Hot-rolled steel sheet, steel pipe, and component having excellent cold bending properties after heating and quenching-tempering heat treatment, and manufacturing method thereof

Abstract

The present invention provides a hot-rolled steel sheet, steel pipe, and member having excellent cold bendability after heating and quenching-tempering heat treatment, and a method for manufacturing the same. The present invention relates to a hot-rolled steel sheet, steel pipe, and member containing, by weight percent, C: 0.20% or more and less than 0.3%, Mn: 0.5 to 1.3%, Si: 0.3% or less (excluding 0%), P: 0.03% or less (including 0%), S: 0.004% or less (including 0%), Al: 0.04% or less (excluding 0%), Cr: 0.3% or less, Ni: 0.1 to 0.4%, Ti: 0.05% (including 0%), B: 0.0005 to 0.0050%, N: 0.01% or less (excluding 0%), with the remainder being Fe and other impurities, and satisfying Relational Formula 1-3.

Description

[Technical field]

[0001] The present invention relates to a hot-rolled steel sheet used for automobile body components such as automobile suspension parts, a steel pipe and a member using the same, and a manufacturing method thereof; and more specifically, to a hot-rolled steel sheet having an excellent hardness difference (ΔHv) value of less than 15 between the surface layer of the steel sheet and a position at t / 4 (t is the thickness of the steel sheet) and exhibiting high strength of 1300 MPa or more and a maximum bending angle of 40 degrees or more after heating and quenching-tempering heat treatment, a steel pipe, a member using the same, and a manufacturing method thereof. [Background technology]

[0002] Among automobile chassis parts, suspension parts are one of the parts that require fatigue resistance, and high-strength thin hot-rolled steel sheets are preferred due to the trend toward lighter parts.

[0003] Meanwhile, such suspension parts are generally manufactured by hot or cold forming and heat treating pipe-shaped materials, but a new manufacturing method has been proposed in which ultra-high strength steel pipes that have been heat-treated unlike conventional methods are manufactured and then cold-formed to manufacture parts in order to achieve environmentally friendly manufacturing and cost reduction. This method involves manufacturing steel pipes with high strength of 1000 MPa or more into parts of a desired shape by applying an external bending force in a cold (≦25°C) or warm (≦550°C) state, and in this case, it is necessary for the heat-treated steel plate or steel pipe itself to exhibit high resistance to bending crack formation or high ductility.

[0004] As a method for improving the bending formability of the steel sheet itself, research has been conducted on uniformly controlling the microstructure of ferrite and pearlite in the final cold-rolled steel sheet, regardless of whether heat treatment is applied, or on controlling the size distribution of prior austenite in 95% or more of the martensite phase and the size distribution of carbides remaining in the martensite.

[0005] Patent Document 1 proposes a method for improving the bendability of steel sheets for hot forming in order to solve the problem of reduced bendability associated with increased strength of hot press formed products. It proposes that by controlling the Mn / Si ratio in the range of 0.05 to 2.0 and adding 0.5% or more silicon (Si) element to steel used for cold rolled steel sheets and performing cold rolling annealing heat treatment, the microstructure of the cold rolled steel sheet, specifically the pearlite phase, can be uniformly distributed, and by performing painting heat treatment on the above cold rolled steel sheet after hot press forming, a retained austenite phase can be formed in the martensite structure, thereby improving the bendability. That is, the document suggests adding 0.5% or more of silicon (Si) to steel used as cold-rolled steel sheet, but in order to directly apply this to steel tube-shaped chassis parts, a large amount of silicon oxide formed in the butt weld or fusion zone during electric arc welding must be effectively discharged. If the oxide cannot be effectively discharged, welding defects may occur, which may reduce formability such as the flattening or expansion characteristics of the steel tube, and therefore there are limitations on use.

[0006] In Patent Document 2, the number of fine carbides of 25 to 60 nm in size in a martensite single-phase structure consisting of fine prior austenite (PAGS) of 5 to 6 μm in size is 2The authors propose a method for controlling the number of particles per area to 0 to 500,000 or less and improving the bendability (limit bending radius (R) / thickness (t)≦2.4) of high-strength cold-rolled steel sheets of 1470 MPa or more. On the other hand, the improvement in bendability or extremely low measured value of R / t of the high-strength cold-rolled steel sheets is considered to be mainly due to the fine size of prior austenite, which can be understood as being due to the delay in the formation of crack sites or the propagation of cracks caused by external bending force at prior austenite grain boundaries or carbide / martensite interfaces. In particular, although the tensile strength of the cold-rolled steel sheets is high at 1470 MPa or more due to the very fine martensite structure, it is expected to show a low elongation of less than 6%, and therefore the formability is considered to be insufficient for manufacturing parts having complex shapes. Meanwhile, even in the case of the high strength steel, a large amount of silicon (Si) of 1.0% or more is added in order to control the size of carbides that precipitate and grow during tempering heat treatment of the martensite structure formed by rapid cooling at 100°C / sec or more, and it is expected that it will be difficult to apply it to steel pipes or steel pipe parts.

[0007] Therefore, from the consideration of the manufacturing process of the steel plate and steel parts proposed in the above patent documents, there is no proposal for a hot-rolled steel plate, steel pipe, member, or manufacturing method thereof, which is a steel plate that can be manufactured into a stabilizer part of a desired shape by bending an electric resistance welded steel pipe or drawn steel pipe that has been previously subjected to heating and quenching-tempering heat treatment in a cold (≦25° C.) state with an external force, and which has a hardness difference (ΔHv) value of less than 15 between the surface layer part and a position at / 4 of the thickness of the steel plate, and which shows a high strength of 1300 MPa or more (elongation of 8% or more) and a maximum bending angle of 40° or more after heat treatment. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Korean Patent No. 10-1568549 [Patent Document 2] Korean Patent Publication No. 10-2015-0105476 Summary of the Invention [Problem to be solved by the invention]

[0009] A preferred embodiment of the present invention provides a hot-rolled steel sheet for cold-formed parts, which has a small hardness difference between the surface layer and the quarter-thickness position of the steel sheet (high decarburization resistance), and which exhibits high strength and excellent bendability with a maximum bending angle of 40° or more after heating and quenching-tempering heat treatment, and a manufacturing method thereof.

[0010] In addition, one preferred embodiment of the present invention provides a steel pipe for cold forming components, which is manufactured using a hot-rolled steel sheet that has a small hardness difference between the surface layer and the quarter thickness position of the steel sheet (high decarburization resistance), and exhibits high strength and excellent bendability with a maximum bending angle of 40° or more after heating and quenching-tempering heat treatment, and a manufacturing method thereof.

[0011] Furthermore, one preferred embodiment of the present invention provides a cold-formed member manufactured using a steel pipe that has a small hardness difference between the surface layer and the quarter-thickness position of the steel plate (high decarburization resistance), and that exhibits high strength and excellent bendability with a maximum bending angle of 40 degrees or more after heating and quenching-tempering heat treatment, and a manufacturing method thereof.

[0012] Furthermore, the technical problems to be achieved by the present invention are not limited to those mentioned above, and other technical problems not mentioned can be clearly understood by a person having ordinary skill in the art to which the present invention pertains from the following description. [Means for solving the problem]

[0013] One embodiment of the present invention is a sintered body having a composition, by weight percent, of C: 0.20% or more and less than 0.35%, Mn: 0.5 to 1.3%, Si: 0.3% or less (excluding 0%), P: 0.03% or less (including 0%), S: 0.004% or less (including 0%), Al: 0.04% or less (excluding 0%), Cr: 0.3% or less, Ni: 0.1 to 0.4%, Ti: 0.05% (including 0%), B: 0.0005 to 0.0050%, N: 0.01% or less (excluding 0%), and the remaining Fe and other elements. The present invention relates to a hot-rolled steel sheet for cold-forming parts, which contains other impurities, satisfies the following relational expressions 1-3, has a hardness difference value of less than 15 at a surface layer portion and a portion at a position of / 4 thickness of the steel sheet, has a microstructure containing, by volume, 20 to 65% ferrite and 35 to 80% pearlite, and has an average crystal grain size of prior austenite of 15 μm or more, and has high strength and excellent bendability with a maximum bending angle of 40° or more after heating and quenching-tempering heat treatment.

