KR102557845B1 - Cold-rolled steel sheet and method of manufacturing the same - Google Patents

Abstract

In the present invention, carbon (C): 0.28 to 0.34% by weight, silicon (Si): 0.1 to 0.5% by weight, manganese (Mn): 0.5 to 1.4% by weight, phosphorus (P): greater than 0 and less than 0.02% by weight, sulfur (S): greater than 0 and less than 0.003% by weight, aluminum (Al): 0.01 to 0.1% by weight, chromium (Cr): greater than 0 1.0% by weight or less, boron (B): 0.001 to 0.003% by weight, nitrogen (N): more than 0 and 0.01% by weight or less, titanium (Ti): 48/14 x [N] to 0.1% by weight (the [N] is the value of nitrogen weight%) and the rest iron (Fe) and other unavoidable impurities, yield strength (YS): 1400 to 1760MPa, tensile strength (TS ): 1760 to 1900 MPa, and elongation (El): provides a cold-rolled steel sheet having 5 to 8%.

Description

Cold-rolled steel sheet and its manufacturing method {COLD-ROLLED STEEL SHEET AND METHOD OF MANUFACTURING THE SAME}

The present invention relates to a cold-rolled steel sheet and a manufacturing method thereof, and more particularly, to a cold-rolled ultra-high strength steel sheet excellent in bending workability and a manufacturing method thereof.

Ultra-high-strength steel for automotive steel is being developed to meet the two factors of vehicle weight reduction in response to environmental regulatory issues and reinforcement of crash safety standards in accordance with strengthened safety regulations. In particular, the application of ultra-high-strength steel sheets with a tensile strength of 1300 MPa or more (more recently, a tensile strength of 1700 MPa or more) is increasing in parts that need to secure collision stability, such as automobile bumper beams. In general, as the tensile strength increases, the elongation and cold formability deteriorate, so for structural parts to which ultra-high strength steel such as bumper beams are applied, hot stamping steel that secures high strength at room temperature by water cooling after high temperature forming has been developed and commercialized. However, it has problems of lack of productivity and increase in process cost. Recently, with the application of the roll forming method, part workability of cold-rolled ultra-high strength steel having tempered martensite as a microstructure has been improved. However, in order to apply the roll forming method, bending workability must be sufficiently secured.

Patent Document 1 proposes a method for manufacturing tempered martensitic steel having a tensile strength of 1700 MPa or more using a water cooling method, but the thickness of the steel is limited to 1 mm or less, and the bending workability R / t is very poor at 3.5 to 4.0 level, so it is difficult to apply the roll forming method. Patent Document 2 proposes a method for producing tempered martensitic steel with a tensile strength of 1700 MPa class having a wave height of 3 mm or less and excellent shape quality, but the Mn content is high, so there is concern about edge cracking and surface quality degradation due to surface oxides such as MnO. Due to the slow cooling rate and the absence of a tempering process, there is concern that the yield strength will be lower than that of conventional quenched martensitic steel.

1. Republic of Korea Patent Publication No. 2015-0061209 2. Korean Patent Publication No. 2019-0077203

A technical problem to be achieved by the present invention is to provide an ultra-high-strength cold-rolled steel sheet having a yield strength of 1400 MPa or more, a tensile strength of 1760 MPa or more, an elongation of 5% or more, and an excellent bending workability of R / t 2.5 or less based on 90 degree bending applicable to automotive parts and a method for manufacturing the same.

Cold-rolled steel sheet according to an embodiment of the present invention for solving the above problems is carbon (C): 0.28 ~ 0.34% by weight, silicon (Si): 0.1 ~ 0.5% by weight, manganese (Mn): 0.5 ~ 1.4% by weight, phosphorus (P): more than 0 and less than 0.02% by weight, sulfur (S): more than 0 and less than 0.003% by weight, aluminum (Al): 0.01 ~ 0.1% by weight, chromium (Cr): greater than 0 and less than or equal to 1.0% by weight, boron (B): 0.001 to 0.003% by weight, nitrogen (N): greater than 0 and less than 0.01% by weight, titanium (Ti): 48/14 x [N] to 0.1% by weight (the [N] is the value of nitrogen weight%), and the remainder includes iron (Fe) and other unavoidable impurities, yield strength (YS): 140 0 to 1760 MPa, tensile strength (TS): 1760 to 1900 MPa, and elongation (El): 5 to 8%.

Bending workability (R/t) of the cold-rolled steel sheet may be 1.5 to 2.5.

The final microstructure of the cold-rolled steel sheet is made of tempered martensite, bainite, and ferrite, and the phase fraction of the tempered martensite may be 95% or more.

