KR20150039008A - Precipitation hardening steel sheet having excellent yield strength and yield ratio and method for manufacturing the same - Google Patents

Abstract

The present invention relates to a precipitation strengthened steel sheet having excellent yield strength and yield ratio, and a method to manufacture the same. An embodiment of the present invention comprises: 0.04-0.14 wt% of C; 0.8-1.8 wt% of Mn; 0.04 wt% or less (excluding 0) of P; 0.01 wt% or less (excluding 0) of S; 0.006-0.015 wt% of N; 0.5 wt% or less (excluding 0) of Si; soluble acid; 0.01-0.07 wt% of Al; 0.02-0.07 wt% of Nb; Fe and other unavoidable remaining impurities; 0.18 wt% or less of total tramp elements (Cu+Ni+Sn+Pb); 20 or more carbides of 20 nm or smaller per μm^2; and a refined structure having ferrites of 7 μm or smaller in crystal grain size covering 80 area% or more, wherein an un-recrystallized rolled structure in the refined structure covers 5-10 area%. According to the present invention, excellent yield strength and yield ratio can be obtained without adding large quantities of alloy elements; thereby reducing the manufacturing costs, and improving machinability.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a precipitation hardening type steel sheet having excellent yield strength and yield ratio,

The present invention relates to a precipitation hardening type steel sheet excellent in yield strength and yield ratio and a method of producing the same. More particularly, the present invention relates to a precipitation hardening type steel sheet excellent in yield strength and yield ratio, which can be used for automotive structural members such as members, A precipitation hardening type steel sheet and a manufacturing method thereof.

In order to improve the impact resistance of the vehicle body due to the regulation of the impact stability of automobiles, a precipitation strengthening type high strength steel plate is widely used for structural members such as members, beams, and pillars. The precipitation-strengthening high-strength steel sheet is designed to absorb impact energy of automobiles, and therefore has a high yield strength to tensile strength, that is, a high yield ratio (tensile strength / yield strength).

On the other hand, methods for strengthening steel usually include strengthening by solid solution, strengthening by precipitation, strengthening by grain refinement, and strengthening of transformation. However, it is very difficult to manufacture a high strength steel having a tensile strength of 500 MPa or more as a strengthening method and a fine grain strengthening method. In addition, a large amount of an alloy component is required for securing strength and forming a transformed structure, Or martensite, it has a drawback that the yield ratio is low and therefore it is not suitable to be applied to parts requiring impact resistance in a vehicle crash.

On the other hand, the precipitation-strengthening high-strength steel is a steel which mainly improves the strength by precipitation strengthening effect and grain refinement effect through addition of carbon and nitride forming elements such as Nb, Ti, V and the like, It has the advantage of being able to. The precipitation strengthening method utilizes the phenomenon that the solution is first subjected to the solution treatment at a high temperature, and then a large number of fine precipitates are formed during cooling to be strengthened by the stress field around the precipitate.

As typical techniques of such precipitation-strengthening high-strength steel, Patent Documents 1 to 3 are known. Patent Documents 1 and 2 propose a method of producing a Ti-B type thin steel sheet having excellent formability and toughness, and more specifically, it relates to a method of producing a Ti-B thin steel sheet excellent in moldability and toughness, So as to improve the incombustibility due to the addition of B by suppressing as much as possible. However, since the above techniques are techniques for securing the strength and toughness by rapidly cooling the steel sheet after it is formed in water, there is a disadvantage that the underwater quenching facility must be additionally introduced, and there is a disadvantage in that the shape is distorted during the quenching.

Patent Document 3 proposes a technique of using Nb or V, which is a precipitate-forming element, to increase the strength by accelerated cooling after hot rolling. However, the above-mentioned technique has a problem that since the coiling temperature is set to 400 占 폚 or less, a bainite or martensite structure is formed instead of forming a uniform ferrite structure, and thus the yield ratio is low, and also a large amount of expensive Nb or Sb So that the manufacturing cost is increased.

On the other hand, Patent Document 4 proposes a method of producing a precipitation hardening type cold rolled steel sheet having a yield strength of 750 MPa or more by adding Nb and Mo. However, in the above technique, the manufacturing cost is increased by adding Mn at 1.8% or more, Nb at 0.06% or more, and Mo at 0.09% or more, and the overhead during cold rolling occurs at a reduction ratio of 50% or more due to high hot- .