[0014] [Equation 1] (Mn / Si)≧2 (weight ratio)

[0015] [Equation 2] (Ni) / (Mn)≧0.05 (weight ratio)

[0016] [Equation 3] (Si+Ni) / (C+Mn)≧0.2 (weight ratio)

[0017] In addition, another embodiment of the present invention is Heating the steel slab having the above composition to a temperature range of 1150 to 1300 ° C.; hot rolling the heated slab at a temperature of Ar3 or higher, including rough rolling and finish rolling, to obtain a hot-rolled steel sheet; and cooling the hot-rolled steel sheet on a run-out table and winding it at a temperature of 550 to 750°C. The method for manufacturing a hot-rolled steel sheet for cold-forming parts has a hardness difference value of less than 15 at the surface layer and at a position at / 4 thickness of the steel sheet, and has high strength and a maximum bending angle of 40° or more after heating and quenching-tempering heat treatments.

[0018] The method may further include pickling the hot-rolled steel sheet to obtain a hot-rolled pickled steel sheet.

[0019] In still another embodiment of the present invention, the composition contains, by weight, C: 0.20% or more and less than 0.35%, Mn: 0.5 to 1.3%, Si: 0.3% or less (excluding 0%), P: 0.03% or less (including 0%), S: 0.004% or less (including 0%), Al: 0.04% or less (excluding 0%), Cr: 0.3% or less, Ni: 0.1 to 0.4%, Ti: 0.05% (including 0%), B: 0.0005 to 0.0050%, and N: 0.01% or less (excluding 0%). The steel pipe for cold-forming components contains ferrite and the remainder is Fe and other impurities, satisfies the following relational expressions 1-3, has a hardness difference value of less than 15 between the surface layer portion and the portion at the thickness / 4 position of the steel pipe, has a microstructure containing, by volume, 20 to 65% ferrite and 35 to 80% pearlite, and has an average crystal grain size of prior austenite of 15 μm or more, and has high strength and excellent bendability with a maximum bending angle of 40° or more after heating and quenching-tempering heat treatment.

[0020] [Equation 1] (Mn / Si)≧2 (weight ratio)

[0021] [Equation 2] (Ni) / (Mn)≧0.05 (weight ratio)

[0022] [Equation 3] (Si+Ni) / (C+Mn)≧0.2 (weight ratio)

[0023] Furthermore, still another embodiment of the present invention is Heating the steel slab having the above composition to a temperature range of 1150 to 1300 ° C.; hot rolling the heated slab at an Ar3 temperature or higher, including rough rolling and finish rolling, to obtain a hot-rolled steel sheet; cooling the hot-rolled steel sheet on a run-out table and coiling it at a temperature of 550 to 750°C; Welding the hot-rolled steel sheets to obtain a steel pipe; and subjecting the steel pipe to annealing heat treatment at a temperature of Ac1-50°C to Ac3+150°C for 3 to 60 minutes.

[0024] The method may further include drawing the annealed steel pipe.

[0025] The method may further include a reheating step of heating the annealed or drawn steel pipe to a temperature of Ar3 to 970°C at a heating rate of 10°C / sec or more and then holding the pipe at that temperature for 60 seconds or less, a quenching step of cooling the reheated steel pipe to room temperature at a cooling rate of 20 to 350°C / sec, and a tempering step of heating the quenched steel pipe to a temperature range of 150 to 350°C at a heating rate of 2 to 20°C / sec and then holding the pipe at that temperature.

[0026] In addition, still another embodiment of the present invention is a steel sheet having, by weight%, C: 0.20% or more and less than 0.35%, Mn: 0.5 to 1.3%, Si: 0.3% or less (excluding 0%), P: 0.03% or less (including 0%), S: 0.004% or less (including 0%), Al: 0.04% or less (excluding 0%), Cr: 0.3% or less, Ni: 0.1 to 0.4%, Ti: 0.05% (including 0%), B: 0.0005 to 0.0050%, The Fe3C carbide contains N: 0.01% or less (excluding 0%), the remainder being Fe and other impurities, satisfies the following relational expressions 1-3, has a microstructure containing, by volume%, at least one of martensite and tempered martensite at 95%, and the remainder being 5% or less of retained austenite, and has an average crystal grain size of prior austenite of 15 μm or more and an average equivalent circle size of 300 nm or less, and is formed in a unit area (μm 2 The present invention relates to a cold-formed member having high strength and excellent bendability, in which the number of defects per one chip is 30 or less.

[0027] [Equation 1] (Mn / Si)≧2 (weight ratio)

[0028] [Equation 2] (Ni) / (Mn)≧0.05 (weight ratio)

[0029] [Equation 3] (Si+Ni) / (C+Mn)≧0.2 (weight ratio)

[0030] Furthermore, still another embodiment of the present invention is Heating the steel slab having the above composition to a temperature range of 1150 to 1300 ° C.; hot rolling the heated slab at a temperature of Ar3 or higher, including rough rolling and finish rolling, to obtain a hot-rolled steel sheet; Cooling the hot-rolled steel sheet on a run-out table and winding it at a temperature of 550 to 750 ° C.; welding the coiled hot-rolled steel sheet to obtain a steel pipe; Annealing and drawing the steel pipe; A reheating step in which the annealed and drawn steel pipe is heated to a temperature of Ar3 to 970 ° C. at a heating rate of 10 ° C. / sec or more and then held for 60 seconds or less; a quenching step of cooling the reheated steel pipe to room temperature at a cooling rate of 20 to 350 ° C / sec; a tempering heat treatment step of heating the quenched steel pipe to a temperature range of 150 to 350 ° C. at a heating rate of 2 to 20 ° C. / sec and then maintaining the pipe at the heated temperature; and cold forming the tempered steel pipe into a component. Effect of the Invention

[0031] According to the present invention having the above-mentioned configuration, it is possible to provide a hot-rolled steel sheet and a steel pipe for forming parts, which have an excellent hardness difference (ΔHv) value of less than 15 between the surface layer of the steel sheet and the position at / 4 of the thickness, and which can exhibit high strength of 1300 MPa or more and a maximum bending angle of 40° or more after heating and quenching-tempering heat treatment, and further, it is possible to manufacture parts by cold forming, which has the effect of reducing the manufacturing costs of the parts. [Brief description of the drawings]

[0032] [Figure 1]FIG. 2 is a graph showing change curves of strength and maximum bending angle of a steel sheet after tempering heat treatment in Example 2 in an embodiment of the present invention. [Diagram 2] 1 is a photograph showing the shape of a quenched and tempered heat treated steel pipe of Example 2 after cold bending in one embodiment of the present invention. [Diagram 3] 1(a) and (b) are photographs showing the presence or absence of cracks in a heat-treated steel pipe according to an embodiment of the present invention, where (a) shows Example 2 of the invention and (b) shows Comparative Example 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] The present invention will now be described.

[0034] First, a hot-rolled steel sheet for cold-formed components according to a preferred embodiment of the present invention will be described, which has a hardness difference (ΔHv) value of less than 15 between the surface layer portion and the portion at a position of / 4 thickness of the steel sheet, and has high strength and excellent bendability with a maximum bending angle of 40° or more after heating and quenching-tempering heat treatment.