A method for manufacturing a cold-rolled steel sheet according to an embodiment of the present invention for solving the above problems is (a) carbon (C): 0.28 to 0.34% by weight, silicon (Si): 0.1 to 0.5% by weight, manganese (Mn): 0.5 to 1.4% by weight, phosphorus (P): more than 0 and less than 0.02% by weight, sulfur (S): more than 0 and less than 0.003% by weight, aluminum (Al): 0.01 to 0.1% by weight, chromium (Cr): greater than 0 and less than 1.0% by weight, boron (B): 0.001 to 0.003% by weight, nitrogen (N): greater than 0 and less than 0.01% by weight, titanium (Ti): 48/14 x [N] ~ 0.1% by weight (the [N] is the value of nitrogen weight%) and the rest iron (Fe) and other unavoidable impurities. steps; (b) hot rolling the reheated steel; (c) cold-rolling the hot-rolled steel material; And (d) sequentially performing an annealing, cooling and tempering process on the cold-rolled steel; includes

In the manufacturing method of the cold-rolled steel sheet, the (d) step is an annealing heat treatment for the steel material at 850 ~ 920 ℃ for 30 ~ 120 seconds; can include

In the cold-rolled steel sheet manufacturing method, the step (d) includes cooling the steel material to 770 to 820 ° C at a rate of 3 to 15 ° C / s; Cooling the steel material to 400 ~ 450 ℃ at a rate of 80 ℃ / s or more; And cooling the steel material to 25 ~ 150 ℃ at a rate of 130 ~ 500 ℃ / s; can be included sequentially.

In the cold-rolled steel sheet manufacturing method, the step (d) includes tempering the steel material at 180 to 220° C. for 50 to 500 seconds; can include

In the cold-rolled steel sheet manufacturing method, step (a) may include reheating the steel material at 1180 to 1300 ° C. for 1 to 4 hours.

In the cold-rolled steel sheet manufacturing method, in the step (b), the finish rolling temperature may be 850 to 950 ° C.

According to an embodiment of the present invention, it has a yield strength of 1400 MPa or more, a tensile strength of 1760 MPa or more, an elongation of 5% or more, and an excellent bending workability of R / t 2.5 or less based on 90 degree bending. An ultra-high strength cold-rolled steel sheet and a manufacturing method thereof can be implemented. Of course, the scope of the present invention is not limited by these effects.

1 is a flowchart schematically illustrating a method for manufacturing a cold-rolled steel sheet according to an embodiment of the present invention.
2 is a view showing an overview of heat treatment including reheating, hot rolling, cold rolling, annealing, cooling and tempering processes in a method for manufacturing a cold rolled steel sheet according to an embodiment of the present invention.
3 is a graph showing a comparison of strength, elongation and bending workability according to some experimental examples of the present invention.
4 to 9 are pictures taken with a scanning electron microscope (SEM) of microstructures of specimens according to some experimental examples of the present invention.

A cold-rolled steel sheet and a manufacturing method thereof according to an embodiment of the present invention will be described in detail. Terms to be described later are terms appropriately selected in consideration of functions in the present invention, and definitions of these terms should be made based on the contents throughout this specification.

Hereinafter, specific details of an ultra-high strength cold-rolled steel sheet having a yield strength of 1400 MPa or more, a tensile strength of 1760 MPa or more, an elongation of 5% or more, and an excellent bending workability of R / t 2.5 or less based on 90 degree bending and a manufacturing method thereof will be provided.

steel plate

In the cold-rolled steel sheet according to an embodiment of the present invention, carbon (C): 0.28 to 0.34% by weight, silicon (Si): 0.1 to 0.5% by weight, manganese (Mn): 0.5 to 1.4% by weight, phosphorus (P): greater than 0 and less than 0.02% by weight, sulfur (S): greater than 0 and less than 0.003% by weight, aluminum (Al): 0.01 to 0.1% by weight, Chromium (Cr): greater than 0 and 1.0% by weight or less, boron (B): 0.001 to 0.003% by weight, nitrogen (N): greater than 0 and 0.01% by weight or less, titanium (Ti): 48/14 x [N] to 0.1% by weight (the [N] is the value of nitrogen weight%), and the rest iron (Fe) and other unavoidable impurities. Hereinafter, the role and content of each component included in the cold-rolled steel sheet will be described.

carbon (C)

Carbon (C) is added to secure the strength of the steel, and the strength increases as the carbon content increases in the martensitic structure. That is, carbon is an alloying element that contributes to improving the martensite fraction and strength. In one embodiment, the carbon (C) is included in an amount of 0.28 to 0.34% by weight based on the total weight of the cold-rolled steel sheet. When the amount of carbon added is less than 0.28% by weight, it is difficult to secure a yield strength of 1400 MPa or more and a tensile strength of 1760 MPa or more, and when the carbon content exceeds 0.34% by weight, the strength increases, but the weldability is disadvantageous and the bendability may be greatly reduced.

Silicon (Si)

Silicon (Si), as a ferrite stabilizing element, delays the formation of carbides in ferrite and has a solid solution strengthening effect. Silicon is preferably added in an amount of 0.1 to 0.5% by weight based on the total weight of the cold-rolled steel sheet. If it is less than 0.10% by weight, the effect of inhibiting carbide _formation cannot _be properly exerted and it is difficult to secure elongation.

