WO2025127409A1 - High-strength cold rolled steel sheet and manufacturing method therefor - Google Patents

Abstract

According to one embodiment of the present invention, provided is a high-strength cold rolled steel sheet comprising, by wt%, 0.08-0.15% of carbon (C), 0.8-1.5% of silicon (Si), 2.0-3.0% of manganese (Mn), more than 0% and less than or equal to 1.0% of aluminum (Al), more than 0% and less than or equal to 0.02% of phosphorus (P), more than 0% and less than or equal to 0.01% of sulfur (S), more than 0% and less than or equal to 0.01% of nitrogen (N), 0.001-0.005% of boron (B), 0.1% or less (excluding 0%) in total of titanium (Ti) and/or niobium (Nb), and the balance of iron (Fe) and other inevitable impurities, wherein the final microstructure is composed of tempered martensite, bainite, fresh martensite, ferrite and retained austenite.

Description

High-strength cold rolled steel sheet and its manufacturing method

The technical idea of the present invention relates to cold rolled steel sheets, and more specifically, to high strength cold rolled steel sheets having excellent formability along with high strength, and a method for manufacturing the same.

High-strength steels for automobile steel plates are being developed to satisfy two factors: vehicle weight reduction due to strengthened energy resource-environmental regulations and improved crash safety due to strengthened safety regulations. Steel plates used as body structural members require a tensile strength of 1 GPa. In addition, the yield ratio (YS/TS) must be high in order to improve the crash performance of the body. In order to improve crash safety and formability of parts, it is necessary to develop materials with high yield strength and excellent ductility. In particular, high-strength steel plates used for parts with complex shapes require steels with excellent properties such as elongation and hole expandability. However, strength and elongation are in a trade-off relationship, and as the strength of the material gradually increases, the problem of difficulty in forming parts occurs, and various studies are being conducted to develop high-strength steels with excellent formability.

For example, there is DP (Dual Phase) steel that evenly distributes soft ferrite and hard martensite in the microstructure to secure both strength and elongation. However, DP steel has the characteristic of having a low yield ratio (YS/TS) due to the introduction of mobile dislocations in ferrite during martensitic transformation. TRIP (Transformation Induced Plasticity) steel is a steel that utilizes the phenomenon of residual austenite transforming into martensite during plastic deformation, and has the advantage of being able to utilize both the properties of austenite with excellent formability and the properties of hard martensite after forming. TRIP steel has the characteristic of high elongation but low hole expandability.

Therefore, a steel grade that has a high yield ratio, excellent elongation, and high hole expandability is required for ultra-high strength steel plates with a strength of 1 GPa or higher.

The technical problem to be achieved by the technical idea of the present invention is to provide an ultra-high-strength cold-rolled galvanized steel sheet having excellent elongation and hole expandability and a method for manufacturing the same.

However, these tasks are exemplary and the technical idea of the present invention is not limited thereto.

According to one aspect of the present invention, a high-strength cold rolled steel sheet is provided.

According to one embodiment of the present invention, the cold rolled steel sheet contains, in wt%, carbon (C): 0.08% to 0.15%, silicon (Si): 0.8% to 1.5%, manganese (Mn): 2.0% to 3.0%, aluminum (Al): more than 0% to 1.0%, phosphorus (P): more than 0% to 0.02% or less, sulfur (S): more than 0% to 0.01% or less, nitrogen (N): more than 0% to 0.01% or less, boron (B): 0.001% to 0.005%, the total of at least one selected from titanium (Ti) and niobium (Nb): 0.1% or less (excluding 0%), and the remainder being iron (Fe) and other unavoidable impurities, and the final microstructure may be formed of tempered martensite, bainite, fresh martensite, ferrite, and retained austenite.

In one embodiment, the high-strength cold-rolled steel sheet can satisfy the following equations 1) and 2).

Equation 1): 10×C + 0.4×(Si + 6×Al) - 0.4×Mn + 15×(Ti+Nb) - 52.5×B ≤ 1.2

Equation 2): Yield ratio (YR) × elongation (EL) × hole expandability (HER) ≥ 400

(Here, the element symbol in formula 1) represents the content of that element in weight%.)

In one embodiment, the high-strength cold-rolled steel sheet may have an area fraction of the sum of the tempered martensite and the bainite of 60 to 80%, an area fraction of the fresh martensite of less than 10%, an area fraction of the ferrite of 10 to 30%, and an area fraction of the retained austenite of 1 to 5%.

In one embodiment, the ratio of the fresh martensite (FM) and the sum of tempered martensite and bainite (TM+B) may be FM/(TM+B) < 0.25.

In one embodiment, the high-strength cold-rolled steel sheet may further contain, in weight %, one or more selected from molybdenum (Mo), chromium (Cr), copper (Cu), and nickel (Ni), in a total amount of 0.5% or less (excluding 0%).

In one embodiment, the crystal grain size of the tempered martensite and bainite may be 5 μm or less.

In one embodiment, the cold rolled steel sheet may have a tensile strength (TS) of 980 MPa or more, and a product of the tensile strength (TS) and the hole expandability (HER) (TSХHER) of 50,000 MPa% or more.

In one embodiment, the titanium (Ti) may be contained in an amount of 0.015% or more and 0.04% or less, and the niobium (Nb) may be contained in an amount of 0.06% or more and 0.085% or less.

According to another aspect of the present invention, a method for manufacturing a high-strength cold rolled steel sheet is provided.

According to one embodiment of the present invention, the method for manufacturing the high-strength cold-rolled steel sheet comprises: (a) a step of hot-rolling a steel material including, in wt%, carbon (C): 0.08% to 0.15%, silicon (Si): 0.8% to 1.5%, manganese (Mn): 2.0% to 3.0%, aluminum (Al): more than 0% to 1.0%, phosphorus (P): more than 0% to 0.02% or less, sulfur (S): more than 0% to 0.01% or less, nitrogen (N): more than 0% to 0.01% or less, boron (B): 0.001% to 0.005%, a total of at least one selected from titanium (Ti) and niobium (Nb): 0.1% or less (excluding 0%), and the remainder being iron (Fe) and other unavoidable impurities; (b) a step of cold-rolling the hot-rolled steel material; (c) a step of annealing the cold-rolled steel; (d) a step of cooling the annealed steel to a temperature lower than or equal to Ms; and (e) a step of plating the cooled steel; wherein the step (a) includes a step of performing at least one pass of rolling at a reduction ratio of 40% or higher per pass and a step of rolling such that the total reduction ratio of the rolling of the first to third passes is higher than the total reduction ratio of the remaining passes, and the step (d) may include a step of first cooling to a first cooling end temperature having a range of 620°C to 720°C at a first cooling rate; and a step of second cooling to a second cooling end temperature that is a temperature lower than or equal to Ms at a second cooling rate that is faster than the first cooling rate after the first cooling.

