KR950007792B1 - Austenite stainless steel - Google Patents

Abstract

The stainless steel comprises (by wt.) 0.045-0.08% C, less than 0.03% P, less than 0.003% S, less than 0.55% Si, less than 0.85% Mn, 18-18.5% Cr, 8-9% Ni, less than 0.15% Mo, less than 3% Cu, less than 0.015% Ti, less than 0.003% B and balance Fe with inevitable impurities, and has an ASTM grain size no. of 6-9, an austenite stability temperature of -10˜-2 deg.C, stacking fault energy of above 35mJ/M2, and yield strength of less than 25 kg/mm2.

Description

Austenitic stainless steel with excellent formability

1 is a graph showing changes in stabilization temperature (Md30), yield strength (Y.S), and stacking fault energy (SFE) of austenitic phases, which are metallurgical factors obtained by calculation of cup height variation after LDR test.

FIG. 2 is a graph showing breakage failure rate of each forming process during primary drawing molding and secondary elongation molding according to cup height change after forming limit ratio (LDR) test during a real sink molding.

3 is a graph showing the correlation between the cup height obtained from the equation (4), which is a formula for calculating the height of the cup after the LDR test, and the cup height measured by the experiment.

4 is a graph showing the forming limit curve of the invention steel and the comparative steel and the amount of deformation for each part after the actual sink molding.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to austenitic stainless steels used in kitchen sinks and the like, and more particularly, to austenitic stainless steels having excellent formability.

STS 304 austenitic stainless cold rolled steel is mainly used as a material for forming the kitchen sink.

The characteristics required for the raw materials in molding by two-stage press molding should be excellent in drawing property in the primary molding and excellent in elongation in secondary molding.

However, when the metal structure of STS 304 steel is cold worked into metastable austenite (γ) phase, a part of the γ phase is transformed into a processing organic martensite (α ') phase, and thus the work hardening degree is increased. The phenomenon is called Transformation Induced Plasticity (TRIP), and STS 304 steel has excellent drawability and stretchability.

However, in recent years, the molding condition of the sink molding company is that the sink bowl is large, deeply molded, and the shape is complicated. Also, in order to save working time and reduce manufacturing atoms during the press process, lubricating vinyl is used. use. Therefore, because of the high frictional resistance during molding often breaks, a material having excellent moldability is required to prevent breakage failure. In addition, since the conventional STS 304 steel has a wide range of components, the failure rate is often high when forming large sinks, and also defective shapes are generated.

Conventionally known techniques for austenitic stainless steel for kitchen sinks include the following.

Staro (16 (1975) 993) reported that the formability of austenitic stainless steels was not consistent with the Rankford (γ) values unlike mild and ferritic stainless steels. Erichsen) reports that the Md30 temperature is the maximum in the range of 0 ° C to + 40 ° C, but the component range is too wide, the Md30 temperature is high, and there is a high risk of aging cracking during drawing, and the amount of processed organic martensite (α ') phase There is a problem in that a shape defect occurs due to bending of the side of the sink due to a spring back phenomenon due to an increase in strength, and K. Nohara [J. Iron and Steel Inst. Of Japan. (1977) 212

F. B. Picking [Proceeding Intern. Conf. on Stainless Steel. '84, Gotenburg (1984) 2, described the stacking fault energy (SFE).

It is reported that the higher the lamination defect energy, the lower the work hardening during molding, and F.B.Picking [Physical Metallurgy & The Desing of Steel, Applied Sci, Pub. LTD, Lodon (1978) 231] shows the yield strength (Y.S) of the material as the following formula, the lower the yield strength has been reported to be advantageous in molding because the easier inflow of the flange portion during molding.

d = diameter of the average grain (mm)

In addition, FBPicking reports that the addition of Ni increases the stabilization of the austenite phase, resulting in less formation of the processing organic martensite phase during molding. There is a conflicting claim that this is effective for improving deep drawing, but it is unclear.

The present invention increases the stability (γd30) and stacking fault energy (SFE) of the γ phase, which is a metallurgical factor that affects the workability of the material, and lowers the yield strength, thereby reducing the amount of expensive Ni added. An object of the present invention is to provide an austenitic stainless steel having excellent moldability.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated.

