Specification
Title of invention: high strength, low toughness cold rolled steel sheet with excellent breaking characteristics and its manufacturing method
Technical field
[One]
The present invention relates to the manufacture of a high-strength, low-toughness cold-rolled steel sheet that is mainly used as a flat tie for construction (nominal tie, long formwork) and is used as a variety of other long-lasting materials. As a result , high-strength, low-toughness cold-rolled steel sheet with a final thickness of 1.5mmt~3.0mmt, tensile strength of 950MPa or more, and room temperature impact toughness (Charpy absorbed energy) of 1.0~5.0J (0.05~0.35J/cm 2 ) based on 2.0mmt thickness , and its manufacturing method. It is about.
[2]
Background
[3]
Flat ties are also called nominal wall ties for long formwork. Flat tie is a tension member that maintains the formwork at regular intervals for construction purposes and finally supports the lateral pressure after concrete placement. In the manufacturing process, a product is produced by slitting and pressing a cold rolled material suitable for the final thickness. Since the required characteristics must support lateral pressure, a tensile strength of 950 MPa or more is required based on the raw material of cold rolled material. In addition, the flat tie part protruding out of the formwork after construction at different wall thicknesses (intervals between the forms) of the construction site should be easily removed with a hammer. In general, the flat tie protrusion must be hit once with a hammer to cut the buried part and the protrusion, and the fracture surface must be cut cleanly in a single shape. In order to satisfy these characteristics, the raw material must have low impact toughness. Because of these unique properties required for flat tie, the raw material also requires unique properties of high strength and low toughness, not the high strength and high toughness required of general steel materials.
[4]
Various methods exist in order to realize the unique low toughness characteristic of flat tie, but there should be no problem in hot rolled steel sheet production due to too low impact toughness, and production processing should be possible without cracking until final press processing. In addition, flat tie is a consumable product and is a product that is buried after final construction, so low-cost design/production should be possible. The characteristics of flat tie requiring such low toughness are not high strength and high toughness required by general steels, so there is no related prior art for realizing low toughness in particular.
[5]
(Prior art)
[6]
(Patent Document 1) Korean Patent Application No. 10-1998-0059176 (applied on December 28, 1998)
[7]
Detailed description of the invention
Technical challenge
[8]
Accordingly, the present invention has been devised in order to solve the limitations of the prior art described above, and an object thereof is to provide a high-strength, low-toughness cold-rolled steel sheet for flat tie and a manufacturing method thereof by controlling a steel composition component and hot rolling and cold rolling processes.
[9]
In addition, the technical problems to be achieved in the present invention are not limited to the technical problems mentioned above, and other technical problems that are not mentioned are clearly understood by those of ordinary skill in the technical field to which the present invention belongs from the following description. Can be.
[10]
Means of solving the task
[11]
The present invention for achieving the above object is, by weight, C: 0.30 to 0.70%, Mn: 0.2 to 1.0%, Si: 0.005 to 0.5%, P: 0.005 to 0.02%, S: 0.01% or less, Al: 0.01 to 0.1%, Cr: 0.005 to 0.1%, contains the balance iron (Fe) and other inevitable impurities, the steel microstructure is composed of 50 to 95% pearlite and residual ferrite, the average grain size of the ferrite structure Is 10 to 50㎛, the average size of the pearlite colonies is 10 to 50㎛, has a thickness of 1.5mmt to 3.0mmt, and satisfies the room temperature impact toughness (Charpy absorbed energy) 1.0 to 5.0J (0.05 to 0.35J/cm 2 ) It relates to a high-strength low-toughness cold-rolled steel sheet.
[12]
[13]
The cold rolled steel sheet may satisfy a yield strength of 700 to 950 MPa, a tensile strength of 950 to 1200 MPa, and an elongation of 2 to 12%.
