KR20030055029A - Workhardenable austenitic stainless seels - Google Patents

Abstract

PURPOSE: Provided is a Cr-Mn-Ni-N based work hardenable austenitic stainless steel with excellent strength and elongation, which exhibits uniform work hardenability without sudden change of fluid stress during cold forming. CONSTITUTION: The austenitic stainless steel comprises C 0.03 to 0.05, 1.0 wt.% or less than Si, Mn 8.0 to 9.5 wt.%, 0.2 wt.% or less of P, 0.1 wt.% or less of S, Cr 15.0 to 17.0 wt.%, Ni 1.5 to 3.5 wt.%, 0.5 wt.% or less of Mo, N 0.2 to 0.35 wt.%, 1.5 wt.% or less of Cu, 0.0070 wt.% or less of B, 0.02 wt.% or less of Ca, a balance of Fe and incidental impurities, wherein 2(wt.%Ni)+45(wt.%C+wt.%N)-(wt.%Cr) is regulated within 0 to 5, so that it is possible to prevent decrease of elongation due to inner pore; austenite stability value calculated by 60(wt.%C+wt.%N)+(wt.%Mn)+1.7(wt.%Cr)+1.5(wt.%Ni) is regulated greater than 55, so that it is possible to keep high elongation and uniform work hardenability by optimally controlling martensite formation during transformation.

Description

Work hardenable austenitic stainless seels

The present invention relates to a work hardening austenitic stainless steel, and more particularly, compared to Cr-Ni metastable work hardening stainless steel, by replacing manganese and nitrogen with a substantial amount of nickel, which is an expensive alloying element, superior strength and excellent strength The present invention relates to a Cr-Mn-Ni-N-based work hardening stainless steel having ductility and having excellent workability due to uniform work hardening without sudden change in flow stress during cold deformation.

In general, the work hardening stainless steel refers to a stainless steel that is formed so as to obtain high strength by cold working such as temper rolling by lowering the stability of the austenite phase to an appropriate level so that the austenite phase is transformed into processed organic martensite during cold deformation.

The most common work hardened stainless steels are metastable austenitic stainless steels that meet the KS STS 301 and STS 301L specifications. These steels can obtain various strengths in the range of 700 ~ 1000 MPa through temper rolling according to the specifications of LT, DLT, ST, MT, HT, etc., and are mainly used for corrosion-resistant structures such as electric vehicle structural parts.

However, since these steels usually contain more than 7% by weight of nickel, an expensive alloying element, the price of the materials becomes expensive, and thus they are not practically used for general structural applications such as automobile structural parts. In addition, since the work hardening of these metastable austenitic stainless steels depends entirely on the martensite transformation, the work hardening during the deformation is not uniform and the work hardening rapidly increases in the specific deformation range where the martensite transformation is actively induced. There is a problem that it is difficult to control the molding accuracy and spring back phenomenon when forming the plate.

As a well-known technique, the Cr-Mn-Ni-N-based hardened stainless steel proposed in US Pat. No. 4,533,391 proposed to solve the price problem of Cr-Ni metastable austenitic stainless steel. In this well-known technique, instead of reducing nickel to 2 to 2.3% by weight, the manganese content is increased to 6.5 to 8.5% by weight and chromium is formed to be in a range of 12 to 15% by weight to Cr-Ni metastable austenitic stainless steel In comparison with the steel, which is inexpensive and has similar strength. However, Cr-Mn-Ni-N-based work hardening stainless steel provided by this known technology has a disadvantage of low ductility compared to the existing STS 301L steel. Substantially, the ductility in 20% temper rolling of STS 301L steel is generally 20% or more based on a gauge length of 50mm, but the ductility of steel provided by the above-mentioned known technology is only in the range of 10 to 13%. In addition, the steel provided in the prior art, like the STS 301L steel, due to the work hardening is completely dependent on the processing organic martensite transformation, the problem of bending the flow curve still occurs.

Therefore, the present invention prevents the deterioration of the ductility and the flow curve, which is a problem of Cr-Mn-Ni-N-based work hardening stainless steel, and is inexpensive compared to conventional Cr-Ni metastable work hardening stainless steel, but also in annealing and temper rolling It is an object of the present invention to provide Cr-Mn-Ni-N-based work hardening stainless steel having excellent molding properties by having uniform strength and ductility compared to STS 301L steel and uniform work hardening.

1 is a photograph showing whether or not pore inside the ingot in the comparative example and the invention example.

