KR970004991B1 - Making method of high carbon steel strip and the same product - Google Patents

Abstract

The high carbon steel rod is produced by (a) reheating the steel containing 0.6-0.9wt% of carbon, 0.1-0.5wt% of silicone, 0.3-0.8wt% of manganese, 0.1-0.5wt% of chromium, 0.005-0.015wt% of aluminum, 0.005-0.015wt% of nitrogen, below 0.006wt% of sulfur, iron and other impurities for less than 250 minutes at 1250 degree C reheating temperature, and then producing hot soft steel tape using hot rolling, (b) winding hot soft steel rod at 580-670 degree C and rinsing in acid, and doing spherophyric annealing in non-oxidizing condition at 650 degree C-A1 critical point for 5-20 hours and cooling at below 60% of draft percentage, (c) doing softness annealing in non-oxidizing condition at 600 degree C-A1 critical point for 5-20 hours, and then heating in non-decarbonization condition at over A3 critical point for 5-30 minutes, cooling and tempering it.

Description

High carbon steel strip with excellent toughness and manufacturing method

The present invention relates to a method for producing a high carbon steel sheet having high hardness and excellent toughness.

In recent years, with the development of the civil, building, landscaping and timber industries, the demand for circular saws for high speed cutting of concrete, stone and wood is increasing. The material of the circular saw is mainly used SK5 carbon alloy, SKS5 alloy alloy.

Since the circular saw is subjected to continuous impact loads on the notch generated when the saw blade is made, the most important material property of the circular saw is impact toughness. In the past, the toughness of the circular saw was not very important because the circular saw was less demanded, the cutting speed was slow, and the disc and saw blade were used as the same material, so the saw blade was worn out first.

Recently, however, the use of diamond chips with excellent wear resistance as the material of the saw blade has greatly increased the service life of the saw blade, the cutting speed is very fast, and the impact load due to the improvement of the productivity is increased. It has come to dominate life.

In addition, when the circular saw blade breaks during high-speed cutting, not only the damage of the circular saw is more likely to occur, but also the workability and productivity can be greatly reduced.

Therefore, conventional techniques for improving the toughness of tool steels used as circular saws include SKS5 in which Ni and Cr are added to SK5, which is a carbon steel ball specified in JIS G 3311, but with addition of alloys such as Ni and Cr. There is a disadvantage that the manufacturing value is increased.

In this regard, there is a demand for a material having excellent toughness without the addition of additional alloying components to existing tool steels.

Accordingly, the present inventors have comprehensively studied the action and manufacturing conditions of Al and N, which are elements affecting the toughness of high carbon steel, to improve the toughness of the high carbon steel in response to the above demands, The present invention has been made in view of the fact that steels having excellent toughness can be produced in the steels contained therein.

The present invention refines austenite grains during heat treatment through proper control and precipitation of the amount of Al added to the deoxidizer and the amount of N contained in the steel, and reduces the harmful S to toughness, thereby reducing toughness at low cost without adding an alloying element. The purpose is to produce an excellent high carbon steel strip.

Hereinafter, the present invention will be described.

In the present invention, by weight, C: 0.6-09%, Si: 0.1-0.5%, Mn: 0.3-0.8%, Cr: 0.1-0.5%, Al: 0.005-0.015%, N: 0.005-0.015%, S : 0.006% or less, and is composed of the remaining Fe and impurities, and relates to a high carbon steel strip having excellent toughness, the structure of which is martensite.

In the present invention, by weight, C: 0.6-09%, Si: 0.1-0.5%, Mn: 0.3-0.8%, Cr: 0.1-0.5%, Al: 0.005-0.015%, N: 0.005-0.015%, S : 0.006% or less, steel composed of the remaining Fe and impurities are reheated to a reheating temperature of 1250 ° C. or less and a reheating time of 250 minutes or less, and then hot rolled by a conventional method to form a hot rolled steel strip. Winding and pickling at a temperature range of 670 ° C., followed by 5-20 hours of spheroidization in a non-oxidizing atmosphere at a temperature range of 650 ° C.-A ₁ , followed by cold rolling at a rolling reduction of 60% or less, followed by 600 ° C.-A ₁ transformation temperature. After softening annealing for 5-20 hours in a non-oxidizing atmosphere in the range, it is temper-rolled by the usual method and maintained at a temperature above the A ₃ transformation point for 5-30 minutes in a non-carburizing atmosphere. It relates to a manufacturing method.

C is the most important element for increasing the hardness of steel. In order to obtain the hardness (HRC: 40-47) required for the circular saw by quenching and tempering, it is necessary to contain at least 0.6%. However, if the amount of C is too high, the cementite cementite precipitates at the grain boundary and the toughness is greatly reduced, and cracking is likely to occur when quenching.

