KR102321269B1 - High strength steel sheet and manufacturing method thereof - Google Patents

Abstract

The present invention consists of a component system including C, Si, Mn, Cr, Al, Nb, Ti, B, P, S, N, the remainder Fe and other unavoidable impurities, and the content of C, Si and Al is calculated using the following equation Equation (1) is satisfied, and the microstructure contains more than 50% and 70% or less of tempered martensite as an area fraction, and the remaining retained austenite, fresh martensite, ferrite and bainite, and the bainite Provided are a high-strength steel sheet in which a cementite phase is precipitated and distributed in an area fraction of 1% or more and 3% or less as a second phase between laths or at the lath or grain boundaries of the tempered martensite phase, and a method for manufacturing the same.
[Equation (1)] [C] + ([Si]+[Al])/5 ≤ 0.35 wt.%
(Here, [C], [Si], and [Al] mean the weight % of C, Si, and Al, respectively.)

Description

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 for manufacturing the same.

Recently, in order to reduce the weight of automobiles, it is being promoted to secure a steel plate manufacturing technology having high strength. Among them, in the case of a steel sheet that has both high strength and formability, productivity can be increased, which is excellent in terms of economic feasibility and is more advantageous in terms of safety of final parts. In particular, since a steel sheet with high tensile strength (TS) has a high bearing load until fracture occurs, there is a growing demand for steel with high tensile strength of 1180 MPa or higher. In the past, many attempts have been made to improve the strength of the existing steel, but when the strength is simply improved, the ductility and hole expansion ratio (HER, hole expansion ratio) are lowered.

On the other hand, as a prior art that overcomes the above disadvantages, there is a TRIP (Transformation Induced Plasticity) steel sheet containing a large amount of Si or Al. However, in TRIP steel sheet, an elongation of 14% or more can be obtained at TS 1180 MPa class, but due to the addition of large amounts of Si and Al, LME (Liquid Metal Embrittlement) resistance deteriorates and weldability deteriorates. have.

In addition, in the same tensile strength grade, various yield ratios are pursued depending on the use and purpose. Because it is usually necessary to introduce a martensite or ferrite phase as a second phase in order to lower the yield ratio, this histological characteristic is a factor impairing the hole expandability.

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

Patent Publication No. 2017-7015003

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

The object of the present invention is not limited to the above. Those of ordinary skill in the art to which the present invention pertains will have no difficulty in understanding the additional problems of the present invention from the general description of the present invention.

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 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 remainder including iron (Fe) and other unavoidable impurities,

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

The microstructure is an area fraction, including more than 50% and less than 70% of tempered martensite, and the remaining residual austenite, fresh martensite, ferrite and bainite,

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

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

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

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

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

The steel sheet may further include 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% can

It may 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 alloying 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 to 0.8%, manganese (Mn): 2.5 to 3.0%, chromium (Cr): 0.4 to 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 remainder including iron (Fe) and other unavoidable impurities, 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 in a finish rolling temperature (FDT) range 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 of 30 to 60%;

continuous annealing of the cold-rolled steel sheet in a temperature range of (Ac3+30°C to Ac3+80°C);

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 ~ 480 ℃ at a temperature increase rate of 5 ℃ / s or less; It is a method of manufacturing a high-strength steel sheet comprising a.

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

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

The slab, in wt%, 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 can

After the reheating step, the step of hot-dip galvanizing treatment at a temperature range of 480 ~ 540 ℃ may be further included.

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

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

According to the present invention, a high-strength steel sheet having a high 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, high hole expandability of 25% or more, and an elongation of 7% to 14%. can

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

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

The terminology used herein is for the purpose of referring to specific embodiments only, and is not intended to limit the invention. As used herein, the singular forms also include the plural forms unless the phrases clearly indicate the opposite.

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

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

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 unless otherwise specified, it means weight %. In addition, the ratio of crystals or tissues is based on the 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.

High-strength steel sheet according to an aspect of the present invention is, by weight%, C: 0.12% or more and less than 0.17%, Si: 0.3 to 0.8%, Mn: 2.5 to 3.0%, Cr: 0.4 to 1.1%, Al: 0.01 to 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, the balance Fe and other unavoidable impurities, The content of C, Si and Al may satisfy Equation (1) below.

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

(Here, [C], [Si], and [Al] mean the weight % 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 a 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, so if the spot welding conditions are severe, cracks may occur due to penetration of molten Zn during the welding process. In addition, if the carbon content is high, arc weldability and laser weldability are deteriorated, the risk of cracking in the weld zone 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 in TRIP (Transformation Induced Plasticity) steel that increases the retained austenite fraction and elongation by inhibiting the precipitation of cementite in the bainite region. When Si is less than 0.3%, residual austenite hardly remains and elongation becomes too low. On the other hand, when Si exceeds 0.8%, it is impossible to prevent deterioration of weld properties due to the formation of LME cracks, and surface properties of steel and poor plating properties. Therefore, in the present invention, it is preferable to limit the Si content 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 becomes difficult to secure strength. On the other hand, when the content exceeds 3.0%, the bainite transformation rate is slowed, and too much fresh martensite is formed, making it difficult to obtain high hole expandability. In addition, if the content of Mn is high, the start 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, it is preferable to limit the content of Mn to 2.5 to 3.0%.

