KR101461715B1 - Ultra high strength cold rolled steel sheet and method for manufacturing the same - Google Patents

Abstract

(Excluding 0%), Mn: 3.0 to 4.0%, P: not more than 0.03% (excluding 0%), S: not more than 0.015% (excluding 0%), , Al: not more than 0.1% (excluding 0%), Cr: not more than 1% (excluding 0%), Ti: 48/14 * The content of C, Mn, Cr and Ti is in the range of 1 to 3756 [C] + 324 [C], and the content of C, Mn, Cr and Ti is less than 0.01% Mn] + 218 [Cr] - 619 [Ti] > 1690, and the microstructure is composed of 90% or more of martensite and 10% or less of ferrite and bainite in a volume fraction, Method is provided.
According to the present invention, an ultra-high strength steel having a tensile strength of 1400 MPa or more can be produced by using a conventional continuous annealing furnace in which a cooling zone is present, and a steel sheet having excellent shape quality as compared with martensitic steel produced by water cooling A cold rolled steel sheet can be provided.

Description

TECHNICAL FIELD [0001] The present invention relates to an ultrahigh strength cold rolled steel sheet,

The present invention relates to an ultra-high-strength cold-rolled steel sheet which is applied to lightweight automobiles and to improve collision performance, and a method of manufacturing the same.

In order to satisfy the contradictory goal of light weight of automotive steel sheet and ensuring collision safety, it has been proposed to use dual phase steel (hereinafter referred to as DP steel), Transformation Induced Plasticity Steel (hereinafter also referred to as TRIP steel) A variety of automotive steel plates such as a composite phase steel (hereinafter also referred to as CP steel) are being developed. However, it is possible to increase the strength by increasing the amount of carbon in the advanced high strength steel. However, considering the practical aspect such as the spot weldability, the tensile strength that can be implemented is limited to the level of about 1200 MPa.

Hot press forming steel, which ensures ultimate strength by quenching by direct contact with a die that is water-cooled after molding at high temperature, is in the spotlight for application to structural members to secure collision safety. However, The increase in investment cost is not large due to excessive investment cost, heat treatment and increase in process cost.

The roll forming method with high productivity compared to general press forming and hot press forming is a method for producing a complicated shape through multi-step roll forming, but the application is widely applied to the forming of an ultra high strength material having a low elongation. It is mainly manufactured in a continuous annealing furnace with water cooling equipment, and the microstructure has a tempered martensite structure tempered with martensite. There is a disadvantage in that workability deterioration and material deviation by position are applied when the roll forming is applied in order to heat the shape quality due to the temperature deviation in the width direction and the longitudinal direction during water cooling. Therefore, there is a need to devise alternatives to quenching by water cooling.

Patent Document 1 proposes a method for producing a cold-rolled steel sheet excellent in plate form after continuously annealing by simultaneously obtaining high strength and high ductility by utilizing tempering martensite. The carbon content is as high as 0.2% or more, There is concern about the possibility of induction.

Japanese Patent Laid-Open No. 2010-090432

One aspect of the present invention is to propose a method of manufacturing an ultrahigh strength cold rolled steel sheet in a continuous annealing furnace having a cooling facility in place of a water cooling facility and an ultra high strength cold rolled steel sheet.

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, according to one aspect of the present invention, there is provided a method of manufacturing a semiconductor device, comprising 0.12 to 0.2% of C, 0.5% or less of Si (excluding 0%), 3.0 to 4.0% of Mn, (Excluding 0%), S: not more than 0.015% (excluding 0%), Al: not more than 0.1% (excluding 0%), Cr: not more than 1% Mn, not more than 0.1% (excluding 0%), B: not more than 0.005% (excluding 0%), N: not more than 0.01% (excluding 0%), the balance Fe and other unavoidable impurities. The content of Cr and Ti satisfies the relational expression 1 (3756 [C] + 324 [Mn] + 218 [Cr] - 619 [Ti]> 1690), and the microstructure contains 90% or more of martensite, 10% Of ferrite and bainite, and a super high strength cold rolled steel sheet.

