductility, and cannot perform good hot forging. In particular, when the heating temperature is 1300°C, the high-temperature ductility reduction value of high-Si duplex stainless steel is 0, and that of steel without Ca addition is 1200°C.
The ductility is low even at temperatures of

[Table] (Note) * indicates that the conditions are outside the conditions of the present invention.

【table】

[Table] (Note) * indicates that the conditions are outside the conditions of the present invention.
This shows that it is difficult. <Overall Effects> As explained above, according to the present invention, hot forging of high-Si duplex stainless steel, which has been extremely difficult until now, can be carried out stably and easily.
This brings about industrially useful effects, such as making it possible to stably supply structural members that exhibit excellent corrosion resistance even in hot concentrated nitric acid environments where Cr ⁶⁺ ions are present.

Claims (1)

[Claims] 1. In terms of weight percentage: C: 0.02% or less, Si: 2 to 5%, Mn: 0.1 to 2.0%, Cr: 20 to 30%, Ni: 5 to 20%, P: 0.02% or less , S: 0.006% or less, N: 0.05 to 0.30%, O: 0.005% or less, and the balance is substantially Fe, and 0.001 to 0.020% Ca is added and contained, and hot forged. The heating temperature is 1100-1220℃.
A hot forging method for high-Si duplex stainless steel, characterized by a temperature range of .