KR20160077857A - Method for manufacturing midium carbon steels excellent machining by controlled rolling - Google Patents

Abstract

The present invention relates to a method to manufacture a medium carbon steel with excellent machinability and, more specifically, relates to a method to manufacture a medium carbon steel with excellent machinability even after omitting a normalizing thermal treatment by applying controlled rolling. Specifically, the present invention provides the method to manufacture the medium carbon steel, comprising: a step of uniformly maintaining a steel material at a temperature of 1,060-1,140°C for 80-100 minutes, where the steel material includes 0.35-0.61 wt% of C, 0.15-0.35 wt% of Si, 0.60-0.90 wt% of Mn, 0.030 wt% or less (excluding 0 wt%) of P, 0.035 wt% or less (excluding 0 wt%) of S, 0.20 wt% or less (excluding 0 wt%) of Ni, 0.20 wt% or less (excluding 0 wt%) of Cr, 0.010-0.030 wt% of Al, and the remaining consisting of Fe and inevitable impurities; and a step of performing rough milling of the steel material at 930-990°C, and performing a main milling of the steel material by cooling the steel material at a finishing milling temperature of 850-900°C using a water cooling facility. The medium carbon steel has tensile strength equal to 70 kgf/mm^2 or less, and a hardness equal to 90 HRB or less.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a method for producing a medium carbon steel having excellent machinability by controlled rolling,

The present invention relates to a method for producing a medium carbon steel having excellent machinability by controlled rolling, and more particularly, to a method for producing a medium carbon steel excellent in machinability by omitting the normalizing heat treatment by applying controlled rolling.

Parts such as automobiles, industrial machines, etc. are generally finished in a final shape by cutting in a raw material state after hot forging. Since the cost for such cutting takes a large proportion of the manufacturing cost, the raw material steel is required to have excellent machinability.

In order to improve the machinability of the steel material, a technique of forming free cutting steel by adding Pb or a technique of forming MnS by adding S is generally known. However, Pb is a harmful component to the human body and its use has been regulated for a long time. In addition, sulfides deteriorate the mechanical properties. For example, there is a limit to the addition of S due to the difference in anisotropy, which is different in toughness and ductility in cross section and length direction.

Since the machining amount of the parts is different between the large diameter part and the small diameter part and is manufactured by a mass production system, the tolerance of the outer diameter dimension of the raw material must be strictly controlled and the cutting length must be strictly controlled. For this reason, the raw material is manufactured by strictly controlling the tolerance of the outer diameter due to peeling or cold drawing.

Cold drawing is used in consideration of economy because manufacturing cost is low compared with peeling process. In addition, since the work is hardened in cold drawing, the cutting angle is increased when cutting, which is advantageous in workability such as reduction of chip deformation. Therefore, in the case of low-carbon steel, the ferrite structure can be processed and cured to improve the workability. However, in the case of medium carbon steel, the hardness may be too high to reduce tool life.

Therefore, in order to minimize the increase in hardness due to cold drawing, it is required to increase the ferrite content before cold drawing. As such treatment, a normalizing heat treatment is used, and a steel material having a fine ferrite + pearlite structure can be produced through a normalizing heat treatment. However, there is a disadvantage of cost increase because additional heat treatment is required. In addition, since the production time is increased in accordance with the normalizing heat treatment, a problem of delivery may occur.

The present invention has been made in order to solve the problems of the prior art described above, and it is an object of the present invention to provide a method of manufacturing a medium carbon steel in which the increase in hardness due to work hardening during cold drawing of medium carbon steel is minimized, And a method thereof. In addition, the present invention establishes optimal controlled rolling conditions to ensure fine and uniform ferrite + pearlite microstructure to exhibit properties equal to or better than normalizing heat treatment, thereby improving machinability in machining of medium carbon steel by cold drawing And to provide a method for producing a medium carbon steel which can be improved.

It is another object of the present invention to provide a method for producing medium carbon steel in which the cost competitiveness is secured by omitting the normalizing heat treatment and the productivity is improved by shortening the delivery time.

The above object is attained by a method for producing a medium carbon steel comprising the steps of: C: 0.35 to 0.61 wt%, Si: 0.15 to 0.35 wt%, Mn: 0.60 to 0.90 wt%, P: 0.030 wt% % Of Ni (not included), 0.20% or less of Ni (not including 0), 0.20% or less of Cr (not including 0%) and 0.010 to 0.030% of Al and the balance of Fe and other unavoidable impurities Maintaining the crack at 1,060 to 1,140 캜 for 80 minutes to 100 minutes; Rolling the steel material at 930 to 990 ° C. and cooling the rough-rolled steel material to a finish rolling temperature of 850 to 900 ° C. by using a water-cooling equipment, wherein the carbon steel has a tensile strength of 70 kgf / , And a hardness of 90 HRB or less.

