ULTRA-HIGH TENSILE COLD-ROLLED STEEL SHEET AND METHOD FOR MANUFACTURING SAME

Abstract

Provided is an ultra-high-strength cold-rolled steel sheet with a balanced improvement in strength and ductility, and a method of manufacturing the same. According to an embodiment of the present disclosure, the ultra-high-strength cold-rolled steel sheet includes carbon (C): 0.1 wt % to 0.3 wt %, silicon (Si): 1.0 wt % to 2.0 wt %, manganese (Mn): 1.5 wt % to 3.0 wt %, aluminum (Al): more than 0 wt % and up to 0.05 wt %, a combination of one or more selected from titanium (Ti), niobium (Nb), and vanadium (V): more than 0 wt % and up to 0.05 wt %, phosphorus (P): more than 0 wt % and up to 0.02 wt %, sulfur (S): more than 0 wt % and up to 0.005 wt %, nitrogen (N): more than 0 wt % and up to 0.006 wt %, and a balance of iron (Fe) and other unavoidable impurities, wherein the ultra-high-strength cold-rolled steel sheet meets a yield strength (YS): 850 MPa or more, a tensile strength (TS): 1180 MPa or more, an elongation (EL): 14% or more, a hole expansion ratio (HER): 25% or more, and TSELHER/1000: 500 or more.

Claims

1. An ultra-high-strength cold-rolled steel sheet comprising carbon (C): 0.1 wt % to 0.3 wt %, silicon (Si): 1.0 wt % to 2.0 wt %, manganese (Mn): 1.5 wt % to 3.0 wt %, aluminum (Al): more than 0 wt % and up to 0.05 wt %, a combination of one or more selected from titanium (Ti), niobium (Nb), and vanadium (V): more than 0 wt % and up to 0.05 wt %, phosphorus (P): more than 0 wt % and up to 0.02 wt %, sulfur (S): more than 0 wt % and up to 0.005 wt %, nitrogen (N): more than 0 wt % and up to 0.006 wt %, and a balance of iron (Fe) and other unavoidable impurities, wherein the ultra-high-strength cold-rolled steel sheet meets a yield strength (YS): 850 MPa or more, a tensile strength (TS): 1180 MPa or more, an elongation (EL): 14% or more, a hole expansion ratio (HER): 25% or more, and TSELHER/1000: 500 or more.

2. The ultra-high-strength cold-rolled steel sheet of claim 1, wherein the ultra-high-strength cold-rolled steel sheet has a mixed structure of ferrite, retained austenite, bainite, fresh martensite, and tempered martensite, wherein an area fraction of ferrite ranges from 10% to 20%, wherein an area fraction of retained austenite ranges from 5% to 20%, wherein an area fraction of bainite ranges from 5% to 20%, and wherein a sum of area fractions of fresh martensite and tempered martensite is a remaining are fraction.

3. The ultra-high-strength cold-rolled steel sheet of claim 2, wherein a ratio (FM/TM) of fresh martensite (FM) to tempered martensite (TM) is 0.1 to 0.6.

4. The ultra-high-strength cold-rolled steel sheet of claim 2, wherein a density of iron carbide particles in tempered martensite is 1.010.sup.6 particles/mm.sup.2 or more.

5. The ultra-high-strength cold-rolled steel sheet of claim 2, wherein a grain size of tempered martensite is 5 m or less.

6. The ultra-high-strength cold-rolled steel sheet of claim 1, further comprising a combination of chromium (Cr) and molybdenum (Mo): more than 0 wt % and up to 1.0 wt %.

7. A method of manufacturing an ultra-high-strength cold-rolled steel sheet, the method comprising: producing a hot-rolled steel sheet with an alloy composition of carbon (C): 0.1 wt % to 0.3 wt %, silicon (Si): 1.0 wt % to 2.0 wt %, manganese (Mn): 1.5 wt % to 3.0 wt %, aluminum (Al): more than 0 wt % and up to 0.05 wt %, a combination of one or more selected from titanium (Ti), niobium (Nb), and vanadium (V): more than 0 wt % and up to 0.05 wt %, phosphorus (P): more than 0 wt % and up to 0.02 wt %, sulfur (S): more than 0 wt % and up to 0.005 wt %, nitrogen (N): more than 0 wt % and up to 0.006 wt %, and a balance of iron (Fe) and other unavoidable impurities; producing a cold-rolled steel sheet by cold rolling the hot-rolled steel sheet; primarily soaking the cold-rolled steel sheet at a primary soaking temperature of Ac3-30 C. to 900 C. for 30 sec. to 200 sec.; primarily cooling the primarily soaked cold-rolled steel sheet at a cooling rate of 5 C./s to 15 C./s to a primary cooling temperature of 620 C. to 720 C.; secondarily cooling the primarily cooled cold-rolled steel sheet at a cooling rate of 15 C./s to 100 C./s to a secondary cooling temperature of 250 C. to 480 C.; secondarily soaking the secondarily cooled cold-rolled steel sheet at a secondary soaking temperature of 250 C. to 480 C. for 50 sec. to 300 sec.; tertiarily cooling the secondarily soaked cold-rolled steel sheet to a tertiary cooling temperature of 150 C. or lower; and tertiarily soaking the tertiarily cooled cold-rolled steel sheet at a tertiary soaking temperature of 150 C. to 300 C. for 100 sec. to 30000 sec.

8. The method of claim 7, wherein the producing of the hot-rolled steel sheet comprises: reheating a steel material with the alloy composition at a slab reheating temperature of 1,150 C. to 1,250 C.; hot rolling the reheated steel material; cooling the hot-rolled steel material at a cooling rate of 10 C./s to 50 C./s; and coiling the cooled steel material at a coiling temperature of 500 C. to 700 C., and wherein the hot rolling comprises: a rough rolling process performed at 1,000 C. to 1,150 C. with a reduction ratio of 40% to 50% in a last pass; and a finishing rolling process performed at a finishing delivery temperature of 880 C. to 980 C., with rolling through a final 3-high stand performed at a temperature of 1020 C. or lower and a total reduction ratio of 40% or more, and a reduction ratio of 40% to 60% in a 1.sup.st pass.

