KR102057777B1 - Ultra high strength cold rolled steel sheet and method for manufacturing the same - Google Patents

Abstract

Disclosed are an ultra high strength cold rolled steel sheet and a method of manufacturing the same. In one embodiment, the ultra-high strength cold rolled steel sheet includes carbon (C): 0.12 to 0.22 wt%, silicon (Si): 1.6 to 2.4 wt%, manganese (Mn): 2.0 to 3.0 wt%, aluminum (Al): 0.01 to 0.05% by weight, the sum of at least one of niobium (Nb), titanium (Ti) and vanadium (V) greater than 0 and 0.05% by weight or less, phosphorus (P): greater than 0 and 0.020% by weight or less, sulfur (S): greater than 0 and 0.005 weight % Or less, and a residual amount of iron (Fe) and other unavoidable impurities, and has a microstructure including martensite and residual austenite at room temperature, the volume fraction of the retained austenite is 10-15% by volume, yield Strength (YP): 850 MPa or more, Tensile Strength (TP): 1180 MPa or more, Elongation (El): 14% or more, and Hole Expansion Device (HER): 30% or more measured according to ISO 16630.

Description

Ultra high strength cold rolled steel sheet and manufacturing method {ULTRA HIGH STRENGTH COLD ROLLED STEEL SHEET AND METHOD FOR MANUFACTURING THE SAME}

The present invention relates to an ultra high strength cold rolled steel sheet and a method of manufacturing the same. More specifically, the present invention relates to an ultra high strength cold rolled steel sheet having improved formability and a method of manufacturing the same.

There is a demand for a steel plate having a higher strength and a higher ductility for the purpose of crash safety of a vehicle and weight reduction of a vehicle body. On the other hand, members and filler parts that affect collision safety among automotive parts have a complicated shape, and a situation in which steel sheets having excellent moldability are required to manufacture them.

Ultra-high strength steels, on the other hand, are dual-phase steels that secure elongation in two phases, ferrite and martensite, and metamorphic organics that secure strength and elongation through phase transformation of residual austenite in the final structure during plastic deformation. Transformation induced plasticity steel.

However, the development of low-strength metamorphic organic-plastic steel base, which is composed of bainite and main steel, which cannot escape the limits of the Rule of mixture (ROM), has reached its limit. Therefore, the development direction of the steel sheet for securing high strength and proper elongation by replacing the main base of the metamorphic organic plastic steel with martensite instead of bainite is attracting attention.

Background art of the present invention is disclosed in Japanese Patent Laid-Open No. 6220745 (December 22, 2017, Published Name: High Strength Cold Rolled Steel Sheet and Manufacturing Method Thereof).

According to one embodiment of the present invention, it is to provide an ultra-high strength cold rolled steel sheet having excellent moldability such as rigidity and hole expandability.

According to one embodiment of the present invention, to provide an ultra-high strength cold rolled steel sheet excellent in productivity and economy.

According to an embodiment of the present invention, to provide a method for producing the ultra-high strength cold rolled steel sheet.

One aspect of the invention relates to ultra high strength cold rolled steel sheet. In one embodiment, the ultra-high strength cold rolled steel sheet includes carbon (C): 0.12 to 0.22 wt%, silicon (Si): 1.6 to 2.4 wt%, manganese (Mn): 2.0 to 3.0 wt%, aluminum (Al): 0.01 to 0.05% by weight, the sum of at least one of niobium (Nb), titanium (Ti) and vanadium (V) greater than 0 and 0.05% by weight or less, phosphorus (P): greater than 0 and 0.020% by weight or less, sulfur (S): greater than 0 and 0.005 weight At least%, and at least one of ferrite and bainite at room temperature, the balance comprising iron (Fe) and other unavoidable impurities; Martensite; And a microstructure including residual austenite, and the volume fraction of the retained austenite is 10-15% by volume, yield strength (YP): 850 MPa or more, tensile strength (TP): 1180 MPa or more, and elongation (El): 14% or more, and HER: 30% or more, measured according to ISO 16630.

In one embodiment the ultra-high strength cold rolled steel sheet is at least one of ferrite and bainite at room temperature 0 to 10% by volume; Martensite 75-90% by volume and residual austenite 10-15% by volume.

In one embodiment, the microstructure of the cold rolled steel sheet may be formed by separating at least a portion of the retained austenite in an island form inside the martensite matrix.

In one embodiment, the ratio of the residual austenite area formed inside the grain boundaries of the martensite to the total area of the residual austenite may be 60% or less.

In one embodiment, the ratio of the residual austenite area having a long reduction ratio of 2.5 or more to the total area of the residual austenite may be 40% or less.

Another aspect of the invention relates to a method for producing the ultra-high strength cold rolled steel sheet. In one embodiment the ultra-high strength cold rolled steel sheet manufacturing method is carbon (C): 0.12 ~ 0.22% by weight, silicon (Si): 1.6 ~ 2.4% by weight, manganese (Mn): 2.0 ~ 3.0% by weight, aluminum (Al): 0.01 to 0.05% by weight, sum of at least one of niobium (Nb), titanium (Ti) and vanadium (V) greater than 0 and 0.05% by weight or less, phosphorus (P): greater than 0 and 0.020% by weight or less, sulfur (S): greater than 0 Manufacturing a hot rolled sheet material using a slab material containing 0.005% by weight or less, and a residual amount of iron (Fe) and other unavoidable impurities; Cold rolling the hot rolled sheet material to prepare a cold rolled sheet material; And heat treating the cold rolled sheet material; wherein the heat treatment step comprises: annealing the cold rolled sheet material by heating and maintaining it at a temperature of 870 to 900 ° C; Cooling the annealed cold rolled sheet; And reheating the cooled cold rolled sheet, wherein the cooling is performed by first cooling the cold rolled sheet to 700 to 800 ° C. at a cooling rate of 5 to 10 ° C./s; And secondly cooling the first cooled cold rolled sheet to 200 to 300 ° C at a cooling rate of 50 ° C / s or more.

