KR970010807B1 - Austenite stainless steel of excellent corrosion resistance & hot-working characteristic - Google Patents

Abstract

An austenitic stainless steel and a manufacturing process therefor are disclosed, in which, instead of the expensive Ni, there are added Cu as an austenite stabilizing element, and tin amounts of ti as a ferrite forming element and B for improvement of high temperature hot workability, so that the optimum austenite stabilizing temperature(Md30) and the optimum delta-ferrite can be controlled. The austenite stainless steel consists, by weight, of up to 0.05% of C, up to 1.0% of Si, up to 2.0% of Mn, 16-18% of Cr, 6-8% of Ni, up to 0.005% of Al, up to 0.5% of P, up to 0.005% of S, up to 0.03% of Ti, up to 0.003% of B, up to 3% of Cu, up to 1% of Mo, up to 0.045% of N and the balance being Fe and inevitable impurities.

Description

Austenitic stainless steel with excellent press formability, age cracking resistance, corrosion resistance, hot workability and high temperature oxidation resistance

1 is a graph showing the cross-sectional reduction rate according to the change in high temperature tensile strength.

The second is a graph showing the weight by the high temperature oxidation according to the change in heating time at 1270 ℃.

The present invention relates to an austenitic stainless steel having excellent press formability, age-resistant balance, corrosion resistance, hot workability and high temperature oxidation resistance.

Generally, austenitic stainless steels, represented by 18% Cr-8% Ni (STS304) steels, have superior moldability, corrosion resistance, and welding characteristics compared to ferritic stainless steels, and thus have a wide range of use for various molding applications.

However, since a large amount of expensive Ni element is added, manufacturing cost increases.

Therefore, attempts have been made to produce stainless steel having excellent moldability even though the amount of expensive Ni added is reduced.

In this case, STS304JI steel (Korean Patent Application No. 1166, No. 16,071) was developed, which improved the formability by adding 2% less expensive Cu than Ni instead of Ni element, but the corrosion resistance was lower than that of the existing STS 304 steel. There is a problem of deterioration and the use is restricted. In addition, as one of attempts to improve moldability and corrosion resistance at the same time, a technique of increasing 2% Cu and Mn content instead of expensive Ni and adding Mo to improve corrosion resistance is disclosed in Japanese Patent Application Laid-Open No. 52-119414, Japan Japanese Patent Laid-Open No. 84-033633, Japanese Patent No. 75-005646 and Japanese Patent Laid-Open No. 83-056746. However, the above technologies have a problem that the aging crack resistance is lowered when the C content and the nitrogen content in the steel are high, and even when the C content is low, the surface defects during hot rolling of the slab due to the high temperature oxidation resistance and the hot workability decrease. It is very likely to occur. In addition, there are drawbacks in that there is a high risk of occurrence of a blue color during bright annealing performed for manufacturing the bright annealing plate.

As another attempt, there is Japanese Patent Application Publication No. 81-01412 in which Cu and Mo are added to 0.1% or less of C and 2% or less of Mn. However, in this case, high surface defects are caused by deterioration of hot workability by addition of Mo and Cu, which are poor in hot workability. There is a problem.

As another attempt, Japanese Patent Publication No. Hei 1-92342 adds a small amount of Ti and B to Cu-containing steel and adds 50 ppm or less of oxygen and CaO or 0.06% or less to improve hot workability and formability by suppressing inclusion formation. The river that has been made is presented. However, in this case, the expensive Ni content is higher than 8%, and thus, the manufacturing cost is high compared to the existing STS304 steel and STS304JI steel, which has an uneconomical disadvantage.

Accordingly, the present inventors have conducted research and experiments to improve all the problems of the prior art, and based on the results, the present invention proposes the present invention. Instead of expensive Ni, an austenite stabilizing element such as Ni is used. In order to improve phosphorus Cu and corrosion resistance, Ti and B were added simultaneously to control the proper M _d30 temperature and delta-ferrite content in slab to improve moldability, aging crack resistance and corrosion resistance. The purpose of the present invention is to provide an austenitic stainless steel which reduces the production cost and reduces the occurrence of surface defects during hot rolling by improving the hot workability and high temperature oxidation resistance which are problematic.

Hereinafter, the present invention will be described.

The present invention is in weight% C: 0.05% or less, Si: 1.0% or less, Mn: 2.0% or less, Cr: 16-18%, Ni: 6-8%, Al: 0.005% o or less, P: 0.5% or less , S: 0.005% or less, Ti: 0.03% or less, B: 0.003% or less, Cu: 3% or less, Mo: 1% or less, N: 0.045% or less, press formability composed of residual Fe and other unavoidable impurities, The present invention relates to an austenitic stainless steel having excellent anti-aging cracking resistance, corrosion resistance, hot workability and high temperature oxidation resistance.