9. The method of claim 8, wherein a time taken for the steel sheet to pass through the final 3-high stand during the finishing rolling process is no longer than 2.0 sec. (and longer than 0 sec.).

10. The method of claim 8, wherein a time taken from an end of the finishing rolling process to a start of cooling of the hot-rolled steel material is no longer than 1.5 sec.

11. The method of claim 7, further comprising softening the hot-rolled steel sheet at a temperature ranging from 500 C. to 650 C., after the hot-rolled steel sheet is produced.

12. The method of claim 7, further comprising hot-dip galvanizing the cold-rolled steel sheet after the cold-rolled steel sheet is secondarily soaked.

13. The method of claim 12, wherein the secondary soaking is hot-dip galvanizing the cold-rolled steel sheet.

14. The method of claim 12, further comprising alloying the cold-rolled steel sheet after the cold-rolled steel sheet is hot-dip galvanized.

Description

DESCRIPTION OF THE DRAWINGS

[0028] FIG. 1 is a flowchart of a method of manufacturing an ultra-high-strength cold-rolled steel sheet, according to an embodiment of the present disclosure.

[0029] FIG. 2 is a flowchart of a method of manufacturing an ultra-high-strength hot-dip galvanized cold-rolled steel sheet, according to an embodiment of the present disclosure.

[0030] FIG. 3 is a graph showing the heat-treatment history over time of an ultra-high-strength cold-rolled steel sheet according to an embodiment of the present disclosure.

[0031] FIG. 4 is a graph showing the heat-treatment history over time of an ultra-high-strength hot-dip galvanized cold-rolled steel sheet according to an embodiment of the present disclosure.

DETAILED DESCRIPTION

[0032] Hereinafter, the present disclosure will be described in detail by explaining embodiments of the disclosure with reference to the attached drawings. The disclosure may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the disclosure to one of ordinary skill in the art. Like reference numerals refer to like elements throughout. Further, various elements and regions in the drawings are schematically illustrated. Therefore, the scope of the present disclosure is not limited by the relative sizes or distances shown in the attached drawings.

[0033] According to the present disclosure, a steel sheet, which is annealed, galvanized, or galvanized and alloyed in a continuous annealing or galvanizing line, is cooled to the martensite start temperature (Ms) or below to form fresh martensite, and then reheated and maintained at a constant temperature for tempering to transform fresh martensite into tempered martensite.

[0034] Tempered martensite is a structure with well-balanced strength and toughness, but toughness may deteriorate when the size is large or iron carbides in tempered martensite coarsen. As such, in preferred aspects, to prevent the above problem, a certain amount of bainite is formed by constantly maintaining an appropriate temperature range during annealing. The formed bainite breaks down austenite, and thus the size of martensite may be reduced when austenite transforms into martensite during subsequent cooling. As such, tempered martensite formed through tempering also has a reduced size.

[0035] In preferred aspects, at the same time, by controlling tempering conditions after annealing, the number or density of iron carbide particles in tempered martensite may be controlled to prevent excessive formation, and thus the desired performance may be achieved.

[0036] In certain aspects, in addition, to enhance the formability of the final steel sheet, a uniform structure needs to be obtained by minimizing the segregation of manganese (Mn) and the like during hot rolling. To this end, the segregation of Mn may be minimized by controlling the temperatures and reduction ratios of rough rolling and finishing rolling.

[0037] An ultra-high-strength cold-rolled steel sheet according to the present disclosure will now be described in detail.

[0038] An ultra-high-strength cold-rolled steel sheet according to an embodiment of the present disclosure includes carbon (C): 0.1 wt % to 0.3 wt %, silicon (Si): 1.0 wt % to 2.0 wt %, manganese (Mn): 1.5 wt % to 3.0 wt %, aluminum (Al): more than 0 wt % and up to 0.05 wt %, a combination of one or more selected from titanium (Ti), niobium (Nb), and vanadium (V): more than 0 wt % and up to 0.05 wt %, phosphorus (P): more than 0 wt % and up to 0.02 wt %, sulfur (S): more than 0 wt % and up to 0.005 wt %, nitrogen (N): more than 0 wt % and up to 0.006 wt %, and the balance of iron (Fe) and other unavoidable impurities.

[0039] The ultra-high-strength cold-rolled steel sheet suitably may further include chromium (Cr): more than 0 wt % and up to 1.0 wt %. The ultra-high-strength cold-rolled steel sheet suitably may further include a combination of Cr and molybdenum (Mo): more than 0 wt % and up to 1.0 wt %.

[0040] The functions and contents of the components included in the ultra-high-strength cold-rolled steel sheet according to the present disclosure will now be described. In this case, the unit for the content of each constituent element is wt % relative to the total weight of the steel sheet.

Carbon (C): 0.1 wt % to 0.3 wt %

[0041] C is added to achieve the strength and control the microstructure of the steel sheet. When the content of C is less than 0.1 wt %, the target strength may not be easily obtained. When the content of C is greater than 0.3 wt %, formability such as elongation and hole expansion ratio may decrease, and spot weldability may deteriorate. Therefore, the content of C may be 0.1 wt % to 0.3 wt % of the total weight of the steel sheet.

Silicon (Si): 1.0 wt % to 2.0 wt %

[0042] S is a ferrite-stabilizing element, delays the formation of carbides in ferrite and tempered martensite, and has a solid solution strengthening effect. When the content of Si is less than 1.0 wt %, the Si addition effect is insufficient. When the content of Si is greater than 2.0 wt %, the formation of oxides such as Mn.sub.2SiO.sub.4 may deteriorate coatability, and the increase in C equivalent may decrease weldability. Therefore, the content of Si may be 1.0 wt % to 2.0 wt % of the total weight of the steel sheet.

