KR20230091218A - High strength steel sheet having excellent formability and high yield ratio and method of manufacturing the same - Google Patents

Abstract

The present invention relates to a high-strength steel sheet having excellent formability and a high yield ratio and a manufacturing method thereof, and more particularly, to a high-strength steel sheet having excellent formability and a high yield ratio usable for various purposes including automobile parts and a manufacturing method thereof. will be.

Description

High-strength steel sheet having excellent formability and high yield ratio and manufacturing method thereof

The present invention relates to a high-strength steel sheet having excellent formability and a high yield ratio and a manufacturing method thereof, and more particularly, to a high-strength steel sheet having excellent formability and a high yield ratio usable for various purposes including automobile parts and a manufacturing method thereof. will be.

Recently, securing the safety of occupants and improving the fuel efficiency of automobiles have received great attention in the automobile industry. For this reason, the application of high-strength steel sheets to meet the safety and lightweight requirements for automotive body materials is increasing.

In order to improve the crash performance of the automobile body, if the yield strength of the steel is increased, the crash energy can be efficiently absorbed even at a low deformation amount. As a method of increasing the yield strength, there are solid solution hardened steel and precipitation hardened steel.

Solid-solution strengthening steel is a steel sheet in which the yield strength is increased by dissolving solid-solution strengthening elements (Mn, Si, Cr, etc.) on ferrite having excellent formability. However, Si or Cr is an element that easily forms an oxide on the surface of a steel sheet in a continuous annealing line or a continuous hot-dip galvanizing line. In addition, Mn is an element that promotes the formation of a low-temperature transformation phase (bainite or martensite), which is characterized by lowering the yield strength. Therefore, solid-solution strengthening steel with a large amount of Mn, Si, and Cr added is not suitable as a method of increasing the yield ratio of a high-strength steel sheet having a tensile strength of 610 MPa or more.

On the other hand, precipitation hardened steel using Nb, Ti, V, etc. is a steel sheet that improves yield strength by precipitating fine carbides in ferrite. Precipitation hardened steel increases the yield ratio without deteriorating workability, so it is a suitable reinforcing mechanism for high-strength steel plates with excellent crash performance and workability with a tensile strength of 610 MPa or more.

As a technique for improving the formability and yield ratio of a steel sheet, methods utilizing the introduction of non-recrystallized ferrite and the addition of Ti or Nb are disclosed in Patent Documents 1 and 2. Precipitation hardening and non-recrystallized ferrite using Ti or Nb directly strengthens ferrite, so it is effective in increasing yield strength without significantly increasing tensile strength.

However, Patent Documents 1 and 2 have a disadvantage in that it is difficult to simultaneously secure excellent strength, elongation, formability and high yield ratio as they contain a large amount of non-recrystallized ferrite. Patent Document 3 is a technology using non-recrystallized ferrite instead of the transformation hard phase (martensite, bainite, etc.) of the existing DP (Dual Phase) steel, that is, by including only the ferrite structure, it has excellent strength, elongation, formability and high yield ratio The disadvantage is that it is difficult to secure both at the same time. Patent Document 4 has a disadvantage in that it is difficult to simultaneously secure excellent strength, elongation, formability and high yield ratio as it includes Mn in the range of 0.15 to 0.45%.

Japanese Patent Laid-Open No. 2009-114523 Japanese Patent Laid-Open No. 2017-002333 Japanese Patent Laid-Open No. 2017-002332 Japanese Patent Laid-Open No. 2015-147965

One aspect of the present invention is to provide a high-strength steel sheet having excellent formability and a high yield ratio and a manufacturing method thereof.