Chromium (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 a target tensile strength, and if it exceeds the upper limit of 1.1%, the transformation rate of bainite becomes slow, making it difficult to obtain high hole expandability. Therefore, in the present invention, it is preferable to limit the content of Cr 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, when Al is added in excess of 0.3%, the castability of the steel is impaired. Therefore, in the present invention, it is preferable to limit the content of Al to 0.01 to 0.3%.

Niobium (Nb): 0.01 to 0.03%

In the present invention, 0.01 to 0.03% of niobium (Nb) may be added in order to increase the strength and hole expandability of the steel through crystal grain refinement and precipitate formation. When the Nb content is less than 0.01%, the effect of refining the structure is lost and the amount of precipitation strengthening is insufficient. On the other hand, when the Nb content is more than 0.03%, the castability of the steel deteriorates. Therefore, in the present invention, it is preferable to limit the content of Nb to 0.01 to 0.03%.

Titanium (Ti): 0.01 to 0.03%, boron (B): 0.001 to 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 is combined with N and the hardenability strengthening effect of B is lost, and when Ti is contained in more than 0.03%, the castability of the steel deteriorates. On the other hand, if the B content is less than 0.001%, an effective hardenability strengthening effect cannot be obtained, and if it contains more than 0.003%, boron carbide may be formed, which may rather impair hardenability. Therefore, in the present invention, it is preferable to limit the Ti content 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 steel. However, if the P is excessively added, the toughness of the steel deteriorates, and 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

Sulfur (S) is present as an impurity in steel like P, and it is advantageous to control its content as low as possible. In addition, since S deteriorates the ductility and impact properties of the steel, it is preferable to limit the upper limit to 0.01% or less.

Nitrogen (N): 0.01% or less

In the present invention, nitrogen (N) is added to the 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 ≤ 0.35 wt.%

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

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

When the value of Equation (1) exceeds 0.35%, as described above, LME resistance is deteriorated during spot welding, so that LME cracks exist after the spot welding, thereby impairing fatigue characteristics and structural safety. On the other hand, as the value of Equation (1) is smaller, the lower limit may not be separately set because the spot weldability and LME resistance are improved. Since it is difficult to obtain a high tensile strength of

High-strength steel sheet according to an aspect of the present invention, in addition to the above-described alloy components, 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 more may 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 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 excessive amounts, they may exceed the target strength grade and are expensive elements. desirable. On the other hand, since the Cu, Ni, and Mo act as a solid solution strengthening element, when added in an amount of 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 may be selectively added to increase the yield strength in the present invention. However, if the content is excessive, the elongation may be too low and cause brittleness of the steel, 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 is effective, but when added in an amount of less than 0.005%, the effect may be insignificant.

In the present invention, in addition to the steel composition described above, the remainder may include Fe and unavoidable impurities. Inevitable impurities may be unintentionally mixed in a typical steel manufacturing process, and this cannot be entirely excluded, and those skilled in the ordinary steel manufacturing field can easily understand the meaning. In addition, the present invention does not entirely exclude the addition of a composition other than the above-mentioned 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, by area fraction, of tempered martensite in an area fraction of more than 50% and 70% or less, and the remaining residual austenite, fresh martensite, ferrite and bainite. The remaining phases other than the tempered martensite, by area fraction, contain more than 1% and less than 4% of retained austenite, more than 10% and less than 20% of the fresh martensite, and more than 0% and less than 5% of the ferrite, The wealth may consist of bainite. In addition, a cementite phase as a second phase between the bainite laths or at the laths or grain boundaries of the tempered martensite phase may be precipitated and distributed in an area fraction of 1% or more and 3% or less.

Because the tempered martensite phase has a fine internal structure, it is an advantageous steel structure for securing the hole expandability of steel. When the fraction of tempered martensite is less than 50%, it is difficult to obtain the target hole expandability. both the elongation and the hole expandability of On the other hand, when tempered martensite exceeds 70 area%, the yield ratio and yield strength of the steel exceed the upper limit of the present invention, and the forming of the steel becomes difficult and problems such as springback after forming may occur.

Meanwhile, the remaining structures other than the tempered martensite may include retained austenite, fresh martensite, ferrite, and bainite.

In the high-strength steel sheet according to the present invention, the upper limits of Si and Al are limited by Equation (1), but since Si and Al are added to some extent, retained 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 steels with very high Si and Al contents.