In one embodiment, the reheating may be performed by heating the cooled cold rolled sheet to 370 to 430 ° C. for 100 to 250 seconds.

In one embodiment, the reheating may be performed by heating the cooled cold rolled sheet to 430-470 ° C. and maintaining it for 30-100 seconds, and then immersing it in a zinc plating bath to form a galvanized layer.

In one embodiment, the reheating may further include, after the forming of the galvanized layer, heating the cold rolled sheet material on which the galvanized layer is formed to 490 ° C. to 530 ° C. to heat and alloy the heat.

Ultra high strength cold rolled steel sheet of the present invention is excellent in rigidity and formability at the same time, it may be excellent in productivity and economic efficiency.

Figure 1 shows an ultra-high strength cold rolled steel sheet manufacturing method according to an embodiment of the present invention.
Figure 2 shows a cross section in the thickness direction after the hole expansion evaluation of the cold rolled steel sheet of the present invention.
Figure 3 shows the microstructure of an embodiment cold rolled steel sheet according to the present invention.
Figure 4 shows the initial shear surface microstructure during the evaluation of the hole expandability of the comparative example cold rolled steel sheet for the present invention.

Hereinafter, the present invention will be described in detail. In this case, when it is determined that the detailed description of the related known technology or configuration may unnecessarily obscure the subject matter of the present invention, the detailed description thereof will be omitted.

The terms to be described below are terms defined in consideration of functions in the present invention, and may be changed according to intentions or customs of users or operators, and the definitions should be made based on the contents throughout the specification for describing the present invention.

Ultra High Strength Cold Rolled Steel Sheets

One aspect of the invention relates to ultra high strength cold rolled steel sheet. In one embodiment, the ultra-high strength cold rolled steel sheet includes carbon (C): 0.12 to 0.22 wt%, silicon (Si): 1.6 to 2.4 wt%, manganese (Mn): 2.0 to 3.0 wt%, aluminum (Al): 0.01 to 0.05% by weight, the sum of at least one of niobium (Nb), titanium (Ti) and vanadium (V) greater than 0 and 0.05% by weight or less, phosphorus (P): greater than 0 and 0.020% by weight or less, sulfur (S): greater than 0 and 0.005 weight At least%, and at least one of ferrite and bainite at room temperature, the balance comprising iron (Fe) and other unavoidable impurities; Martensite; And a microstructure including residual austenite, and the volume fraction of the retained austenite is 10-15% by volume, yield strength (YP): 850 MPa or more, tensile strength (TP): 1180 MPa or more, and elongation (El): 14% or more, and HER: 30% or more, measured according to ISO 16630.

Hereinafter, the components included in the ultra-high strength cold rolled steel sheet of the present invention will be described in detail.

Carbon (C)

The carbon is added for the purpose of stabilizing austenite while securing the strength of the steel. In one embodiment, the carbon is included 0.12 to 0.22% by weight based on the total weight of the cold rolled steel sheet. By improving the stability of the austenite in the above range, it may be easy to secure an appropriate austenite structure for improving the material properties. When the carbon is included in less than 0.12% by weight it is difficult to secure the strength, and when included in excess of 0.22% by weight may lower the low-temperature impact toughness and weldability.

silicon( Si )

The silicon is an element that suppresses the formation of carbide, and is included for the purpose of preventing material degradation due to the formation of Fe ₃ C. In one embodiment, the silicon is included 1.6 to 2.4% by weight based on the total weight of the cold rolled steel sheet. When the silicon is included in less than 1.6% by weight, its addition effect is insignificant, and when it contains more than 2.4% by weight, oxide (SiO ₂ ) is formed on the surface of the steel sheet, and the plating property may be degraded according to the wettability inferior part of the part. have.

Manganese (Mn)

The manganese is an element contributing to austenite stabilization. As the manganese is added, Ms, which is a martensite formation starting temperature, is gradually lowered, thereby increasing the residual austenite fraction during the continuous annealing process.

In one embodiment the manganese is included 2.0 to 3.0% by weight based on the total weight of the cold rolled steel sheet. When the manganese is included in an amount less than 2.0% by weight, the addition effect is insignificant, and when it is included in an amount exceeding 3.0% by weight, the weldability decreases as the carbon equivalent increases, and an oxide (MnO) is formed on the surface of the steel sheet during the process. The plating property may be degraded due to the wettability inferiority of the part.

Aluminum (Al)

The aluminum is an element that plays a role of inhibiting cementite precipitation and stabilizing austenite, such as silicon (Si). Since aluminum makes fine grain boundaries and carbides of the hot rolled sheet material, it precipitates unnecessary solid solution nitrogen (N) in steel as AlN. Therefore, it has the effect of raising the strength. In one embodiment, the aluminum is included 0.01 to 0.05% by weight or less based on the total weight of the cold rolled steel sheet. When the aluminum is included in less than 0.01% by weight, the effect of addition is insignificant, and when included in excess of 0.05% by weight may reduce the continuous castability.