One embodiment of the present invention, by weight%, C: 0.05 to 0.25%, Si: 0.7% or less (excluding 0%), Mn: 0.46 to 1.8%, Al: 0.7% or less (excluding 0%), P : 0.05% or less (excluding 0%), S: 0.03% or less (excluding 0%), N: 0.03% or less (excluding 0%), the total amount of one or more of Ti, Nb, and V: 0.22% or less, Excellent formability and high yield ratio, including the balance Fe and other unavoidable impurities, microstructure by area%, non-recrystallized ferrite: 1-13%, recrystallized ferrite: 67-98% and cementite: 1-20% It provides a high-strength steel sheet having

Another embodiment of the present invention, in weight percent, C: 0.05 to 0.25%, Si: 0.7% or less (excluding 0%), Mn: 0.46 to 1.8%, Al: 0.7% or less (excluding 0%), P : 0.05% or less (excluding 0%), S: 0.03% or less (excluding 0%), N: 0.03% or less (excluding 0%), the total amount of one or more of Ti, Nb, and V: 0.22% or less, heating the steel ingot or slab containing the remainder of Fe and other unavoidable impurities at 1000 to 1350° C.; Obtaining a hot-rolled steel sheet by hot-rolling the heated steel ingot or slab at a finish rolling temperature of 800 to 1000° C.; winding the hot-rolled steel sheet at 300 to 600° C.; heat-treating the rolled hot-rolled steel sheet at 650-800° C. for 600-1700 seconds; Obtaining a cold-rolled steel sheet by cold-rolling the heat-treated hot-rolled steel sheet at a cold rolling reduction ratio of 30 to 90%; Reheating the cold-rolled steel sheet at 720 to 860° C. and maintaining the cold-rolled steel sheet for 50 seconds or more; firstly cooling the reheated and maintained cold-rolled steel sheet to 600 to 760° C. at an average cooling rate of 1° C./s or more; Secondarily cooling the primary cooled cold-rolled steel sheet to 450 to 550 ° C at an average cooling rate of 2 ° C / s or more, and holding for 50 seconds or more; And it provides a method for manufacturing a high-strength steel sheet having excellent formability and a high yield ratio, including the step of tertiary cooling the secondary cooling and maintained cold-rolled steel sheet to room temperature.

According to one aspect of the present invention, it is possible to provide a high-strength steel sheet having excellent formability and a high yield ratio and a manufacturing method thereof.

Hereinafter, a high-strength steel sheet having excellent formability and a high yield ratio according to an embodiment of the present invention will be described. First, the alloy composition will be described. The content of the alloy composition described below refers to % by weight.

C: 0.05 to 0.25%

C is an element for forming a precipitate together with Ti, Nb or V in the ferrite phase to impart strength to the steel sheet. If the C content is less than 0.05%, it is difficult to secure a tensile strength of 610 MPa or more. On the other hand, if the C content exceeds 0.25%, it is difficult to secure sufficient weld strength. Therefore, the content of C is preferably in the range of 0.05 to 0.25%. The lower limit of the C content is more preferably 0.06%, and even more preferably 0.07%. The upper limit of the C content is more preferably 0.24%, and even more preferably 0.23%.

Si: 0.7% or less (excluding 0%)

Si is an element that has an effect of improving strength by solid solution strengthening, and is an element that strengthens ferrite, homogenizes microstructure, and improves workability. In addition, it is an element necessary for deoxidation during steelmaking. When the content of Si exceeds 0.7, plating defects such as non-plating in the plating process and deterioration of weldability of the steel sheet. Therefore, the Si content is preferably in the range of 0.7% or less. The lower limit of the Si content is more preferably 0.001%, and even more preferably 0.002%. The upper limit of the Si content is more preferably 0.69%, and even more preferably 0.68%.

Mn: 0.46~1.8%

Mn is a useful element for increasing both strength and ductility. If the Mn content is less than 0.46%, it is difficult to sufficiently obtain the above effect, and if it exceeds 1.8%, the formation of low-temperature transformation phases such as martensite or bainite is promoted from austenite, thereby lowering the yield ratio of the steel sheet. Therefore, the Mn content is preferably in the range of 0.46 to 1.8%. The lower limit of the Mn content is more preferably 0.47%, and even more preferably 0.48%. The upper limit of the Mn content is more preferably 1.79%, and even more preferably 1.78%.

