KR100285258B1 - Spheroidizing method of high ally steel for wire rod - Google Patents

Abstract

PURPOSE: A method of spheroidizing a high ally steel for wire rod is provided to rapidly obtain spheroidizing speed and uniform bainite microstructure. CONSTITUTION: The spheroidizing method of high alloy steel for wire rod includes the steps of (i) hot rolling a high alloy steel billet comprising C 0.2-0.6wt.%, Si 0.1-1.0wt.%, Mn 0.1-1.5wt.%, at least one element selected from the group consisting of Cr 0.1-2.0wt.%, Ni 0.1-5.0wt.%, Mo 0.1-1.5wt.%, B 5-200ppm, a balance of Fe and other inevitable impurities(wherein, the impurities includes Al less than 0.1wt.%, N less than 0.01wt.%, P less than 0.02wt.%, S less than 0.02wt.%); (ii) cooling the hot rolled high alloy steel wire in a cooling rate of 0.5 to 5deg.C/sec to control bainitic fraction of microstructure of the steel wire over 70%.

Description

Fabrication of High Alloy Steel Wire for Homogenization

TECHNICAL FIELD The present invention relates to a high alloy steel wire rod subjected to spheroidizing heat treatment before hot or cold forging for use in mechanical structures, bolts and nuts, and a method of manufacturing the same. More particularly, the present invention relates to a high alloy steel wire rod for spheroidizing heat treatment And a method of manufacturing the same. [0002] The present invention relates to a high alloy steel wire rod and a method of manufacturing the same.

Generally, since high alloy steel wire has excellent hardenability, it can be hot-rolled and controlled-cooled to obtain desired final shape through hot or cold forging in a secondary processing company, and then quenching or tempering treatment to increase toughness and strength And is manufactured as a final product having a tempered martensite structure. On the other hand, since a very high deformation occurs in such hot and cold forging processes, defects such as cracks are likely to occur during processing unless the ductility of the material is sufficiently secured. Therefore, prior to hot or cold forging of high alloy steel, spheroidizing heat treatment is generally performed to secure ductility of the material. Here, the spheroidizing heat treatment refers to a heat treatment process that distributes spherical cementite particles to a ferrite base so that a ferrite base having excellent ductility can deal with deformation during hot or cold working so that defects such as cracks do not occur.

There are many ways to perform such spheroidizing heat treatment, but the most commonly applied method is to heat the material directly on the A1 transformation temperature and dissolve the carbides on the austenite base at this temperature. If the holding time of the flock is made too long, the carbon content dissolved in the austenite base will be uniform without any change depending on the position, and will be transformed into pearlite structure during the maintenance period immediately below the A1 temperature. In order to prevent this, it is necessary to adjust the holding time at the temperature just above A1 so that the carbon content in the austenite is cooled down to a temperature just below the A1 temperature while the carbon content in the austenite changes depending on the position, And induces them to serve as the nucleation site of the tight. As the spherical cementite particles and ferrite grains grow during the holding at the temperature directly below the A1, the strength of the material gradually decreases. Therefore, after the appropriate time is maintained as required, the annealing process is finished after cooling.

On pages 331-344 of the Proceeding of the International Symposium on Accelerated Cooling of Rolled Steel, held in Winnipeg, Canada, from August 24 to August 25, 1987, As reported, the high alloy wire for performing the spheroidizing heat treatment generally has a microstructure composed mainly of ferrite and pearlite through controlled cooling after hot rolling. However, when the microstructure is made of ferrite and pearlite, it takes a very long time to change the planar cementite inside the pearlite into a spherical shape. Since the spherical cementite is concentrated only in the original pearlite region after the spheroidizing heat treatment, the microstructure after the spheroidizing heat treatment is uneven, and there is still a high possibility that defects such as cracks are generated in the subsequent hot or cold forging.

In the case where the microstructure of the wire is made of martensite, since the carbide inside the martensite is very finely distributed, it can be easily dissolved into the austenite matrix during the spheroidization heat treatment. However, since the velocity is too fast, The carbon component is homogenized while the temperature is maintained above the temperature. For this reason, when cooling is performed at a temperature of not more than A1, a region having a relatively high carbon content, which serves as a nucleation site of spherical cementite, disappears and transformation into pearlite occurs again. This is called normal regenerated pearlite. When the material is made of martensite, there is almost no ductility, which causes a disconnection in handling or straightening of the wire coil. Therefore, it is advantageous to lower the fraction of martensite in the microstructure of the high alloy steel wire as much as possible.