Manganese (Mn)

Manganese (Mn) has a solid solution strengthening effect and contributes to strength improvement by increasing hardenability. That is, manganese is an element that facilitates the formation of a low-temperature transformation phase and provides an effect of increasing strength through solid solution strengthening. Some of manganese is dissolved in steel and some is combined with sulfur contained in steel to form MnS, which is a non-metallic inclusion. This MnS is ductile and elongates in the processing direction during plastic processing. However, the formation of MnS reduces the sulfur component in the steel, making the crystal grains brittle and suppressing the formation of FeS, a low melting point compound. It inhibits the acid resistance and oxidation resistance of steel, but improves the yield strength by making pearlite finer and strengthening ferrite by solid solution. Manganese is preferably added in an amount of 0.5 to 1.4% by weight based on the total weight of the cold-rolled steel sheet. If the content of manganese is less than 0.5% by weight, the effect is not sufficient, making it difficult to secure strength, and if it exceeds 1.4% by weight, workability and delayed fracture resistance decrease due to formation or segregation of inclusions such as MnS, and carbon equivalent. Increase weldability.

Phosphorus (P)

Phosphorus (P) can increase the strength by solid solution strengthening and suppress the formation of carbides. The phosphorus may be added in a content ratio of more than 0 and 0.02% by weight or less based on the total weight of the cold-rolled steel sheet. When phosphorus is added, it can help improve strength by solid solution strengthening, but when the phosphorus content exceeds 0.02% by weight, the weld is embrittled, low-temperature brittleness is induced, press formability is lowered, and impact resistance is lowered. Problems may occur.

Sulfur (S)

Sulfur (S) can combine with manganese, titanium, etc. to improve the machinability of steel and form precipitates of fine MnS to improve workability, but is an element that generally inhibits ductility and weldability. The sulfur may be added in a content ratio of more than 0 and 0.003% by weight or less based on the total weight of the cold-rolled steel sheet. When the sulfur content exceeds 0.003% by weight, the number of MnS inclusions increases, resulting in poor workability and weldability, and segregation during continuous casting solidification may cause high-temperature cracks.

Aluminum (Al)

Aluminum (Al) is an element mainly used as a deoxidizer, promotes ferrite formation, improves elongation, suppresses carbide formation, and promotes carbon concentration in austenite to stabilize austenite. In addition, aluminum is an element effective in suppressing the formation of manganese bands in hot-rolled coils. The aluminum (Al) is preferably added in a content ratio of 0.01 to 0.1% by weight based on the total weight of the cold-rolled steel sheet. When the content of aluminum (Al) is less than 0.01% by weight, the above-described effect of adding aluminum may be properly exhibited. On the contrary, when the content of aluminum (Al) exceeds 0.1% by weight and is excessively added, aluminum inclusions increase, lowering the playability, concentrating on the surface of the steel sheet, lowering the plating properties, and forming AlN in the slab to cause cracking during casting or hot rolling.

Chromium (Cr)

Chromium (Cr) is an alloying element that contributes to improving the strength of steel through solid solution hardening and increasing hardenability. The chromium is included in an amount greater than 0 and 1.0% by weight or less based on the total weight of the cold-rolled steel sheet. When the chromium is not included, it is difficult to secure the strength of the present invention, and when it is included in an amount exceeding 1.0% by weight, weldability may be impaired.

Boron (B)

Boron (B) is an element added to increase the hardenability of steel by suppressing the formation of ferrite. In addition, boron, as a strong hardenable element, greatly contributes to the formation of martensite after cooling after annealing, and serves to improve strength by preventing segregation of phosphorus (P). If segregation of phosphorus (P) occurs, secondary processing brittleness may occur, so boron is added to prevent segregation of phosphorus (P) to increase resistance to processing brittleness. The boron is preferably added in a content ratio of 0.001 to 0.003% by weight based on the total weight of the cold-rolled steel sheet. If the content of boron is less than 0.001% by weight, the above-mentioned effect is insufficient and it is difficult to secure martensite, and if it is added in excess of 0.003% by weight, toughness and weldability deteriorate, and the surface quality of the steel is deteriorated due to the formation of boron oxide. It can cause problems.

Nitrogen (N)

Nitrogen (N) inhibits the elongation and deteriorates the formability of steel. The lower the nitrogen content, the better, but if the nitrogen content is managed at a low level, the manufacturing cost of the steel may increase. The nitrogen may be included in an amount greater than 0 and 0.01% by weight or less based on the total weight of the cold-rolled steel sheet. When the nitrogen content exceeds 0.01% by weight, the elongation of the cold-rolled steel sheet may decrease.

Titanium (Ti)

Titanium (Ti) is a precipitate-forming element, and has the effect of precipitating TiN and refining crystal grains. In particular, the nitrogen content in the steel can be lowered through the precipitation of TiN, and the precipitation of BN can be prevented when added together with boron. Titanium is preferably added in an amount of 48/14 × [N] to 0.1% by weight based on the total weight of the cold-rolled steel sheet. Here, [N] expressed in the formula defining the lower limit of the titanium content represents the nitrogen content (unit: wt%) with respect to the total weight of the cold-rolled steel sheet. When the content of titanium is less than 48/14 × [N] wt%, BN is precipitated and the effect of adding Ti is insufficient, and when it exceeds 0.1 wt%, it is difficult to secure strength by reducing carbon solubility in the base material.