Manganese (Mn): 1.5 wt % to 3.0 wt %

[0043] Mn has a solid solution strengthening effect and contributes to strength enhancement by increasing hardenability. Although the strength, toughness, and yield ratio may be controlled depending on the Mn content, an excessive amount of Mn may lead to the formation of MnS inclusions and cause center segregation during casting, thereby decreasing the toughness of steel. When the content of Mn is less than 1.5 wt %, the strength may not be easily achieved due to the insufficient hardenability, and the Mn addition effect is insufficient. When the content of Mn is greater than 3.0 wt %, the formation of inclusions such as MnS or the segregation of Mn may deteriorate formability, and the increase in C equivalent may decrease weldability. Therefore, the content of Mn may be 1.5 wt % to 3.0 wt % of the total weight of the steel sheet.

Aluminum (Al): More than 0 wt % and Up to 0.05 wt %

[0044] Al is used as a deoxidizer and may contribute to ferrite purification. When the content of Al is greater than 0.05 wt %, the formation of AlN during slab production may cause cracks during casting or hot rolling. Therefore, the content of Al may be more than 0 wt % and up to 0.05 wt % of the total weight of the steel sheet.

Combination of Titanium (Ti), Niobium (Nb), and Vanadium (V): More than 0 wt % and Up to 0.05 wt %

[0045] Ti, V, and Nb are major elements precipitated in the form of carbides inside steel. Ti, V, and Nb are added to achieve the stability of retained austenite and enhance strength by refining initial austenite grains through the formation of precipitates, and to enable precipitation hardening through the refinement of ferrite grains and the presence of precipitates in ferrite. When the total content of Ti, V, and Nb is greater than 0.05 wt %, a degradation in material properties and an increase in production costs may be caused. Therefore, the total content of Ti, Nb, and V may be more than 0 wt % and up to 0.05 wt % of the total weight of the steel sheet.

[0046] The steel sheet may include at least one of Ti, Nb, and V. As such, the content of Ti may be 0 wt % to 0.05 wt % of the total weight of the steel sheet, the content of Nb may be 0 wt % to 0.05 wt % of the total weight of the steel sheet, and the content of V may be 0 wt % to 0.05 wt % of the total weight of the steel sheet.

Combination of Chromium (Cr) and Molybdenum (Mo): More than 0 wt % and Up to 1.0 wt %

[0047] Cr and Mo serve as hardenability elements and contribute to the formation of a dual-phase structure. When the total content of Cr and Mo is greater than 1.0 wt %, the effect may converge, and the production costs may increase. Therefore, the total content of Cr and Mo may be 0 wt % to 1.0 wt % of the total weight of the steel sheet.

[0048] The steel sheet may further include at least one of Cr and Mo. As such, the content of Cr may be more than 0 wt % and up to 1.0 wt % of the total weight of the steel sheet, and the content of Mo may be more than 0 wt % and up to 1.0 wt % of the total weight of the steel sheet.

Phosphorus (P): More than 0 wt % and Up to 0.02 wt %

[0049] P is an impurity introduced while producing steel, and may contribute to strength enhancement based on solid solution strengthening. However, an excessive amount of P may cause low-temperature brittleness. Therefore, the content of P needs to be limited to more than 0 wt % and up to 0.02 wt % of the total weight of the steel sheet.

Sulfur (S): More than 0 wt % and Up to 0.005 wt %

[0050] S is an impurity introduced while producing steel, and may decrease toughness and weldability by forming non-metallic inclusions such as FeS and MnS. Therefore, the content of S needs to be limited to more than 0 wt % and up to 0.005 wt % of the total weight of the steel sheet.

Nitrogen (N): More than 0 wt % and Up to 0.006 wt %

[0051] N is an element inevitably introduced while producing steel, and an excessive amount of N may lead to the precipitation of nitrides in a large amount and a decrease in ductility. Therefore, the content of N needs to be limited to more than 0 wt % and up to 0.006 wt % of the total weight of the steel sheet.

[0052] The remainder of the ultra-high-strength cold-rolled steel sheet is iron (Fe). However, due to the inevitable introduction of unintended impurities from raw materials or the surrounding environment during the typical steelmaking process, the addition of impurities may not be completely excluded. These impurities are known to anyone of ordinary skill in the art and, therefore, are not particularly mentioned in this specification.

[0053] A ultra-high-strength cold-rolled steel sheet manufactured by controlling specific components of the above-described alloy composition and the content ranges thereof, and performing the following manufacturing method may meet, for example, a yield strength (YS): 850 MPa or more, a tensile strength (TS): 1180 MPa or more, an elongation (EL): 14% or more, a hole expansion ratio (HER): 25% or more, and TSELHER/1000: 500 or more. The ultra-high-strength cold-rolled steel sheet may meet, for example, an YS: 850 MPa to 1,000 MPa, a TS: 1180 MPa to 1300 MPa, an EL: 14% to 20%, a HER: 25% to 40%, and TSELHER/1000: 500 to 900.

[0054] Herein, TSELHER/1000 refers to the product of tensile strength, elongation, and hole expansion ratio, divided by 1000.

[0055] The ultra-high-strength cold-rolled steel sheet may have a mixed structure of ferrite, retained austenite, bainite, fresh martensite, and tempered martensite.

[0056] The area fraction of ferrite may range, for example, from 10% to 20%. The area fraction of retained austenite may range, for example, from 5% to 20%. The area fraction of bainite may range, for example, from 5% to 20%. The remaining area fraction may consist of martensite, and the martensite may include both fresh martensite and tempered martensite. The area fraction refers to an area percentage derived from a microstructural image using an image analyzer.

