KR101607011B1 - Steel sheet and method of manufacturing the same - Google Patents

Abstract

Disclosed are a steel sheet having excellent bake hardening and non-aging properties and a manufacturing method thereof. The steel sheet, according to the present invention, comprises: one or more from less than or equal to 0.2 wt% of C, less than or equal to 0.5 wt% of Si, 0.3-3.0 wt% of Mn, less than or equal to 0.10 wt% of P, less than or equal to 0.05 wt% of S, 0.1-3.0 wt% of Al, and 0.3 <= [Cr]+0.3[Mo] <= 3.0 of Cr and Mo, less than or equal to 0.02 wt% of N, and the remainder consisting of Fe and inevitable impurities.

Description

Technical Field [0001] The present invention relates to a steel sheet and a method of manufacturing the steel sheet.

TECHNICAL FIELD The present invention relates to a steel sheet manufacturing technique, and more particularly, to a steel sheet having excellent bake hardening characteristics and anti-aging properties, and a manufacturing method thereof.

The automotive exterior sheathing member is required to have a low resistance for ensuring the shape fixability during molding. On the other hand, in automobiles, which are finished products after molding, it is necessary to have dent resistance which is not easily deformed against externally applied stress.

Sintered hardened steel is a type of steel that can satisfy both of these requirements. It can retain the solid carbon in the steel and ensure the dent resistance by increasing the yield strength of the final product by using the carbon diffusion at the top during the sintering process. Normally, the hardened steel is guaranteed to have an increase in yield strength of ³ kgf / mm ² or more after coating.

However, the activated carbon has activity to some extent at room temperature other than the coating furnace condition, causing the aging phenomenon and yield point elongation at room temperature.

The aging phenomenon is caused by the fact that the solid carbon is fixed to the movable potential and disturb the movement of the potential. The aging phenomenon also increases proportionally to the amount of solute carbon, and in order to suppress the aging phenomenon, a method of controlling the amount of solute carbon in steel to about 0.001 wt% has been widely used. However, the amount of solid carbon in the steel varies due to various process parameters of the component and the manufacturing process, and is exposed to conditions that can cause aging at any time depending on the stored temperature conditions.

Normally, the internal aging guarantee of hardened steel has been recognized as 3 months at room temperature. However, considering the transportation time and the point of use, it requires a longer aging period of 6 to 12 months.

On the other hand, there is a method of improving the dislocation density in the steel through temper rolling by a method for improving the endurance of the hardened steel. However, in case of this method, if proper temper rolling can not be carried out according to the conditions of high carbon concentration, low rolling reduction or working conditions, the yield point stretching may be finely remained to cause surface defects, Which causes the quality of the steel to deteriorate.

BACKGROUND ART [0002] The background art related to the present invention is a cold stamped cold-rolled steel sheet excellent in endurance and disclosed in Korean Patent Laid-Open Publication No. 10-2000-0016460 (published on Mar. 25, 2000).

SUMMARY OF THE INVENTION An object of the present invention is to provide a steel sheet excellent in endurance and resistance to abrasion and a method for producing the same.

In order to achieve the above object, a steel sheet according to an embodiment of the present invention comprises 0.2% or less of carbon (C), 0.5% or less of silicon (Si), 0.3-3.0% of manganese (Mn) ) Of not more than 0.10%, sulfur (S) of not more than 0.05%, aluminum (Al) of 0.1 to 3.0%, 0.3? [Cr] +0.3 [Mo]? 3.0 Cr) and molybdenum (Mo), 0.02% or less of nitrogen (N), and the balance of iron (Fe) and unavoidable impurities.

At this time, the steel sheet may contain, by weight%, 0.01% or less of niobium (Nb), 0.01% or less of titanium (Ti), 0.03% or less of vanadium (V), 0.005% or less of calcium, (Sn): 0.05% or less; tungsten (W): 0.1% or less; and (3) And boron (B): 0.005% or less.

The steel sheet may have a ferrite single phase structure in the hot rolled steel sheet state, and may have a composite structure including ferrite and martensite in the cold rolled steel sheet state.

Further, in the cold rolled steel sheet, the steel sheet may include 1.0 to 10.0% of the area ratio of the martensite.

