KR20140030970A

KR20140030970A - High strength steel sheet having excellent yield strength

Info

Publication number: KR20140030970A
Application number: KR1020120097808A
Authority: KR
Inventors: 이규영; 구민서; 김성우; 주세돈; 곽재현
Original assignee: 주식회사 포스코
Priority date: 2012-09-04
Filing date: 2012-09-04
Publication date: 2014-03-12

Abstract

Provided are a high strength steel sheet having excellent yield strength comprising in wt%, 0.12-0.2% of C, 0.5% or less of Si (0% is excluded), 2.6-4.0% of Mn, 0.03% or less of P (0% is excluded), 0.015% or less of S (0% is excluded), 0.1% or less of Al (0% is excluded), 1% or less of Cr (0% is excluded), 48/14*′N′-0.1% of Ti, 0.1% or less of Nb (0% is excluded), 0.005% or less of B (0% is excluded), 0.01% or less of N (0% is excluded), and residual Fe and unavoidable impurities, and a method for manufacturing the same. A microstructure is obtained by heat treating a steel sheet comprising, in a volume fraction, 90% or more of martensite, and ferrite and bainite of 10% or less. The ratio of yield strength to tensile strength is 0.75 or more. According to the present invention, the yield strength of martensite steel having a microstructure fraction of 90% or more can be improved through annealing heat treatment in an annealing furnace or a continuous annealing type hot dipping line where a slow cooling section exists. [Reference numerals] (AA) Tensile strength (MPa)

Description

Ultra high strength steel sheet with excellent yield strength and manufacturing method thereof {HIGH STRENGTH STEEL SHEET HAVING EXCELLENT YIELD STRENGTH}

The present invention relates to an ultra high strength steel sheet excellent in yield strength and a method of manufacturing the same.

In order to meet the contradictory goals of reducing the weight of automobile steels and securing collision safety, dual phase steel (also referred to as DP steel), transformation-induced plasticity steel (also referred to as TRIP steel), composite Various automotive steels such as complex phase steel (hereinafter referred to as CP steel) are being developed. However, in the advanced high-strength steel, the strength can be increased by increasing the amount of carbon, but considering the practical aspects such as spot weldability, the tensile strength that can be implemented is limited to about 1200 Mpa level. Application to structural members to secure collision safety has been widely recognized as a method of securing final strength by quenching through direct contact with a die, which is water-cooled after molding at high temperatures. Because of this high application expansion.

Slow cooling is generally used as an alternative to quenching through water cooling.

However, in the continuous annealing furnace and continuous annealing hot-dip galvanizing line in which there is a slow cooling section, martensitic steel having a microstructure fraction of more than 90% after annealing heat treatment has a disadvantage in that the yield strength is lowered because the yield strength and tensile strength ratio are less than 0.75. have. In order to increase the resistance in the crash of the vehicle it is desirable to increase the yield strength, it is required to improve for this. In general, the tempering of martensitic steel is performed to improve the lack of ductility and toughness of martensitic steel, and a method of increasing yield strength while suppressing a drop in tensile strength is needed.

One aspect of the present invention is to propose an ultra-high strength steel sheet and a method of manufacturing the improved yield strength through additional heat treatment of a steel sheet with a low yield strength.

However, the problems to be solved by the present invention are not limited to the above-mentioned problems, and other problems not mentioned can be clearly understood by those skilled in the art from the following description.

In order to achieve the above object, one aspect of the present invention, in weight%, C: 0.12 ~ 0.2%, Si: 0.5% or less (excluding 0%), Mn: 2.6 ~ 4.0%, P: 0.03% or less (Excluding 0%), S: 0.015% or less (excluding 0%), Al: 0.1% or less (excluding 0%), Cr: 1% or less (excluding 0%), Ti: 48/14 * [N] ~ 0.1 %, Nb: 0.1% or less (except 0%), B: 0.005% or less (except 0%), N: 0.01% or less (except 0%), balance Fe and other impurities. It is obtained by heat-treating a steel plate composed of 90% or more martensite, 10% or less ferrite and bainite, the ratio of yield strength to tensile strength of 0.75 or more, to provide an excellent high strength steel sheet.