Si is a very important element as a deoxidizer. If the amount of Si is too small, since the effect of deoxidation is small, it contains 0.1% or more. However, if the amount of Si is too high, the surface decarburization becomes severe during reheating, and since red scale is generated, pickling is difficult and surface defects are likely to occur, so it is limited to 0.5% or less.

Mn is an important element for improving the hardenability and an element that inhibits the generation of graphite during annealing of high carbon steel. In order to obtain such an effect, it must contain 0.3% or more.

However, when the amount of Mn is too large, the nodularization of cementite during spheroidizing annealing becomes difficult, and the spheroidizing annealing time needs to be lengthened, and the toughness also decreases, so the upper limit is set to 0.8%.

Cr is an important element for suppressing surface decarburization during reheating for hot rolling. In addition, like Mn, it is an element that improves hardenability and inhibits graphite generation. To this end, at least 0.1% must be contained. However, if the amount of Cr is too high, spheroidizing annealing is difficult, pickling deteriorates and toughness is also reduced, so 0.5% is set as an upper limit.

Al is not only an important element as a deoxidizer, but also forms a AIN during heat treatment and plays a large role as a grain refining element. For this purpose, the Al content is preferably contained 0.005% or more. However, if the amount of Al is too large, it is easy to generate graphite during annealing and the toughness is degraded by the alumina inclusions, so it is set to 0.015% or less.

N is a very important element that precipitates AIN together with Al to refine the grains during heat treatment. To this end, 0.005% or more must be contained. However, if the amount of N is too large, toughness is lowered, so it is set at 0.015% N or less.

S is an element which produces MnS together with Mn and greatly affects the toughness reduction. The smaller the amount of S, the better the toughness, but especially at 0.006% or more, the toughness deteriorated greatly, so the upper limit was set to 0.006%.

Hereinafter, the manufacturing method of the high carbon steel strip of this invention is demonstrated.

In the present invention, the steel having the above components is reheated and then hot rolled in a conventional manner. In the hot rolling process, it is important to prevent surface decarburization and to refine the structure.

Surface decarburization usually increases with a higher reheating temperature before hot rolling, and with a longer time. If the surface decarburization is less than 0.5% of the thickness of the hot rolled sheet, it does not significantly affect the hardness and deformation of the final heat treatment material.

However, further surface decarburization reduces the surface hardness of the high carbon steel, which not only inhibits wear resistance but also causes deformation during heat treatment. In order to prevent this, the reheating temperature before hot rolling should be lowered and the time should be shortened. In the present invention, the reheating temperature is preferably 1250 ° C. or less, and the holding time, which is a holding time in a heating furnace, is limited to 250 minutes or less.

The reheated steel as described above is hot rolled and wound in a conventional manner, in which tissue micronization is important for improving spheroidizing annealing, cold rolling and toughness. That is, the finer the texture, the more spherical and finer the spheroidization is. Thus, the cold rolling property is improved and the heat treatment temperature and time can be shortened, so that the heat treatment structure is fine and the toughness is greatly improved.

In general, the microstructure of the hot rolled steel sheet is most dependent on the coiling temperature. The higher the coiling temperature is, the more coarse the tissue is, the more difficult it is to form. However, if the coiling temperature is too low, the yield strength is too high, the winding is difficult, and cracking easily occurs during winding. Therefore, it is preferable to limit the winding temperature to 580-670 ℃, because the winding temperature becomes coarse structure above 670 ℃ and bainite structure below 580 ℃, and the winding property deteriorates and edge cracks in the hot rolled sheet. This is easy to occur.

The hot rolled steel coil wound in the above temperature range is pickled and then subjected to spheroidizing annealing. When the treatment temperature is higher than A ₁ transformation point (vacant transformation temperature, 720 ° C.) in the nodular annealing process, the size of spheroidized cementite is coarsened. Therefore, in order to solidify the carbide in the austenite phase during heating for heat treatment, toughness is lowered because a long time is required at high temperature. However, below 650 ° C, spheroidization annealing is difficult, so long time is required and spheroidization rate is lowered. Therefore, the nodular annealing temperature was set at 650 ° C-A ₁ transformation point.

At this time, the spheroidizing annealing is preferably performed for 5-20 hours in a non-oxidizing atmosphere.

Spherical annealing steel strip is cold rolled, wherein the cold reduction rate is preferably 60% or less. This is because edge cracking is likely to occur due to work hardening when the cold reduction ratio is 60% or more.

In the soft annealing process, if the temperature is too high, the cementite becomes coarse. If the temperature is too low, the work hardened ferrite by cold rolling cannot be sufficiently recrystallized and the hardness becomes high after annealing, thereby deteriorating the cold rolling property. Therefore, the softening annealing temperature was set to 600 ° C-A ₁ transformation point or less.

