JP5797461B2 - Stainless steel and manufacturing method thereof - Google Patents

Description

  The present invention relates to a stainless steel and a method for producing the same, which have improved corrosion resistance, hot workability, and strength even when components such as Mo, Nb, and Ti are not included by increasing the Cr content.

  In general, in order to increase the corrosion resistance of stainless steel, Mo is added or an element having a strong affinity for C, such as Ti or Nb, is added. SUS316L, N321, N347, and N317 are known as typical steel types. Alternatively, as disclosed in Patent Document 1, austenite is stabilized by increasing the Ni content. Super stainless steel such as SUS836L is known as a typical steel type. However, Mo and Ni are expensive raw materials and have large price fluctuations. When Ti is added, there may be a problem that the immersion nozzle is clogged in continuous casting. Moreover, since Nb reduces hot workability, there exists a problem that a crack generate | occur | produces in a hot rolling process.

JP2004-149830

  With respect to the above problems, it is possible to improve the corrosion resistance by increasing the content of Cr, which has a relatively small price fluctuation and is inexpensive. However, if the Cr content is increased, the amount of δ ferrite increases, and during hot rolling, ear cracks may occur along the ferrite present at the austenite grain boundaries. Furthermore, at the final stage of the hot rolling process, the ferrite phase changes to a σ phase that is more likely to cause embrittlement, and the hot workability is poor, and troubles such as fracture of the hot strip may occur. Thus, high Cr content steel is a difficult-to-work material and is not easy to process into a thin plate during manufacture.

  Accordingly, the present invention provides a stainless steel that can be manufactured at a low raw material cost while maintaining hot workability and maintaining strength so as not to cause cracking or breaking during hot rolling while maintaining excellent corrosion resistance, and a method for manufacturing the same. The purpose is to provide.

  In order to solve the above-mentioned problems, the present inventors have investigated and studied the corrosion resistance, the hot workability (σ precipitation behavior) and the strength of the high Cr content stainless steel. First, a steel ingot with various chemical components is melted by a high-frequency melting furnace, and hot forging, cold rolling and solidification treatment are performed on this to produce a cold-rolled sheet, particularly from the viewpoint of hot workability From this, manufacturability was evaluated. In addition, the productivity was evaluated from the evaluation of hot workability by the ultra-high temperature tensile test and the yield rate until the hot forging / cold rolling process.

  In particular, for stainless steel to which Cr of 22 mass% or more was added, the balance between Ni equivalent and Cr equivalent was examined, and the components were adjusted with C, Si, Mn, and N so that δ ferrite was 19 volume% or less. In general, as the amount of δ ferrite increases, cracks are likely to occur during hot working. When δ ferrite exceeds 15% by volume, hot working cracks become prominent and manufacturing becomes difficult. On the other hand, when the Cr content is 26% or more, intermetallic compounds such as the σ phase are likely to precipitate during the cooling process of slabs and hot-rolled sheets and the like, resulting in embrittlement and easy cracking, making manufacturing difficult. Therefore, ingots or slabs adjusted with components of C, Si, Mn, and N so that δ ferrite is 19% by volume or less in the range of Ni equivalents from 15% to 22% and Cr equivalents from 23% to 32%. The hot workability (σ precipitation behavior), corrosion resistance, and strength were investigated. The amount of δ ferrite in the steel was determined by measuring with a ferrite meter. According to this measurement result, it was found that the following equation was established within the range of the amount of δ ferrite as described above. In addition, the chemical symbol in a formula shows content (mass%) of the said chemical component.

  δ ferrite (volume%) = 2.8 (1.5Si + Cr) −2.5 (30C + 30N + 0.5Mn + Ni) −14.1

  Furthermore, the productivity was examined by changing the cooling rate of the cast ingot and changing the cooling history at the time of hot rolling. When the cooling rate is slow, in the chemical composition in which δ ferrite is 15% by volume or more, It was also found that sigma phase precipitation was observed.