[14]
[15]
In addition, the present invention,
[16]
In% by weight, C: 0.30 to 0.70%, Mn: 0.2 to 1.0%, Si: 0.005 to 0.5%, P: 0.005 to 0.02%, S: 0.01% or less, Al: 0.01 to 0.1%, Cr: 0.005 to 0.1 %, and preparing a steel slab containing the balance iron (Fe) and other inevitable impurities;
[17]
Reheating step of heating the steel slab to a temperature of 1100 ~ 1300 ℃;
[18]
After rough-rolling the reheated slab at 1000 to 1100°C, finishing hot rolling at a temperature range of 850 to 950°C;
[19]
Cooling the hot-rolled steel sheet at a rate of 10 to 200°C/s, and then winding it at a temperature range of 550 to 750°C;
[20]
After pickling the wound steel sheet, the steel microstructure is composed of 50-95% pearlite and residual ferrite by cold rolling at a reduction ratio of 50-70%, and the average grain size of the ferrite structure is 10-50 μm. It relates to a method for manufacturing a high-strength low-toughness cold-rolled steel sheet comprising; the step of preparing a cold-rolled steel sheet having a thickness of 1.5mmt to 3.0mmt having an average size of the pearlite colonies of 10 to 50 μm.
[21]
[22]
The cold rolled steel sheet has a yield strength of 700 to 950 MPa, a tensile strength of 950 to 1200 MPa, an elongation of 2 to 12%, and a room temperature impact toughness (Charpy absorbed energy) of 1.0 to 5.0 J (0.05 to 0.35 J/cm 2 ). have.
[23]
[24]
The wound hot-rolled steel sheet may have a thickness of 2.5 to 4.5 mmt.
[25]
Effects of the Invention
[26]
The present invention having the configuration as described above can effectively provide a high-carbon steel having high strength and low toughness for flat tie for construction and for other tightening by optimizing the range of steel components and manufacturing process conditions.
[27]
Brief description of the drawing
[28]
1 is a photograph of a microstructure of Inventive Example 1 in an embodiment of the present invention.
[29]
Best mode for carrying out the invention
[30]
Hereinafter, the present invention will be described.
[31]
The high-strength low-toughness cold-rolled steel sheet of the present invention, by weight, C: 0.30 to 0.70%, Mn: 0.2 to 1.0%, Si: 0.005 to 0.5%, P: 0.005 to 0.02%, S: 0.01% or less, Al: 0.01 ~0.1%, Cr:0.005~0.1%, contains the balance iron (Fe) and other inevitable impurities, the steel microstructure is composed of 50~95% pearlite and residual ferrite, and the average grain size of the ferrite structure is It is 10-50㎛, the average size of the pearlite colonies is 10-50㎛, the thickness is 1.5mmt ~ 3.0mmt, and satisfies the room temperature impact toughness (Charpy absorbed energy) 1.0 ~ 5.0J (0.05 ~ 0.35J / cm 2 ) .
[32]
That is, the present invention is characterized by providing very low toughness in order to secure excellent fracture characteristics of the flat tie.To this end, based on the final cold-rolled material, the pearlite fraction is 50 to 95%, and the average grain size of the ferrite structure is 10 to By setting the average size of 50 μm and pearlite colonies to 10 to 50 μm, very coarse ferrite grains and pearlite colony sizes were secured, resulting in low toughness. Specifically, the cold-rolled steel sheet of the present invention has a thickness of 1.5mmt to 3.0mmt, a yield strength of 700 to 950 MPa, a tensile strength of 950 to 1200 MPa, an elongation of 2 to 12%, and a room temperature impact toughness (Charpy absorbed energy). This satisfies 1.0~5.0J (0.05~0.35J/cm 2 ).
[33]
[34]
Hereinafter, the alloy components of the cold-rolled steel sheet of the present invention and reasons for limiting the content thereof will be described.
[35]
Carbon (C): 0.30 to 0.70% by weight
[36]
Carbon is an element that affects strength and toughness. When the carbon content is less than 0.30% by weight, it is difficult to secure a target strength. On the other hand, when the carbon content exceeds 0.7% by weight, there is a problem in that the formability is deteriorated due to excessive increase in strength and formation of cementite. In addition, since it is necessary to form a single-shape clean fracture surface upon fracture, excessive cementite formation may adversely affect fracture characteristics. Therefore, the carbon content is preferably limited to 0.30 ~ 0.70% by weight.