In one embodiment, the steel material can satisfy the following equation 1).

Equation 1): 10×C + 0.4×(Si + 6×Al) - 0.4×Mn + 15×(Ti+Nb) - 52.5×B ≤ 1.2

(Here, the element symbol in formula 1) represents the content of that element in weight%.)

According to one embodiment, the method for manufacturing the cold rolled steel sheet may further contain, in weight %, at least one selected from molybdenum (Mo), chromium (Cr), copper (Cu), and nickel (Ni), in a total amount of 0.5% or less (excluding 0%).

In one embodiment, the secondary cooling end temperature may be a temperature of Ms - 140°C or higher and Ms - 30°C or lower.

In one embodiment, the titanium (Ti) may be contained in an amount of 0.015% or more and 0.04% or less, and the niobium (Nb) may be contained in an amount of 0.06% or more and 0.085% or less.

In one embodiment, the first cooling rate can have a range of 3°C/sec to 20°C/sec.

In one embodiment, the second cooling rate can range from 15°C/sec to 100°C/sec.

In one embodiment, the step (a) may include: (a-1) a step of reheating the steel at 1150 to 1300°C; (a-2) a step of hot-rolling the steel under the condition that the finishing rolling temperature is 850 to 950°C; and (a-3) a step of coiling the steel at 400 to 600°C.

In one embodiment, after step (a-2) and before step (a-3), a step of cooling to 680°C or lower at a cooling rate of 80°C/s or higher can be performed.

According to the technical idea of the present invention, a steel plate having high strength and excellent ductility can be manufactured, and thus can be suitably applied as a material for automobile structural members, etc. In addition, a cold-rolled steel plate having excellent hole expandability, crash resistance, and formability can be manufactured because the yield ratio is high compared to existing DP steel. The above-described effects of the present invention have been described as examples, and the scope of the present invention is not limited by these effects.

FIG. 1 is a flow chart schematically showing a method for manufacturing a cold rolled steel sheet according to one embodiment of the present invention, and FIG. 2 is a drawing showing an outline of a heat treatment process including annealing, cooling, and plating processes in a method for manufacturing a cold rolled steel sheet according to one embodiment of the present invention.

Figure 3 is a graph showing the properties of a cold rolled steel sheet according to an experimental example of the present invention.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings. The embodiments of the present invention are provided to more completely explain the technical idea of the present invention to those skilled in the art, and the following embodiments may be modified in various different forms, and the scope of the technical idea of the present invention is not limited to the following embodiments. Rather, these embodiments are provided to more faithfully and completely convey the technical idea of the present invention to those skilled in the art. Like reference numerals throughout this specification denote like elements. Furthermore, various elements and areas in the drawings are schematically drawn. Therefore, the technical idea of the present invention is not limited by the relative sizes or intervals drawn in the attached drawings.

In this specification and claims, the phase fraction means the area ratio (area %) derived from a microstructure photograph using an image analyzer. In addition, the content or concentration of a specific component means weight % unless otherwise specified.

The technical idea of the present invention is to provide a cold-rolled high-strength steel sheet having excellent formability, a tensile strength (TS) of 980 MPa or more and a yield ratio (YR) × elongation (EL) × hole expandability (HER) ≥ 400, and a method for manufacturing such a cold-rolled steel sheet.

In addition, according to an embodiment of the present invention, a cold rolled steel sheet having a product of tensile strength (TS) and hole expandability (HER) (TS × HER) of 50,000 MPa% or more can be provided.

Below, the alloy amount and suitable heat treatment conditions for securing the tensile strength, elongation, yield ratio, and hole expandability targeted in the present invention are described.

First, a high-strength cold-rolled steel plate according to the technical idea of the present invention will be described in detail.

A high-strength cold-rolled steel sheet according to an embodiment of the present invention contains, in wt%, carbon (C): 0.08% to 0.15%, silicon (Si): 0.8% to 1.5%, manganese (Mn): 2.0% to 3.0%, aluminum (Al): more than 0% to 1.0%, phosphorus (P): more than 0% to 0.02% or less, sulfur (S): more than 0% to 0.01% or less, nitrogen (N): more than 0% to 0.01% or less, boron (B): 0.001% to 0.005%, the sum of at least one selected from titanium (Ti) and niobium (Nb): 0.1% or less (excluding 0%), and the remainder being iron (Fe) and other unavoidable impurities. Optionally, one or more selected from molybdenum (Mo), chromium (Cr), copper (Cu), and nickel (Ni) may be further included in a total amount of 0.5% or less (excluding 0%).

Hereinafter, the role and content of each component included in the high-strength cold-rolled steel sheet according to the present invention will be described as follows. In this case, the content of the component elements all refers to the weight% of the entire steel sheet.

Carbon (C): 0.08% to 0.15%

It is added to secure the appropriate fraction and stability of retained austenite. The carbon content ranges from 0.08% to 0.15% in weight%. If the carbon content is less than 0.08%, the residual austenite fraction in the final microstructure is insufficient, making it difficult to obtain the target ductility and making it difficult to form fresh martensite. On the other hand, if it exceeds 0.15%, the strength increases excessively and carbides are likely to be formed, which may have disadvantages in weldability.

Silicon (Si): 0.8% to 1.5%

Silicon is a ferrite stabilizing element that delays the formation of carbides in ferrite and has a solid solution strengthening effect. The silicon content ranges from 0.8% to 1.5% in weight %. If the silicon content is less than 0.8%, it is difficult to exhibit the above-mentioned effect, while if it exceeds 1.5%, oxides such as Mn ₂ SiO ₄ are formed during the manufacturing process, which hinders the plating property and increases the carbon equivalent, which may lower the weldability.

