KR100666727B1 - 304H austenite stainless steel for spring - Google Patents

Abstract

304H austenitic stainless steel for springs is provided.

The stainless steel of the present invention is, by weight, C: 0.060-0.08%, Si: 0.3-0.6%, Mn: 1.2-1.4%, P: 0.035% or less, S: 0.01% or less, Cr: 18.3-18.9% , Ni: 8.3-8.7%, Mo: 0.5% or less, Cu: 0.7% or less, N: 0.02-0.05%, balance Fe and other unavoidable impurities, and (C + N) ^1/2 is 0.28-0.35 Satisfies.

304H, stainless steel, spring, solid

Description

304H austenite stainless steel for springs {304H austenite stainless steel for spring}

1 is a graph comparing the saturation magnetization value according to the freshness of the invention material and the comparative material

Figure 2 is a graph comparing the tensile strength according to the draw rate of the invention and the comparative material

3 is a graph comparing the tensile strength according to the (C + N) ^1/2 concentration of the invention and the comparative material

The present invention relates to a 304H austenitic stainless steel for spring, and more particularly, even if the concentration of Mo and Cu is adjusted higher than the normal value, there is no problem in manufacturing the 5.5 mmφ intermediate material, The present invention relates to austenitic stainless steel for steelmaking cost-saving precision springs, which is not only used but also hardly reduced in strength of fresh wire.

Stainless 304H wire is mainly used to improve mechanical properties such as tensile strength, elongation, and reduction of section, and is manufactured and used as precision spring through various production and processing processes. In the AISI standard, 304H wire rod is C: 0.04-0.1 wt.%, Si: 1 wt.% Or less, Mn: 2 wt.% Or less, P: 0.045 wt.% Or less, S: 0.03 wt.% Or less, Ni: 8-10.5 wt.%, Cr: 18-20 wt.%, The remainder is defined as Fe and other unavoidable impurities, and there are no specific regulations for Mo and Cu concentrations.

Referring to the spring manufacturing process of 304H wire as follows. In other words, after the production of the billet billet through the steelmaking process in the electric furnace maker to produce a wire rod of about 5.5 mmφ by wire rod rolling, and then to produce a wire rod as an intermediate material through a solid solution heat treatment and pickling process. In the subsequent process, a process of drawing the wire rod with a wire diameter suitable for manufacturing a spring is made in a fresh maker, and then, a spring manufacture maker completes the final product of the spring with the fresh wire.

However, when the spring is manufactured in such a series of processes, a critical defect occurs in which the spacing between the rings, that is, the pitch between pitches, is not constant in the spring, which is a finished product. This problem is primarily attributable to 5.5 mmφ intermediate material makers, which makes demanding material quality difficult. Therefore, in general, 5.5 mmφ intermediate material makers operate strict management standards in detail on steelmaking residual elements, rolling conditions, heat treatment, inspection, etc. of wire rods produced. As the primary factor to determine, the concentration limit of steel composition is strictly defined. For example, at present, the 5.5 mmφ intermediate material maker has a wire composition of 304H, C: 0.065-0.085 wt.%, Si: 0.3-0.6 wt.%, Mn: 1.2-1.4 wt.%, P: 0.035 wt.% Or less, S: 0.01 wt.% Or less, Ni: 8.3-8.7 wt.%, Cr: 18.3-18.9 wt.%, N: 0.02-0.05 wt.%, The rest is defined as Fe and other unavoidable impurities. The concentrations of Mo and Cu are 0.2 wt.% Or less and 0.35 wt.% Or less, respectively, and are much more strictly limited than those of AISI.

However, the increase of the concentration of C in such a component regulation does not have any problem in the technology of the steelmaking industry, but the control of the concentration of Mo, Cu requires a significant additional burden of steelmaking cost. In detail, in the case of an electric furnace maker, since 60-70% of raw materials are filled with scrap metal during steelmaking, the trace amounts of Mo and Cu, which are usually trace components, are relatively high at 0.45 wt.% And 0.6 wt.%, Respectively. Therefore, in order to lower the concentration of these components, the steelmaking industry using the subsidiary materials is required, and there is a problem in that an additional economic cost for this is increased.

On the other hand, a method of suppressing inclusions as a conventional technique for increasing productivity by suppressing disconnection when drawing a stainless wire (Japanese Patent Application Laid-Open No. 3-61322), processing organic necessity that is essential in processing austenitic stainless steel by addition of alloying elements A method of suppressing the production of martensite (Japanese Patent Laid-Open No. 5-271771) and the like have been proposed. In addition, a technique for high strength austenitic stainless steel (Korean Patent Registration No. 0380725), in which the bright annealing process can be omitted during the drawing process due to the complex addition of Cu and N, and the strength of the drawing wire is not reduced.

