KR19980034485A - Manufacturing method of hard steel wire for drawing high manganese - Google Patents

Abstract

In order to improve the hardenability of Mn and to suppress martensite generation due to central segregation during the production of high Mn hard steel wire, the direct drawing of the total cross-sectional reduction rate up to 80 ~ 85% without patenting heat treatment is applied by applying stepwise continuous cooling. It is an object of the present invention to provide a method for producing a high Mn-containing hard steel wire which is as excellent as possible.

In order to achieve the above object, the present invention is a method for producing a steel wire rod for drawing, in weight%, C: 0.6 ~ 0.8%, Si: 0.1 ~ 0.3%, Mn: 0.9 ~ 1.2%, P: 0.015% Or less, S: 0.01% or less, Cu: 0.05% or less, hot rolled steel composed of residual Fe and unavoidable impurities with a wire rod, and then austenite → ferrite + pearlite (austenite → ferrite +) at a cooling start temperature of 800 to 900 ° C. Perlite) After cooling to 20 ~ 25 ℃ / sec to the transformation start temperature, after cooling, the cooling rate is lowered to 4 ~ 9 ℃ / sec to manufacture cold MW-containing wire rods It is the point about the method.

Description

Manufacturing method of high strength steel wire rod containing Mn

The present invention relates to a method for producing a hard steel wire for drawing, and more particularly, to a method for producing a high Mn-containing hard steel wire having a microstructure suitable for direct drawing.

Generally, coil springs for automobile parts are made of hard steel wire containing 0.4 to 0.8% of carbon except for special purposes such as suspension. Spring material is subjected to very severe plastic deformation during coiling forming spring shape, so the mechanical properties of material such as strength and toughness must be uniform, and straightness is required to express spring characteristics well. . For this purpose, after direct drawing to the required diameter in the wire state, austenitized heat treatment at 900 ℃ or more in order to give uniform mechanical properties and microstructure, and quenched in oil (60) to 70 ℃ , Coiling molding by tempering (tempering) at a temperature of 450 ~ 500 ℃. Direct drawing during the process is an essential step in order to obtain the diameter of the material exhibiting the most uniform mechanical properties and microstructure under the given oil quenching-tempering conditions. In addition, the direct drawing is a process that has the greatest impact on productivity since disconnection occurs in the direct drawing process when the microstructure of the raw material wire is poor.

In general, steel mills manufacture wire rods for direct drawing using the continuous cooling method based on the continuous cooling curve according to the alloy composition of steel. Therefore, inadequate cooling rate, a large amount of hardenability-enhancing alloying elements, or a central segregation caused by these elements generate low-temperature structures such as martensite, which acts as a disconnection factor when drawing, and thus suppression thereof is required.

Generally, the steel wire rod for drawing usually contains 0.3 ~ 0.9% of Mn. However, in the case of spring steel wire rod, an attempt has been made to increase Mn to 0.9% or more to improve the hardenability during oil quenching and tempering. have. However, Mn not only improves the hardenability but also facilitates central segregation, so that when it is contained in a large amount, Mn has a high probability of locally improving the hardenability at the center of the wire, thereby generating martensite structure at the center of the wire. Therefore, in the production of high Mn-containing steel wire for general spring through the oil quenching-tempering process, it is necessary to apply a cooling method or to set a cooling rate that suppresses martensite generation that acts as a disconnection factor during direct drawing as a pretreatment. It is the most important condition.

Conventional techniques for producing high Mn-containing hard steel wires using the continuous cooling method include Japanese Patent Application Laid-Open Nos. 61-128789, 6-57261, US Patent Publication No. US771028 and Japanese Ochiai et al. Iron and Steel, '' 74 (1998) p.1625, `` Influence of the central segregation on the freshness of high-carbon steel wire rods ''.

61-128789 is a steel containing C: 0.7-1.0%, Si: 0.5-2.0%, Mn: 0.5-1.5%, N: 0.003-0.015%, Al: 0.02-0.1%. The present invention relates to a method of directly cooling a die during drawing by patenting heat treatment to secure strength of 138 kg / mm2 or more and at the same time obtaining excellent ductility. This is not suitable for direct drawing because it performs the intermediate heat treatment.

