KR20210014055A - High strength steel sheet and manufacturing method thereof - Google Patents

Abstract

The present invention provides a high-strength steel sheet with high hole expandability and satisfactory weldability and a manufacturing method thereof. The high-strength steel sheet comprises C, Si, Mn, Cr, Al, Nb, Ti, B, P, S, N, and the remainder consisting of Fe and other inevitable impurities. The contents of C, Si, and Al satisfy the following equation (1). The microstructure includes 50-70 area% (excluding 50 area%) of tempered martensite and the remainder consisting of residual austenite, fresh martensite, ferrite, and bainite. 1-3 area% of a cementite phase as a second phase is deposited and distributed between laths of the bainite or on a crystal grain boundary or laths of the tempered martensite phase. The equation (1) is [C] + ([Si]+[Al])/5 <= 0.35 wt%, wherein [C], [Si], and [Al] mean wt% of C, Si, and Al, respectively.

Description

High strength steel sheet and its manufacturing method {HIGH STRENGTH STEEL SHEET AND MANUFACTURING METHOD THEREOF}

The present invention relates to a high-strength steel sheet having high hole expandability and a method of manufacturing the same.

Recently, securing a steel plate manufacturing technology having high strength has been promoted to reduce the weight of automobiles. Among them, a steel plate that has both high strength and formability can increase productivity, so it is excellent in terms of economy and is more advantageous in terms of safety of final parts. In particular, the demand for steel with high tensile strength of 1180 MPa or higher is increasing because a steel plate with high tensile strength (TS) has a high bearing load until fracture occurs. Conventionally, many attempts have been made to improve the strength of existing steel materials, but when the strength is simply improved, it has been found that the ductility and hole expansion ratio (HER) are deteriorated.

Meanwhile, as a conventional technology that overcomes the above disadvantages, there is a TRIP (Transformation Induced Plasticity) steel sheet in which a large amount of Si or Al is added. However, in the case of TRIP steel sheet, elongation of more than 14% can be obtained at TS 1180MPa class, but due to the addition of large amounts of Si and Al, LME (Liquid Metal Embrittlement) resistance is deteriorated and weldability deteriorates, so practical use as a material for automobile structural is limited. have.

In addition, in the same tensile strength class, various yield ratios are pursued according to use and purpose. In the case of a steel plate with a low yield ratio, it is not easy to make a steel with high hole expansion. This is because it is usually necessary to introduce a martensite or ferrite phase as the second phase in order to lower the yield ratio, because this histological characteristic is a factor that impairs the hole expandability.

Patent Literature 1 discloses a high-strength cold-rolled steel sheet having a yield ratio, strength, hole expandability, and delayed fracture characteristics, and having a high elongation of 17.5% or more. However, in Patent Document 1, there is a disadvantage in that the weldability is poor due to the occurrence of LME due to high Si addition.

Patent Publication No. 2017-7015003

An object of the present invention is to solve the limitations of the prior art described above, and to provide a high-strength steel sheet having high strength and resistance compounding ratio, suitable elongation for processing, high hole expansion, and good weldability, and a method for manufacturing the same. .

The subject of the present invention is not limited to the above description. Those of ordinary skill in the art to which the present invention pertains will not have any difficulty in understanding the additional subject of the present invention from the general details of the present specification.

One aspect of the present invention is by weight %, carbon (C): 0.12% or more and less than 0.17%, silicon (Si): 0.3 to 0.8%, manganese (Mn): 2.5 to 3.0%, chromium (Cr): 0.4 to 1.1 %, Aluminum (Al): 0.01~0.3%, Niobium (Nb): 0.01~0.03%, Titanium (Ti): 0.01~0.03%, Boron (B): 0.001~0.003%, Phosphorus (P): 0.04 % Or less, sulfur (S): 0.01% or less, nitrogen (N): 0.01% or less, the balance iron (Fe) and other inevitable impurities,

The content of carbon (C), silicon (Si) and aluminum (Al) satisfies the following equation (1),

The microstructure contains more than 50% and 70% or less of tempered martensite in area fraction, and the remaining austenite, fresh martensite, ferrite and bainite,

It is a high-strength steel sheet in which a cementite phase as a second phase is deposited between the bainite laths or at the lath or grain boundary of the tempered martensite phase in an area fraction of 1% or more and 3% or less.

[Equation (1)] [C] + ([Si]+[Al])/5 ≤ 0.35wt.%

(Here, [C], [Si], and [Al] mean the weight percent of C, Si, and Al, respectively.)

The residual austenite may be more than 1% and 4% or less, the fresh martensite may be more than 10% and 20% or less, and the ferrite may be more than 0% and 5% or less, and the balance may be bainite.

When the micro Vickers hardness test is performed, the difference between the 25%-th hardness value and the 75%-th hardness value may be distributed in a range between 100 and 150.

The steel sheet, in weight%, copper (Cu): 0.1% or less, nickel (Ni): 0.1% or less, molybdenum (Mo): 0.3% or less, and vanadium (V): 0.03% or less of one or more of I can.