[0035] The hot rolled steel sheet for cold forming members of the present invention contains, by weight, C: 0.20% or more and less than 0.35%, Mn: 0.5 to 1.3%, Si: 0.3% or less (excluding 0%), P: 0.03% or less (including 0%), S: 0.004% or less (including 0%), Al: 0.04% or less (excluding 0%), Cr: 0.3% or less, Ni: 0.1 to 0.4%, Ti: 0.05% (including 0%), B: 0.0005 to The steel plate has a microstructure containing, by volume, 20 to 65% ferrite and 35 to 80% pearlite, and the hardness difference value at the surface layer and at a position at / 4 of the thickness of the steel plate is less than 15, and the average crystal grain size of prior austenite is 15 μm or more.

[0036] First, the alloy composition of the steel plate and steel pipe for cold-formed components according to the present invention and the reasons for limiting the content thereof will be described. In the following, "%" means "weight %" unless otherwise specified.

[0037] C: 0.20% or more and less than 0.35% The carbon (C) is an element effective in improving the strength of steel, and increases the strength after quenching and tempering heat treatment. If the carbon (C) content is less than 0.20%, it is difficult to secure a sufficient strength of 1300 MPa or more after tempering heat treatment, whereas if the carbon (C) content is 0.35% or more, martensite having excessive hardness or strength is formed, making it difficult to secure a bending angle of 40° or more during cold bending of the steel plate material or steel pipe part after heat treatment. If the tempering heating temperature can be increased to 350°C or more to increase the bending angle, there is a restriction in securing a strength of 1300 MPa or more. Therefore, it is preferable to limit the carbon (C) content to 0.20% or more and less than 0.35%.

[0038] Mn: 0.5-1.3% The manganese (Mn) is an essential element for improving the strength of steel, and increases the strength of steel after quenching heat treatment. If the Mn content is less than 0.5%, it is difficult to ensure sufficient strength of 1300 MPa or more after tempering heat treatment, whereas if the Mn content exceeds 1.3%, it is advantageous for ensuring strength, but it is difficult to ensure a bending angle of 40° or more for the steel plate and steel pipe after heat treatment. In this case, the size of the prior austenite may be appropriately controlled to improve cold bending property, or a relatively large amount of nickel (Ni) may be required to suppress the growth of carbides, which may lead to an increase in manufacturing costs. In addition, there is a possibility that segregation zones may be formed inside and / or outside the continuous cast slab and hot rolled steel plate, which may lead to frequent processing defects during pipe making. Therefore, it is preferable to limit the Mn content to 0.5 to 1.3%.

[0039] Si: 0.3% or less (excluding 0%) The silicon (Si) is an element added to improve strength or ductility, and is added in a range that does not cause problems with surface scaling of hot-rolled steel sheets and hot-rolled pickled steel sheets. If the content exceeds 0.3%, silicon oxide is generated, causing surface defects that are not easy to remove by pickling. In addition, if oxides are not smoothly discharged from the molten part of the steel pipe weld during the manufacture of steel pipes, the formability of the steel pipe may be reduced. Therefore, the silicon (Si) content is limited to 0.3% or less.

[0040] Equation 1 The Mn and Si contents should satisfy the following relational expression 1.

[0041] [Equation 1] (Mn / Si)≧2 (weight ratio)

[0042] The Mn / Si ratio is an important parameter that determines the quality of the welded parts of steel pipes. If the Mn / Si ratio is less than 2, the Si content is relatively high, and if silicon oxide is formed in the molten metal of the welded parts and is not forcibly discharged, defects may form in the welded parts, leading to poor pipe manufacturing. Therefore, the Mn / Si ratio is limited to 2 or more.

[0043] P: 0.03% or less (including 0%) The phosphorus (P) may segregate at the austenite grain boundaries and / or interphase grain boundaries, which may induce embrittlement. Therefore, the phosphorus (P) content should be kept as low as possible, with the upper limit limited to 0.03%. The preferred phosphorus (P) content is 0.02% or less.

[0044] S: 0.004% or less (including 0%) The sulfur (S) may induce high-temperature cracks by segregating into MnS nonmetallic inclusions in steel or during solidification in continuous casting. It is also necessary to control the sulfur (S) content as low as possible because it may deteriorate the impact toughness of heat-treated steel plate or steel pipe. Therefore, in the present invention, the sulfur (S) content is preferably kept as low as possible, with the upper limit being limited to 0.004%.

[0045] Al: 0.04% or less (excluding 0%) The aluminum (Al) is an element added as a deoxidizer. On the other hand, it reacts with nitrogen (N) in steel to precipitate as AlN. During the production of slabs, these precipitates may induce slab cracks under the cooling conditions of the slab, which may deteriorate the quality of the slab or hot-rolled steel sheet. In addition, if Al-rich inclusions or oxides are present inside the steel sheet or steel pipe, they may deteriorate the fatigue durability of the final part, so the content must be kept as low as possible. Therefore, it is preferable to limit the aluminum (Al) content to 0.04% or less (excluding 0%).

[0046] Cr: 0.3% or less (excluding 0%) Chromium (Cr) is an element that delays the transformation of austenite to ferrite, thereby increasing the hardenability during quenching heat treatment of steel, and also improving heat treatment strength. If chromium (Cr) is added in excess of 0.3% to steel containing 0.30% or more carbon (C), it may induce excessive hardenability of the steel, so the content is limited to 0.3% or less (excluding 0%).

[0047] Ni: 0.1-0.4% The nickel (Ni) is an element that simultaneously increases the hardenability and toughness of steel. Meanwhile, in the present invention, when the tensile properties and maximum bending angle of a steel plate or steel pipe having an increased nickel (Ni) content in the basic components are evaluated after quenching-tempering heat treatment, the yield strength after heat treatment decreases as the Ni content increases. This is considered to be because the nickel (Ni) element promotes the movement of dislocations introduced into martensite, or inhibits the size of prior austenite from excessively coarsening to an average of 26 μm or more during heat treatment, and further controls the size of carbides precipitated in the martensite structure to less than 300 nm during tempering heat treatment. However, when the content is less than 0.1%, the effect of decreasing the yield strength and increasing the maximum bending angle is insufficient. Meanwhile, when the content exceeds 0.4%, there is a possibility that the manufacturing cost of the steel plate will increase rapidly despite the above advantages. Therefore, the content is limited to the range of 0.1 to 0.4%.

[0048] Equation 2 The above Mn and Ni should satisfy the following relational expression 2.

[0049] [Equation 2] (Ni / Mn)≧0.05 (weight ratio)

[0050] The above (Ni / Mn) ratio is a necessary condition for securing a maximum bending angle of 40° or more while securing a strength of 1300 MPa or more after quenching-tempering heat treatment. If the (Ni / Mn) ratio is less than 0.05, it falls outside the range of manganese (Mn) or nickel (Ni) content proposed in the present invention. In addition, when the silicon (Si) content is low or the manganese (Mn) content is high, a band structure with a high manganese (Mn) content is likely to be formed in the microstructure of the hot-rolled steel sheet, and the bendability may deteriorate after heating and quenching-tempering heat treatment. Meanwhile, during the heating heat treatment process, nickel (Ni) element can prevent the size of prior austenite from growing excessively by decomposing the band structure or segregating to the grain boundaries of carbides, thereby hindering the complete decomposition or dissolution of carbides. In addition, during the tempering process, nickel (Ni) can segregate to the adjacent parts of carbides precipitated in martensite, suppressing the growth of carbides, and allowing them to exist in fine sizes. Therefore, as a method for improving the cold bendability of steel plate or steel pipe after heating and quenching-tempering heat treatment, the (Ni / Mn) ratio is limited to 0.05 or more in the present invention.

[0051] Equation 3 The above C, Mn, Si and Ni should satisfy the following relational expression 3.