Niobium ( Nb ), titanium( Ti ) And vanadium (V)

The niobium (Nb), titanium (Ti) and vanadium (V) are the main elements precipitated in the form of carbides in the steel, securing the stability and strength of the retained austenite through the refinement of the initial austenite grains due to the formation of precipitates, precipitates, precipitates Precipitation hardening by the presence of is included.

When one or more of the niobium, titanium and vanadium are included in an appropriate amount, an appropriate amount of carbide can be formed to refine the grains of the retained austenite without a significant decrease in formability and elongation. This ensures the stability of the retained austenite appropriately to improve the strength, elongation, and formability of the transformation organic plastic mechanism can be advantageous for material compensation. Elongation and moldability that can fall through carbide formation can be compensated stably through the above-mentioned martensite-residual austenite microstructure distribution and microstructure shape.

In one embodiment, the sum of one or more of niobium, titanium, and vanadium is included in an amount of more than 0 and 0.05% by weight or less based on the total weight of the cold rolled steel sheet. When the sum of one or more of niobium and titanium is more than 0.05% by weight, it is difficult to exert more effects, and the material may be degraded and the manufacturing cost may be increased.

Phosphorus (P)

The phosphor plays a role similar to that of silicon. In one embodiment, the phosphorus is included more than 0 and 0.020% by weight or less based on the total weight of the cold rolled steel sheet. When the phosphorus is included in excess of 0.020% by weight, weldability is lowered, and material degradation may occur due to brittleness of the material.

Sulfur (S)

The sulfur should be controlled to the lowest possible content as an impurity element. In one embodiment, the sulfur is included in more than 0 0.005% by weight based on the total weight of the cold rolled steel sheet.

Nitrogen (N)

In another embodiment of the present invention, the cold rolled steel sheet may further include nitrogen (N) as an unavoidable impurity. When the nitrogen (N) is present in excess in the steel, a large amount of nitride is precipitated, there is a fear that ductility deterioration. In one embodiment the nitrogen may further comprise more than 0 to 0.006% by weight or less based on the total weight of the cold rolled steel sheet. When included in the content, it is possible to prevent ductile deterioration.

The cold rolled steel sheet is at least one of ferrite and bainite at room temperature; Martensite; And a microstructure comprising residual austenite. The volume fraction of the retained austenite is 10-15% by volume. When the residual austenite volume fraction is included in less than 10% by volume, it is difficult to secure elongation and formability of the cold rolled steel sheet of the present invention, and when included in excess of 15% by volume, hydrogen embrittlement resistance may decrease.

In one embodiment the ultra-high strength cold rolled steel sheet is at least one of ferrite and bainite at room temperature 0 to 10% by volume; Martensite 75-90% by volume and residual austenite 10-15% by volume. When including the microstructure in the above range, the strength and formability of the present invention can be excellent at the same time.

At least one of the ferrite and the bainite may be included in an amount of 0 to 10% by volume, depending on the process conditions in the first cooling section to be described later. The ferrite may play a role of securing the elongation of the steel sheet when it is distributed in the final structure, but it is necessary to secure the elongation because the strength decreases and too many cracks may be formed and propagated at the initial stage of deformation due to too many interphase interfaces. Otherwise it may not be included in the final organization.

The bainite may serve to alleviate the difference in strength between phases of martensite, residual austenite, and ferrite in the final tissue, but may inhibit the strength compensating ability of martensite when formed.

Martensite and residual austenite constituting the final microstructure of the cold rolled steel sheets are hard and soft tissues, respectively. Thus, in the case of a composite tissue steel having two or more phases, there is a possibility that fracture may occur due to the difference in the strength of the tissue due to crack formation at the interface between the tissues during molding.

However, both phases of martensite and retained austenite are essential to determine the material of the material. Therefore, it is necessary to minimize the formation and propagation of cracks by implementing the final microstructure in an ideal form to secure the material.

The microstructure of the cold rolled steel sheet of the present invention may be formed inside the martensite matrix, at least a part of the retained austenite in an island form.

As described above, when forming the microstructure in which the retained austenite is isolated and disconnected in the form of islands in the martensite matrix as described above, cracks may be formed at the interface between the initial deformations. That is, even if a crack is formed on the interface between phases, it is possible to prevent the propagation of additional cracks. In addition, by controlling the content of residual austenite and ferrite, it is possible to reduce the overall interphase interface length to lower the probability of crack formation, propagation and breakdown, while increasing the minimum secured strength.

Residual austenite present in the martensite matrix of the cold rolled steel sheet of the present invention may exist on grain boundaries and laths. In the case of residual austenite formed inside the martensite grain boundary, when fracture occurs at an interphase interface, continuous fracture may occur along the martensite grain boundary. Therefore, if the fraction of the retained austenite formed inside the grain boundary is limited as much as possible, there is a possibility of delaying continuous fracture.

In one embodiment, the ratio of the residual austenite area (= grain boundary residual austenite area / total residual austenite area) formed inside the grain boundary of the martensite to the total area of the residual austenite may be 60% or less. Under the above conditions, when interphase boundary cracks occur, the effect of delaying and preventing continuous fracture is excellent, and the hole expandability can be secured by improving the shear rate. For example, it may be 50% or less. For example 40% or less.