  From the above examination results, if the steel has the following components, the stainless steel having excellent corrosion resistance aimed by the present invention can be stably obtained without cracking during hot working, and the strength can be increased. It became clear.

The stainless steel of the present invention has been made based on the above findings, and in mass%, C: 0.02 to 0.08%, Si: 0.2 to 1.0%, Mn: 1.2 to 2. 0% P: 0.03% or less, S: 0.005% or less, Ni: 13~15%, Cr: 25.1 ~30%, N: it contains 0.07 to 0.20%, the balance Consists of Fe and inevitable impurities, and satisfies the following formulas 1 and 2.

Moreover, the manufacturing method of the stainless steel of this invention is the mass%, C: 0.02-0.08%, Si: 0.2-1.0%, Mn: 1.2-2.0%, P : 0.03% or less, S: 0.005% or less, Ni: 13-15%, Cr: 22-30%, N: 0.07-0.20%, the balance is Fe and inevitable impurities A method for producing stainless steel that satisfies the following formulas 3 and 4, wherein the raw material is melted in an electric furnace, refined by the AOD method and / or the VOD method, and the molten steel is cast by continuous casting. The slab obtained by the continuous casting is provided with a step of obtaining a slab, hot-rolling the slab to obtain a hot-rolled sheet, then annealing pickling and cold-rolling to obtain a cold-rolled sheet The cooling rate at the time of cooling is set to a cooling rate of 1 ° C./min or more until reaching 700 ° C. In this case, it is preferable that the temperature range in which the steel is cooled from 900 ° C. to 700 ° C. after hot rolling is cooled at a cooling rate of 10 ° C./min or more.

  Hereinafter, the grounds for limiting the numerical values of the stainless steel of the present invention will be described together with the operation of the present invention. In the following description, “%” means “mass%”.

C: 0.02 to 0.08%
When the C content decreases, the production of carbides decreases and the workability of stainless steel improves, so the C content is set to 0.08% or less. On the other hand, C is an element that suppresses excessive generation of δ ferrite, so 0.02% or more is necessary. Therefore, the C content is 0.02 to 0.08%.

Si: 0.2 to 1.0%
Since Si is necessary for deoxidation, it is necessary to add 0.2% or more. On the other hand, Si needs to be reduced to 1.0% or less in order to suppress precipitation of intermetallic compounds such as σ phase. The Si content is 0.2 to 1.0%.

Mn: 1.2 to 2.0%
Mn, like Si, is necessary for deoxidation and has the effect of suppressing excessive generation of δ ferrite, so it is necessary to add 1.2% or more. On the other hand, Mn needs to be reduced to 2.0% or less in order to suppress precipitation of intermetallic compounds such as σ phase. Therefore, the Mn content is set to 1.2 to 2.0%.

P: 0.03% or less P is an element that is inevitably mixed as an impurity, is easily segregated at the grain boundary, and is preferably as small as possible from the viewpoint of hot workability. However, extremely reducing the P content causes an increase in raw material costs and manufacturing costs such as refining. Therefore, the P content is 0.03% or less.

S: 0.005% or less S is an element that is inevitably mixed as an impurity like P, and is easily segregated at the crystal grain boundary, and is preferably smaller from the viewpoint of hot workability. Therefore, the S content is 0.005% or less. The content of S is preferably 0.003% or less.

Ni: 13-15%
Ni is an element effective in suppressing intermetallic compounds such as the σ phase. If the content is less than 13%, the σ phase is likely to precipitate, δ ferrite is excessively generated, Workability is reduced. On the other hand, when the Ni content exceeds 15%, the occurrence of stress corrosion cracking is promoted. Therefore, the Ni content is 13 to 15%.

Cr: 22-30%
Cr is an effective element for improving corrosion resistance, and is the most important and effective element in the present invention. However, if it exceeds 30%, intermetallic compounds such as σ phase are likely to precipitate during the cooling process of slabs, etc., and become brittle and cracks easily occur. Therefore, the Cr content is 22 to 30%. The content of Cr is preferably 23 to 28%, more preferably 24 to 27%.