It can have a tensile strength of 1180 MPa or more, a yield strength of 740 MPa to 980 MPa, a yield ratio of 0.65 to 0.85, a hole expandability (HER) of 25% or more, and an elongation of 7 to 14%.

The steel sheet may be a cold rolled steel sheet.

A hot-dip galvanizing layer may be formed on at least one surface of the steel sheet.

An alloyed hot dip galvanizing layer may be formed on at least one surface of the steel sheet.

Another aspect of the present invention is by weight %, carbon (C): 0.12% or more and less than 0.17%, silicon (Si): 0.3-0.8%, manganese (Mn): 2.5-3.0%, chromium (Cr): 0.4- 1.1%, aluminum (Al): 0.01 to 0.3%, niobium (Nb): 0.01 to 0.03%, titanium (Ti): 0.01 to 0.03%, boron (B): 0.001 to 0.003%, phosphorus (P): 0.04% or less, sulfur (S): 0.01% or less, nitrogen (N): 0.01% or less, the balance iron (Fe) and other inevitable impurities, and the carbon (C), silicon (Si) and aluminum (Al) Preparing a slab whose content satisfies the following equation (1);

Heating the slab to a temperature range of 1150 to 1250°C;

Finishing hot rolling the reheated slab at a finish rolling temperature (FDT) of 900 to 980°C;

Cooling at an average cooling rate of 10 to 100° C./sec after the finish hot rolling;

Winding in a temperature range of 500 ~ 700 ℃;

Obtaining a cold rolled steel sheet by cold rolling at a cold rolling reduction rate of 30-60%;

Continuously annealing the cold-rolled steel sheet in a temperature range of (Ac3+30℃~Ac3+80℃);

The step of first cooling the continuously annealed steel sheet at an average cooling rate of 10°C/s or less to a temperature range of 560 to 700°C, and secondary cooling at an average cooling rate of 10°C/s or more to a temperature range of 280 to 380°C. ; And

Reheating the cooled steel sheet to a temperature range of 380 to 480°C at a temperature increase rate of 5°C/s or less; It is a method of manufacturing a high-strength steel sheet comprising a.

[Equation (1)] [C] + ([Si]+[Al])/5 ≤ 0.35wt.%

(Here, [C], [Si], and [Al] mean the weight percent of C, Si, and Al, respectively.)

The slab may further contain at least one of copper (Cu): 0.1% or less, nickel (Ni): 0.1% or less, molybdenum (Mo): 0.3% or less, and vanadium (V): 0.03% or less, by weight% I can.

After the reheating step, it may further include a step of hot-dip galvanizing treatment at a temperature range of 480 to 540°C.

After the hot-dip galvanizing treatment, after performing an alloying heat treatment, cooling to room temperature may be performed.

After cooling to room temperature, temper rolling of less than 1% can be performed.

According to the present invention, it is possible to provide a high-strength steel sheet having a high tensile strength of 1180 MPa or more, a yield strength of 740 MPa to 980 MPa, a low yield ratio of 0.65 to 0.85, and a high hole expandability of 25% or more, and an elongation of 7% to 14%. I can.

In addition, the galvanized steel sheet manufactured by using the high-strength steel sheet of the present invention has excellent resistance to LME (Liquid Metal Embrittlement) after galvanizing, and has an effect of showing excellent weldability.

The various and beneficial advantages and effects of the present invention are not limited to the above description, and will be more easily understood in the course of describing specific embodiments of the present invention.

The terminology used herein is for reference only to specific embodiments and is not intended to limit the invention. Singular forms as used herein also include plural forms unless the phrases clearly indicate the opposite.

The meaning of "comprising" as used in the specification specifies a specific characteristic, region, integer, step, action, element and/or component, and other specific characteristic, region, integer, step, action, element, component and/or group It does not exclude the existence or addition of

Although not defined differently, all terms including technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present invention belongs. Terms defined in a commonly used dictionary are additionally interpreted as having a meaning consistent with the related technical literature and the presently disclosed content, and are not interpreted in an ideal or very formal meaning unless defined.

Hereinafter, a high-strength steel sheet according to an aspect of the present invention will be described in detail. In the present invention, when expressing the content of each element, it is necessary to note that it means weight% unless otherwise specified. In addition, the ratio of crystals or tissues is based on area unless otherwise indicated.

First, the component system of the high-strength steel sheet according to an aspect of the present invention will be described in detail.

The high-strength steel sheet according to an aspect of the present invention is in wt%, C: 0.12% or more and less than 0.17%, Si: 0.3-0.8%, Mn: 2.5-3.0%, Cr: 0.4-1.1%, Al: 0.01-0.3% , Nb: 0.01 to 0.03%, Ti: 0.01 to 0.03%, B: 0.001 to 0.003%, P: 0.04% or less, S: 0.01% or less, N: 0.01% or less, balance Fe and other inevitable impurities, The contents of C, Si and Al may satisfy the following equation (1).

[Equation (1)] [C] + ([Si]+[Al])/5 ≤ 0.35wt.%

(Here, [C], [Si], and [Al] mean the weight percent of C, Si, and Al, respectively.)