Japanese Patent Application Laid-Open No. 04-221015 Japanese Patent Application Laid-Open No. 05-098357 Japanese Patent Application Laid-Open No. 04-221015 Korean Patent Publication No. 2006-0072701

The present invention aims to provide a precipitation hardening type steel sheet having an excellent yield strength and yield ratio without adding a large amount of alloying elements by appropriately controlling the alloy composition and the manufacturing conditions, and a manufacturing method thereof.

In one embodiment of the present invention, the steel sheet contains 0.04 to 0.14% of C, 0.8 to 1.8% of Mn, 0.04% or less of P (excluding 0), S of 0.01% (Cu + Ni + Sn) containing 0.015% or less of Si, not more than 0.5% of Si (excluding 0), 0.01 to 0.07% of acid soluble Al, 0.02 to 0.07% of Nb, the balance Fe and other unavoidable impurities. + Pb) of 0.18% or less, 20 or more carbides having a size of 20 nm or less per 탆 ² and having a microstructure containing 80% or more of ferrite having a grain size of 7 탆 or less, A precipitation hardened steel sheet excellent in yield strength and yield ratio of 5 to 10% by area of the non-recrystallized rolled structure in the microstructure is provided.

Another embodiment of the present invention is a steel sheet comprising, by weight%, 0.04 to 0.14% of C, 0.8 to 1.8% of Mn, 0.04% or less of P (excluding 0) (Cu + Ni + Sn) containing 0.015% or less of Si, not more than 0.5% of Si (excluding 0), 0.01 to 0.07% of acid soluble Al, 0.02 to 0.07% of Nb, the balance Fe and other unavoidable impurities. + Pb) of 0.18% or less is continuously cast at a peripheral speed of 4.5 to 8 mpm to obtain a thin slab; Rolling the thin slab through the roughing mill immediately after the continuous casting so that the surface temperature of the thin slab is 950 to 1100 ° C at the side of the roughing mill to obtain a metal bar; Heating or heating the metal bar to 920 to 1150 ° C; Rolling the heated or heat-treated metal bar to a finish rolling temperature of Ar3 to Ar3 + 30 占 폚 to obtain a hot-rolled steel sheet; Winding the hot-rolled steel sheet at 550 to 680 占 폚; Cold rolling the wound hot rolled steel sheet at a reduction ratio of 35 to 65% to obtain a cold rolled steel sheet; And continuously annealing the cold-rolled steel sheet at a temperature of 760 to 820 캜, wherein the yield strength and the yield ratio are excellent.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide an excellent yield strength and yield ratio without adding a large amount of alloying elements, thereby reducing the manufacturing cost and also providing a steel sheet with excellent processability.

Hereinafter, the present invention will be described.

First, the alloy composition of the present invention will be described. % Of the alloy composition described below means% by weight.

C: 0.04 to 0.14%

C plays a very important role as a precipitate forming element. When the content is less than 0.04%, a sufficient precipitation effect can not be obtained and sufficient strength can not be secured, and the yield ratio of TiC or NbC carbide is coarsened and the yield ratio tends to be low. On the other hand, when the C content exceeds 0.14%, not only the possibility of cracking of the cast steel in the steelmaking process is increased, but also the bainite structure is formed at the time of winding the coil after hot rolling so that the strength of the hot- Therefore, the content of C is preferably in the range of 0.04 to 0.14%.

Mn: 0.8 to 1.8%

Mn not only contributes to the strength enhancement with the solid solution strengthening element but also precipitates the steel S into MnS and plays an important role in suppressing plate breakage and high temperature embrittlement caused by S during hot rolling. When the content of Mn is less than 0.8%, it is difficult to obtain the desired strength because the effect of strengthening the steel is not sufficient to secure the yield strength. When the Mn content exceeds 1.8%, the desired strength can be secured. However, , The content of Mn is preferably in the range of 0.8 to 1.8%.

P: 0.04% or less (excluding 0)

P is the most favorable element for securing strength without greatly deteriorating the formability. However, when P exceeds 0.04%, the possibility of occurrence of brittle fracture is remarkably increased to increase the possibility of plate breakage of the slab during hot rolling, It is preferable that the content of P is 0.04% or less.

S: 0.01% or less (excluding 0)

S is inevitably added as an impurity element. In order to secure good weldability, it is preferable to control the content of S as low as possible. In the present invention, the upper limit is controlled to 0.01%.