As described above, the cold-rolled steel sheet according to one embodiment of the present invention having an alloy element composition has a yield strength of 1400 MPa or more, a tensile strength of 1760 MPa or more, an elongation of 5% or more, and an R / t based on 90 degree bending. It may have excellent bending workability of 2.5 or less. For example, the cold-rolled steel sheet may have yield strength (YS): 1400 to 1760 MPa, tensile strength (TS): 1760 to 1900 MPa, elongation (El): 5 to 8%, and bending workability (R/t): 1.5 to 2.5.

As described above, the final microstructure of the cold-rolled steel sheet according to an embodiment of the present invention having an alloy element composition is composed of tempered martensite, bainite, and ferrite, and the phase fraction of the tempered martensite is 95% or more, and the balance may be the bainite and ferrite. Furthermore, fine-sized transition carbides may be precipitated in the martensite lath.

Hereinafter, a method for manufacturing a cold-rolled steel sheet according to an embodiment of the present invention having the above-described composition and microstructure will be described.

Manufacturing method of cold-rolled steel sheet

1 is a flowchart schematically showing a method for manufacturing a cold-rolled steel sheet according to an embodiment of the present invention, and FIG. 2 is a diagram showing an overview of heat treatment including reheating, hot rolling, cold rolling, annealing, cooling and tempering processes in a method for manufacturing a cold-rolled steel sheet according to an embodiment of the present invention.

1 and 2, a method for manufacturing a steel sheet according to an embodiment of the present invention includes (a) carbon (C): 0.28 to 0.34% by weight, silicon (Si): 0.1 to 0.5% by weight, manganese (Mn): 0.5 to 1.4% by weight, phosphorus (P): greater than 0 and less than 0.02% by weight, sulfur (S): greater than 0 and less than 0.003% by weight, aluminum (Al) ): 0.01 to 0.1% by weight, chromium (Cr): greater than 0 and less than 1.0% by weight, boron (B): 0.001 to 0.003% by weight, nitrogen (N): greater than 0 and less than 0.01% by weight, titanium (Ti): 48/14 × [N]% by weight to 0.1% by weight (provided that [N] is the value of nitrogen weight%) and the rest consisting of iron (Fe) and other unavoidable impurities Reheating the steel (S10); (b) hot rolling the reheated steel material (S20); (c) cold-rolling the hot-rolled steel material (S30); And (d) sequentially performing an annealing, cooling and tempering process on the cold-rolled steel material (S40); includes

The step (a) (S10) may include reheating the steel material at 1180 to 1300 ° C. for 1 to 4 hours. For example, the slab is manufactured in the form of a semi-finished product by continuously casting molten steel obtained through a steelmaking process, homogenizes component segregation generated in the casting process through a reheating process, and makes it into a state that can be hot rolled.

If the slab reheating temperature (SRT) is less than 1180 ° C, there is a problem that the segregation of the slab is not sufficiently re-dissolved, and if it exceeds 1300 ° C, the size of austenite grains increases, and process costs may increase. On the other hand, when the reheating time is less than 1 hour, the segregation zone is not sufficiently reduced, and when the reheating time exceeds 4 hours, the grain size increases and the process cost may increase.

The step (b) (S20) includes hot rolling (HR) the reheated slab. Hot rolling is hot rolling at a finish delivery temperature (FDT) of 850 ~ 950 ℃.

When the finish rolling temperature is lower than 850 ° C., the rolling load increases rapidly and productivity decreases, and when the temperature exceeds 950 ° C., the size of grains increases and strength may decrease.

After hot rolling, it is cooled to a temperature of 450 ~ 650 ℃ and then wound up. If the coiling temperature is less than 450 ° C, the shape of the hot-rolled coil is non-uniform and the cold rolling load increases, and if it exceeds 650 ° C, a non-uniform microstructure is caused due to a difference in cooling rate between the center and the edge of the steel sheet, and the inside of the grain boundary is oxidized.

The step (c) (S30) includes performing cold rolling (CR) on the hot-rolled steel material. The hot-rolled coil is pickled to remove the surface scale layer, and then cold-rolled. The thickness reduction rate during cold rolling is approximately 40 to 70%.

Step (d) (S40) includes sequentially performing annealing, cooling and tempering processes on the cold-rolled steel material.

The annealing process includes a step of annealing (Soaking) heat treatment for the steel material at 850 ~ 920 ℃ for 30 ~ 120 seconds. The cold-rolled cold-rolled steel is heated and annealed in a temperature range of Ae3 or higher to secure an austenite single-phase structure.

At this time, the heating rate is set to 3 ° C./sec or more, and maintained at an annealing temperature of 850 to 920 ° C. for 30 to 120 seconds. When the temperature increase rate is less than 3° C./s, it takes a lot of time to reach the annealing temperature, so heat treatment efficiency decreases and austenite crystal grains may become excessively large.

The annealing temperature is preferably performed at A3 or higher in order to secure an austenite single-phase structure, but if the annealing temperature exceeds 920 ° C., the austenite crystal size becomes too coarse, which may cause a decrease in strength. If the annealing temperature is less than 850 ° C or the holding time is less than 30 seconds, austenite may not be sufficiently homogenized, and if the annealing temperature exceeds 920 ° C or the holding time exceeds 120 seconds, the austenite crystal size becomes excessively coarse, resulting in a decrease in strength, and heat treatment efficiency and productivity may decrease.