Manganese (Mn): 2.0% to 3.0%

Manganese has a strengthening effect and contributes to the improvement of strength by increasing hardenability. The manganese content ranges from 2.0% to 3.0% in weight percent. If the manganese content is less than 2.0%, the effect is not sufficient and it is difficult to secure strength. If it exceeds 3.0%, the processability is reduced due to the formation or segregation of inclusions such as MnS, and the delayed fracture resistance is reduced. In addition, the carbon equivalent is increased, which can reduce the weldability.

Aluminum (Al): greater than 0% to 1.0%

Aluminum is used as a deoxidizer and can help purify ferrite. Aluminum has a range of 0% to 1.0% in weight percent. If aluminum is not contained, the deoxidation effect is insufficient, while if it exceeds 1.0%, AlN may be formed during slab manufacturing in the continuous casting step, which may cause cracks during continuous casting or hot rolling.

P: More than 0% but less than or equal to 0.02%

Phosphorus is an impurity included in the steel manufacturing process, and although it can help improve strength through solid solution strengthening, it can cause low-temperature brittleness when included in large amounts. Therefore, it is desirable to limit the phosphorus content to 0.02% or less in weight percent.

Sulfur (S): more than 0% and less than or equal to 0.01%

Sulfur is an impurity included in the steel manufacturing process, and can form non-metallic inclusions such as FeS and MnS, thereby reducing toughness and weldability. Therefore, it is desirable to limit the sulfur content to 0.01% or less in weight percent.

Nitrogen (N): More than 0% but less than or equal to 0.01%

Nitrogen is an element that is inevitably included in the production of steel. If it exceeds 0.01%, the elongation may decrease due to a delay in recrystallization, so it is desirable to reduce it as much as possible. Therefore, it is desirable to limit the nitrogen content to 0% to 0.01% of the total weight of the steel sheet.

Boron (B): 0.001% to 0.005% or less

Boron is a quenching element that suppresses the formation of polygonal ferrite and facilitates the formation of fresh martensite. Boron has a range of 0.001% to 0.005% in weight percent. When the boron content is less than 0.001%, the effect is minimal, whereas when it exceeds 0.005%, the workability deteriorates.

Total of titanium (Ti) and niobium (Nb): 0.1% or less (excluding 0%)

Titanium and niobium are strong carbonitride forming elements, and when hot rolled, they form fine precipitates with carbon and nitrogen present in the steel, thereby inhibiting grain growth and improving strength. The total content of titanium and niobium is in the range of 0.1% or less in weight%. If neither titanium nor niobium is contained in the steel, it is difficult to exhibit the precipitation strengthening effect, whereas if it exceeds 0.1%, the strength becomes excessively high and the ductility decreases.

At this time, when adding titanium, it is preferable to add titanium with a content of 0.015% to 0.04%. In order to sufficiently obtain the effect of boron, since the effect of boron cannot be obtained if boron is lost as BN by combining with nitrogen in the steel, it is preferable to induce precipitation of TiN by adding titanium in a certain amount or more. However, if the content of titanium exceeds 0.04% at this time, defects such as nozzle clogging may occur due to the formation of coarse TiN, which may deteriorate continuous castability.

The remaining components of the above high-strength cold-rolled steel sheet are iron (Fe). However, since unintended impurities may inevitably be mixed in from raw materials or the surrounding environment during the normal steelmaking process, this cannot be ruled out. Since these impurities are known to anyone skilled in the normal manufacturing process, not all of their contents are specifically mentioned in this specification.

Meanwhile, the high-strength cold-rolled steel sheet may optionally further include one or more selected from molybdenum (Mo), chromium (Cr), copper (Cu), and nickel (Ni), in a total amount of 0.5% or less (excluding 0%) in weight %.

Sum of one or more of molybdenum (Mo), chromium (Cr), copper (Cu), and nickel (Ni): 0.5% or less (excluding 0%)

Molybdenum increases the hardenability and suppresses the formation of pearlite. It also refines martensite. On the other hand, when molybdenum segregates at grain boundaries, the grain growth of ferrite stops, so the ferrite fraction decreases. To suppress this, when containing molybdenum, it is set to 0.50% or less.

Chromium, similar to manganese, has a solid-solution strengthening effect and contributes to improving strength by increasing hardenability. When the chromium content exceeds 0.5%, hardenability is excessive, the residual austenite fraction decreases, and the martensite fraction increases, which reduces ductility.

Copper is an effective element for securing corrosion resistance, and can block the corrosive environment through surface concentration. If copper is added in excess of 0.5%, the corrosion rate of the corroded steel plate may increase.

Nickel is added together with copper in the steel plate to increase corrosion resistance. When nickel is added at 0.5% or less, the density of the Cu-concentrated layer in the steel plate increases, which has the effect of increasing corrosion resistance.

The steel sheet according to the present invention having the alloy composition as described above may have a carbon equivalent defined by 10×C + 0.4×(Si + 6×Al) - 0.4×Mn + 15×(Ti+Nb) - 52.5×B (wherein C, Si, Al, Mn, Ti, Nb, and B are weight ratios of carbon, silicon, aluminum, manganese, titanium, niobium, and boron, respectively) of 1.2 or less.

Microstructure of steel plate

The microstructure of a high-strength cold-rolled steel sheet according to the technical idea of the present invention may include tempered martensite, bainite, fresh martensite, ferrite, and retained austenite. In this case, the total area fraction of tempered martensite and bainite may be 60 to 80%, the area fraction of fresh martensite may be less than 10%, the area fraction of ferrite may be 10 to 30%, and the area fraction of the retained austenite may be 1 to 5%.

The above area fraction refers to the area fraction derived from the image of the microstructure of the steel obtained by electron backscatter diffraction (EBSD) through an image analyzer. The EBSD specimen can be prepared by polishing a cold-rolled steel plate and removing surface defects. The above area fraction can be derived by calculating the image area fraction of the image obtained by EBSD through FE-SEM.

Since the hardness of tempered martensite and bainite is higher than that of ferrite and lower than that of fresh martensite, when the total fraction of tempered martensite and bainite is secured at 60 to 80%, voids between the hard and soft phases are unlikely to occur.