Further, the steel sheet may have a dislocation density in the ferrite matrix of 1 x ¹⁰ &lt; ¹³ &gt; pieces / m &lt; ² &gt;

In order to accomplish the above object, a steel sheet manufacturing method according to an embodiment of the present invention is characterized in that the steel sheet comprises 0.2 wt% or less of carbon (C), 0.5 wt% or less of silicon (Si), 0.3 to 3.0 wt% of manganese (P): not more than 0.10%, sulfur (S): not more than 0.05%, aluminum (Al): 0.1 to 3.0%, 0.3 ≦ [Cr] +0.3 [Mo] ≦ 3.0 Hot rolling a slab plate made of at least one of chromium (Cr) and molybdenum (Mo), 0.02% or less of nitrogen (N) and the balance of iron (Fe) and unavoidable impurities; A step of cold picking the picked-up plate material; And annealing the cold-rolled sheet material at a temperature of 800 ° C or higher for 30-200 seconds, and then cooling the sheet to 550 ° C or lower at an average cooling rate of 5 ° C / sec or more.

At this time, the annealing treatment and the cooled plate may be further subjected to temper rolling at a reduction ratio of 3.0% or less.

Further, the cold plate is subjected to constant temperature transformation processing; And cooling the plate subjected to the constant temperature transformation to a temperature of Ms point or less.

According to the steel sheet manufacturing method of the present invention, ferrite single phase structure in the hot rolled steel sheet state, ferrite and martensite abnormal structure in the cold rolled steel sheet state are controlled through the hot rolling process control and the annealing heat treatment control together with the alloy component adjustment of chromium and aluminum .

In particular, the steel sheet according to the present invention contains martensite in an area ratio of 1% or more in the state of a cold-rolled steel sheet, and as a result shows a high dislocation density of 1x10 ¹³ / m ² or more in the ferrite matrix. Thus, according to the steel sheet manufacturing method of the present invention, a steel sheet having excellent aging resistance characteristics over 12 months can be produced without performing temper rolling.

Further, according to the steel sheet manufacturing method of the present invention, the steel sheet having excellent moldability and processability can be produced by controlling the coiling temperature to 650 ° C or higher in the hot rolled steel sheet production process.

Hereinafter, a steel sheet and a manufacturing method thereof according to embodiments of the present invention will be described in detail.

Steel plate

The steel sheet according to the present invention is characterized in that it contains 0.2% or less of carbon (C), 0.5% or less of silicon (Si), 0.3-3.0% of manganese (Mn) (Cr) and molybdenum (Mo) so as to be 0.3% or less and 0.3% or less (Cr) or 0.3% (Mo) , And nitrogen (N): 0.02% or less.

If necessary, the steel sheet may contain, as the weight%, 0.01% or less of niobium (Nb), 0.01% or less of titanium (Ti), 0.03% or less of vanadium (V), 0.005% or less of calcium (Mg): not more than 0.005%, nickel (Ni): not more than 1.0%, copper (Cu): not more than 1.0%, antimony (Sb): not more than 0.05%, tin (Sn) By weight or less and boron (B): 0.005% or less.

The rest of the alloy components are composed of iron (Fe) and impurities inevitably included in the steelmaking process.

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

Carbon (C)

The martensite structure is a structure containing supersaturated carbon by the non-diffusive transformation in the austenite structure, and carbon contributes to the formation of this martensite structure.

The carbon is preferably contained in an amount of 0.2% by weight or less based on the total weight of the steel sheet, more preferably 0.005 to 0.05% by weight. When the carbon content is more than 0.2% by weight, the strength is excessively high and the elongation is decreased, so that the moldability may be greatly lowered.

Silicon (Si)

Silicon (Si) is added as a deoxidizer to remove oxygen in steel during the steelmaking process. Silicon also contributes to the strength improvement of the steel sheet through solid solution strengthening.

The silicon is preferably contained in an amount of 0.5% by weight or less based on the total weight of the steel sheet, more preferably 0.2% by weight or less. When the amount of silicon added is more than 0.5% by weight, a large amount of oxides are formed on the surface of the steel sheet, thereby deteriorating processability.

Manganese (Mn)

Manganese is an effective ingot element and contributes to the formation of martensite upon cooling after annealing.

The manganese is preferably contained in an amount of 0.3 to 3.0% by weight based on the total weight of the steel sheet. If the content of manganese is less than 0.3% by weight, the effect of the addition is insufficient. On the contrary, when the content of manganese exceeds 3.0 wt%, the phase transformation starting temperature becomes too low and the phase change occurs before the development of the {111} // ND aggregate structure by recrystallization, resulting in deterioration of moldability, Which may cause surface quality problems.