Carbon (C): 0.12% or more and less than 0.17%

Carbon (C) is a basic element that supports the strength of steel through solid solution strengthening and precipitation strengthening. If the amount of C is less than 0.12%, it is difficult to secure a tempered martensite fraction of 50% or more, and it is difficult to obtain a strength equivalent to the tensile strength (TS) of 1180 MPa class. On the other hand, when the amount of C is 0.17% or more, it is difficult to have high LME resistance, and when the spot welding condition is severe, cracks are generated due to the penetration of molten Zn during the welding process. In addition, when the carbon content is high, the arc weldability and laser weldability deteriorate, the risk of occurrence of a weld crack due to low-temperature brittleness increases, and it becomes difficult to obtain a target hole expandability value. Therefore, in the present invention, the content of C is preferably limited to 0.12% or more and less than 0.17%.

Silicon (Si): 0.3~0.8%

Silicon (Si) is a key element of TRIP (Transformation Induced Plasticity) steel, which acts to increase the residual austenite fraction and elongation by inhibiting the precipitation of cementite in the bainite region. When Si is less than 0.3%, the elongation is too low as residual austenite hardly remains. On the other hand, when Si exceeds 0.8%, it is impossible to prevent the deterioration of the properties of the weld due to the formation of LME cracks, and the surface characteristics of the steel material. And plating properties deteriorate. Therefore, in the present invention, the content of Si is preferably limited to 0.3 to 0.8%.

Manganese (Mn): 2.5~3.0%

In the present invention, the content of manganese (Mn) may be 2.5 to 3.0%. When the content of Mn is less than 2.5%, it is difficult to secure strength, whereas when the content exceeds 3.0%, the bainite transformation rate is slowed to form too much fresh martensite, making it difficult to obtain high hole expandability. In addition, when the content of Mn is high, the starting temperature of martensite formation is lowered, and the cooling end temperature required to obtain the initial martensite phase in the annealing water cooling step is too low. Therefore, in the present invention, the content of Mn is preferably limited to 2.5 to 3.0%.

Chrome (Cr): 0.4~1.1%

In the present invention, the content of chromium (Cr) may be 0.4 to 1.1%. If the amount of Cr is less than 0.4%, it becomes difficult to obtain the target tensile strength, and if it exceeds the upper limit of 1.1%, the transformation speed of bainite becomes slow, making it difficult to obtain high hole expandability. Therefore, in the present invention, the content of Cr is preferably limited to 0.4 to 1.1%.

Aluminum (Al): 0.01~0.3%

In the present invention, the content of aluminum (Al) may be 0.01 to 0.3%. If the amount of Al is less than 0.01%, deoxidation of the steel is not sufficiently performed and cleanliness is impaired. On the other hand, if Al is added in excess of 0.3%, the castability of the steel is impaired. Therefore, in the present invention, the content of Al is preferably limited to 0.01 to 0.3%.

Niobium (Nb): 0.01~0.03%

In the present invention, 0.01 to 0.03% of niobium (Nb) may be added to increase the strength and hole expandability of the steel material through grain refinement and formation of precipitates. If the Nb content is less than 0.01%, the effect of finer structure is lost and the amount of precipitation strengthening is insufficient. On the other hand, if the Nb content is more than 0.03%, the castability of the steel material is deteriorated. Therefore, in the present invention, the content of Nb is preferably limited to 0.01 to 0.03%.

Titanium (Ti): 0.01~0.03%, Boron (B): 0.001~0.003%

In the present invention, 0.01 to 0.03% of titanium (Ti) and 0.001 to 0.003% of boron (B) may be added to increase the hardenability of the steel. When the Ti content is less than 0.01%, B bonds with N, so that the effect of strengthening the hardenability of B is lost, and when the content of Ti is more than 0.03%, the castability of the steel material is deteriorated. On the other hand, if the B content is less than 0.001%, an effective hardenability enhancing effect cannot be obtained, and if it is contained in excess of 0.003%, boron carbide may be formed, which may rather impair hardenability. Therefore, in the present invention, the Ti content is preferably limited to 0.01 to 0.03% and the B content to 0.001 to 0.003%.

Phosphorus (P): 0.04% or less

Phosphorus (P) exists as an impurity in steel and it is advantageous to control its content as low as possible, but it is also intentionally added to increase the strength of the steel. However, if the P is excessively added, the toughness of the steel material is deteriorated, so in the present invention, it is preferable to limit the upper limit to 0.04% in order to prevent this.

Sulfur (S): 0.01% or less

Like P, sulfur (S) exists as an impurity in steel, and it is advantageous to control its content as low as possible. In addition, since S deteriorates the ductility and impact characteristics of the steel material, it is preferable to limit the upper limit thereof to 0.01% or less.

Nitrogen (N): 0.01% or less

In the present invention, nitrogen (N) is added to steel as an impurity, and its upper limit is limited to 0.01% or less.

In addition to the above-described contents of C, Si and Al, C, Si and Al may satisfy Equation (1).

[Equation (1)] [C] + ([Si]+[Al])/5 ≤ 0.35wt.%

(Here, [C], [Si], and [Al] mean the weight percent of C, Si, and Al, respectively.)