[37]
[38]
Manganese (Mn): 0.2 to 1.0% by weight
[39]
Manganese is a solid solution strengthening element and is added to increase the strength and prevent the slab red heat embrittlement due to the formation of FeS. For this effect, 0.2% by weight or more should be added, and when it is contained in an amount exceeding 1.0% by weight, central segregation and micro-segregation are severe, and the final carbide becomes coarse. In the steel for flat tie, where low cost design is important, the manganese content is limited to 0.2 to 1.0% by weight because excessive Mn addition increases the cost.
[40]
[41]
Silicon (Si): 0.005 to 0.5% by weight
[42]
Silicon has an effect of improving the strength by solid solution strengthening. If it is less than 0.005% by weight, the effect of improving the strength is insufficient, and if a large amount is added, the surface quality is adversely affected due to an increase in red-scale defects. Accordingly, the content of silicon is preferably limited to 0.005 to 0.5% by weight.
[43]
[44]
Phosphorus (P): 0.005 to 0.02% by weight
[45]
Phosphorus is an element that has a large solid solution strengthening effect. In order to secure strength, phosphorus should be added at least 0.005% by weight, whereas if it exceeds 0.02% by weight, there is a problem of impairing workability by P segregation, so the lower and upper limits are limited to 0.005% by weight and 0.02% by weight, respectively.
[46]
[47]
Sulfur (S): 0.01% by weight or less
[48]
Sulfur is an element that tends to form non-metallic inclusions, and since it is an impurity that increases the amount of precipitates, it is necessary to keep the sulfur content low. Accordingly, the upper limit is limited to 0.01% by weight, and the lower the sulfur content is, the better the moldability is, so the lower limit is not limited.
[49]
[50]
Aluminum (Al): 0.01 to 0.1% by weight
[51]
Aluminum is mainly added to deoxidize and trap nitrogen into AlN. Since the aluminum content is less than 0.01% by weight and cannot achieve the purpose of addition, the addition amount is 0.1% by weight or more, and excessive strength increase and slab defect problems may occur during playing, the content is limited to 0.01 to 0.1% by weight.
[52]
[53]
Chrome (Cr): 0.005 to 0.1% by weight
[54]
Chromium should be added at least 0.005% by weight for solid solution strengthening effect. On the other hand, if it is added in excess of 0.1% by weight, central segregation and unnecessary inclusions may be formed, and it is preferable to limit the upper limit to 0.1% by weight because it also increases the cost.
[55]
[56]
The cold rolled steel sheet of the present invention consists of the above composition, and the remaining component not mentioned is iron (Fe). In addition, impurities that may inevitably be mixed in a conventional manufacturing process cannot be excluded, but this is known to anyone of ordinary skill in the art, and thus is not specifically mentioned in the present specification.
[57]
[58]
On the other hand, the cold rolled steel sheet of the present invention is composed of 50 to 95% of pearlite and residual ferrite. And the average grain size of the ferrite structure is 10 ~ 50㎛, the average size of the pearlite colonies 10 ~ 50㎛ by securing the size of very coarse ferrite grains and pearlite colonies, having low toughness, thickness of 1.5mmt ~ 3.0mmt A cold rolled steel sheet can be effectively provided.
[59]
The steel microstructure of the cold-rolled steel sheet presented in the present invention is a mixed structure of pearlite and ferrite. The pearlite has higher strength than ferrite, but lacks toughness, so that when an external impact is applied, crack formation and propagation are much easier than that of ferrite. Therefore, in the present invention, it is possible to secure a low impact toughness between 1 and 5J only when 50 to 95% of pearlite is secured in the microstructure of the final cold-rolled steel sheet. In addition, it is more advantageous to realize low toughness when the average pearlite colony size is 5 to 40 μm and the average grain size of the ferrite structure is 10 to 50 μm.