[0052] [Equation 3] (Si+Ni) / (C+Mn)≧0.2 (weight ratio)

[0053] The above (Si+Ni) / (C+Mn) ratio is a necessary condition for securing a strength of 1300 MPa or more after quenching-tempering heat treatment while securing a maximum bending angle of 40° or more. In general, it is considered that the cold bending property of a steel plate or steel pipe after quenching-tempering heat treatment is inversely proportional to the tensile strength. Meanwhile, in the present invention, Relational Equation 3 has been devised to simultaneously satisfy the characteristics of high strength and high formability after heat treatment. When the (Si+Ni) / (C+Mn) ratio is less than 0.2, it corresponds to a case where the (C+Mn) content is higher than the (Si+Ni) content, in which case the hardening ability is high and the strength after heat treatment is very high, but the maximum bending angle is very low and bending cracks may frequently occur during bending. In addition, when a precipitate-forming element is added, the size of prior austenite (PAGS) during heat treatment is fine at less than 15 μm, so that the tensile strength is high but the bending property is poor.

[0054] Ti: 0.05% or less (excluding 0%) The titanium (Ti) is an element that forms precipitates (TiC, TiCN, TiNbCN, etc.) in the hot-rolled steel sheet, and inhibits the growth of austenite grains to increase the strength of the hot-rolled steel sheet.

[0055] If its content exceeds 0.05%, it may be effective in increasing the strength of quenched and tempered heat treated steel, but if it exists in the form of coarse precipitates rather than fine precipitates in the hot rolled steel sheet, it may deteriorate the toughness or act as a crack initiation point during cold bending, thereby reducing the cold formability of the heat treated steel sheet and steel pipe parts, or reducing the fatigue durability of the final parts. Therefore, its content is limited to 0.05% or less (excluding 0%).

[0056] B: 0.0005 to 0.005% or less (excluding 0%) The boron (B) is a beneficial element that significantly increases the hardenability of steel even at a low content. When an appropriate content is added, it is effective in increasing the hardenability by suppressing the formation of ferrite, but if it is added in excess, it increases the recrystallization temperature of austenite and deteriorates weldability. If the boron (B) content is less than 0.0005%, it is difficult to ensure the above effects of the steel, and if it exceeds 0.005%, the above effects are saturated or it is difficult to ensure appropriate strength and toughness. Therefore, the content is limited to 0.0005 to 0.005% or less. More preferably, limiting the content to 0.003% or less is effective in simultaneously ensuring the strength and formability of the heat-treated steel.

[0057] N: 0.01% or less (excluding 0%) The nitrogen (N) is an element that contributes to austenite stabilization and nitride formation. If the nitrogen (N) content exceeds 0.01%, it forms coarse AlN nitrides, which may act as the origin of fatigue cracks during fatigue durability evaluation of heat-treated steel sheets or steel pipe parts, thereby deteriorating fatigue durability. Therefore, the nitrogen content is limited to 0.01% or less (excluding 0%). More preferably, the nitrogen content is limited to 0.006% or less.

[0058] In addition, when boron (B) element is added, it is necessary to control the nitrogen (N) content as low as possible in order to increase the effective boron (B) content.

[0059] The steel of the present invention basically contains the above-mentioned components, with the balance being substantially Fe and other impurities. However, the following allowable components can be selectively added within limits that do not impair the present invention.

[0060] Mo, Cu, Nb and V In the present invention, one or more of Mo: 0.01-0.2%, Cu: 0.05-0.2%, Nb: 0.005-0.02%, and V: 0.01-0.05% may be selectively included. These elements increase the hardenability of the steel, or refine the crystal grain size of the prior austenite, and refine the lath size constituting the martensite or tempered martensite structure of the final part. This can therefore increase the tensile strength of the steel, and further contribute to improving the bending angle or bendability.

[0061] On the other hand, the above-mentioned hot-rolled steel sheet and steel pipe of the present invention have a microstructure containing, by volume, 20-65% ferrite and 35-80% pearlite. If the ferrite fraction is less than 20%, the pearlite content increases too much, making it difficult to ensure high strength and bending angle due to the development of band structure. Therefore, it is preferable to limit the ferrite fraction to 20% or more. The preferred ferrite fraction is 20-60%. On the other hand, if the ferrite fraction exceeds 65%, the total amount of hardening elements added to the hot-rolled steel sheet may be insufficient, in which case it is difficult to ensure sufficient strength after heating and quenching-tempering heat treatment.

[0062] Furthermore, the hot-rolled steel sheet of the present invention has a hardness difference (ΔHv) value between the surface layer and the quarter-thickness position of less than 15, and can have a tensile strength of 320 to 950 MPa.

[0063] Then, by heating and quenching-tempering the above-mentioned hot-rolled steel sheet, a formed part having a microstructure containing, by volume, 95% of one or more of martensite and tempered martensite, and the remainder being 5% or less of retained austenite, and having high strength and a maximum bending angle of 40° or more can be obtained.

[0064] Next, a method for producing a hot-rolled steel sheet according to an embodiment of the present invention will be described.

[0065] The method for producing a hot-rolled steel sheet of the present invention includes the steps of heating a steel slab having the above-mentioned compositional components to a temperature range of 1150 to 1300°C, hot rolling the heated slab including rough rolling and finish rolling at a temperature of Ar3 or higher to obtain a hot-rolled steel sheet, and cooling the hot-rolled steel sheet on a run-out table and coiling it at a temperature of 550 to 750°C.

[0066] Heating stages of steel slabs First, in the present invention, the steel slab having the above composition is heated to a temperature range of 1150 to 1300°C.

[0067] The reason for heating the steel slab to a temperature range of 1150-1300°C is to provide a uniform structure and composition distribution within the slab. If the slab heating temperature is lower than 1150°C, the precipitates formed in the continuous cast slab are not dissolved, and composition uniformity cannot be ensured. On the other hand, if the slab heating temperature exceeds 1300°C, an excessive increase in decarburization depth and grain growth occur, making it difficult to ensure the target material properties and surface quality of the hot-rolled steel sheet. Therefore, the slab heating temperature is limited to the range of 1150-1300°C.

[0068] Obtaining hot rolled steel sheets Next, in the present invention, the heated slab is subjected to hot rolling including rough rolling and finish rolling at a temperature of Ar3 or higher to obtain a hot-rolled steel sheet.

[0069] The hot rolling is preferably hot finish rolling at Ar3 or higher. If the hot rolling is performed at a temperature lower than Ar3, a part of the austenite is transformed into ferrite, which makes the deformation resistance of the material in the hot rolling non-uniform, which deteriorates the sheet passing properties including the straightness of the steel sheet, and there is a high possibility of occurrence of operational defects such as sheet breakage. However, if the finish rolling temperature exceeds 950°C, scale defects and the like occur, so the hot finish rolling temperature is preferably limited to 950°C or lower.

[0070] Winding stage In the present invention, the hot-rolled steel sheet obtained by hot rolling as described above is cooled on a run-out table and coiled at a temperature of 550 to 750°C.

[0071] After the above hot rolling, the hot rolled steel sheet is cooled on a run-out table and coiled in a temperature range of 550 to 750°C in order to ensure uniform material properties of the hot rolled steel sheet. If the coiling temperature is too low, below 550°C, a low-temperature transformation phase such as bainite or martensite is introduced into the edge portion of the steel sheet in the width direction, and there is a risk that the strength of the steel sheet will increase rapidly, resulting in increased variation in hot rolled strength in the width direction.

[0072] On the other hand, if the coiling temperature exceeds 750°C, internal oxidation is promoted in the surface layer of the steel sheet, and surface scratches such as cracks or surface irregularities may occur on the surface after hot rolling and pickling. In addition, the surface hardness of the steel sheet may vary due to coarsening of pearlite. Therefore, it is preferable to limit the coiling temperature after cooling of the hot-rolled steel sheet to 550 to 750°C.