The present invention provides a good deep drawing in the primary drawing process, and the metallurgical factor of the metallurgy factor calculated from the chemical composition in STS 304 steel in order to reduce the failure rate at the time of sink molding requiring good elongation formation during the secondary elongation molding. By examining the influence and adjusting these factors, an austenitic stainless steel having excellent formability is provided. That is, in the austenitic stainless steel, the present invention, in terms of weight percent, C: 0.045 to 0.08%, P: 0.03% or less, S: 0.003% or less, Si: 0.55% or less, Mn: 0.85% or less, Cr: 18-18.5%, Ni: 8-9%, Mo: 0.15% or less, Cu: 3% or less, Ti: 0.015% or less, B: 0.003% or less, balance Fe and other inevitable impurities The lattice defect energy (SFE) whose grain size is ASTM crystal grain size number 6-9, and the austenite stabilization temperature (Md30) calculated as in the following formula (1) is -10 to 2 ° C, and calculated as in the following formula (2): ) Is at least 35 (mJ / m ² ), and the yield strength calculated as shown in the following formula (3) relates to austenitic stainless steel is configured to be 25 (Kg / mm ² ) or less.

Austenitic stability temperature

d = diameter of the average grain (mm)

In addition, the present invention relates to an austenitic stainless steel configured to have a cup height (mm) of 29.0 or more calculated in the following formula (4) in the austenitic stainless steel configured as described above.

Hereinafter, the reason for limitation of the component range and the metallurgical factor value will be described. In the case of C, since the content of delta-ferrite phase is reduced as a strong austenite phase stabilizing element, the hot workability is improved, and the amount of expensive Ni is added, and the additive energy is increased as high as possible. It is advantageous to improve moldability. It is preferable to limit C to 0.08 (%) or less, which is a limit addition amount which can prevent carbide precipitation from 0.045 (%) or more.

Ni is added in an amount of 8.0 (%) or more in consideration of stability, processability, and age cracking of the austenite phase, but if the amount is too large, the Md30 temperature is lowered, resulting in poor elongation-forming properties, and also increasing manufacturing cost. It is preferable to limit to the following.

The Cr is required to be 18% or more from the viewpoint of corrosion resistance, it is preferable to limit to 18.5 (%) or less because too much addition of the workability and delta-ferrite content increases.

Cu is an element 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. However, when Cu is more than 3 (%), there is a risk of cracking during workability and hot rolling. It is preferable to limit to%) or less.

P is preferably limited to 0.03 (%) or less because the workability and the corrosion resistance decrease when the amount is large.

It is preferable to limit S to 0.003 (%) or less because it decreases hot workability, in particular, segregates austenite grain boundaries during solidification and causes linear slivers during hot rolling.

The Ti is preferably limited to 0.015 (%) or less because it serves to prevent surface defects during hot rolling by preventing high temperature oxidation during high temperature heating of the slab for hot rolling. Do.

The B is effective for preventing surface defects generated during hot rolling by improving high temperature hot workability, but when it is added in a large amount, it is limited to 0.003 (%) or less because it forms a process B compound and significantly lowers the melting point by lowering the melting point. desirable.

In STS 304 steel, if the grain size increases after cold-rolling annealing, the moldability is improved, but if the grain size is less than ASTM particle number 6, the grain size is too coarse and orange peel occurs during molding, resulting in streaks on the surface. It is preferable to limit the number to ASTM grain size 6-9.

Metallurgical factors affecting the work hardening of materials include the stabilization temperature (Md30), lamination defect energy and yield strength of austenite phase. The Md30 temperature is a temperature at which 50% processed organic martensite phase is formed when the material is 30% deformed. A high temperature indicates that the processed organic martensite phase is easily formed during deformation. Therefore, when the deep drawing is not easy to flow into the flange portion, the LDR value is lowered, and a shape defect occurs in the sink side due to spring tightness after molding. On the other hand, if the Md30 temperature is too low, it is uneconomical because the elongation moldability is lowered and the expensive Ni content must be increased. Therefore, it is preferable to limit Md30 temperature computed by said Formula (1) to -10--2 (degreeC).

In addition, when the lamination defect energy is large, the energy required for the intersection or dislocation of dislocation during deformation is small, so that the cross slip easily occurs, and thus the work hardening of the material during molding is lowered. The lamination defect energy value is preferably limited to 35 (mJ / M ² ) or more.