Figure 2 is a graph showing the stress-strain behavior at room temperature strain in Comparative Examples and Inventive Examples.

In order to achieve the above object, the work hardening austenitic stainless steel provided by the present invention is in weight%, C 0.03 to 0.05, Si up to 1.0%, Mn 8.0 to 9.5%, P 0.2% or less, S 0.1% or less, Cr 15.0 ~ 17.0%, Ni 1.5 ~ 3.5%, Mo 0.5% or less, N 0.2 ~ 0.35%, Cu 1.5% or less, B 0.0070% or less, Ca 0.02% or less, the balance consists of iron and ordinary impurities. By setting the value of 2 (% Ni) + 45 (% C +% N)-(% Cr) to satisfy the value between 0 and 5, the reduction of ductility due to internal pores is prevented, and at the same time 60 (% C +% N A) + (% Mn) + 1.7 (% Cr) + 1.5 (% Ni) so that the austenite stability value is 55 or more. It is characterized by having the characteristics.

Hereinafter, the present invention will be described in detail.

In order to provide economical structural stainless steel, it is essential to replace most of nickel, an expensive alloying element added as an austenite forming element, with manganese and nitrogen in the conventional austenitic stainless steel. However, in this case, if the alloy composition is not very strictly controlled, the ductility is severely lowered compared to Cr-Ni metastable stainless steel as in the above-described known technique. The present inventors investigated the metallurgical factors affecting the ductility of the Cr-Mn-Ni-N stainless steel sheet in the annealing state and the temper rolled state, first of all to prevent the defect caused by the pores introduced into the plate during the manufacturing process. The ductility of the plate is improved, and the ductility is improved while maintaining the high strength when the work hardening by martensite transformation and the known work hardening improvement effect by nitrogen occur simultaneously. It has been found that ductility improves when the carbon content is properly controlled. In addition, compared to the work hardening by martensitic transformation, it was found that the work hardening effect by nitrogen was maintained at a certain level or more, so that work hardening occurred uniformly during cold plastic deformation and there was no rapid flow stress inflection. Based on the present invention.

Hereinafter, the reason for limiting the alloy composition in the present invention will be described.

Chromium improves the corrosion resistance of the overall alloy and at the same time increases the solubility of nitrogen. To ensure proper corrosion resistance and nitrogen concentrations as structural stainless steels, chromium must be contained at least 15%. However, in the case of excessive chromium, since the hot workability is embrittled by the residual ferrite remaining after solidification, the content is limited to 17% or less.

Manganese is an element that increases the solubility of nitrogen and at the same time plays a role in stabilizing austenite phase. In order to simultaneously generate the martensite transformation and the known work hardening effect by nitrogen, the austenitic stability must be maintained at a certain level or higher. For this purpose, at least 8% of manganese must be contained in the alloy. However, if the amount of manganese exceeds 9.5% or more, since the corrosion resistance is lowered due to excessive manganese emulsion production, the content is limited to 9.5% or less.

Nickel, like manganese, plays a role in stabilizing austenite, so it must be contained 1.5% or more to control austenite stability. However, nickel is not only a very expensive element of the alloying elements, but when excessively added, it lowers the solubility of nitrogen in the liquid phase to generate pores, so its content is limited to 3.5% or less.

Nitrogen is an important element for obtaining high strength and at the same time, it is an element that suppresses martensite transformation. It should contain 0.2% or more to obtain sufficient known solid solution strengthening and work hardening effect by nitrogen. However, when the content of nitrogen is excessive, the formation of nitrogen pores during coagulation increases, so the ductility is reduced, the content is limited to 0.35% or less.

Carbon is a strong austenite stabilizing element and an element that provides high solid-solution strengthening effect and greatly increases stacking defect energy. In steels with a high nitrogen content, the lamination defect energy is greatly reduced by nitrogen, and the ductility is reduced by encouraging planar slip during cold deformation. In this case, carbon offsets the decrease in lamination defect energy and thus reduces the ductility. It acts as a deterrent. Carbon content in the present invention is one of the important features of the invention steel should contain at least 0.03% of carbon for sufficient lamination defect energy canceling effect and austenite stabilization effect. However, if the carbon content is excessive, the corrosion resistance is lowered so that the content is limited to 0.05% or less.

Silicon is an element that is inevitably contained in the alloy, if the content is excessive, the ferrite content is high and the ductility and hot workability is reduced, so it is limited to 1% or less.

Phosphorus is inevitably contained in the alloy, and if its content is excessive, hot workability is lowered, so it is limited to 0.2% or less.