N: 0.07 to 0.20%
N, like Cr, is an effective element that improves corrosion resistance and suppresses the formation of δ ferrite, and is an important element. Furthermore, it is an effective element for increasing the strength of steel. However, when N is contained excessively, hot deformation resistance is increased and hot workability is hindered. On the other hand, if the N content is large, the Ni equivalent described later increases and the balance with the Cr equivalent is lost. Therefore, the N content is set to 0.07 to 0.20%. The content of N is preferably 0.08 to 0.17%.

Ni equivalent: 15-22
The Ni equivalent is determined by Ni equivalent = Ni + 30 × C + 0.5 × Mn + 30 × N, but if it is less than 15, the σ phase is likely to precipitate, and it becomes brittle and easily cracked. On the other hand, if the Ni equivalent exceeds 22, the desired δ ferrite is difficult to obtain.

Cr equivalent: 23-32
The Cr equivalent is determined by Cr equivalent = Cr + 1.5 × Si. When the Cr equivalent is less than 23, it is difficult to obtain a desired amount of δ ferrite. On the other hand, if the Cr equivalent exceeds 32, the σ phase precipitates and becomes brittle.

δ Ferrite Amount: 19% by Volume or Less Considering conventional alloys, it can be judged that the stainless steel containing high δ ferrite exceeding 19% by volume has deteriorated hot workability. In order to overcome this problem, by controlling the addition amount of Ni and N within the range of the present invention described above, δ ferrite can be suppressed to 19% by volume or less, and hot workability that can be sufficiently processed is achieved. Can be secured. The δ ferrite is preferably contained in an amount of 5% by volume or more. The reason is that when the δ ferrite is lower than 5% by volume, the segregation of P and S becomes strong, and conversely, the hot workability is lowered.

  Next, the manufacturing method of the stainless steel of this invention is demonstrated. The most industrially desirable is efficient and economical to manufacture on a real scale such as 60 tons. First, raw materials such as iron scrap, stainless steel scrap, ferrochrome, ferronickel, pure nickel, and metallic chromium are melted in an electric furnace. Thereafter, oxygen is blown and decarburized and refined in the AOD method and / or the VOD method. The furnace brick used in the AOD method is preferably a MgO-containing refractory such as magchrom or dolomite. The ladle used in the VOD method is preferably a MgO-containing refractory such as magchrom or dolomite. In the case of the AOD method, it is preferable to use Ar and / or nitrogen as the dilution gas.

In the AOD method, a Cr oxide formed in the slag is covered with a lid capable of exhausting the inside of the furnace for depressurization from above and stirred vigorously while blowing Ar gas under depressurization at the end of decarburization. And C in the molten steel can be positively reacted to decarburize. After decarburization, FeSi alloy and / or Al is added to reduce Cr oxide in the slag. Thereafter, limestone and fluorite are added to form a CaO—SiO ₂ —Al ₂ O ₃ —MgO-based slag, deoxidation and desulfurization are performed, and nitrogen gas is blown as necessary to increase the nitrogen concentration in the molten steel. Is controlled within the scope of the present invention.

In order to control the S content to 0.005% or less, the basicity (CaO% / SiO ₂ %) in the slag is desirably 2 to 6. If the basicity is less than 2, the S permissible capacity of the slag is insufficient, so S exceeds 0.005%, and hot workability deteriorates. If the basicity exceeds 6, the hatchability is inferior, the fluidity of the slag is deteriorated and disadvantageous for desulfurization, so that S exceeds 0.005%.