However, none of the above-mentioned prior arts has disclosed a technique for solving the problems associated with the concentration control of Mo and Cu as described above in 304H steel, which is an austenitic stainless steel for springs.

Therefore, the present invention was devised to solve the above-mentioned problems of the prior art, and even if the concentration of Mo and Cu in the 304H steel, which is an austenitic stainless steel for springs, is controlled to be higher than the normal value, it is not suitable for the production of 5.5 mmφ intermediate materials. There is no problem, and there is no disconnection at the time of drawing, and the purpose is to provide austenitic stainless steel for steelmaking cost-saving spring hardly reducing the strength of the fresh wire.

Hereinafter, the present invention will be described.

The inventors have conducted research and experiments on the method of maintaining the strength of the wire rod even if the concentration of Mo and Cu is increased above the prescribed value in the 304H steel, which is a spring stainless steel, and as a result, the concentration of Mo and Cu at the same time Increasing the amount of martensite that is organically produced during freshness decreases the strength, but maintaining the concentration of C and N in an appropriate range results in a relatively large strengthening effect of C and N in the produced martensite. It is to confirm the fact that the decrease in strength caused by the addition of a large amount of Cu and to propose the present invention.

Therefore, the present invention prepared from this point of view, in weight%, C: 0.060-0.08%, Si: 0.3-0.6%, Mn: 1.2-1.4%, P: 0.035% or less, S: 0.01% or less, Cr: 18.3- 18.9%, Ni: 8.3-8.7%, Mo: 0.5% or less, Cu: 0.7% or less, N: 0.02-0.05%, balance Fe and other unavoidable impurities, and (C + N) ^1/2 is 0.28- Austenitic stainless steel for springs satisfying 0.35.

Hereinafter, the austenitic stainless steel for spring manufacture of this invention is demonstrated in detail.

First, the stainless steel of the present invention in weight%, C: 0.060-0.08%, Si: 0.3-0.6%, Mn: 1.2-1.4%, P: 0.035% or less, S: 0.01% or less, Cr: 18.3-18.9 %, Ni: 8.3-8.7%, N: 0.02-0.05%, remaining Fe and other unavoidably contained impurities. This composition is a common component of 304H austenitic stainless steels in electric furnace manufacturers.

The present invention is characterized in that the basic 304H steel component to control Mo and Cu to an appropriate value, and to control the value of (C + N) ^1/2 to a predetermined value, the specific component content and The reason for limitation is as follows.

In general, Mo is a solid solution strengthening element that promotes the formation of delta ferrite (d-ferrite) and is known to increase hot deformation resistance by combining with C and N when added, and also to reduce the martensite transformation temperature during processing. (Irvine, KJ, DT Llewellyn, and FBPickering, J. Iron Steel Inst., London, Vol. 192 (1959) p.227-228) and are known to have the opposite effect of lowering strength upon freshness.

In the present invention, it is preferable to limit the content of Mo to 0.5% by weight or less. This is because it is not necessary to adjust the expensive Mo concentration artificially because the Mo content is usually 0.45% by weight without melting the steel for the steelmaking industry in the furnace maker using a large amount of scrap iron. In addition, if the content exceeds the actual amount of delta ferrite (d-ferrite) to generate a large amount of defects on the surface of the wire rod during hot rolling for the manufacture of 5.5 mmφ intermediate material may increase the error rate, and also due to substitution This is because the strength of the wire rod may be greatly reduced due to the production of a small amount of martensite rather than the strengthening effect.

More preferably, the Mo content is limited to 0.2 to 0.5%.

Cu is an element that increases corrosion resistance, and is known to decrease the strength of wire rods during processing, especially during drawing, by increasing austenite stability and increasing stacking defect energy. , Which does not significantly change the effect of solidification by substitution (Irvine, KJ, DT Llewellyn, and FBPickering, J. Iron Steel Inst., London, Vol.192 (1959) p.227-228 and Irvine, JJ , et. al., J. Iron Steel Inst. London, vol. 199 (1961) p.153-175).

In the present invention, it is preferable to limit the addition amount of the Cu to 0.7% by weight or less. This is because Cu, like Mo, does not usually exceed about 0.6 wt% of Cu concentration without any component adjustment when melting with an electric furnace using a large amount of scrap iron. More preferably, the Cu content is limited to 0.35 to 0.7% by weight.