Sulfur is an element that is inevitably contained in the alloy, if its content is excessive, hot workability and corrosion resistance are lowered, so it is limited to 0.1% or less.

Molybdenum is an element that improves the corrosion resistance of the alloy, but in the case of an alloy containing a large amount of nitrogen as in the present invention, the content is limited to 0.5% or less since it reacts with nitrogen to inhibit the recrystallization of the alloy severely, thereby severely inhibiting hot workability. do.

Copper is an element that improves the cold formability of the alloy, but when excessively contained, it causes high temperature embrittlement, making it difficult to hot work, so the content is limited to 1.5% or less.

Boron is an element that improves hot workability. In order to suppress the occurrence of surface defects due to cracks in rolling the steel provided by the present invention by a conventional hot rolling method, a small amount of boron may be added. However, if the boron content is excessive, the local melting point of the segregation region is lowered, which impairs the hot workability. Therefore, the content is limited to 0.007% or less.

Calcium is an element that improves hot workability, similar to boron. In rolling the steel provided by the present invention by a conventional hot rolling method, a trace amount of calcium can be added to suppress the occurrence of surface defects due to cracking. However, if the calcium content is excessive, the ductility is reduced by the formation of excessive inclusions, so the content is limited to 0.02% or less.

In the present invention, when forming Cr-Mn-Ni-N stainless steel in the above-mentioned range, pore defects are prevented, and the processing organic martensite transformation and known work hardening by nitrogen occur simultaneously at an appropriate ratio, and the lamination defect energy by nitrogen is reduced. Is sufficiently offset to have ductility equal to or higher than that of STS 301L, and exhibits uniform work hardening behavior. However, even in this composition range, the ductility is not sufficiently improved if the combination of each element is not preferable, and the present inventors are in the above composition range. In the following, the combination method of component elements to secure excellent ductility is presented as follows.

First of all, the alloy composition must satisfy the following formula (1) to prevent ductility deterioration due to internal pores during melt-solidification under atmospheric pressure.

0 ≤ 2 (% Ni) + 45 (% C +% N)-(% Cr) ≤ 5 ---------------- (Equation 1)

When the value of formula (1) exceeds 5, the solubility of nitrogen in the liquid phase decreases, and large nitrogen pores are formed in the final solidification site inside the ingot, and when the value of formula (1) is 0 or less, the ferrite solidification section during solidification This excess leads to the formation of pinhole-shaped pores below the surface of the ingot. When the value of Equation (1) is between 0 and 5, pinholes and internal pores do not occur in the alloy composition range. In this condition, therefore, no reduction in ductility due to internal defects caused by pores occurs after the ingot is made of a rolled sheet material.

In addition, the austenite stability value calculated by Equation (2) should be 55 or more as a requirement for enhancing the martensite transformation and improving the known work hardening effect by nitrogen.

60 (% C +% N) + (% Mn) + 1.7 (% Cr) + 1.5 (% Ni) ≥ 55 --------------- (Equation 2)

When the austenite stability is secured under the condition of Equation (2), martensite transformation is partially suppressed during deformation in the above alloy composition range, thereby improving the work hardening and mar-based process of martensite transformation, that is, the improvement of work hardening of austenite. It happens at the same time. In case where work hardening by martensitic transformation is predominant in stainless steel with high nitrogen content, martensite is embrittled due to containing a large amount of carbon and nitrogen and ductility is drastically deteriorated.

However, by increasing the austenite stability and reducing the martensite transformation, embrittlement caused by these factors is greatly reduced. At this time, most of the nitrogen serves to promote the work hardening of the austenite matrix, thereby canceling the strength decrease caused by the reduction of martensite transformation to maintain a sufficient work hardening rate. The processing organic martensite transformation starts to occur after 10-15% or more plastic deformation, so if the work hardening depends only on the martensite transformation, there is a sharp increase in flow stress at this amount of deformation. However, in the case of reducing the martensite transformation and facilitating the known work hardening as in the present invention, since the effect of improving the work hardening of nitrogen appears from the initial deformation regardless of the amount of deformation, it shows a high flow stress even in the low deformation region and 10 to 15 Abrupt changes in flow stress at% strain will not occur.