Fresh martensite is hard and is difficult to deform when sheared, so it can suppress the occurrence of defects in press-processed products. However, if the area fraction of fresh martensite is 10% or more, voids are likely to be generated during processing, which reduces the workability. Therefore, the area fraction of fresh martensite is set to less than 10%.

Ferrite is a soft phase, and it is effective to configure the metal structure with ferrite crystal grains that have low dislocation density and excellent ductility. To obtain this effect, the area ratio should be 10% or more. On the other hand, if the area ratio exceeds 30%, ferrite is easy to deform, so defects may occur during press processing. For this reason, the area ratio of ferrite is set to 10 to 30%.

The retained austenite phase is transformed into martensite during processing such as punching, which is likely to result in poor hole expandability. Therefore, it is recommended to control the retained austenite phase to 5% or less. In addition, it is preferably 1% or more.

Furthermore, the grain size of tempered martensite and bainite can be less than 5 ㎛. In this case, the grain size is measured as the equivalent diameter of a circle in a region surrounded by boundaries with an orientation difference of 10 degrees or more using EBSD.

The ratio of the fresh martensite (FM) and the sum of tempered martensite and bainite (TM+B), FM/(TM+B), shall be less than 0.25. If the FM/(TM+B) value is 0.25 or more, voids between the hard phase and the soft phase may easily occur during processing of cold rolled steel sheets, which may reduce processability.

The high-strength cold-rolled steel sheet of the present invention can be implemented as a cold-rolled high-strength steel sheet with excellent formability, having a tensile strength (TS) of 980 MPa or more and a yield ratio (YR) × elongation (EL) × hole expandability (HER) ≥ 400.

In addition, the cold rolled steel sheet may preferably have a product of tensile strength (TS) and hole expandability (HER) (TS × HER) of 50,000 MPa% or more.

Hereinafter, with reference to the attached drawings, a method for manufacturing a high-strength cold-rolled steel sheet according to the technical idea of the present invention and having the above-described composition range will be described.

Method for manufacturing cold rolled steel sheet

In the manufacturing method according to the present invention, the semi-finished product to be subjected to the hot rolling process may be, for example, a slab. The slab in the semi-finished state can be obtained through a continuous casting process after obtaining molten steel of a predetermined composition through a steelmaking process.

Figure 1 is a flow chart showing step-by-step a method for manufacturing a high-strength cold-rolled steel sheet according to an embodiment of the present invention.

A method for manufacturing a high-strength cold-rolled steel sheet according to an embodiment of the present invention comprises a step (S10) of hot-rolling a steel material having the above composition; a step (S20) of cold-rolling the hot-rolled steel material; a step (S30) of annealing the cold-rolled steel material; a step (S40) of first-cooling the annealed steel material; a step (S50) of second-cooling the steel material; and a step (S60) of plating the cooled steel material.

Hot rolling stage

The steel slab having the alloy composition described above is reheated to a temperature of 1150 to 1300°C. The slab is manufactured in the form of a semi-finished product by continuously casting molten steel obtained through a steelmaking process, and the reheating process homogenizes the component segregation caused by the casting process and makes it into a state that can be hot rolled. If the slab reheating temperature (SRT) is less than 1150°C, there is a problem that the segregation of the slab is not sufficiently reused, and if it exceeds 1300°C, the size of the austenite grains increases, and the process cost may increase. The reheating of the slab can be performed for 1 to 2 hours. If the reheating time is less than 1 hour, the segregation zone is not sufficiently reduced, and if it exceeds 2 hours, the grain size increases, and the process cost may increase.

After the above reheating, hot rolling including rough rolling and finish rolling is performed, and hot finish rolling is performed at a finish delivery temperature (FDT) of, for example, 850°C to 950°C to manufacture a hot rolled steel sheet. If the finish rolling temperature is lower than 850°C, the rolling load increases rapidly, which reduces productivity, and if it exceeds 950°C, the grain size may increase, which may reduce the strength.

The above rough rolling temperature is preferably set to be higher than the temperature (Tnr) at which recrystallization of austenite stops, and can be performed at, for example, 1000°C to 1150°C. In addition, when the rough rolling is performed in two or more passes, the reduction ratio of the last pass is preferably 40% or higher. Here, the reduction ratio (%) means {[thickness of material before rolling - thickness of material after rolling] / thickness of material before rolling} × 100.

During rough rolling, the grain size of the recrystallized structure due to the initial rolling is increased due to the high temperature. However, when the final pass is performed, the grain size is delayed, so the reduction ratio of the final pass has a significant effect on the grain size of the final microstructure. In addition, if the reduction ratio of the final pass of rough rolling is low, sufficient strain is not transmitted to the center, which may result in a decrease in toughness due to coarsening of the center. Therefore, it is desirable to set the reduction ratio of the final pass to 40% or more for the purpose of refining austenite and reducing manganese segregation.

The above-mentioned rough-rolled steel is subjected to finishing rolling in 5 to 7 passes. When performing the finishing rolling, the reduction ratio of the first pass is set to 40% or more, and the total reduction ratio of the rolling of the 1st to 3rd passes is set higher than the total reduction ratio of the remaining passes. This is for the purpose of refining austenite grains and reducing manganese segregation. If the total reduction ratio of the initial 1st to 3rd passes is lower than the total reduction ratio of the remaining passes, coarse austenite grains are formed and the desired microstructure cannot be obtained, it is difficult to secure strength, and a uniform microstructure cannot be obtained.

After hot rolling, it is cooled to a temperature of 400 to 600℃ and then coiled. If the coiling temperature is less than 400℃, the strength increases and the rolling load increases during cold rolling. If it exceeds 600℃, ferrite and pearlite structures are excessively generated, which may result in poor hole expandability and surface oxidation, which may cause defects in subsequent processes.

In the cooling process after the above hot rolling, cooling (rapid cooling) can be performed at a rapid cooling rate of 80°C/s or more from the finishing rolling end temperature to, for example, a temperature of 680°C, and then cooling (slow cooling) can be performed at a low cooling rate of 5°C/s or more to a coiling temperature of 600°C or lower.

The above rapid cooling step is to prevent the ferrite transformation from becoming too excessive. In addition, pearlite may be generated at a temperature higher than 680°C, and structural inhomogeneity may occur in the subsequent process, resulting in poor hole expandability. Therefore, it is preferable to apply the rapid cooling step up to a temperature of 680°C.