[0057] The value (FM/TM) obtained by dividing the area fraction of tempered martensite (TM) by the area fraction of fresh martensite (FM) may range from 0.1 to 0.6.

[0058] The density of iron carbide particles in tempered martensite may be, for example, 1.010.sup.6 particles/mm.sup.2 or more, and more specifically, 1.010.sup.6 particles/mm.sup.2 to 2010.sup.6 particles/mm.sup.2.

[0059] The grain size of tempered martensite may be, for example, 5 m or less, and more specifically, 1 m to 5 m. The grain size in this case is measured as an equivalent circle diameter of a region enclosed by boundaries with an orientation difference of 10 degrees or more, using electron backscatter diffraction (EBSD).

[0060] A method of manufacturing an ultra-high-strength cold-rolled steel sheet according to the present disclosure will now be described with reference to the attached drawings.

Method of Manufacturing Ultra-High-Strength Cold-Rolled Steel Sheet

[0061] FIG. 1 is a flowchart of a method of manufacturing an ultra-high-strength cold-rolled steel sheet, according to an embodiment of the present disclosure, and relates to a method of manufacturing an uncoated cold-rolled steel sheet.

[0062] Referring to FIG. 1, the method according to an embodiment of the present disclosure includes a hot-rolled steel sheet production step S110, a cold-rolled steel sheet production step S120, a primary soaking step S130, a primary cooling step S140, a secondary cooling step S150, a secondary soaking step S160, a tertiary cooling step S170, and a tertiary soaking step S180.

Hot-Rolled Steel Sheet Production (S110)

[0063] In the hot-rolled steel sheet production step S110, a steel material including C: 0.1 wt % to 0.3 wt %, Si: 1.0 wt % to 2.0 wt %, Mn: 1.5 wt % to 3.0 wt %, Al: more than 0 wt % and up to 0.05 wt %, a combination of one or more selected from Nb, Ti, and V: more than 0 wt % and up to 0.05 wt %, P: more than 0 wt % and up to 0.02 wt %, S: more than 0 wt % and up to 0.005 wt %, N: more than 0 wt % and up to 0.006 wt %, and the balance of Fe and other unavoidable impurities. The steel material may further include a combination of Cr and Mo: more than 0 wt % and up to 1.0 wt %.

[0064] In the method according to the present disclosure, the semi-finished product to be hot-rolled may be, for example, a slab. The slab provided as a semi-finished product may be produced by continuously casting molten steel with a certain composition obtained through a steelmaking process.

[0065] The steel material, e.g., a slab, is reheated at a slab reheating temperature (SRT) of, for example, 1,150 C. to 1,250 C. for, for example, 1 hour to 5 hours. Through the reheating process, the components segregated during casting and the precipitates may redissolve. As such, the steel material is homogenized and ready for hot rolling. When the SRT is lower than 1,150 C., the components segregated during casting may not be sufficiently redissolved, leading to uneven distribution. When the SRT is higher than 1,250 C., the coarsening of austenite grains may cause a decrease in yield strength. In addition, as the SRT increases, the heating costs and the additional time required to reach the hot rolling temperature may result in higher production costs and reduced productivity. When the reheating time is shorter than 1 hour, the segregation zones may not be reduced sufficiently. When the reheating time is longer than 5 hours, the grain size may increase, and the process costs may rise.

[0066] Subsequently, the reheated steel material is heated and then hot-rolled to adjust its shape. The hot rolling process may be performed continuously through rough rolling and finishing rolling. Due to the hot rolling process, the steel material may be formed into a hot-rolled steel material. The hot-rolled steel material may be a hot-rolled steel sheet.

[0067] As described above, to enhance the formability of the final steel sheet, a uniform structure needs to be obtained by minimizing the segregation of Mn and the like during the hot rolling process. To this end, the segregation of Mn may be controlled by appropriately adjusting control conditions such as the temperatures and reduction ratios of rough rolling and finishing rolling, and Mn may be uniformly diffused by inhibiting the formation of a band structure. Thus, an ultra-high-strength steel including a hard phase with a uniform hardness after final tempering may be manufactured.

[0068] For this purpose, the rough rolling process may be performed at, for example, 1,000 C. to 1,150 C. The rough rolling process may be performed in multiple passes while moving back and forth through the roughing mill. In this case, the reduction ratio in the last pass may be 40% or more, e.g., 40% to 50%. By setting a high reduction ratio of 40% or more in the last pass of the rough rolling process, austenite may be refined, and the segregation of Mn may be reduced.

[0069] In the finishing rolling process, rolling through the final 3-high stand is performed at a temperature of 1020 C. or lower, e.g., 880 C. to 1020 C. The reduction ratio in the first pass is 40% or more, and the total reduction ratio through the final 3-high stand needs to be controlled to be 40% to 60%. In this case, the time taken for the steel sheet to pass through the final 3-high stand needs to be controlled to be as short as possible, for example, no longer than 2.0 sec. (and longer than 0 sec.). By controlling the reduction ratio in the finishing rolling process as described above, the segregation of Mn and P may be reduced.

[0070] The finishing delivery temperature (FDT) ranges from 880 C. to 980 C. When the FDT is lower than 880 C., the rolling load may increase rapidly to cause a decrease in productivity. When the FDT is higher than 980 C., grains may coarsen to cause a decrease in strength.

[0071] Then, the hot-rolled steel material is cooled. The time taken from the final pass of finishing rolling to the start of cooling needs to be controlled to be as short as possible, for example, no longer than 1.5 sec. (and longer than 0 sec.). The cooling process may be performed using air cooling or water cooling at a cooling rate of, for example, 10 C./s to 50 C./s. The cooling process may be performed to a coiling temperature of, for example, 500 C. to 700 C. When the cooling rate is less than 10 C./s, the average particle size of precipitates may increase and thus strength may not be easily achieved. On the other hand, when the cooling rate is greater than 50 C./s, the microstructure of the steel material may become harder and thus impact toughness may decrease.