[60]
That is, the steel sheet of the present invention having the steel microstructure as described above has a yield strength of 700 to 950 MPa, a tensile strength of 950 to 1200 MPa, an elongation of 2 to 12%, and a room temperature impact toughness (Charpy absorbed energy) of 1.0 to 5.0. J(0.05~0.35J/cm 2 ) can be satisfied.
[61]
[62]
Next, a method of manufacturing a high-strength low-toughness cold-rolled steel sheet of the present invention will be described.
[63]
The method for manufacturing a cold-rolled steel sheet having high strength and low toughness of the present invention comprises: preparing a steel slab having the composition as described above; Reheating step of heating the steel slab to a temperature of 1100 ~ 1300 ℃; After rough rolling the reheated slab at 1000 to 1100°C, finishing hot rolling at a temperature range of 850 to 950°C; Cooling the hot-rolled steel sheet at a rate of 10 to 200°C/s, and then winding it at a temperature range of 550 to 750°C; After pickling the wound steel sheet, the steel microstructure is composed of 50-95% pearlite and residual ferrite by cold rolling at a reduction ratio of 50-70%, and the average grain size of the ferrite structure is 10-50 μm. And manufacturing a cold-rolled steel sheet having a thickness of 1.5mmt to 3.0mmt having an average size of the pearlite colonies of 10 to 50 μm.
[64]
[65]
Slab reheating and hot rolling step
[66]
In the present invention, first, the steel slab having the alloy composition as described above is reheated, and at this time, the reheating temperature is preferably between 1100°C and 1300°C, which is a typical level. If the temperature is less than 1100℃, it is difficult to secure sufficient temperature of the slab plate required for mail order. If it exceeds 1300℃, abnormal austenite growth and surface defects due to scale may occur. It is desirable.
[67]
Subsequently, in the present invention, hot rolling is performed on the reheated slab as described above. That is, after going through a conventional rough rolling process between 1000 and 1100°C, hot rolling is finished. At this time, in the present invention, the hot finish rolling is preferably performed between 850 and 950°C, and more preferably between 900 and 950°C. At a temperature of 900°C or higher, austenite grains can be grown to coarsen the final ferrite grains and pearlite colonies. The hot-rolled finishing should be done at the Ar3 transformation point or higher, because this is to prevent two-phase rolling, and when the two-phase rolling is performed, cornerstone ferrite without carbides is generated. In addition, a finishing temperature of 850°C or less has a large rolling load, which makes subsequent processes difficult, and a finishing temperature of 950°C or higher may cause scalability defects on the surface, so the hot rolling finishing temperature is limited to 850-950°C.
[68]
[69]
Cooling and winding stage
[70]
Cool the hot-rolled steel sheet as above. At this time, the cooling rate is limited to the range of 10 ℃ / s ~ 200 ℃ / s. By cooling at the above cooling rate and maintaining a little more on a run-out table (ROT), pearlite transformation, colony size, and ferrite grain size can be maximized. At a cooling rate of less than 10℃/s, it is difficult to secure a pearlite fraction of 50% or more due to insufficient time to maintain on the ROT, and a cooling rate exceeding 200℃/s makes it difficult to achieve uniform cooling due to uneven temperature in the width direction. The shape of the coil can be very bad. Therefore, the cooling rate is preferably limited to 10 ~ 200 ℃ / s.
[71]
Subsequently, the cooled hot-rolled steel sheet is wound between 550 and 750°C, and more preferably maintains a high winding temperature around 700°C. The reason why the coiling temperature is limited to 550 to 750°C is that the temperature section is a section that can make the pearlite colony size the most coarse. Specifically, if the coiling temperature is less than 550°C, a bainite or martensite structure, which is a low-temperature transformation structure, is produced, so that a uniform pearlite cannot be obtained, whereas if the coiling temperature exceeds 750°C, surface defects such as scale may be severely generated. to be.