Another aspect of the present invention is a steel sheet comprising 0.12 to 0.2% of C, 0.5% or less of Si (excluding 0%), 3.0 to 4.0% of Mn, 0.03% or less of P Ti: 48/14 * [N] to 0.1%, Nb: 0.1% or less (excluding 0%), Al: not more than 0.1% Mn, Cr, and Ti in the steel material are not more than 0.005% (excluding 0%), N: not more than 0.01% (excluding 0%), the balance Fe and other unavoidable impurities. Reheating the steel slab satisfying the following conditions: 3756 [C] + 324 [Mn] + 218 [Cr] - 619 [Ti]> 1690 to a temperature of 1100 to 1300 ° C; heating the reheated steel slab to a temperature of 850 to 950 ° C Hot rolling the hot rolled steel sheet at a temperature of 720 占 폚 or less, cold rolling after pickling, annealing at a temperature in the range of 780 to 880 占 폚, Cooling to 5 deg. C / s or less, and cooling to 700 deg. C, to obtain a super high strength cold rolled steel sheet The ball.

According to one aspect of the present invention, an ultra-high strength steel having a tensile strength of 1400 MPa or more can be manufactured by using a conventional continuous annealing furnace in which a cooling zone is present, and a superior shape A cold-rolled steel sheet having quality can be provided.

1 is a microstructure photograph of an ultra-high strength cold rolled steel sheet according to an embodiment of the present invention.
2 is a microstructure photograph of a cold-rolled steel sheet according to a comparative example of the present invention.

Hereinafter, the ultrahigh-strength cold-rolled steel sheet of the present invention and its manufacturing method will be described in detail so that those skilled in the art can readily carry out the present invention.

One aspect of the present invention is a steel sheet comprising 0.12 to 0.2% of C, 0.5% or less of Si (excluding 0%), 3.0 to 4.0% of Mn, 0.03% or less of P Ti: 48/14 * [N] to 0.1%, Nb: 0.1% or less (excluding 0%), Al: not more than 0.1% Mn, Cr, and Ti in the steel material are not more than 0.005% (excluding 0%), N: not more than 0.01% (excluding 0%), the balance Fe and other unavoidable impurities. And the microstructure has a volume fraction of 90% or more of martensite, 10% or less of ferrite and bainite, and the ratio of the microstructure is 3756 [C] + 324 [Mn] + 218 [Cr] - 619 [Ti] High-strength cold-rolled steel sheet.

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%

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 promoting the formation of cold ferrite after annealing in a conventional continuous annealing furnace in which a cooling section is present, and a large amount of Mn is added for suppressing the phase transformation as in the present invention The upper limit is limited because there is a risk of surface enrichment due to Si and induction of dent defects due to oxidation during annealing.

Mn : 3.0 to 4.0%

The content of manganese (Mn) is preferably 3.0 to 4.0%. Mn of steel is well known as an element which inhibits formation of ferrite and facilitates the formation of austenite. When Mn is less than 3%, ferrite is easily produced upon cooling, and when Mn is more than 4% And the increase in the amount of alloy iron due to excessive alloying input during the transformation process is limited.

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 alloying element that expands the ferrite phase. When the continuous annealing process in which the cooling is present is utilized as in the present invention, there is a disadvantage that ferrite formation is accelerated. Since hot Al rolling can be deteriorated by AlN formation, .

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. When using a continuous annealing process in which there is cooling as in the present invention, Cr is advantageous in suppressing ferrite formation, , It is limited to an increase in the cost of alloy iron 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.

The content of C, Mn, Cr and Ti should satisfy the relationship 1 (3756 [C] + 324 [Mn] + 218 [Cr] - 619 [Ti]> 1690).

In the case of a continuous annealing furnace including a normal annealing zone, there is a continuous annealing zone of 100 to 200 m after annealing. Such annealing zone exists to promote the precipitation of carbide of low carbon steel. However, the production of super high strength steel It is difficult to remove it from unnecessary equipment or continuous mass production based continuous annealing furnace. In the presence of such a cooling zone, there is a disadvantage in that it is difficult to manufacture an ultra-high strength steel due to transformation of a soft phase such as ferrite by the quenching at a high temperature after annealing. For example, in the presence of a 160-m-long standing cooling zone, when the passing speed of the thin steel sheet is 160 m / min, the holding time in the cooling zone is 60 seconds, and the annealing temperature is 830 [ The cooling rate in the cooling section is very low at 3 ° C per second, which greatly increases the likelihood that a soft phase such as ferrite will be produced. The above-mentioned relational expression 1 is a composition for deriving an alloy component for reducing generation of a soft phase such as ferrite and obtaining a tensile strength of 1400 MPa in a general continuous annealing furnace in which a cooling section is present.