Al: 0.7% or less (excluding 0%)

Al is an element that acts as a deoxidizer by combining with oxygen in steel. Also, like Si, it is an element that strengthens ferrite, homogenizes the microstructure, and improves workability. If the Al content exceeds 0.7%, plating defects such as non-plating occur in the plating process, and the weldability of the steel sheet is deteriorated. Therefore, the Si content is preferably in the range of 0.7% or less. The lower limit of the Al content is more preferably 0.001%, and even more preferably 0.002%. The upper limit of the Al content is more preferably 0.69%, and even more preferably 0.68%.

P: 0.05% or less (excluding 0%)

P is an element that is contained as an impurity and deteriorates the impact toughness. Therefore, the content of P is preferably controlled to 0.05% or less. The P content is more preferably 0.04% or less, and even more preferably 0.03% or less.

S: 0.03% or less (excluding 0%)

S is an element that is contained as an impurity to form MnS in the steel sheet and deteriorates ductility. Therefore, the content of S is preferably controlled to 0.03% or less. The S content is more preferably 0.02% or less, and even more preferably 0.01% or less.

N: 0.03% or less (excluding 0%)

N is an element that is contained as an impurity and causes cracks in the slab by creating nitride during continuous casting. Therefore, the N content is preferably controlled to 0.03% or less. The N content is more preferably 0.02% or less, and even more preferably 0.01% or less.

Total amount of at least one of Ti, Nb, and V: 0.22% or less

Ti, Nb, and V are important elements that form precipitates in steel sheets. It may be contained in order to improve the strength and impact toughness of the steel sheet. When the total amount of one or more of Ti, Nb, and V exceeds 0.22%, the unrecrystallized ferrite fraction exceeds 13% due to excessive precipitate formation, making it difficult to obtain the physical properties desired by the present invention and causing an increase in manufacturing cost. becomes Therefore, the total amount of at least one of Ti, Nb, and V preferably has a range of 0.22% or less. The lower limit of the total amount of at least one of Ti, Nb and V is more preferably 0.03%, and still more preferably 0.05%. The upper limit of the total amount of at least one of Ti, Nb and V is more preferably 0.21%, and still more preferably 0.20%.

In addition to the above-described steel composition, the rest may include Fe and unavoidable impurities. Inevitable impurities can be unintentionally mixed in the normal steel manufacturing process, and cannot be completely excluded, and those skilled in the ordinary steel manufacturing field can easily understand the meaning. Further, the present invention does not entirely exclude the addition of other compositions than the aforementioned steel composition.

Meanwhile, the steel sheet of the present invention may further include a total amount of at least one of Cr and Mo: 0.8% or less.

Cr and Mo are elements that suppress austenite decomposition during alloying and stabilize austenite in the same way as Mn. When the total amount of at least one of Cr and Mo exceeds 0.8%, formation of a low-temperature transformation phase such as martensite or bainite is promoted and the yield ratio of the steel sheet is lowered. Therefore, the total amount of at least one of Cr and Mo preferably has a range of 0.8% or less. The lower limit of the total amount of at least one of Cr and Mo is more preferably 0.0001%, and even more preferably 0.001%. The upper limit of the total amount of at least one of Cr and Mo is more preferably 0.7%, still more preferably 0.6%, and most preferably 0.53%.

In addition, the steel sheet of the present invention may further include a total amount of at least one of Cu and Ni: 0.8% or less.

Cu and Ni are elements that stabilize austenite and inhibit corrosion. In addition, the Cu and Ni are concentrated on the surface of the steel sheet to prevent hydrogen penetration into the steel sheet, thereby suppressing delayed hydrogen destruction. If the total amount of one or more of the Cu and Ni exceeds 0.8%, it may be difficult to obtain the physical properties desired by the present invention and cause an increase in manufacturing cost. Therefore, the total amount of at least one of Cu and Ni is preferably within a range of 0.8% or less. The lower limit of the total amount of one or more of the above Cu and Ni is more preferably 0.0001%, and even more preferably 0.001%. The upper limit of the total amount of at least one of Cu and Ni is more preferably 0.7%, still more preferably 0.6%, and most preferably 0.54%.