In addition, the residual austenite having a longer or shorter ratio (longer axis length / shorter length) of 2.5 or more of the residual austenite in the final microstructure of the cold rolled steel sheet of the present invention has a higher stability than that of the spherical residual austenite. There is a possibility that stable phase transformation does not occur. If metamorphic organic machinery is not expressed, the possibility of destruction due to the difference in phase boundary between phases can be increased.

In one embodiment, the ratio of the residual austenite area having a major axis length of 2.5 or more relative to the total area of the residual austenite (= a long minority ratio of 2.5 or more austenite area / total residual austenite area) is 40% or less. Can be formed. Stable phase transformation occurs during plastic deformation under the above conditions, thereby improving the shear rate and securing hole expandability. For example, it may be 30% or less. For example, it may be formed from 0 to 20%.

In the present invention, the microstructure formed by reducing the interphase boundary, isolated structure, and ensuring appropriate stability prevents microcracking and cavities on the interphase boundary that may occur during material punching before the hole expansion test, thereby inducing a healthy surface state. In addition, even if a crack is formed and propagated, it is possible to secure sound hole expandability by delaying the point of time at which fatal breakdown can be reached.

In one embodiment, the cold rolled steel sheet has a yield strength (YP): 850 MPa or more, tensile strength (TP): 1180 MPa or more and elongation (El): 14% or more, and hole expansion equipment (HER) measured according to ISO 16630 standard: Have more than 30%. For example, the cold rolled steel sheet has a yield strength (YP): 850 to 1200 MPa, tensile strength (TP): 1180 to 1400 MPa and elongation (El): 14 to 18%, and an ISO 16630 standard. ): It can be 30 ~ 50%.

Ultra High Strength Cold Rolled Steel Sheet

Another aspect of the invention relates to a method for producing the ultra-high strength cold rolled steel sheet. Figure 1 shows a method of manufacturing an ultra-high strength cold rolled steel sheet according to an embodiment of the present invention.

1, the ultra-high strength cold rolled steel sheet manufacturing method (S10) hot rolled sheet manufacturing step; (S20) cold rolled sheet manufacturing step; And (S30) heat treatment step. More specifically, the ultra-high strength cold rolled steel sheet manufacturing method (S10) carbon (C): 0.12 ~ 0.22 wt%, silicon (Si): 1.6 ~ 2.4 wt%, manganese (Mn): 2.0 ~ 3.0 wt%, aluminum (Al): 0.01 to 0.05% by weight, at least one of niobium (Nb), titanium (Ti) and vanadium (V) more than 0 up to 0.05% by weight, phosphorus (P): more than 0 to 0.020% by weight, sulfur (S ): Manufacturing a hot rolled sheet material using a slab material containing more than 0 and 0.005 wt% or less, and a balance of iron (Fe) and other unavoidable impurities; (S20) cold rolling the hot rolled sheet material to prepare a cold rolled sheet material; And (S30) heat treating the cold rolled sheet material; wherein the heat treatment step includes: annealing the cold rolled sheet material by heating and maintaining it at a temperature of 870 to 900 ° C; Cooling the annealed cold rolled sheet; And reheating the cooled cold rolled sheet, wherein the cooling is performed by first cooling the cold rolled sheet to 700 to 800 ° C. at a cooling rate of 5 to 10 ° C./s; And secondly cooling the first cooled cold rolled sheet to 200 to 300 ° C at a cooling rate of 50 ° C / s or more.

Hereinafter, the ultra-high strength cold rolled steel sheet manufacturing method will be described in detail step by step.

(S10) Hot Rolled Sheet Manufacturing Step

The step is carbon (C): 0.12-0.22% by weight, silicon (Si): 1.6-2.4% by weight, manganese (Mn): 2.0-3.0% by weight, aluminum (Al): 0.01-0.05% by weight, niobium (Nb) ), A sum of at least one of titanium (Ti) and vanadium (V) greater than 0 and 0.05 wt% or less, phosphorus (P) and greater than 0 and 0.020 wt% or less, sulfur (S) and greater than 0 and 0.005 wt% or less, and the balance of iron It is a step of producing a hot rolled sheet using a slab material containing (Fe) and other unavoidable impurities. Components and contents of the slab material is the same as the above-described cold rolled steel sheet components, detailed description thereof will be omitted.

In one embodiment, the hot rolled sheet material is a step of reheating the slab material to 1150 ~ 1250 ℃; Manufacturing a hot rolled material by hot rolling the reheated slab material at a finish rolling temperature of 880 ° C to 930 ° C; And winding the hot rolled material at a coiling temperature of 550 to 650 ° C.

In one embodiment the reheat may be reheated to a temperature of Ac3 or higher. For example, the slab reheating temperature: may be made at 1150 ~ 1250 ℃ conditions. In such conditions, the segregation component may be sufficiently reclaimed to have good product quality. If the reheating temperature is less than 1150 ℃ hot rolling load may be large, if it exceeds 1250 ℃ due to the coarsening of the initial austenite grains it may be difficult to secure the strength of the final production steel sheet. In one embodiment, the hot rolling may be performed by performing the reheated slab material at a finish rolling temperature of 880 ° C to 930 ° C to produce a hot rolled material. Under the above conditions, the crystal grain refining effect can be excellent to secure the target strength.

In one embodiment, the winding may be performed by cooling the hot rolled material at a cooling rate of 10 to 30 ° C./s, at a winding temperature of 550 to 650 ° C. Under the above conditions, the surface quality and mechanical strength of the hot rolled sheet may be excellent. In one embodiment, the hot rolled sheet material may be prepared, further comprising the step of uncoiling after the pickled hot rolled coil.