In the present invention, in order to obtain a low yield ratio, a fresh martensite tissue may be introduced at a level of more than 10 area% and 20 area% or less. When the austenite phase fraction is high after secondary cooling and reheating, the carbon content in the austenite is low and the stability is insufficient, and some of the austenite is transformed into fresh martensite in the subsequent cooling process, thereby lowering the yield ratio.

In addition, although the ferrite structure in the present invention has poor hole expandability, it may exist at a level of more than 0 area% and 5 area% or less during 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 is precipitated and grown in the microstructure by limiting the contents of Si and Al, which stabilize austenite by suppressing cementite growth, according to the conditions of Equation (1). will do This cementite precipitates at martensite laths or grain boundaries when martensite formed by secondary cooling is reheated, or carbon between bainitic ferrite laths is concentrated when bainite transformation occurs during reheating after secondary cooling. formed from the part

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

On the other hand, the difference between the 25% hardness value and the 75% hardness value in the micro Vickers hardness test with a maximum load of 100 g or less can be distributed in the range of 100 to 150. The method for obtaining the difference in the hardness values 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 calculating the difference between the hardness values corresponding to the 75% and 25%. If the difference in hardness value is less than 100, higher hole expandability can be expected, but the yield strength is increased and can exceed 980 MPa. On the other hand, when the difference in hardness value is greater than 150, the yield strength is lower than the level desired in the present invention, and it is difficult to expect high hole expandability.

By having the above component composition and microstructure, the high-strength steel sheet of the present invention can exhibit a 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. % or more, it was confirmed that it can be obtained.

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 shows 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 by a hot-dip galvanizing method may be formed on at least one surface of the high-strength steel sheet according to the present invention. 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 may be preferably applied to the present invention. In addition, the hot-dip galvanized layer may be an alloyed hot-dip galvanized layer alloyed with some alloy components of the steel sheet.

Next, a method for 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 a reheating process, and the details are as follows.

First, having the above-described alloy composition, preparing a slab satisfying Equation (1), and reheating the slab to a temperature of 1150 ° C. to 1250 ° C. At this time, if the slab temperature is less than 1150 ℃, it is impossible to perform the next step, hot rolling, whereas if it exceeds 1250 ℃, 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 the intended 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 the shape defect is increased, and thus productivity is deteriorated. On the other hand, when the finish rolling temperature exceeds 980 °C, the surface quality is deteriorated due to an increase in oxides due to excessively 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 carried out in a temperature range of 500 to 700 °C. Then, after winding, cold rolling is performed at a cold rolling reduction of 30 to 60% to obtain a cold rolled steel sheet.

When the cold rolling reduction ratio is less than 30%, it is difficult to secure the target thickness precision, as well as difficult to correct the shape of the steel sheet. On the other hand, when the cold rolling reduction ratio exceeds 60%, the possibility of cracks occurring in the edge portion of the steel sheet increases, and a problem occurs in that the cold rolling load becomes excessively large. Therefore, it is preferable to limit the cold rolling reduction in the cold rolling step to 30 to 60%.

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

In the case of 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 performed in a continuous alloying hot-dip plating continuous furnace.

The continuously annealed steel sheet is first cooled at an average cooling rate of 10°C/s or less to a primary cooling termination temperature of 560°C to 700°C (hereinafter also referred to as 'SCS'), and a secondary cooling termination temperature of 280°C to 380°C. (hereinafter also referred to as 'RCS') to introduce martensite into the microstructure of the steel sheet by secondary cooling at an average cooling rate of 10°C/s or more. 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 executed step by step, it is possible to reduce the final temperature and material deviation by making the temperature distribution of the steel sheet uniform in the slow cooling step, and it is also advantageous to obtain the necessary phase composition.

Primary cooling is slow cooling at an average cooling rate of 10 °C/s or less, and the cooling end temperature may be in a temperature range of 560 to 700 °C. When the end temperature of the primary cooling is lower than 560℃, the ferrite phase is excessively precipitated and the final hole expandability is bad. can go down

Secondary cooling may be additionally applied to a quenching facility not applied in the primary cooling, and as a preferred embodiment _{, a hydrogen quenching facility using H 2} gas may be used, but is not limited thereto.

At this time, it is important to control the cooling end temperature of the secondary cooling to be 280 ~ 380 ℃ at which an appropriate initial martensite fraction can be obtained. There is no space to obtain various necessary phase transformations, and the shape and workability of the steel sheet deteriorate. On the other hand, when the secondary cooling termination temperature exceeds 380 °C, it may be difficult to obtain high hole expandability due to a low initial martensite fraction.