Cold rolling stage

The hot-rolled steel sheet is subjected to an acid pickling treatment to remove a surface scale layer. Subsequently, the hot-rolled steel sheet is cold-rolled at an average reduction ratio of, for example, 30% to 80% to form a cold-rolled steel sheet. The higher the average reduction ratio, the more the formability is improved due to the effect of refining the microstructure. When the average reduction ratio is less than 30%, it is difficult to obtain a uniform microstructure. When the average reduction ratio exceeds 80%, the roll force increases, thereby increasing the process load. The structure of the cold-rolled steel sheet may have a structure in which the structure of the hot-rolled steel sheet is elongated.

After cold rolling is completed, the cold rolled steel plate undergoes annealing heat treatment and plating processes.

FIG. 2 is a drawing showing an outline of heat treatment including annealing, cooling, and plating processes in a method for manufacturing a cold rolled steel sheet according to one embodiment of the present invention.

Referring to Figure 2 below, the heat treatment after cold rolling is described step by step.

Annealing heat treatment step

s11 to s12 in Fig. 2 represent the annealing heat treatment steps.

Referring to Figure 2, the cold rolled steel sheet is heated from room temperature to 800℃ to 900℃ (S11). The heating step (S11) is a step in which austenite is nucleated from the initial microstructure, martensite, to determine the microstructure shape. The heating rate is preferably 2℃/s or higher in consideration of productivity.

Continuing, primary cracking is performed at a temperature of 800℃ to 900℃ (S12). During annealing, the low-temperature phase is reversely transformed into ferrite/austenite, and carbon (C) and manganese (Mn) are redistributed into the austenite. For sufficient reverse transformation and alloy element redistribution, the longer the annealing time is desirable, but if the annealing time is too long, there is a concern about reduced productivity, so the annealing holding time is limited to 30 to 180 seconds.

Thereafter, the annealed steel plate is subjected to first cooling (S13) and second cooling (S14). The first cooling section (S13) can be divided into a slow cooling section (SCS), and the second cooling section (S14) can be divided into a rapid cooling section (RCS).

When cooling the above annealed steel plate, a slow cooling section may be included depending on the heat treatment equipment. When a slow cooling section is included, the slow cooling end point temperature should be 620°C to 720°C to control the ferrite phase fraction, and the cooling rate may be in the range of 3°C/sec to 20°C/sec. When the temperature and cooling rate are outside the above, the ferrite fraction decreases, thereby reducing ductility.

A steel plate that has undergone a first cooling step (S13) of slow cooling is subjected to a second cooling step (S14) of rapid cooling. The cooling end temperature of the second cooling step (S14) may be Ms-140°C to Ms-30°C. The cooling rate of the second cooling step (S14) may be set to be greater than the cooling rate of the first cooling step (S13), and may be cooled at a rate of, for example, 15°C/s or more. When the cooling rate is less than 15°C/s, polygonal ferrite or pearlite is generated during cooling, which causes deterioration of the tensile properties of the final steel, and therefore the cooling rate may be in a range of, for example, 15°C/s to 100°C/s. The end point temperature of the second cooling can effectively increase austenite stability.

In the present invention, Ms can be determined by the following equation 3), but may vary somewhat depending on process conditions, etc.

Equation 3): Ms(℃) = 539-423C-30.4Mn-12.1Cr-17.7Ni-7.5Mo

If the cooling end point temperature of the second cooling stage (S14) exceeds Ms-30℃, the redistribution of alloy elements is insufficient, making it difficult to sufficiently secure austenite stability, and as the cooling end point temperature increases, ferrite transformation occurs, which causes a decrease in strength and elongation.

After the secondary cooling end point temperature is reached, secondary cracking is performed and maintained for 10 to 100 seconds in a range of ±20℃ (S15). In the initial stage of the maintenance after rapid cooling, the temperature of the steel is homogenized, and during the isothermal maintenance, some of the retained austenite may be transformed into lower bainite, etc.

Next, reheat to a temperature of 350 to 550°C and maintain it in that temperature range for 30 to 500 seconds (S16).

Thereafter, the steel material is immersed in a molten zinc plating bath to perform plating (GI) (S17-1). The temperature of the molten zinc plating bath can be in the range of 400 to 600°C. If necessary, the steel material immersed in the plating bath is subjected to alloying heat treatment (GA) (S17-2) at a temperature range of, for example, 500 to 570°C. After the above process, the steel material is finally cooled to 100°C or less at a cooling rate of 10°C/s or more.

At this time, the plating (S17-1) and alloying heat treatment (S17-2) times may also be included in the reheating holding time.

If necessary, temper rolling can also be performed. The elongation during temper rolling is in the range of 0.1% to 1.0%.

The final microstructure of the cold rolled steel sheet implemented by the above-described manufacturing method may include tempered martensite, bainite, fresh martensite, ferrite, and retained austenite. In this case, the total area fraction of tempered martensite and bainite may be 60 to 80%, the area fraction of fresh martensite may be less than 10%, the area fraction of ferrite may be 10 to 30%, and the area fraction of the retained austenite may be 1 to 5%.

Furthermore, the ratio of the fresh martensite (FM) and the sum of tempered martensite and bainite (TM+B), FM/(TM+B), may be less than 0.25. When the FM/(TM+B) value is 0.25 or more, voids between the hard phase and the soft phase may easily occur during processing of the cold rolled steel sheet, which may reduce the processability.

The steel grade composed of the heat treatment process described above and the microstructure obtained therefrom within the composition range described in the present invention can be realized as a cold-rolled high-strength steel sheet having excellent formability, with a tensile strength (TS) of 980 MPa or more and a yield ratio (YR) × elongation (EL) × hole expandability (HER) ≥ 400.

In addition, the cold rolled steel sheet may preferably have a product of tensile strength (TS) and hole expandability (HER) (TS × HER) of 50,000 MPa% or more.

Experimental example

Hereinafter, preferred experimental examples are presented to help understand the present invention. However, the following experimental examples are provided only to help understand the present invention, and the present invention is not limited to the following experimental examples.

Regarding the analysis and measurement of this experimental example, the microstructure was analyzed using a scanning electron microscope (SEM), and the XRD analysis method was used to analyze the retained austenite fraction and the carbon content in the retained austenite. The mechanical properties were evaluated by performing a tensile test according to the KS No. 5 standard using Zwick/Roell Corp. Z100.