The Patent Publication No. 6-57261 is C: 0.7-1.1%, Si: 0.1-1.5%, Mn: 0.1-1.5%, P: 0.02%, S: 0.02% as the basic chemical composition Cr: 0.3 It relates to a method of increasing the freshness by controlling the aspect ratio (length / thickness) of the inclusions inevitably formed in steels in which at least one element of%, Ni: 1.0% and Cu: 0.8% is added. . It is impossible to control the aspect ratio because the strength of martensite is very high when the inclusion is martensite. Therefore, it is difficult to apply this method when martensite is present in the form of inclusions due to segregation or hardenability improvement due to high Mn content.

US Patent Publication No. US771028 includes C: 0.4-1.5%, Si: 0.05-1.0%, Mn: 0.05-1.2%, Cr: 0.01-1.4%, Co: 0.3-1.6%, Ni: 0.01-0.5% After the plated heat treatment at 540 ~ 620 ℃ of steel material to produce a wire rod with a final reduction of 60 ~ 98%. However, since it contains Cr and Co, which have a large effect of improving hardenability in addition to high Mn, it is difficult to suppress martensite generation under the conditions of wire manufacturing, so that patenting heat treatment is inevitable.

In addition, the above three techniques do not mention the suppression of martensite generation in the center of the wire rod by the central segregation due to the high Mn content.

Influence of central segregation on the freshness of high-carbon steel wire rods, which was published in Iron and Steel 74 (1998) p.1625, is a technique for reducing the central segregation of Mn in performance billets, which are pre-rolled wire rods. When reheating the valet before wire rod rolling, the temperature and material time of the furnace are increased to reduce central segregation due to the diffusion of Mn. However, this technique can not completely eliminate the Mn segregation zone, there is a problem that martensite is likely to occur during continuous cooling of the wire rod by the remaining Mn segregation zone.

The present inventors have proposed the present invention based on the results of repeated research and experiments to solve the problems of the above techniques, the present invention is to improve the hardenability and the central segregation of Mn when manufacturing high Mn hard steel wire In order to suppress the martensite generation by the step, by applying a continuous continuous cooling method to provide a method of manufacturing a high-strength high Mn-containing hard wire material that can be drawn directly to 80 ~ 85% of the total cross-sectional reduction rate without the patented heat treatment. have.

1 is a continuous cooling curve used in the present invention

Figure 2 is a photograph of the cross section of the invention material

The present invention is a method for producing a steel wire rod for drawing, in weight%, C: 0.6 ~ 0.8%, Si: 0.1 ~ 0.3%, Mn: 0.9 ~ 1.2% or less, P: 0.015%, S: 0.01% or less , Cu: 0.05% or less, hot rolled steel composed of residual Fe and unavoidable impurities, followed by austenite → ferrite + pearlite (austenite → ferrite + pearlite) transformation start temperature at the cooling start temperature of 800 ~ 900 ℃ After cooling at the cooling rate of 20-25 degreeC / sec, the cooling rate is reduced to 4-9 degreeC / sec after a transformation start, and it is related with the method of manufacturing the direct-stretchable high Mn containing hard steel wire for drawing.

Hereinafter, the reason for component limitation of this invention is demonstrated.

The carbon is a component known as the most effective element for increasing the strength, and the carbon content is limited to 0.6 to 0.8% in the present invention. This is because, when the carbon content is less than 0.6%, the strength decreases with the increase in the fraction of ferrite, and thus, it is not possible to control the characteristics of the spring, which is the use of steels having the chemical composition. In addition, if it exceeds 0.8%, the cornerstone cementite is easily precipitated in the range of the cooling rate, which may cause disconnection during drawing.

The silicon is an element essential for deoxidation of the steel, and the content is limited to 0.1 to 0.3%. This is because if the content is less than 0.1%, the deoxidation effect is insufficient, and the effect of improving the strength of the pearlite due to solid solution in the ferrite in the pearlite is not sufficient. In addition, when the content is more than 0.3%, silicon exhibits a hardening effect similar to that of Mn, so that in high Mn-containing steels such as the above components, synergistic effect is shown in addition to Mn for improving hardenability. This is because the increase in freshness is caused by the decrease in ductility.