Preferably, the heavy carbon steel may have a hardness of 100 HRB or less after cold drawing with a reduction ratio of 6.3%.

Also, preferably, the medium carbon steel is formed by rolling to form a crystal grain boundary and a strain band in the crystal grain to form a fine ferrite and a pearlite structure.

Since the medium carbon steel produced according to the method of the present invention has a fine and uniform ferrite + pearlite structure, it can have workability, microstructure and hardness equal to or higher than that of the normalizing heat treatment. Since the medium carbon steel produced according to the method of the present invention has an improved machinability, a separate normalizing heat treatment step can be omitted when machining into parts. By omitting this normalizing heat treatment process, the cost competitiveness, prevention of environmental pollution, and shortening the delivery time by shortening the manufacturing process time can be obtained.

Figure 1 shows the microstructure after controlled rolling, ordinary rolling and normalizing heat treatment.
FIG. 2 shows the hardness in the control rolling after rolling, ordinary rolling, and normalizing heat treatment.
Fig. 3 shows the mechanical properties (tensile strength, hardness) in the control rolling and the normalizing heat treatment before cold drawing.
Fig. 4 shows the results of the machinability evaluation (cutting force) after the control rolling, the general rolling, and the normalizing heat treatment.
Fig. 5 shows the results of the workability evaluation (surface roughness) after the control rolling, the general rolling and the normalizing heat treatment.

The present invention relates to a method for producing medium carbon steels having excellent machinability by controlled rolling, which comprises 0.35 to 0.61 wt% of C, 0.15 to 0.35 wt% of Si, 0.60 to 0.90 wt% of Mn, 0.030 wt% or less of P (Not including 0), S: not more than 0.035 wt% (not including 0), Ni: not more than 0.20 wt% (not included), Cr: not more than 0.20 wt% Cracking the steel material made of Fe and other unavoidable impurities at 1,060 to 1,140 DEG C for 80 minutes to 100 minutes; Subjecting the steel material to rough rolling at 930 to 990 ° C; And subjecting the rough-rolled steel material to main rolling at a finishing rolling temperature of 850 to 900 캜 using a water-cooling facility.

Through the above-mentioned control rolling, a fine ferrite + pearlite structure similar to the case of performing the normalizing heat treatment process is obtained. As a result, it is possible to reduce the increase in hardness due to work hardening during cold drawing for part machining, thereby preventing deterioration of tool life during drilling. The component is not particularly limited as long as it is produced by using cold carbon steel and cold drawn. An example of such a component is a camshaft component, which is subjected to cutting, particularly drilling, in the end piece of the camshaft. Since the medium carbon steel of the present invention has improved machinability through controlled rolling, it is possible to prevent a reduction in tool life at the time of part processing. Further, by increasing the fraction of ferrite before cold drawing through controlled rolling, the increase in hardness due to work hardening can be minimized and the roughness of the inner diameter can be improved, thereby omitting the normalizing heat treatment at the time of part processing.

The reason for adding the alloy component of the present invention and limiting the range of the component will be described below.

C: 0.35 wt% to 0.61 wt%

C is an important element that increases strength and hardness as an austenite stabilizing element. When it is added in an excessive amount, the hardness of the material is high during drilling, causing tool abrasion, and the machinability is lowered, so that it is limited to 0.61 wt%. When C is low, it is difficult to secure the required strength of the component, so the lower limit is limited to 0.35 wt%.

Si : 0.15 wt% to 0.35 wt%

Si is used as an effective deoxidizer for steelmaking and is not preferable because it increases the carbon activity of the steel or the ferrite strengthening element or the ferrite structure of the material is strengthened when the content of Si is high. Therefore, the Si upper limit is limited to 0.35 wt%. When the Si content is low, the carbon activity is low, and since it is difficult to obtain the desired strength in the product, the lower limit is limited to 0.15 wt%.

Mn : 0.60 wt% to 0.90 wt%

Mn is a deoxidizer, which improves the denseness and strength, and is added to prevent the harmfulness of S present in the steel to form MnS, thereby preventing red-hot brittleness and improving cutting workability. If the Mn is 0.60 wt% or less, the desired strength of the product is difficult to obtain. If the Mn is excessively added in excess of 0.90 wt%, the toughness may be lowered. Therefore, it is preferable to limit the Mn content to 0.60 wt% to 0.90 wt%.

P: 0.030 wt% or less (not including 0)

If the addition amount is larger than 0.030% by weight, segregation occurs at the austenite grain boundary and the toughness is lowered. Therefore, the content is limited to 0.030% by weight or less.

S: 0.035% by weight or less (not including 0)

S bonds with Mn in the steel to form MnS inclusions to increase the machinability of the steel, and when the amount of the additive is increased, surface defects occur due to large inclusions and pathways, so it is limited to 0.035 wt% or less.

