Title of Invention: Steel with excellent workability and manufacturing method therefor
technology field
[One]
The present invention relates to a steel material particularly suitable as a material for tools due to excellent workability and a method for manufacturing the same.
background art
[2]
In general, among the physical properties of steel, hardness and workability are widely known as incompatible properties. This is because an increase in the strength of the steel causes an increase in hardness, whereas when the strength of the steel increases, the workability of the steel deteriorates.
[3]
[4]
In the case of tool steel used for manufacturing tool parts, excellent workability is required when manufacturing parts into the shape of parts, whereas parts after final machining require high hardness in order to secure wear resistance and impact resistance characteristics. In particular, in the case of a tool steel used for manufacturing tool parts, a steel material containing a relatively large amount of carbon (C) is mainly used to secure a certain level of hardness and strength, so it is difficult to secure a desired level of workability. to be.
[5]
[6]
In the case of steel materials for tools, the method of securing the machinability of the steel through spheroidizing annealing, processing it into a part shape, and then introducing a martensitic structure into the steel through quenching to ensure hardness is generally applied. Spheroidizing annealing is a heat treatment performed at high temperature to make plate-shaped lamellar cementite into a spherical shape.
[7]
[8]
In order to shorten the annealing heat treatment time, Patent Document 1 proposes an annealing heat treatment process condition in which heat treatment is performed for a short time at a temperature higher than A1 and then heat treated for a long time at a temperature lower than A1. However, such a heating pattern is not only difficult to implement in a conventional heating furnace, but also has a problem that it takes a long time to secure the workability of high-carbon steel, so it is not evaluated as a realistic method for manufacturing high-carbon steel for tools. does not
[9]
[10]
(Prior art literature)
[11]
(Patent Document 1) Republic of Korea Patent Publication No. 10-2015-0075290 (2015.07.03, published)
DETAILED DESCRIPTION OF THE INVENTION
technical challenge
[12]
According to one aspect of the present invention, a steel material having excellent workability and a method for manufacturing the same may be provided.
[13]
[14]
The subject of the present invention is not limited to the above. A person of ordinary skill in the art will have no difficulty in understanding the further problems of the present invention from the general contents of the present specification.
means of solving the problem
[15]
The steel material having excellent workability according to an aspect of the present invention is, by weight, C: 0.8 to 1.0%, Si: 0.1 to 0.3%, Mn: 0.2 to 0.5%, Cr: 0.1 to 0.3%, P: 0.03% or less, S: 0.005% or less, the remaining Fe and other unavoidable impurities are included, and the microstructure is a ferrite single-phase structure containing spheroidized carbides, the average particle size of the carbides is 0.8 μm or less, and the number density of the carbides is 2 * 10 5 ~ It can be 7*10 5pcs/mm 2
[16]
The spheroidization ratio of the carbide may be 95% or more.
[17]
The room temperature surface hardness of the steel may be 230 ~ 270HV.
[18]
The steel may have a burr height of 20 μm or less after press working, and the bendability (R/t) of the steel may be 2 or less.
[19]
The average particle size of the carbide may be 0.55 μm or more.
[20]
The steel is heated to 800 ~ 950 ℃ and maintained for 30 minutes or less, cooled to a temperature range of 50 ℃ or less at a cooling rate of 50 ~ 150 ℃ / s, heat treated at 200 ~ 300 ℃ 10 ~ 60 minutes After the surface hardness of the steel may be 56HRC or more.
[21]
[22]
The steel material having excellent workability according to another aspect of the present invention is, by weight, C: 0.8 to 1.0%, Si: 0.1 to 0.3%, Mn: 0.2 to 0.5%, Cr: 0.1 to 0.3%, P: 0.03% or less , S: 0.005% or less, reheating, hot rolling, and winding the slab containing the remaining Fe and other unavoidable impurities; Segmenting the carbide of the steel by applying a mechanical external force to the wound steel; And after heating the steel material in which the carbide is segmented, the step of spheroidizing annealing by maintaining for 5 to 20 hours in a temperature range of 650 ~ 700 ℃; may include.