[0073] In the present invention, the hot-rolled steel sheet produced as described above can be further subjected to pickling treatment to produce a hot-rolled pickled steel sheet. The pickling method is not limited to a specific method, and any pickling method generally used in a hot-rolling pickling process can be used.

[0074] Meanwhile, in the present invention, it is preferable to selectively use one of the following processes in the coiling process: (1) controlling the coiling temperature in the range of 550 to 750°C as low as possible to minimize the occurrence of decarburization; (2) preventing the coiled coil from being held at high temperature for a long time by placing it in a water-cooled bath instead of naturally cooling it; or (3) removing the decarburized region on the coil surface by cooling the coil to 200 to 250°C and then subjecting it to superpickling. By selectively using such processes, the hardness difference (ΔHv) between the surface layer and the t / 4 position of the steel sheet can be effectively reduced to less than 15.

[0075] The hot-rolled steel sheet of the present invention produced by the above-mentioned production process can have a microstructure containing, by volume, 20 to 65% ferrite and 35 to 80% pearlite.

[0076] In addition, the hardness difference (ΔHv) between the surface layer and the t / 4 position of the hot-rolled steel sheet is controlled to less than 15, and the hot-rolled steel sheet can have a tensile strength of 320 to 950 MPa.

[0077] The hot-rolled steel sheet of the present invention produced by the above-mentioned production process can have a fine structure in which the average crystal grain size of prior austenite is 15 μm or more.

[0078] When a hot-rolled steel sheet having an average crystal grain size of prior austenite of 15 μm or more is subjected to a heating and quenching-tempering heat treatment for manufacturing a formed part described later, the martensite structure formed becomes relatively coarse, and the initiation of crack generation during bending is delayed, so that the bendability can be improved or the bending angle can be increased.

[0079] In the present invention, in the case of a hot-rolled steel sheet satisfying the above-mentioned Relational Expressions 2 to 3, the average crystal grain size of prior austenite can be effectively controlled to 15 μm or more, and as a result, appropriate yield strength or tensile strength and a high bending angle can be simultaneously ensured after heat treatment after quenching and tempering.

[0080] If a hot-rolled steel sheet or steel pipe containing a prior austenite structure having an average grain size of less than 15 μm is subjected to heating and quenching-tempering heat treatment for manufacturing the above-mentioned formed part, a fine martensite structure is formed, so that the strength after heat treatment is high, and as a result, cracks may occur in a short time during bending, resulting in a decrease in bendability or a decrease in the bending angle. Therefore, in this case, there may be restrictions on realizing the shape when manufacturing parts by cold forming.

[0081] Hereinafter, a method for producing a steel pipe according to one embodiment of the present invention will be described.

[0082] A preferred method for producing a steel pipe of the present invention includes the steps of obtaining a steel pipe by welding hot-rolled steel sheets produced by the method for producing a hot-rolled steel sheet of the present invention described above, and subjecting the steel pipe to an annealing heat treatment.

[0083] Obtaining steel pipes The hot-rolled steel sheets produced by the above-mentioned method for producing a hot-rolled steel sheet of the present invention are welded to obtain a steel pipe.

[0084] The above-mentioned hot-rolled steel sheet or hot-rolled pickled steel sheet is made into a steel pipe by, for example, electric resistance welding or induction heating welding.

[0085] Annealing heat treatment stage of steel pipe The steel pipe obtained by the above-mentioned pipe making process is subjected to annealing heat treatment.

[0086] In the present invention, the above-mentioned hot-rolled steel sheet or hot-rolled pickled steel sheet can be used to manufacture a small-diameter steel pipe by a conventional cold forming method including the steps of forming a steel pipe by electric resistance welding or induction welding, annealing heating, and cold drawing.

[0087] The annealing heat treatment of the above steel pipe is preferably carried out at a temperature of Ac1-50°C to Ac3+150°C for 3 to 60 minutes.

[0088] The annealing heat treatment may include furnace cooling and air cooling.

[0089] In this case, the present invention may further include a step of drawing the annealed steel pipe. The steel pipe may be cold drawn to reduce the diameter of the steel pipe. The drawing method may be a cold drawing method.

[0090] The steel pipe of the present invention manufactured as described above has a microstructure containing, by volume, 20 to 65% ferrite and 35 to 80% pearlite, and preferably, the microstructure of the steel pipe contains, by volume, 20 to 50% ferrite.

[0091] Next, the present invention may further include a reheating step of heating the cooled steel pipe or the drawn steel pipe to a temperature of Ar3 to 970°C at a heating rate of 10°C / sec or more and then holding the temperature for 60 seconds or less, a quenching step of cooling the reheated steel pipe to room temperature at a cooling rate of 20 to 350°C / sec, and a tempering heat treatment step of heating the quenched steel pipe to a temperature range of 150 to 350°C at a heating rate of 2 to 20°C / sec and then holding the temperature. This will be described in detail below.

[0092] Next, a method for producing a cold-formed member according to an embodiment of the present invention will be described.

[0093] The method for manufacturing a cold-formed part of the present invention includes the steps of reheating the steel pipe obtained by the above-mentioned steel pipe manufacturing method, subjecting the reheated steel pipe to a quenching-tempering heat treatment, and manufacturing a part by cold forming the quenched-tempered heat treated steel pipe.

[0094] The above steel pipe is first subjected to the following heat treatment before being manufactured into a chassis part such as a stabilizer by cold bending.

[0095] Steel pipe reheating stage In the present invention, the above-mentioned annealed heat treated steel pipe or drawn steel pipe is reheated to manufacture it as a component for cold forming applications.

[0096] The reheating temperature may be Ar3 to 970°C. That is, a steel pipe of a certain length is passed through a high-frequency induction heating furnace at a moving speed of less than 100 mpm, heated to a target temperature range at a heating rate of 10°C / sec or more, and maintained under the condition of 60 seconds or less. At this time, the moving speed may be varied under the condition of less than 100 mpm so that a thickness of 3 to 6 mm of the steel pipe inner wall may have a uniform temperature. When the steel pipe is rapidly heated to the target temperature at a heating rate of 10°C / sec or more, the depth of the decarburization layer that may occur on the outer or inner wall of the steel pipe can be minimized, which is advantageous in that the durability of the final part can be improved.

[0097] Quenching and tempering heat treatment stage of steel pipe Next, in the present invention, the reheated steel pipe is subjected to a quenching-tempering heat treatment.

[0098] In the case of the quench cooling, water or oil is sprayed onto a specific length of a steel pipe heated to a temperature below 970°C to cool it to room temperature. The cooling rate of the entire steel pipe can be varied from 20 to 350°C / sec, and is not limited to a specific range as long as the entire inner wall of the steel pipe can have a martensite structure. In this case, the cooling rate is controlled by appropriately controlling the amount of sprayed water or oil and the moving speed of the steel pipe.

[0099] The steel pipe that has been heated and quenched as described above is then subjected to a tempering heat treatment in order to impart toughness thereto.

[0100] Under the above tempering heat treatment temperature conditions, the microstructure of the steel can be formed as a tempered martensite structure formed to correspond to a prior austenite grain size of 15 μm or more as the main phase, which increases the resistance to grain boundary cracking along the prior austenite grain boundaries during bending, or allows sufficient plastic deformation without cracking, allowing a bending angle of 40° or more to be exhibited.