The manganese has a great deoxidation effect in the production of steel and reacts with sulfur, which is essentially contained in the production of steel, to form manganese sulfide (MnS), thereby preventing red brittleness due to grain boundary segregation of sulfur. In general hard steel wire, considering the improvement effect of hardening and limiting the Mn content to 0.9% or less, in the present invention, it increases the linearity, which is the characteristic of the spring, and guarantees the hardenability during oil quenching-tempering. In order to do this, 0.9% or more is added. However, when it is over-added, segregation zones with an average Mn concentration of 1.5 times or more are formed in the center of the wire rod, so even if the lowest cooling rate is applied under continuous cooling facilities currently in use, marsenite is inevitable, so it is limited to 1.2% or less. Therefore, in this invention, Mn is limited to 0.9 to 1.2%.

The phosphorus is most likely to segregate in the center of steelmaking and forms the segregation zone in the center even after wire rod rolling as an element having the greatest effect on improving hardenability. In addition, since it is easy to generate co-segregation with Mn and easily generates low-temperature tissues even under normal slow cooling conditions, the present invention manages 0.015% or less.

The sulfur is an element that is easy to segregate in the grain boundary, when the grain is sewn into a grain crater makes the grain boundary brittle, and in particular, the thermal brittleness occurs during high temperature rolling.

Cu is added for the purpose of preventing corrosion. However, Cu is precipitated as ε-Cu at the cementite / ferrite interface in the ferrite during the ferrite transformation when excessively added, thereby suppressing the growth of the ferrite, and therefore, the present invention is limited to 0.05% or less.

Hereinafter, in the present invention, the reason for limitation of the cooling start temperature range and the reason for limiting the cooling rate will be described.

The present invention is hot-rolled steel as described above and then cooled to austenite → ferrite + perlite transformation start temperature at the cooling start temperature of 800 ~ 900 ℃. When cold steel is started at a relative low temperature of 800 ℃ or less, cold start temperature is inevitable because cold start temperature is directly related to wire rolling temperature, which causes rolling failure such as uneven shape of material due to rolling overload during rolling. Above 900 ° C, the strength decreases due to the decrease of the fraction of aulite due to coarse austenite, and the wire rods start to cool because it causes deterioration of wire quality such as defects due to surface defects and winding problems during hot rolling. Is limited to 800 to 900 ° C.

In addition, the present invention is limited to the cooling rate from the wire rod cooling start temperature to the pearlite transformation start temperature to 20 ~ 25 ℃ / sec.

When the cooling rate to the start of the perlite transformation temperature is less than 20 ℃ / sec, the perlite transformation starts at a temperature higher than the nose temperature of the continuous cooling curve, at which time the supercooling, the owner of determining the lamellar spacing of the perlite Becomes small, forming a coarse resolveable pearlite structure, deteriorating freshness and causing a decrease in strength. Subcooling means the difference between equilibrium transformation temperature and actual transformation temperature, and resolvable pearlite is a pearlite with a wide lamellar spacing, and the structure of perlite and fermentite that can distinguish ferrite and cementite under the magnification of a general optical microscope It is called. In the case of 25 ° C./sec or more, bainite transformation, which is a part of the low temperature tissue, starts before the start of the perlite transformation, so that the cooling rate to the starting temperature of the pearlite transformation is limited to 25 ° C./sec or less.

In addition, the present invention is limited to 4 ~ 9 ℃ / sec cooling rate after the start of the perlite transformation.

The reason for limiting the cooling rate to 9 degrees C / sec or less is as follows. In the silicon and manganese contents of the steel, the critical cooling rate of martensite generation on the continuous cooling curve is 15 ° C./sec, but when a cooling rate of 9 to 15 ° C./sec is given, the ferrite transformation is performed at the center of the wire with severe Mn segregation. Some austenite is non-diffusion transformed into low temperature bainite or martensite before it is completed. Therefore, in order to suppress the low temperature structure generation at the center of the wire rod by the Mn central segregation, the cooling rate after the start of the pearlite transformation is limited to 9 ° C / sec or less. In addition, the cooling rate after the start of the ferrite transformation is limited to 4 ℃ / sec or more for the following reasons. When slow cooling is performed at 4 ° C / sec or less to suppress martensite generation at the center of the wire, the supercooling degree is reduced as described above when the untransformed austenite is transformed into a pearlite. Form. Therefore, the cooling rate after the start of the perlite transformation is limited to 4 ~ 9 ℃ / sec.