[0072] Thereafter, the hot-rolled steel sheet is coiled at a coiling temperature (CT) ranging, for example, from 500 C. to 700 C. When the CT is lower than 500 C., the significant difference between the FDT and CT may degrade the surface quality of the steel material, and the increased strength may increase the rolling load during cold rolling. When the CT is higher than 700 C., carbonitride elements may not remain in a solid solution form, undesired precipitates may be formed, and defects may occur in subsequent processes due to surface oxidation or the like. The coiled steel material may be cooled to room temperature.

Softening

[0073] Optionally, after the hot-rolled steel sheet is produced, the hot-rolled steel sheet may be softened at a temperature ranging, for example, from 500 C. to 650 C. for, for example, 1 hour to 10 hours. The softening step may effectively control the influence on the microstructure of the hot-rolled steel sheet after hot rolling and before cold rolling.

[0074] In general, the coiling temperature during the hot rolling process influences the microstructure and properties of the hot-rolled steel sheet. Additionally, the cooling rate of the coil after coiling may have a similar effect. The coiling temperature may not be easy to control uniformly across the entire width/length of the steel sheet, Furthermore, during cooling in the yard after coiling, seasonal factors or the proximity of other coils may vary the cooling rate, causing significant material property deviations in the hot-rolled steel sheet. These variations in material properties may continuously affect the subsequent cold rolling process, significantly influencing the quality of the final product. To eliminate the material property deviations or the influence on the microstructure of the hot-rolled steel sheet, the softening step may be performed.

[0075] Due to softening, the hot-rolled steel sheet may become softened, and the rolling load during the subsequent cold rolling process may decrease. Additionally, the thickness variation commonly observed when cold rolling high-strength steel may be reduced, and shape control may be easily achieved.

[0076] When the softening temperature is lower than 500 C., the hot-rolled steel sheet is not sufficiently softened, and the influence on the microstructure of the finally obtained cold-rolled steel sheet after hot rolling may not be eliminated. In addition, the microstructure after softening may become non-uniform.

[0077] When the softening temperature is higher than 650 C., a non-uniform austenite phase may be formed, and unnecessary phases may be formed during cooling to affect the annealing conditions of the finally produced steel sheet.

[0078] When softening is performed for a long time at 600 C. or higher, various alloy carbides may precipitate during heat treatment and may not be easily redissolved during subsequent continuous annealing, preventing the desired mechanical properties from being achieved. Thus, the softening time may be within 10 hours.

Cold-Rolled Steel Sheet Production (S120)

[0079] The cold-rolled steel sheet production step S120 is performed to obtain the thickness of the finally produced steel sheet by using the hot-rolled steel sheet. The coiled hot-rolled steel sheet is pickled with acid. Then, a cold-rolled steel sheet is formed by cold rolling the pickled hot-rolled steel sheet with a cold rolling reduction ratio of 40% to 60%. When the cold rolling reduction ratio is less than 40%, because nucleation for recrystallization during subsequent soaking is insufficient, grains may grow excessively during soaking and thus strength may rapidly decrease. When the cold rolling reduction ratio is greater than 60%, because nucleation occurs excessively, grains formed during soaking may become excessively fine, and ductility and formability may decrease.

[0080] After cold rolling is completed, the desired final microstructure may be obtained through certain heat treatment processes. FIG. 3 is a graph showing the heat-treatment history of an ultra-high-strength cold-rolled steel sheet according to an embodiment of the present disclosure. The heat treatment processes for the cold-rolled steel sheet will now be described in detail with reference to FIGS. 1 and 3.

Primary Soaking (S130)

[0081] In the primary soaking step S130, the cold-rolled steel sheet may be soaked in a typical continuous annealing furnace with a slow cooling period. In the primary soaking process, the cold-rolled steel sheet is heated at a heating rate of, for example, 1 C./s or more, and more specifically, 1 C./s to 10 C./s, to a primary soaking temperature of, for example, Ac3-30 C. to 900 C. The steel sheet is maintained at the primary soaking temperature for, for example, 30 sec. to 200 sec. Due to the primary soaking process, the desired austenite fraction may be achieved. When the primary soaking temperature is lower than Ac3-30 C. or the holding time is shorter than 30 sec., sufficient austenite may not be easily formed, and the increased ferrite fraction may result in a decrease in strength. When the primary soaking temperature is higher than 900 C. or the holding time is longer than 200 sec., the austenite grain size may coarsen, or productivity may decrease excessively.

Primary Cooling (S140)

[0082] In the primary cooling step S140, the primarily soaked cold-rolled steel sheet is primarily cooled at a cooling rate of, for example, 5 C./s to 15 C./s, to a primary cooling temperature of, for example, 620 C. to 720 C. The cooling process may be performed using air cooling or water cooling. The primary cooling process may also be called a slow cooling process. The primary cooling process is performed to achieve plasticity by obtaining a certain amount of ferrite in the final microstructure. When the primary cooling temperature is lower than 620 C., excessive ferrite transformation may lead to a decrease in strength. When the primary cooling temperature is higher than 720 C., the significant temperature difference with the subsequent secondary cooling process may cause quality degradation or a reduction in productivity.

Secondary Cooling (S150)

[0083] In the secondary cooling step S150, the primarily cooled cold-rolled steel sheet is secondarily cooled at a cooling rate of, for example, 15 C./s to 100 C./s to a secondary cooling temperature of, for example, 250 C. to 480 C. The secondary cooling process may also be called a rapid cooling process. During the secondary cooling process, additional ferrite transformation needs to be inhibited.