[23]
In the step of segmenting the carbide, the carbide of the steel may be segmented by cold rolling the wound steel material at a reduction ratio of 30-50%.
[24]
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 can be wound in a temperature range of 600 to 650 ° C.
Effects of the Invention
[25]
According to a preferred aspect of the present invention, it is possible to provide a steel material for a tool that has excellent hardness properties as well as excellent workability and a method for manufacturing the same.
Brief description of the drawing
[26]
1 is a photograph of observing the microstructure of specimen A, and FIG. 2 is a photograph of observing the microstructure of specimen H.
BEST MODE FOR CARRYING OUT THE INVENTION
[27]
The present invention relates to a steel material having excellent workability and a method for manufacturing the same, and preferred embodiments of the present invention will be described below. Embodiments of the present invention may be modified in various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The present embodiments are provided in order to further detailed the present invention to those of ordinary skill in the art to which the present invention pertains.
[28]
[29]
Hereinafter, a steel material having excellent workability according to an aspect of the present invention will be described in more detail.
[30]
[31]
The steel material having excellent workability according to an aspect of the present invention is, by weight, C: 0.8 to 1.0%, Si: 0.1 to 0.3%, Mn: 0.2 to 0.5%, Cr: 0.1 to 0.3%, P: 0.03% or less, S: 0.005% or less, the remaining Fe and other unavoidable impurities are included, and the microstructure is a ferrite single-phase structure containing spheroidized carbides, the average particle size of the carbides is 0.8 μm or less, and the number density of the carbides is 2 * 10 5 ~ It can be 7*10 5pcs/mm 2
[32]
[33]
Hereinafter, the alloy composition of the present invention will be described in more detail. Hereinafter, unless otherwise specified, % and ppm related to the content of the alloy composition are based on weight.
[34]
[35]
The steel material having excellent workability according to an aspect of the present invention is, by weight, C: 0.8 to 1.0%, Si: 0.1 to 0.3%, Mn: 0.2 to 0.5%, Cr: 0.1 to 0.3%, P: 0.03% or less, S: 0.005% or less, the remaining Fe and other unavoidable impurities may be included.
[36]
[37]
Carbon (C): 0.8~1.0%
[38]
Carbon (C) is a representative hardenability improving element, and in the present invention, it is an element essential to secure hardness after quenching. Therefore, the present invention may contain 0.8% or more of carbon (C) for this effect. A preferred carbon (C) content may be greater than 0.8%, and a more preferred carbon (C) content may be 0.82% or higher. On the other hand, when the carbon (C) content in the steel exceeds a certain range, the carbide fraction in the steel is excessively high, there is a risk of promoting brittle fracture. Therefore, the present invention may limit the upper limit of the carbon (C) content to 1.0%. A preferred carbon (C) content may be less than 1.0%, and a more preferred carbon (C) content may be 0.98% or less.
[39]
[40]
Silicon (Si): 0.1~0.3%
[41]
Since silicon (Si) is a component that contributes to the improvement of the strength of steel, the present invention may include 0.1% or more of silicon (Si) to achieve such an effect. A preferable lower limit of the silicon (Si) content may be 0.12%, and a more preferable lower limit of the silicon (Si) content may be 0.15%. However, when the silicon (Si) content in the steel exceeds a certain range, not only the cold rolling property is inferior, but also the possibility of decarburization during heat treatment increases, and since it may cause an increase in surface scale defects on the surface of the steel, the present invention provides silicon The upper limit of the (Si) content may be limited to 0.3%. 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%.