In (P)

Phosphorus (P) contributes partly to the strength improvement and is an element capable of exhibiting the effect of improving texture. In addition, phosphorus is particularly effective in controlling the r value in the 45 ° direction. However, if phosphorus is contained in an excess amount exceeding 0.1 wt% of the total weight of the steel sheet, surface defects due to segregation and a problem of workable brittleness can be caused.

Therefore, phosphorus is preferably added in an amount of 0.1% by weight or less, more preferably 0.03 to 0.08% by weight based on the total weight of the steel sheet.

Sulfur (S)

Sulfur (S) forms MnS to reduce the effective manganese content and can cause surface defects attributable to MnS.

Therefore, in the present invention, the content of sulfur is limited to 0.05 wt% or less of the total weight of the steel sheet.

Aluminum (Al)

In the present invention, aluminum (Al) is not only used as a deoxidizing agent but also an element capable of increasing the degree of carbon concentration in austenite by delaying Ac3 transformation, and also produces a hard phase martensite in a cooling process after annealing with a low carbon content It is an effective element.

The aluminum is preferably contained in an amount of 0.1 to 3.0% by weight, more preferably 0.3 to 1.0% by weight based on the total weight of the steel sheet. When the content of aluminum is less than 0.1% by weight, the austenite fraction increases sharply at an abnormal temperature range during annealing, and not only the material deviation increases, but also the carbon concentration in the austenite decreases. The same carbide structure may be formed to increase the yield strength and deteriorate the anti-aging property, and the hardness of the martensite may be lowered. On the contrary, when the content of aluminum exceeds 3.0% by weight, the Ac3 temperature is increased to reduce the anomaly fraction during annealing, and finally the formation of martensite structure is suppressed, and the risk of inclusion increase and the surface oxidation phenomenon There is a problem that the plating quality is deteriorated.

Chromium (Cr), molybdenum (Mo)

Chromium (Cr) and molybdenum (Mo) are elements that can secure the martensite structure by strengthening the ingotability of the steel sheet. However, when the content of chromium is excessive, the austenite fraction during annealing sharply increases and the degree of carbon concentration decreases. Also, when the molybdenum content is excessive, the Ac3 temperature is increased to decrease the austenite fraction, and the increase in Ac3 temperature is a factor to lower the productivity in the conventional continuous annealing line. In addition, the effect of chromium and molybdenum changes is remarkable in the case of chromium.

Taking this into consideration, the inventors of the present invention have found that, as a result of a long study, when chromium and molybdenum satisfy the following equations in the alloy composition of the steel sheet according to the present invention, they do not cause problems due to excessive chromium and austenite content And contributes to securing martensite structure.

0.3? [Cr] + 0.3 [Mo]? 3.0 ([] is the weight percentage of the component)

When [Cr] +0.3 [Mo] is less than 0.3, it is difficult to sufficiently exert the effect of strengthening the ingot due to the addition of chromium and molybdenum. On the other hand, when [Cr] +0.3 [Mo] exceeds 3.0, problems of over-chromium addition or problems of over-molybdenum addition may occur. In the case of the above-mentioned [Cr] +0.3 [Mo], 0.5?? [Cr] +0.3 [Mo]? 1.5 is more preferable in terms of securing stable martensite.

Nitrogen (N)

Nitrogen (N) generates inclusions in the steel to deteriorate the inner quality of the steel sheet.

Therefore, in the present invention, the content of nitrogen is limited to not more than 0.02 wt% of the total weight of the steel sheet.

Other

If necessary, the steel sheet according to the present invention may contain 0.01% or less of niobium (Nb), 0.01% or less of titanium (Ti) and 0.03% or less of vanadium (V) (Ca): not more than 0.005%, magnesium (Mg): not more than 0.005%, nickel (Ni): not more than 1.0%, copper (Cu): not more than 1.0%, antimony (Sb) (W): 0.1 wt% or less, and boron (B): 0.005 wt% or less. These elements are known to play roles such as improvement of strength, improvement of purification and improvement of corrosion resistance However, when it is added excessively beyond the above-mentioned range, it may cause other problems such as deterioration of moldability and toughness, and therefore it is preferable that the addition is made within a range not exceeding the above-mentioned range.