(S20) Cold Rolled Sheet Manufacturing Step

The step is cold rolling the hot rolled sheet material, to produce a cold rolled sheet material. The cold rolling is performed to match the thickness of the final production steel sheet using a hot rolled sheet material, and the pickling is performed before rolling. Since the microstructure of the final production steel sheet is determined in a subsequent heat treatment process of the cold-rolled final structure, the hot-rolled sheet structure forms an elongated structure. For example, the cold rolling can be carried out on the hot rolled sheet material under conditions of a reduction ratio of 40 to 60%. Under these conditions, the strength and workability may be excellent.

(S30) heat treatment step

The step is a step of heat-treating the cold rolled sheet material. In one embodiment, the heat treatment, the cold-rolled sheet material is heated and maintained to a temperature of 870 ~ 900 ℃ annealing; Cooling the annealed cold rolled sheet; And reheating the cooled cold rolled sheet material.

In the present invention, a high martensite fraction is required in order to secure the strength of the final steel sheet. For this purpose, it is preferable to form as much austenite as possible at the initial stage of annealing to induce phase transformation into martensite upon cooling. Therefore, the annealing is carried out at 870 ~ 900 ℃ in consideration of the temperature deviation caused by the austenite single-phase reverse heat treatment, and at the actual process line.

When the annealing is carried out at a temperature of less than 870 ℃ proceeds when the initial structure is composed of a ferrite and austenitic composite structure, the ferrite is present in the final cold-rolled steel sheet is difficult to secure the strength, the annealing exceeds 900 ℃ When carried out at temperature, the initial austenite grain size becomes excessively coarse, and the martensite matrix microstructure formed during rapid quenching also forms coarse grains, thereby lowering the strength of the final annealing material.

In one embodiment, the annealing may be performed by heating the cold rolled sheet to a temperature of 870 ~ 900 ℃ at a temperature increase rate of 3 ~ 10 ℃ / s and maintained for more than 50 seconds. When annealing at the temperature rising rate and holding time, sufficient austenite single phase reverse heat treatment may be possible. For example, the holding time during the annealing may be 60 to 500 seconds.

In one embodiment, the cooling, the cold-rolled sheet material is first cooled to 700 ~ 800 ℃ at a cooling rate of 5 ~ 10 ℃ / s; And secondly cooling the first cooled cold rolled sheet to 200 to 300 ° C at a cooling rate of 50 ° C / s or more.

After the annealing, when the cold rolled sheet is cooled to a first cooling end temperature of 700 to 800 ° C. at a cooling rate of 5 to 10 ° C./s, a final amount of ferrite is secured by attempting to secure a certain amount of ferrite in the final microstructure during the heat treatment process. When the elongation of the tissue is insufficient, the elongation can be secured, but a decrease in formability may occur due to an increase in interphase interface.

When the primary cooled cold rolled sheet material is quenched at a cooling rate of 50 ° C./s or more to a secondary cooling end temperature of 200 ° C. to 300 ° C., the final material is secured by transforming austenite in the microstructure to martensite after the primary cooling. It can be done easily.

In one embodiment, the second cooling may be carried out at a cooling rate of 50 ℃ / s or more to suppress the phase transformation that may occur during the quenching process. For example, it may be 50 ~ 500 ℃ / s.

In one embodiment, the reheating may be performed by heating the cooled cold rolled sheet to 370 to 430 ° C. and maintaining the same for 100 to 250 seconds.

Under the above conditions, carbon concentration and martensite tempering in the retained austenite may be easily proceeded to secure strength and elongation. When the reheating is performed at a temperature of less than 370 ° C., carbon partitioning between martensite and residual austenite through reheating does not proceed properly, and the austenite stability is lowered so that phase transformation does not proceed at an appropriate plastic deformation point. Sex may decrease. When the reheating is performed at a temperature exceeding 430 ° C., the matrix softens excessively during martensite tempering, and thus the strength may be reduced.

In addition, when the reheating is maintained in the temperature range to maintain less than 100 seconds, carbon partitioning does not proceed properly, the elongation is lowered, if it is maintained for more than 250 seconds, the martensite base is softened due to tempering and mechanical strength Can be lowered.

In another embodiment, the reheating may be performed by heating the cooled cold rolled sheet to 430-470 ° C. and maintaining it for 30-100 seconds, and then immersing it in a zinc plating bath to form a galvanized layer. In the above conditions, the zinc plating layer may be easily formed.

When the cold rolled sheet is heated to a temperature below 430 ° C., the temperature of the bath drops when entering the hot dip galvanizing bath, and in this case, the solubility of iron (Fe) and aluminum (Al) in the hot dip galvanizing bath is decreased, thereby forming. When the foreign matters are formed together during the formation of the plating layer, the zinc plating layer is not easily formed, and when heated to a temperature exceeding 470 ° C, the solubility of iron and aluminum in the hot dip galvanizing bath is increased so that alloying elements in the steel sheet escape. The phenomenon occurs, foreign matters are formed again when the temperature drops, and the adhesion strength and surface properties of the zinc plated layer may be reduced.

In addition, when the cold rolled sheet is heated to the temperature range and maintained at less than 30 seconds during reheating, elongation is lowered due to a decrease in residual austenite stability because carbon partitioning to residual austenite does not proceed sufficiently, resulting in 100 seconds. If it is maintained in excess of the mechanical strength of the cold rolled steel sheet of the present invention due to excessive tempering of the martensite matrix.