[42]
[43]
Manganese (Mn): 0.2-0.5%
[44]
Manganese (Mn) is an element that not only contributes to the improvement of hardenability, but also effectively contributes to the improvement of the strength of steel by solid solution strengthening. In addition, manganese (Mn) combines with sulfur (S) in steel and precipitates as MnS, so that red hot brittleness caused by sulfur (S) can be effectively prevented. The present invention may contain 0.2% or more of manganese (Mn) to achieve such an effect. A preferred lower limit of the manganese (Mn) content may be 0.25%, and a more preferred lower limit of the manganese (Mn) content may be 0.3%. However, when the manganese (Mn) content in the steel exceeds a certain range, cold rolling is poor as well as may cause a decrease in workability due to central segregation, so the present invention sets the upper limit of the manganese (Mn) content by 0.5% can be limited to A preferable upper limit of the manganese (Mn) content may be 0.45%, and a more preferable upper limit of the manganese (Mn) content may be 0.4%.
[45]
[46]
Chromium (Cr): 0.1~0.3%
[47]
Chromium (Cr), like manganese (Mn), is an element that effectively contributes to the improvement of hardenability. Therefore, the present invention may contain 0.1% or more of chromium (Cr) for this effect. A preferred lower limit of the chromium (Cr) content may be 0.13%, and a more preferred lower limit of the chromium (Cr) content may be 0.16%. However, when the chromium (Cr) content in the steel exceeds a certain range, not only the cold rolling ductility may decrease, but also the decomposition of the carbide by heat treatment is delayed, so the spheroidization of the carbide may not be completed even by the spheroidizing annealing. this exists Accordingly, the present invention may limit the upper limit of the chromium (Cr) content to 0.3%. A preferable upper limit of the chromium (Cr) content may be 0.28%, and a more preferable upper limit of the chromium (Cr) content may be 0.25%.
[48]
[49]
Phosphorus (P): 0.03% or less (including 0%)
[50]
Phosphorus (P) in steel is a typical impurity Although water is an element, it is also the most advantageous element for securing strength without significantly impairing formability. However, when phosphorus (P) is excessively added, the possibility of brittle fracture increases, which can cause plate breakage of the slab during hot rolling, and can also significantly degrade the surface properties of the coated steel sheet. Therefore, the present invention may limit the upper limit of the phosphorus (P) content to 0.03%.
[51]
[52]
Sulfur (S): 0.005% or less (including 0%)
[53]
Sulfur (S) is an impurity element that inevitably flows into steel, and it is desirable to keep its content as low as possible. In particular, since sulfur (S) in steel may cause red heat brittleness, the present invention may limit the upper limit of the sulfur (S) content to 0.005%.
[54]
[55]
The steel material having excellent workability according to one aspect of the present invention may include Fe and other unavoidable impurities in addition to the above components. However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in the normal manufacturing process, it cannot be entirely excluded. Since these impurities can be known to anyone skilled in the art, all of them are not specifically mentioned in the present specification. In addition, the addition of effective ingredients other than the above composition is not excluded.
[56]
[57]
The microstructure of the steel material according to one aspect of the present invention may be a ferrite single phase structure including spheroidized carbide. The spheroidized carbide of the present invention may mean not only a case where all carbides are spheroidized, but also a case where some of the carbides are spheroidized. The average particle 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 . That is, in the steel material according to one aspect of the present invention, since spheroidized carbide is finely formed in the steel and a large amount of carbide is evenly distributed, workability of the steel material can be effectively secured.
[58]
[59]
Carbide contained in the steel according to an aspect of the present invention may have a spheroidization rate of 95% or more, and a preferred carbide spheroidization rate may be 99% or more. Here, the carbide spheroidization ratio means the area ratio of spheroidized carbides having an aspect ratio (ratio of major axis and minor axis) of 2 or less to the area of ​​all carbides. That is, in the steel material according to one aspect of the present invention, since most of the carbides included in the steel material are spheroidized carbides, workability of the steel material can be effectively secured.
[60]
[61]
In addition, when the average grain size of the carbide contained in the steel material is below a certain level, it may mean that the spheroidization of the carbide is not sufficiently performed, and the present invention may limit the lower limit of the average grain size of the carbide to 0.55 μm.