In addition, in the present invention, when adding Mo and Cu as described above, it is preferable to limit the content ratio of Mo to Cu, that is, the Mo / Cu ratio to 0.6-0.75. This is due to the fact that the Mo content ratio to Cu is usually about 0.7 without any component adjustment in the melting operation for steelmaking in the electric furnace maker.

On the other hand, the solid solution strengthening effect of the invasive elements C and N in the solid solution is generally very large, but the increase in the content of these components has the opposite effect of significantly lowering the martensite transformation temperature to significantly reduce the strength of the wire rods when drawing. That is, the yield and tensile strength of the wire rod during austenite cold working (ie, freshness) should be considered simultaneously with the contribution of martensite fraction and the strengthening effect of martensite itself. The reinforcing of has a great influence on the content of invasive elements, especially C and N, and the extent of the same is the same. And the strength of martensite is generally known to be proportional to the (intrusive element content) ^1/2 (PG Wincheel and M. Cohen, Trans ASM, 55 (1962) p.347, and MJ Roberts and WS Owen, J.). iron and Steel Inst., 206 (1968) p.1419).

Therefore, the present invention limits the value of (C + N) ^{1/2 in} the 304H steel having a C, N content range as described above in consideration of the above point to 0.28-0.35. If the value of (C + N) ^1/2 is less than 0.28, the effect of solid solution strengthening of the martensite itself, which can offset the decrease in strength due to the addition of a large amount of Mo and Cu, is small, resulting in a decrease in strength of the fresh wire. This is because when the strength of the martensite itself is much greater than that, the strength of the fresh wire is increased more than necessary, which adversely affects the wire drawing process itself.

As described above, the present invention can solve the problems such as disconnection during drawing due to the increased strength by optimally controlling the contents of C and N while increasing the Mo and Cu content as compared to the conventional 304H steel, Therefore, the additional steelmaking cost due to the Cu content control can be effectively reduced.

Hereinafter, the present invention will be described in more detail with reference to Examples.

(Example)

TABLE 1

Ingredients (% by weight) Si Mn P S Cr Ni Mo Cu C N Invention One 0.424 1.31 0.031 0.001 18.66 8.49 0.235 0.381 0.063 0.0265 2 0.413 1.31 0.031 0.001 18.50 8.51 0.358 0.490 0.067 0.0294 3 0.415 1.31 0.030 0.001 18.34 8.47 0.462 0.690 0.067 0.0296 Comparison One 0.417 1.27 0.016 0.002 18.36 8.38 0.124 0.217 0.057 0.0238 2 0.40 1.24 0.024 0.004 18.02 8.43 0.120 0.240 0.070 0.0280 3 0.36 1.29 0.030 0.003 18.18 8.39 0.110 0.260 0.077 0.0420

After preparing the 5.5 mmφ intermediate material composition as shown in Table 1, it was cold drawn directly (cold drawing) without an intermediate heat treatment process so that it is 1.04 mmφ fine wire.

Specifically, Inventive Materials 1-3 and Comparative Material 1 in Table 1 produced a rolled plate of 20 mm thickness through hot rolling after producing a 25 kg ingot with the composition of the table. Subsequently, after heat-treating the rolled plate, a 5.5 mmφ intermediate material was manufactured by processing in the rolling direction at the center of the width of the plate. And the comparative material 2-3 in Table 1 represents a 5.5 mm diameter 304H intermediate material manufactured by wire rod rolling and heat treatment heat treatment in the actual electric furnace maker. On the other hand, as described above, in the manufacture of 5.5 mmφ intermediate material of the invention material 1-3 and the comparative material 1, the process conditions for applying the reheat treatment conditions and the heat treatment heat treatment conditions after production of the ingot to wire rod production in a conventional electric furnace maker And metallurgical properties of the intermediates as much as possible were similar. That is, the intermediate material of Inventive Materials 1-3 and Comparative Material 1 was prepared by performing crack heat treatment at 1215 ^o C for 30 minutes and crack heat treatment at 1230 ^o C for 1 hour after ingot manufacturing, which is a process condition of an electric furnace maker.