Also in the austenitic stainless steel of the present invention, which is formulated as described above, [M _d30 (° C.) = 551-462 (C% + N%)-9.2 (Si%)-8.1 (Mn%)-29 (Ni % + Cu%)-13.7 (Cr%)-18.5 (Mo%)-1.42 (crystal grain size-8.0)] to limit the austenite stabilization temperature [M _d30 (° C)] to -25 ° C to + 10 ° C. Delta-ferrite content (%) = [{Cr% + Mo% + 1.5Si% + 18}] / (Ni% + 0,52Cu% + 30C% + 30N% + 0.5Mn% + 36)} + It is characterized by limiting the delta-ferrite content in the slab, which is defined as 0.262 × 161-161 ″, to 8.0 Vol% or less.

The reason for limitation of the range of components presented in the present invention will be described.

C is a strong austenite phase stabilizing element that reduces the content of the delta-ferrite phase during slab-casting to improve hot workability and to reduce the amount of expensive Ni added, and also to increase the layer defect energy to improve moldability. If the content is too high, the strength of the temporary covalent martensite phase formed during molding on the molded article after deep drawing increases, and the residual stress is increased, thereby reducing the age cracking resistance, and the corrosion resistance by the precipitation of carbides during annealing is feared. The content is preferably limited to 0.05% or less.

The Si is advantageous for high temperature oxidation resistance, but when the addition amount is too high, the delta-flight content is increased to decrease the hot workability, and the inclusion of linear inclusion defects is increased due to the increase of Si inclusions. The content of is preferably limited to 1.0% or less.

When Mn is added too much, the corrosion resistance and high temperature oxidation resistance may be lowered, and in particular, the glossiness defect may be generated due to the generation of blue color during light annealing. Therefore, the amount of Mn is preferably limited to 2.0% or less.

When the Cr content is too small, the corrosion resistance and high temperature oxidation resistance are lowered. If the Cr content is too large, the delta-ferrite content is increased so that the hot workability and moldability are deteriorated. For this reason, the amount of Cr added is preferably limited to 16-18%.

The amount of Ni is controlled in consideration of the degree of recognition and moldability of the austenite phase, the aging resistance, and the manufacturing cost. If the amount is too high, the M _d30 temperature is low, so that moldability such as elongation property is lowered and manufacturing cost is also reduced. In the case of too small an increase in the amount of the processed organic martensite phase during molding, which lowers the aging crack resistance, it is preferable to limit the amount of Ni added to 6-8%.

The Al is a component that improves high temperature oxidation resistance, and as the amount of addition increases, the amount of inclusions by Al oxide increases during steelmaking to decrease surface defects and formability, so the amount of addition is preferably limited to 0.005% or less.

Cu is a component that can be used as a substitute for Ni element because it softens the material, increases the lamination defect energy, and increases the stability of the austenite phase. When the added amount is 3% or more, the moldability decreases and the grain boundary when slab casting is used. Since the occurrence of cracking during hot rolling due to the occurrence of Cu segregation at the melting point, the addition amount is preferably limited to 3% or less.

When the P content is large, moldability and corrosion resistance are lowered, and therefore the amount of P is preferably limited to 0.05% or less.

S is deteriorated in hot workability, and in particular, when solidified, segregates at the grain boundaries of the austenite phase and causes defects in the hot rolling line. Therefore, the content thereof is preferably limited to 0.005% or less.

The Ti serves to prevent surface bonding during hot rolling by preventing high temperature oxidation during slab high temperature heating for hot rolling and to suppress orange peel formation during molding by grain refinement, and to stabilize ferrite at the same M _d30 temperature. Steels containing a small amount of phosphorus Ti improve the formability because the amount of formation of the processed organic martensite phase during molding increases, thereby increasing the breaking strength and the work hardening index n value of the high deformation region. If the amount is too large, surface defects are caused by Ti oxide, so the amount is preferably limited to 0.03% or less.

The B is effective for preventing surface defects generated during hot rolling because it has an effect of improving hot workability at high temperatures, but when the amount is too large, the B process compound is formed to significantly lower the melting point to lower the hot workability. The amount of addition is preferably limited to 0.003% or less.

The content of N is good for reducing delta-ferrite, but it has the effect of increasing the yield strength of the material to twice the C, and thus the moldability is lowered. It is preferable to limit the content of N to 0.045% or less.

The Mo content is effective to improve the corrosion resistance, but if the addition amount is too high, the increase in manufacturing cost and delta-ferrite content, and recrystallization during hot rolling suppresses the hot workability, causing surface defects, M _d30 Since the temperature and the work hardening roll are significantly increased to lower the moldability and the aging crack resistance, the addition amount is preferably limited to 1.0% or less.