[62]
[63]
Room temperature surface hardness of the steel according to one aspect of the present invention may be 230 ~ 270HV. In addition, the steel material having excellent workability according to one aspect of the present invention may have a burr height of 20 μm or less after press working, and the bending workability (R/t) of the steel material may be 2 or less. The height of the burr can be obtained by measuring the height difference of the surface edge with a roughness meter after blanking under the clearance condition of 5°. It can be measured according to whether cracks occur on the surface of the material when bent at 90°. t of bending workability means the steel thickness (mm).
[64]
[65]
Steel according to one aspect of the present invention, the above-described steel material is heated to 800 ~ 950 ℃ and maintained for a time of 30 minutes or less, cooled to a temperature range of less than 50 ℃ at a cooling rate of 50 ~ 150 ℃ / s, Surface hardness of the steel after heat treatment at 200 to 300 ° C. for 10 to 60 minutes may be 56 HRC or more. That is, the steel material according to one aspect of the present invention, while ensuring excellent workability before quenching, can secure excellent hardness characteristics after quenching.
[66]
[67]
Hereinafter, a method for manufacturing a steel having excellent workability according to an aspect of the present invention will be described in more detail.
[68]
[69]
In the method for manufacturing a steel having excellent workability according to an aspect of the present invention, 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, reheating, hot rolling, and winding a slab containing the remaining Fe and other unavoidable impurities; Segmenting the carbide of the steel by applying a mechanical external force to the wound steel; And after heating the steel material in which the carbide is segmented, the step of spheroidizing annealing by maintaining for 5 to 20 hours in a temperature range of 650 ~ 700 ℃; may include.
[70]
[71]
Slab reheating, hot rolling and winding
[72]
After preparing a slab provided with a predetermined alloy composition content, reheating may be performed. Since the alloy composition of the slab of the present invention corresponds to the alloy composition of the above-described steel material, the description of the alloy composition of the slab of the present invention is replaced with the description of the alloy composition of the above-described steel material. In addition, the slab reheating temperature of the present invention may be subject to conditions applied to normal slab reheating, but as a non-limiting example, the slab reheating temperature of the present invention may be in the range of 1000 to 1300 ° C.
[73]
[74]
A hot-rolled steel material may be provided by performing hot rolling on the reheated slab in a temperature range of 850 to 1150 ° C. When the hot rolling temperature is excessively high, since there is a problem in that desired physical properties cannot be secured due to coarsening of the microstructure, the present invention may limit the upper limit of the hot rolling temperature range to 1150 ° C. On the other hand, if the hot rolling temperature is less than a certain level, excessive rolling load may be a problem, so the lower limit of the hot rolling temperature may be limited to 850 ° C.
[75]
[76]
Hot-rolled steel can be wound in a temperature range of 600 to 650 ° C. When the coiling temperature is excessively high, the thickness of the cementite in the pearlite structure becomes thick, and shape defects may occur due to phase transformation after coiling, so the upper limit of the coiling temperature 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 about plate breakage in the process after winding, so the lower limit of the winding temperature may be limited to 600 ° C. In addition, in order to prevent sheet breakage due to material variation in the carbide segmentation step described later, the temperature variation in the longitudinal direction of the hot-rolled coil may be controlled to 20° C. or less.
[77]
[78]
Carbide fragmentation by application of mechanical force
[79]
After uncoiling the rolled steel material, a pickling process may be selectively applied according to the surface quality of the uncoiled steel material, and then carbide (lamella cementite) may be mechanically segmented by applying a mechanical external force to the steel material. As a method of applying a mechanical external force to the steel, any method may be used as long as the lamellar cementite can be segmented, and cold rolling or forging may be applied as a non-limiting example. As an example, when a mechanical external force is applied to steel by applying cold rolling, a cold reduction of 30 to 50% may be applied in consideration of effective segmentation of cementite.