The steel sheet according to the present invention may have a ferrite single phase structure in the hot rolled steel sheet state and a ferrite and martensite abnormal structure in the cold rolled steel sheet state by the alloy components and the process control described below. More specifically, in the steel sheet according to the present invention, in the cold rolled steel sheet, the martensite may be contained in an area ratio of 1.0 to 10.0%, and the remainder may be substantially composed of ferrite. As a result, the steel sheet according to the present invention can exhibit a yield point elongation of less than 0.2% in a cold rolled steel sheet. Through this, the steel sheet according to the present invention can be endowed with an age guarantee of 12 months or more. When the yield point elongation is 0.2% or more, surface defects due to stretcher strain occur during processing, and the aging tends to proceed rapidly.

In addition, the steel sheet according to the present invention may exhibit an internal dislocation density of 1 x ^{10 13} / m ² or more in the ferrite base. Through this high dislocation density, a sufficient movable potential can be ensured and the aging phenomenon at room temperature can be suppressed. Therefore, the steel sheet according to the present invention can have excellent endurance.

Further, the steel sheet according to the present invention can exhibit a yield ratio (YP / TS) of 0.45 or less through the control of the alloy composition and omission of the temper rolling process described below.

In addition, the steel sheet according to the present invention may exhibit an elongation (El) of 38% or more when the carbon content is adjusted to 0.025% by weight or less.

In addition, the steel sheet according to the present invention can exhibit an r-value of 1.2 or more as a result of controlling the coiling temperature to 650 ° C or higher in a manufacturing process described later.

Steel plate manufacturing method

The steel sheet manufacturing method according to the present invention includes a slab reheating step, a hot rolling step, a winding step, a cold rolling step and a annealing heat treatment step.

First, in the slab reheating step, the slab plate having the above-described alloy composition is reheated to a temperature of approximately 1100 to 1250 ° C.

Next, in the hot rolling step, the reheated plate is hot-rolled at a finishing rolling temperature (approximately 870 ° C or higher) of Ar 3 or higher. Next, in the winding step, the hot-rolled plate is cooled and then wound.

At this time, the coiling temperature is preferably 650 占 폚 or higher, more preferably 680 占 폚 to 750 占 폚. Secondary phase carbides such as pearlite and cementite remain during rolling at a temperature of less than 650 ° C to cause a shear band which causes deterioration of the aggregate structure during cold rolling, , The austenite having a high carbon concentration is produced and the strength is rapidly increased and the elongation rate is lowered. Therefore, the hot rolled structure is controlled to a ferrite single phase structure by winding at a high temperature of 650 deg. In the present invention, the term "single-phase tissue" means a case where the ratio of one tissue is 99% or more, including the case where the ratio of one tissue is 100% as an area ratio.

As described above, the steel sheet according to the present invention has a ferrite single phase structure in the hot rolled steel sheet state. This is possible by controlling the coiling temperature of 650 ° C or more together with the alloy composition.

Next, in the cold rolling step, the rolled plate is pickled and cold-rolled at a reduction ratio of approximately 50 to 80%.

Next, in the annealing heat treatment step, the cold-rolled plate is annealed to control the austenite fraction to control the microstructure of the finally produced steel sheet, and then cooled.

At this time, the annealing treatment is preferably performed at 800 ° C or higher, and more preferably at 820 to 860 ° C for 30-200 seconds. When the annealing treatment temperature is less than 800 ° C, it is difficult to secure a sufficient austenite fraction, and as a result, it is difficult to obtain a martensite phase of 1% or more at an areal ratio. However, if the annealing temperature exceeds 860 ° C, a martensite phase exceeding 10% in area ratio may be formed in the microstructure of the final steel sheet due to an excessive austenite fraction.

In the cooling step, the annealed sheet material is cooled to obtain the target microstructure. At this time, the cooling is preferably performed at an average cooling rate of 5 DEG C / sec or more, more preferably 15 to 30 DEG C / sec. When cooling at an average cooling rate of 5 ° C / sec or more, martensite is generated during cooling, and the dislocation density during the phase change process may increase.

As an example, cooling may be performed at a temperature of 550 DEG C or lower to a temperature of Ms point or higher. In this case, after the cooling, the plate can be subjected to the constant temperature transformation process and then cooled to a temperature of Ms point or less. Strength and elongation can be controlled through constant temperature transformation process.

As another example, cooling may be performed to a temperature below the Ms point. In this case as well, a constant temperature transformation process can be further performed.