Secondary Soaking (S160)

[0084] In the secondary soaking step S160, the secondarily cooled cold-rolled steel sheet is secondarily soaked at a secondary soaking temperature of 250 C. to 480 C. for 50 sec. to 300 sec. In the secondary soaking step, a certain amount of bainite is formed by constantly maintaining the temperature within the range from 250 C. to 480 C. Bainite formed at this time breaks down austenite. Therefore, when austenite transforms into martensite during the subsequent cooling process, the size of martensite may decrease. As such, the size of tempered martensite formed after tempering may also decrease to contribute to the refinement of tempered martensite.

Tertiary Cooling (S170)

[0085] In the tertiary cooling step S170, the cold-rolled steel sheet is tertiarily cooled to a tertiary cooling temperature of, for example, room temperature (e.g., 0 C. to 40 C.) to 150 C. In the tertiary cooling step S170, austenite transforms into fresh martensite while cooling to a temperature at or below Ms.

Tertiary Soaking (S180)

[0086] In the tertiary soaking step S180, the tertiarily cooled cold-rolled steel sheet is heated at a heating rate of, for example, 1 C./s or more, and more specifically, 1 C./s to 10 C./s, to a tertiary soaking temperature of, for example, 150 C. to 300 C., and the tertiary soaking temperature is maintained for, for example, 100 sec. to 30000 sec. In the tertiary soaking step, fresh martensite formed during the secondary cooling step transforms into tempered martensite, the number of carbide particles may be controlled, and C may be enriched in retained austenite for stabilization. As such, high strength and high elongation may be achieved, and the configuration of the final microstructure may be maintained. When the tertiary soaking temperature is lower than 150 C. or the holding time is shorter than 100 sec., martensite may be insufficiently tempered or retained austenite may be insufficiently stabilized. When the tertiary soaking temperature is higher than 300 C. or the holding time is longer than 30000 sec., excessively tempering of martensite may lead to a decrease in strength, and phase transformation of retained austenite may cause a deterioration in formability.

[0087] After the tertiary soaking step S180 is completed, the cold-rolled steel sheet is cooled at a cooling rate of 1 C./s to 100 C./s to room temperature, for example, 0 C. to 40 C.

[0088] According to an embodiment of the present disclosure, after the primary soaking step S130, instead of directly dropping the temperature to Ms or below during rapid cooling after slow cooling, secondary soaking is performed to form a certain amount of bainite and then cooling is performed to Ms or below.

[0089] FIG. 2 is a flowchart of a method of manufacturing an ultra-high-strength hot-dip galvanized cold-rolled steel sheet, according to an embodiment of the present disclosure, and FIG. 4 is a graph showing the heat-treatment history over time of an ultra-high-strength hot-dip galvanized cold-rolled steel sheet according to an embodiment of the present disclosure. FIG. 4 shows both a hot-dip galvanizing step S161 and an alloying step S162.

[0090] In the current embodiment, steps S110 to S150 are the same as those of the above-described method of manufacturing an uncoated ultra-high-strength cold-rolled steel sheet.

[0091] In the current embodiment, as shown in FIG. 4, after the secondary soaking step is completed, the cold-rolled steel sheet is inserted into a galvanizing bath and hot-dip galvanized (see S161 of FIG. 2).

[0092] Alternatively, immediately after the secondary soaking temperature is reached, the cold-rolled steel sheet may be inserted into the galvanizing bath and hot-dip galvanized. In this case, the secondary soaking step S160 may be regarded as the hot-dip galvanizing step.

[0093] When necessary, an alloying step (see S165 of FIG. 4) may be additionally performed after the hot-dip galvanizing step to form a hot-dip galvanized steel material and an alloyed hot-dip galvanized steel material.

Hot-Dip Galvanizing (S161)

[0094] In the hot-dip galvanizing step, the cold-rolled steel sheet is dipped into the galvanizing bath to form a hot-dip galvanized layer. The temperature of the galvanizing bath may vary depending on the types and ratio of alloying elements for configuring the galvanized layer, and the composition system of the cold-rolled steel sheet, and may be, for example, 450 C. to 480 C. The cold-rolled steel sheet is dipped into the galvanizing bath and held for, for example, 30 sec. to 100 sec. Under the above conditions of the galvanizing bath, the hot-dip galvanized layer may be easily formed on the surface of the cold-rolled steel sheet, and the adhesion of the galvanized layer may be enhanced.

[0095] Although FIG. 4 shows both the hot-dip galvanizing step and the alloying step, when the alloying step is not performed after the hot-dip galvanizing step, the galvanized steel sheet removed from the galvanizing bath directly undergoes the tertiary cooling step S170. Therefore, austenite transforms into martensite in the current step. Then, the tertiary soaking step S180 is performed. The description of the tertiary soaking step S180 has been already provided above, and thus will not be repeated here to avoid redundancy.

Alloying (S165)

[0096] When necessary, the cold-rolled steel sheet with the hot-dip galvanized layer may be alloyed. For the alloying step, the galvanized steel sheet removed from the galvanizing bath may be loaded into heat treatment equipment, and alloyed. The alloying step may be performed by maintaining a temperature of, for example, 500 C. to 600 C. for, for example, 1 sec. to 20 sec. When the alloying step is performed under the above conditions, the hot-dip galvanized layer may grow stably, and the adhesion of the galvanized layer may be enhanced. When the alloying temperature is lower than 500 C., insufficient alloying may compromise the integrity of the hot-dip galvanized layer. When the alloying temperature is higher than 600 C., entering the intercritical temperature range may change the material properties. The alloyed steel sheet undergoes the tertiary cooling step S170. Therefore, austenite transforms into martensite in the current step. Then, the tertiary soaking step S180 is performed. The description of the tertiary soaking step S180 has been already provided above, and thus will not be repeated here to avoid redundancy.

