KR20190074842A - Steel sheet having ultra high strength and high yield ratio and method of manufacturing the same - Google Patents

Abstract

The present invention relates to an ultra high strength high-yield ratio steel sheet and a manufacturing method thereof. According to an embodiment of the present invention, the ultra high strength high-yield ratio steel sheet includes: 0.12-0.4 wt% of C; no more than 0.5 wt% (except 0 wt%) of Si; 2.6-4.0 wt% of Mn; no more than 0.03 wt% (except 0 wt%) of P; no more than 0.015 wt% (except 0 wt%) of S; no more than 0.1 wt% (except 0 wt%) of Al; no more than 1 wt% of Cr (except 0 wt%); 48/14*[N]-0.1 wt% of Ti; no more than 0.1 wt% (except 0 wt%) of Nb; no more than 0.005 wt% (except 0 wt%) of B; no more than 0.01 wt% (except 0 wt%) of N; and the remaining of Fe and inevitable impurities. A microstructure includes: no less than 90% (including 100%) of the sum of martensite and tempered martensite in terms of area fraction; and no more than 10% (including 0%) of the sum of one or two kinds of ferrite and bainite. The yield ratio is no less than 0.8, and the deviation between before and after tempering is no more than 100 MPa.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel sheet having a high strength and a high strength,

BACKGROUND OF THE INVENTION 1. Field of the Invention [0002] The present invention relates to a super high strength and high strength steel plate and a manufacturing method thereof.

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) And 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. As a structural member for ensuring collision safety, a method of securing a final strength by quenching through direct contact with a die that is water-cooled after molding at a high temperature is attracting attention, and Patent Literature 1 is a typical technique . However, in Patent Document 1, since it is necessary to quench the steel sheet to room temperature after annealing, there is a problem that it can not be manufactured without a line having a special facility capable of quenching the steel sheet between the annealing furnace and the overexposure furnace. That is, it is not so large that the investment cost is high and the heat treatment and the process cost are high.

As an alternative to the quenching method through water cooling, a technique using a slow cooling method has been developed. However, in the continuous annealing furnace and the continuous annealing type hot-dip plating line in which the slow cooling section exists, the martensitic steel having a microstructure fraction of 90% or more after annealing has a drawback that the yield ratio is less than 0.75 . Accordingly, it is desirable to increase the yield strength in order to increase the resistance of the vehicle in the event of a collision of the vehicle. In addition, to reduce the weight of automobiles, it is necessary to minimize the decrease of the tensile strength.

On the other hand, Patent Document 2 discloses a technique of using the slow cooling method, in which martensite transformation is caused to a steel sheet having reached Ms point, that is, a martensitic transformation starting temperature, and at the same time, And a high strength is ensured. However, Patent Document 2 has a problem in manufacturing stability because the heat treatment conditions under the temperature of Ms must be strictly controlled. In addition, since it has to be cooled at a very low speed in the tempering temperature range or must be maintained for a long time, the productivity is not good and the yield ratio is still low.

Japanese Patent Publication No. 2528387 Korean Patent Publication No. 2010-0116608

An aspect of the present invention is to provide an ultra high strength high strength steel sheet and a manufacturing method thereof.

An embodiment of the present invention is a steel sheet comprising, by weight%, 0.12 to 0.4% of C, 0.5% or less of Si (excluding 0%), 2.6 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% %), B: not more than 0.005% (excluding 0%), N: not more than 0.01% (excluding 0%), the balance Fe and other unavoidable impurities. The microstructure contains martensite and tempered martensite : 90% or more (including 100%); And a sum of one or both of ferrite and bainite: 10% or less (including 0%), having a yield ratio of 0.8 or more, and a tensile strength difference before and after tempering of 100 MPa or less do.

In another embodiment of the present invention, there is provided a method of manufacturing a semiconductor device, comprising: 0.12 to 0.4% of C, 0.5% or less of Si (excluding 0%), 2.6 to 4.0% of Mn, 0.03% Ti: 48/14 * [N] to 0.1%, Nb: 0.1% or less (excluding 0%), Al: 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; Continuously annealing the cold-rolled steel sheet; Slowly cooling the continuous annealed cold rolled steel sheet to room temperature; And annealing the slowly cooled cold-rolled steel sheet at 150 to 300 ° C for at least 20 seconds by induction heating the cold-rolled cold-rolled steel sheet.