[80]
[81]
In the case of the present invention, since the lamellar cementite is segmented by applying a mechanical external force to the hot-rolled steel, it is possible to effectively improve the spheroidizing efficiency in the spheroidizing annealing performed later. That is, in the present invention, since the spheroidization annealing is started in a state in which a large amount of finely segmented carbide is distributed, the carbide can be effectively spheroidized within a relatively short time.
[82]
[83]
spheroidization annealing
[84]
By applying a mechanical external force, spheroidization annealing may be performed by heating the carbide-segmented steel to a temperature range of 650 to 700 ° C and holding for 5 to 20 hours. If the spheroidization annealing temperature and time are less than a certain level, there is a risk that sufficient spheroidization of carbides may not be achieved, so the present invention may limit the lower limits of the spheroidization annealing temperature and time to 650 ° C. and 5 hours, respectively. On the other hand, when the temperature and time of spheroidization annealing exceed a certain level, not only the carbide becomes excessively coarse, but there is a concern about the decrease in the hardness of the steel. can be limited
[85]
[86]
The microstructure of the steel material manufactured by the above-described manufacturing method may be a ferrite single phase structure including spheroidized carbide. 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.
[87]
[88]
The room temperature surface hardness of the steel produced by the above-described manufacturing method is 230 to 270 HV, the burr height after press working is 20 μm or less, and the bending workability (R / t) may be 2 or less.
[89]
Mode for Carrying Out the Invention
[90]
Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the embodiments to be described later are only for exemplifying and more specific the present invention, and are not intended to limit the scope of the present invention.
[91]
[92]
(Example)
[93]
After preparing a slab provided with the alloy composition of Table 1, it was heated at a temperature range of 1200 ° C, and hot rolling was performed at a temperature range of 950 ° C. After pickling, cold rolling was performed at a reduction ratio of 50%, and the resultant was rolled into a thin steel sheet having a thickness of 1.0 mm. Rollability during cold rolling for each type of steel was evaluated and listed in Table 1 together. A case where plate breakage and edge cracks did not occur during cold rolling, or when less than 5 cracks with a size of less than 10 mm were formed even when edge cracks occurred was evaluated as ○. In addition, plate breakage and edge cracks occurred during cold rolling, and the case where the edge crack size was 10 mm or more or 5 or more cracks with a size of less than 10 mm were formed was evaluated as X.
[94]
[95]
[Table 1]
Steel type Alloy composition (% by weight) Rollability
C Si Mn Cr P S
1 0.84 0.23 0.37 0.22 0.0126 0.0023 ○
2 0.96 0.23 0.39 0.21 0.0118 0.0020 ○
3 0.60 0.21 0.37 0.21 0.0129 0.0022 ○
4 1.17 0.17 0.36 0.19 0.0121 0.0022 X
[96]
[97]
After Table 2 Spheroidization annealing was performed under the conditions of, and the hardness and microstructure of each specimen were compared and analyzed, and the results are shown in Table 2 together. However, specimen K means a specimen directly subjected to spheroidizing annealing without performing cold rolling. At this time, the hardness of each specimen was converted into HV after measuring HRC using a Brinell hardness tester, and the microstructure of each specimen was etched after specimen cutting and mirror polishing, and the cross-sectional structure was measured using a scanning electron microscope. Observed. In addition, the height of the burr was measured after press working was performed on each specimen under a clearance condition of 5%, and the bending workability (R/t) was measured by performing a 90° bending test. In addition, after the quenching treatment of heating and rapid cooling at a temperature of 900 ° C. and the tempering treatment of heating at a temperature of 250 ° C. were sequentially performed on each specimen, the surface hardness hardness was measured and the results are shown in Table 2 together. The surface hardness at this time was also measured for HRC using a Brinell hardness tester.
