KR20130034202A - High strength steel sheet and method for manufacturing the same - Google Patents

Abstract

PURPOSE: A high strength steel sheet and a manufacturing method thereof are provided to manufacture dual phase steel sheets composed of 38-42 vol% of hard martensite and the rest of soft ferrite by controlling alloy elements and process conditions. CONSTITUTION: A manufacturing method for a high strength steel sheet is as follows: a step of hot-rolling a steel sheet composed of 0.1-0.12 wt% of C, 0.1-0.3 wt% of Si, 1.8-2.3 wt% of Mn, 0.25-0.4 wt% of Cr, 0.02-0.08 wt% of Mo, 0.3-0.6 wt% of Al, and the rest of Fe and other impurities(S110); a step of cold-rolling the hot-rolled steel sheet(S120); a step of sintering the cold-rolled steel sheet at 770-810 deg. C(S130); a step of maintaining the sintered steel sheet for 30 seconds after the sintered steel sheet is cooled to 490-590 deg. C(S140); and a step of secondarily cooling the steel sheet to a martensite zone(S150). [Reference numerals] (AA) Start; (BB) End; (S110) Manufacturing a hot-rolled steel sheet; (S120) Cold-rolling; (S130) Annealing; (S140) Primary cooling and maintaining; (S150) Secondary cooling;

Description

High strength steel sheet and manufacturing method thereof {HIGH STRENGTH STEEL SHEET AND METHOD FOR MANUFACTURING THE SAME}

The present invention relates to a high strength steel sheet, and more particularly, to a high strength steel sheet having a high strength of 1 GPa or more and an elongation of 15% or more through control of process conditions such as an alloying component and annealing and cooling, and a manufacturing method thereof.

As the competition intensifies day by day, the demand for high quality and diversification of car quality is increasing. In addition, in order to satisfy the stricter regulations on passenger safety and environmental regulations and to improve fuel efficiency, we are pursuing weight reduction and high strength.

The steel sheets applied to automotive exterior materials are mainly cold rolled steel sheets with excellent workability and elongation.

In general, the cold rolled steel sheet is manufactured through a hot rolling process, a cold rolling process and an annealing process. In the hot rolling process, a hot rolled steel sheet is manufactured, including a slab reheating step, a hot rolling step, and a cooling step. In the cold rolling process, the hot rolled steel sheet is cold rolled at a predetermined rolling rate to be processed to the final thickness of the product. In the annealing process, by heating to an annealing temperature of more than A1 transformation point to secure the austenite fraction and then cooled to control the final microstructure of the steel sheet.

Prior art related to the present invention is Republic of Korea Patent Publication No. 10-2003-0055530 (2003.07.04. Publication).

It is an object of the present invention to provide a high strength steel sheet having a high strength of 1 GPa or more of tensile strength and having an excellent elongation and also having excellent plating properties.

Another object of the present invention is to provide a method for manufacturing the high strength steel sheet through the control of alloy components and process conditions control.

High-strength steel sheet manufacturing method according to an embodiment of the present invention for achieving the above one object by weight, carbon (C): 0.1 ~ 0.12%, silicon (Si): 0.1 ~ 0.3%, manganese (Mn): 1.8 ~ 2.3%, Chromium (Cr): 0.25 ~ 0.4%, Molybdenum (Mo): 0.02 ~ 0.08%, Aluminum (Al): 0.3 ~ 0.6% and hot rolled steel slab consisting of remaining iron (Fe) and unavoidable impurities Cooling to prepare a hot rolled steel sheet; Cold rolling the hot rolled steel sheet; Annealing the cold rolled steel sheet at 770 ° C. to 810 ° C .; First cooling the annealed steel sheet to 490˜590 ° C. and then maintaining the steel sheet for 30 seconds or more; And secondary cooling the first cooled and maintained steel sheet to the martensite region.

High-strength steel sheet manufacturing method according to another embodiment of the present invention for achieving the above object is by weight, carbon (C): 0.1 ~ 0.12%, silicon (Si): 0.1 ~ 0.3%, manganese (Mn): 1.8 ~ 2.3%, Chromium (Cr): 0.25 ~ 0.4%, Molybdenum (Mo): 0.02 ~ 0.08%, Aluminum (Al): 0.3 ~ 0.6% and hot rolled steel slab composed of remaining iron (Fe) and unavoidable impurities And cooling to produce a hot rolled steel sheet; Cold rolling the hot rolled steel sheet; Annealing the cold rolled steel sheet at 770 ° C. to 810 ° C .; Cooling the annealed steel sheet to a temperature of 490 to 590 캜 for a period of at least 30 seconds; Hot-dip plating the first cooled and maintained steel sheet; And secondary cooling the molten plated steel sheet to the martensite region.