According to an aspect of the present invention, there is an effect that a super high strength and high strength composite steel sheet can be manufactured with relatively simple and high productivity.

Hereinafter, the present invention will be described in detail. First, the alloy composition of the present invention will be described. The alloy composition of the inventive steel sheet described below means weight%.

C: 0.12 to 0.4%

The content of carbon (C) is preferably 0.12 to 0.4%. C is required for securing the strength of martensite, so it should be added by 0.12% or more. However, if the content exceeds 0.4%, the weldability becomes poor, so the upper limit is limited to 0.4%.

Si: 0.5% or less (excluding 0%)

The content of silicon (Si) is preferably 0.5% or less (excluding 0%). Si has a disadvantage of weakening 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 as a ferrite stabilizing element. Further, when a large amount of Mn is added for inhibiting phase transformation as in the present invention, there is a risk of deterioration of the molten plating property due to the formation of surface oxides by Si during annealing and generation of dent defects due to surface oxidation and oxidation of Si, To 0.5%.

Mn: 2.6 to 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 the continuous annealing type hot-dip coating heat treatment furnace, when Mn is less than 2.6%, generation of ferrite is easy during cooling, and when Mn is more than 4%, band formation due to segregation caused by slab and hot rolling process is excessive There is a problem in that the amount of alloy iron is increased due to an excessive amount of the alloy when the converter is operated.

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 to 0.1%.

Cr: 1% or less (excluding 0%)

The content of chromium (Cr) is preferably 1% or less (excluding 0%). Cr is an alloying element which facilitates securing low-temperature transformation structure by suppressing ferrite transformation. In the present invention, when a continuous annealing type hot-dip coating heat treatment process is used, there is an advantage of suppressing ferrite formation, If it exceeds 1%, it is limited to the increase of alloy iron cost due to excessive alloying amount.

Ti: 48/14 * [N] to 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 added in an amount less than 48/14 * [N], it is not only difficult to obtain the above effect, but cracks may occur during continuous casting by AlN formation. On the other hand, if it exceeds 0.1%, the martensite strength may be reduced by additional carbide precipitation in addition to removal of solid solution N.

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 is segregated 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%, the cost increases due to an excessive amount of alloy.

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 inhibiting ferrite formation and is an element that inhibits the formation of ferrite upon cooling after annealing. If the content of B exceeds 0.005%, ferrite formation is promoted by precipitation of Fe (C, B) 6.

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 of the steel sheet of the present invention is Fe, and may contain impurities that are inevitably contained in other manufacturing processes.

The microstructure of the steel sheet of the present invention preferably has a sum of martensite and tempered martensite of 100%. However, microstructures such as ferrite or bainite can inevitably be formed inevitably in the manufacturing process. Therefore, in the present invention, the microstructure has an area fraction of at least 90% (including 100%) of martensite and tempered martensite; And the sum of one or both of ferrite and bainite: 10% or less (inclusive of 0%). As a result, excellent tensile strength and yield strength can be secured by controlling the microstructure as described above, and the yield ratio can be secured at a high level.

The steel sheet of the present invention as described above has a tensile strength of 1300 MPa or more, a yield strength of 1040 MPa or more, and a yield ratio of 0.8 or more, thereby securing a very high strength and a high yield ratio. In addition, the steel sheet of the present invention may have a tensile strength difference before and after tempering of 100 MPa or less.

The steel sheet of the present invention may be one of a cold rolled steel sheet, a hot-dip galvanized steel sheet and a galvannealed galvanized steel sheet.

Hereinafter, the method of manufacturing the steel sheet of the present invention will be described in detail.

The steel material having the above-described alloy composition is cold-rolled to obtain a cold-rolled steel sheet. The cold rolling can be applied without any particular limitations as long as it is a method commonly applied in the related art.

The cold-rolled steel sheet is continuously annealed. The continuous annealing is preferably performed at 780 to 880 캜. If the annealing temperature is lower than 780 占 폚, a large amount of ferrite may be formed and the strength may be lowered. On the other hand, if the annealing temperature exceeds 880 DEG C, the durability of the continuous annealing furnace may be deteriorated and production may be difficult.