At this time, the soft annealing is preferably heat-treated for 5-20 hours in an atmosphere where oxidation does not occur.

After soft annealing, decarburization does not occur so that cementite is sufficiently employed at temperatures above the A ₃ transformation point (the temperature at which ferrite to austenite is transformed) to obtain the hardness (HRC: 40-47) required for the rough-rolled cold rolled sheet. Hold for 5-30 minutes in the atmosphere, then quench and temper the oil.

The tempering is preferably heat treated in the atmosphere by adjusting the heat treatment temperature and time according to the hardness required in the final product.

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

Example

The steel having the chemical composition shown in Table 1 was reheated at a reheating temperature of 1240 ° C. and a zero time of 230 minutes, and then wound up to 650 ° C. after finishing rolling to a thickness of 4.0 mm using normal hot rolling. The hot rolled sheet thus wound was pickled and spheroidized annealing was performed in a non-oxidizing atmosphere. At this time, the spheroidizing annealing is A ₁ transformation point or less was performed at 720 ℃ 10 hours. The spheroidized material as described above was cold-rolled at a rolling reduction of 40%, followed by temper rolling after softening annealing at 700 ° C. for 7 hours to prepare a 2.3 mm tempered rolled plate. Thus, the tempered rolled sheet was heat-treated at 825 ° C. for 15 minutes or more at A ₃ transformation point, and then quenched in oil at 60% temperature. After quenching the material was tempered at 450 ° C. for 30 minutes.

In the following Table 1, steel grade (1-5) is an invention material within the chemical component range prescribed | regulated by this invention, and steel grade (6-11) is the comparative material melt | dissolved for comparison in this invention.

After reheating the invention material (1) and the comparative material (11) of the steel grades of Table 1 under the condition of reheating temperature of 1270 ℃, 260 minutes of reheating time and the thickness of the decarburized layer of the hot rolled steel sheet to 4.0mm, 230 at 1240 ℃ The decarburized layer depth in the case of reheating was measured and the results are shown in Table 2 below.

As can be seen from Table 2, in the case of the comparative material 11 without Cr added, the decarburized layer was much larger than that of the inventive material 1, and the reheating condition was 1270 ° C × 260 for comparison. It can be seen that the depth of the decarburized layer is greatly increased when the powder is divided into minutes. Therefore, in order to obtain a decarburized layer in the hot rolled sheet to a target depth of 20 μm or less, the reheating conditions should be limited to a reheating temperature of 1250 ° C. or less, 250 min or less of the present invention.

In order to know the effect of the winding temperature on the crack formation and spheroidization of the hot rolled sheet, after winding the steel grade of the invention material (1) of Table 1 to the winding temperature of the following Table 3 and observed the crack occurrence, 10 hours at 720 ℃ After the spheroidizing annealing treatment, the spheroidization rate was measured and the results are shown in Table 3 below.

(* Spherical annealing condition: 720 ℃ × 10 hours)

As shown in Table 3, when the coiling temperature is lower than the range of the present invention, the spheroidization rate is increased but edge cracking occurs. On the other hand, when the coiling temperature is higher than the range of the present invention, cracking is suppressed, but the spheroidization rate is It can be seen that the decrease.

After spheroidizing the inventive material (1, 2, 4) steel grade and the comparative material (9, 10) steel of Table 1 under the spheroidizing annealing conditions as shown in Table 4, respectively, the spheroidization rate was measured and the results are shown in the following table. 4 is shown.

As can be seen from Table 4, it can be seen that No. 9 and No. 10 having a high Mn and Cr content significantly decreased the spheroidization rate. In the nodularization conditions, the nodularization rate drops even when the nodularization temperature is low. If the spheroidization rate is low, there is a high possibility of cracking during cold rolling, and the heat treatment is inferior.

As can be seen in Table 4, in the case of the comparative materials (9, 10) steel grades outside the steel grade range of the present invention, the spheroidization rate satisfies the steel grade range of the present invention due to the high content of Mn and Cr ( 1, 2, 4) It can be seen that the steel sheet is significantly lower than the case, and also the spheroidizing annealing temperature in each steel class is lower than the nodular annealing temperature of the present invention. As such, the low spheroidization rate increases the likelihood of cracking during cold rolling and lowers the heat treatment property. Therefore, in the present invention, the spheroidizing annealing temperature is preferably set to a temperature range of 650 ° C-A transformation point.

The invention (1) steel grade of Table 1 was cold-rolled at the cold rolling reduction rate as shown in Table 5 and observed for the occurrence of cracks are shown in Table 5 below.