N: 0.006 to 0.015%

N is an element exhibiting the same phenomenon as C and plays a role of greatly enhancing solubility enhancement when a steel sheet is manufactured using a thin slab continuous casting method (hereinafter, also referred to as "high mill"). When the N content is less than 0.006%, the solid solution strengthening effect is small, and the TiN precipitation effect is not effective. Therefore, when the N content is more than 0.015%, excessive N content causes cracking during performance, Is preferably in the range of 0.006 to 0.015%.

Si: 0.5% or less (excluding 0)

Si is an element included in the scrap used for manufacturing the steel sheet of the present invention, and is not intentionally added in the present invention. However, when it exceeds 0.5%, it is very disadvantageous to the plating surface characteristics, and therefore, it is preferable to control the content to 0.5% or less. On the other hand, since Si is an element favorable in ensuring strength, it is preferable that Si is contained in an amount of 0.02% or more.

Acid soluble Al: 0.01 to 0.07%

Acid soluble Al is an element added for grain size reduction and deoxidation of steel. When the amount of the acid soluble Al is less than 0.01%, a killed steel can not be produced in a normal stable state. When the content is more than 0.07 wt%, the strength is increased due to grain refinement effect. However, It is preferable that the amount of the acid soluble Al is in the range of 0.01 to 0.07% since the possibility of occurrence of the surface defect of the coated steel sheet due to excessive formation is increased and the production cost is increased.

Nb: 0.02 to 0.07%

Nb is an element that plays an important role in increasing the strength by precipitating NbC by acting with solid C during hot rolling. If the content of Nb is less than 0.02%, the fine precipitates for securing strength can not be sufficiently precipitated and the strength to be obtained can not be secured. If the content exceeds 0.07%, the rolling load tends to cause plate breakage , The content of Nb is preferably in the range of 0.02 to 0.07%.

Total tramp element (Cu + Ni + Sn + Pb): 0.18% or less

The tramp element (Cu + Ni + Sn + Pb) is a kind of impurity element originating from scrap used as a raw material in the steelmaking process, but the effect of increasing the strength is very high. However, if the content of the tramp element (Cu + Ni + Sn + Pb) exceeds 0.18%, the surface crack of the slab- The content is preferably controlled to 0.18% or less,

The alloy composition of the present invention contains, in addition to the above-mentioned components, Fe and an impurity inevitably contained in the manufacturing process. In addition, although the above-mentioned alloy composition alone can achieve a desired effect, one or two of Ti: 0.01 to 0.1% and B: 0.002% or less (excluding 0) .

Ti: 0.01 to 0.1%

Ti is an element exhibiting a precipitation effect together with Nb. When the content is less than 0.01%, there is no sufficient precipitation effect. When the content exceeds 0.1%, the recrystallization temperature is rapidly increased and the manufacturing cost increase effect in the steelmaking / , The content of Ti is preferably in the range of 0.01 to 0.1%.

B: 0.002% or less (excluding 0)

B is an element which is very advantageous for grain refinement. When the content exceeds 0.002%, the cost rise and rolling load are caused. Therefore, the content of B is preferably 0.002% or less.

In the steel sheet proposed by the present invention, it is preferable that 20 or more carbides having a size of 20 nm or less are distributed per ² 탆. The fine carbides are very effective for the strength contribution, and for the above effect, the upper limit of the carbide size is controlled to 20 nm in the present invention. In order to maximize the effect of precipitation strengthening, it is preferable that 20 or more carbides are distributed per 탆 ² . The larger the number of carbides, the more advantageous is the securing of the precipitation strengthening effect. Therefore, in the present invention, the upper limit of the number of carbides is not particularly limited. However, the number of the carbides is difficult to exceed 2,000 due to the content of alloying elements to be added or the inevitability of the process. Furthermore, when too much carbide is precipitated in this way, the precipitate may be rather coarse, which may be disadvantageous to increase the strength. On the other hand, the carbide may be at least one of an Nb-based or a Ti-based carbide.

The steel sheet of the present invention preferably has a ferrite single-phase structure. It is preferable that at least 80% by area of the ferrite has a grain size of 7 탆 or less, thereby achieving a high yield strength and a high porosity . If the grain size of the ferrite exceeding 7 탆 is more than 20% by area, it may be difficult to achieve the desired strength and yield ratio of the present invention.

In the steel sheet of the present invention, the non-recrystallized rolled structure in the microstructure is preferably 5 to 10% by area. When the non-recrystallized rolled structure is less than 5% by area, the recrystallization of the steel is completed and the strength improving effect can be reduced. When the area is more than 10% by area, the elongation rate may be drastically lowered due to excessive non- , And the non-recrystallized rolled structure preferably has a range of 5 to 10 area%.