Steel having the composition (unit: weight%) shown in Table 1 below was prepared, and cold-rolled steel sheets according to examples and comparative examples were prepared through the specified hot rolling, cold rolling, and heat treatment processes. The remainder is iron (Fe).

River species C Si Mn Al P S Ti Nb Cr Mo B Ms(℃) Equation 1 Invention Lecture 1 0.10 0.9 2.5 0.03 0.010 0.002 0.020 - 0.3 - 0.0020 417 0.6 Invention lecture 2 0.12 1.3 2.7 0.03 0.012 0.002 0.020 - - - 0.0020 406 0.9 Invention Lecture 3 0.12 0.9 2.5 0.03 0.010 0.002 0.020 - - - 0.0020 412 0.8 Invention lecture 4 0.12 0.8 2.4 0.025 0.012 0.002 0.015 0.02 0.2 0.4 0.0013 410 1.1 Invention lecture 5 0.11 0.8 2.6 0.025 0.012 0.002 0.015 0.02 0.5 0.2 0.0014 406 0.9 Invention lecture 6 0.12 0.8 2.4 0.025 0.012 0.002 0.015 0.02 0.5 0.2 0.0015 408 1.1 Comparative Lecture 1 0.13 0.9 2.5 0.03 0.012 0.002 0.020 0.025 0.3 - 0.0005 404 1.4 Comparative Lecture 2 0.16 0.9 2.5 0.03 0.010 0.002 0.020 0.025 - - 0.0008 395 1.7 Comparative Lecture 3 0.14 0.6 2.3 0.047 0.012 0.002 0.040 0.02 0.2 0.2 0.0018 406 1.6 Comparative Lecture 4 0.12 0.6 2.3 0.5 0.012 0.002 0.010 　 0.8 0.1 0.0008 408 1.8 Comparative Lecture 5 0.13 0.6 2.3 0.054 0.012 0.002 0.040 0.02 0.2 0.06 0.0019 411 1.5 Comparative Lecture 6 0.14 0.6 2.2 0.5 0.012 0.002 0.015 　 0.8 0.1 0.0007 402 2.1 Comparative Lecture 7 0.10 0.5 2.4 0.5 0.012 0.002 0.015 　 0.8 0.1 0.0005 413 1.6 Comparative Lecture 8 0.14 0.6 2.6 0.04 0.012 0.002 0.030 0.02 0.2 　 0.0009 398 1.4 Comparative Lecture 9 0.14 0.6 2.4 0.04 0.012 0.002 0.020 0.02 0.2 0.1 0.0006 404 1.3

Table 2 shows the hot rolling process conditions for the steel grades in Table 1.

River species Hot acting division Heater Pre-rolled Finish rolling Winding Slavic heating temperature (℃) Rolling mill outlet temperature (℃) Last pass of pre-rolling reduction ratio (%) Final output temperature (℃) 1st pass compression rate (%) Total pressure reduction rate of 1~3 passes (%) Total remaining pass pressure rate (%) 1st cooling rate (℃/s) Coiling temperature (℃) Invention Lecture 1 1200 1090 45 925 44 119 101 133 560 Experimental Example 1 Invention lecture 2 1197 1060 46 920 45 120 100 97 570 Experimental example 2 Invention Lecture 3 1198 1045 46 915 45 122 98 128 550 Experimental Example 3 Invention lecture 4 1200 1078 45 905 46 123 97 93 540 Experimental example 4 Invention lecture 5 1202 1082 45 908 45 118 102 126 570 Experimental Example 5 Invention lecture 6 1206 1064 46 911 46 119 101 86 570 Experimental example 6 Comparative Lecture 1 1198 1062 45 907 46 120 100 102 550 Comparative Example 1 Comparative Lecture 2 1195 1059 46 906 45 120 100 115 550 Comparative Example 2 Comparative Lecture 3 1199 1080 45 904 45 121 99 116 550 Comparative Example 3 Comparative Lecture 4 1201 1090 46 912 46 125 95 123 550 Comparative Example 4 Comparative Lecture 5 1201 1063 45 909 45 119 101 121 550 Comparative Example 5 Comparative Lecture 6 1202 1078 44 908 45 122 98 119 550 Comparative Example 6 Comparative Lecture 7 1206 1085 45 903 44 121 99 134 560 Comparative Example 7 Comparative Lecture 8 1198 1071 46 905 46 120 100 128 550 Comparative Example 8 Comparative Lecture 9 1199 1080 46 904 45 119 101 126 550 Comparative Example 9 Invention lecture 2 1197 1060 46 907 35 105 115 134 570 Comparative Example 10 Invention Lecture 3 1198 1045 46 903 45 121 99 133 550 Comparative Example 11

After hot rolling the slabs of the above-mentioned steel grades according to the conditions in Table 2, the surface oxide scale was removed through pickling, and then cold rolled at a reduction ratio of 30 to 80%. Thereafter, the cold-rolled steel sheets were heat treated according to the conditions in Table 3.

River species Heat treatment note 1st crack Primary cooling Secondary cooling Secondary crack Reheat Plating temperature Retention time temperature speed temperature speed Retention time temperature Retention time Invention Lecture 1 840 80 680 15 280 35 35 450 55 GA Experimental Example 1 Invention lecture 2 830 80 680 12 270 35 35 450 55 GA Experimental example 2 Invention Lecture 3 840 80 680 15 280 35 35 450 55 GA Experimental Example 3 Invention lecture 4 830 80 650 5 300 28 35 440 55 GA Experimental example 4 Invention lecture 5 815 80 650 5 350 23 35 460 55 GA Experimental Example 5 Invention lecture 6 830 80 650 5 330 23 35 460 55 GA Experimental example 6 Comparative Lecture 1 840 80 680 15 250 39 35 450 55 GA Comparative Example 1 Comparative Lecture 2 810 80 680 10 280 35 35 450 55 GA Comparative Example 2 Comparative Lecture 3 830 80 660 5 320 25 35 480 55 GA Comparative Example 3 Comparative Lecture 4 815 80 620 6 300 23 35 460 55 GA Comparative Example 4 Comparative Lecture 5 810 80 650 5 450 15 35 480 55 GA Comparative Example 5 Comparative Lecture 6 840 80 650 6 350 20 35 460 55 GA Comparative Example 6 Comparative Lecture 7 850 80 650 6 450 15 35 460 55 GA Comparative Example 7 Comparative Lecture 8 830 80 650 6 350 22 35 480 55 GA Comparative Example 8 Comparative Lecture 9 850 80 670 6 350 23 35 460 55 GA Comparative Example 9 Invention lecture 2 830 80 680 12 270 35 35 450 55 GA Comparative Example 10 Invention Lecture 3 840 80 680 15 400 25 35 450 55 GI Comparative Example 11