[0101] If the heating temperature during the tempering is less than 150°C, the formation of the tempered martensite structure is insufficient, or the dislocation density in the martensite is high, and the strength after the heat treatment is relatively high, so that the bending angle may be low. On the other hand, if the heating temperature during the tempering exceeds 350°C, a high bending angle can be ensured due to the excessive tempering effect of the martensite structure, but it is difficult to ensure a strength of 1300 MPa or more. Therefore, it is preferable to limit the tempering heat treatment temperature to the range of 150 to 350°C. More preferably, the heat treatment is performed in the temperature range of 200 to 250°C, which can avoid temper brittleness.

[0102] On the other hand, under the above heating rate conditions for the heat treatment, the microstructure of the steel has Fe3C carbides of various sizes within and at the grain boundaries of the tempered martensite crystal grains. If the heating rate for the tempering heat treatment is less than 2°C / sec, the heating rate is too slow, causing excessive growth of Fe3C due to the excessive tempering softening effect, and the strength after the heat treatment tends to be low. In addition, under the above conditions, the heating rate is slow, so the productivity of the steel pipe is low and it is not economical. On the other hand, if the heating rate exceeds 20°C, the growth of Fe3C in the martensite is suppressed, and the strength after the heat treatment tends to be excessively high, which may make it difficult to simultaneously secure appropriate strength and a high bending angle. That is, if the size of Fe3C is fine, there are few crack sites including the grain boundaries between Fe3C and tempered martensite, but the strength tends to be excessively high to ensure a high bending angle. Therefore, it is preferable to limit the heating rate of the tempering heat treatment to a range of 2 to 20°C / sec, and to select the heating rate in consideration of the range of the heating temperature. In consideration of the above-mentioned circumstances, in one embodiment of the present invention, the number of Fe3C carbides having an average circle equivalent size of 300 nm or less is calculated per unit area (μm 2 ) may be limited to 30 or less per

[0103] Cold forming stage The steel pipe that has been reheated and quenched-tempered as described above is cold formed to manufacture a component.

[0104] The forming of the steel pipe is carried out by a cold forming method. For example, the heat-treated steel pipe can be formed at room temperature using a cold forming machine having dies with various bending radii (R, Radius). An example of the above-mentioned member is a suspension part such as a stabilizer.

[0105] The cold forming of the steel pipe is preferably performed by inserting a steel pipe of a specific length into a die having a bending radius of 30 to 60R and performing minimum to maximum bending to obtain a member. In the present invention, the bending radius means the degree to which a straight steel pipe is bent (curvature) and the radius of the curve formed by a circle. Therefore, a bending radius of 30R indicates that the curvature radius is small but the curvature or bending angle is large, and a bending radius of 60R indicates that the curvature radius is large but the curvature or bending angle is small. In other words, a bending radius of 60R means that the bending is relatively gentle. Meanwhile, in the present invention, there is no restriction on the specific range of the bending speed and the friction coefficient between the die and the steel pipe as long as the member can be manufactured by adjusting the bending speed and the friction coefficient between the die and the steel pipe within a range in which bending cracks do not occur during the manufacturing process of the member.

[0106] On the other hand, the bending angle can be measured not only for steel pipes but also for flat plates after heat treatment. In the present invention, the maximum bending angle of quenched and tempered heat-treated flat plate material having various thicknesses was evaluated by the three-point bending test VDA238-100 standard test.

[0107] According to the method for manufacturing a member of the present invention, it is possible to manufacture a member that has a tensile strength of 1,300 MPa or more after heat treatment while having a bending angle of 40° or more, or does not generate cracks even at a bending radius of less than R50, and that simultaneously has high strength and excellent cold bending formability after heat treatment.

[0108] As described above, in the present invention, in order to manufacture a member having high strength and excellent cold bending formability after heat treatment, in which cracks do not occur even when the bending angle is 40° or more or the bending radius is less than R50 while having a tensile strength of 1300 MPa or more after heat treatment, the number of Fe3C carbides having an average crystal grain size of prior austenite of 15 μm or more and an average equivalent circle size of 300 nm or less per unit area (μm 2It is preferable to limit the number of carbides per unit area to 30 or less per unit area. This is because this ensures appropriate strength, toughness or plastic deformation characteristics of a tempered martensite structure steel having carbides of appropriate size in the tempered martensite structure, and increases resistance to the occurrence of grain boundary cracks along the prior austenite grain boundaries when bending a steel plate or steel pipe, or makes it possible to induce plastic deformation sufficiently before cracks occur.

[0109] As described above, the present invention ensures a strength of 1300 to 1800 MPa by subjecting a drawn steel pipe to heating and quenching-tempering heat treatment, and can fabricate such ultra-high strength steel pipe into parts with complex shapes such as stabilizers by cold forming. Although such cold-formed parts do not differ significantly in shape / performance from parts manufactured by conventional hot forming processes, the shape of the ultra-high strength steel pipe is relatively simple, and furthermore, the manufacturing cost of parts can be significantly reduced.

[0110] The present invention will be described in more detail below with reference to examples. However, it should be noted that the following examples are intended to illustrate and explain the present invention in more detail, and are not intended to limit the scope of the present invention. The scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom. EXAMPLES

[0111] [Table 1]

[0112] [Table 2]

[0113] Using steel having the composition shown in Table 1-2 above, hot rolling was performed under the conditions shown in Table 3 below to produce hot-rolled steel sheets having a thickness of 3.6 mm, which were then pickled. Specifically, slabs or lapped ingots having the composition shown in Table 1-2 above were heated at a temperature range of 1200±20°C for 200 minutes for homogenization, and then each slab or ingot was subjected to rough rolling and finish rolling, and then coiled at a temperature of 550 to 750°C to produce hot-rolled steel sheets having a thickness of 3.6 mm.

[0114] [Table 3]

[0115] The yield strength (YS), tensile strength (TS), elongation (EL), Vickers hardness difference between the surface layer and t / 4, and fraction of microstructure were measured for the hot-rolled steel sheet manufactured as described above, and the results are shown in Table 3. The microstructure other than pearlite is ferrite. Meanwhile, in this example, the yield strength (YS), tensile strength (TS), and elongation (EL) of the hot-rolled steel sheet were measured using JIS5 standard for test pieces taken in a direction parallel to the rolling direction, and the fraction of pearlite was measured using an image analysis program under the condition of X500 magnification optical microscope for nital-etched test pieces.

[0116] The hot-rolled steel sheets were then electric resistance welded to produce steel pipes with a diameter of 28 mm, which were then subjected to annealing and cold drawing to produce drawn steel pipes with a diameter of 22.2 mm. The steel pipes were then subjected to heating-quenching-tempering heat treatment under the conditions shown in Table 4 below, and then cold forming was performed to produce components.

[0117] In this case, the quenching heat treatment was performed by heating the steel pipe to a temperature of 930 to 970°C, and then cooling the steel pipe to a temperature of 200°C or less, and then cooling the steel pipe by immersing it in water or oil or spraying it with water so as to cool it as completely as possible to room temperature. In addition, the tempering heat treatment was performed by heating the steel pipe to a temperature of 200 to 300°C at a heating rate in the range of 2 to 20°C / sec, and then cooling it.

[0118] [Table 4]

[0119] The tensile properties and three-point bending test were performed on the steel pipes after the quenching and tempering heat treatment, and the microstructure was observed, and the results are shown in Table 5. Specifically, the steel pipes were heated and quenched, and then the tensile properties of the steel plate and the maximum bending angle in a three-point bending test were measured.

[0120] The average size of the prior austenite grains was calculated by measuring the size of at least 10 grains at a magnification of X500 on the cross section of the same test piece where the optical microstructure was observed, after polishing and etching with picric acid, and calculating the average of the results. The number of Fe3C carbides per unit area was calculated by using the same test piece and measuring the number of Fe3C carbides present within an area of ​​2.9 μm horizontal × 3.1 μm vertical at magnifications of X5,000 to X10,000 using a scanning electron microscope, and the result was taken as the number of carbides per unit area. In detail, the number of Fe3C carbides present within tempered martensite grains was measured as much as possible. In order to understand the correlation between the change in the microstructure after heating and quenching-tempering heat treatment and the bending angle, the microstructure was measured in detail as described above.