The temperature-annealing heat treatment section affects the austenite single phase structure and the austenite grain size, which is important in the tensile strength of the final steel sheet.

The cooling process includes a step (I) of cooling the steel to 770 to 820 ° C at 3 to 15 ° C / s; Step (II) cooling the steel to 400 ~ 450 ℃ at a rate of 80 ℃ / s or more; And a step (III) of cooling the steel to room temperature (25 ° C) to 150 ° C at a rate of 130 to 500 ° C / s; includes sequentially.

Specifically, the annealed cold-rolled steel is slowly cooled at a rate of 3 to 15 °C/sec to a temperature of 770 to 820 °C to prevent ferrite transformation during cooling. If the slow cooling temperature is less than 770 ° C, ferrite transformation may occur, and if the slow cooling temperature exceeds 820 ° C, the temperature difference to be cooled in the subsequent quenching section greatly increases, which may cause quality problems.

In general, the continuous annealing furnace includes a slow cooling section, and when transformation of the soft ferrite phase occurs during slow cooling, it becomes difficult to achieve the final target tensile strength. In the alloy system proposed in this embodiment, when slowly cooling below 750 ° C, a soft ferrite phase may occur, so it is preferable to limit the slow cooling end point temperature to 770 ~ 820 ° C.

After slow cooling, the first rapid cooling is performed at a cooling rate of 80 ° C / sec or more (eg, 450 ~ 600 ° C / s) to a temperature of 400 to 450 ° C, and then the second rapid cooling is performed at a cooling rate of 130 ° C / sec or more to a temperature of 150 ° C or less.

In the case of primary quenching, if the cooling rate is less than 80 ° C / sec, ferrite or bainite transformation occurs during cooling, making it difficult to secure strength, and in secondary quenching, if the cooling rate is less than 130 ° C / sec, tempering of martensite occurs during cooling. It is difficult to secure strength.

Specifically, in the first cooling step of rapidly cooling the slow-cooled cold-rolled steel sheet to a temperature of 400 ° C (M _s ) or higher at a cooling rate of 80 ° C / s or higher, rapid cooling from the slow cooling end point temperature to a temperature of Ms or higher is performed. It is important to suppress ferrite and bainite phase transformation during cooling. For this purpose, it is preferable to cool at a cooling rate of 80° C./s or more, and if the cooling rate is insufficient, it may cause a decrease in tensile strength of the final steel due to ferrite or bainite phase transformation.

The secondary cooling step of rapidly cooling the primary cooled cold-rolled steel sheet from 150 ° C to room temperature (25 ° C) at _a cooling rate of 130 ° C / s or more and 500 ° C / s or less is 250 ° C. Auto-tempering may occur during the secondary cooling, and if the auto-tempering is excessive due to the slow cooling rate, the carbon content in martensite is reduced due to excessive precipitation of carbides, resulting in a decrease in tensile strength, and even after tempering. It may be difficult to sufficiently secure the yield strength. On the other hand, if the secondary cooling rate is very high, it is preferable to cool the secondary cooling section at 500 ° C./s or less, since the shape imbalance of the plate is expected due to severe material wave due to temperature imbalance.

The tempering process includes reheating the secondary cooled cold-rolled steel sheet at 3° C./s or higher and tempering it at 180 to 220° C. for 50 to 500 seconds. That is, the cooled cold-rolled steel is reheated to a temperature of 180 to 220° C. and tempered for 50 to 500 seconds.

At this time, when the tempering temperature is less than 180 ° C., it is difficult to sufficiently achieve the effect of tempering, and when the tempering temperature exceeds 220 ° C., the size of carbides becomes coarse during tempering, making it difficult to secure strength. In addition, if the holding time is less than 50 seconds, the tempering effect may not be sufficient, and if the holding time exceeds 500 seconds, the size of the carbide becomes coarse, making it difficult to secure strength and reducing productivity.

During the secondary cooling, it is difficult to achieve both the target yield strength of 1400 MPa or more and the bendability R / t of 2.5 or less with the softening furnace by auto tempering, so subsequent tempering is performed. It is a heat treatment section that improves yield strength, elongation, and bending workability by tempering martensite produced by secondary cooling to precipitate fine-sized transition carbides. If the tempering temperature is too low, carbide precipitation is insufficient. If the temperature is too high, cementite precipitation is excessive, making it difficult to achieve the bending workability required by the present invention. Accordingly, the tempering temperature is preferably limited to between 180 and 220°C. Since the tempering time has a smaller effect on the degree of tempering than the tempering temperature, the tempering time is not significantly limited, but if the tempering time is too short, the tempering is not sufficient, so the tempering time is preferably limited to 50 to 500 seconds.

Through the heat treatment process, a microstructure composed of tempered martensite of 95% or more, the balance of ferrite and bainite, and transition carbides of fine size in martensite lath can be finally obtained, and a cold-rolled ultra-high strength steel sheet with excellent bending workability having a yield strength of 1400 MPa or more, a tensile strength of 1760 MPa or more, an elongation of 5% or more, and a bendability (R/t) of 2.5 or less can be realized.