In addition, when the yield strength is low, the deformation resistance of the flange portion during molding is low, so deformation easily occurs and the final yield ratio (yield strength / tensile strength ratio) is also lowered. It is preferable to limit the yield strength value calculated by the above formula (3) to 25 (kg / mm ² ) or less.

On the other hand, if the cup height after the LDR test (molding limit ratio test) with a diameter of 3 mm is 29 mm or more, defects are not generated during the actual sink molding, and the present inventors pay attention to these facts to study the cup height and steel. The correlation between component range and grain size was derived, which is the above equation (4).

By constructing the steel so that the cup height obtained by the above formula (4) becomes 29 mm or more, it has excellent moldability and can prevent breakage failure during sink molding. Hereinafter, the present invention will be described in more detail with reference to Examples.

EXAMPLE

In this embodiment, a specimen having a chemical composition, grain size, metallurgical ruler, and cup height as shown in Table 1 was used.

These specimens were prepared as follows.

That is, the austenitic stainless steels prepared in Table 1 below were dissolved in a 100 ton electric furnace, and slabs were manufactured by continuous casting, and hot rolling and cold rolling were performed by a conventional cold rolling plate manufacturing method. At this time, the cold rolling rate was constant at 60% of all specimens to make a cold rolled annealing plate having a thickness of 0.6mm. The cold rolled annealing plate was subjected to temper rolling at 1 (%) cold rolling rate, and the surface condition of the specimen was used in 2B state.

TABLE 1

Using 33 (mm) diameter punch, each specimen prepared as above, and after the limit mold ratio (LDR) test, the height of the cup is not broken from the bottom of the cup to the valley of the cup (valley) After four measurements, the average value is determined, and the relationship between the average value and the lamination defect energy, yield strength, and austenite-phase stabilization temperature (Md30) calculated from the chemical composition of each specimen is shown in FIG. It was.

As shown in FIG. 1, when the stacking defect energy is 35 (mJ / m ² ) or more, Md30 temperature is in the range of -10 (° C.) to 2 (° C.), and the yield strength is 25 (Kg / mm ² ) or less [Inventive Steel (34-39)], it can be seen that the measured cup height is more than 29mm.

In addition, the change in the failure rate of breakage by each forming process is determined by the first drawing molding and the second restriking molding according to the cup height change after the LDR test in the real sink molding. As shown in FIG. 2, when the cup height is 29 mm or more, as shown in FIG. 2, it can be seen that failure failure does not occur.

In addition, the correlation between the cup height calculated by Equation (4) and the measured cup height is shown in FIG. 3, and as shown in FIG. 3, the correlation coefficient value (r ² ) is 0.99. It can be seen that the calculated cup height and the measured cup height are in good agreement.

On the other hand, the molding limit curve and the deformation amount of each sink portion of the inventive steel 39 and the comparative steel 7 were obtained, and the results are shown in FIG.

As shown in FIG. 4, it can be seen that the molding limit of the inventive steel 39 is improved compared to the comparative steel 7.

As described above, the present invention can provide an austenitic stainless steel having excellent moldability even if expensive nickel is reduced, so that it can be more economically applied to kitchen sinks and the like requiring excellent drawing and elongation properties. There is.

Claims (2)

In austenitic stainless steel, in weight%, C: 0.045 to 0.08%, P: 0.03% or less, S: 0.003% or less, Si: 0.55% or less, Mn: 0.85% or less, Cr: 18-18.5%, Ni: 8-9%, Mo: 0.15% or less, Cu: 3% or less, Ti: 0.015% or less, B: 0.003% or less, balance Fe and other inevitable impurities; The grain size is ASTM grain size 6-9; The austenite stabilization temperature (Md30) calculated as in the following formula (1) is -10 to -2 ° C; The stacking fault energy (SFE) calculated as in the following formula (2), 35 (mJ / m ² ) or more; And the yield strength (kg / mm ² ) calculated as in the following equation (3), Where d = diameter of the average grain size in mm Austenitic stainless steel with excellent formability, characterized by being less than 25 (kg / mm ² ). The cup height (mm) of Claim 1 computed like following formula (4), Austenitic stainless steel having excellent moldability, characterized in that the component and grain size are controlled to be 29 (mm) or more.