JP2023507727A - Steel material with excellent workability and its manufacturing method - Google Patents

Abstract

According to one aspect of the present invention, the steel material having excellent workability contains, by weight %, C: 0.8-1.0%, Si: 0.1-0.3%, Mn: 0.2-0. 5%, Cr: 0.1 to 0.3%, P: 0.03% or less, S: 0.005% or less, the balance contains Fe and other inevitable impurities, and the microstructure is ferrite containing spheroidized carbides The carbide may have a single-phase structure, the average grain size of the carbide may be 0.8 μm or less, and the number density of the carbide may be 2*10 ⁵ to 7*10 ⁵ /mm ² .

Description

TECHNICAL FIELD The present invention relates to a steel material which has excellent workability and is particularly suitable as a material for tools, and a method for producing the same.

In general, hardness and workability are widely known as physical properties of steel materials that are difficult to achieve at the same time. This is because an increase in the strength of the steel material results in an increase in hardness, but the higher the strength of the steel material, the lower the workability of the steel material.

Tool steel materials used to manufacture tool parts also require excellent workability when manufacturing into the shape of the part. high hardness is required. In particular, tool steel materials used to manufacture tool parts are mainly steel materials containing a relatively large amount of carbon (C) in order to ensure a certain level of hardness and strength. It is difficult to secure workability at the level of

For steel materials for tools, a method is generally applied in which the workability of the steel material is secured by spheroidizing annealing, then it is processed into a part shape, and then hardness is secured by incorporating a martensitic structure into the steel material by quenching. . Spheroidizing annealing is a heat treatment performed at a high temperature to spheroidize plate-like lamellar cementite, and it takes a long time to secure the desired level of workability. Undesirable.

In order to shorten the annealing heat treatment time, Patent Document 1 proposes an annealing heat treatment process condition in which a short time heat treatment is performed at a temperature of A1 or higher, followed by a long time heat treatment at a temperature lower than A1. However, such a heating pattern is not only difficult to achieve using a normal heating furnace, but also has the problem that it still takes a long time to ensure the workability of the high-carbon steel material. Therefore, it is not desirable as a practical method for manufacturing high-carbon tool steel.

Korean Patent Publication No. 10-2015-0075290

According to one aspect of the present invention, it is possible to provide a steel material with excellent workability and a method for producing the same.

The subject of the present invention is not limited to what has been described above. A person of ordinary skill in the art should have no difficulty in understanding the additional subject matter of the present invention from the overall content of this specification.

According to one aspect of the present invention, the steel material having excellent workability contains, by weight %, C: 0.8-1.0%, Si: 0.1-0.3%, Mn: 0.2-0. Ferrite containing 5%, Cr: 0.1 to 0.3%, P: 0.03% or less, S: 0.005% or less, balance Fe, and other inevitable impurities, and the microstructure contains spheroidized carbides The carbide may have a single-phase structure, an average particle size of the carbide of 0.8 μm or less, and a number density of the carbide of 2*10 ⁵ to 7*10 ⁵ /mm ² .

The spheroidization rate of the carbide may be 95% or more.

The normal temperature surface hardness of the steel material may be 230 to 270 HV.

The steel material may have a burr height of 20 μm or less after press working, and a bending workability (R/t) of the steel material may be 2 or less.

The carbide may have an average particle size of 0.55 μm or more.

The steel material was heated to 800 to 950° C. and maintained for 30 minutes or less, cooled to a temperature range of 50° C. or less at a cooling rate of 50 to 150° C./s, and heat-treated at 200 to 300° C. for 10 to 60 minutes. The surface hardness of the subsequent steel material may be 56 HRC or more.

According to another aspect of the present invention, a method for producing a steel material with excellent workability comprises, in weight percent, C: 0.8 to 1.0%, Si: 0.1 to 0.3%, Mn: 0.1%, 0.1% to 1.0%, and 0.1% to 1.0% by weight. 2 to 0.5%, Cr: 0.1 to 0.3%, P: 0.03% or less, S: 0.005% or less, the balance Fe and other inevitable impurities Reheat the slab, heat rolling and winding; applying a mechanical external force to the wound steel to segment the carbides of the steel; and a spheroidizing anneal in a range of 5 to 20 hours.

In the step of segmenting the carbides, the wound steel material may be cold-rolled at a rolling reduction of 30 to 50% to segment the carbides of the steel material.