In one embodiment the temperature of the zinc plating bath may be 430 ~ 470 ℃. In the above conditions, the zinc plating layer may be easily formed.

In one embodiment, the reheating step may further include, after the forming of the galvanized layer, heating the cold rolled sheet material on which the galvanized layer is formed to 490 ° C. to 530 ° C., followed by alloying heat treatment. Alloying can be easily performed under the above conditions.

In metamorphic organic plastic steel, strength and elongation are secured through the martensite phase transformation of the retained austenite in the final structure during plastic deformation, but the main base is composed of bainite, which is relatively low in strength. Substituting for martensite instead of to ensure high strength and proper elongation. The present invention can ensure strength, elongation, and formability using only an appropriate alloy composition and a continuous annealing process.

In addition, the present invention is effective in terms of strength compensation by initial austenite grain refining and precipitation hardening by forming a precipitate by adding at least one strong precipitate forming element of titanium, niobium and vanadium. In addition, it is possible to improve the strength and elongation of the metamorphic organic plastic mechanism by miniaturizing the crystal grains of the retained austenite to ensure adequate stability of the retained austenite.

On the other hand, in the case of steel having two or more phases, there is a possibility that fracture may occur due to crack formation at the interface between phases during molding due to the difference in strength between phases.

However, in the present invention, the problem that cracks may be formed at the interface between the initial stages of deformation is realized by the formation of microstructure in which residual austenite in the martensite matrix is isolated and disconnected in the form of islands, even if uniformity is formed at the interface between phases. It can prevent further destruction by preventing the propagation of. In addition, the present invention limits the fraction of retained austenite present on the grain boundary to 60% or less, and furthermore, the residual austenite present in the final structure has a residual austenite ratio of 2.5 or more (long axis length / short axis length) of 2.5 or more. It can be formed in% or less to prevent the possibility of further propagation in case of cracking.

Hereinafter, the configuration and operation of the present invention through the preferred embodiment of the present invention will be described in more detail. However, this is presented as a preferred example of the present invention and in no sense can be construed as limiting the present invention.

Example  And Comparative example

Example  1 and 2 and Comparative example  1-10

Cold rolled sheet material manufacture: carbon (C): 0.12 to 0.22 wt%, silicon (Si): 1.6 to 2.4 wt%, manganese (Mn): 2.0 to 3.0 wt%, aluminum (Al): 0.01 to 0.05 wt%, niobium ( Nb), the sum of one or more of titanium (Ti) and vanadium (V) greater than 0 and 0.05% by weight or less, phosphorus (P) and greater than 0 and 0.020% by weight or less, sulfur (S) and greater than 0 and 0.005% by weight or less, and the balance of A slab material containing iron (Fe) and other unavoidable impurities was prepared. After reheating the slab material under the condition of 1150 ~ 1250 ° C., hot rolling is carried out under the condition of finishing rolling temperature: 900 ° C. to manufacture the hot rolled material, and cooling the hot rolled material at a cooling rate of 10˜30 ° C./s. Winding temperature: winding to 550 ℃ conditions; including, to prepare a hot rolled sheet material. Then, the hot rolled sheet material was cold rolled at a reduction ratio of 40 to 60% to prepare a cold rolled sheet material.

The cold rolled sheet material was annealed under the heat treatment conditions according to the following Table 1 (heating rate: 3 ~ 10 ℃ / s), the primary cooling, secondary cooling and reheating, and then cooled to room temperature to prepare a cold rolled steel sheet.

The cold rolled steel sheets of the above Examples and Comparative Examples were measured based on yield strength (YP), tensile strength (TP), elongation (total elongation (T.El) and uniform elongation (U.El)), and ISO 16630. One pore expander (HER) and the residual austenite content was measured and the results are shown in Table 2 below. At this time, the uniform elongation (U.El) and the residual austenite fraction were measured only for Examples 1-2 and Comparative Examples 1-4.

Referring to the results of Table 2, Comparative Examples 1 and 4, which are less than the reheating holding time of the present invention, Comparative Example 2, which is less than the reheating temperature, and Comparative Example 3, which exceeds the reheating holding time are Examples 1 to 2 above. Compared to the target material could not be seen. In addition, in Comparative Example 2 which does not reach the reheating temperature of the present invention, carbon partitioning between martensite and residual austenite through reheating does not proceed properly, resulting in relatively low austenite stability, and thus, phase transformation does not proceed at an appropriate plastic deformation point. Elongation and formability decrease occurred due to the hard material generated first. In addition, when the reheating holding time is exceeded as in Comparative Example 3, it was also confirmed that the martensite matrix was softened due to the tempering, and thus the tensile strength was lowered compared to the example proceeding at the same reheating temperature.

In Comparative Examples 5 to 10, the annealing was performed at 825 ° C and 850 ° C and the process simulation was performed by setting the secondary cooling end temperatures to 260 ° C, 280 ° C, and 300 ° C.

Referring to Table 2, in the case of Comparative Examples 5 to 7 after completing the second cooling at 260 ℃, 280 ℃ and 300 ℃ after annealing at 825 ℃, compared to Examples and Comparative Examples 8-10 It can be confirmed that it has yield strength and tensile strength. This is due to two factors, such as a decrease in the overall strength due to the ferrite formed when the heat treatment is performed in the ideal region rather than the single phase region during annealing, and a decrease in strength due to the decrease in the martensite fraction as the quenching end temperature increases.