Another aspect of the present invention is, by weight, C: 0.12 to 0.2%, Si: 0.5% or less (excluding 0%), Mn: 2.6 to 4.0%, P: 0.03% or less (excluding 0%), S: 0.015 % Or less (excluding 0%), Al: 0.1% or less (excluding 0%), Cr: 1% or less (excluding 0%), Ti: 48/14 * [N] ~ 0.1%, Nb: 0.1% or less (0 %), B: 0.005% or less (except 0%), N: 0.01% or less (except 0%), balance Fe and other impurities, and the microstructure is 90% or more martensite by volume fraction, 10% Preparing a steel sheet composed of the following ferrite and bainite, and the yield strength comprising the step of heat-treating the steel sheet under the condition of the following relation 1 ((T + 273.15) x (20 + logt) <11000) It provides an excellent method for producing an ultra high strength steel sheet. Where T is the heat treatment temperature (℃) and t is the heat treatment time (hour)

According to an aspect of the present invention, the yield strength of martensitic steel having a martensite fraction of 90% or more through additional heat treatment of a low yield strength steel sheet produced in a continuous annealing furnace or a continuous annealing hot dip plating line having a slow cooling section. You can.

1 is a graph showing a change in tensile strength according to a change in heat treatment conditions according to an embodiment of the present invention.

Hereinafter, the ultra-high strength steel sheet excellent in the yield strength of the present invention and its manufacturing method will be described in detail so that a person skilled in the art can easily carry out the present invention.

One aspect of the invention, in weight%, C: 0.12 ~ 0.2%, Si: 0.5% or less (excluding 0%), Mn: 2.6 ~ 4.0%, P: 0.03% or less (excluding 0%), S: 0.015 % Or less (excluding 0%), Al: 0.1% or less (excluding 0%), Cr: 1% or less (excluding 0%), Ti: 48/14 * [N] ~ 0.1%, Nb: 0.1% or less (0 %), B: 0.005% or less (except 0%), N: 0.01% or less (except 0%), balance Fe and other impurities, and the microstructure is 90% or more martensite by volume fraction, 10% It is obtained by heat-treating a steel plate composed of the following ferrite and bainite, and provides an ultra high strength steel sheet having excellent yield strength, in which the ratio of yield strength to tensile strength is 0.75 or more.

For example, the slow cooling conditions of a continuous annealing furnace or continuous annealing hot-dip galvanizing line in which a slow cooling section is present are generally cooled to 3 ° C./s after cooling annealing to 650 ° C. or 460 ° C. which is a hot dip deposition temperature. The steel sheet having the component system of the present invention manufactured under such conditions is a steel having a martensite fraction of 90% or more of microstructure, and has an initial yield strength of 1000 to 1250 MPa, an initial tensile strength of 1400 to 1700 MPa, and yield strength and tensile strength. Yield strength is inferior with ratio of less than 0.75. The present invention aims to improve the yield strength by selecting a steel sheet of such a yield strength.

The reason for limiting the numerical values of the above components will be described as follows. Hereinafter, it is necessary to pay attention that the content unit of each component is weight% unless otherwise stated.

C: 0.12 to 0.2%

The content of carbon (C) is preferably 0.12 to 0.2%. C is required for securing the strength of martensite, so it should be added by 0.12% or more. However, if the content exceeds 0.2%, the weldability becomes poor, so the upper limit is limited to 0.2%.

Si : 0.5% or less (excluding 0%)

The content of silicon (Si) is preferably 0.5% or less (excluding 0%). Si is a ferrite stabilizing element and has a disadvantage in that it weakens the strength by accelerating the generation of cold ferrite after annealing in a conventional continuous annealing type hot dip galvanizing heat treatment furnace in which a cooling section exists and also has a disadvantage in that a large amount Mn is added, there is a risk of deterioration of the molten plating property due to the formation of the surface oxide by the Si during the annealing and a risk of induction of the dent defect due to surface enrichment and oxidation of the Si, so that the upper limit is limited.