The manufacture of the work hardening austenitic stainless steel provided by the present invention does not require special techniques in melting, casting and hot and cold working. Hot water is produced by melting the furnace by argon-oxygen decarburization and the slab by the continuous casting process under atmospheric pressure, and reheating to a temperature of 1100 ~ 1300 ℃. Through annealing to produce a hot rolled steel strip, it can be produced by cold rolling and annealing again to produce a cold rolled steel strip. It may also be prepared by rolling or forging after melting the ingot, and may also be produced by strip casting under atmospheric pressure as a more economical method.

The present invention will be described using the following examples.

In order to illustrate the properties of the alloy of the present invention, alloys of the composition shown in Table 1 were prepared. Here, Comparative Example 10 is STS 301L steel, and these alloys were melted by induction at atmospheric pressure and then cast into 50 kg ingots. The cast ingot was reheated at a temperature of 1250 ° C., and then hot rolled to a thickness of 3 mm to prepare a hot rolled sheet, annealing, and then cold rolling to a thickness of 1 mm. The cold rolled sheet was annealed and rinsed again, and then mechanical properties were measured through a tensile test of 50 mm in length. In addition, the degree of martensite formation was measured in the cold rolled annealing plate at 20% temper rolling, and the mechanical properties were measured in the same manner.

Compared to the STS 301L steel of Comparative Example 10, most of the test specimens have high yield strength and similar tensile strength in the annealing state and the temper rolling state. However, the elongation showed a big difference depending on the alloy composition.

First, Comparative Example 1 and Comparative Example 2 due to the lack of carbon content, the elongation after temper rolling was reduced compared to the STS 301L steel of Comparative Example 10. In addition, although the manganese, nickel, and nitrogen satisfy the composition ranges of Comparative Example 3, Comparative Example 5, Comparative Example 7, and the present invention, which are insufficient compared to the composition range in the present invention, respectively, they violate the austenite stability requirements according to Formula (2). In Comparative Example 9, the martensite transformation occurred excessively during cold working, thereby reducing the ductility in the rough rolling state.

In addition, in Comparative Example 8, which violates the requirements of Comparative Example 4, Comparative Example 6, and Equation (1), respectively, in which nickel and nitrogen were exceeded, large nitrogen pores were found in the ingot state, and thus, as shown in Table 2 below. Elongation has already been reduced in the annealing state compared to STS 301L, and the elongation was relatively low even in the rough rolling state. Including the steel grades in which the lack of austenite stability and the generation of pores are overlapped, it can be seen that the steel grades of Comparative Examples 4 to 9 all contain pores and thus have low elongation at annealing.

Inventive Examples 1 to 6 are steels satisfying the alloy composition shown in the present invention, and have no lowering in elongation at annealing due to pores and have superior ductility than STS 301L.

1 shows ingot cross-sections of Inventive Example 1 without pores as compared to Comparative Example 3 having internal pores and Comparative Example 9 having pinholes under the surface. In addition, the steels of Inventive Examples 1 to 6 were sufficiently suppressed in martensite transformation, so that it was found that less than half martensite was formed compared to STS 301L steel in a 20% temper rolled state, which is superior to STS 301L steel in temper rolled state. Maintain ductility

In addition, as shown representatively in FIG. 2, Example 1 of the present invention shows a characteristic that high and uniform work hardening occurs from the beginning of deformation without rapid change of flow stress during cold deformation, as compared with Comparative Examples 7,10.

division C Si Mn P S Cr Ni N Etc Expression (1) value Expression (2) Comparative Example 1 0.015 0.50 8.72 0.024 0.0027 15.98 2.57 0.250 - 1.09 55.64 Comparative Example 2 0.027 0.52 8.38 0.031 0.0025 16.27 2.68 0.245 - 1.33 56.38 Comparative Example 3 0.034 0.51 7.23 0.024 0.0027 16.13 2.40 0.230 - 0.53 54.06 Comparative Example 4 0.043 0.49 8.30 0.025 0.0031 16.81 4.38 0.260 - 5.59 61.63 Comparative Example 5 0.040 0.52 8.82 0.026 0.0034 16.05 1.34 0.240 - -0.77 54.92 Comparative Example 6 0.038 0.52 8.71 0.023 0.0030 15.87 2.64 0.380 - 8.22 64.73 Comparative Example 7 0.041 0.46 8.81 0.030 0.0025 15.94 2.66 0.170 - -1.13 52.56 Comparative Example 8 0.048 0.52 9.46 0.027 0.0033 16.88 3.32 0.310 - 5.87 64.62 Comparative Example 9 0.027 0.48 8.13 0.024 0.0029 15.23 1.72 0.210 - -1.13 50.82 Comparative Example 10 0.026 0.56 1.30 0.023 0.0025 17.05 7.36 0.120 - 4.24 50.09 Inventive Example 1 0.039 0.50 9.16 0.024 0.0040 15.89 3.16 0.280 - 4.79 60.05 Inventive Example 2 0.038 0.49 9.01 0.023 0.0035 15.90 3.19 0.210 - 1.64 55.71 Inventive Example 3 0.043 0.49 9.48 0.024 0.0030 16.11 2.23 0.260 - 1.99 58.39 Inventive Example 4 0.035 0.44 8.93 0.026 0.0031 16.10 1.99 0.250 Cu 0.82 0.70 56.39 Inventive Example 5 0.031 0.50 8.86 0.025 0.0028 15.92 3.29 0.250 B 0.0029 3.31 57.72 Inventive Example 6 0.044 0.70 8.24 0.019 0.0021 16.85 3.25 0.230 Ca 0.010 1.98 58.20