[72]
[73]
Pickling and cold rolling steps
[74]
The wound hot-rolled coil is pickled. When pickling, the temperature is naturally cooled in the range of room temperature to 200℃, and then the scale is removed by pickling. At this time, if the pickling temperature of the hot-rolled steel sheet exceeds 200°C, the surface layer portion of the hot-rolled steel sheet is over-pickled and the roughness of the surface layer portion is deteriorated. Therefore, the pickling temperature is limited to room temperature to 200°C.
[75]
Then, the pickled hot-rolled steel sheet is cold-rolled at a reduction ratio of 50 to 70%. Since the tensile strength of the cold-rolled steel sheet is proportional to the rolling reduction rate, the tensile strength of the final cold-rolled steel sheet is 950 MPa or more when the rolling reduction rate is high. Therefore, for a tensile strength of 950 MPa or more, a cold reduction ratio of 50% or more is required. However, since excessive reduction ratio increases the load of the facility and cannot be produced, the upper limit of the reduction ratio is set at 70% in consideration of the rolling load and production efficiency. In cold-rolled materials, the impact toughness has a maximum value at a specific reduction rate, and the impact toughness decreases at low or high pressure reduction rates. For 0.3~0.7C steel grades, the impact toughness is usually maximum around 40% of the cold reduction ratio, and the impact toughness decreases again when the low reduction ratio of 30% or the high pressure reduction ratio of 70% is achieved. This is a factor related to the formation of shear-lip and is a general characteristic of cold rolled materials. In the present invention, since it is necessary to secure high strength and low toughness, a reduction ratio of 50% or more is advantageous. Preferably, in terms of securing strength and low toughness, a high pressure drop rate of 50 to 70% is used.
[76]
[77]
The cold-rolled steel sheet manufactured through the above cold rolling is composed of 50-95% pearlite and residual ferrite in the steel microstructure. And since the average grain size of the ferrite structure satisfies the range of 10 to 50 μm and the average size of pearlite colonies in the range of 10 to 50 μm, very coarse ferrite grains and pearlite colony sizes may be secured, thereby maintaining low toughness.
[78]
That is, the cold-rolled steel sheet of the present invention having the steel microstructure as described above has a yield strength of 700 to 950 MPa, a tensile strength of 950 to 1200 MPa, an elongation of 2 to 12%, and a room temperature impact toughness (Charpy absorbed energy) of 1.0 to 5.0J (0.05~0.35J/cm 2 ) can be satisfied.
[79]
Mode for carrying out the invention
[80]
Hereinafter, the present invention will be described in more detail through examples.
[81]
(Example)
[82]
[Table 1]
Steel grade C Mn Si P S Al Cr Remark
One 0.21 0.64 0.16 0.011 0.005 0.06 0.05 Comparative steel
2 0.50 0.15 0.20 0.013 0.004 0.04 0.06 Comparative steel
3 0.82 0.59 0.23 0.012 0.004 0.03 0.07 Comparative steel
4 0.55 0.70 0.10 0.012 0.005 0.03 0.05 Invention
5 0.25 0.15 0.15 0.011 0.004 0.04 0.06 Comparative steel
6 0.45 1.15 0.19 0.012 0.005 0.03 0.07 Comparative steel
7 0.82 1.20 0.21 0.011 0.004 0.03 0.05 Comparative steel
[83]
* In Table 1, the content unit of each component element is an increase in %.
[84]
After reheating the steel slab satisfying the alloy component system described in Table 1 at 1200° C. for 2 hours, it was hot-rolled under the conditions shown in Table 2 below, and at this time, the thickness of the hot-rolled material is also shown in Table 2 below. As shown in Table 2, the finish was hot-rolled, cooled at a cooling rate of 20 to 50°C/s to a coiling temperature (CT), and then wound at a coiling temperature (CT). Thereafter, the wound hot-rolled coil was pickled and then cold-rolled under the conditions shown in Table 2.