The slab is reheated in a temperature range of 1000 to 1300 ° C., the reheated slab is hot rolled in a temperature range of 850 to 1150 ° C., and the hot rolled steel is rolled in a temperature range of 600 to 650 ° C. can be rolled up.

According to a preferred aspect of the present invention, it is possible to provide a steel material for tools having not only excellent hardness properties but also excellent workability, and a method for producing the same.

4 is a photograph of observation of the microstructure of test piece A. FIG. 4 is a photograph of observation of the microstructure of test piece H. FIG.

The present invention relates to a steel material with excellent workability and a method for producing the same, and preferred embodiments of the present invention will be described below. Embodiments of the invention may be varied in many forms and should not be construed as limiting the scope of the invention to the embodiments set forth below. The present embodiments are provided to make the present invention more detailed for those skilled in the art to which the present invention pertains.

Hereinafter, a steel material excellent in workability will be described in more detail as one aspect of the present invention.

According to one aspect of the present invention, the steel material having excellent workability contains, by weight %, C: 0.8-1.0%, Si: 0.1-0.3%, Mn: 0.2-0. 5%, Cr: 0.1 to 0.3%, P: 0.03% or less, S: 0.005% or less, the balance containing Fe and other unavoidable impurities, and the microstructure is a single ferrite containing spheroidized carbides. The carbide may have a phase structure, the average grain size of the carbide may be 0.8 μm or less, and the number density of the carbide may be 2*10 ⁵ to 7*10 ⁵ /mm ² .

The alloy composition of the present invention will be described in detail below. In the following, unless otherwise stated, percentages and ppm regarding contents of alloy compositions are based on weight.

According to one aspect of the present invention, the steel material having excellent workability contains, by weight %, C: 0.8-1.0%, Si: 0.1-0.3%, Mn: 0.2-0. 5%, Cr: 0.1 to 0.3%, P: 0.03% or less, S: 0.005% or less, the balance being Fe and other unavoidable impurities.

Carbon (C): 0.8-1.0%
Carbon (C) is a representative element that contributes to the improvement of hardenability, and in the present invention, is an essential element added to ensure hardness after quenching. Therefore, in the present invention, 0.8% or more of carbon (C) can be included for such effects. A preferred carbon (C) content may be greater than 0.8%, and a more preferred carbon (C) content may be 0.82% or more. On the other hand, when the carbon (C) content in the steel exceeds a certain range, the fraction of carbides in the steel becomes excessively high, which may promote brittle fracture. Therefore, in the present invention, the upper limit of carbon (C) content can be limited to 1.0%. A preferred carbon (C) content is less than 1.0%, and a more preferred carbon (C) content may be 0.98% or less.

Silicon (Si): 0.1-0.3%
Silicon (Si) is a component that contributes to improving the strength of steel. Therefore, in the present invention, 0.1% or more of silicon (Si) can be included in order to achieve such effects. A preferred lower limit of the silicon (Si) content is 0.12%, and a more preferred lower limit of the silicon (Si) content is 0.15%. However, if the content of silicon (Si) in the steel exceeds a certain range, not only does the cold rolling property deteriorate, but also the possibility of decarburization during heat treatment increases, causing surface scale defects on the surface of the steel material. In the present invention, the upper limit of the content of silicon (Si) can be limited to 0.3% because it may induce an increase. A preferable upper limit of the silicon (Si) content may be 0.28%, and a more preferable upper limit of the silicon (Si) content may be 0.25%.

Manganese (Mn): 0.2-0.5%
Manganese (Mn) is an element that not only contributes to the improvement of hardenability, but also effectively contributes to the strength improvement of steel through solid-solution strengthening. Manganese (Mn) is combined with sulfur (S) in steel and precipitates as MnS, so that red shortness due to sulfur (S) can be effectively prevented. In the present invention, manganese (Mn) of 0.2% or more can be included to achieve such effects. A preferable lower limit of manganese (Mn) content may be 0.25%, and a more preferable lower limit of manganese (Mn) content may be 0.3%. However, when the content of manganese (Mn) in the steel exceeds a certain range, not only the cold-rollability is deteriorated, but also the workability may be deteriorated due to center segregation. Mn) content can be limited to 0.5%. A preferable upper limit of manganese (Mn) content may be 0.45%, and a more preferable upper limit of manganese (Mn) content may be 0.4%.