[0121] Meanwhile, the steel pipes were simply Zig-Zag formed using dies with various bending radii, or cold formed using a cold forming machine. After bending the steel pipes under various conditions using a cold forming machine, the presence or absence of cracks and the minimum bending radius without cracks were investigated, and the results are shown in Table 6 below. Here, the mechanical properties of the heat-treated steel sheets were measured by processing and heat treating test pieces taken in a direction parallel to the rolling direction according to the JIS5 standard, and the maximum bending test value was measured according to the VDA238-100 standard test, and both edges of the long side of the heat-treated test pieces were surface-grinded to eliminate the edge characteristics of the test pieces.

[0122] [Table 5]

[0123] [Table 6]

[0124] As shown in Table 1-6 above, inventive examples 1-4, 3-1, and 4-1 manufactured using inventive steel 1-4 that satisfies the steel composition and relational formula 1-3, no bending cracks occurred even when the maximum bending angle exceeded 40° or the bending radius was 50R or less.

[0125] Also, Examples 1-4, 3-1 and 4-1 all have a yield strength of 1200 to 1400 MPa, a tensile strength of 1300 to 1700 MPa, a yield ratio of 0.8 or more and a maximum bending angle of 40° or more, showing excellent bending formability.

[0126] In addition, in the case of the hot-rolled steel sheet before heat treatment, the hardness difference between the surface layer and t / 4 in the examples of the present invention is less than 15, which is relatively small compared to Comparative Examples 1 to 6. This is because the hardness difference depending on the position in the thickness direction of the hot-rolled steel sheet is small, or it is understood that the occurrence of decarburization in the surface layer is small.

[0127] In contrast, Comparative Example 1-10, which was manufactured using Comparative Steel 1-10 that does not satisfy at least one of the alloy components and Relational Formulas 1 to 3 of the present invention, corresponds to a case where the maximum bending angle of the steel plate after heat treatment is relatively less than 40°, or the bending radius at which no cracks occur in the steel pipe is 55R or more.

[0128] On the other hand, Figure 1 is a graph showing the change curve of strength and maximum bending angle of the steel plate after tempering heat treatment of Example 2 in one embodiment of the present invention, Figure 2 is a photograph showing the shape of the quenched-tempered heat-treated steel pipe of Example 2 after cold bending in one embodiment of the present invention, and Figure 3 (a) and (b) are photographs showing the presence or absence of cracks in the heat-treated steel pipe of one embodiment of the present invention, where (a) shows Example 2 and (b) shows Comparative Example 2.

[0129] That is, the changes in strength and maximum bending angle of heat-treated steel sheets of various embodiments of the invention and comparative examples were investigated, and as a representative example, the results of the strength and maximum bending angle of the steel sheet after tempering heat treatment of invention example 2 are shown in Figure 1.

[0130] In addition, bending tests were conducted on the heat-treated steel pipes of the invention steels and comparative steels of various examples using a cold forming machine having a die with a bending radius of 30 to 60R. As a representative example, the results of the bending test on the final shape of the steel pipe of invention example 2 are shown in Figure 2.

[0131] When various steel pipes are cold-bent, it has been observed that cracks occur in the surface layer of heat-treated steel pipes when the bending radius is small. Figure 3 shows Example 2 of the invention and Comparative Example 2 as representative examples of the presence or absence of cracks.

[0132] As shown in FIG. 1, a tensile strength of 1300 MPa or more and a high bending angle of 40° or more can be obtained at a tempering temperature of 200 to 250°C.

[0133] Furthermore, as shown in Figures 2 and 3, in the case of the heat-treated steel pipe manufactured as an example of the invention that satisfies the conditions of the present invention with a bending radius of 50R or less, it can be confirmed that components can be manufactured without the occurrence of cracks during cold forming.

[0134] As described above, the reason why no bending cracks occur in steel pipes or the flat plate three-point bending angle is measured high after QT heat treatment in the present invention is believed to be that the steel type of the present invention has relatively coarse prior austenite grains, so that the initiation of cracking is delayed in response to external bending force, or the size of Fe3C carbide does not grow sufficiently in the coarse tempered martensite grains due to the application of a rapid heating rate during tempering. Also, the effect of delaying the growth of Fe3C is believed to be further promoted when nickel (Ni) elements segregate at the interface of adjacent parts of Fe3C during tempering.

[0135] As described above, the detailed description of the present invention has been given of the preferred embodiment of the present invention, but it is obvious that those skilled in the art can make various modifications without departing from the scope of the present invention. Therefore, the scope of the present invention should not be limited to the described embodiment, but should be determined by the claims below as well as equivalents thereto.

Claims

1. the steel sheet contains, by weight, C: 0.20% or more and less than 0.35%, Mn: 0.5 to 1.3%, Si: 0.3% or less (excluding 0%), P: 0.03% or less (including 0%), S: 0.004% or less (including 0%), Al: 0.04% or less (excluding 0%), Cr: 0.3% or less, Ni: 0.1 to 0.4%, Ti: 0.05% (including 0%), B: 0.0005 to 0.0050%, N: 0.01% or less (excluding 0%), with the remainder being Fe and other impurities, and satisfies the following relational formula 1-3, and the hardness difference value between the surface layer portion and the portion at a position of thickness / 4 of the steel sheet is less than 15, A hot-rolled steel sheet for cold forming components having a microstructure containing, by volume, 20 to 65% ferrite and 35 to 80% pearlite, and an average crystal grain size of prior austenite of 15 μm or more. [Relationship 1] (Mn / Si)≧2 (weight ratio) [Relationship 2] (Ni) / (Mn)≧0.05 (weight ratio) [Relationship 3] (Si+Ni) / (C+Mn)≧0.2 (weight ratio)

2. The hot-rolled steel sheet for cold-formed members according to claim 1, further comprising one or more of Mo: 0.01 to 0.2%, Cu: 0.05 to 0.2%, Nb: 0.005 to 0.02%, and V: 0.01 to 0.05%.

3. The hot-rolled steel sheet for cold-formed components according to claim 1, characterized in that the hot-rolled steel sheet has a tensile strength of 320 to 950 MPa.

4. A step of heating a slab containing, by weight, C: 0.20% or more and less than 0.35%, Mn: 0.5 to 1.3%, Si: 0.3% or less (excluding 0%), P: 0.03% or less (including 0%), S: 0.004% or less (including 0%), Al: 0.04% or less (excluding 0%), Cr: 0.3% or less, Ni: 0.1 to 0.4%, Ti: 0.05% (including 0%), B: 0.0005 to 0.0050%, N: 0.01% or less (excluding 0%), the remainder being Fe and other impurities, and satisfying the following relations 1 to 3, to a temperature range of 1150 to 1300 ° C.; The heated slab was then cooled to 100° C. ３ performing hot rolling including rough rolling and finish rolling at a temperature equal to or higher to obtain a hot-rolled steel sheet; and cooling the hot-rolled steel sheet on a run-out table and coiling it at a temperature of 550 to 750°C. A method for producing a hot-rolled steel sheet for cold-formed components, the hot-rolled steel sheet having a hardness difference value of less than 15 between the surface layer portion and a portion at a position equal to / 4 of the thickness of the steel sheet. [Relationship 1] (Mn / Si)≧2 (weight ratio) [Relationship 2] (Ni) / (Mn)≧0.05 (weight ratio) [Relationship 3] (Si+Ni) / (C+Mn)≧0.2 (weight ratio)

5. The method for producing a hot-rolled steel sheet for cold-formed members according to claim 4, further comprising one or more of Mo: 0.01 to 0.2%, Cu: 0.05 to 0.2%, Nb: 0.005 to 0.02%, and V: 0.01 to 0.05%.