[85]
[Table 2]
[86]
By observing the microstructure of the cold-rolled steel sheet prepared as described above, the average ferrite grain size (µm), pearlite fraction (%), and pearlite colony size (µm) were measured, and the results are shown in Table 3 below. In addition, the yield strength (YS), tensile strength (TS) and elongation (El) of the prepared cold-rolled steel sheet were measured, and the results are also shown in Table 3 below. In this example, the tensile strength is a value obtained by taking a tensile test according to JIS No.5 standard based on the vertical direction of the rolling direction of the rolled plate, and the impact toughness is a value measured by converting the thickness to 1.9mmt by a V-notch Charpy impact test at room temperature. Is shown.
[87]
[Table 3]
[88]
As shown in Table 1-3, steel grade 1 is a steel grade whose C content is less than the component range of the present invention. Comparative Example 1-2 prepared with a component system of steel type 1 satisfies the scope of the present invention in terms of FDT, CT, cold rolling reduction ratio, etc. It was not good at 893 MPa and 910 MPa, respectively. In addition, the impact toughness Comparative Example 1-2 did not satisfy the allowable range of 1.0 to 5.0J as 22J and 19J, respectively.This is because the carbon content is low, so sufficient strength cannot be secured. It is analyzed that this is because it is difficult.
[89]
[90]
Steel grade 2 is a steel grade whose Mn content is less than the range of components of the present invention. Comparative Example 3-4 prepared with a component system of steel type 2 satisfies the scope of the present invention in terms of FDT, CT, and cold rolling reduction ratio, but the Mn content is outside the allowable range of the present invention and the tensile strength of the final material is 920, respectively. MPa, 931 MPa was not good. This is because the Mn content is low, so it is impossible to secure sufficient strength.
[91]
[92]
Steel grade 3 is a steel grade whose C content exceeds the component range of the present invention. Comparative Example 5-6 prepared with a component system of steel type 3 satisfies the scope of the present invention in terms of FDT, CT, and cold rolling reduction ratio, but the carbon content exceeds the allowable range of the present invention, so that the yield strength of the final material is respectively 980 MPa and 976 MPa did not satisfy the acceptable range of 750 to 950 MPa of the present invention. In addition, the tensile strength of Comparative Examples 5-6 was 1240 MPa and 1283 MPa, respectively, out of the allowable range of 950 to 1200 MPa, which is because the strength was increased too much due to excessive addition of carbon.
[93]
[94]
Steel grade 4 is a steel grade that satisfies the component range of the present invention. In Comparative Example 7-10 having a component system of steel type 4, the FDT, CT, and cold rolling reduction ratio did not satisfy the conditions of the present invention, and thus the final required material was not satisfied. Specifically, Comparative Example 7 had an FDT of 830°C, out of the range of 850 to 950°C, which is a condition of the present invention, and the final material had an excessively high impact toughness of 21J, making it impossible to implement low toughness. In Comparative Example 8, the CT was 520°C, which was out of the range of 550 to 750°C, which is the condition of the present invention, and the final material also had an impact toughness of 18J, which was excessively high, making it impossible to implement low toughness. In Comparative Example 9, the cold rolling reduction ratio was 16%, out of the 50-70% condition of the present invention, and the final material also had a tensile strength of 915 MPa, which did not satisfy the allowable tensile strength range of 950-1200 MPa. And Comparative Example 10 had a cold rolling reduction ratio of 76%, out of 50-70%, the condition of the present invention, and the final material also had a yield strength of 1030 MPa, which did not satisfy the allowable yield strength range of 700-950 MPa, and the tensile strength was also It did not satisfy the allowable range of 950 to 1200 MPa with 1278 MPa.
[95]
Inventive Examples 1 to 3 can be seen that by satisfying the hot rolling conditions and cold rolling conditions presented in the present invention, it is possible to manufacture high-strength, low-toughness high-carbon steel that satisfies the given final material requirements. The final microstructure picture of the steel species is shown.