In addition, Examples 8 to 10, which were subjected to annealing at 850 ° C, had a tendency to change strength according to ferrite formation and martensite fractions similarly to Comparative Examples 5 to 7, but the annealing temperature was higher than 825 ° C. As the amount is reduced by more than about 6%, it can be confirmed that it can have a higher yield strength than 1 to 3 times.

Eventually, Comparative Examples 5 to 10, which were rapidly quenched under two annealing conditions at 825 ° C and 850 ° C, are not easily secured by the heat treatment in the above-described zone, so that the austenitic single-phase heat treatment may be performed at 875 to 900 ° C. It was confirmed that the conditions were preferable.

In addition, in Comparative Examples 5 to 10, although the ferrite may be formed through an abnormal reverse heat treatment, it may be determined that the effect of improving the elongation of the ferrite is insignificant because the elongation does not show a significant improvement. It was found that it is desirable to suppress as much as possible unless excluded from the final tissue.

Figure 2 shows a cross section in the thickness direction after the hole expansion evaluation of the cold-rolled steel sheet of the present invention. Meanwhile, the cupability and bi-axial properties, which can be found in the general forming limit diagram, of the breakage of parts during molding of existing high-strength materials during molding of existing body parts. There is a case that cannot be explained by evaluation criteria such as stretchability.

This can be explained by the new evaluation criterion, Stretch-flangeability, and the hole expansion ratio test is used to estimate the fracture and the likelihood of forming the part when forming parts. Can be.

The hole expandability evaluation test according to the ISO standard 16630 is made by drilling a hole in a plate with a punch, inserting a punch into the hole, and ending the test when a crack in which the crack is completely propagated in the sheet thickness direction is observed. The hole expandability is evaluated by the ratio of the size and the size of the hole after the end of the test (initial hole size / break hole size × 100 (%)). In the test, the punched hole surface consists of three areas (Roll-over Portion; RP, Shearing Section; SS. Break Section; BS) in FIG. Among them, SS means a clean cut surface free of surface defects due to shear, and BS means a cut surface where surface defects exist due to fracture. In most hole extensibility evaluation tests, the crack area of the test piece starts and propagates in the BS area, which means that the local tensile stress of the specimen under test is in the BS area through the stress triaxial ratio which can be confirmed by the finite element analysis method. This is because the cracks grow and propagate in the microcrack and the microvoid (see FIG. 4), and finally, the fracture occurs. This can be reconfirmed as a phenomenon in which the hole expandability is greatly increased when the wire or reamer is processed to remove surface defects. Therefore, in order to improve the hole expandability of the ultra-high strength steel sheet inferior to the hole expandability, it is desirable to have a microstructure that can minimize the surface defects of the shear surface when punching.

Figure 3 shows the microstructure of Example 1 cold rolled steel sheet according to the present invention, Figure 4 shows the initial shear surface microstructure during the evaluation of the hole expandability of Comparative Example 1 cold rolled steel sheet for the present invention. 3 and 4, the cold rolled steel sheet 1 of the present invention is formed inside the martensite matrix, at least a part of the retained austenite is isolated in an island form, and inside the grain boundary of martensite. By increasing the residual austenite area formed, it can be seen that even if a uniformity is formed in the interface between phases, it is excellent in preventing further fracture by preventing propagation of additional cracks. On the other hand, in the case of Comparative Example 1 cold-rolled steel sheet, a fine crack occurs even in the initial deformation, it was found that the moldability is inferior to the Example.

In the present invention, it was confirmed that improved hole expandability and material can be obtained by controlling the final microstructure by controlling the process conditions so that the surface defect area can be reduced to maintain a relatively healthy hole surface state.

Example  3 and Comparative example  11-13

A cold rolled steel sheet was manufactured in the same manner as in Example 1, except that the cold rolled sheet material was annealed at a temperature shown in Table 3 below (heating rate: 3 to 10 ° C./s).

For Example 1, Example 3, and Comparative Examples 11 to 13, the microstructure of the cold rolled sheet at the time when the annealing was completed, and the results are shown in Table 3 below.

Referring to the results of Table 3, in the case of Examples 1 and 3 of the present invention, while the single-phase reverse heat treatment proceeds, all of them are austenitized to secure the target strength and elongation, but it is less than the annealing temperature of the present invention. In the case of Comparative Examples 11 to 13, the abnormal reverse heat treatment proceeds, and thus, the initial structure is composed of a composite structure of ferrite and austenite and the ferrite content is increased in the final steel sheet, thereby securing the strength and formability. there was.

Example  4 and Comparative example  14

Cold rolled sheet material manufacturing: carbon (C): 0.12 to 0.22 wt%, silicon (Si): 1.6 to 2.4 wt%, manganese (Mn): 2.0 to 3.0 wt%, aluminum (Al): 0.01 to 0.05 wt%, niobium ( Nb), the sum of one or more of titanium (Ti) and vanadium (V) greater than 0 and 0.05% by weight or less, phosphorus (P) and greater than 0 and 0.020% by weight or less, sulfur (S): greater than 0 and 0.005% by weight or less, and the balance A slab material containing iron (Fe) and other unavoidable impurities was prepared. After reheating the slab material under the condition of 1150 ~ 1250 ° C., hot rolling is carried out under the condition of finishing rolling temperature: 900 ° C. to manufacture the hot rolled material, and cooling the hot rolled material at a cooling rate of 10˜30 ° C./s. Winding temperature: winding to 550 ℃ conditions; including, to prepare a hot rolled sheet material. Then, the hot rolled sheet material was cold rolled at a reduction ratio of 40 to 60% to prepare a cold rolled sheet material.