[98]
[99]
[Table 2]
Psalter
No steel grade annealing
temperature
(℃) Annealing
hour
(hr) nodularization rate
(%) Carbide
size
(μm) Carbide
number density
(pcs/mm 2) Annealed material
Hardness
(HV) Burr
Height
(μm) bending
machinability
(R/t) quenching
Hardness
(HRC)
A 1 660 15 100 0.58 6.9 x10 5 243 14 0 57.1
B 1 690 15 100 0.71 4.6 x10 5 232 12 0 56.5
C 2 660 15 99 0.62 6.6 x10 5 250 8 0 59.6
D 2 690 15 99 0.68 5.5 x 10 5 236 15 0 59.4
E 3 660 15 100 0.61 4.9 x 10 5 214 20 0 52.1
F 3 690 15 99 0.73 3.2 x 10 5 196 25 0 50.8
G 1 630 15 78 0.51 8.8 x10 5 264 2 3.5 56.2
H 1 720 15 100 0.93 2.7 x10 5 195 26 2.5 56.2
I 2 630 15 83 0.53 9.2 x10 5 275 3 8.0 59.4
J 2 720 15 100 0.89 3.2 x10 5 205 22 5.0 58.2
K 1 730 15 98 1.29 1.4 x 10 5 235 19 4.5 56.9
[100]
[101]
Specimens satisfying both the alloy composition and process conditions limited by the present invention satisfy both excellent hardness characteristics and workability, whereas in the case of specimens that do not satisfy any of the alloy composition and process conditions limited by the present invention, the present invention It can be seen that it is impossible to simultaneously secure the desired level of hardness characteristics and workability.
[102]
[103]
In the microstructure observation photograph of specimen A in FIG. 1, it can be seen that a large amount of spheroidized fine carbide is uniformly distributed. On the other hand, FIG. 2 is a microstructure observation photograph of specimen H, and it can be confirmed that not only the spheroidization rate of carbides is low, but also that coarse carbides are locally distributed.
[104]
[105]
Although the present invention has been described in detail through examples above, other types of embodiments are also possible. Therefore, the spirit and scope of the claims set forth below are not limited to the embodiments.

claims
[Claim 1]
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 rest Fe and other unavoidable impurities Including, the microstructure is a ferrite single-phase structure containing spheroidized carbide, the average grain size of the carbide is 0.8㎛ or less, the number density of the carbide is 2 * 10 5 ~ 7 * 10 5 / mm 2 , excellent workability steel.
[Claim 2]
According to claim 1, wherein the spheroidization rate of the carbide is 95% or more, excellent workability steel.
[Claim 3]
According to claim 1, The room temperature surface hardness of the steel material is 230 ~ 270HV, excellent workability steel material.
[Claim 4]
The steel material having excellent workability according to claim 1, wherein the steel material has a burr height of 20 μm or less after press working, and a bending workability (R/t) of the steel material is 2 or less.
[Claim 5]
According to claim 1, The average grain size of the carbide is 0.55㎛ or more, excellent workability steel.
[Claim 6]
The steel material according to any one of claims 1 to 5 is heated to 800 ~ 950 ℃ and maintained for 30 minutes or less, cooled to a temperature range of 50 ℃ or less at a cooling rate of 50 ~ 150 ℃ / s, 200 A steel material with excellent workability, wherein the surface hardness of the steel material after heat treatment at ~300 ° C. for 10 to 60 minutes is 56 HRC or more.
[Claim 7]
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 rest Fe and other unavoidable impurities Reheating, hot rolling, and winding the slab comprising; Segmenting the carbide of the steel by applying a mechanical external force to the wound steel; And After heating the carbide-segmented steel material, maintaining it at a temperature range of 650 ~ 700 ℃ for 5 to 20 hours to perform spheroidizing annealing; Method for producing a steel material having excellent workability, including.
[Claim 8]
The method of claim 7, wherein in the step of segmenting the carbide, the carbide of the steel is segmented by cold rolling the coiled steel at a reduction ratio of 30 to 50%.
[Claim 9]
The method of claim 7, wherein 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 material is heated in a temperature range of 600 to 650 ° C Winding from, a method for producing steel with excellent workability.