Through the annealing heat treatment as described above, it is possible to secure a microstructure in which the area ratio of martensite is 1.0 to 10.0% and the remainder is ferrite.

On the other hand, the plate subjected to the annealing heat treatment and cooling is subjected to a skin pass mill (SPM) to increase the dislocation density.

At this time, the temper rolling is preferably performed at a reduction ratio of 3.0% or less, more preferably 0.5 to 3.0%. If the reduction ratio of the temper rolling exceeds 3.0%, the yield strength may increase and deterioration of the shape-fixation may occur.

Further, the method of manufacturing a steel sheet according to the present invention may further include a step of hot-dipping the cooled or plate-shaped plate material. The hot-dip coating may be performed by a method of hot-dip galvanizing at approximately 450 to 510 ° C, or by alloying heat treatment at approximately 500 to 550 ° C after hot-dip galvanizing at approximately 450 to 510 ° C.

When temper rolling is included, temper rolling may be performed after hot-dip coating.

Example

Hereinafter, the configuration and operation of the present invention will be described in more detail with reference to preferred embodiments of the present invention. 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. The contents not described here are sufficiently technically inferior to those skilled in the art, and a description thereof will be omitted.

1. Preparation of steel plate specimens

In order to prepare specimens 1 to 8, the slab plate containing the components listed in Table 1 and composed of the remaining iron and impurities was reheated at 1200 ° C for 2 hours and then hot-rolled. The hot rolling was carried out under finishing rolling conditions at a temperature of 870 DEG C corresponding to a temperature of Ar3 point or higher. The hot rolled plate was cooled and rolled at 700 ° C. Thereafter, pickling and cold rolling at a reduction ratio of 60%, annealing at 830 ° C for 100 seconds, and cooling to 300 ° C at 20 ° C / sec were conducted to prepare steel plate specimens 1 to 5 and 8.

In the case of the steel plate specimen 6, the other conditions were the same as those of the specimen 1, except that the annealing was performed at 790 ° C, cooled to 300 ° C at 20 ° C / sec, and then tempered at a reduction ratio of 0.5%.

In the case of the steel plate specimen 7, the other conditions were the same as those of the specimen 1, but the coiling temperature was 600 ° C.

[Table 1] (unit:% by weight)

Further, in order to produce specimens 9 to 17, the slab plate containing the components listed in Table 2 and composed of the remaining iron and impurities was reheated at 1180 占 폚 for 2 hours and then hot-rolled. The hot rolling was carried out under the condition of finish rolling at 900 캜 corresponding to the temperature of Ar 3 point or more. The hot rolled plate was cooled and wound at 700 ° C.

Thereafter, the steel sheet was pickled and cold-rolled, annealed at 820 ° C for 60 seconds, cooled to 480 ° C at 20 ° C / sec, subjected to constant temperature transformation at 480 ° C and then immersed in a zinc bath at 465 ° C, At 520 캜, and then cooled to 300 캜, which corresponds to a temperature below the Ms point.

Thereafter, temper rolling was performed at a reduction ratio of 0.5%.

[Table 2] (unit:% by weight)

2. Evaluation of mechanical properties

Table 3 shows the microstructure and mechanical properties of specimens 1 to 8.

EBSD (Electron Back Scatter Diffraction) was used for microstructure and dislocation density.

The dislocation density was evaluated by crystallographic misorientation analysis using Electron Back-Scatter Diffraction (EBSD). The dislocation density was calculated as follows.

KAM [θ] = 1 / 6n × Σ (θ 1 + θ 2 + ............... + θ n)

L = a (2n + 1)

ρ (θ) = 2 * θ / L * b

K: Average degree of misorientation, θ: misorientation angle, L: unit length, a: step length, n: Number of Kernel, ρ (θ): dislocation density, b: burgers vector)

[Table 3]

Referring to Table 3, in the case of the steel sheet specimens 1 to 3 and 8 satisfying the conditions of the present invention, the ferrite single phase structure (ferrite 99% or more) in the hot rolled steel sheet state, the ferrite and martensite abnormal structure in the cold rolled steel sheet state , And r-bar showed the highest value for specimen 8 containing 0.05 wt% phosphorus. More specifically, in the case of the steel plate specimens 1 to 3, the area ratio of the martensite structure was 5% or more, the endurance test duration was 12 months or more, and the yield ratio was 0.45 or less. .