[96]
[97]
And steel grade 5 is the case where the C and Mn components are less than the scope of the present invention, steel grade 6 is the case where the C content satisfies the scope of the present invention but the Mn content is exceeded, and the steel grade 7 is the target content of both C and Mn content. Shows the steel type when it exceeds the range. Comparative Examples 11-13 are cases where the steel composition components were manufactured through the manufacturing process (hot rolling operating conditions and cold rolling conditions) conditions of the present invention using each of the steel grades 5-7 outside the scope of the present invention, all of the present invention It can be seen that it does not satisfy the strength/toughness goals required by.
[98]
[99]
On the other hand, in the present invention, the value of impact toughness at room temperature (Shaffie absorbed energy) required by the final product is in the range of 1 to 5J, and breaking/cutting characteristics deteriorate when it exceeds 5J. Therefore, in the present invention, the low-temperature toughness was referred to as the low-temperature toughness compared to the range of 1 to 5J, which has excellent fracture/cutting characteristics, in comparison to more than 5J.
[100]
In general, in order for the steel plate to have good breaking characteristics, the breaking surface should not be uneven when cutting the steel plate by hammer strike, and it should be neatly cut in a single shape. Therefore, the breaking characteristic in the present invention means that the cutting surface can be cut with one hit of a hammer, and the cutting surface is cut in a single shape as if cut with a knife, and this is a characteristic obtained when the impact toughness value is between 1 and 5 J. If the impact toughness value is higher than 5J, it is difficult to cut with a single blow, and the fracture surface is increased due to ductile fracture, so it is not cut neatly.
[101]
[102]
As described above, in the detailed description of the present invention, preferred embodiments of the present invention have been described, but those of ordinary skill in the art to which the present invention pertains, various modifications without departing from the scope of the present invention Of course this is possible. Therefore, the scope of the present invention is limited to the described embodiments and should not be determined, and should not be determined by the claims to be described later, as well as those equivalent thereto.
Claims
[Claim 1]
In% by weight, C: 0.30 to 0.70%, Mn: 0.2 to 1.0%, Si: 0.005 to 0.5%, P: 0.005 to 0.02%, S: 0.01% or less, Al: 0.01 to 0.1%, Cr: 0.005 to 0.1 %, contains the balance iron (Fe) and other inevitable impurities, the steel microstructure is composed of 50-95% pearlite and residual ferrite, the average grain size of the ferrite structure is 10-50㎛, the pearlite colonies High-strength low-toughness cold-rolled steel sheet with an average size of 10 to 50㎛, thickness of 1.5mmt to 3.0mmt, and satisfies the room temperature impact toughness (Charpy absorbed energy) of 1.0 to 5.0J (0.05 to 0.35J/cm 2 ).
[Claim 2]
The high-strength low-toughness cold-rolled steel sheet according to claim 1, wherein the cold-rolled steel sheet satisfies a yield strength of 700 to 950 MPa, a tensile strength of 950 to 1200 MPa, and an elongation of 2 to 12%.
[Claim 3]
In% by weight, C: 0.30 to 0.70%, Mn: 0.2 to 1.0%, Si: 0.005 to 0.5%, P: 0.005 to 0.02%, S: 0.01% or less, Al: 0.01 to 0.1%, Cr: 0.005 to 0.1 %, and preparing a steel slab containing the balance iron (Fe) and other inevitable impurities; Reheating step of heating the steel slab to a temperature of 1100 ~ 1300 ℃; After rough-rolling the reheated slab at 1000 to 1100°C, finishing hot rolling at a temperature range of 850 to 950°C; Cooling the hot-rolled steel sheet at a rate of 10 to 200°C/s, and then winding it at a temperature range of 550 to 750°C; After pickling the wound steel sheet, the steel microstructure is composed of 50-95% pearlite and residual ferrite by cold rolling at a reduction ratio of 30-70%, and the average grain size of the ferrite structure is 10-50 μm. , Preparing a cold-rolled steel sheet having a thickness of 1.5mmt to 3.0mmt having an average size of the pearlite colonies of 10 to 50 μm.
[Claim 4]
4. cm 2 ) A method of manufacturing a high-strength low-toughness cold-rolled steel sheet, characterized in that satisfying.