The steel sheet of the present invention provided as described above can secure an excellent yield strength of 500 MPa or more and a high cost ratio of 0.8 or more.

Hereinafter, the production method of the present invention will be described.

First, molten steel having the above-mentioned alloy composition is prepared and then continuously cast at a peripheral speed of 4.5 to 8 mpm (m / min) to obtain a thin slab. Since the high-mill produced steel has many elements added to the steel for the purpose of securing strength such as C, Mn, Si, etc., as the casting speed is slower, there is a risk of segregation from the cast steel. The casting speed is preferably 4.5 mPm or more. However, when it exceeds 8 mpm, there is a possibility of plate breakage due to the limit of the facility capability and the high-speed rolling, so that the casting speed is preferably in the range of 4.5 to 8 mpm.

Meanwhile, the present invention is characterized by using a high mill, that is, a continuous thin slab casting method. The thin slab obtained by the continuous thin slab casting method may have a thickness of 30 to 150 mm.

After the continuous casting, the thin slab is allowed to pass through the roughing mill, and the surface of the thin slab is roughly rolled at a temperature of 950 to 1100 ° C to obtain a metal bar. When the surface temperature of the thin slab is less than 950 ° C, the rolling load increases greatly during rough rolling, and the risk of edge cracks increases. On the other hand, when the surface temperature exceeds 1100 ° C, The surface temperature of the slab is preferably in the range of 950 to 1100 ° C. The arithmetic scale mentioned above means Fe oxide which is not removed directly under the surface and means a scale which causes defects on the surface of the steel sheet. Meanwhile, the roughing mill may be a 2 to 4 stand mill.

It is preferable that the cumulative rolling reduction ratio in the rough rolling is in the range of 65 to 90%. The rolling reduction control during rough rolling plays an important role in obtaining a product having a uniform material of the present invention. That is, the higher the reduction rate in the rough rolling, the smaller the temperature gradient in the width direction and the thickness direction of the bar becomes, so that it is very effective to obtain a uniform material. However, if the cumulative reduction rate is less than 65%, the above effect may not be sufficient. If the cumulative reduction rate is more than 90%, the rolling deformation resistance may greatly increase and the cost may increase considerably. 90%. ≪ / RTI >

Thereafter, it is preferable to heat or heat the metal bar obtained as described above to 920 to 1150 占 폚. This is for imparting a suitable temperature to the metal bar to perform finish rolling, which is a subsequent process, and a heating device such as an induction heater can be used. If the heating or the heat retaining temperature is less than 920 占 폚, the rolling deformation resistance may increase greatly. If the heating or the heat retaining temperature is more than 1150 占 폚, It is preferable that the temperature is in the range of 920 to 1150 ° C.

Subsequently, the heated or heated metal bar is rolled to a finish rolling temperature of Ar3 to Ar3 + 30 占 폚 to obtain a hot-rolled steel sheet. If the finish rolling temperature is lower than Ar 3, long elongated microstructure may be generated and elongation may be reduced. When the finish rolling temperature is higher than Ar 3 + 30 ° C, there is a problem that a material variation occurs due to unevenness of the structure. The temperature is preferably in the range of Ar 3 to Ar 3 + 30 ° C.

At the finish rolling, the rolling speed difference in one strip is preferably 15% or less. When the rolling speed difference is large, the difference in deformation rate during finish rolling causes a difference in material anisotropy, which may result in a poor shape. That is, if the rolling speed is different, it is difficult to obtain a uniform cooling rate and a target winding temperature on the run-out table, which may result in a large material deviation. Therefore, the rolling speed difference is preferably 15%. Here, the above strip means a length corresponding to one winding coil.

The hot-rolled steel sheet thus obtained is rolled at 550 to 680 캜. In order to maximize the strength enhancement effect through the fine precipitates, low-temperature coiling is preferable. However, if the coiling temperature is excessively low, the shape may become poor, and if it is excessively high, Lt; 0 > C.

Thereafter, the rolled hot-rolled steel sheet is cold-rolled at a reduction ratio of 35 to 65% to obtain a cold-rolled steel sheet. The higher the rolling reduction rate in the cold rolling, the earlier the recrystallization is completed, so that it is difficult to obtain the desired level of the non-recrystallized rolled structure desired by the present invention. Therefore, it is difficult to secure the target strength and the cold rolling load is also high, The reduction rate is preferably 65% or less. However, when it is less than 35%, it is out of the lower limit range of the ordinary cold rolling reduction ratio, and thus the workability becomes very poor and the temperature for completion of recrystallization becomes very high, so that the reduction rate is preferably in the range of 35 to 65%.