Liquid metal embrittlement (LME) of plated steel is caused by the penetration of liquid zinc into the austenite grain boundary while tensile stress is formed at the austenite grain interface of the steel plate while the plated zinc becomes liquid during spot welding. . Since this LME phenomenon is particularly severe in the steel sheet to which Si and Al are added, in the present invention, the addition amount of Si and Al is controlled through the above equation (1). In addition, when the C content is high, the A3 temperature of the steel material is lowered, so that the austenite region vulnerable to LME is expanded, and the toughness of the material is weakened. Thus, the amount of addition was limited through Equation (1).

When the value of Equation (1) exceeds 0.35%, the LME resistance is deteriorated during spot welding as described above, and thus LME cracks exist after the spot welding, thereby impairing fatigue characteristics and structural safety. On the other hand, the smaller the value of Equation (1) is, the better the spot weldability and LME resistance, so the lower limit may not be separately set.If the value is less than 0.20, the spot weldability and LME resistance are improved, but 1180 MPa class with excellent hole expansion. Since it becomes difficult to obtain a high tensile strength of, in some cases, the lower limit may be limited to 0.20% or more.

The high-strength steel sheet according to an aspect of the present invention further comprises at least one of Cu: 0.1% by weight or less, Ni: 0.1% by weight or less, Mo: 0.3% by weight or less, and V: 0.03% by weight or less, in addition to the alloy components described above. Can include.

Copper (Cu): 0.1% or less, Nickel (Ni): 0.1% or less, Molybdenum (Mo): 0.3% or less

Copper (Cu), nickel (Ni) and molybdenum (Mo) are elements that increase the strength of steel, and are included as optional components in the present invention, and the upper limit of addition of each element is limited to 0.1%, 0.1%, and 0.3%, respectively. These elements are elements that increase the strength and hardenability of steel, but if they are added in an excessive amount, they may exceed the target strength class and are expensive elements, so it is economical to limit the upper limit of addition to 0.1% or 0.3%. desirable. On the other hand, since Cu, Ni, and Mo act as solid solution strengthening elements, when added in less than 0.03%, the solid solution strengthening effect may be insignificant, and when added, the lower limit thereof may be limited to 0.03% or more.

Vanadium (V): 0.03% or less

Vanadium (V) is an element that increases the yield strength of steel through precipitation hardening, and in the present invention, it may be selectively added to increase the yield strength. However, if the content is excessive, the elongation may be too low and brittleness of the steel may be caused, so the upper limit of V is limited to 0.03% or less in the present invention. On the other hand, in the case of V, since it causes precipitation hardening, even a small amount of addition is effective, but when it is added in less than 0.005%, the effect may be insignificant. When added, the lower limit can be limited to 0.005% or more.

In the present invention, in addition to the above-described steel composition, the remainder may contain Fe and unavoidable impurities. Unavoidable impurities may be unintentionally incorporated in a conventional steel manufacturing process, and cannot be completely excluded, and those skilled in the ordinary steel manufacturing field can easily understand the meaning. In addition, the present invention does not entirely exclude addition of a composition other than the aforementioned steel composition.

On the other hand, the high-strength steel sheet according to an aspect of the present invention that satisfies the above-described steel composition has a microstructure, in an area fraction, containing more than 50% and 70% of tempered martensite, and remaining residual austenite, fresh martensite, It may include ferrite and bainite. The remaining phases other than the tempered martensite, by area fraction, contain more than 1% and 4% or less of residual austenite, more than 10% and 20% or less of the fresh martensite, and more than 0% and 5% or less of the ferrite. The pour can be made of bainite. In addition, the cementite phase as a second phase may be deposited between the bainite laths, or at the lath or grain boundary of the tempered martensite phase in an area fraction of 1% or more and 3% or less.

Since the tempered martensite phase has a fine internal structure, it is a steel structure that is advantageous for securing the hole expandability of steel materials. If the fraction of tempered martensite is less than 50%, it is difficult to obtain the target hole expandability, and if the amount of tempered martensite is insufficient, the amount of phase transformation before the final cooling step becomes insufficient, and finally, excessive fresh martensite is formed and steel materials. It hurts the elongation and expandability of the hole together. On the other hand, when the tempered martensite exceeds 70% by area, the yield ratio and yield strength of the steel material exceed the upper limit of the present invention, making it difficult to form the steel, and problems such as springback after molding may occur.

Meanwhile, the remaining tissues other than the tempered martensite may be made of retained austenite, fresh martensite, ferrite, and bainite.

In the high-strength steel sheet according to the present invention, the upper limit of Si and Al is limited by Equation (1), but since Si and Al are added to some extent, residual austenite may exist at a level of more than 1 area% and 4 area% or less. . However, a high fraction of retained austenite is not distributed as in typical TRIP steel with very high Si and Al content.

In the present invention, in order to obtain a low yield ratio, a fresh martensite structure may be introduced at a level of more than 10 area% and less than 20 area%. If the austenite phase fraction is high after the secondary cooling and reheating is completed, the carbon content in the austenite is low and stability is insufficient, and in the subsequent cooling process, a part of the austenite is transformed into fresh martensite, resulting in a lower yield ratio.

