JP4495106B2 - Steel pipe for machine structure excellent in machinability and manufacturing method thereof - Google Patents

Description

  The present invention relates to a steel pipe for machine structure suitable for machine structure members, particularly machine parts such as gears and cylinders and hollow structure members such as shafts, and a method for producing the same.

  Conventionally, machine parts used for automobiles and industrial machines are manufactured by using steel bar as a raw material, forging, cutting, and tempering heat treatment. In recent years, hollowing of machine parts is being promoted for the purpose of reducing the weight of automobiles and the like, and a change from a steel bar to a steel pipe is being studied.

  Since many machine parts are machined, steel pipes for machine structures excellent in machining are proposed (for example, Patent Documents 1 to 5). In Patent Document 1, the hardness is increased by tube drawing and the machinability is improved. However, in this method, an increase in manufacturing cost due to an increase in processes is inevitable. Patent Documents 2 and 3 aim to improve machinability by adding Ca, and Patent Document 4 aims to improve machinability by adding Bi, Pb, and Te. Both add impurity elements to such an extent that an effect of improving machinability is obtained. Deterioration of toughness and fatigue characteristics is inevitable. Also, addition of environmentally hazardous substances such as Bi, Pb, Te, etc. is not preferable from the viewpoint of recycling.

  Patent document 5 manufactures the steel tube for cylinder rods which was excellent in machinability by making a metal structure a bainite by a drawn pipe and stress relief annealing. However, when the metal structure is bainite, there is a limit to improving the strength. In addition, since a tube extension is required, an increase in manufacturing cost is inevitable. In addition, none of the above proposals systematically evaluate machinability from the viewpoint of tissue control.

Japanese Patent Laid-Open No. 3-81008 JP-A-3-177539 JP-A-3-177540 JP-A-5-5157 JP-A-4-191323

  The present invention provides a steel pipe for machine structure in which machining, particularly cutting of an inner surface of a steel pipe, is improved by controlling the metal structure without adding an expensive alloy, and a manufacturing method thereof.

  In order to solve the above problems, the present inventors have intensively studied the relationship between the metal structure of steel and machinability, and in particular, the optimum metal structure for broaching is a mixed structure of tempered martensite and tempered bainite. It was clarified that some ferrite may be included. In addition, in order to reduce the residual stress of steel pipes and to manufacture them at low cost, we aim to apply a process for accelerated cooling of high-temperature steel pipes, study various optimum cooling conditions, and ensure a stable metal-cutting structure. The optimum combination of C, Mn, Cr, and Mo, the cooling rate during quenching, and the tempering temperature were found.

  This invention is made | formed based on the said knowledge, The place made into the summary is as follows.

(1) By mass%, C: 0.15-0.45%, Si: 0.1-0.4%, Mn: 0.5-1.0%, Cr: 0.8-1.5% , Mo: 0.05 to 0.5%, S: 0.005 to 0.03%, α [%] defined by the following formula (1) is 580 or more and 640 or less, and the balance is Cutting made of iron and inevitable impurities, and having a metal structure composed of tempered bainite with an area ratio of 20 to 70%, ferrite with an area ratio of 25% or less (including 0%), and the balance tempered martensite. Excellent steel pipe for machine structure.
α = 830-270C-90Mn-70Cr-83Mo (1)
Here, C, M, Cr, and Mo are the contents of each element.

(2) The steel pipe for machine structure having excellent machinability according to (1) above, wherein the area ratio of tempered bainite is 20 to 50% and the area ratio of ferrite is 1 to 20%.

(3) The steel pipe for machine structure excellent in machinability according to (1) or (2) above, wherein an absolute value of residual stress in a region from the outer surface of the steel pipe to 1 mm is 150 MPa or less.

(4) The method of manufacturing a steel pipe for machine structure having excellent machinability according to any one of (1) to (3) above, comprising the chemical component according to claim 1, and the following (1 ) While satisfying the following formula (2) while rotating the steel pipe defined by the formula [alpha] [%] from 580 to 640 in the circumferential direction from a temperature of 800 ° C. to 200 ° C. A method for producing a steel pipe for machine structure excellent in machinability, wherein the steel pipe is forcibly cooled from the outer surface of the steel pipe at a cooling rate V [° C / s] and then reheated to 550 to 700 ° C.
α = 830-270C-90Mn-70Cr-83Mo (1)
1.0 <V <0.4159α-231.95 (2)
Here, C, M, Cr, and Mo are the contents of each element.