Chromium (Cr): 0.1-0.3%
Chromium (Cr), like manganese (Mn), is an element that effectively contributes to improving hardenability. Therefore, in the present invention, 0.1% or more of chromium (Cr) can be included for such effects. A preferable lower limit of the chromium (Cr) content may be 0.13%, and a more preferable lower limit of the chromium (Cr) content may be 0.16%. However, if the content of chromium (Cr) in the steel exceeds a certain range, not only is there a risk of deterioration in cold rolling performance, but also the decomposition of carbides due to heat treatment is delayed, and the carbides are spheroidized by spheroidizing annealing. may not be completed. Therefore, in the present invention, the upper limit of chromium (Cr) content can be limited to 0.3%. A preferred upper limit of the chromium (Cr) content may be 0.28%, and a more preferred upper limit of the chromium (Cr) content may be 0.25%.

Phosphorus (P): 0.03% or less (including 0%)
Phosphorus (P) in steel is a typical impurity element, but it is the most advantageous element for securing strength without greatly impairing formability. However, if phosphorus (P) is excessively added, the possibility of brittle fracture increases, which may cause slab breakage during hot rolling. may be greatly reduced. Therefore, in the present invention, the upper limit of phosphorus (P) content can be limited to 0.03%.

Sulfur (S): 0.005% or less (including 0%)
Sulfur (S) is an impurity element that is inevitably mixed in steel, and it is preferable to manage its content as low as possible. In particular, sulfur (S) in steel may induce red shortness, so in the present invention, the upper limit of sulfur (S) content can be limited to 0.005%.

According to one aspect of the present invention, the steel material having excellent workability may contain the balance Fe and other unavoidable impurities in addition to the above components. However, unintended impurities from the raw materials or the surrounding environment may inevitably be mixed in during normal manufacturing processes, and it is impossible to completely eliminate them. Since these impurities are well known to anyone having ordinary knowledge in this technical field, the full content thereof is not specifically mentioned herein. It should be noted that addition of effective components other than the above composition is not excluded.

According to one aspect of the invention, the microstructure of the steel material may be a ferritic single-phase structure containing spheroidized carbides. The spheroidized carbide of the present invention means not only the case where all the carbides are spheroidized but also the case where some of the carbides are spheroidized. The average particle size of the carbides may be 0.8 μm or less, and the number density of the carbides may be 2*10 ⁵ to 7*10 ⁵ pieces/mm ² . That is, according to one aspect of the present invention, the steel material has not only fine spheroidal carbides formed in the steel, but also a large amount of carbides are uniformly distributed, so that the workability of the steel material is effectively secured. can do.

According to one aspect of the present invention, the carbides contained in the steel material may have a spheroidization rate of 95% or more, and preferably the carbides may have a spheroidization rate of 99% or more. Here, the spheroidization rate of carbide means the ratio of the area of spheroidized carbide having an aspect ratio (ratio of major axis to minor axis) of 2 or less to the area of all carbides. That is, according to one aspect of the present invention, since most of the carbides contained in the steel material are spheroidized carbides, the workability of the steel material can be effectively ensured.

In addition, when the average grain size of carbides contained in the steel material is below a certain level, it means that the carbides are not sufficiently spheroidized, so in the present invention, the lower limit of the average grain size of carbides is limited to 0.55 μm can do.

According to one aspect of the present invention, the steel may have a room temperature surface hardness of 230-270 HV. Further, according to one aspect of the present invention, the steel material having excellent workability has a burr height of 20 μm or less after press working, and the bending workability (R/t) of the steel material is 2 or less. It's okay. The height of the burr can be obtained by measuring the height difference of the surface edge with a roughness measuring instrument after blanking under the condition of a clearance of 5°. Bending workability can be measured by whether or not cracks occur on the surface of a material when the material is bent at 90 degrees while being pressed with a triangular prismatic cemented carbide with a radius of curvature of R (mm) at the end portion. can. Bendability t means the thickness (mm) of the steel material.

According to one aspect of the present invention, the steel material is obtained by heating the above steel material to 800 to 950° C., maintaining it for 30 minutes or less, and cooling it to a temperature range of 50° C. or less at a cooling rate of 50 to 150° C./s. and a surface hardness of 56 HRC or more after heat treatment at 200 to 300° C. for 10 to 60 minutes. That is, according to one aspect of the present invention, the steel material can ensure excellent workability before quenching, and can ensure excellent hardness properties after quenching.

Hereinafter, as one aspect of the present invention, a method for manufacturing a steel material having excellent workability will be described in more detail.