Thereafter, the cold-rolled steel sheet continuously annealed is slowly cooled to room temperature. Normally, quenching is carried out at a cooling rate of several hundreds of degrees Celsius per second in order to obtain a martensite structure. In the present invention, however, martensite of 90% or more (including 100%) is formed . In the present invention, the slow cooling rate is not particularly limited. For example, it may have a cooling rate of 5 DEG C / s or less from 700 to 650 DEG C and a cooling rate of 20 DEG C / s or less from 320 to 460 DEG C Lt; / RTI >

Thereafter, the continuously annealed cold rolled steel sheet is subjected to induction heating and tempered at 150 to 300 DEG C for 20 seconds or more. Generally, a dislocation is introduced at the time of forming the martensite structure. In the present invention, the carbon in the steel is diffused through the rapid tempering process according to the induction heating method, so that the solidified carbon is fixed to the potential to improve the yield strength. However, if the tempering temperature is less than 150 ° C, it is difficult to obtain the above-mentioned effect sufficiently, and it is difficult to secure a yield ratio of 0.8 or more. When the tempering temperature is more than 300 ° C, the yield strength and tensile strength are decreased . The tempering time is preferably 20 seconds or more, but if the tempering time is less than 20 seconds, the tempering effect can not be sufficiently obtained and the yield ratio can not be ensured. In the present invention, the upper limit of the tempering time is not particularly limited. However, in view of productivity, the tempering is preferably performed for 30 seconds or less. As described above, in the present invention, productivity can be improved through short tempering.

On the other hand, the heating rate during the induction heating is preferably 15 ° C / s or higher. If the heating rate is less than 15 ° C / s, tempering can not be performed sufficiently and it is difficult to obtain a high specific gravity. The heating rate is more preferably 18 ° C / s or more, and more preferably 20 ° C / s or more. The upper limit of the heating rate is not particularly limited, but the heating rate is not more than 35 DEG C / s at the limit of the facility.

In this case, the induction heating may be performed using an induction heater, and the induction heater may be rapidly tempered through the induction heater.

On the other hand, before the induction heating, the step of hot-dipping the continuously annealed steel sheet may be further included. Normally, the above hot-dip coating is performed at about 460 캜.

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

(Example)

The steel material having the alloy composition shown in the following Table 1 was cold-rolled, then continuously annealed at 800 ° C, heated at a heating rate of 20 ° C / s using an induction heater, tempered under the conditions shown in Table 2, . The cold-rolled steel sheet thus produced was measured for physical properties, and the results are shown in Table 2 below. However, the deviation of the tensile strength in Table 2 means the difference in tensile strength of the steel species after the tempering treatment with respect to the tensile strength of the steel species not subjected to the tempering treatment.

division Alloy composition (% by weight) C Si Mn P S Al Cr Ti Nb B N Inventive Steel 1 0.18 0.11 3.6 0.012 0.0036 0.022 0.05 0.02 0.039 0.0016 0.004 Invention river 2 0.15 0.06 3.1 0.011 0.0031 0.023 0.78 0.02 0.03 0.0015 0.003 Comparative River 1 0.17 0.07 2.2 0.012 0.0030 0.022 0.06 0.02 0.029 0.0018 0.004 Comparative River 2 0.11 0.07 2.5 0.013 0.0028 0.023 0.5 0.021 0.032 0.0019 0.004

division Grade Nr. Microstructure
(area%) Tempering
Temperature
(° C) Tempering
time
(s) Yield strength
(YS, MPa) Tensile strength
(TS, MPa) Seal
Intensity deviation
(? TS) gun
Elongation
(%
Uniformity
Elongation
(%) Yield ratio
(YS / TS) M + TM F B Comparative Example 1 Inventive Steel 1 98 One One - - 1158 1558 - 7.4 4.4 0.74 Comparative Example 2 98 2 0 100 20 1219 1561 3 7.9 4.3 0.78 Inventory 1 97 One 2 150 20 1234 1550 8 7.8 4.2 0.80 Inventory 2 99 0 One 200 20 1321 1554 4 7.5 4.1 0.85 Inventory 3 98 One One 250 20 1322 1536 22 7.1 4.0 0.86 Honorable 4 97 One 2 300 20 1377 1507 51 6.2 3.3 0.91 Comparative Example 3 96 2 2 330 20 1307 1443 115 6.1 3.1 0.91 Comparative Example 4 97 One 2 425 20 1268 1335 223 5.8 2.8 0.95 Comparative Example 5 99 One 0 200 10 1205 1555 3 7.2 4.3 0.77 Comparative Example 6 Invention river 2 98 One One - - 1058 1395 - 6.9 3.4 0.76 Comparative Example 7 97 One 2 100 20 1089 1391 4 6.8 4.1 0.78 Inventory 5 99 0 One 200 20 1212 1390 5 7.1 3.8 0.87 Comparative Example 8 98 2 0 200 10 1089 1388 7 6.8 3.7 0.78 Comparative Example 9 Comparative River 1 92 2 6 - - 958 1265 - 10.3 6.2 0.76 Comparative Example 10 93 2 5 200 20 1088 1258 7 9.7 5.8 0.86 Comparative Example 11 Comparative River 2
88 2 10 - - 750 998 - 13.2 7.5 0.75 Comparative Example 12 87 2 11 200 20 814 992 6 12.8 6.9 0.82 In the above microstructure, M means martensite, TM means tempered martensite, F means ferrite, and B means bainite.