(5) The machinability according to (4) above, wherein the steel pipe formed in the hot drawing process is accelerated and cooled from the outer surface of the steel pipe while rotating in the circumferential direction as it is, and then reheated. Method for manufacturing steel pipes for machine structures with excellent performance.

(6) The steel structure having excellent machinability according to (5) above, wherein the steel pipe is formed by hot piercing and rolling a steel slab and further pipe forming by a hot drawing process. Steel pipe manufacturing method.

  According to the present invention, it is possible to provide a machine structural steel pipe excellent in machinability on the inner surface at low cost, and the industrial contribution is extremely remarkable.

  The present inventors performed various heat treatments on a steel pipe having an outer diameter of 146 mm and a wall thickness of 12 mm, and investigated the influence of the metal structure on the machinability of the steel by changing the metal structure. The machinability was evaluated by broaching the inner surface of a steel pipe into a gear shape and investigating the accuracy of the cut surface. A sample was collected by cutting a part of the steel pipe, and the unevenness of the cut surface on the inner surface of the steel pipe was measured with a stylus type surface roughness measuring instrument. The metal structure was observed with a scanning electron microscope, and the area ratio of each structure was determined by image analysis.

  The accuracy of the cut surface was good when the unevenness of the cut surface was 10 μm or less, and the ratio of the good number of 100 processed products was evaluated as a pass rate [%]. “○” indicates that the ratio of the processed product having a cutting surface unevenness of 10 μm or less that exhibits good cutting surface accuracy is 95 to 99%, “◎” indicates that it is 100%, and “94” indicates that it is 94% or less. It showed in Table 1 as x.

  As shown in Table 1, the metal structure having a cutting surface accuracy pass rate of 95% or more is a mixed structure of tempered bainite and tempered martensite having an area ratio of 20 to 70%, and further includes ferrite. Recognize. Furthermore, in the mixed structure of tempered bainite of 20 to 50%, ferrite of 1 to 20% and tempered martensite by area ratio, the pass rate becomes 100%.

  From the above, as the metal structure of the steel pipe, the area ratio of tempered bainite was 20 to 70%, the area ratio of ferrite was 0 to 25%, and the remainder was limited to tempered martensite. In order to further improve the machinability, it is preferable that the area ratio of tempered bainite is 20 to 50%, the area ratio of ferrite is 1 to 20%, and the balance is tempered martensite.

  In the present invention, tempered martensite and tempered bainite can be distinguished from each other by the form of cementite deposited in the lath by structural observation with a scanning electron microscope. That is, tempered martensite has a plurality of cementite major axis directions, and tempered bainite has one major axis direction of cementite. In addition, ferrite is not a lath like bainite but a lump, and pearlite has plate-like cementite precipitated at grain boundaries.

  In the present invention, the hardness is not particularly defined, but the Vickers hardness of at least 3 mm from the inner surface of the steel pipe is preferably 220 or more and 280 or less. When the Vickers hardness exceeds 280, the tool life may be reduced. On the other hand, when the Vickers hardness is 220 or less, peeling may occur and the accuracy of the cutting surface may be impaired.

  Next, the reason why the chemical composition of the steel pipe is limited in the present invention will be described. Note that “%” shown below means “% by mass” unless otherwise specified.

  C: C is an extremely effective element for improving strength, and at least 0.15% is necessary to obtain sufficient strength as a structure such as a gear or a cylinder, which is a main application destination of the steel pipe of the present invention. . However, if it exceeds 0.45%, there will be a problem of cracking due to cooling during quenching. Therefore, C is limited to 0.15 to 0.45%.

  Si: Si is a deoxidizing element and contributes to solid solution strengthening. In the present invention, in order to obtain sufficient strength, the lower limit of the Si amount is set to 0.1%. However, if Si is added excessively, the workability is impaired, so the upper limit was limited to 0.4%.

  Mn: Mn is an essential element for improving the strength, and its lower limit is 0.5%. However, if it exceeds 1.0%, sufficient martensite and bainite are not generated, and the accuracy of the cut surface of the processed product is greatly deteriorated, so 1.0% was made the upper limit.