In addition, the ferrite structure in the present invention is poor in hole expandability, but may exist at a level of more than 0 area% and 5 area% or less in the manufacturing process. In addition, the microstructure of the present invention may be composed of bainite.

In the high-strength steel sheet according to an aspect of the present invention, some cementite precipitates and grows in the microstructure by limiting the content of Si and Al that stabilizes austenite by inhibiting cementite growth according to the conditions of Equation (1). Is done. This cementite precipitates at the martensite lath or grain boundary when martensite formed by secondary cooling is reheated, or when bainite transformation occurs during reheating after secondary cooling, carbon between the bainitic ferrite laths is concentrated. Formed in the part

In the high-strength steel sheet according to the present invention, by limiting the upper limit of Si and Al by Equation (1), cementite at a level of 1% or more is precipitated in terms of area fraction, but nonetheless, due to the presence of some Si and Al Since knight remains and carbon is distributed in the retained austenite, the amount of cementite precipitated may be less than 3% by area.

Meanwhile, in a micro Vickers hardness test with a maximum load of 100 g or less, the difference between the 25%-th hardness value and the 75%-th hardness value may be distributed in a range between 100 and 150. The method of obtaining the difference in the hardness value is not specifically limited, but as a non-limiting example, after measuring the microhardness 100 times or more with a load of 100 g or less of the maximum load on the microstructure, the measured hardness value is in the order of hardness. It can be calculated by listing the difference between the 75% and 25% hardness values. If the difference in hardness value is less than 100, a higher hole expandability can be expected, but the yield strength can be increased to exceed 980 MPa. On the other hand, when the difference in the hardness value is greater than 150, the yield strength is lower than the desired level in the present invention, and it is difficult to expect a high hole expandability.

By having the above component composition and microstructure, the high-strength steel sheet of the present invention can exhibit high hole expandability of 25% or more even at a tensile strength of 1180 MPa or more, a yield strength of 740 MPa to 980 MPa, and a low yield ratio of 0.65 to 0.85.

As described above, the low yield ratio of the high-strength steel sheet according to the present invention is due to the introduction of fresh martensite, and the present inventors believe that the hole expandability is 25 even in the presence of fresh martensite under the alloy composition and structure control conditions according to the present invention. It was confirmed that it can be obtained in% or more.

In addition, since the high-strength steel sheet according to the present invention limits the content of Si and Al, the TRIP effect is weak and exhibits an elongation of 7% or more and 14% or less.

The high-strength steel sheet according to the present invention may be a cold rolled steel sheet.

A hot-dip galvanizing layer may be formed on at least one surface of the high-strength steel sheet according to the present invention by a hot-dip galvanizing method. In the present invention, the configuration of the hot-dip galvanized layer is not particularly limited, and any hot-dip galvanized layer commonly applied in the art can be preferably applied to the present invention. In addition, the hot-dip galvanizing layer may be an alloyed hot-dip galvanizing layer alloyed with some alloy components of a steel sheet.

Next, a method of manufacturing a high-strength steel sheet according to another aspect of the present invention will be described in detail.

High-strength steel sheet according to an aspect of the present invention prepares a steel slab that satisfies the above-described steel composition and equation (1)-slab reheating-hot rolling-winding-cold rolling-continuous annealing-primary and secondary cooling -It can be manufactured by going through the reheating process, and the details are as follows.

First, a slab having the above-described alloy composition and satisfying Equation (1) is prepared, and the slab is reheated to a temperature of 1150°C to 1250°C. At this time, if the slab temperature is less than 1150°C, the next step, hot rolling, becomes impossible, whereas if the slab temperature exceeds 1250°C, a lot of energy is unnecessary to increase the slab temperature. Therefore, the heating temperature is preferably limited to a temperature of 1150 ~ 1250 ℃.

The reheated slab is hot-rolled to a thickness suitable for a desired purpose under the condition that the finish rolling temperature (FDT) is 900 to 980°C. When the finish rolling temperature (FDT) is less than 900°C, the rolling load is large and shape defects increase, resulting in poor productivity. On the other hand, when the finish rolling temperature exceeds 980°C, the surface quality deteriorates due to an increase in oxides due to excessive high-temperature operation. Therefore, it is preferable to perform hot rolling under the condition that the finish rolling temperature is 900 to 980°C.

After hot rolling, it is cooled to the coiling temperature at an average cooling rate of 10 to 100°C/s, and winding is performed in a temperature range of 500 to 700°C. And after winding, cold rolling is performed at a cold rolling reduction ratio of 30 to 60% to obtain a cold rolled steel sheet.

If the cold reduction ratio is less than 30%, it is difficult to secure a target thickness accuracy and it is difficult to correct the shape of the steel sheet. On the other hand, when the cold-rolling reduction ratio exceeds 60%, the possibility of cracking in the edge of the steel sheet increases, and the cold-rolling load is excessively large. Therefore, it is preferable to limit the cold rolling reduction rate in the cold rolling step to 30 to 60%.