Experimental example

Hereinafter, preferred experimental examples are presented to aid understanding of the present invention. However, the following experimental examples are only for helping understanding of the present invention, and the present invention is not limited by the following experimental examples.

1. Composition of the specimen

In this experimental example, specimens having the alloy element composition (unit: weight%) of Table 1 are provided. Each alloy was reheated at 1250 ° C for 4 hours, then hot rolled to a thickness of 3.5 mm at a finish rolling temperature (FDT) of 850 ° C, and then wound at a coiling temperature of 600 ° C. Thereafter, surface oxide scale was removed through pickling, and cold rolling was performed to a thickness of 1.2 mm.

steel grade C Si Mn Ti B Cr P S Al N A 0.30 0.20 0.80 0.04 0.002 0.1 0.01 0.001 0.03 0.004 B 0.30 0.80 1.00 0.04 0.002 - 0.01 0.001 0.03 0.004 C 0.30 0.20 0.50 0.04 0.002 0.2 0.01 0.001 0.03 0.004 D 0.28 0.20 0.50 0.025 - - 0.01 0.001 0.03 0.004 E 0.26 0.20 1.00 0.03 0.002 - 0.01 0.001 0.03 0.004

Steel grades A and C in Table 1 are carbon (C): 0.28 to 0.34% by weight, silicon (Si): 0.1 to 0.5% by weight, manganese (Mn): 0.5 to 1.4% by weight, phosphorus (P): more than 0 and less than 0.02% by weight, sulfur (S): more than 0 and less than 0.003% by weight, aluminum (Al): 0.01 to 0.1% by weight, chromium (Cr): greater than 0 and less than 1.0% by weight, boron (B): 0.001 to 0.003% by weight, nitrogen (N): greater than 0 and less than 0.01% by weight, titanium (Ti): 48/14 x [N] ~ 0.1% by weight (where [N] is the value of nitrogen weight%) and the rest iron (Fe).

However, steel type B exceeds the composition range of silicon (Si): 0.1 to 0.5% by weight, steel type D is below the composition range of boron (B): 0.001 to 0.003% by weight, and steel grade E is carbon (C): 0.28 ~ 0.34% by weight.

2. Evaluation of process conditions and physical properties

Table 2 shows heat treatment process conditions applied to the specimens having the compositions disclosed in Table 1.

In Table 2, the unit of annealing temperature, slow cooling temperature, and tempering temperature is ° C, the unit of annealing time and tempering time is seconds, and the unit of slow cooling rate, primary cooling rate, and secondary cooling rate is ° C / s.

division steel grade annealing temperature annealing time annealing rate annealing temperature 1st cold speed 2nd cold speed tempering temperature tempering time Comparative Example 1 A 900 60 5 800 600 480 - - Comparative Example 2 A 900 60 5 800 450 130 - - Invention example 1 A 900 60 5 800 450 130 200 240 Comparative Example 3 A 900 60 5 800 600 480 160 240 Invention Example 2 A 900 60 5 800 600 480 180 240 Invention Example 3 A 900 60 5 800 600 480 200 60 Invention Example 4 A 900 60 5 800 600 480 200 240 Comparative Example 4 A 900 60 5 800 600 480 240 240 Comparative Example 5 A 900 60 5 800 600 480 300 240 Comparative Example 6 A 900 60 5 800 50 50 240 240 Comparative Example 7 B 850 60 5 800 450 130 200 240 Invention Example 5 C 850 60 5 800 450 130 200 240 Comparative Example 8 D 890 60 5 800 600 480 200 240 Comparative Example 9 E 890 60 5 800 600 480 200 240

Invention Examples 1, 2, 3, 4, and 5 not only satisfy the composition of the cold-rolled steel sheet according to the embodiment of the present invention described above, but also annealing heat treatment at 850 to 920 ° C. for 30 to 120 seconds; Slowly cooling the steel from 3 to 15 °C/s to 770 to 820 °C; Step of first rapid cooling at a rate of 80 ℃ / s or more to 400 ~ 450 ℃ with respect to the steel; and performing secondary rapid cooling at a rate of 130 to 500 °C/s to 25 to 150 °C for the steel; tempering at 180 to 220° C. for 50 to 500 seconds; Satisfies all heat treatment conditions of the method for manufacturing a cold-rolled steel sheet according to an embodiment of the present invention to perform.

In contrast, Comparative Example 1, Comparative Example 2, Comparative Example 3, Comparative Example 4, Comparative Example 5, and Comparative Example 6 satisfy the composition (steel type A) of the cold-rolled steel sheet according to the above-described embodiment of the present invention, but do not satisfy all of the heat treatment conditions of the manufacturing method of the cold-rolled steel sheet according to the embodiment of the present invention. For example, in Comparative Example 1 and Comparative Example 2, tempering at 180 to 220° C. for 50 to 500 seconds is not performed. Comparative Example 3 is below the tempering temperature range of 180 to 220 ° C., and Comparative Example 4 and Comparative Example 5 exceed the tempering temperature range of 180 to 220 ° C. Comparative Example 6 is below the cooling rate of 80 ° C / s or more in the first rapid cooling, and below the cooling rate of 130 to 500 ° C / s in the second rapid cooling.