Test Examples

[0097] Test examples will now be described for better understanding of the present disclosure. However, the following test examples are merely to promote understanding of the present disclosure, and the present disclosure is not limited to thereto. The details not described herein may be easily inferred by one of ordinary skill in the art, and therefore, further explanation is omitted.

[0098] Steel materials with the compositions (unit: wt %) of Tables 1 and 2 were prepared. In Tables 1 and 2, the balance consists of Fe and impurities that are inevitably introduced during the steelmaking process or the like. The unit for the content of each component is wt %.

TABLE-US-00001 TABLE 1 Steel Type C Si Mn P S Al N Cr A 0.163 1.02 2.66 0.011 0.0023 0.030 0.0037 0 B 0.230 1.88 2.02 0.009 0.0020 0.015 0.0024 0 C 0.116 0.63 3.32 0.016 0.0030 0.030 0.0025 0 D 0.212 1.49 2.17 0.012 0.0015 0.024 0.0031 0 E 0.186 1.81 2.76 0.014 0.0016 0.037 0.0033 0.04

TABLE-US-00002 TABLE 2 Steel Ac Ac Ms Type Nb Ti V B 1( C.) 3( C.) ( C.) A 0 0 0.028 0.0005 724 843 361 B 0 0 0 0.0005 756 848 380 C 0 0 0 0.0005 706 793 389 D 0.012 0 0 0.0005 743 836 383 E 0.001 0.018 0.003 0.0007 747 852 376

[0099] Referring to Tables 1 and 2, steel types A, B, D, and E meet the composition range of the present disclosure. Steel type C has a Si content lower than the lower limit of the composition range of the present disclosure, and an Mn content higher than the upper limit of the composition range of the present disclosure.

[0100] Cold-rolled steel sheets were produced by hot rolling and cold rolling steel types A to E.

[0101] Table 3 shows the hot rolling condition values of the ultra-high-strength cold-rolled steel sheets of the test examples.

TABLE-US-00003 TABLE 3 Finishing Rolling Time from Final Finishing Rough 3- 1.sup.st Rolling Rolling High Pass End to urnace Last Pass Reduction Reduction Cooling ooling oiling Softening teel RT DT Reduction DT Ratio Ratio Start Rate T Temp. No. Type C.) C.) Ratio C.) (%) (%) (sec.) C./s) C.) ( C.) Test 220 090 42 60 45 41 0.88 0 00 Example 1 Test 220 060 41 20 42 42 0.88 0 00 Example 2 Test 250 010 40 40 53 46 0.88 0 00 Example 3 Test 250 005 43 00 55 48 0.88 0 00 Example 4 Test 230 005 45 00 48 52 0.88 0 70 600 Example 5 Test 230 030 45 00 49 51 0.88 0 70 600 Example 6 Test 210 050 41 40 45 43 0.88 0 30 Example 7 Test 210 050 42 20 43 45 0.88 0 30 Example 8 Test 207 080 45 57 49 46 0.88 0 00 600 Example 9 Test 205 090 43 34 52 43 0.88 0 00 600 Example 10 indicates data missing or illegible when filed

[0102] Referring to Table 3, Test Examples 1 to 10 were produced while meeting the hot rolling conditions proposed in the present disclosure. Test Examples 5, 6, 9, and 10 were produced by further performing softening at 600 C. for 2 hours.

[0103] Table 4 shows the heat treatment condition values of the test examples after cold rolling.

TABLE-US-00004 TABLE 4 Primary Primary Secondary Secondary Galvanizing ertiary Teriary Soaking Cooling Cooling Soaking alvanizing lloying Cooling Soaking teel emp. ime emp. ate emp. ate emp. ime Temp. Temp. emp. emp. ime No. Type C.) sec.) C.) C./s) C.) C./s) ( C.) sec.) C.) C.) C.) C.) sec.) Test 25 0 80 0 80 0 80 50 60 01 0 80 00 Example 1 Test 25 0 80 0 80 0 80 50 60 06 50 50 00 Example 2 Test 40 0 80 0 00 0 00 00 60 12 0 00 00 Example 3 Test 40 0 80 0 00 0 00 00 60 05 0 00 00 Example 4 Test 10 0 80 0 00 0 00 50 60 03 0 10 00 Example 5 Test 10 0 80 0 00 0 00 50 60 15 0 50 0 Example 6 Test 30 0 80 0 90 0 90 00 60 11 0 80 0000 Example 7 Test 30 0 80 0 90 0 90 0 60 16 0 00 0 Example 8 Test 50 0 80 00 0 00 00 0 00 0000 Example 9 Test 50 0 80 60 0 60 0 60 20 0 00 5000 Example 10 indicates data missing or illegible when filed

[0104] Referring to Table 4, among Test Examples 1 to 10, the final product of Test Example 9 was a cold-rolled steel sheet (CR) without a galvanized layer, and the final product of the others was an alloyed hot-dip galvanized steel sheet (GA). Test Examples 1, 7, 9, and 10 were produced while meeting all the process conditions proposed in the present disclosure. The galvanizing temperature in Table 4 indicates the temperature of molten zinc in the galvanizing bath.

[0105] Test Example 2 has a tertiary cooling temperature higher than the upper limit of the process range of the present disclosure, Test Example 3 has a tertiary soaking temperature higher than the upper limit of the process range of the present disclosure, Test Example 4 has a primary soaking temperature lower than the lower limit of the process range of the present disclosure, and a tertiary soaking temperature higher than the upper limit of the process range of the present disclosure, Test Example 5 has a tertiary soaking temperature higher than the upper limit of the process range of the present disclosure, Test Example 6 has a tertiary soaking holding time lower than the lower limit of the process range of the present disclosure, and Test Example 8 has a tertiary soaking temperature higher than the upper limit of the process range of the present disclosure, and a tertiary soaking holding time lower than the lower limit of the process range of the present disclosure.