6. A steel pipe for cold-formed components, which contains, by weight%, C: 0.20% or more and less than 0.35%, Mn: 0.5 to 1.3%, Si: 0.3% or less (excluding 0%), P: 0.03% or less (including 0%), S: 0.004% or less (including 0%), Al: 0.04% or less (excluding 0%), Cr: 0.3% or less, Ni: 0.1 to 0.4%, Ti: 0.05% (including 0%), B: 0.0005 to 0.0050%, N: 0.01% or less (excluding 0%), with the remainder being Fe and other impurities, satisfies the following relational formula 1-3, has a hardness difference value of less than 15 between a surface layer portion and a portion at a position of thickness / 4 of the steel pipe, has a fine structure containing, by volume%, 20 to 65% ferrite and 35 to 80% pearlite, and has an average crystal grain size of prior austenite of 15 μm or more. [Relationship 1] (Mn / Si)≧2 (weight ratio) [Relationship 2] (Ni) / (Mn)≧0.05 (weight ratio) [Relationship 3] (Si+Ni) / (C+Mn)≧0.2 (weight ratio)

7. The steel pipe for cold forming members according to claim 6, further comprising one or more of Mo: 0.01 to 0.2%, Cu: 0.05 to 0.2%, Nb: 0.005 to 0.02%, and V: 0.01 to 0.05%.

8. A step of heating a slab containing, by weight, C: 0.20% or more and less than 0.35%, Mn: 0.5 to 1.3%, Si: 0.3% or less (excluding 0%), P: 0.03% or less (including 0%), S: 0.004% or less (including 0%), Al: 0.04% or less (excluding 0%), Cr: 0.3% or less, Ni: 0.1 to 0.4%, Ti: 0.05% (including 0%), B: 0.0005 to 0.0050%, N: 0.01% or less (excluding 0%), the remainder being Fe and other impurities, and satisfying the following relations 1 to 3, to a temperature range of 1150 to 1300 ° C.; The heated slab was then cooled to 100° C. ３ performing hot rolling including rough rolling and finish rolling at a temperature equal to or higher to obtain a hot-rolled steel sheet; cooling the hot-rolled steel sheet on a run-out table and coiling it at a temperature of 550 to 750°C; welding the hot-rolled steel sheets to obtain a steel pipe; The steel pipe is Ac １ -50℃~Ac ３ and subjecting the steel pipe to an annealing heat treatment at a temperature of +150°C for 3 to 60 minutes. [Relationship 1] (Mn / Si)≧2 (weight ratio) [Relationship 2] (Ni) / (Mn)≧0.05 (weight ratio) [Relationship 3] (Si+Ni) / (C+Mn)≧0.2 (weight ratio)

9. The method for producing a steel pipe for cold forming members according to claim 8, further comprising one or more of Mo: 0.01 to 0.2%, Cu: 0.05 to 0.2%, Nb: 0.005 to 0.02%, and V: 0.01 to 0.05%.

10. The method for producing a steel pipe for cold forming components according to claim 8, further comprising the step of drawing the annealed steel pipe.

11. The annealed and drawn steel pipe was subjected to Ar ３ A reheating step of heating to a temperature of 970° C. at a heating rate of 10° C. / sec or more and then holding for 60 seconds or less; a quenching step of cooling the reheated steel pipe to room temperature at a cooling rate of 20 to 350 ° C. / sec; The method for producing a steel pipe for cold forming components according to claim 10, further comprising a tempering heat treatment step of heating the quenched steel pipe to a temperature range of 150 to 350 ° C. at a heating rate of 2 to 20 ° C. / sec and then holding the steel pipe at the temperature.

12. Fe, which contains, by weight %, C: 0.20% or more and less than 0.35%, Mn: 0.5 to 1.3%, Si: 0.3% or less (excluding 0%), P: 0.03% or less (including 0%), S: 0.004% or less (including 0%), Al: 0.04% or less (excluding 0%), Cr: 0.3% or less, Ni: 0.1 to 0.4%, Ti: 0.05% (including 0%), B: 0.0005 to 0.0050%, N: 0.01% or less (excluding 0%), and the remainder is Fe and other impurities, and satisfies the following relational expressions 1-3, has a fine structure containing, by volume %, 95% of at least one of martensite and tempered martensite, and the remainder is 5% or less of retained austenite, and the average crystal grain size of prior austenite is 15 μm or more, and the average equivalent circle size is 300 nm or less. ３ C carbide is divided into 100 units of C carbide per unit area (μm ２ 30 or less per 100 mm diameter cold formed member. [Relationship 1] (Mn / Si)≧2 (weight ratio) [Relationship 2] (Ni) / (Mn)≧0.05 (weight ratio) [Relationship 3] (Si+Ni) / (C+Mn)≧0.2 (weight ratio)

13. The cold-formed member according to claim 12, further comprising one or more of Mo: 0.01-0.2%, Cu: 0.05-0.2%, Nb: 0.005-0.02%, and V: 0.01-0.05%.

14. 13. The cold-formed part according to claim 12, wherein the formed part has a tensile strength of 1300 MPa or more and a maximum bending angle of 40° or more.

15. A step of heating a slab containing, by weight, C: 0.20% or more and less than 0.35%, Mn: 0.5 to 1.3%, Si: 0.3% or less (excluding 0%), P: 0.03% or less (including 0%), S: 0.004% or less (including 0%), Al: 0.04% or less (excluding 0%), Cr: 0.3% or less, Ni: 0.1 to 0.4%, Ti: 0.05% (including 0%), B: 0.0005 to 0.0050%, N: 0.01% or less (excluding 0%), the remainder being Fe and other impurities, and satisfying the following relations 1 to 3, to a temperature range of 1150 to 1300 ° C.; The heated slab was then cooled to 100° C. ３ performing hot rolling including rough rolling and finish rolling at a temperature equal to or higher to obtain a hot-rolled steel sheet; cooling the hot-rolled steel sheet on a run-out table and coiling it at a temperature of 550 to 750°C; welding the hot-rolled steel sheets to obtain a steel pipe; Annealing and drawing the steel pipe; The annealed or drawn steel pipe is subjected to Ar ３ A reheating step of heating to a temperature of 970° C. at a heating rate of 10° C. / sec or more and then holding for 60 seconds or less; a quenching step of cooling the reheated steel pipe to room temperature at a cooling rate of 20 to 350 ° C. / sec or more; a tempering heat treatment step of heating the quenched steel pipe to a temperature range of 150 to 350 ° C. at a heating rate of 2 to 20 ° C. / sec and then holding the pipe at this temperature; and cold forming the tempered steel pipe into a component. [Relationship 1] (Mn / Si)≧2 (weight ratio) [Relationship 2] (Ni) / (Mn)≧0.05 (weight ratio) [Relationship 3] (Si+Ni) / (C+Mn)≧0.2 (weight ratio)

16. The method for producing a cold-formed member according to claim 15, further comprising one or more of Mo: 0.01 to 0.2%, Cu: 0.05 to 0.2%, Nb: 0.005 to 0.02%, and V: 0.01 to 0.05%.

17. The method for producing a cold-formed part according to claim 15, wherein the formed part has a tensile strength of 1300 MPa or more and a maximum bending angle of 40° or more.

18. The steel pipe is Ac １ -50℃~Ac ３ The method for producing a cold-formed part according to claim 15, characterized in that the annealing heat treatment is performed at a temperature of +150° C. for 3 to 60 minutes.

19. The method for manufacturing a cold-formed member according to claim 15, characterized in that when the heat-treated steel pipe is cold-formed, the cold-formation is performed with a bending radius in the range of 30 to 60R.

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