High-strength steel sheet according to an embodiment of the present invention for achieving the above another object is a cold rolled steel sheet or a hot-dip galvanized steel sheet, by weight, carbon (C): 0.1 ~ 0.12%, silicon (Si): 0.1 ~ 0.3%, manganese (Mn): 1.8 ~ 2.3%, Chromium (Cr): 0.25 ~ 0.4%, Molybdenum (Mo): 0.02 ~ 0.08%, Aluminum (Al): 0.3 ~ 0.6% and other iron (Fe) and unavoidable impurities Including ferrite and martensite, the martensite is characterized in that 38 to 42vol%, elongation is more than 15%.

In addition, the steel sheet may have a tensile strength of 1 GPa or more and a yield ratio of 0.5 or less.

On the other hand, the steel sheet may include phosphorus (P): 0.03% by weight or less, sulfur (S): 0.003% by weight or less and nitrogen (N): 50 ppm or less.

The method of manufacturing high strength steel sheet according to the present invention includes 38 to 42 vol% of hard martensite through controlling alloy components such as carbon, silicon, chromium, aluminum, and controlling process conditions such as annealing and cooling, and the rest is soft ferrite. It is possible to manufacture a dual phase steel sheet consisting of. As a result, the steel sheet produced by the method according to the present invention may exhibit a tensile strength of at least 1 GPa and an elongation of at least 15%.

In addition, the high-strength steel sheet according to the present invention has a relatively low content of silicon which forms an oxide which adversely affects the plating property of 0.3% by weight or less, while the content of aluminum capable of forming an oxide that is advantageous for plating property is 0.3% by weight. By including a relatively large number above it can exhibit excellent plating properties.

1 is a flow chart schematically showing a high strength steel sheet manufacturing method according to an embodiment of the present invention.
2 is a flowchart schematically showing a method for manufacturing a high strength steel sheet according to another embodiment of the present invention.

Advantages and features of the present invention and methods for achieving them will be apparent with reference to the embodiments described below in detail with the accompanying drawings. It should be understood, however, that the invention is not limited to the disclosed embodiments, but is capable of many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, To fully disclose the scope of the invention to those skilled in the art, and the invention is only defined by the scope of the claims. Like reference numerals refer to like elements throughout the specification.

Hereinafter, a high strength steel sheet according to a preferred embodiment of the present invention and a method of manufacturing the same will be described in detail with reference to the accompanying drawings.

The high strength steel sheet according to the present invention is a cold rolled steel sheet or a hot dip steel sheet containing ferrite and martensite in a microstructure.

High strength steel sheet according to the present invention by weight, carbon (C): 0.1 ~ 0.12%, silicon (Si): 0.1 ~ 0.3%, manganese (Mn): 1.8 ~ 2.3%, chromium (Cr): 0.25 ~ 0.4% , Molybdenum (Mo): 0.02 ~ 0.08% and aluminum (Al): 0.3 ~ 0.6%.

In addition, the high strength steel sheet according to the present invention may include phosphorus (P): 0.03% by weight or less, sulfur (S): 0.003% by weight or less, and nitrogen (N): 50 ppm or less.

The remainder of the above components consist of iron (Fe) and unavoidable impurities.

Hereinafter, the role and content of each component contained in the high-strength steel sheet according to the present invention will be described.

Carbon (C)

Carbon (C) contributes to the improvement of martensite fraction and hardness in composite steel.

The carbon is preferably added in 0.1 ~ 0.12% by weight of the total weight of the steel sheet. When the amount of carbon added is less than 0.1% by weight, it is difficult to secure a target martensite fraction of 38 to 42 vol%. On the other hand, when the carbon content exceeds 0.12% by weight, the formation of carbide in the steel is promoted, there is a problem that it is difficult to secure the target elongation of more than 15%.

Silicon (Si)

Silicon (Si) is a solid solution strengthening element which accelerates the purification of steel and carbon enrichment in austenite. It improves the flowability of molten metal during welding among steels to which manganese (Mn) is added, to be. In addition, since silicon improves strength without slowing the balance between yield ratio and elongation, and slows the diffusion rate of carbon in ferrite, silicon inhibits carbide growth and contributes to stabilizing ferrite to improve elongation.