Mn : 2.6 ~ 4.0%

The content of manganese (Mn) is preferably 2.6 to 4.0%. Mn in steel is well known as an element that inhibits ferrite formation and facilitates the formation of austenite. In the case of continuous annealing hot-dip heat treatment furnace, ferrite is easily produced when Mn is less than 2.6%, and when Mn is more than 4%, band formation due to segregation caused by slab and hot rolling process is excessive. It is also limited to the increase in the cost of ferroalloy due to excessive alloy input during converter operation.

P: 0.03% or less (excluding 0%)

The content of phosphorus (P) is preferably 0.03% or less (excluding 0%). If the content of P is an impurity element and the content thereof exceeds 0.03%, the weldability decreases and the risk of brittleness of steel increases, and the possibility of occurrence of dent defects increases. Therefore, the upper limit of P is preferably limited to 0.03%.

S: 0.015% or less (excluding 0%)

The content of sulfur (S) is preferably 0.015% or less (excluding 0%). S, like P, is an impurity element in steel and is an element that hinders ductility and weldability of the steel sheet. If the content exceeds 0.015%, the ductility and weldability of the steel sheet are likely to be deteriorated. Therefore, the upper limit is preferably limited to 0.015%.

Al : 0.1% or less (excluding 0%)

The content of aluminum (Al) is preferably 0.1% or less (excluding 0%). Al is an alloy element for expanding the ferrite phase. When the continuous annealing type hot-dip coating heat treatment process in which the cooling is present as in the present invention is utilized, there is a disadvantage that ferrite formation is promoted, and the hot- The upper limit is limited.

Cr : Less than 1% (except 0%)

The content of chromium (Cr) is preferably 1% or less (excluding 0%). Cr is an alloying element that facilitates securing low temperature transformation structure by suppressing ferrite transformation, and there is an advantage of suppressing ferrite formation when utilizing the continuous annealing hot dip heat treatment process in which slow cooling exists as in the present invention. If it exceeds 1%, it is limited to increase of iron alloy cost due to excessive alloy input.

Ti : 48/14 * [N] - 0.1%

The content of titanium (Ti) is preferably 48/14 * [N] to 0.1%. Ti is a nitride-forming element, and it is necessary to add a chemical equivalent of 48/14 * [N] in order to precipitate N in steel and scavenging it with TiN. When Ti is not added, cracking is likely to occur in the continuous casting due to AlN formation. Therefore, addition of more than 0.1% is required because addition of solid solution carbide precipitates in addition to elimination of solid solution N, thereby reducing martensite strength.

Nb : 0.1% or less (excluding 0%)

The content of niobium (Nb) is preferably 0.1% or less (excluding 0%). Nb is an element which segregates in the austenite grain boundaries and inhibits the coarsening of austenite grains during the annealing heat treatment. Therefore, when Nb is added in an amount exceeding 0.1%, it is limited to an increase in the amount of alloy iron due to excessive alloying amount.

B: 0.005% or less (excluding 0%)

The content of boron (B) is preferably 0.005% or less (excluding 0%). B has an advantage of suppressing ferrite formation, and has an advantage of suppressing the formation of ferrite upon cooling after annealing. If the content of B exceeds 0.005%, ferrite formation is promoted by precipitation of Fe ₂₃ (C, B) ₆ , which is limited.

N: 0.01% or less (excluding 0%)

The content of nitrogen (N) is preferably 0.01% or less (excluding 0%). If N is more than 0.01%, the risk of cracking during performance through AlN formation or the like is greatly increased, so that the upper limit is preferably limited to 0.01%.

The remainder consists of Fe and unavoidable impurities.

In addition, the steel sheet prepared to prepare the ultra-high strength steel sheet of the present invention while satisfying the above component system, the microstructure is composed of 90% or more martensite, 10% or less ferrite and bainite by volume fraction. The ultra-high strength steel sheet of the present invention finally obtained by heat treatment also has the same microstructure.