The reason for _{scouring of} the stabilization temperature (M _d30 , ° C) and delta-ferrite content of the austenite phase of the metallurgical factor is explained below.

The higher the M _d30 (° C) temperature (the temperature at which 50% processed organic martensite (α ') phase is formed after 30% stretching), which indicates the stability of the austenite phase, the higher the amount of processed organic martensite phase is produced during molding. Proper M _d30 temperature control is important for improved performance.

In the Cu-added steel, when the M _d30 temperature is low, the aging cracking resistance is improved, but there is a problem in that the manufacturing cost is increased by increasing the amount of expensive Ni added. Particularly, when the M _d30 temperature is too low below -25 ° C, moldability such as elongation property is increased. Lowers. In addition, if the M _d30 temperature is too high above + 10 ° C, the moldability is lowered, but there is a problem in that aging cracking occurs in the molded article after molding, in particular, the aging crack resistance is lowered.

Therefore, in order to obtain better moldability and age cracking resistance than the present invention steel containing Mo, it is preferable to limit the M _d30 temperature in the range of -25 to +10 (° C).

On the other hand, when the delta-ferrite content in the slab is increased, hot workability decreases, causing side cracks and surface defects during the production of hot rolled plates. It is important to adjust the titration delta-ferrite content. Therefore, in consideration of hot workability and formability, in the present invention, the delta-ferrite content is preferably limited to 8.0 vol% or less.

The present invention will be described in detail through the following examples.

Example 1

To prepare an ingot (ingot) by dissolving austenitic stainless steel composition as shown in Table 1 in a 30kg vacuum induction melting furnace, and heated at 1270 ℃ for 2 hours and hot rolled to prepare a 2.5mm hot rolled sheet 1100 After annealing at ℃, the hot rolled sheet was cold rolled to produce a cold rolled sheet having a thickness of 0.7mm, cold-rolled at 1100 ℃ to make crystal introduction 7, then pickling and temper rolling to cold rolled sheet After the preparation, a moldability evaluation test, a tensile test, and a hardness test were performed, and the results are shown in Table 2 below.

TABLE 1

TABLE 2

As shown in Table 2, according to the present invention, the inventive steel (1-3) to which 0.5Mo, Ti, and B were added was compared with the comparative steel (A) and STS 304 raceway steel (without Ti and B added to 0.5Mo) Compared with F, G), Mo drawing shows excellent deep drawing resistance (limit forming ratio: LDR), elongation resistance formation (Eric wire) and aging cracking resistance, and deteriorates moldability and aging cracking resistance to improve corrosion resistance. Inventive steel (1-3) containing 0.5% exhibits the same level of moldability and anti-aging cracking properties compared to comparative steel (BE) containing a small amount of Mo.

The reason why the moldability and aging crack resistance is improved when a small amount of Ti is added in the 2% Cu-added steel is that the Ti is not added at the same M _d30 temperature when the grain refinement effect by Ti and the ferrite stabilizing element Ti are added. Compared to that, the amount of processing organic martensite is increased during processing, thereby increasing the processing hardening index n value of the high deformation region to prevent local necking and to increase the breaking strength, thereby improving moldability and aging crack resistance. Is considered.

Compared with 0.5% Mo added steel and non-added steel, Mo-added steel has higher work hardening effect due to the solid solution effect of Mo. Therefore, _Md30 temperature is lowered than Mo-added steel to reduce work hardening. It can be seen that it is at the same level as Mo unadded steel.

Example 2

As shown in Table 3, Joseo Audi dissolves austenitic stainless steel in a 30kg vacuum induction melting furnace to produce an ingot, and then heats it at 1270 ° C. for 2 hours to hot roll it to produce a 2.5 mm hot rolled plate, and then at 1100 ° C. After annealing, the hot rolled sheet was cold rolled to produce a cold rolled sheet having a thickness of 0.7 mm, cold rolled at 1100 ° C. to have a constant grain size of 7, and then subjected to pickling and controlled rolling to perform cold rolled annealing. After the preparation, the surface was polished with Noj. # 600 sand paper, and then measured at 1000 ppm Cl ⁻ solution, 30 ° C., in order to measure the official potential of each steel type, and potentiostat (EG G. Formula potential was measured using Model 273), and the results are shown in Table 4 below.

TABLE 3

TABLE 4

As shown in Table 4, according to the present invention, the official potential of the inventive steel (1-3) added 0.5% of Mo, which improves the official potential in the C1 ^- based solution, was compared with the comparative steel (C, D) without adding Mo. It is much higher than STS304JI steel, which improves corrosion resistance. It has a formula potential almost equal to that of STS304 steel, which is a conventional steel (F, G), in which Cr content is more than 18% and added more than the invention steel.