  Cr: Cr is an element that improves the strength and is effective in increasing the surface hardness by nitriding treatment, and it is necessary to add 0.8% or more. However, if it exceeds 1.5%, the generation of martensite becomes excessive and the cutting surface accuracy of the workpiece is deteriorated, so the upper limit was made 1.5%.

  Mo: Mo is an element that improves the hardenability and contributes to an increase in strength. To obtain the effect, addition of at least 0.05% is necessary. However, if it exceeds 0.5%, the generation of martensite and bainite becomes insufficient and the machined surface accuracy is greatly deteriorated, so the upper limit was made 0.5%.

  S: S is an element effective for improving the machinability, and 0.005% or more must be added to obtain the effect. However, if added excessively, the problem of cracking after tempering occurs, so the upper limit was made 0.03%.

Furthermore, in the present invention, in order to obtain a metal structure optimal for machinability, α [%] defined by the following formula (1) for C, Mn, Cr, and Mo satisfies 580 ≦ α ≦ 640. It was limited as follows.
α = 830-270C-90Mn-70Cr-83Mo (1)
α is an index of hardenability, and when it exceeds 640, hardenability is low, and ferrite is excessively generated, so that tempered martensite and tempered bainite necessary for improving machinability cannot be secured. On the other hand, if α is less than 580, the hardenability is high, and the tempered bainite fraction necessary for improving the machinability cannot be ensured. Therefore, α is limited to 580 or more and 640 or less.

  In steel pipes manufactured in the quenching / tempering process, there is residual stress generated by uneven heat during cooling during quenching. The occurrence of residual stress is greatly influenced by the cooling uniformity during tempering and the tempering temperature. If the residual stress is high, the residual stress is released during machining, so the accuracy of the part shape is impaired. In order to suppress the change in the part shape accompanying the release of the residual stress during processing, the absolute value of the residual stress in the region from the outer surface of the steel pipe to the depth of 1 mm is preferably 150 MPa or less. Here, the absolute value of the residual stress being 150 MPa or less means that the residual stress is in the range of −150 MPa to +150 MPa, and in the present invention, the residual stress value is positive when the residual stress value is positive. Negative tension is defined as tensile residual stress.

  Next, a manufacturing method will be described. In the present invention, the condition when the steel pipe having the above chemical components is hot-worked or heated and cooled from 800 ° C. or higher is important. In particular, the cooling rate of the accelerated cooling is controlled by the cutting surface accuracy of the machining surface. This is a fundamental technology for improvement. In addition, a cooling rate is a thing of the inner surface position of a steel pipe.

  When the cooling rate of accelerated cooling is less than 1.0 ° C./s, the formation of ferrite becomes remarkable, and tempered martensite and tempered bainite necessary for ensuring machinability cannot be obtained. Therefore, the lower limit was limited to 1.0 ° C./s. On the other hand, the upper limit is determined by α [%] defined by the above equation (1), which is closely related, in order to secure the optimum metal structure for improving the processing accuracy.

  As a result of intensive investigations on the relationship between cutting surface accuracy, chemical composition, and accelerated cooling rate, the present inventors have determined that the upper limit of the cooling rate V [° C./s] of accelerated cooling that can ensure cutting surface accuracy is 231.95). From the above, the cooling rate V [° C./s] of the accelerated cooling is limited to 1.0 or more and (0.4159α-231.95) or less.

  The accelerated cooling method was defined as cooling from the outer surface only while rotating the steel pipe in the circumferential direction. This is to cool uniformly in the circumferential direction and the longitudinal direction. If the steel pipe is not rotated, the lower surface of the steel pipe will be excessively cooled, and even when cooling from the inner surface side of the steel pipe, water will accumulate on the lower surface and sufficient cooling will occur. This is because there is a problem that speed cannot be obtained. The cooling method can be arbitrarily selected, for example, a method in which water is directly applied to the outer surface of the steel pipe, a method in which the water is applied in a tangential direction on the outer periphery of the steel pipe, or mist cooling.

  The reason for limiting the temperature of the steel pipe before the start of accelerated cooling to 800 ° C. or more is that the metal structure at the start of accelerated cooling is an austenite single phase. If the temperature of the steel pipe is too high, the austenite grains become coarse and it is difficult to produce ferrite, so 900 ° C. or lower is desirable.