division pore Cold Rolled Annealing Material (1mm thick) 20% temper roll (0.8mm thickness) Yield strength (MPa) Tensile Strength (MPa) Elongation (50mm,%) Flow curve inflection Yield strength (MPa) Tensile Strength (MPa) Elongation (50mm,%) Martensite (%) Comparative Example 1 Not Occurred 420 835 48.5 Not Occurred 873 1201 10.8 12.6 Comparative Example 2 Not Occurred 426 837 51.6 Not Occurred 885 1186 17.5 10.9 Comparative Example 3 Not Occurred 430 867 52.6 Occur 874 1173 18.5 15.8 Comparative Example 4 Occur 494 823 43.3 Not Occurred 891 1120 10.1 1.5 Comparative Example 5 Occur 486 832 48.5 Occur 826 1199 11.3 13.8 Comparative Example 6 Occur 568 837 37.1 Not Occurred 925 1113 6.5 0 Comparative Example 7 Occur 403 815 48.3 Occur 853 1069 9.4 16.8 Comparative Example 8 Occur 505 816 39.6 Not Occurred 912 1098 5.4 0 Comparative Example 9 Occur 479 828 46.2 Occur 877 1223 7.8 19.6 Comparative Example 10 Not Occurred 418 848 47.2 Occur 834 1156 20.9 20.8 Inventive Example 1 Not Occurred 488 839 52.6 Not Occurred 896 1196 23.7 4.4 Inventive Example 2 Not Occurred 476 855 53.8 Not Occurred 893 1205 20.5 10.5 Inventive Example 3 Not Occurred 499 871 52.7 Not Occurred 907 1214 23.6 7.17 Inventive Example 4 Not Occurred 458 799 50.1 Not Occurred 880 1123 22.8 9.4 Inventive Example 5 Not Occurred 475 840 51.7 Not Occurred 886 1164 23.4 8.2 Inventive Example 6 Not Occurred 482 828 52.5 Not Occurred 892 1150 24.1 7.1

Compared to Cr-Ni metastable austenitic stainless steels, the present invention is less expensive and has the same or more strength and excellent ductility in both annealing and temper rolling conditions. Provides free Cr-Mn-Ni-N based hardened stainless steel. Steel provided by the present invention has excellent mechanical properties and price competitiveness can be used for a variety of structural applications, such as automotive structural parts, building structures, as well as electric vehicle structures. The steel of the present invention in each application is fundamentally superior in formability compared to the existing carbon steel structural material, which enables the design of complex and diverse structures, and also improves the structural safety of automobiles due to its excellent toughness and absorbing energy of impact energy. It is possible to reduce the weight of the structure and the effect of extending the life of the structure due to excellent corrosion resistance.

Claims (1)

By weight%, C 0.03 ~ 0.05, Si max 1.0%, Mn 8.0 ~ 9.5%, P 0.2% or less, S 0.1% or less, Cr 15.0 ~ 17.0%, Ni 1.5 ~ 3.5%, Mo 0.5% or less, N 0.2 ~ 0.35%, Cu 1.5% or less, B 0.0070% or less, Ca 0.02% or less, and the balance consists of iron and ordinary impurities, and in this range 2 (% Ni) + 45 (% C +% N)-(% Cr) The value of satisfies a value between 0 and 5, and at the same time the austenite stability value calculated as 60 (% C +% N) + (% Mn) + 1.7 (% Cr) + 1.5 (% Ni) is 55 or more. The work hardening type austenitic stainless steel, characterized by low cost, excellent elongation and uniform work hardening properties.