[0106] Table 5 shows the results of measuring the mechanical properties of the test examples, e.g., a tensile strength (TS), an elongation (EL), a hole expansion ratio (HER), and a product of tensile strength, elongation, and hole expansion ratio (TSELHER/1000).

TABLE-US-00005 TABLE 5 teel TS L ER TS EL HER/1000 No. Type Galvanizing (MPa) %) %) (MPa %.sup.2) Remarks Test GA 1185 6 2 607 Inventive Example Example 1 Test GA 1151 6 8 331 Comparative Example Example 2 Test GA 1078 2 5 830 Comparative Example Example 3 Test GA 690 0 0 414 Comparative Example Example 4 Test GA 1036 7 1 370 Comparative Example Example 5 Test GA 1280 0 205 Comparative Example Example 6 Test GA 1230 6 3 649 Inventive Example Example 7 Test GA 1320 4 5 277 Comparative Example Example 8 Test CR 1223 4 5 599 Inventive Example Example 9 Test GA 1219 5 8 512 Inventive Example Example 10 indicates data missing or illegible when filed

[0107] Referring to Table 5, Test Examples 1, 7, 9, and 10 meet all the process conditions proposed in the present disclosure, and exhibit TS, EL, HER, and TSELHER/1000 values which fall within the target ranges of the present disclosure.

[0108] On the contrary, Test Examples 2, 3, 4, 5, 6, and 8 do not meet the process conditions proposed in the present disclosure, and exhibit mechanical properties which fall outside the target ranges of the present disclosure.

[0109] Specifically, Test Examples 2, 4, and 5 exhibit TS and TSELHER/100 values lower than the lower limits of the target ranges of the present disclosure, Test Example 3 exhibits a TS value lower than the lower limit of the target range of the present disclosure, Test Example 6 exhibits EL and TSELHER/1000 values lower than the lower limits of the target ranges of the present disclosure, and Test Example 8 exhibits a TS value higher than the upper limit of the target range of the present disclosure, and a TSELHER/1000 value lower than the lower limit of the target range of the present disclosure.

[0110] Table 6 shows the microstructures of the test examples, e.g., area fractions of ferrite, retained austenite, bainite, fresh martensite (FM), and tempered martensite (TM), a fresh martensite/tempered martensite (FM/TM) ratio, and the number of tempered martensite particles5 m.

TABLE-US-00006 TABLE 6 No. of Tempered Martensite Retained Tempered Fresh Particles 5 m teel Ferrite Austenite Martensite Martensite Bainite (10.sup.6 M/TM No. Type (area %) (area %) (area %) (area %) (area %) p/mm.sup.2) Ratio Remarks Test 18 10 47 8 17 2 .2 Inventive Example Example 1 Test 18 10 25 17 30 0 .7 Comparative Example Example 2 Test 34 15 35 3 13 3 .1 Comparative Example Example 3 Test 77 2 5 5 11 0 .0 Comparative Example Example 4 Test 10 6 70 6 8 10 .1 Comparative Example Example 5 Test 10 3 10 70 7 10 .0 Comparative Example Example 6 Test 5 14 53 10 18 2 .2 Inventive Example Example 7 Test 5 12 15 45 23 3 .0 Comparative Example Example 8 Test 15 10 58 6 11 4 .1 Inventive Example Example 9 Test 14 10 52 6 18 2 .1 Inventive Example Example 10 indicates data missing or illegible when filed

[0111] Test Examples 1, 7, 9, and 10 correspond to embodiments of the present disclosure, and exhibit area fractions of ferrite, retained austenite, bainite, fresh martensite (FM), and tempered martensite (TM), the number of tempered martensite particles5 m, and a fresh martensite/tempered martensite (FM/TM) ratio, which fall within the target ranges of the present disclosure.

[0112] On the contrary, Test Example 2 exhibit a bainite content and FM/TM ratio higher than the upper limits of the target ranges of the present disclosure. It is analyzed that Test Example 2 exhibits a low mechanical strength due to the relatively high content of bainite because the tertiary cooling temperature is higher than the upper limit of the process range of the present disclosure.

[0113] Test Example 3 has a tertiary soaking temperature higher than the upper limit of the process range of the present disclosure, and exhibits mechanical properties lower than the target range due to the area fraction of ferrite higher than the upper limit of the target range of the present disclosure.

[0114] Test Example 4 has a primary soaking temperature lower than the lower limit of the process range of the present disclosure, and a tertiary soaking temperature higher than the upper limit of the process range of the present disclosure, and thus exhibits very low TS and TSELHER values due to the 77% area fraction of ferrite, which indicates that ferrite is the dominant phase of the microstructure.

[0115] Test Example 5 has a Si content lower than and an Mn content higher than the composition of the present disclosure, and a tertiary soaking temperature higher than the upper limit of the process range of the present disclosure. As such, Test Example 5 exhibits a low FM/TM ratio and TS and TSELHER/1000 values lower than the lower limits of the target ranges of the present disclosure.

[0116] Test Example 6 has a Si content lower than and an Mn content higher than the composition of the present disclosure, and a tertiary soaking holding time lower than the lower limit of the process range of the present disclosure. As such, Test Example 6 exhibits an FM/TM ratio higher than the upper limit of the target range of the present disclosure due to insufficient tempering, and thus exhibits EL and TSELHER/1000 values lower than the lower limits of the target ranges of the present disclosure.

[0117] Test Example 8 has a tertiary soaking temperature higher than the upper limit of the process range of the present disclosure. As such, Test Example 8 exhibits an area fraction of bainite higher than the upper limit of the target range of the present disclosure due to excessive tempering, and thus exhibits TS and TSELHER/1000 values lower than the target ranges of the present disclosure.

[0118] While the present disclosure has been particularly shown and described with reference to embodiments thereof, it will be understood by one of ordinary skill in the art that various changes in form and details may be made therein without departing from the scope of the present disclosure as defined by the following claims.