Ni : Not more than 0.20% by weight (not including 0)

Ni is an element for increasing the fineness of the steel structure and increasing the incombustibility. If it is more than 0.20% by weight, the toughness is improved but the machinability is lowered and the production cost of the component is increased.

Cr  : Not more than 0.20% by weight (not including 0)

When Cr is added as a cementite stabilizing element and an element improving the strength and strength of the cementite stabilizer by 0.20 wt% or more, the strength is increased and the tool life can be lowered during drilling, so the upper limit is limited to 0.20 wt%.

Al  : 0.010 wt% to 0.030 wt%

Al acts as a strong deoxidizing agent and binds with N to refine the crystal grains. However, if less than 0.010 wt% is added, deoxidization or grain refinement becomes small, and if it is added in excess of 0.030 wt% Increasing the amount of non-metallic inclusions such as Al ₂ O ₃ may have detrimental effects such as lowering toughness. Therefore, the optimum content range of Al is limited to 0.010 wt% to 0.030 wt%.

Hereinafter, a method for producing heavy carbon steel excellent in machinability capable of omitting the normalizing heat treatment by controlled rolling of the present invention will be described in detail.

The method for producing heavy carbon steel of the present invention comprises the steps of: maintaining a steel material having the above composition at a temperature of 1,060 to 1,140 DEG C; And subjecting the steel material to rough rolling at 930 to 990 캜 and controlled rolling with the roughly rolled steel material lowered to 850 to 900 캜.

Crack holding step

In the crack holding step, the steel material having the above composition is maintained at a temperature of 1,060 to 1,140 DEG C for 80 minutes or more, preferably 80 to 100 minutes. The above-mentioned crack temperature region refers to the time for which the charged steel material is maintained after reaching the target heating temperature. If the crack holding temperature is lower than 1,060 ° C, the hardness and microstructure of the material may improve, but the surface quality of the material may deteriorate. If the temperature is higher than 1,140 ° C, the surface quality may be improved. However, There is a problem that material deflection occurs. Therefore, it is preferable that the crack holding temperature is in the range of 1,060 to 1,140 占 폚.

Controlled rolling  step

In the control rolling step, the billet is subjected to rough rolling at a temperature of 930 to 990 캜 by extracting the billet from a walking beam heated furnace at a temperature of 1,060 to 1,140 캜. At this time, a control cooling facility can be installed behind the roughing mill to control the rolling temperature. As the control cooling facility, for example, a water zone in which water is injected and filled by water pressure control (water-cooled zone) may be installed.

The steel subjected to rough rolling is cooled by passing through a control cooling facility, that is, a water zone (water jacket), and is subjected to the main rolling in a state where it is lowered to a temperature of 850 to 900 ° C. In the present invention, by performing rolling in a state of being lowered to a temperature of 850 to 900 DEG C by using a water zone (water-cooled bar), a fine ferrite + pearlite structure is formed due to low temperature deformation and the workability, hardness, mechanical Properties can be secured.

In addition, the conventional hot rolling is generally completed at a temperature higher than the recrystallization temperature, and the austenite grains are refined due to repeated deformation and recrystallization, and the fine austenite grains obtained by recrystallization in the deformation step are ferrite grains . The size of the recrystallized austenite grains decreases sharply with increasing rolling reduction amount, but the ferrite nucleation site only occurs at the austenite grain boundary. However, rolling in controlled rolling and controlled cooling is completed in the normalizing heat treatment temperature range, which is the temperature range in the region below the recrystallization temperature. The austenitic grain boundaries by controlled rolling are stretched by deformation and deformation bands are generated in the grain boundaries to provide ferrite nucleation sites not only in the grain boundaries but also in the grain boundaries to provide a more uniform and fine ferrite + pearlite structure Can be obtained.

Hereinafter, the present invention will be described in detail with reference to specific examples, but the scope of the present invention is not limited by the examples.

Example

Table 1 below shows the composition of heavy carbon steel used in this example, the balance being Fe and inevitable impurities.

(Unit: wt%) division C Si Mn P S Ni Cr Al Medium carbon steel 0.45 0.24 0.70 0.017 0.023 0.16 0.07 0.016

The medium carbon steels having the composition shown in Table 1 were rolled under the rolling method (crack holding temperature, holding time, temperature condition after rough rolling) shown in Table 2 below. In Examples 1 and 2, a fine ferrite + pearlite structure was formed in the austenite structure through controlled rolling, and a low-temperature heating in the range of 1,060-1,140 ° C was maintained at the crack-to- Cold rolling is carried out at a temperature of 850 to 900 ° C through cooling.