The microstructure of the cold-rolled steel sheet of the present invention is composed of 90% or more of martensite, 10% or less of ferrite and bainite in a volume fraction.

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. Since the martensite, which is a hard phase, is a main phase, there is an advantage that it is easy to secure an ultra high strength steel due to the structure of the microstructure.

In order to produce an ultrahigh-strength cold-rolled steel sheet having the above-mentioned component system and microstructure, the following process is performed.

First, in terms of% by weight, 0.12 to 0.2% of C, 0.5% or less of Si (excluding 0%), 3.0 to 4.0% of Mn, 0.03% or less of P, (Excluding 0%), Ti: 48/14 * [N] to 0.1%, Nb: 0.1% or less (excluding 0%), Al: not more than 0.1% , The content of C, Mn, Cr, and Ti is in the range of 1: 3756 (C) + 0.005% (excluding 0%), N: not more than 0.01% (excluding 0%), the balance Fe and other unavoidable impurities. A steel slab satisfying 324 [Mn] + 218 [Cr] - 619 [Ti]> 1690 is prepared and reheated to a temperature of 1100 to 1300 ° C.

If the reheating temperature is lower than 1100 ° C, there is a problem that the hot rolling load sharply increases. When the reheating temperature is higher than 1300 ° C, the amount of the surface scale increases, leading to loss of the material.

The reheated steel slab is subjected to finish hot rolling at 850 to 950 ° C. When the temperature of the steel sheet is lower than 850 ° C, there is a high possibility that the problem of plate twisting occurs during hot rolling, and thus the above limitation is imposed. In the case of exceeding 950 캜, finishing rolling is likely to cause scale-induced surface defects and is therefore limited.

After the hot rolling, the steel sheet is wound at a temperature of 720 DEG C or less. If the coiling temperature exceeds 720 占 폚, the oxide film on the surface of the steel sheet may be excessively generated to cause defects. The lower the coiling temperature is, the higher the strength of the hot-rolled steel sheet is, and the lower the rolling load of the cold rolling, which is a post-process, is increased.

The hot-rolled steel sheet thus produced is pickled, cold-rolled, and then annealed in a temperature range of 780 to 880 캜 in a continuous annealing furnace having a usual continuous cooling section. The reason why the lower limit of the annealing temperature in the continuous annealing is set to 780 占 폚 is that when the work is performed at less than 780 占 폚, the strength decreases due to the formation of a large amount of ferrite and at the time of connecting with other steel products annealed at 800 占 폚 or more, This is because material deviation may be caused by temperature gradient of the top and end portions. The reason why the upper limit of the annealing temperature is set at 880 캜 is that when the temperature exceeds 880 캜, the production is difficult due to deterioration of the durability of the continuous annealing furnace.

After the annealing, the steel sheet is cooled to a cooling rate of 5 ° C / s or less in the cooling zone. The preheating period is up to 700 ° C, more preferably up to 680 ° C, and even more preferably up to 650 ° C after annealing.

In the case of a continuous annealing furnace including a conventional annealing zone, there is a standing cooling zone of 100 to 200 m after annealing. After the annealing, the soft phase such as ferrite is transformed by the cooling at high temperature, Making it difficult to manufacture. As described above, when the passing speed of the thin steel sheet is 160 m per minute in the presence of the 160 m long standing cooling section, the holding time in the cooling section means 60 seconds and the annealing temperature is 830 ° C When the final temperature of the cooling zone is 650 ° C, the cooling rate in the cooling zone is very low at 3 ° C per second, which greatly increases the likelihood of a soft phase such as ferrite. Relation 1 is a restriction on the condition of the alloy component to reduce the generation of ferrite and obtain a tensile strength of 1400 MPa in a general continuous annealing furnace in which the cooling zone is present. In order to secure a cooling rate higher than 5 ° C / sec after annealing, an additional cooling device should be introduced to limit this.

It is preferable that the cooling rate is 5 [deg.] C / s or less and 5 [deg.] C / s or more at a lower temperature than the preheating section.

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  One]

The steel having the composition shown in the following Table 1 was vacuum-melted with a 34 kg ingot, and then subjected to sizing rolling to produce a hot-rolled slab. The hot rolled coils were maintained at a temperature of 1200 ° C. for 1 hour, finishing at 900 ° C., and heated to 680 ° C. for 1 hour. After annealing at 820 ° C after pickling and 50% cold rolling, annealing was performed at 3 ° C / sec to 650 ° C / sec. The specimens were cooled to 440 ° C at 20 ° C / sec, and the mechanical properties were measured Table 2 shows the results and the satisfaction of the condition of the relational expression 1 for securing strength in the continuous annealing furnace with the cooling section in the present invention.