Next, for the control of the structure to be obtained in the present invention, the cold-rolled steel sheet obtained as described above is preferably continuously annealed at 760 to 820 占 폚. When the annealing temperature is less than 760 DEG C, the recrystallization ratio is low and the elongation is likely to decrease. When the annealing temperature is more than 820 DEG C, the precipitates become coarse and the grain growth is carried out, It is preferable to have a range of 760 to 820 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)

Molten steel having the alloy composition shown in the following Table 1 was cast to obtain a thin slab, and then the thin slab was subjected to rough rolling under the conditions described in Table 2 to obtain a metal bar. The above-mentioned metal bar was heated to 1150 캜 and then subjected to hot rolling and cold rolling under the conditions shown in Table 2 below to produce a cold-rolled steel sheet. The cold-rolled steel sheet thus prepared was measured for carbide distribution, microstructure and mechanical properties, and the results are shown in Table 3 below.

division Alloy composition (% by weight) C Mn P S N Si Sol.Al Nb Ti B Cu Ni Sn Pb Inventive Steel 1 0.042 1.7 0.035 0.006 0.009 0.15 0.035 0.06 0.03 - 0.05 0.03 0.03 0.01 Invention river 2 0.063 1.3 0.026 0.007 0.011 0.2 0.032 0.052 0.025 - 0.04 0.03 0.02 0.01 Invention steel 3 0.085 1.1 0.03 0.0065 0.014 0.18 0.025 0.048 - 0.0015 0.051 0.03 0.03 0.01 Inventive Steel 4 0.112 0.9 0.031 0.007 0.009 0.2 0.031 0.061 - 0.0018 0.035 0.03 0.02 0.01 Invention steel 5 0.138 0.8 0.036 0.006 0.012 0.21 0.036 0.038 - - 0.051 0.02 0.03 0.01 Comparative River 1 0.085 0.5 0.028 0.005 0.18 0.18 0.035 - - - - - - -

division Grade Nr. Slab
thickness
(mm) casting
speed
(mpm) Slab
Surface temperature
(° C) accumulate
Reduction rate
(%) Wrap-up
Rolling temperature
(° C)
Rolling
Speed difference
(%) Cold
Reduction rate
(%) Coiling
Temperature
(° C) Annealing
Temperature
(° C) Inventory 1 Inventive Steel 1 84 6.0 1010 70 911 5 48 580 780 Comparative Example 1 Inventive Steel 1 84 6.0 1005 72 905 5 47 575 840 Inventory 2 Inventive Steel 1 84 6.0 1012 71 906 5 50 580 800 Comparative Example 2 Inventive Steel 1 84 6.0 1008 68 902 5 51 700 800 Inventory 3 Invention river 2 84 6.0 1006 75 896 5 50 600 800 Comparative Example 3 Invention river 2 84 6.0 1011 73 897 5 50 700 800 Comparative Example 4 Invention river 2 84 6.0 1013 78 886 5 32 600 820 Honorable 4 Invention river 2 84 6.0 1015 77 889 5 60 630 800 Inventory 5 Invention steel 3 84 6.0 1012 76 896 5 62 630 800 Comparative Example 5 Invention steel 3 84 2.5 1009 75 895 5 59 630 800 Inventory 6 Invention steel 3 84 6.0 1021 79 894 55 61 630 800 Honorable 7 Inventive Steel 4 84 6.0 1013 73 886 5 61 580 800 Honors 8 Invention steel 5 84 6.0 1014 72 876 5 62 580 800 Comparative Example 6 Comparative River 1 230 1.0 1011 70 901 30 58 580 780

division Carbide distribution
(Number / 탆 ² ) The grain size is
Ferrite having a thickness of 7 μm or less
Fraction (area%) Yield strength
(YS)
(MPa) The tensile strength
(TS)
(MPa) Yield ratio
(TS / YS) Elongation
(%) Unrecognition
Rolling Tissue Fraction
(area%) Inventory 1 23 82 523 595 0.88 28 7 Comparative Example 1 17 65 460 680 0.68 21 0 Inventory 2 25 85 538 601 0.90 27 6 Comparative Example 2 13 68 486 676 0.72 24 6 Inventory 3 26 83 536 615 0.87 28 7 Comparative Example 3 15 81 562 756 0.74 16 6 Comparative Example 4 19 82 480 675 0.71 18 7 Honorable 4 23 86 526 602 0.87 23 7 Inventory 5 24 83 515 603 0.85 24 8 Comparative Example 5 18 75 468 625 0.75 18 6 Inventory 6 23 83 516 602 0.86 23 7 Honorable 7 22 85 526 611 0.86 22 7 Honors 8 24 85 520 615 0.85 24 7 Comparative Example 6 10 70 380 498 0.76 10 0