Subsequently, the six kinds of 5.5 mmφ intermediate materials manufactured as described above were subjected to the fresh processing under the same conditions without the intermediate heat treatment process so as to be 1.04 mmφ fine wire. At this time, the disconnection of the material was observed but no disconnection occurred. In other words, unlike the comparative materials as in Inventive Materials 1-3, even if the concentration of Mo and Cu was increased, there was no problem in drawing workability, and bright annealing was caused by increasing the strength of drawing wires as the drawing strain increased. It was found that no separate heat treatment is required. Here, the drawing rate is expressed as Ln (Ai / Af), where Ai is the cross-sectional area of the wire before drawing and Af is the cross-sectional area of the wire after drawing.

1 is a graph showing a change in the saturation magnetization value of the magnetic properties of each material with an increase in fresh processing rate for the intermediate materials.

Austenitic stainless steel is a non-magnetic material and its magnetic properties do not change due to the external magnetic field, so no change in the saturation magnetization value is observed, but martensite, which is induced in austenite by processing, has magnetic properties. Will change. Such characteristics can be detected by a high sensitivity VSM (Vibtaring Sample Magnetometer), and the amount of martensite released can be evaluated by measuring the saturation magnetization value which is proportional to the amount of magnetic material among the magnetic properties. That is, the vertical axis of FIG. 1 is a saturation magnetization value measured when 16 KOe magnetic fields are applied to each material through the VSM, and this value corresponds to the amount of martensite generated inside the material according to the freshness. do. As shown in Figure 1, the saturation magnetization value of each material is shown to increase with the increase in the fresh working rate, it can be seen that as the content of Mo and Cu increases in each fresh processing rate. This means that the higher the amount of Mo and Cu, the less the amount of martensite is produced. Therefore, the higher the content of Mo and Cu, the lower the amount of martensite.

Figure 2 is a graph showing the measured tensile strength of each material changed according to the fresh working rate in step of cold drawing against the six kinds of intermediate materials. As shown in FIG. 2, the tensile strength of each material was increased in a manner almost similar to the change in magnetization value of FIG. 1 as the drawing rate increased, which is closely related to the amount of martensite generated in each material. I think there is. It should be noted that the tensile strength of each material is not significantly different for each material. That is, it can be seen that the invention material 1-3 having an increased content of Mo and Cu, although the amount of martensite produced was smaller than that of the comparative material 1-3, as shown in FIG. This shows that even if the content of Mo and Cu is higher than the specified value, there is no problem in the strength of the wire rod. Therefore, the present invention can effectively solve the problem of steel cost increase required in the comparative material in order to reduce the content of Mo and Cu. It can be seen that.

Meanwhile, although the amount of martensite generated as the Mo and Cu content increases as shown in FIG. 1 decreases the strength of the material, the reason for showing the tensile strength characteristics as shown in FIG. 2 is shown in FIG. 3.

3 is a graph showing the change in tensile strength with respect to the (C + N) ^1/2 value of the steel grade. As shown in Figure 3, it can be seen that there is almost no change in the tensile strength with the increase in the (C + N) ^1/2 value at the same ductility. However, as described above, when the austenitic cold working (ie, the fresh working), the yield of the wire rod and the increase in tensile strength should simultaneously consider the contribution of the martensite fraction and the strengthening effect of the martensite itself. Therefore, Figure 3 shows the strength reduction of the fresh wire produced by the reduction of the amount of martensite generated by increasing the Mo and Cu content of the present invention materials 1-3, specifically, (C + N) ^{1 /} By controlling the value of ² to satisfy 0.28-0.35, the effect of solid solution strengthening on the martensite itself is increased, indicating that the tensile strength is equivalent to that of the comparative material as a whole.

As described above, the present invention has no problem in manufacturing the 5.5 mmφ intermediate material even when the content of Mo and Cu in the 304H steel, which is austenitic stainless steel for springs, is higher than the specified value, and there is no disconnection at the time of drawing. In the strength of the fresh wire, there is a useful effect in providing a steelmaking cost-saving austenitic stainless steel with almost no decrease in strength.

Claims (4)

By weight%, C: 0.060-0.08%, Si: 0.3-0.6%, Mn: 1.2-1.4%, P: 0.035% or less, S: 0.01% or less, Cr: 18.3-18.9%, Ni: 8.3-3.8% , O: 0.2-0.5%, Cu: 0.35-0.70%, N: 0.02-0.05%, remainder Fe and other unavoidable impurities, (C + N) ^1/2 for spring austerity meets 0.28-0.35 Knight stainless steel. delete delete The austenitic stainless steel for springs as claimed in claim 1, wherein the Mo / Cu ratio satisfies 0.6-0.75.

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