Table 4 shows the microstructure and mechanical properties of the finally manufactured cold rolled steel sheet by completing the process according to Tables 2 and 3. In Table 4, the structure TM+B means that it is composed of tempered martensite and bainite, F means ferrite, FM means fresh martensite, RA means retained austenite, and FM/(TM+B) means the ratio of fresh martensite (FM) and the sum of tempered martensite and bainite (TM+B).

In addition, it represents the yield strength (YS), tensile strength (TS), elongation (EL), and hole extensibility (HER), and YRХELХHER represents the yield ratio (YR) × elongation (EL) × hole extensibility (HER).

No Microstructure Mechanical properties note TM+B F FM RA FM/(TM+B) YS TS EL HER YRХELХHER Invention Lecture 1 76 14 8 2 0.11 890 990 12.0 62 669 Experimental Example 1 Invention lecture 2 75 13 9 3 0.12 788 997 12.6 53 528 Experimental example 2 Invention Lecture 3 72 18 9 1 0.13 873 980 10.5 57 533 Experimental Example 3 Invention lecture 4 72 15 8 5 0.11 958 1090 12.1 43 457 Experimental example 4 Invention lecture 5 74 14 9 3 0.12 892 1103 12.7 50 514 Experimental Example 5 Invention lecture 6 75 14 9 2 0.12 872 1062 12.8 47 494 Experimental example 6 Comparative Lecture 1 82 12 3 3 0.04 788 1030 12.7 37 359 Comparative Example 1 Comparative Lecture 2 58 21 18 3 0.31 612 974 14.7 32 296 Comparative Example 2 Comparative Lecture 3 81 8 9 2 0.11 863 1118 12.1 38 355 Comparative Example 3 Comparative Lecture 4 28 42 29 1 1.04 519 930 15.1 25 211 Comparative Example 4 Comparative Lecture 5 30 33 37 0 1.23 519 1040 12.5 22 137 Comparative Example 5 Comparative Lecture 6 27 38 33 2 1.22 517 1046 12.9 21 134 Comparative Example 6 Comparative Lecture 7 35 36 29 0 0.83 516 987 14.2 20 148 Comparative Example 7 Comparative Lecture 8 41 24 33 2 0.80 579 960 14.5 27 236 Comparative Example 8 Comparative Lecture 9 38 24 37 1 0.97 564 970 15.6 25 227 Comparative Example 9 Invention lecture 2 77 11 9 3 0.12 832 1025 13.1 37 393 Comparative Example 10 Invention Lecture 3 54 18 24 4 0.44 612 907 14.1 31 295 Comparative Example 11

Figure 3 is a graph showing the properties of a cold rolled steel sheet according to an experimental example of the present invention.

Referring to Table 1 and Figure 3, invention steels 1 to 6 satisfy the alloy composition proposed in the present invention, thereby satisfying the range of formula 1) to be achieved in the present invention.

On the other hand, Comparative Steel 1 to Comparative Steel 9 do not satisfy the alloy composition proposed in the present invention, and thus are outside the range of Formula 1) that the present invention seeks to achieve.

In addition, referring to Table 4 and Fig. 3, all of Comparative Steels 1 to 9 showed yield ratio (YR) × elongation (EL) × hole expandability (HER) of less than 400, and all of Inventive Steels 1 to 6 showed yield ratio (YR) × elongation (EL) × hole expandability (HER) of 400 or more.

Referring to Table 2, Comparative Example 10 deviates from the process conditions proposed by the present invention since the sum of the reduction ratios of the 1st to 3rd passes during the final rolling is lower than the sum of the reduction ratios of the remaining passes. Referring to Tables 1 and 3, Comparative Example 11 does not satisfy the temperature range (264°C to 374°C) of Ms-140°C to Ms-30°C for the end of the secondary cooling as proposed by the present invention.

Referring to Table 4, when comparing Experimental Example 2 and Comparative Example 10 using the same steel grade, Invention Steel 2, Experimental Example 2 satisfied all the mechanical properties to be achieved in the present invention, whereas Comparative Example 10 showed a yield ratio (YR) × elongation (EL) × Hole expandability (HER) of less than 400. This is thought to be because Comparative Example 10 exceeds the reduction ratio suggested in the present invention, and thus the austenite grain refinement effect is minimal.

In addition, when comparing Experimental Example 3 and Comparative Example 11 using the same steel grade, Invention Steel 3, Experimental Example 3 satisfied all the mechanical properties that the present invention intends to achieve, whereas Comparative Example 11 did not satisfy the tensile strength (TS): 980 MPa or more that the present invention intends to achieve, and the yield ratio (YR) × elongation (EL) × Hole expandability (HER) was less than 400.

In addition, while Experimental Example 3 satisfies the target fraction of the area fraction of each microstructure component in the present invention, Comparative Example 11 does not satisfy the composition requirements of the microstructure proposed in the present invention, namely, the sum of tempered martensite and bainite: 60 to 80% and fresh martensite less than 10%, and FM/(TM+B) < 0.25.

It appears that Comparative Example 11 failed to secure the desired microstructure because the secondary cooling end point temperature exceeded Ms-30℃, causing ferrite transformation, which reduced strength and elongation, and did not sufficiently secure austenite stability.

Comparative Examples 5 and 7 did not satisfy the target area fraction of retained austenite of 1 to 5% of the present invention because the secondary cooling end point temperature exceeded Ms-30℃ and austenite stability was not sufficiently secured.

Through this, it can be seen that when the alloy composition and process conditions proposed in the present invention are all satisfied, a steel plate having high strength and excellent ductility can be manufactured, a cold rolled steel plate having a high yield ratio and thus excellent hole expandability, and excellent crash resistance and formability can be manufactured.