  The accelerated cooling stop temperature was limited to 200 ° C. or lower. This is because if the accelerated cooling stop temperature exceeds 200 ° C., carbides are finely precipitated during quenching, and as a result, the tool life of machining after tempering is reduced.

  The cooling start temperature and cooling stop temperature of the inner surface of the steel pipe may be measured before and after accelerated cooling, that is, the temperature on the inner surface of the steel pipe with a contact thermometer before and after the cooling device. The cooling rate can be calculated from the passing speed. The temperature of the outer surface of the steel pipe may be measured by a radiation thermometer, and the temperature of the inner surface of the steel pipe may be obtained by heat conduction calculation. Also, thermocouples are attached to the inner and outer surfaces of steel pipes having various outer diameters and thicknesses, and cooling curves corresponding to various heating temperatures, refrigerant ejection conditions, and cooling times are created, and are within the scope of the present invention. Conditions can also be determined.

  Tempering temperature was limited to 550 ° C. or higher and 700 ° C. or lower. If the temperature is lower than 550 ° C., the residual stress cannot be released by uniform tempering, so the lower limit is set to 550 ° C. On the other hand, when the temperature exceeds 700 ° C., recrystallization proceeds and the tempered structure is lost, so the upper limit was set to 700 ° C.

  The steel pipe of the present invention is preferably a seamless steel pipe, and the pipe forming process is generally hot piercing-rolling-stretching. Also, drilling cold or hot, seamless steel pipes manufactured by hot extrusion press, steel plates such as hot coils, etc. are formed into a tubular shape with a roll cold or hot, then both end faces are welded It may be a welded steel pipe manufactured by:

  The steel pipe may be heated by a heating furnace or induction heating once the steel pipe manufacturing process is finished, and the steel slab is pierced and rolled hot, and at the final stage immediately after pipe forming by the drawing process, 800 ° C. or higher. If so, it is possible to cool in-line as it is. The steel sheet may be cold-formed, piped by ERW welding and heated, or the steel sheet may be cold-formed and heated by ERW welding, then cooled by heating, or piped by a hot drawing process. Immediately afterwards, it may be cooled as it is in-line.

  In the present invention, the steel pipe shape is not particularly limited, but the wall thickness is preferably 5 mm or more and 20 mm or less. The reason is that when the thickness is 20 mm or more, the difference in cooling rate between the outer surface side and the inner surface side becomes large. Further, when the thickness of the steel pipe is 5 mm or less, if the machining allowance on the inner and outer surfaces is subtracted, the thickness is too thin and may not be suitable for machine parts.

  The length of the steel pipe is preferably at least 5 times the outer diameter. This is because the steel pipe is uniformly cooled when water-cooled from the outer surface. If the length of the steel pipe is smaller than 5 times the outer diameter, the water sprayed from the outer surface enters the inner surface, and the cooling varies greatly in the longitudinal direction and the circumferential direction of the steel pipe.

Example 1
Steel pipes having the chemical components shown in Table 2 and having an outer diameter of 156 mm and a wall thickness of 12 mm were manufactured, and machinability was evaluated. The pipe making method includes an electric resistance welding process (ERW) in which a steel sheet is cold-formed and electro-welded, and a seamless process (SML) in which a steel piece is hot drilled, rolled and stretched. After heating these steel pipes from room temperature in a heating furnace, they were cooled from the outer surface of the steel pipes at a predetermined cooling rate by ring water cooling. Thereafter, the steel pipe was reheated to a predetermined temperature in a heating furnace and cooled to room temperature by air cooling. The steel pipe forming method, the heating temperature before accelerated cooling, the cooling rate and stop temperature of accelerated cooling, and the reheating were performed under the conditions shown in Table 3. Vmax in Table 3 is a calculated value of 0.4159α-231.95, which means the upper limit of the cooling rate.

  The metal structure at the center of the thickness of the manufactured steel pipe is a small piece taken from any position in the circumferential direction except the welded part of the ERW steel pipe, except for the 100 mm long end, and polished and etched. And observed using a scanning electron microscope and an optical microscope. The metal structure was observed with a scanning electron microscope at a maximum magnification of 5000, and was classified into tempered martensite (M), tempered bainite (B), ferrite (F), and pearlite (P). The area ratio of tempered bainite and ferrite was determined by image analysis.