Since it is not easy to measure the volume fraction, which is a substantially three-dimensional concept, it is replaced with an area fraction measurement through a cross-sectional observation used in ordinary microstructure observation. Due to the constitution of the microstructures, the singular point of effect is that the hard phase martensite has a microstructure in which the main phase has a merit that it is easy to secure super strength.

In addition, it is obtained by heat-treating the steel sheet having the component system and the microstructure, it is preferable that the decrease in the tensile strength of the steel sheet after the heat treatment is less than 150MPa than before the heat treatment. Through this, the target of the yield strength and the tensile strength ratio of the steel sheet of the present invention can be 0.75 or more.

The steel sheet may be a cold rolled steel sheet, a hot dip galvanized steel sheet, or a hot dip galvanized steel sheet.

In order to manufacture an ultra-high strength steel sheet having excellent yield strength having a component system and a microstructure as described above, the following process is performed.

First, in weight percent, C: 0.12 ~ 0.2%, Si: 0.5% or less (excluding 0%), Mn: 2.6 ~ 4.0%, P: 0.03% or less (excluding 0%), S: 0.015% or less (0% Al: 0.1% or less (excluding 0%), Cr: 1% or less (excluding 0%), Ti: 48/14 * [N] ~ 0.1%, Nb: 0.1% or less (excluding 0%), B : Less than 0.005% (except 0%), N: Less than 0.01% (except 0%), remainder Fe and other impurities, microstructure consists of more than 90% martensite, less than 10% ferrite and bay Prepare a steel sheet composed of knight.

The steel sheet is then heat treated under the condition of relation 1 ((T + 273.15) x (20 + logt) <11000). However, in relation 1, T is the heat treatment temperature (° C), and t represents the heat treatment time (hour).

The reason for setting the relation 1 is as follows.

The ultra-high strength steel sheet manufactured by passing through the continuous annealing furnace or the continuous annealing alloy plating furnace in which the slow cooling section exists has a ratio of yield strength and tensile strength of less than 0.75 by forming a martensite-based microstructure. This is due to the fixation of solid solution carbon at the potential introduced at the time. Thus, the condition of the heat treatment to freely diffuse the carbon stuck to the potential is derived from the above equation 1 through careful experiment in the present invention.

When the fixed carbon is freely diffused, fixing the dislocation inhibits deformation of the material and consequently increases the yield strength. The freeing of the fixed carbon is a function of temperature and time as in the normal diffusion behavior. The higher the temperature, the longer the time can be freely diffused. However, if the temperature is too high, yield strength and tensile strength are rather increased due to the formation of carbides. Since the strength is reduced, it is preferable to satisfy the above equation 1.

In addition, it is preferable that the ratio of the yield strength to the tensile strength of the steel sheet after the heat treatment is 0.75 or more. This is because the yield strength is advantageous due to the characteristics of the steel sheet applied to the collision member.

In addition, the tensile strength of the steel sheet after the heat treatment is preferably lower than 150MPa than the tensile strength of the steel sheet before the heat treatment. This is because it is advantageous that the tensile strength of the steel sheet applied to the collision member is also high.

Hereinafter, the present invention will be described in detail with reference to Examples. However, the following examples are only for illustrating the present invention in more detail and do not limit the scope of the present invention.

[ Example ]

To the heat treatment conditions of Table 1, the initial yield strength of 1117MPa, the initial tensile strength of 1535MPa heat treatment of the steel, and the mechanical properties change according to the heat treatment is shown in Table 2. The steel used was 0.16% C, 0.11% Si, 3.12% Mn, 0.012% P, 0.69% Cr, 0.02% Ti, 0.039% Nb, 0.0016% B, 0.0036% S, 0.022% Al, 0.004% N It contained.