It will be apparent to a person skilled in the art that the technical idea of the present invention described above is not limited to the above-described embodiments and the attached drawings, and that various substitutions, modifications, and changes are possible within a scope that does not depart from the technical idea of the present invention.

Claims (18)

In weight %, carbon (C): 0.08% to 0.15%, silicon (Si): 0.8% to 1.5%, manganese (Mn): 2.0% to 3.0%, aluminum (Al): more than 0% to 1.0%, phosphorus (P): more than 0% to 0.02% or less, sulfur (S): more than 0% to 0.01% or less, nitrogen (N): more than 0% to 0.01% or less, boron (B): 0.001% to 0.005%, the sum of at least one selected from titanium (Ti) and niobium (Nb): 0.1% or less (excluding 0%), and the remainder including iron (Fe) and other unavoidable impurities. The final microstructure consists of tempered martensite, bainite, fresh martensite, ferrite, and retained austenite. High strength cold rolled steel sheet. In paragraph 1, Characterized in that it satisfies the following equations 1) and 2). High strength cold rolled steel sheet. Equation 1): 10×C + 0.4×(Si + 6×Al) - 0.4×Mn + 15×(Ti+Nb) - 52.5×B ≤ 1.2 Equation 2): Yield ratio (YR) × elongation (EL) × hole expandability (HER) ≥ 400 (Here, the element symbol in formula 1) represents the content of that element in weight%.) In paragraph 1, In terms of area fraction, the total fraction of the tempered martensite and the bainite is 60 to 80%, the fresh martensite is less than 10%, the fraction of the ferrite is 10 to 30%, and the fraction of the retained austenite is 1 to 5%. High strength cold rolled steel sheet. In paragraph 1, The ratio of the fresh martensite (FM) and the sum of the tempered martensite and the bainite (TM+B) is FM/(TM+B) < 0.25. High strength cold rolled steel sheet. In paragraph 1, Containing, in weight %, one or more selected from molybdenum (Mo), chromium (Cr), copper (Cu), and nickel (Ni), in a total amount of 0.5% or less (excluding 0%), High strength cold rolled steel sheet. In paragraph 1, The crystal grain size of the above tempered martensite and the above bainite is 5㎛ or less. High strength cold rolled steel sheet. In paragraph 1, The tensile strength (TS) is 980 MPa or more, and the product of tensile strength (TS) and hole extensibility (HER) (TSХHER) is in the range of 50,000 MPa% or more. High strength cold rolled steel sheet. In paragraph 1, The above titanium (Ti) is contained in an amount of 0.015% or more and 0.04% or less, and niobium (Nb) is contained in an amount of 0.06% or more and 0.085% or less. High strength cold rolled steel sheet. (a) a step of hot rolling a steel material including, in wt%, carbon (C): 0.08% to 0.15%, silicon (Si): 0.8% to 1.5%, manganese (Mn): 2.0% to 3.0%, aluminum (Al): more than 0% to 1.0%, phosphorus (P): more than 0% to 0.02% or less, sulfur (S): more than 0% to 0.01% or less, nitrogen (N): more than 0% to 0.01% or less, boron (B): 0.001% to 0.005%, the sum of at least one selected from titanium (Ti) and niobium (Nb): 0.1% or less (excluding 0%), and the remainder being iron (Fe) and other unavoidable impurities; (b) a step of cold rolling the hot-rolled steel; (c) a step of annealing the cold rolled steel; (d) a step of cooling the annealed heat-treated steel to a temperature below the Ms temperature; and (e) a step of plating the cooled steel material; including, Step (a) above, It includes a step of performing at least one pass of rolling at a reduction ratio of 40% or more per pass, and performing rolling so that the total reduction ratio of the rolling of the first to third passes is higher than the total reduction ratio of the remaining passes. Step (d) above, A first cooling step having a first cooling end temperature in the range of 620℃ to 720℃ at the first cooling rate; and A step of performing a second cooling to a second cooling end temperature of Ms or lower at a second cooling rate faster than the first cooling rate after the first cooling; including; Method for manufacturing high-strength cold rolled steel sheet. In Article 9, The above steel satisfies the following equation 1): Method for manufacturing high-strength cold rolled steel sheet. Equation 1): 10×C + 0.4×(Si + 6×Al) - 0.4×Mn + 15×(Ti+Nb) - 52.5×B ≤ 1.2 (Here, the element symbol in formula 1) represents the content of that element in weight%.) In Article 9, Containing, in weight %, one or more selected from molybdenum (Mo), chromium (Cr), copper (Cu), and nickel (Ni), in a total amount of 0.5% or less (excluding 0%), Method for manufacturing high-strength cold rolled steel sheet. In Article 9, The above secondary cooling end temperature is a temperature of Ms - 140℃ or higher and Ms - 30℃ or lower. Method for manufacturing high-strength cold rolled steel sheet. In Article 9, The above titanium (Ti) is contained in an amount of 0.015% or more and 0.04% or less, and niobium (Nb) is contained in an amount of 0.06% or more and 0.085% or less. Method for manufacturing high-strength cold rolled steel sheet. In Article 9, The above first cooling rate has a range of 3°C/sec to 20°C/sec. Method for manufacturing high-strength cold rolled steel sheet. In Article 9, The second cooling rate has a range of 15°C/sec to 100°C/sec. Method for manufacturing high-strength cold rolled steel sheet. In Article 9, Step (a) above, (a-1) a step of reheating the above steel at 1150 to 1300℃; (a-2) a step of hot rolling the above steel material under the condition that the finishing rolling temperature is 850 to 950℃; and (a-3) a step of coiling the steel material at 400 to 600°C; including; Method for manufacturing high-strength cold rolled steel sheet. In Article 16, After step (a-2) above and before step (a-3), A step of cooling to 680℃ or lower at a cooling rate of 80℃/s or higher is performed. Method for manufacturing high-strength cold rolled steel sheet. In Article 9, The above Ms temperature is defined by the following equation 3). Method for manufacturing high-strength cold rolled steel sheet. Equation 3): Ms = 539-423C-30.4Mn-12.1Cr-17.7Ni-7.5Mo (Here, the element symbol in Equation 3) represents the content of that element in weight%.)

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