The content of MgO in the slag is desirably 3 to 8%. If the content of MgO is less than 3%, brick erosion progresses and the life of the furnace is shortened. On the other hand, when the content of MgO exceeds 8%, MgO is reduced, Mg is supplied into the molten steel, and MgO · Al ₂ O ₃ spinel is easily generated as nonmetallic inclusions. The spinel inclusions adhere to and accumulate on the inner wall of the immersion nozzle that pours from a tundish into a continuous casting mold. When the spinel inclusions fall off, the spinel inclusions are trapped in the slab as large inclusions having a size of several hundred μm to several mm. In this case, a defect is generated on the surface of the steel plate or inside the steel plate. Therefore, the content of MgO in the slag is desirably 3 to 8%.

  The molten steel, which is refined as described above and whose chemical components are controlled within the range of the present invention, is cast by a continuous casting machine to obtain a slab. The cooling rate at the time of cooling the continuously cast slab is desirably 1 ° C./min or more until 700 ° C. is reached. This is because when the cooling is performed at a cooling rate of less than 1 ° C./min up to 700 ° C., a σ phase is formed in the slab and becomes brittle. Furthermore, the obtained slab is hot-rolled to produce a hot-rolled sheet. In hot rolling, it is desirable to cool at 10 ° C./min or more in a temperature range where the steel is cooled from 900 to 700 ° C. during the hot rolling process. When cooled at a cooling rate of less than 10 ° C./min in the temperature range cooled from 900 to 700 ° C., a σ phase is formed and embrittled. Note that a steel ingot can be produced by melting raw materials such as electrolytic iron, pure chromium, pure nickel, ferrochromium nitride, ferrosilicon, and pure manganese using a high-frequency induction furnace.

  According to the present invention, by controlling the component balance of Si, Mn, Cr, Ni, N without using rare resources such as Mo, Ti, Nb, etc., it is excellent in corrosion resistance, hot workability, and high strength. Stainless steel is offered at a low raw material cost.

Hereinafter, the present invention will be described in more detail with reference to examples. First, raw materials such as iron scrap, stainless steel scrap, ferrochrome, ferronickel, pure nickel, and metallic chromium were melted in a 60-ton electric furnace. Thereafter, in the VOD, oxygen was blown and decarburized and refined. Mugkuro refractory was put on the ladle of VOD. After decarburization, FeSi alloy and Al were added to reduce Cr oxide in the slag. Furthermore, limestone and fluorite were added to form a CaO—SiO ₂ —Al ₂ O ₃ —MgO slag, deoxidation and desulfurization were performed, nitrogen gas was blown, and the nitrogen concentration in the molten steel was controlled. The molten steel refined in this manner was cast with a vertical continuous casting machine to obtain slabs (samples 1 to 23) having the compositions shown in Table 1.

In Table 1, the values calculated from the following formulas 5 and 6 are also shown.

  Each slab was hot-rolled and cold-rolled and subjected to a solution treatment to produce a cold-rolled sheet having a thickness of 2.0 mm. And productivity was evaluated from the yield rate to a hot forging and cold rolling process. Yield rates were evaluated as 85% or more as ◯, 84 to 70% as Δ, and 69% or less as x. In addition, the strength of each sample was evaluated by measuring Vickers hardness. The case where the hardness was Hv200 or more was evaluated as ◯, the case where Hv180 or more and less than Hv200 was Δ, and the case where the hardness was less than Hv180 was evaluated as x.

  Corrosion resistance was evaluated by ASTM G48 Method C, and the case where CPT was 15 ° C. or higher was evaluated as “◯” and the case where it was lower than 15 ° C. was evaluated as “X”. Furthermore, PRE was calculated | required as a parameter | index of corrosion resistance. PRE is obtained from PRE =% Cr + 16 ×% N. Since SUS316L having high corrosion resistance is 25, it is presumed that 25 or more samples have good corrosion resistance. The above evaluation results are shown in Table 2.

  As shown in Table 1, it was confirmed that the tentress steel satisfying the conditions of the present invention was prevented from hot cracking and had good production yield and excellent corrosion resistance. On the other hand, in the comparative example, any of corrosion resistance, hot workability, and strength was inferior to that of the inventive steel.