The silicon is preferably added in 0.1 to 0.3% by weight of the total weight of the steel sheet. When the addition amount of silicon is less than 0.1% by weight, the effect of the addition is insufficient. On the contrary, when the addition amount of silicon exceeds 0.3% by weight, there is a problem in that the Mn ₂ SiO ₄ phase and the SiO ₂ phase are formed on the surface of the material to lower the plating property and weldability.

Manganese (Mn)

Manganese (Mn) is a solid solution strengthening element which stabilizes austenite to lower the annealing temperature of the two-phase region, and martensite is easily produced even at a low cooling rate.

The manganese is preferably added in 1.8 to 2.3% by weight of the total weight of the steel sheet. When the content of manganese is less than 1.8% by weight, the effect of the addition is insufficient. On the contrary, when the content of manganese exceeds 2.3% by weight, there is a problem in that the elongation is lowered due to the development of manganese band in the center of the material thickness.

Chrome (Cr)

Chromium (Cr) is an austenite stabilizing element and contributes to the improvement of quenchability.

The chromium is preferably added at 0.25 to 0.4% by weight of the total weight of the steel sheet. If the chromium is added at less than 0.25% by weight, the effect of addition is insufficient. On the other hand, if chromium is added in an amount exceeding 0.4% by weight, the plating ability is deteriorated.

Molybdenum (Mo)

Molybdenum (Mo) together with the chromium as an austenite stabilizing element contributes to the improvement of quenchability.

The molybdenum is preferably added in an amount of 0.02 to 0.08% by weight based on the total weight of the steel sheet. When the addition amount of molybdenum is less than 0.02 wt%, the effect of addition is insufficient. On the other hand, when the added amount of molybdenum exceeds 0.08 wt%, the toughness of the steel sheet is deteriorated.

Aluminum (Al)

Aluminum (Al) is an element mainly used as a deoxidizer. It contributes to stabilize austenite by improving ferrite and improving elongation rate and increasing carbon concentration in austenite. In addition, aluminum acts as a layer between the iron and the zinc plated layer to improve the plating property, and is an effective element for suppressing the formation of manganese bands in the hot-rolled coil.

The aluminum is preferably added in an amount of 0.3 to 0.6% by weight based on the total weight of the steel sheet. If the addition amount of aluminum is less than 0.3% by weight, it is difficult to sufficiently obtain the above effect. On the other hand, when the addition amount of aluminum exceeds 0.6% by weight, the performance is deteriorated and AlN in the slab is formed to cause hot cracking.

Phosphorus (P)

Phosphorus (P) contributes to the improvement of the strength of the steel sheet by solid solution strengthening, and serves as an effective element for inhibiting carbide formation, and serves to prevent elongation due to carbide formation in the overaging section after annealing and cooling. It is also effective in obtaining martensite by improving manganese equivalents. However, when too much phosphorus is added, it becomes a cause of hot brittleness by forming steritic Fe3P.

In the present invention, the content of phosphorus is limited to 0.03% by weight or less based on the total weight of the steel sheet.

Sulfur (S)

Sulfur (S) inhibits toughness and weldability, and increases MnS nonmetallic inclusions, which hinders the effect of incombustibility of Mn and causes cracks. In addition, excessive sulfur content increases coarse inclusions and degrades fatigue properties.

In the present invention, the content of sulfur is limited to 0.003% or less of the total weight of the steel sheet.

Nitrogen (N)

Nitrogen (N) contributes to grain refinement due to the formation of AlN or the like, but is over-saturated in the cooling process after the galvanizing step of the zinc plating layer during hot-dip galvanizing, thereby lowering the uniform elongation. Therefore, in the present invention, the content of nitrogen was limited to 50 ppm or less of the total weight of the steel sheet.

The high strength steel sheet according to the present invention may have a microstructure including ferrite and martensite by controlling the alloy components and controlling process conditions described later. The higher the fraction of hard martensite, the higher the strength, and the higher the fraction of soft ferrite, the higher the elongation can be.

At this time, in order to secure an elongation of 15% or more with a tensile strength of 1 GPa or more of the present invention, it is preferable that the hard martensite exhibits 38 to 42 vol%. As a result of the experiment, when the martensite fraction was 38 to 42 vol%, both the strength and the elongation could be satisfied. On the other hand, when the martensite fraction is less than 38%, it is difficult to secure a tensile strength of 1 GPa or more, and when the martensite fraction is more than 42%, it is difficult to secure an elongation of 15% or more due to a decrease in the soft ferrite fraction.