On the contrary, in the case of the specimen 4 of the steel sheet to which niobium was added instead of chromium, molybdenum and aluminum, the martensite phase was not sufficiently obtained even in the cold rolled steel sheet, and thus the aging resistance characteristics were relatively poor.

Further, in the case of the steel sheet specimen 5 in which chromium and molybdenum were not sufficiently added, the martensite was formed in a small amount less than 1% in area ratio, and thus the anti-aging properties were relatively poor.

In the case of the steel plate specimen 6, in which the annealing treatment temperature was relatively low at 790 캜, martensite was relatively less formed. However, in the case of the steel plate specimen 6, it was possible to exhibit the aging characteristics up to 12 months or more by performing the temper rolling, but the resistance-versus-ratio characteristics were relatively poor. On the other hand, in the case of Specimens 1 to 3 and 8, excellent resistance-to-brittleness characteristics can be exhibited without temper rolling, and their aging properties are also excellent.

Further, in the case of the specimen 7 of the steel sheet having the coiling temperature of less than 650 占 폚, the workability was relatively poor as compared with the specimens 1 to 3 of the steel sheet.

Table 4 shows the mechanical properties of specimens 9-17.

[Table 4]

Referring to Table 4, the specimens 11 to 12 and 15 to 17 satisfying the alloy composition shown in the present invention exhibited a yield ratio of less than 60% and r-bar of 1.2 or more.

However, for specimens 9 and 10, which did not contain chromium and had a relatively low aluminum content, the yield ratio was very high. For specimens 13 and 14, where the other conditions were satisfied but the aluminum content was relatively low, , And r-bar was relatively low for specimen 14.

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.

Claims (8)

(C): not less than 0.2%, silicon (Si): not less than 0.5%, manganese (Mn): not more than 0.3 to 3.0%, phosphorus (P): not less than 0.10% (Cr) and molybdenum (Mo) so as to be in a range of more than 0 to 0.05%, aluminum (Al): 0.1 to 3.0% and 0.3? [Cr] +0.3 [Mo]? 3.0 (N): more than 0 and not more than 0.02%, and at least one of the following additional elements, the balance being iron (Fe) and inevitable impurities,
Wherein the additional element is at least one selected from the group consisting of niobium (Nb): greater than 0 and less than 0.01%, vanadium (V) greater than 0 and less than 0.03%, calcium: greater than 0 and less than 0.005%, magnesium (Mg) (Sn): more than 0 and not more than 0.05%, and tungsten (W): more than 0: 0.05% 0.1% by weight or less,
Wherein the steel sheet has a dislocation density in the ferrite matrix of 1 x 10 &lt; ¹³ &gt; pieces / m &lt; ² &gt; or more in a cold rolled steel sheet.
delete The method according to claim 1,
The steel sheet
A ferrite single phase structure in a hot rolled steel sheet state,
And a composite structure including ferrite and martensite in a cold rolled steel sheet.
The method of claim 3,
Wherein the steel sheet comprises 1.0 to 10.0% of the area ratio of martensite in the state of a cold-rolled steel sheet.
delete (C): not less than 0.2%, silicon (Si): not less than 0.5%, manganese (Mn): not more than 0.3 to 3.0%, phosphorus (P): not less than 0.10% (Cr) and molybdenum (Mo) so as to be in a range of more than 0 to 0.05%, aluminum (Al): 0.1 to 3.0% and 0.3? [Cr] +0.3 [Mo]? 3.0 (N): more than 0 and not more than 0.02%, and at least one of the following additional elements, wherein the slab plate made of the remaining iron (Fe) and unavoidable impurities is hot-rolled, A step of winding up the apparatus;
A step of cold picking the picked-up plate material; And
Annealing the cold-rolled sheet material at a temperature of 800 ° C or higher for 30-200 seconds, and then cooling the sheet to 550 ° C or lower at an average cooling rate of 5 ° C / sec or higher;
Subjecting the cooled plate to constant temperature transformation processing;
Cooling the plate subjected to the constant temperature transformation to a temperature of Ms point or less; And
And subjecting the annealed and cooled plate to temper rolling at a reduction ratio of 3.0% or less,
Wherein the additional element is at least one selected from the group consisting of niobium (Nb): greater than 0 and less than 0.01%, vanadium (V) greater than 0 and less than 0.03%, calcium: greater than 0 and less than 0.005%, magnesium (Mg) (Sn): more than 0 and not more than 0.05%, and tungsten (W): more than 0: 0.05% 0.1% by weight or less. delete delete