Ultra-High Tensile Cold-Rolled Steel Sheet and Method for Manufacturing Same

This application is a continuation of International Application No. PCT/KR2023/00874 filed on Dec. 18, 2023, which claims under 35 U.S.C. § 119(a) the benefit of Korean Patent Application No. 10-2022-089704 filed on Dec. 29, 2022, the entire contents of which applications are incorporated by reference herein.

The present disclosure relates to a steel material, and more particularly, to an ultra-high-strength cold-rolled steel sheet with a balanced improvement in strength and ductility, and a method of manufacturing the same.

Recently, the automotive industry has been applying various ultra-high-strength steel sheets to vehicles to meet conflicting goals such as vehicle lightweighting and crash safety. In general, ultra-high-strength steel sheets exhibit a decrease in ductility as strength increases. Both strength and ductility need to be enhanced to achieve these goals.

A conventional method for manufacturing an ultra-high-strength galvanized steel sheet with a strength of 1.0 GPa or more includes annealing or hot-dip galvanizing a cold-rolled steel sheet in a continuous annealing line (CAL) or a continuous galvanizing line (CGL), and cooling the cold-rolled steel sheet to the martensite start temperature (Ms) or below during final cooling to form martensite, thereby achieving strength. However, the martensite formed at this time is mainly fresh martensite, which is in a quenched state and does not contain iron carbides. This contributes to strength enhancement but significantly deteriorates hydrogen embrittlement resistance or toughness. Tempered martensite, formed by tempering the martensite, contains iron carbides and thus may contribute to improved hydrogen embrittlement resistance and toughness.

To form tempered martensite, after annealing, the steel sheet is cooled to a temperature at or below Ms, reheated for tempering, and then cooled again. For a galvanized cold-rolled steel sheet, the tempered cold-rolled steel sheet may be dipped in a galvanizing bath for galvanizing. Additionally, the galvanized cold-rolled steel sheet may be alloyed and then cooled to room temperature during final cooling to produce an ultra-high-strength galvanized cold-rolled steel sheet. However, these methods require modifications to existing equipment or the construction of new lines to configure rapid cooling and reheating systems. Furthermore, martensite formed in the previous cooling process may be excessively tempered during galvanizing and alloying, resulting in material degradation and a decrease in the stability of retained austenite. Thus, the transformation-induced plasticity (TRIP) effect may not be expected.

The present disclosure provides an ultra-high-strength cold-rolled steel sheet with a balanced improvement in strength and ductility, and a method of manufacturing the same.

However, the above description is an example, and the scope of the present disclosure is not limited thereto.

The present disclosure provides an ultra-high-strength cold-rolled steel sheet and a method of manufacturing the same.

According to an aspect of the present disclosure, there is provided an ultra-high-strength cold-rolled steel sheet including 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, preferably 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 TS×EL×HER/1000: 500 or more.

The ultra-high-strength cold-rolled steel sheet may have a mixed structure of ferrite, retained austenite, bainite, fresh martensite, and tempered martensite, an area fraction of ferrite may range from 10% to 20%, an area fraction of retained austenite may range from 5% to 20%, an area fraction of bainite may range from 5% to 20%, and a sum of area fractions of fresh martensite and tempered martensite may be a remaining are fraction.

A ratio (FM/TM) of fresh martensite (FM) to tempered martensite (TM) may be 0.1 to 0.6.

A density of iron carbide particles in tempered martensite may be 1.0×10particles/mmor more.

A grain size of tempered martensite may be 5 μm or less.

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

According to another aspect of the present disclosure, there is provided a method of manufacturing an ultra-high-strength cold-rolled steel sheet, the method including 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.

The producing of the hot-rolled steel sheet may include 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.

The hot rolling may include 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 1pass.

In certain preferred aspects, a time taken for the steel sheet to pass through the final 3-high stand during the finishing rolling process may be no longer than 2.0 sec. (and longer than 0 sec.).

In certain preferred aspects, a time taken from an end of the finishing rolling process to a start of cooling of the hot-rolled steel material may be no longer than 1.5 sec.

In aspects, the method suitably may further include softening the hot-rolled steel sheet at a temperature ranging from 500° C. to 650° C., after the hot-rolled steel sheet is produced.

In aspects, the method suitably may further include hot-dip galvanizing the cold-rolled steel sheet after the cold-rolled steel sheet is secondarily soaked.

The secondary soaking suitably may be for example hot-dip galvanizing the cold-rolled steel sheet.

In aspects, the method suitably may further include alloying the cold-rolled steel sheet after the cold-rolled steel sheet is hot-dip galvanized.

According to preferred aspects, a steel sheet, which is annealed, galvanized, or galvanized and alloyed in a continuous annealing or galvanizing line, may be cooled to the martensite start temperature (Ms) or below to form martensite, and then reheated and maintained at a constant temperature to appropriately temper martensite and stabilize retained austenite so as to prevent excessive tempering of martensite. As such, an ultra-high-strength cold-rolled steel sheet with a balanced improvement in strength and ductility may be provided.

The above-described effects of the present disclosure are examples, and the scope of the present disclosure is not limited thereto.

As referred to herein, yield strength (YP) and tensile stress (TS) and elongation (EL) can be measured using a commercially available tensile tester and according to the ISO standard ISO 6892-1, published in October 2009.

As referred to herein, a hole expansion ratio (HER) of a sample (e.g. steel sheet) can be determined by the following protocol: a test piece (e.g. a size of 100 mm×100 mm or other dimensions) is obtained from steel sheet, and a 10 mm diameter hole made in the test sample. A hole expanding test is then conducted on the perforated test sample, such as a conical punch with a vertex angle of 60° is inserted into the 10 mm hole from the punch side. The diameter d (mm) of the hole when a crack runs through the test sample (e.g. steel sheet) is measured, and the hole expansion ratio λ (%) is calculated according to the following formula.

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.

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.

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.

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.

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.

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

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.

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 %.

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.

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.

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 MnSiOmay 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.

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 %

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 %

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.

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 %

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.

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 %

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.