In addition, in the case of the high strength steel sheet according to the present invention, it was possible to exhibit a resistance ratio of less than 0.5. This contributes to the high strength steel sheet according to the present invention, which does not include niobium (Nb) and vanadium (V), which contribute to the improvement of tensile strength and the refinement of ferrite, but also affect the yield strength.

That is, in the case of the high strength steel sheet according to the present invention, without adjusting niobium (Nb), vanadium (V), and the like, the alloy components such as carbon (C), chromium (Cr), and the like are controlled by controlling the process conditions described below. The tensile strength of 1GPa or more can be secured, and the yield ratio can be achieved with an elongation of 15% or more.

Hereinafter, a high strength steel sheet manufacturing method according to the present invention having the composition and mechanical properties will be described.

1 is a flowchart schematically showing a method for manufacturing a high strength steel sheet according to an embodiment of the present invention, and more specifically, a method for manufacturing a cold rolled steel sheet.

Referring to Figure 1, the high strength steel sheet manufacturing method shown is a hot rolled steel sheet manufacturing step (S110), cold rolling step (S120), annealing step (S130), the first cooling / maintenance step (S140) and the second cooling step (S150) ).

Hot-rolled steel sheet manufacturing

In the hot-rolled steel sheet manufacturing step (S110), the hot-rolled steel sheet is produced by hot rolling and cooling the steel slab having the above composition. The hot-rolled steel sheet may be prepared by various known process conditions, and more preferably, the following process may be employed.

First, the steel slab is reheated at 1180 to 1250 占 폚. If the slab reheating temperature is less than 1180 ° C, the segregated components may become difficult to reuse during casting. On the other hand, when the slab reheating temperature exceeds 1250 ° C, the austenite grain size increases and the ferrite grain size can be coarsened.

Next, the reheated steel slab is hot-rolled to a finish rolling temperature of 810 to 910 캜. When the finishing rolling temperature exceeds 910 ° C, strength and ductility may be reduced due to an increase in ferrite grain size. On the other hand, if the finishing rolling temperature is lower than 810 ° C, a problem may arise, such as a complicated structure due to abnormal reverse rolling.

Next, the hot rolled steel is cooled to 490 to 590 ° C. The hot rolled steel sheet produced after cooling is finished can be wound. Various methods such as natural cooling and forced cooling can be applied to the cooling. Ductility and workability may fall when cooling end temperature is less than 490 degreeC. On the other hand, when the cooling end temperature exceeds 590 ° C, manganese silicon may be segregated.

Cold rolled

In the cold rolling step (S120), the hot-rolled steel sheet is cold-rolled to a final thickness of the steel sheet. The reduction rate of the cold rolling can be set to about 50 to 70% depending on the thickness of the hot-rolled steel sheet and the final thickness of the steel sheet to be targeted.

Acid pickling may be performed to remove scale of the hot rolled steel sheet before cold rolling, and then coated with oil to prevent oxidation of the steel sheet surface.

Annealing

In the annealing step (S130), the cold rolled steel sheet is heated to annealing by 770 ~ 810 ℃. It is possible to control the austenite phase fraction through annealing, through which the target martensite of 38 to 42 vol% can be secured through primary cooling and secondary cooling, which will be described later.

It is preferable that annealing temperature is 770-810 degreeC. If the annealing temperature is less than 770 캜, it may become difficult to secure sufficient austenite. On the other hand, when the annealing temperature exceeds 810 ° C., the physical properties of the steel sheet may decrease due to an increase in austenite grain size.

Primary Cooling / Maintaining

In the primary cooling / maintenance step (S140), the annealing steel sheet is first cooled to 490 to 590 ° C., and then maintained at 490 to 590 ° C. for 30 seconds or more.

If the primary cooling end temperature or holding temperature exceeds 590 ° C, it may be difficult to secure sufficient tensile strength, and if the primary cooling end temperature or holding temperature is less than 490 ° C, it may be difficult to secure an elongation of 15% or more. have.

Meanwhile, primary cooling may be performed at a cooling rate of 5 to 20 ° C./sec, and a cooling method may include a roll quenching method, a gas jet method, or the like. If the cooling rate is less than 5 ℃ / sec, austenite may be transformed into ferrite, pearlite, etc. during the cooling process, it may be difficult to obtain the target martensite fraction, and the cooling rate is too fast, exceeding 20 ℃ / sec In this case, a problem of material unevenness may occur.