As can be seen in Table 5, if the cold reduction rate does not satisfy the range of the present invention, there was no cracking, but if the cold reduction rate did not satisfy the range of the present invention, the cold reduction rate was increased to increase the cold reduction rate. It can be seen that edge cracking occurs.

Therefore, in the present invention, it is preferable that the cold reduction rate is 60% or less.

After the temper rolling of the inventive material (1, 2) and the comparative material (8) of Table 1, the presence of graphite was observed and the results are shown in Table 6 below.

As can be seen in Table 6, the invention (1, 2) steel grades satisfying the steel grade range of the present invention, there was no graphite generation, especially among the comparative steel grades outside the scope of the present invention, the Al content is high It can be seen that graphite is generated in the steel of the comparative material 8. As such, there is a problem in that the heat treatment property in which graphite is generated in the cold rolled sheet is greatly degraded.

Therefore, the content range of Al in the steel species in the present invention is preferably set to 0.005-0.015% by weight.

After the heat treatment of the inventive material (1-5) steel and the comparative material (6, 7) of the Table 1 and the conventional SK5 steel, SKS5 steel of the conventional material of Table 7 and then measured the grain size and the hardness and impact value after tempering It is shown in Table 7 below.

In Table 7, the impact test value test temperature after tempering was 0 ° C. The hardness and impact values of the conventional SK5 and SKS5 steels after tempering were 吉 井, 藤 藤: 住友 金 屬, Vol. 42, No. 5 (1990) P. From 71.

As can be seen in Table 7, the grain size of the austenite and the comparative material containing Al and N were found in the grain size of the austenite under heat treatment conditions in which cementite was sufficiently dissolved in austenite. 6) Although the grain size is fine, the grain size is coarse in the comparative steel material (7) having little Al content. The reason why the grain size is small in the steel containing Al and N is that grain growth is suppressed by precipitation of AlN. Since the toughness of the tempered martensite is governed by the size of the austenite grains, the toughness decreases when the austenite grain size increases. Therefore, it can be seen that excellent toughness can be obtained in the case of the inventive steel (1-5).

In Table 7, it can be seen that the hardness after the tempering of the present invention material (1-5) is comparable to that of the comparative material (6-7) and the conventional SK5 and SKS5 steel grades.

Since the hardness after tempering is almost independent of the size of the austenite hard grain and depends on the carbon content, the steel of the invention material (5) having a low carbon content has a somewhat low hardness.

However, when the carbon content is more than 0.8, there is little change in hardness as the carbon content increases. In the impact value after tempering, the impact value of the invention materials (2) and (4) steels, which have a large amount of Cr and Mn, is slightly lower, but the invention materials have improved the impact value by more than 40% compared to the comparative materials and SK5, and Ni is 1.2%. It can be seen that it has a value similar to the SKS5 added above. This indicates that the high-carbon steel sheet which is cheap and excellent in toughness can be manufactured by the method developed in the present invention.

As described above, the present invention properly manages Al added as a deoxidizer and N present in the steel, and by adding a small amount of alloying elements, a high-carbon steel sheet excellent in toughness can be manufactured at low cost, thereby improving the safety of the circular saw. As a result, it has the effect of greatly increasing the demand for high carbon steel strips.

Claims (2)

By weight, C: 0.6-09%, Si: 0.1-0.5%, Mn: 0.3-0.8%, Cr: 0.1-0.5%, Al: 0.005-0.015%, N: 0.005-0.015%, S: 0.006% Hereinafter, a high carbon steel strip having excellent toughness, which is composed of the remaining Fe and impurities, and whose structure is a martensite structure. By weight, C: 0.6-09%, Si: 0.1-0.5%, Mn: 0.3-0.8%, Cr: 0.1-0.5%, Al: 0.005-0.015%, N: 0.005-0.015%, S: 0.006% The steel, which is composed of the remaining Fe and impurities, is then reheated to a reheating temperature of 1250 ° C. or lower and a 250 min or less of time, and then hot rolled in a conventional manner to form a hot rolled steel strip, and the hot rolled steel strips of 580-670 ° C. Winding and pickling in the range, and then annealed for 5-20 hours in a non-oxidizing atmosphere in the 650 ℃ -A ₁ transformation temperature range, cold-rolled to a rolling reduction of less than 60%, then scattered in the 600 ℃ -A ₁ transformation temperature range. After softening annealing for 5-20 hours in a Martian atmosphere, the method of manufacturing high-carbon steel sheet having excellent toughness, characterized in that it is maintained by heating for 5-30 minutes in a non-carburizing atmosphere at a temperature not lower than A ₃ transformation point, which is a common method, and then cooled and tempered. .