Steel grade Yield strength (MPa) Tensile Strength (MPa) Elongation (%) Relationship 1 Comparative River 1 736 1128 8.2 1415.8 Comparative River 2 751 1260 7.6 1525.1 Comparative Steel 3 785 1235 7.0 1527.6 Comparative Steel 4 884 1365 5.1 1682.7 Comparative Steel 5 872 1360 6.3 1650.9 Inventive Steel 1 891 1478 6.7 1771.2 Comparative Steel 6 876 1191 7.2 1494.3 Comparative Steel 7 897 1291 8.3 1608.7 Comparative Steel 8 958 1353 7.7 1643.0 Invention river 2 1113 1516 7.6 1762.6 Invention steel 3 1141 1495 6.2 1778.9 Inventive Steel 4 1158 1580 6.8 1888.5 Comparative Steel 9 411 531 27.1 1119.0

In Table 1, the contents of Mn in Comparative Steel 1, Comparative Steel 2, Comparative Steel 6, Comparative Steel 7 and Comparative Steel 9 are out of the range of the present invention. Mn is well known as an element for suppressing ferrite transformation and is a very important element for suppressing ferrite transformation of a steel sheet produced in a continuous annealing furnace in which a cooling section is present as in the present invention. Therefore, as can be seen from Table 2, the tensile strength of these steels is very low, especially the Mn content is very low, and the comparative steel 9, to which B is not added, has a very low tensile strength of less than 600 MPa Respectively. Its microstructure is shown in Fig.

In contrast, Comparative Steel 3, Comparative Steel 4, Comparative Steel 5 and Comparative Steel 8 correspond to the constituent system of the invention steel, but the relationship 1 (3756 [C] + 324 [Mn] + 218 [Cr] - 619 [Ti] . Therefore, as can be seen from Table 2, these steel types show a tensile strength of 1400 MPa or less.

In the case of the inventive steel, as shown in Table 2, it satisfies the relational expression 1 (3756 [C] +324 [Mn] + 218 [Cr] - 619 [Ti]> 1690) Or more tensile strength is secured. 1 shows a typical scanning electron microscope (SEM) micrograph of an inventive steel. The main phase is martensite and contains a small amount (less than 10%) of ferrite and bainite. This second phase is essential for a continuous annealing furnace And it is believed that a transformation occurs in the cooling and overflowing of the water.

Claims (2)

(Excluding 0%), Mn: 3.0 to 4.0%, P: not more than 0.03% (excluding 0%), S: not more than 0.015% (excluding 0%), , Al: not more than 0.1% (excluding 0%), Cr: not more than 1% (excluding 0%), Ti: 48/14 * Mn, Cr, and Ti satisfy the following relational expression (1), and the microstructure is composed of at least one of the following elements: A high-strength cold-rolled steel sheet excellent in shape quality, comprising 90% or more of martensite, residual ferrite and bainite in a volume fraction.
[Relation 1]
3756 [C] + 324 [Mn] + 218 [Cr] - 619 [Ti] > 1690
([N], [C], [Mn], [Cr] and [Ti] (Excluding 0%), Mn: 3.0 to 4.0%, P: not more than 0.03% (excluding 0%), S: not more than 0.015% (excluding 0%), , Al: not more than 0.1% (excluding 0%), Cr: not more than 1% (excluding 0%), Ti: 48/14 * Mn, Cr, and Ti satisfying the following relational expression 1 is added to the steel slab at a ratio of 1100 (excluding 0%), N: 0.01% or less (excluding 0%), the balance Fe and other unavoidable impurities. Reheating to a temperature of ~ 1300 ° C;
Finishing the reheated steel slab to a temperature of 850 to 950 ° C or less;
Rolling the hot-rolled steel sheet at a temperature of 720 占 폚 or lower;
Cold rolling after pickling;
Performing annealing annealing in a temperature range of 780 to 880 캜; And
And cooling the steel sheet at a cooling rate of 5 占 폚 / s or less after annealing to slowly cool to 700 占 폚.
[Relation 1]
3756 [C] + 324 [Mn] + 218 [Cr] - 619 [Ti] > 1690
([N], [C], [Mn], [Cr] and [Ti]

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