According to one aspect of the present invention, a method for producing a steel material with excellent workability is as follows: C: 0.8 to 1.0%, Si: 0.1 to 0.3%, Mn: 0.2% ~ 0.5%, Cr: 0.1 to 0.3%, P: 0.03% or less, S: 0.005% or less, the balance Fe and other inevitable impurities Reheat the slab, hot rolling and winding; applying a mechanical external force to the wound steel to segment the carbides of the steel; and spheroidizing annealing for 5 to 20 hours at.

(slab reheating, hot rolling, and coiling)
After providing a slab with a given alloy composition content, reheating can be performed. Since the alloy composition of the slab of the present invention corresponds to the alloy composition of the steel material described above, the description of the alloy composition of the slab of the present invention is replaced by the description of the alloy composition of the steel material described above. In addition, the reheating temperature of the slab of the present invention can be applied to the conditions that are applied to normal reheating of slabs, but as a non-limiting example, the reheating temperature of the slab in the present invention is 1000 to 1300 ° C. may be in the range of

A hot-rolled steel material can be provided by subjecting the reheated slab to hot rolling in a temperature range of 850 to 1150°C. If the hot rolling temperature is too high, there is a problem that the target physical properties cannot be secured due to coarsening of the fine structure. Therefore, in the present invention, the upper limit of the hot rolling temperature range is set to 1150 ° C. can be limited to On the other hand, if the hot rolling temperature is below a certain level, excessive rolling load may become a problem, so in the present invention, the lower limit of the hot rolling temperature is limited to 850 ° C. can be done.

Hot rolled steel can be coiled in the temperature range of 600-650°C. If the coiling temperature is too high, not only will the thickness of the cementite in the pearlite structure increase, but there is also a risk of shape defects due to phase transformation after coiling. The upper limit can be limited to 650°C. On the other hand, if the winding temperature is less than a certain level, the strength is too high and there is a concern that the plate will break in the process after winding. can be restricted. In addition, in order to prevent breakage of the sheet due to variations in material properties, the temperature deviation in the longitudinal direction over the entire length of the hot-rolled coil can be controlled to 20° C. or less at the step of segmenting carbides, which will be described later.

(Segmentation of Carbide by Application of Mechanical External Force)
After unwinding the wound steel, a pickling process can be selectively applied according to the surface quality of the unwound steel, and then mechanical external force is applied to the steel to remove carbides (lamellar cementite). It can be mechanically segmented. As a method of applying a mechanical external force to the steel material, any method can be used as long as it can segment the lamellar cementite, but cold rolling or forging can be applied as non-limiting examples. be. As an example, when cold rolling is applied to apply a mechanical external force to a steel material, a cold rolling reduction of 30 to 50% can be applied in consideration of effective segmentation of cementite.

In the present invention, since the lamellar cementite is segmented by applying a mechanical external force to the hot-rolled steel material, the spheroidizing efficiency in the subsequent spheroidizing annealing can be effectively improved. That is, in the present invention, since the spheroidizing annealing is started in a state in which a large amount of fine segmented carbides are distributed, the carbides can be effectively spheroidized within a relatively short time.

(Spheroidizing annealing)
Spheroidizing annealing can be performed by heating a steel material in which carbides are segmented by applying a mechanical external force in a temperature range of 650 to 700° C. and maintaining it for 5 to 20 hours. If the temperature and time of spheroidizing annealing are below a certain level, the carbide may not be sufficiently spheroidized. can be restricted. On the other hand, if the temperature and time of the spheroidizing annealing exceed a certain level, not only the carbides become excessively coarse, but also the hardness of the steel material may decrease. and time can be limited to 700° C. and 20 hours, respectively.

The microstructure of the steel manufactured by the manufacturing method described above may be a ferrite single-phase structure containing spheroidized carbides. The average particle size of the carbide may be 0.55 μm or more, and the spheroidization rate of the carbide may be 95% or more.

The steel material manufactured by the above-described manufacturing method has a normal temperature surface hardness of 230 to 270 HV, a burr height after press working of 20 μm or less, and a bending workability (R/t) of 2 or less. can be

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to Examples. However, it should be noted that the examples described below are merely for illustrating and embodying the present invention, and are not intended to limit the scope of rights of the present invention.

(Example)
A slab having the alloy composition shown in Table 1 was prepared, heated in the temperature range of 1200°C, and hot rolled in the temperature range of 950°C. After pickling, cold rolling was performed at a rolling reduction of 50%, and a thin steel plate with a thickness of 1.0 mm was obtained. The rollability at the time of cold rolling was evaluated for each steel type, and these are listed in Table 1. The evaluation was given as "Good" when no plate breakage and edge cracks occurred during cold rolling, or when less than 5 cracks with a size of less than 10 mm occurred even when edge cracks occurred. In addition, when sheet breakage and edge cracks occurred during cold rolling, and the edge crack size was 10 mm or more, or the number of cracks having a size of less than 10 mm was 5 or more, the evaluation was given as "X".