On the other hand, Comparative Example 7, Comparative Example 8, and Comparative Example 9 satisfy all of the heat treatment conditions of the cold-rolled steel sheet manufacturing method according to the embodiment of the present invention, but do not satisfy all of the composition (steel type A) of the cold-rolled steel sheet according to the embodiment of the present invention. For example, referring to Table 1 together, Comparative Example 7 exceeds the composition range of silicon (Si): 0.1 to 0.5% by weight, Comparative Example 8 is below the composition range of boron (B): 0.001 to 0.003% by weight, and Comparative Example 9 is less than the composition range of carbon (C): 0.28 to 0.34% by weight.

Table 3 shows the microstructure, tensile properties, and bending properties realized as a result of applying the composition and heat treatment process conditions disclosed in Tables 1 and 2.

In Table 3, the microstructure (T.M.) is the phase fraction (%) of tempered martensite in the final microstructure, the microstructure (F) is the phase fraction (%) of ferrite in the final microstructure, the units of yield strength (YS) and tensile strength (TS) are MPa, the unit of tensile elongation (T.El) is %, and the yield ratio (YR) represents the ratio (%) of yield strength and tensile strength.

4 to 9 are photographs taken with a scanning electron microscope (SEM) of microstructures of specimens according to some experimental examples of the present invention. Specifically, FIG. 4 corresponds to Inventive Example 4, and FIG. 5 corresponds to Comparative Example 6. 6 corresponds to Comparative Example 1, FIG. 7 corresponds to Comparative Example 2, FIG. 8 corresponds to Invention Example 4, and FIG.

division microstructure
(TM) microstructure
(F) YS TS T.El YR R/t Comparative Example 1 100 - 1527 1683 1.2 90.7 >7.0 Comparative Example 2 100 - 1268 1838 7.2 69.0 2.17 Invention example 1 100 - 1461 1809 7.0 80.8 2.17 Comparative Example 3 100 - 1542 1950 6.4 79.1 2.83 Invention Example 2 100 - 1522 1886 5.3 80.7 2.17 Invention Example 3 97 3 1513 1846 5.7 82.0 2.50 Invention example 4 100 - 1530 1820 5.9 84.1 2.17 Comparative Example 4 100 - 1466 1681 5.9 87.2 2.67 Comparative Example 5 100 - 1433 1530 5.7 93.7 >3.7 Comparative Example 6 35 65 661 998 5.4 66.2 >3.7 Comparative Example 7 100 - 1526 1846 6.1 82.7 2.67 Invention example 5 100 - 1463 1803 6.5 81.1 2.33 Comparative Example 8 71 29 663 1053 3.0 62.9 >3.7 Comparative Example 9 100 - 1376 1683 4.7 81.7 3.00

Referring to FIG. 3 and Tables 1 to 3, in Comparative Examples 3, 4, and 5 and Inventive Examples 2, 3, and 4, the softness of the martensite microstructure varies depending on the tempering temperature and time, so that the tensile properties and bending properties are different. The physical properties react sensitively to tempering conditions, so that the satisfaction of the goal to be achieved in the present invention varies. Comparative Example 3 has a very high tensile strength of 1950 MPa because the tempering temperature is lower than the appropriate temperature (180 to 220 ° C.), but the bendability is 2.83, which does not meet the standard (2.5 or less) of the present invention. On the other hand, in Comparative Examples 4 and 5, the tempering temperature is higher than the proper temperature (180 ~ 220 ° C.), for example, when tempering at 300 ° C. (Comparative Example 5), cementite precipitation of a relatively large size is observed as shown in FIG. 9. As the carbide precipitation increases, the martensite strength decreases, so the tensile strength does not reach 1760 MPa, and the carbide acts as a crack initiation site, and the bendability also deteriorates.

Inventive Examples 2, 3, and 4 were tempered at an appropriate tempering temperature of 180 to 200 ° C, and a tempered martensitic microstructure with an area fraction of 95% or more as shown in FIG. 4 and transition carbides of a fine size in the structure were precipitated as shown in FIG. All of the bending workability are satisfied. In addition, it can be confirmed that the bending workability responds sensitively to the tempering temperature during the tempering temperature and time.

In the case of Comparative Examples 1 and 2, when tempering heat treatment was not performed, the amount of carbide precipitated during cooling by auto-tempering was changed according to the secondary quenching rate, and there was a significant difference in tensile properties and bendability. In particular, Comparative Example 1 has a very high secondary cooling rate, so auto-tempering hardly occurs, and as shown in FIG. 6, precipitation of transition carbides is not well observed, and physical properties and brittleness characteristics are not good. On the other hand, in Comparative Example 2, the cooling rate is relatively slow, and as shown in FIG. 7, a sufficient amount of fine-sized transition carbide is precipitated by auto tempering, and the bendability satisfies the level (2.5 or less) claimed in the present invention. However, the yield strength does not reach 1400 MPa due to the absence of subsequent tempering. When tempering at 200 ° C. as in Inventive Example 1 compared to Comparative Example 2, there is no additional improvement in bendability, but the yield strength increases to 1400 MPa or more, satisfying all the physical properties claimed in the present invention.