On the other hand, the inventors of the present invention found that since the bainite is formed in the indefinite form of carbide, the dissolution rate into the austenite base is faster than that of pearlite during the holding time immediately above the temperature of A1, and the microstructure after the spheroidizing heat treatment is also highly uniform And since it has considerable ductility unlike martensite, there is no possibility of causing disconnection during the handling or straightening process of the wire coil. In the process of controlling and cooling the high alloy steel wire rod by hot rolling, As a result of in-depth study on the method of increasing the speed and uniformizing the structure of the spheroidizing structure, a control cooling technique capable of obtaining an optimal microstructure by hot-rolling the billets containing the above alloy components and then cooling them by blowing Based on this, the present invention has been proposed.

Accordingly, it is an object of the present invention to provide a high alloy steel wire for machine structure, bolt and nut, and a method for manufacturing the same, which can obtain a significantly faster sphere velocity and a uniform spheroidizing structure in a spheroidizing heat treatment in a subsequent step.

1 is a graph showing a continuous cooling transformation curve of a high alloy steel wire rod having an alloy component of Table 1 in an embodiment of the present invention.

FIG. 2 is a process flow chart showing a processing step before the spheroidizing heat treatment. FIG.

FIG. 3 is a graph showing temperature fluctuation conditions with time during spheroidizing heat treatment. FIG.

FIG. 4 is a photograph of spherical microstructure observed by a scanning electron microscope after spheroidizing heat treatment of various wire materials shown in (Table 2) in the embodiment of the present invention.

In order to attain the above object, the present invention provides, as a first aspect, a wire rod for performing a spheroidizing heat treatment before hot or cold forging for use in mechanical structure, bolt and nut, etc., At least one member selected from the group consisting of Cr: 0.1 to 2.0%, nickel: 0.1 to 5.0%, molybdenum: 0.1 to 1.5%, and boron: 5 to 200 ppm, containing 0.1-1.0% silicon and 0.1-1.5% manganese, And more than 70% of the microstructure is contained in the bainite, and the remainder is iron and unavoidable impurities (in the impurity, the content of aluminum is 0.1% or less, the content of nitrogen is 0.01% or less, the content of phosphorus is 0.02% Wherein the high-alloy steel wire comprises a high-alloy steel wire.

The second aspect of the present invention is a method for producing a steel sheet comprising the steps of: mixing 0.2 to 0.6% of carbon, 0.1 to 1.0% of silicon, 0.1 to 1.5% of manganese, 0.1 to 2.0% of chromium, 0.1 to 5.0% of nickel, At least one element selected from the group consisting of molybdenum: 0.1 to 1.5% and boron: 5 to 200 ppm, and the balance: iron and unavoidable impurities (provided that the content of aluminum is 0.1% or less, the content of nitrogen is 0.01 or less, the content of phosphorus is 0.02% ) And then controlled cooling at a cooling rate in the range of from 0.5 to 5 DEG C per second, so that at least 70% of the microstructure is made of bainite. There is provided a method for manufacturing a high alloy steel wire rod which is subjected to spheroidizing heat treatment before hot or cold forging for use as a nut.

Hereinafter, the reason for limiting the above numerical values will be described.

Carbon is the most effective and important element in increasing the strength of steel. When carbon is added in an amount of 0.2% or less, a sufficiently high strength can not be obtained in the tempered martensite structure which is the microstructure of the final product.

Therefore, the carbon content should be 0.2% or more. If the carbon content is more than 0.6%, the impression of the martensite is remarkably decreased, so that it can not be used for the purpose of the mechanical structure bolts and nuts.

Since silicon is an element required for deoxidation of steel, if the content is too small, deoxidation effect is not sufficient, so it should be added by 0.1% or more. However, if silicon is added excessively, surface decarburization will become severe during the wire manufacturing process, so it is desirable to regulate it to 1.0% or less.

Manganese not only has a deoxidation effect in the production of steel but also forms manganese sulphide (MnS) together with sulfur in the material to prevent the brittle phenomenon caused by sulfur. Therefore, the manganese should be added by 0.1% or more, The bainite of the desired microstructure can not be obtained and the martensite is formed at the time of blowing and cooling after rolling. Therefore, it is a cause of recrystallization pearlite in the spheroidizing heat treatment.