  Residual stress was measured using X-rays at a position 0.5 mm away from the outer surface of the steel pipe. The direction of stress is the circumferential direction. A positive value in the column of residual stress in Table 3 means a compressive residual stress, and a negative value means a tensile residual stress.

  Regarding the machinability, the steel pipe was cut to a length of 50 mm and the inner and outer surfaces were cut by 1 mm, and then the inner surface was broached to form a gear shape, and the broached surface accuracy was investigated and evaluated. Regarding the surface accuracy, if the unevenness of the cut surface was 10 μm or less, it was considered good, and the ratio of a good number in 100 processes was evaluated for the pass rate [%] of the machinability. Further, the outer diameter before and after broaching was measured, and if the difference was within 50 μm, it was considered good, and the percentage of good number in 100 machining was evaluated as the pass rate [%] of deformation.

  If both the machinability and the deformation amount have a pass rate of 95 to 99%, the judgment is “good”, the judgment that the pass rate is 100% is “◎”, and the judgment that any pass rate is 94% or less is ×. It was.

  No. which is an example of the present invention. 1 to 8 are steel pipes manufactured with appropriate chemical components and heat treatment conditions, and had an appropriate metal structure and excellent cutting properties.

  No. No. 9 had a high C content and a high hardenability with α of 580 or less, and the cooling rate of forced cooling was faster than specified. Therefore, it became a tempered martensite single phase and had poor machinability, high residual stress, and large deformation. It is an example. No. No. 10 is an example in which the amount of C is too low and α is 640 or more and the hardenability is low, so that it becomes a structure of ferrite and pearlite and the machinability is difficult. No. No. 11 is an example in which the area ratio of tempered bainite is low and the machinability is difficult because the amount of Mn is high and α is 580 or less and the hardenability is high.

  No. No. 12, since the Si content is high, the Mn content is low, the hardenability is low at 640 or higher, and the forced cooling stop temperature is high, the ferrite area ratio is high and the tempered bainite area ratio is low. This is an example of difficulty in sex. No. No. 13 is an example in which the amount of Cr is too low and the forced cooling start temperature is low, resulting in a structure of ferrite and pearlite and difficult to cut.

  No. No. 14, because the amount of Cr and Mo was too high, α was 580 or less and the hardenability was high, the cooling rate was faster than specified, the tempered martensite single phase was difficult to cut, and the reheating temperature was low. This is an example in which the residual stress is high and the deformation is large.

  No. No. 15 is an example in which the cooling rate was too slow, resulting in a structure of ferrite and pearlite and difficult to cut. No. No. 16 is an example in which the cooling rate was too high, so that it became a tempered martensite single phase, and the machinability was difficult, and the residual stress was high and the deformation was large. No. No. 17 is an example in which the heating area was low and the forced cooling stop temperature was high, so that the ferrite area ratio was high and the machinability was difficult. No. No. 18 is an example in which the reheating temperature was high, the ferrite fraction was high, the area ratio of tempered bainite was low, and the machinability was difficult. No. No. 19 is an example in which the cooling rate is too fast and the tempered martensite single phase is obtained, the machinability is difficult, and the reheating temperature is too low so that the residual stress is high and the deformation is large.

(Example 2)
Steels having chemical components shown in Table 2 were melted and a 170 mm diameter bloom was cast by a converter-continuous casting process. These blooms were heated to 1240 ° C. and punched and rolled by the Mannesmann-plug mill method, or hot rolled steel sheets with the chemical components shown in Table 2 were cold formed into hollow shapes and then electro-welded and welded. The material was reheated to 950 ° C., reduced in diameter, and then cooled from the outer surface side by ring cooling.

  The steel pipe size after the diameter reduction rolling was 156 mm in outer diameter and 12 mm in thickness. Thereafter, the steel pipe was heated to a predetermined temperature in a heating furnace and cooled to room temperature by air cooling. Observation and classification of the manufactured steel pipe, measurement of area ratio of tempered bainite and ferrite, measurement of residual stress, and evaluation of machinability were performed in the same manner as in Example 1. The results are shown in Table 4. Vmax in Table 4 is a calculated value of 0.4159α-231.95, which means the upper limit of the cooling rate.

  No. which is an example of the present invention. Nos. 19 to 26 are steel pipes manufactured with appropriate chemical components and heat treatment conditions, and had an appropriate metal structure and excellent cutting properties.