The cold-rolled steel sheet is subjected to continuous annealing in the temperature range of (Ac3+30℃~Ac3+80℃) (hereinafter also referred to as'SS' or'continuous annealing temperature'). The continuous annealing step is to form austenite close to 100% by heating up to a single phase of austenite and use it for subsequent phase transformation. If the continuous annealing temperature is less than Ac3+30°C, sufficient austenite transformation is not performed, so that the desired martensite and bainite fractions cannot be secured after annealing. On the other hand, when the continuous annealing temperature exceeds Ac3+80°C, productivity may decrease and coarse austenite may be formed, resulting in deterioration of the material, and oxides may grow during annealing, making it difficult to secure the surface quality of the plated material.

If there are circumstances such as difficulty in knowing the Ac3 temperature of the steel sheet being manufactured during actual manufacturing, continuous annealing can be performed in the temperature range of 830 to 880°C. In addition, the continuous annealing may be carried out in a continuous alloying hot dip plating furnace.

Continuously annealed steel sheet is first cooled at an average cooling rate of 10°C/s or less to a primary cooling end temperature of 560~700℃ (hereinafter, also referred to as'SCS'), and a secondary cooling end temperature of 280~380℃ (Hereinafter, also referred to as'RCS') by secondary cooling at an average cooling rate of 10°C/s or more, martensite is introduced into the microstructure of the steel sheet. Here, the primary cooling end temperature may be defined as a time point at which rapid cooling is started by additionally applying a quenching facility not applied in the primary cooling. When the cooling process is divided into primary and secondary cooling and is carried out step by step, the temperature distribution of the steel sheet is uniform in the slow cooling step to reduce the final temperature and material deviation, and it is also advantageous to obtain the required phase composition.

The primary cooling is slow cooling at an average cooling rate of 10°C/s or less, and the cooling end temperature may range from 560 to 700°C. If the primary cooling end temperature is lower than 560℃, the ferrite phase is excessively precipitated and the final hole expandability is deteriorated. On the other hand, if it exceeds 700℃, the secondary cooling is excessively loaded and the speed of delivery of the continuous annealing line has to be slowed. It can fall.

For the secondary cooling, a quenching facility that is not applied in the primary cooling may be additionally applied, and a hydrogen quenching facility using H ₂ gas may be used as a preferred embodiment, but is not limited thereto.

At this time, it is important to control the cooling end temperature of the secondary cooling to 280 to 380°C, where an appropriate initial martensite fraction can be obtained.If it is lower than 280°C, the initial martensite fraction transformed during the secondary cooling is too high, so that in the subsequent process. There is no space to obtain necessary various phase transformations, and the shape and workability of the steel sheet deteriorate. On the other hand, when the secondary cooling end temperature exceeds 380° C., the initial martensite fraction is low, so it may be difficult to obtain high hole expandability.

The cooled steel sheet is reheated again to a temperature range of 380 to 480°C (hereinafter, also referred to as'annealing material heating temperature' or'RHS') at a heating rate of 5°C/s or less to temper the martensite obtained in the previous step, Induction of bainite transformation and concentration of carbon in untransformed austenite adjacent to bainite.

At this time, it is important to control the reheating temperature to 380~480℃, and if it is lower than 380℃ or exceeding 480℃, the amount of phase transformation of bainite is small, so too much fresh martensite is formed during the final cooling process, resulting in elongation and hole expansion. It hurts greatly.

If necessary, hot-dip galvanizing treatment may be performed on the reheated steel sheet at a temperature in the range of 480 to 540°C to form a hot-dip galvanizing layer on at least one surface of the steel sheet.

In addition, it may be cooled to room temperature after hot-dip galvanizing treatment and then alloying heat treatment to obtain an alloyed hot-dip galvanized layer as needed.

In addition, the process of performing temper rolling of less than 1% after cooling to room temperature to correct the shape of the steel sheet and adjust the yield strength may be further included.

Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are only for exemplifying the present invention and not for limiting the scope of the present invention. This is because the scope of the present invention is determined by matters described in the claims and matters reasonably inferred therefrom.

(Example)

First, five kinds of steel sheets of A to E satisfying the component system described in Table 1 below were prepared. In addition, for each example, the thickness of the steel sheet, FDT (finish rolling temperature), CT (hot rolled winding temperature) process conditions and SS (continuous annealing temperature), SCS (primary cooling end temperature), RCS ( The results of measuring the material and phase fraction according to the secondary cooling end temperature) and RHS (annealed material heating temperature) are shown in Tables 2 and 3. The cooling rate after finish rolling, the cold rolling reduction rate, and the heating rate during reheating after cooling, not separately indicated in Table 2 below, were all controlled within the range satisfying the conditions of the present invention. In addition, the Ac3 temperature of each example was calculated using Thermocalc, a commercial thermodynamic software.

The method of measuring the material and phase fraction applied in this example is as follows.

Tensile strength (TS), yield strength (YS), and elongation (EL) of this example were measured through a tensile test in the direction perpendicular to the rolling direction, and a test piece standard having a gauge length of 50 mm and a width of a tensile test piece of 25 mm was used. .

The hole expandability was measured according to ISO 16330 standard, and the hole was sheared with a 12% clearance using a 10mm diameter punch.