The holding time after completion of cooling is preferably 30 seconds or more, more preferably 100 to 200 seconds. If the holding time after the end of cooling is less than 30 seconds, it may be difficult to secure a sufficient elongation.

Secondary cooling

In the second cooling step (S150), in order to secure the martensite fraction of the steel sheet to be finally manufactured, the first cooled and maintained steel sheet is cooled to a temperature corresponding to the martensite region, about Ms point to Ms-100 ° C.

At this time, the secondary cooling rate is preferably 5 to 100 ° C / sec. If the secondary cooling rate is less than 5 占 폚, pearlite, bainite transformation and the like may occur. On the other hand, if the secondary cooling rate exceeds 100 캜 / sec, the elongation of the steel sheet to be produced may be lowered.

2 is a flow chart schematically showing a method for manufacturing a high strength steel sheet according to another embodiment of the present invention, and more specifically, a method for manufacturing a hot dip steel sheet.

Referring to Figure 2, the high strength steel sheet manufacturing method shown is a hot rolled steel sheet manufacturing step (S210), cold rolling step (S220), annealing step (S230), primary cooling / maintenance step (S240), hot-dip plating step (S250) And a secondary cooling step (S260).

In the hot-rolled steel sheet manufacturing step (S210), hot-rolled and cooled steel slabs having the above-described composition are manufactured to produce hot-rolled steel sheets. In the cold rolling step (S220), the hot rolled steel sheet is cold rolled. In the annealing step S230, the cold rolled steel sheet is subjected to annealing at 770 to 810 ° C. In the primary cooling / maintenance step (S240), the annealing steel sheet is first cooled to 490 to 590 ° C. and then maintained for 30 seconds or more. Since the processes are the same as each process (S110 to S140) shown in FIG. 1, a detailed description thereof will be omitted.

In the hot-dip plating step (S250) is hot-plated steel sheet that is primary cooled and maintained. The hot dip plating may be applied to various hot dip plating methods such as hot dip galvanizing and alloying hot dip galvanizing.

In the second cooling step (S260), the hot-dipped steel sheet is second cooled to the martensite region to secure a target martensite fraction.

Example

Hereinafter, the configuration and operation of the present invention through the preferred embodiment of the present invention will be described in more detail. It is to be understood, however, that the same is by way of illustration and example only and is not to be construed in a limiting sense.

Details that are not described herein will be omitted since those skilled in the art can sufficiently infer technically.

1. Preparation of specimens

After preparing the ingots having the compositions of Examples 1 to 3 and Comparative Examples 1 to 7 shown in Table 1, reheating at 1220 ℃ for 2 hours, hot rolling to a finish rolling temperature of 860 ℃ and a cooling rate of 5 ℃ / sec After cooling to 540 ℃, air cooled to room temperature to prepare a hot-rolled specimen.

Each of the prepared hot-rolled specimens was pickled and then cold-rolled at a reduction ratio of 60%, followed by annealing at 790 ° C. After the first cooling to 540 ℃ at a cooling rate of 10 ℃ / sec, and maintained for 120 seconds. Thereafter, each steel sheet was hot-dip galvanized at 570 ° C., and then cooled to 300 ° C. at a cooling rate of 20 ° C./sec.

[Table 1] (unit:% by weight)

2. Evaluation of mechanical properties

Table 2 shows the tensile test and the plating property evaluation results of the specimen prepared according to Comparative Examples 1 to 7 and Examples 1 to 3.

Plating was evaluated by the ratio of the unplated area after hot dip galvanizing. More specifically, when the unplated area is less than 0.1%, it is excellent (O), when the unplated area is not less than 0.1% and less than 0.5%, normal (Δ), and when the unplated area is not less than 0.5%, (X) Evaluated.

[Table 2]

Referring to Table 2, in the case of specimens 8 to 10 corresponding to the examples of the present invention, the martensite fraction was 38 to 42 vol%, and as a result, tensile strength (TS) of 1 GPa or more and elongation (El) of 15% or more Indicated.

On the other hand, in specimens 1 and 2, the tensile strength (TS) was 1GPa or more, but the martensite fraction was less than 38 vol%, and the yield ratio was rather high.