The present invention constituted as described above improves the hardenability associated with high Mn content and suppresses the occurrence of martensite, a microstructure harmful to freshness due to the central segregation of Mn, and reduces the total cross-sectional reduction rate by 80 to 85% without patenting heat treatment. Can be directly drawn by

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

Example 1

In order to measure the critical cooling rate of martensite generation during continuous cooling, rod-shaped specimens of 10 mm length x 3 mm length were processed at the center of 7 mm diameter wire rod having the components shown in Table 1, followed by austenitic heat treatment at 900 ° C. for 3 minutes to 830 ° C. After quenching by using a dilatometry (continuous cooling simulation apparatus), a continuous cooling curve prepared by applying a cooling rate of 1 ~ 50 ℃ / sec is shown in FIG. (A) of FIG. 1 is the continuous cooling curve of the steel grade A of Table 1, and (b) of FIG. 1 is the continuous cooling curve of the steel grade B of the following Table. As shown in (a) and (b) of FIG. 1, in the steel having two kinds of components, there is no significant difference in the continuous cooling curve, and the temperature corresponding to the nose is about 540 ° C. and the cooling rate through the nose is shown. Was 20-25 degreeC / sec. In addition, when a constant cooling rate was applied until the transformation was completed, the critical cooling rate of martensite generation was 15 ° C / sec.

Example 2

On the basis of the continuous cooling curve obtained in Example 1, the cooling rate from the cooling start temperature of 830 ° C. to the start of the perlite transformation at 20 ° C. / After the start of sec and perlite transformation, the cooling rate was applied to 5, 7, 12, and 17 ℃ / sec, respectively, to produce wire rods with a diameter of 7mm in the stelmor continuous cooling facility.

In addition, the wire rod was manufactured by applying a constant cooling rate of 12 ℃ / sec from the start of cooling to the completion of transformation in order to determine the effect of the stepwise continuous cooling. At each cooling rate, the cross section of the wire was cut and observed with an optical microscope at 24x magnification to investigate the occurrence of martensite due to the central segregation due to the high Mn content in the center of the wire. Processing was performed. Drawing was done in 10 passes with a drawing speed of 3m / min. The average section reduction per pass was 20% and the total section reduction was 82%. FIG. 2 (a) is a structure photograph of the cross section of the comparative material (1) to which the cooling rate of 20 ° C./sec before the start of the perlite transformation and 12 ° C./sec after the start of transformation is applied. It shows that martensite occurred due to segregation of Mn. In Fig. 2 (a), the white part of the central part is martensite. Figure 2 (b) is a cross-sectional structure of the invention material (2) applying a cooling rate of 20 ℃ / sec before the start of the transformation, and 7 ℃ / sec after the start of the transformation, applying a stepwise continuous cooling method and step-by-step cooling rate When it is adjusted within the above range it can be seen that the generation of martensite in high Mn-containing hard steel wire. In accordance with the cooling rate applied to each of the invention and the comparative material whether the occurrence of the martensite in the center and whether the disconnection occurs in the drawing is shown in Table 2, respectively. In the microstructure of Table 2, P represents ferrite, F represents ferrite, and M represents martensite.

Martensite is generated at the cooling rate of 12 ℃ / sec below the critical cooling rate due to Mn segregation in the center when the continuous continuous cooling method is not applied as in Comparative Materials (3) and (4). It was. In addition, even if the continuous continuous cooling method is applied as in Comparative Materials (1), (2), (4) and (5), when the cooling rate range is out of the range of the present invention, martensite is generated, and Shows a direct cause. The continuous cooling curve of FIG. 1 shows the stepwise continuous cooling method and the trajectory of the cooling rate of the present invention.

As can be seen from the above, according to the present invention, it is possible to suppress the martensite generation of the microstructure caused by the improvement of the hardenability accompanying the high Mn content and the central segregation of Mn wires, and the total cross-sectional reduction rate of 80 ~ It is possible to produce high Mn-containing hard steel wire which can be directly drawn to 85%.

Claims (1)

In the method of manufacturing a hard steel wire for drawing, By weight%, C: 0.6 ~ 0.8%, Si: 0.1 ~ 0.3%, Mn: 0.9 ~ 1.2%, P: 0.015% or less, S: 0.01% or less, Cu: 0.05% or less, remainder Fe and unavoidable impurities After hot rolled steel with wire rod, it is cooled to austenite → ferrite + pearlite (austenite → ferrite + pearlite) transformation start temperature at the cooling start temperature of 800 ~ 900 ℃, and then cooled at 20 ~ 25 ℃ / sec. After the transformation start, the cooling rate is reduced to 4 ~ 9 ℃ / sec to produce a high Mn-containing hard steel wire for drawing.