Then, the cold rolled sheet material was annealed and cooled under the conditions of the following Table 4, and then the cooled cold rolled sheet was heated and maintained under the conditions of the following Table 4, and dipped and passed through a zinc plating bath to form a galvanized layer. . Next, the cold rolled sheet material on which the zinc plated layer was formed was heated to 490 to 530 ° C. to perform alloying heat treatment.

Yield strength (YP), tensile strength (TP), elongation (El), and hole expansion equipment (HER) measured in accordance with the ISO 16630 standard of the cold rolled steel sheet of Example 4 and Comparative Example 14, and retained austenite content The measurement results are shown in Table 5 below.

Referring to the results of Table 5, in the case of Example 4 according to the present invention secured the target strength and elongation, when forming a galvanized layer of the present invention, elongation and molding for Comparative Example 14 exceeding the heating time range It was found that the sex is degraded.

Simple modifications and variations of the present invention can be easily made by those skilled in the art, and all such modifications or changes can be seen to be included in the scope of the present invention.

Claims (8)

Carbon (C): 0.12 to 0.22 wt%, Silicon (Si): 1.6 to 2.4 wt%, Manganese (Mn): 2.0 to 3.0 wt%, Aluminum (Al): 0.01 to 0.05 wt%, Niobium (Nb), Titanium Sum of at least one of (Ti) and vanadium (V) greater than 0 and 0.05% by weight or less, phosphorus (P): greater than 0 and 0.020% by weight or less, sulfur (S): greater than 0 and 0.005% by weight or less, and the balance of iron (Fe) And other unavoidable impurities,
At room temperature, the microstructure is greater than 0 and 10% by volume of at least one of ferrite and bainite; Martensitic 75 to 90% by volume and residual austenite 10 to 15% by volume,
Inside the martensite matrix, at least a portion of the retained austenite is isolated in island form,
The ratio of the residual austenite area formed in the grain boundaries of the martensite to the total area of the residual austenite is 60% or less,
Regarding the total area of the retained austenite, the ratio of the retained austenite area having a long-to-short ratio of 2.5 or more is 40% or less,
Yield strength (YP): 850 ~ 1200MPa, tensile strength (TP): 1180 ~ 1400MPa and elongation (El): 14-18%, and hole expansion equipment (HER) measured according to ISO 16630 standard: 30-50% Ultra-high strength cold rolled steel sheet, characterized in that.
delete delete delete Carbon (C): 0.12 to 0.22 wt%, Silicon (Si): 1.6 to 2.4 wt%, Manganese (Mn): 2.0 to 3.0 wt%, Aluminum (Al): 0.01 to 0.05 wt%, Niobium (Nb), Titanium Sum of at least one of (Ti) and vanadium (V) greater than 0 and 0.05% by weight or less, phosphorus (P): greater than 0 and 0.020% by weight or less, sulfur (S): greater than 0 and 0.005% by weight or less, and the balance of iron (Fe) Manufacturing a hot rolled sheet material by using a slab material containing a material and other unavoidable impurities;
Cold rolling the hot rolled sheet material to prepare a cold rolled sheet material; And
And heat treating the cold rolled sheet material.
In the heat treatment step, the cold rolled sheet material is heated and maintained to a temperature of 870 ~ 900 ℃ annealing;
Cooling the annealed cold rolled sheet; And,
Reheating the cold cold rolled sheet material; is made, including;
The cooling, the cold rolled sheet material is first cooled to 700 ~ 800 ℃ at a cooling rate of 5 ~ 10 ℃ / s; And
Comprising the second step of cooling the first cold-rolled cold rolled material to 200 ~ 300 ℃ at a cooling rate of 50 ℃ / s or more, Ultra high strength cold rolled steel sheet manufacturing method
The prepared cold rolled steel sheet has a microstructure at room temperature of one or more of ferrite and bainite more than 0 to 10% by volume or less; Martensitic 75 to 90% by volume and residual austenite 10 to 15% by volume,
Inside the martensite matrix, at least a portion of the retained austenite is isolated in island form,
The ratio of the residual austenite area formed in the grain boundaries of the martensite to the total area of the residual austenite is 60% or less,
Regarding the total area of the retained austenite, the proportion of the retained austenite having a long and short ratio of 2.5 or more is 40% or less
Yield strength (YP): 850 ~ 1200MPa, Tensile strength (TP): 1180 ~ 1400MPa and Elongation (El): 14 ~ 18% and hole expansion equipment (HER) measured according to ISO 16630 Characterized in the ultra-high strength cold rolled steel sheet manufacturing method.
The method of claim 5,
The reheating, heating the cooled cold-rolled sheet to 370 ~ 430 ℃ to maintain for 100 to 250 seconds; characterized in that it comprises, ultra-high strength cold rolled steel sheet manufacturing method.
The method of claim 5,
The reheating is performed by heating the cooled cold rolled sheet to 430 ~ 470 ℃ and maintained for 30 to 100 seconds, and then immersed in a zinc plating bath to form a galvanized layer; characterized in that the ultra high strength Cold rolled steel sheet manufacturing method.
The method of claim 7, wherein
The reheating, after the step of forming the galvanized layer, the step of heating the cold-rolled sheet material on which the galvanized layer is formed to 490 ~ 530 ℃ alloying heat treatment; further comprising, ultra-high strength cold rolled steel sheet manufacturing method.

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