In addition, for specimen 3 having a carbon content of 0.09% by weight, the elongation was excellent, but the tensile strength and martensite fraction were slightly below the target value. In addition, in the case of specimen 4 having a carbon content of 0.14% by weight, the elongation was lower than the target value with an excessive martensite fraction.

In addition, in the case of specimen 5 having a chromium content of 0.2% by weight, the elongation was satisfied with the target value, but the martensite fraction was only 35.8 vol%, and thus the tensile strength did not reach 1 GPa. In addition, in the case of specimen 6 having a chromium content of 0.6% by weight, the tensile strength satisfied the target value, but the moldability and the plating property were not good.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments. Such changes and modifications are intended to fall within the scope of the present invention unless they depart from the scope of the present invention. Accordingly, the scope of the present invention should be determined by the following claims.

S110: Hot-rolled steel sheet manufacturing step
S120: Cold rolling step
S130: annealing step
S140: primary cooling / maintenance step
S150: Secondary cooling step
S210: Hot Rolled Steel Sheet Manufacturing Steps
S220: cold rolling stage
S230: Annealing Step
S240: primary cooling / maintenance step
S250: Hot dip plating
S260: second cooling stage

Claims (9)

By weight%, carbon (C): 0.1 to 0.12%, silicon (Si): 0.1 to 0.3%, manganese (Mn): 1.8 to 2.3%, chromium (Cr): 0.25 to 0.4%, molybdenum (Mo): 0.02 ~ 0.08%, aluminum (Al): hot rolling and cooling the steel slab consisting of 0.3 ~ 0.6% and the remaining iron (Fe) and inevitable impurities to produce a hot rolled steel sheet;
Cold rolling the hot rolled steel sheet;
Annealing the cold rolled steel sheet at 770 ° C. to 810 ° C .;
Cooling the annealed steel sheet to a temperature of 490 to 590 캜 for a period of at least 30 seconds; And
And secondly cooling the first cooled and maintained steel sheet to the martensite region.
By weight%, carbon (C): 0.1 to 0.12%, silicon (Si): 0.1 to 0.3%, manganese (Mn): 1.8 to 2.3%, chromium (Cr): 0.25 to 0.4%, molybdenum (Mo): 0.02 ~ 0.08%, aluminum (Al): hot rolling and cooling the steel slab consisting of 0.3 ~ 0.6% and the remaining iron (Fe) and inevitable impurities to produce a hot rolled steel sheet;
Cold rolling the hot rolled steel sheet;
Annealing the cold rolled steel sheet at 770 ° C. to 810 ° C .;
Cooling the annealed steel sheet to a temperature of 490 to 590 캜 for a period of at least 30 seconds;
Hot-dip plating the first cooled and maintained steel sheet;
And secondly cooling the hot-dip plated steel sheet to the martensite region.
The method according to claim 1 or 2,
The steel slab
Phosphorus (P): 0.03% by weight or less, sulfur (S): 0.003% by weight or less and nitrogen (N): 50 ppm or less are contained.
The method according to claim 1 or 2,
The hot rolled steel sheet manufacturing step
Wherein the steel slab is reheated at 1180 to 1250 占 폚 and the reheated steel slab is hot-rolled at a finishing rolling temperature of 810 to 910 占 폚 and then cooled to 490 to 590 占 폚.
The method according to claim 1 or 2,
The primary cooling is
Wherein the heat treatment is carried out at a cooling rate of 5 to 20 占 폚 / sec.
The method according to claim 1 or 2,
The secondary cooling
Wherein the heat treatment is carried out at a cooling rate of 5 to 100 占 폚 / sec.
As cold rolled steel sheet or hot dip galvanized steel sheet,
By weight%, carbon (C): 0.1 to 0.12%, silicon (Si): 0.1 to 0.3%, manganese (Mn): 1.8 to 2.3%, chromium (Cr): 0.25 to 0.4%, molybdenum (Mo): 0.02 ~ 0.08%, aluminum (Al): 0.3 ~ 0.6% and the remaining iron (Fe) and inevitable impurities,
High strength steel sheet comprising ferrite and martensite, but the martensite is 38 ~ 42vol%, elongation is more than 15%.
The method of claim 7, wherein
The steel plate
A high strength steel sheet comprising phosphorus (P): 0.03% by weight or less, sulfur (S): 0.003% by weight or less and nitrogen (N): 50 ppm or less.
The method of claim 7, wherein
The steel sheet
A high strength steel sheet having a tensile strength of 1 GPa or more and a yield ratio of 0.5 or less.

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