Example
One Example
2 Example
3 Example
4 Example
5 Example
6 Example
7 Example
8 Example
9 Comparative Example
One Comparative Example
2 Heat treatment temperature
(℃) 100 100 100 100 200 200 200 200 300 400 500 Heat treatment time
(hr) One 3 10 20 One 3 10 20 0.033 0.033 0.033 Value of relation 1 7463 7641 7836 7948 9463 9689 9936 10079 10616 12469 14321

Example
One Example
2 Example
3 Example
4 Example
5 Example
6 Example
7 Example
8 Example
9 Comparative Example
One Comparative Example
2 Yield strength (MPa) 1181 1216 1230 1241 1289 1304 1304 1307 1328 1115 900 Tensile Strength (MPa) 1532 1525 1522 1516 1505 1483 1468 1466 1433 1123 905 Surrender / tension 0.77 0.80 0.81 0.82 0.86 0.88 0.89 0.89 0.93 0.99 0.99 △ tensile strength
(MPa) 3 10 13 19 30 52 67 69 102 412 630

In the case of the embodiment, the ratio of yield strength and tensile strength is excellent, and the degree of deterioration of tensile strength is low, so it is suitable for application to structural members.

However, Comparative Example 1 and Comparative Example 2 is very excellent in the ratio of yield strength and tensile strength, but deterioration of the tensile strength is very high, such as 400MPa or more is not suitable for application to the structural member.

Figure 1 shows the change in tensile strength with the change in (T + 273.15) x (20 + logt).

If the value of (T + 273.15) x (20 + logt) is less than or equal to 11000, there is only a drop of 135 MPa compared to the initial tensile strength. However, if the value of (T + 273.15) x (20 + logt) is less than 11,000, the tensile strength is very large, making it difficult to apply to a collision member. It can be seen that.

Claims

By weight, C: 0.12 ~ 0.2%, Si: 0.5% or less (excluding 0%), Mn: 2.6 ~ 4.0%, P: 0.03% or less (excluding 0%), S: 0.015% or less (excluding 0%) , Al: 0.1% or less (excluding 0%), Cr: 1% or less (excluding 0%), Ti: 48/14 * [N] ~ 0.1%, Nb: 0.1% or less (excluding 0%), B: 0.005 Less than 0% (except 0%), N: Less than 0.01% (except 0%), remainder Fe and other impurities, microstructure consists of 90% or more martensite, 10% or less ferrite and bainite An ultra-high strength steel sheet having excellent yield strength, obtained by heat-treating a steel sheet, which has a ratio of yield strength to tensile strength of 0.75 or more.

The method of claim 1,
The steel sheet is cold rolled steel sheet, hot-dip galvanized steel sheet or hot-dip zinc alloy plated steel sheet, ultra high strength steel sheet excellent in yield strength.

The method of claim 1,
Ultra-high strength steel sheet excellent in yield strength, the width of the tensile strength after the heat treatment to the tensile strength before heat treatment of the steel sheet is 150MPa or less.

By weight, C: 0.12 ~ 0.2%, Si: 0.5% or less (excluding 0%), Mn: 2.6 ~ 4.0%, P: 0.03% or less (excluding 0%), S: 0.015% or less (excluding 0%) , Al: 0.1% or less (excluding 0%), Cr: 1% or less (excluding 0%), Ti: 48/14 * [N] ~ 0.1%, Nb: 0.1% or less (excluding 0%), B: 0.005 Less than 0% (except 0%), N: Less than 0.01% (except 0%), remainder Fe and other impurities, microstructure consists of 90% or more martensite, 10% or less ferrite and bainite Preparing a steel sheet that is constructed; And
The method of manufacturing an ultra-high strength steel sheet excellent in yield strength, comprising the step of heat-treating the steel sheet under the condition of the following relation 1.
[Relation 1]
(T + 273.15) x (20 + logt) <11000, where T is the heat treatment temperature (° C) and t is the heat treatment time (hour)

5. The method of claim 4,
Yield strength ratio to the tensile strength of the heat-treated steel sheet is 0.75 or more, the method of manufacturing a super high strength steel sheet excellent in yield strength.

5. The method of claim 4,
The method of manufacturing a super high strength steel sheet having excellent yield strength, wherein the drop width of the tensile strength after heat treatment to the tensile strength before heat treatment of the steel sheet is 150 MPa or less.