In Comparative Examples 1 and 2, ordinary rolling was carried out at a temperature of 950 to 1,050 ° C, without using a cold-rolled steel sheet which was placed behind the rough rolling at a temperature of 1,150 to 1,180 ° C. The steel of Comparative Example 2 was subjected to a normalizing heat treatment. The normalizing heat treatment conditions were maintained at 850 ± 5 ℃ for 1 hour and then air-cooled.

division Rolling method Crack temperature () Holding time (minutes) Temperature after rough rolling () Heat treatment Example 1 Controlled rolling 1,080 95 860 - Example 2 1,120 90 880 - Comparative Example 1 General rolling 1,160 100 960 - Comparative Example 2 1,180 95 1,000 Normalizing heat treatment

Fig. 1 shows the microstructure of the medium carbon steel produced in Example 1 (a), Example 2 (b), Comparative Example 1 (c) and Comparative Example 2 (d). The pearlite color sizes of Examples 1 and 2 in which the controlled rolling was carried out showed a fine ferrite + pearlite structure as compared with Comparative Example 1 in which general rolling was performed due to the controlled rolling, and Comparative Example 2 in which the normalizing heat treatment was performed .

Therefore, the medium carbon steel having a fine ferrite + pearlite structure obtained through the controlled rolling can minimize the hardness increase compared to the coarse ferrite + pearlite in the ordinary rolling when cold drawn at the same reduction ratio.

2 shows the hardness (HRB) of the medium carbon steels having the composition shown in Table 1 prepared by the rolling method of Table 2 and evaluated after cold drawing at a reduction ratio of 6.3%. The hardness values of Examples 1 and 2 subjected to controlled rolling were lower than those of Comparative Example 1 subjected to ordinary rolling and were equivalent to those of Comparative Example 2 subjected to normalizing heat treatment.

Fig. 3 shows the mechanical properties (tensile strength, hardness) evaluated before cold drawing at a reduction ratio of 6.3% in Example 1 and Comparative Example 2. Fig. The tensile strength and hardness of the Example 1 subjected to the controlled rolling were comparable to those of the normal rolling and normalizing heat treatment.

Fig. 4 shows the machinability evaluation (cutting force) of Examples 1 and 2 and Comparative Examples 1 and 2. The cutting force can be measured using a tool dynamometer used in formability evaluation. The resistance value (cutting force) generated when drilling under the following conditions can be obtained through the tool dynamometer:

Drill tool: 4.0 HSS GUEHRING 46515

Drilling method: wet

Drill Speed: 2,700 ~ 3,000 RPM

Drill depth: 46 mm, drill feet 160.

The larger the cutting force value, the more tool resistance is generated during drilling. The cutting force values in Examples 1 and 2 in which the controlled rolling was performed are lower than those in Comparative Example 1 in which general rolling was performed and are comparable to those in Comparative Example 2 in which the normalizing heat treatment is performed.

Fig. 5 shows the formability evaluation (surface roughness) of Examples 1 and 2 and Comparative Examples 1 and 2. The surface roughness evaluation can be performed by cutting the drilled material through the above method and observing and observing the corresponding drilled surface image. As shown in FIG. 5, the burr formation and the surface roughness of the drill surface are compared with each other.

5, it can be seen that the surface roughnesses of Examples 1 and 2 and Comparative Example 2 in which control rolling was performed are better than Comparative Example 1 in which general rolling was performed.

Therefore, the mechanical properties (tensile strength, hardness), the machinability evaluation (cutting force, surface roughness) and the like which can prevent the deterioration of the tool life during the machining to obtain the fine ferrite + pearlite structure by the controlled rolling according to the manufacturing method of the present invention, The results are obtained and it can be seen that the normalizing heat treatment can be omitted.

Claims (3)

A method of producing a medium carbon steel excellent in machinability,
(C): 0.35 to 0.61 wt% of C, 0.15 to 0.35 wt% of Si, 0.60 to 0.90 wt% of Mn, 0.030 wt% or less of P, 0 wt% or less of S, , The balance being Fe and other unavoidable impurities, at a temperature of 1,060 to 1,140 DEG C for a period of from 80 minutes to 100 minutes (inclusive), a Cr content of 0.20% or less (not including 0%) and an Al content of 0.010 to 0.030% Maintaining a crack;
Subjecting the steel material to rough rolling at 930 to 990 占 폚, cooling the finish rolling temperature to 850 to 900 占 폚 using a water-cooling equipment,
Wherein the medium carbon steel has a tensile strength of 70 kgf / mm 2 or less and a hardness of 90 HRB or less. The method of claim 1, wherein the medium carbon steel has a hardness of 100 HRB or less after cold drawing with a reduction ratio of 6.3%. The method for producing medium carbon steel according to claim 1, wherein the heavy carbon steel is formed by grain boundaries and strain bands in the crystal grain by the rolling to form fine ferrite and pearlite structure.

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