Sample No. During refining with VOD during the manufacturing process of No. 3, when the basicity of slag (CaO mass% / SiO ₂ mass%) was refined to 1.5, S was as high as 0.009 mass%. Sample No. During refining at VOD during the production process of No. 5, S was refined with a basicity of slag of 8, and S was as high as 0.011% by mass. With these stainless steels, slabs could be produced, but the cracks were large during rolling and did not become products. From the above results, the superiority of setting the basicity of slag in refining to 2 to 6 was confirmed.

  Sample No. The effect of the cooling rate on five slabs manufactured under the condition that the amount of δ ferrite of 1 was as high as 17.3% by volume was investigated. One of the five slabs was cooled at 0.8 ° C./min until the slab temperature reached 700 ° C., and the others were cooled at 1.5 to 3 ° C./min. Such control of the cooling rate was performed by changing the amount of water in the secondary cooling zone.

  Furthermore, the cooling conditions for hot rolling were also changed to investigate the influence. The above results are shown in Table 3. Sample No. A steel plate cooled at 10 ° C./min or more and a plate cooled at 7 ° C./min in the temperature range in which the steel was cooled from 900 ° C. to 700 ° C. in the hot rolling process of 1 were evaluated. Slab No. In No. 1, since the cooling rate was slow cooling of 0.8 ° C./min, as a result of microstructural observation, it was determined that a σ phase was formed in the steel ingot and it was impossible to proceed to the hot rolling process. Also, slab no. In No. 5, since the temperature range in which the steel was cooled from 900 ° C. to 700 ° C. was gradually cooled to 7 ° C./min, the σ phase was formed and embrittled. Product yield was as low as 66%. From the above, the cooling rate when cooling the slab obtained by continuous casting is set to a cooling rate of 1 ° C./min or more until reaching 700 ° C., and the steel is heated from 900 ° C. to 700 ° C. after hot rolling. The effect by cooling the temperature range cooled to 10 degreeC / min or more at the cooling rate was confirmed.

Claims (7)

In mass%, C: 0.02-0.08%, Si: 0.2-1.0%, Mn: 1.2-2.0%, P: 0.03% or less, S: 0.005 % or less, Ni: 13~15%, Cr: 25.1 ~30%, N: contains 0.07 to 0.20%, balance being Fe and unavoidable impurities, and the following equations 1 and 2 Stainless steel characterized by satisfying

The stainless steel according to claim 1, wherein the δ ferrite is 5% by volume or more. In mass%, C: 0.02-0.08%, Si: 0.2-1.0%, Mn: 1.2-2.0%, P: 0.03% or less, S: 0.005 %: Ni: 13 to 15%, Cr: 22 to 30%, N: 0.07 to 0.20%, the balance is composed of Fe and inevitable impurities, and satisfies the following formulas 3 and 4 A method for producing stainless steel, in which raw materials are melted in an electric furnace, refined by the AOD method and / or VOD method, and the molten steel is cast by continuous casting to obtain a slab. A hot-rolled sheet is obtained by rolling, followed by annealing pickling and cold-rolling to obtain a cold-rolled sheet. The cooling rate when cooling the slab obtained by the continuous casting is set to 700 ° C. The manufacturing method of stainless steel characterized by setting it as the cooling rate of 1 degree-C / min or more until it reaches.

The said delta ferrite is 5 volume% or more, The manufacturing method of the stainless steel of Claim 3 characterized by the above-mentioned. The method for producing stainless steel according to claim 3 or 4 , wherein a temperature range in which the steel is cooled from 900 ° C to 700 ° C after the hot rolling is cooled at a cooling rate of 10 ° C / min or more. Method for producing stainless steel according to any one of claims 3-5, characterized in that a 2-6 basicity of slag (CaO mass% / SiO ₂ mass%) in the refining. The method for producing stainless steel according to any one of claims 3 to 6, wherein the content of MgO in the slag in the refining is 3 to 8 mass%.