As can be seen from Tables 1 and 2, in Examples 1 to 5, which satisfy the alloy composition and manufacturing conditions proposed by the present invention, not only the microstructure to be obtained in the present invention but also excellent mechanical properties are secured , And the lowering of the tensile strength after tempering is also low.

On the other hand, in the case of Comparative Examples 1 and 2, the alloy composition proposed by the present invention is satisfied, but it can be seen that the tempering temperature is low or the tempering temperature is low, the yield ratio of 0.8 or more is not obtained.

In the case of Comparative Examples 3 and 4, it was confirmed that the alloy composition proposed by the present invention was satisfied but the tensile strength after tempering was lowered by 100 MPa or more before the tempering, as the tempering temperature was high.

In the case of Comparative Example 5, it is understood that the alloy composition proposed by the present invention is satisfied, but the yield ratio is not 0.8 or more because of insufficient tempering time.

In the case of Comparative Examples 6 and 7, it is understood that the alloy composition proposed by the present invention is satisfied but the annealing temperature is not lowered or the tempering temperature is low.

In the case of Comparative Example 8, it is found that the alloy composition proposed by the present invention is satisfied but the yield ratio is not 0.8 or more because of insufficient tempering time.

In the case of Comparative Examples 9 to 12, the alloy composition of the present invention is not satisfied and it can be seen that the tensile strength of 1300 MPa or more, the yield strength of 1040 MPa or more or the yield ratio of 0.8 or more, which the present invention desires to obtain, can not be secured. In particular, in the case of Comparative Examples 11 and 12, it can be seen that the strength is remarkably low as the microstructure suggested by the present invention can not be secured.

Claims (8)

(Excluding 0%), Mn: 2.6 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 * % (Excluding 0%), N: 0.01% or less (excluding 0%), the balance Fe and other unavoidable impurities,
Microstructure is the area fraction, the sum of martensite and tempered martensite: 90% or more (including 100%); And a sum of one or two of ferrite and bainite: 10% or less (including 0%),
Wherein the yield ratio is not less than 0.8, and the tensile strength difference before and after tempering is not more than 100 MPa.
The method according to claim 1,
The steel sheet has a tensile strength of 1300 MPa or more and a yield strength of 1040 MPa or more.
The method according to claim 1,
The steel sheet is one of a cold rolled steel sheet, a hot-dip galvanized steel sheet, and an alloyed hot-dip galvanized steel sheet.
(Excluding 0%), Mn: 2.6 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 * % Or less (excluding 0%), N: 0.01% or less (excluding 0%), the remainder Fe, and other unavoidable impurities to obtain a cold-rolled steel sheet;
Continuously annealing the cold-rolled steel sheet;
Slowly cooling the continuous annealed cold rolled steel sheet to room temperature; And
And annealing the slowly cooled cold-rolled steel sheet at 150 to 300 ° C for at least 20 seconds by induction heating the rolled cold-rolled steel sheet.
The method of claim 4,
Wherein the continuous annealing is performed in a temperature range of 780 to 880 占 폚.
The method of claim 4,
Wherein the induction heating is performed using an induction heater.
The method of claim 4,
Wherein the heating speed is 15 占 폚 / s or more during the induction heating.
The method of claim 4,
Further comprising a step of hot-dipping the continuously annealed steel sheet before the induction heating.