  No. No. 27 is an example in which the amount of C is high and α is 580 or less, the hardenability is high, and the cooling rate is faster than specified, so that it becomes a tempered martensite single phase, the machinability is difficult, the residual stress is high, and the deformation is large. . No. No. 28 is an example where the amount of C is too low and α is 640 or more and the hardenability is low, so that it becomes a structure of ferrite and pearlite and the machinability is difficult.

  No. No. 29 is an example in which the area ratio of tempered bainite is low and the machinability is difficult because the amount of Mn is high and α is 580 or less and the hardenability is high. No. No. 30 has a high Si content, a low Mn content and a low hardenability with α of 640 or higher, and a high forced cooling stop temperature, so that the area ratio of ferrite is high and the area ratio of tempered bainite is low and machinability is improved. This is an example of difficulties.

  No. No. 31 is an example in which the amount of Cr is too low and the forced cooling start temperature is low, so that it becomes a structure of ferrite and pearlite and the machinability is difficult. No. No. 32, because the amount of Cr and Mo was too high, α was 580 or less, the hardenability was high, the cooling rate was faster than specified, the tempered martensite single phase was difficult to cut, and the reheating temperature was low. This is an example in which the residual stress is high and the deformation is large.

  No. No. 33 is an example in which the cooling rate was too slow, resulting in a structure of ferrite and pearlite and difficult to cut. No. No. 34 is an example in which the cooling rate was too high, so that it became a tempered martensite single phase and had poor machinability, high residual stress, and large deformation.

  No. No. 35 is an example in which the forced cooling start temperature was low and the forced cooling stop temperature was high, so that the ferrite area ratio was high and the machinability was difficult. No. No. 36 is an example in which the reheating temperature was high, the ferrite fraction was high, the area ratio of tempered bainite was low, and machinability was difficult. No. No. 37 is an example in which the cooling rate is too high, the tempered martensite single phase is obtained, the machinability is difficult, and the reheating temperature is too low so that the residual stress is high and the deformation is large.

Claims (6)

% By mass
C: 0.15-0.45%,
Si: 0.1 to 0.4%,
Mn: 0.5 to 1.0%
Cr: 0.8 to 1.5%,
Mo: 0.05-0.5%
S: 0.005 to 0.03%
Tempered bainite containing [alpha] [%] defined by the following formula (1) is 580 or more and 640 or less, the balance is made of iron and inevitable impurities, and the metal structure is 20 to 70% in area ratio A steel tube for machine structure excellent in machinability, comprising ferrite having an area ratio of 25% or less (including 0%) and the balance tempered martensite.
α = 830-270C-90Mn-70Cr-83Mo (1)
Here, C, M, Cr, and Mo are the contents of each element.   The steel pipe for machine structure excellent in machinability according to claim 1, wherein the area ratio of tempered bainite is 20 to 50% and the area ratio of ferrite is 1 to 20%.   The steel pipe for machine structure excellent in machinability according to claim 1 or 2, wherein an absolute value of residual stress in a region from the outer surface of the steel pipe to 1 mm is 150 MPa or less. It is a manufacturing method of the steel pipe for machine structure excellent in machinability of any one of Claims 1-3, Comprising: It has a chemical component of Claim 1, and is defined by following (1) Formula A cooling rate V [° C / ° C] satisfying the following expression (2) while rotating a steel pipe having α [%] of 580 or more and 640 or less in a circumferential direction from a temperature of 800 ° C or higher to a temperature of 200 ° C or lower s], forcibly cooling from the outer surface of the steel pipe, and then reheating to 550 to 700 ° C. A method for producing a steel pipe for machine structure having excellent machinability.
α = 830-270C-90Mn-70Cr-83Mo (1)
1.0 <V <0.4159α-231.95 (2)
Here, C, M, Cr, and Mo are the contents of each element.   5. The machine with excellent machinability according to claim 4, wherein the steel pipe formed in the hot drawing step is accelerated and cooled from the outer surface of the steel pipe while rotating in the circumferential direction as it is, and then reheated. Manufacturing method of structural steel pipe. 6. The steel pipe for machine structural use with excellent machinability according to claim 5, wherein the steel pipe is formed by hot piercing and rolling a steel slab and further making a pipe by a hot drawing process. Method.