After that, spheroidizing annealing was performed under the conditions shown in Table 2, and the hardness and microstructure of each test piece were comparatively analyzed. However, test piece K means a test piece immediately subjected to spheroidizing annealing without cold rolling. At this time, the hardness of each test piece was converted to HV after measuring HRC using a Brinell hardness tester. As for the microstructure of each test piece, the test piece was cut, mirror-polished, etched, and the cross-sectional structure was observed using a scanning electron microscope. In addition, each test piece was subjected to press working under conditions of a clearance of 5%, the height of burrs was measured, and a 90° bending test was performed to measure bending workability (R/t). . Each test piece was subjected to quenching treatment by heating to a temperature of 900° C. and rapid cooling, and tempering treatment by heating to a temperature of 250° C., and then the surface hardness was measured. These results are also shown in Table 2. The HRC of the surface hardness at this time was also measured using a Brinell hardness tester.

A test piece that satisfies all the alloy compositions and process conditions restricted by the present invention can ensure excellent hardness properties and workability, whereas any one of the alloy compositions and process conditions restricted by the present invention It has been confirmed that it is impossible to ensure both the hardness characteristics and workability at the levels aimed at by the present invention for test pieces that do not satisfy .

FIG. 1 is an observation photograph of the microstructure of test piece A, and it can be confirmed that a large amount of spherical fine carbides are uniformly distributed. On the other hand, FIG. 2 is an observation photograph of the fine structure of the test piece H, and it can be confirmed that not only the spheroidization rate of carbides is low but also coarse carbides are locally distributed.

Although the present invention has been described in detail with reference to embodiments, different embodiments are possible. Therefore, the spirit and scope of the appended claims should not be limited to the embodiments described above.

Claims (9)

% by weight, C: 0.8-1.0%, Si: 0.1-0.3%, Mn: 0.2-0.5%, Cr: 0.1-0.3%, P: 0.03% or less, S: 0.005% or less, the balance containing Fe and other inevitable impurities,
The microstructure is a ferrite single-phase structure containing spheroidized carbides,
The average particle size of the carbide is 0.8 μm or less,
A steel material having excellent workability, wherein the number density of the carbides is 2*10 ⁵ to 7*10 ⁵ /mm ² . 2. The steel material with excellent workability according to claim 1, wherein said carbide has a spheroidization rate of 95% or more. The steel material with excellent workability according to claim 1, wherein the normal temperature surface hardness of the steel material is 230 to 270 HV. The steel material has a burr height of 20 μm or less after press working,
The steel material with excellent workability according to claim 1, wherein the bending workability (R/t) of the steel material is 2 or less. The steel material with excellent workability according to claim 1, wherein the carbide has an average grain size of 0.55 µm or more. The steel material according to any one of claims 1 to 5 is heated to 800 to 950 ° C., maintained for 30 minutes or less, and cooled to a temperature range of 50 ° C. or less at a cooling rate of 50 to 150 ° C./s. , a steel having excellent workability, having a surface hardness of 56 HRC or more after heat treatment at 200 to 300° C. for 10 to 60 minutes. % by weight, C: 0.8-1.0%, Si: 0.1-0.3%, Mn: 0.2-0.5%, Cr: 0.1-0.3%, P: reheating, hot rolling and coiling a slab containing 0.03% or less, S: 0.005% or less, balance Fe and other inevitable impurities;
applying a mechanical external force to the wound steel material to segment carbides of the steel material;
A method for producing a steel material having excellent workability, comprising: heating the steel material in which the carbides are segmented, and then spheroidizing the steel material by maintaining the steel material in a temperature range of 650 to 700° C. for 5 to 20 hours. 8. A steel material with excellent workability according to claim 7, wherein in the step of segmenting the carbides, the carbides of the steel material are segmented by cold rolling the wound steel material at a rolling reduction of 30 to 50%. Production method. reheating the slab at a temperature range of 1000 to 1300° C.;
hot rolling the reheated slab at a temperature range of 850 to 1150° C.;
8. The method for producing a steel material with excellent workability according to claim 7, wherein the hot-rolled steel material is coiled at a temperature range of 600 to 650°C.

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