Chromium has the effect of improving the corrosion resistance of steel, and at least 0.1% should be added in order to increase hardenability during quenching, which is the final heat treatment process of high alloy steel wire. However, if the content of chromium is exceeded, not only the martensite is generated during the cooling of the air after the rolling of the wire, but also the secondary scale is excessively dense to deteriorate the mechanical descaling ability or the pickling scalability. Therefore, it is necessary to limit the content of chromium to 2.0% or less.

Nickel is an element that improves the toughness of tempered martensite structure. However, if it is added below 0.1%, the effect can not be obtained, and when it exceeds 5.0%, the effect is saturated.

Molybdenum has the effect of inhibiting the phosphorus segregation in the austenite grain boundaries and improving the toughness of the final product. At least 0.1% should be added in order to obtain the effect of increasing the hardenability of the high alloy steel wire by quenching. On the other hand, if the content of molybdenum exceeds 1.5%, martensite is generated upon cooling the air after blowing the wire.

Boron is an element effective for obtaining martensite during quenching heat treatment, but if it is added in an amount of 5 ppm or less, the effect can not be obtained. If more than 200 ppm is added, the effect is saturated.

The high alloy steel wire rod subjected to the spheroidizing heat treatment before the hot or cold forging process according to the present invention contains impurities which are unavoidable as the remainder in the above-mentioned circle. The content of aluminum, nitrogen, phosphorus, and sulfur in the impurities should be limited as follows.

Aluminum is an effective element for deoxidation in a melting process and to prevent coarsening of austenite grains. However, if the content of aluminum is excessive, a large amount of nonmetallic inclusions of alumina (Al ₂ O ₃ ) system is generated and the fatigue property of the final product is remarkably lowered. Therefore, aluminum should be controlled to be less than 0.1%.

It is an element which is easily segregated in the center part during the coagulation of phosphorus steel of phosphorus. It plays a role to reduce ductility and toughness when it is employed in steel. Especially, high strength steel has a large deterioration in ductility. Therefore, it is necessary to reduce phosphorus as much as possible, and it should be managed at a maximum of 0.02% or less.

Sulfur degrades the toughness and ductility of the steel when segregated, so it should be controlled to a maximum of 0.02% or less.

Nitrogen, together with aluminum, forms aluminum nitride (AlN) to effectively refine the austenite grain boundaries. However, when the content is 0.01% or more, the effect is saturated, and if it is contained excessively, the ductility may be deteriorated during forging.

When 30% or more of the microstructure other than bainite is made of ferrite and pearlite in controlling the microstructure through hot-air cooling and cooling process of the hot-rolled wire of the high alloy steel containing the alloy component in the above range, In addition, when 30% or more of the microstructure other than bainite is made of martensite, the spheroidization rate is remarkably decreased due to the generation of regenerated pearlite after spheroidizing heat treatment, Should be controlled to be at least 70% above the bainite fraction.

Hereinafter, the present invention will be described more specifically by way of examples.

[Example]

A high alloy steel wire rod having a weight percentage of the following composition (Table 1) was formed into a billet having a length of 160 mm and a length of about 10 m by continuous casting and heated at a temperature of 1100 캜 or higher for 1 hour and 30 minutes After removal of the casting structure, hot rolling was carried out in a hot rolling mill to a final diameter of 8.5 mm by means of a continuous rolling mill.

The hot-rolled material was continuously passed through a water-cooling apparatus to adjust the coiling temperature to 830 캜. Thereafter, the air blowing amount of the forced air cooling device was adjusted to cool the material to a room temperature while controlling the cooling rate of the material, thereby producing a wire coil.

At this time, by controlling the cooling rate of the wire rod in the range of 0.5 to 5 ° C per second, it is possible to adjust the bareite to 70% or more of the microstructure of the final wire rod. This was confirmed from the continuous cooling transformation curve in the case where the material having the alloy component of the above (Table 1) shown in FIG. 1 was hot-rolled to have a diameter of 8.5 mm. That is, in the case of the cooling curve D in FIG. 1, the cooling rate is 5 ° C. per second. The microstructure was analyzed by an image analyzer, and it was found that 73% of bareite and 27% of meldensite were formed.

In the case of the cooling curve G in FIG. 1, the cooling rate was 0.5 ° C. per second, and the final microstructure was composed of 75% of bainite and 25% of ferrite and pearlite. When the cooling rate increases to 5 ° C or more per second, the fraction of martensite increases to 30% or more. On the contrary, when the cooling rate decreases to 0.5 ° C or less per second, the fraction of ferrite and pearlite increases to 30% Is reduced to 70% or less.