Tempering the martensite obtained in the previous step by reheating the cooled steel sheet at a temperature increase rate of 5 °C / s or less to a temperature range of 380 to 480 ° C (hereinafter, also referred to as ‘annealing material heating temperature’ or ‘RHS’), 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℃. If it is lower than 380℃ or exceeds 480℃, the amount of phase transformation of bainite is small, so too much fresh martensite is formed in the final cooling process, so elongation and hole expandability are reduced. will do great harm

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

In addition, if necessary, in order to obtain an alloyed hot-dip galvanizing layer, after hot-dip galvanizing, alloying heat treatment may be performed, and then cooling to room temperature may be performed.

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

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

(Example)

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

The material and the phase fraction measurement method applied in this embodiment are 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 rolling, and the gauge length was 50 mm and the width of the tensile specimen was 25 mm. .

Hole expandability was measured according to ISO 16330 standard, and holes were sheared with a clearance of 12% 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 are bainite.

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

steel grade Alloy composition (wt%) 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 Invention 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
(%) tempered
martensite
fraction
(%) cementite
fraction
(%) residual
austenite
fraction
(%) Fresh martensite
fraction
(%) blow
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 Invention 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: Difference between 25%th hardness value and 75%th hardness value when micro Vickers hardness test is performed

First, Comparative Examples 1 and 2 are cases in which steel grades A and B are applied, respectively. In steel grades A and B, the content of carbon (C) or manganese (Mn) was 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% hardness value and the 25% Vickers hardness value was less than 100, so that the hole expandability (HER) value was high, but the yield strength and yield ratio exceeded the scope of the present invention.

In addition, in 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 low. The difference between the 75% hardness value and the 25% Vickers hardness value exceeded 150.

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

In invention Examples 1 to 3, steel grades C and D satisfying the alloy composition of the present invention were applied, and all process conditions were satisfied, and at a low yield ratio of 0.65 to 0.85, hole expandability of 25% or more and 7% to 14% It was possible to obtain an appropriate elongation for the processing of

Although it has been described with reference to the above embodiments, it will be understood by those skilled in the art that various modifications and changes can be made to the present invention without departing from the spirit and scope of the present invention as set forth in the following claims. will be able

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 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 contains iron (Fe) and other unavoidable impurities,
The content of carbon (C), silicon (Si) and aluminum (Al) satisfies the following formula (1),
The microstructure contains more than 50% and 70% or less of tempered martensite as an area fraction, and the remaining residual austenite, fresh martensite, ferrite and bainite,
A high-strength steel sheet in which a cementite phase is precipitated and distributed in an area fraction of 1% or more and 3% or less as a second phase between the bainite laths or at the laths or grain boundaries of the tempered martensite phase.
[Equation (1)] 0.20 wt.% ≤ [C] + ([Si]+[Al])/5 ≤ 0.35 wt.%
(Here, [C], [Si], and [Al] mean the weight % of C, Si, and Al, respectively.)
The method of claim 1,
High-strength steel sheet, characterized in that the retained austenite is more than 1% and less than 4%, the fresh martensite is more than 10% and less than 20%, and the ferrite is more than 0% and less than 5%, and the balance is bainite.
The method of claim 1,
High-strength steel sheet, characterized in that the difference between the 25% hardness value and the 75% hardness value is distributed in the range of 100 to 150 when the micro Vickers hardness test is performed.
The method of claim 1,
The steel sheet, by 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 further comprising one or more 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 alloying hot-dip galvanizing 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 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 remainder including iron (Fe) and other unavoidable impurities, and the content of carbon (C), silicon (Si) and aluminum (Al) is expressed by the following formula ( 1) preparing a slab that satisfies;
heating the slab to a temperature range of 1150 to 1250 °C;
Finishing hot rolling the reheated slab in a finish rolling temperature (FDT) range 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 of 30 to 60%;
continuous annealing of the cold-rolled steel sheet in a temperature range of (Ac3+30°C to Ac3+80°C);
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 ~ 480 ℃ at a temperature increase rate of 5 ℃ / s or less; A method of manufacturing a high-strength steel sheet comprising a.
[Equation (1)] 0.20 wt.% ≤ [C] + ([Si]+[Al])/5 ≤ 0.35 wt.%
(Here, [C], [Si], and [Al] mean the weight % of C, Si, and Al, respectively.)
10. The method of claim 9,
The slab, by 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 further comprising one or more A method of manufacturing a high-strength steel sheet, characterized in that
10. The method of claim 9,
After the reheating step, the method of manufacturing a high-strength steel sheet, characterized in that it further comprises the step of hot-dip galvanizing in a temperature range of 480 ~ 540 ℃.
12. The method of claim 11,
After the hot-dip galvanizing step, the method of manufacturing a high-strength steel sheet, characterized in that cooling to room temperature after performing an alloying heat treatment.
12. The method of claim 11,
After cooling to room temperature, a method for producing a high-strength steel sheet, characterized in that performing temper rolling of less than 1%.