In Comparative Example 6, both the cooling rates of the first quenching and the second quenching proposed in the present invention were insufficient, so ferrite transformation occurred excessively during cooling, so that the tempered martensite fraction was less than 95%, and the tensile strength was 1700 MPa or less, and the bendability R/t exceeded 2.5.

Comparative Examples 7, 8, and 9 are steels in which the alloy amount is out of the range of the present invention, and Comparative Example 7 has a high silicon (Si) content, so that the strength increase due to solid solution strengthening and the precipitation of carbides during tempering according to the silicon content decrease, so that the bendability of the present invention (2.5 or less) is not satisfied. In Comparative Example 8, it can be seen that the tensile strength and elongation are remarkably low due to excessive formation of ferrite in the microstructure due to insufficient hardenability due to low boron (B) content. In Comparative Example 9, it can be confirmed that the content of steel or carbon having a 100% tempered martensitic structure deviated from the range of the present invention (0.28 to 0.34% by weight) and did not secure sufficient tensile strength.

So far, the cold-rolled steel sheet and its manufacturing method according to the spirit of the present invention have been described.

The present invention proposes a method for manufacturing ultra-high strength steel having martensite as a base structure similar to the conventional technology disclosed in the above-mentioned prior art literature, but has the following differences compared to the conventional technology. A homogeneous structure can be implemented by reducing the Mn segregation band by designing a steel grade in which the content of manganese is reduced to 1.4% by weight or less. In addition, it is possible to improve the yield strength and bending workability of the martensitic structure by performing subsequent tempering to precipitate fine transition carbides. As a result, it is possible to realize cold-rolled ultra-high-strength steel that secures a yield strength of 1400 MPa or more, a tensile strength of 1760 MPa or more, and bending workability of R/t 2.5 or less based on 90 degree bending during steel production.

Although the above has been described based on the embodiments of the present invention, various changes or modifications may be made at the level of those skilled in the art. As long as these changes and modifications do not depart from the scope of the present invention, it can be said to belong to the present invention. Therefore, the scope of the present invention will be determined by the claims described below.

Claims (9)

Carbon (C): 0.28 to 0.34 wt%, Silicon (Si): 0.1 to 0.5 wt%, Manganese (Mn): 0.5 to 1.4 wt%, Phosphorus (P): greater than 0 and 0.02 wt% or less, Sulfur (S): greater than 0 and 0.003 wt% or less, aluminum (Al): 0.01 to 0.1 wt%, chromium (Cr): greater than 0 1.0 % by weight or less, boron (B): 0.001 to 0.003% by weight, nitrogen (N): more than 0 to 0.01% by weight or less, titanium (Ti): 48/14 x [N] to 0.1% by weight (the [N] is the value of nitrogen weight%), and the remainder includes iron (Fe) and other unavoidable impurities,
Bending workability (R / t) is 1.5 ~ 2.5, yield strength (YS): 1400 ~ 1760MPa, tensile strength (TS): 1760 ~ 1900MPa, elongation (El): characterized in that 5 ~ 8%,
cold rolled steel. delete According to claim 1,
The final microstructure of the cold-rolled steel sheet is made of tempered martensite, bainite and ferrite,
Characterized in that the phase fraction of the tempered martensite is 95 area% or more,
cold rolled steel. (a) Carbon (c): 0.28 to 0.34%by weight, silicon (SI): 0.1 to 0.5%by weight, manganese (Mn): 0.5 to 1.4 weight%, 0 and 0.02%by weight or less, sulfur (s): 0 and 0 weaver and less than 0.003%by weight, aluminum (AL): 0.01 to 0.1%by weight, chrome (cr): 0 And 1.0 weight%, boron (B): 0.001 to 0.003%by weight, nitrogen (n): 0.01%or less, titanium (Ti): 48/14 x [n] ~ 0.1%(N] to 0.1 weight%(the value of nitrogen weight%) Re -heating for 1 to 4 hours;
(b) hot-rolling the reheated steel material under conditions of a finish rolling temperature of 850 to 950° C.;
(c) cold-rolling the hot-rolled steel material; and
(d) sequentially performing an annealing, cooling and tempering process on the cold-rolled steel; Including,
Step (d) is an annealing heat treatment for the steel material at 850 to 920° C. for 30 to 120 seconds; Slowly cooling the steel to 770 ~ 820 ℃ at 3 ~ 15 ℃ / s; First rapid cooling at a rate of 80 ° C / s or more to 400 ~ 450 ° C with respect to the steel; and secondarily quenching the steel at a rate of 130 to 500 °C/s to 25 to 150 °C; Including; tempering at 180 ~ 220 ℃ with respect to the steel material,
The cold-rolled steel sheet realized by performing the above steps has a bending workability (R / t) of 1.5 to 2.5, a yield strength (YS) of 1400 to 1760 MPa, a tensile strength (TS) of 1760 to 1900 MPa, and an elongation (El) of 5 to 8%.
A method for producing cold-rolled steel sheet.
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