The phase fraction of each example was measured by a point counting method from a scanning electron microscope (SEM) photograph, but the fraction of retained austenite was measured by XRD. And the rest other than the phases listed in Table 3 below is bainite.

The difference in hardness of each example is from the difference between the hardness values corresponding to the 75% and 25% after measuring the microhardness 100 times or more with a load of 1 gf for each specimen, and then listing the measured hardness values in order of hardness. Obtained. This hardness difference value represents the hardness difference between the phases in the entire microstructure, and when the hardness difference between the phases is low, the possibility of obtaining high hole expandability increases.

Steel grade Alloy composition (% by weight) Equation 1 Ac3
Temperature
(℃) C Si Mn Cr Al Ti B P S Cu Ni Mo Nb V N C+(Si+Al)/5 A 0.103 0.569 2.38 0.87 0.075 0.020 0.0018 0.006 0.001 0.03 0.01 0.05 0.021 0.002 0.003 0.23 815 B 0.125 0.72 2.36 0.83 0.021 0.018 0.0011 0.005 0.002 0.02 0.00 0.00 0.017 0.001 0.003 0.27 807 C 0.146 0.513 2.9 0.97 0.088 0.024 0.0022 0.007 0.003 0.01 0.00 0.01 0.016 0.001 0.004 0.27 789 D 0.162 0.51 2.78 0.735 0.065 0.024 0.0019 0.006 0.002 0.03 0.01 0.00 0.014 0.001 0.003 0.28 787 E 0.215 0.85 3.21 0.91 0.031 0.021 0.0017 0.005 0.003 0.01 0.00 0.01 0.021 0.001 0.003 0.39 768

division Steel grade Hot rolled
thickness
(mm) FDT
(℃) CT
(℃) Cold rolled
thickness
(mm) SS
(℃) Primary
Cooling
Average
Cooling
speed
(℃/s) SCS
(℃) Secondary
Cooling
Average
Cooling
speed
(℃/s) RCS
(℃) RHS
(℃) Comparative Example 1 A 2.4 946 605 1.5 833 3.3 643 18.0 332 422 Comparative Example 2 B 2.5 938 598 1.6 852 3.8 621 17.6 297 447 Invention Example 1 C 2.5 952 611 1.6 832 4.0 598 16.1 301 442 Inventive Example 2 C 2.2 944 588 1.4 821 4.2 611 12.9 318 432 Invention Example 3 D 2.1 932 572 1.3 822 3.3 633 14.8 305 428 Comparative Example 3 D 2.5 951 611 1.6 851 3.5 612 12.7 362 446 Comparative Example 4 E 2.3 941 621 1.4 837 3.4 622 17.4 302 438

division Steel grade YS
(MPa) TS
(MPa) El
(%) YR HER
(%) Tempord
Martensite
Fraction
(%) Cementite
Fraction
(%) Residual
Austenite
Fraction
(%) Fresh martensite
Fraction
(%) Blowjob
Site
Fraction
(%) 75%thHV-25%thHV* Comparative Example 1 A 791 1057 12.5 0.75 56 56 One 2 5 27 135 Comparative Example 2 B 1062 1172 10.9 0.91 53 75 One 2 6 4 90 Invention Example 1 C 912 1210 10.9 0.75 35 68 2 3 14 2 121 Inventive Example 2 C 852 1240 10.0 0.69 29 65 One 3 18 One 133 Invention Example 3 D 921 1205 9.6 0.76 31 69 One 4 18 2 118 Comparative Example 3 D 732 1312 9.6 0.56 18 37 One 2 37 2 175 Comparative Example 4 E 963 1232 9.6 0.78 22 56 2 6 11 One 142

*75%thHV-25%thHV: The difference between the 25%th hardness value and the 75%th hardness value when the micro Vickers hardness test is performed

First, Comparative Examples 1 to 2 are cases in which steel grades A and B are applied, respectively. Steel grades A and B are cases where the content of carbon (C) or manganese (Mn) is lower than the range of the present invention, and the strength of TS 1180 MPa class could not be obtained. In addition, in the case of steel type B, the difference between the 75%-th hardness value and the 25%-th Vickers hardness was less than 100, resulting in a high hole expandability (HER) value, but the yield strength and the yield ratio exceeded the range of the present invention.

In addition, in the case of Comparative Example 3, the tempered martensite fraction did not exceed 50 area% and the fresh martensite fraction exceeded 20 area%, the yield strength did not reach 740 MPa and the hole expandability (HER) value was also obtained, The difference between the 75%-th hardness value and the 25%-th Vickers hardness exceeded 150.

In the case of Comparative Example 4, the carbon (C) content of steel type E exceeded the component range of the present invention, and the hole expandability (HER) value was obtained as low as 25% even though other conditions were satisfied.

Inventive Examples 1 to 3 are cases in which steel grades C and D satisfying the alloy composition of the present invention are applied, and all process conditions are satisfied, and hole expandability of 25% or more and 7% to 14% at a low yield ratio of 0.65 to 0.85 An elongation suitable for processing of was obtained.

Although described with reference to the above embodiments, those skilled in the art will understand that various modifications and changes can be made to the present invention without departing from the spirit and scope of the present invention described in the following claims. I will be able to.