Table 2 below shows the final microstructural fractions of the wire rods at each cooling rate when the wire rods were tested at different cooling rates in order to examine the effect of the final microstructure on the roughening behavior. In the case of the A steel shown below (Table 2), the average cooling rate was 10.9 ° C per second, which was very fast, so that the fraction of martensite was 89% and the fraction of bainite was 11%, and ferrite and pearlite did not occur. On the other hand, in the case of the C steel of Table 2, the cooling rate was too slow to 0.3 ° C. per second, so that the percentage of ferrite and pearlite in the final microstructure was 65% and that of bainite was 35%. In the case of the inventive steel B, the cooling rate was 1.2 ° C. per second, and a microstructure consisting of 96% bainite and 4% ferrite and pearlite was obtained.

As shown in Fig. 2, the roughening heat treatment was carried out through the process using the wire rods thus produced. Here, the drawing process is a process which is usually performed for the purpose of precisely adjusting the diameter of the final product and promoting the spheroidizing heat treatment in the wire rod processing company, and the low temperature annealing process is usually performed at about 700 ° C Annealing heat treatment at a temperature not higher than the above temperature. FIG. 3 shows the temperature change conditions with time in the spheroidizing heat treatment.

The morphology of spheroidized tissues of various wire rods of the above-mentioned spheroidized (Table 2) was observed with a scanning electron microscope, and the results are shown in FIGS. 4 (a) to 4 (c). Figure 4 (a) is for the A river in Table 2, Figure 4 (b) is for the B river, and 4 (c) is for the C river. As can be seen from FIGS. 4 (a) to 4 (c), in the case of the A steel having an excessively high fraction of martensite in the wire rod state, regenerated pearlite was generated after spheroidizing heat treatment. As described above, during the spheroidizing heat treatment, while the temperature of the material is raised to the A1 transformation point or higher, the carbon component in the martensite dissolves very quickly in the austenite and becomes homogenized, so that a new pearlite is generated . In case of the C steel of FIG. 4, the structure after the spheroidizing heat treatment in the case where the fraction of the ferrite and the pearlite is excessively high. In the case of the plate-shaped cementite inside the pearlite, as described above, in order to be dissolved in the austenite at the temperature of A1 or higher, It takes a lot of time to dissolve because it is necessary to give priority to the segmentation process of the tight. Therefore, in the spheroidizing heat treatment pattern as shown in FIG. 3, the cementite in the pearlite is not sufficiently dissolved, and a part thereof remains as it is and is observed in the inside of the structure as a plate or stick cementite even after cooling. As a result, as shown in FIG. 3, in the region where the original structure was ferrite, the content of carbon is very small, so that cementite particles are hardly observed even after spheroidizing heat treatment. This causes nonuniformity of the spheroidizing structure, which may promote the occurrence of defects in the subsequent hot or cold forging.

On the other hand, in the case of the B steel of FIG. 4, it can be seen that most of the cementite exists not only in a spherical form but also in a very uniform distribution state. Therefore, when the bainite structure of 70% or more of the microstructure of the high alloy steel wire having the above-described alloy component is used, it is possible to simultaneously obtain the homogeneity of the structure after spheroidization as well as to accelerate the rate of spheroidization in the spheroidizing heat treatment. There is a number.

INDUSTRIAL APPLICABILITY As described above, according to the present invention, it is possible to obtain a high-alloy steel wire rod for a mechanical structure or a bolt and a nut which can achieve a significantly faster spheroidization rate and a uniform spheroidizing structure in a heat treatment in a subsequent step.

Claims (1)

The steel sheet according to any one of claims 1 to 3, which contains 0.2 to 0.6% by weight of carbon, 0.1 to 1.0% of silicon, 0.1 to 1.5% of manganese, 0.1 to 1.5% of chromium, 0.1 to 2.0% of chromium, 0.1 to 5.0% of nickel, 0.1 to 1.5% of molybdenum, And the balance: iron and unavoidable impurities (provided that the content of aluminum is 0.1% or less, nitrogen: 0.01% or less, phosphorus: 0.02% or less, and sulfur: 0.02% or less in the impurities) Hot rolling the billet of the alloy steel; Controlled cooling at a cooling rate of 0.5 to 5 占 폚 per second; Wherein at least 70% of the microstructure is made of bainite.