As can be seen from Tables 1 to 3, in Inventive Examples 1 to 8, which satisfy the alloy composition and manufacturing conditions proposed by the present invention, a large amount of carbide is distributed at a rate of 20 or more per 탆 ² and a grain size is 7 탆 or less It was confirmed that not only the ferrite fraction was 80% by area but also the non-recrystallized rolled structure fraction of 5 to 10% by area, excellent tensile strength and excellent yield strength of 500 MPa or more were secured and a high yield ratio of 0.80 or more was secured .

However, in the case of Comparative Examples 1 to 5, the alloy composition of the present invention is satisfied. However, since the annealing temperature, coiling temperature, cold rolling reduction rate and casting speed proposed by the present invention are not satisfied, Grain growth occurs or a non-recrystallized rolled structure is not formed. Thus, it can be seen that the target yield strength and yield ratio are not secured.

In the case of Comparative Example 6, both the yield strength and the yield ratio are low because the alloy composition and the manufacturing conditions of the present invention are not satisfied.

Claims (8)

(Excluding 0), S: not more than 0.01% (excluding 0), N: 0.006 to 0.015%, Si: 0.5% or less, C: 0.04 to 0.14%, Mn: 0.8 to 1.8% 0.01 to 0.07% of acid soluble Al, 0.02 to 0.07% of Nb, the balance Fe and other unavoidable impurities,
The total tramp element (Cu + Ni + Sn + Pb) is 0.18% or less,
20 or more carbides having a size of 20 nm or less are distributed per 탆 ² ,
Having a microstructure containing not less than 80% by area of ferrite having a grain size of 7 mu m or less,
Wherein the non-recrystallized rolled structure of the microstructure has a yield strength and yield ratio of 5 to 10% by area. The method according to claim 1,
Wherein the steel sheet further comprises one or two of Ti: 0.01 to 0.1% and B: 0.002% or less (excluding 0), and further having a yield strength and a yield ratio. The method according to claim 1,
Wherein the steel sheet has a yield strength of 500 MPa or more and a yield strength and yield ratio of 0.8 or more. (Excluding 0), S: not more than 0.01% (excluding 0), N: 0.006 to 0.015%, Si: 0.5% or less, C: 0.04 to 0.14%, Mn: 0.8 to 1.8% (Cu + Ni + Sn + Pb) of not more than 0.18% (excluding 0), acid soluble Al: 0.01 to 0.07%, Nb: 0.02 to 0.07%, the balance Fe and other unavoidable impurities Is continuously cast at a circumferential speed of 4.5 to 8 mpm to obtain a thin slab;
Rolling the thin slab through the roughing mill immediately after the continuous casting so that the surface temperature of the thin slab is 950 to 1100 ° C at the side of the roughing mill to obtain a metal bar;
Heating or heating the metal bar to 920 to 1150 ° C;
Rolling the heated or heat-treated metal bar to a finish rolling temperature of Ar3 to Ar3 + 30 占 폚 to obtain a hot-rolled steel sheet;
Winding the hot-rolled steel sheet at 550 to 680 占 폚;
Cold rolling the wound hot rolled steel sheet at a reduction ratio of 35 to 65% to obtain a cold rolled steel sheet; And
And continuously annealing the cold-rolled steel sheet at 760 to 820 占 폚, wherein the yield strength and the yield ratio are excellent. The method of claim 4,
Wherein the molten steel further comprises one or two of Ti: 0.01 to 0.1% and B: 0.002% or less (excluding 0), and further wherein the yield strength and yield ratio are excellent. The method of claim 4,
Wherein the thin slab has a thickness of 30 to 150 mm and is excellent in yield strength and yield ratio. The method of claim 4,
Wherein the cumulative rolling reduction ratio in the rough rolling is 65 to 90% and the yield strength and yield ratio are excellent. The method of claim 4,
Wherein the difference in the rolling speed in one strip during the finish rolling is excellent in yield strength and yield ratio of 15% or less.