Claims (13)

By weight%, carbon (C): 0.12% or more and less than 0.17%, silicon (Si): 0.3 to 0.8%, manganese (Mn): 2.5 to 3.0%, chromium (Cr): 0.4 to 1.1%, aluminum (Al) : 0.01~0.3%, niobium (Nb): 0.01~0.03%, titanium (Ti): 0.01~0.03%, boron (B): 0.001~0.003%, phosphorus (P): 0.04% or less, sulfur (S ): 0.01% or less, nitrogen (N): 0.01% or less, the balance contains iron (Fe) and other inevitable impurities,
The content of carbon (C), silicon (Si) and aluminum (Al) satisfies the following equation (1),
The microstructure contains more than 50% and 70% or less of tempered martensite in area fraction, and the remaining austenite, fresh martensite, ferrite and bainite,
A high-strength steel sheet in which a cementite phase as a second phase is deposited between the bainite laths or at the lath or grain boundary of the tempered martensite phase in an area fraction of 1% or more and 3% or less.
[Equation (1)] [C] + ([Si]+[Al])/5 ≤ 0.35wt.%
(Here, [C], [Si], and [Al] mean the weight percent of C, Si, and Al, respectively.)
The method of claim 1,
A high-strength steel sheet comprising more than 1% and 4% or less of the retained austenite, more than 10% and 20% or less of the fresh martensite, and more than 0% and 5% or less of the ferrite, and the balance being bainite.
The method of claim 1,
A high-strength steel sheet, characterized in that the difference between the 25%-th hardness value and the 75%-th hardness value is distributed in a range between 100 and 150 when the micro Vickers hardness test is performed.
The method of claim 1,
The steel sheet further comprises at least one of copper (Cu): 0.1% or less, nickel (Ni): 0.1% or less, molybdenum (Mo): 0.3% or less, and vanadium (V): 0.03% or less, by weight% High-strength steel sheet, characterized in that.
The method of claim 1,
High-strength steel sheet having a tensile strength of 1180 MPa or more, a yield strength of 740 MPa to 980 MPa, a yield ratio of 0.65 to 0.85, a hole expandability (HER) of 25% or more, and an elongation of 7 to 14%.
The method of claim 1,
The steel sheet is a high strength steel sheet, characterized in that the cold rolled steel sheet.
The method of claim 1,
A high-strength steel sheet, characterized in that a hot-dip galvanized layer is formed on at least one surface of the steel sheet.
The method of claim 1,
A high-strength steel sheet, characterized in that an alloyed hot-dip galvanized layer is formed on at least one surface of the steel sheet.
By weight%, carbon (C): 0.12% or more and less than 0.17%, silicon (Si): 0.3 to 0.8%, manganese (Mn): 2.5 to 3.0%, chromium (Cr): 0.4 to 1.1%, aluminum (Al) : 0.01~0.3%, niobium (Nb): 0.01~0.03%, titanium (Ti): 0.01~0.03%, boron (B): 0.001~0.003%, phosphorus (P): 0.04% or less, sulfur (S ): 0.01% or less, nitrogen (N): 0.01% or less, the balance iron (Fe) and other inevitable impurities are included, and the content of the carbon (C), silicon (Si) and aluminum (Al) is the following equation ( Preparing a slab that satisfies 1);
Heating the slab to a temperature range of 1150 to 1250°C;
Finishing hot rolling the reheated slab at a finish rolling temperature (FDT) of 900 to 980°C;
Cooling at an average cooling rate of 10 to 100°C/sec after the finish hot rolling;
Winding in a temperature range of 500 ~ 700 ℃;
Obtaining a cold rolled steel sheet by cold rolling at a cold rolling reduction rate of 30-60%;
Continuously annealing the cold-rolled steel sheet in a temperature range of (Ac3+30℃~Ac3+80℃);
The step of first cooling the continuously annealed steel sheet at an average cooling rate of 10°C/s or less to a temperature range of 560 to 700°C, and secondary cooling at an average cooling rate of 10°C/s or more to a temperature range of 280 to 380°C. ; And
Reheating the cooled steel sheet to a temperature range of 380 to 480°C at a temperature increase rate of 5°C/s or less; Method for producing a high-strength steel sheet comprising a.
[Equation (1)] [C] + ([Si]+[Al])/5 ≤ 0.35wt.%
(Here, [C], [Si], and [Al] mean the weight percent of C, Si, and Al, respectively.)
The method of claim 9,
The slab further comprises at least one of copper (Cu): 0.1% or less, nickel (Ni): 0.1% or less, molybdenum (Mo): 0.3% or less, and vanadium (V): 0.03% or less, by weight% Method for producing a high-strength steel sheet, characterized in that.
The method of claim 9,
After the reheating step, the method of manufacturing a high-strength steel sheet, further comprising the step of hot-dip galvanizing treatment at a temperature range of 480 to 540°C.
The method of claim 11,
After the hot-dip galvanizing treatment, after performing an alloying heat treatment, cooling to room temperature is performed.
The method of claim 11,
After cooling to room temperature, temper rolling of less than 1% is performed.