Example 3

Ingot prepared by dissolving austenitic stainless steels as shown in Table 5 in a 30kg vacuum induction melting furnace, heated at 1270 ° C. for 2 hours, and hot rolled to prepare a 2.5 mm hot rolled plate, followed by annealing at 1100 ° C. Then, the hot rolled sheet was cold rolled to prepare a cold rolled sheet having a thickness of 0.7 mm, cold-rolled annealing at 1100 ° C. to make the grain size constant to 7, and then pickling and controlled rolling to prepare a cold-rolled annealing sheet. After polishing the surface with No. # 660 sand paper, each steel grade was immersed in each solution, and the official corrosion, frontal corrosion test and grain boundary corrosion test were shown in Table 6 below. As a result, formal corrosion in 4% NaCl + 2% H ₂ SO ₄ solution of Inventive Steel (1-3) with 0.5% Mo improved the formula potential in C1 ^- group solution and the front surface in IN H ₂ SO ₄ solution The corrosion rate is lower than that of the comparative steels (C, D) STS 304JI steel and the conventional steels (F, G) STS 304 steel, which has a large Cr content of 18% or more.

In addition, the grain boundary corrosion resistance was evaluated by the Double EPR test in 0.5 mol H ₂ SO ₄ +0.01 mol KSCN solution, which shows equivalent intergranular corrosion resistance compared to the invention steel and the conventional steel.

TABLE 5

TABLE 6

Example 4

Ingot was prepared by dissolving austenitic stainless steel, adjusted as shown in Table 7, as a 30 kg vacuum induction melting solution, heated at 1270 ° C. for 2 hours, and then hot rolled to 15 mm hot rolled to obtain a diameter of 10 (mm). gleeble) specimens were processed and evaluated for hot workability using a gleeble tester and the results are shown in FIG. At this time, the hot workability test by the gleeble test was carried out, and the diameter of each fractured fractured surface was measured to calculate the cross-sectional reduction rate.

As a result, invented steel (1) containing a small amount of Ti and B in order to improve hot workability in steels containing Cu and Mo deteriorating hot workability, was compared with conventional steel (F), which is an STS 304 steel without addition of Cu and Mo. Although hot workability is bad at low temperature, compared with the comparative steel (A) which contains Cu and Mo components, and is not a Ti and B additive steel, it turns out that hot workability is improved at low temperature. As described above, when Cu, which is a low melting point component, is added as in the invention steel 1, the grain boundary bonding force decreases at the time of 1270 ° C. high temperature heating, which is the primary incoat heating temperature, and thus the hot workability is lowered. The effect of reducing the nitrogen content in the steel, which combines with the nitrogen in the molten steel and decreases the hot workability, is also deteriorated. B is added together with Ti to prevent the segregation of grain boundaries at the time of hot rolling, thereby suppressing the cavitation of grain boundaries and delaying decohesion of grain boundaries, and the interaction between B and vancancy in solid solution. It is believed that the hot workability is improved by the action.

TABLE 7

Example 5

Ingot was prepared by dissolving austenitic stainless steels prepared as shown in Table 8 as 30 kg vacuum induction melting, heating at 1270 ° C. for 2 hours, hot rolling, and high temperature oxidation test on a 2.5 mm hot rolled plate (TGP: Fig. 2 shows the process of processing the specimen for thermo-gravimetric-analysis and subjecting it to high temperature oxidation.

At high temperature oxidation test, high temperature oxidation test atmosphere: mixed gas (C.O.G + B.F.G) excess coral volume ratio: 3%, and oxidation test temperature is 1270 ℃.

TABLE 8

As shown in FIG. 2, it can be seen that the inventive steel (1) is superior in high temperature oxidation resistance compared to the comparative steel (A), which does not impart oxidation resistance by concentrating Ti in the scale, but rather in the base metal. It is considered that oxygen at the grain boundary prevents the migration of metal into the matrix metal.

Claims (1)

By weight% C: 0.05% or less, Si: 1.0% or less, Mn: 2.0% or less, Cr: 16-18%, Ni: 6-8%, Al: 0.005% or less, P: 0.5% or less, S: 0.005 Press-molding, characterized in that it is composed of% or less, Ti: 0.03% or less, B: 0.003% or less, Cu: 3% or less, Mo: 1% or less, N: 0.045% or less, balance Fe and other unavoidable impurities Austenitic stainless steel with excellent ageing cracking resistance, corrosion resistance, hot workability and high temperature oxidation resistance.