In addition, the steel sheet of the present invention may further include B: 0.005% or less.

B is an element that improves hardenability to increase strength and suppresses nucleation of grain boundaries. If the content of B exceeds 0.005%, it may be difficult to obtain the physical properties desired by the present invention and cause an increase in manufacturing cost. Therefore, the content of B is preferably in the range of 0.005% or less. The lower limit of the B content is more preferably 0.0001%, and even more preferably 0.0003%. The upper limit of the B content is more preferably 0.0045%, and even more preferably 0.004%.

In addition, the steel sheet of the present invention may further include a total amount of one or more of Ca, REM (excluding Y), and Mg: 0.05% or less.

REMs other than Ca, Mg, and Y are elements that improve the ductility of a steel sheet by spheroidizing sulfides. If the total amount of one or more of Ca, REM (excluding Y) and Mg exceeds 0.05%, it may be difficult to obtain physical properties desired by the present invention and cause an increase in manufacturing cost. Therefore, the total amount of one or more of Ca, REM (excluding Y) and Mg is preferably within a range of 0.05% or less. The lower limit of the total amount of at least one of Ca, REM (excluding Y) and Mg is more preferably 0.0001%, and still more preferably 0.0003%. The upper limit of the total amount of at least one of Ca, REM (excluding Y) and Mg is more preferably 0.04%, still more preferably 0.03%, and most preferably 0.02%. Meanwhile, REM means 17 elements including Sc, Y, and lanthanoids.

In addition, the steel sheet of the present invention may further include a total amount of one or more of W and Zr: 0.5% or less.

W and Zr are elements that increase the strength of a steel sheet by improving hardenability. If the total amount of one or more of the W and Zr exceeds 0.5%, it may be difficult to obtain physical properties desired by the present invention and cause an increase in manufacturing cost. Therefore, the total amount of at least one of W and Zr is preferably within a range of 0.5% or less. The lower limit of the total amount of at least one of W and Zr is more preferably 0.0001%, still more preferably 0.001%, and most preferably 0.01%. The upper limit of the total amount of at least one of W and Zr is more preferably 0.4%, still more preferably 0.35%, and most preferably 0.3%.

In addition, the steel sheet of the present invention may further include a total amount of at least one of Sb and Sn: 0.5% or less.

Sb and Sn are elements that improve plating wettability and plating adhesion of steel sheets. If the total amount of one or more of the Sb and Sn exceeds 0.5%, the brittleness of the steel sheet increases and cracks may occur during hot working or cold working. Therefore, the total amount of at least one of Sb and Sn preferably has a range of 0.5% or less. The lower limit of the total amount of at least one of Sb and Sn is more preferably 0.0001%, still more preferably 0.001%, and most preferably 0.005%. The upper limit of the total amount of at least one of Sb and Sn is more preferably 0.4%, still more preferably 0.3%, and most preferably 0.2%.

In addition, the steel sheet of the present invention may further include a total amount of at least one of Y and Hf: 0.2% or less.

Y and Hf are elements that improve the corrosion resistance of steel sheets. If the total amount of at least one of Y and Hf exceeds 0.2%, the ductility of the steel sheet may deteriorate. Therefore, the total amount of at least one of Y and Hf is preferably in the range of 0.2% or less. The lower limit of the total amount of at least one of Y and Hf is more preferably 0.0001%, still more preferably 0.001%, and most preferably 0.005%. The upper limit of the total amount of at least one of Y and Hf is more preferably 0.15%, still more preferably 0.12%, and most preferably 0.1%.

Hereinafter, the microstructure will be described. The fraction of microstructure described below refers to area %

Non-recrystallized ferrite: 1 to 13%

In general, non-recrystallized ferrite contains many dislocations and exhibits low ductility and hole expandability. However, the present inventors have confirmed that when the fraction of non-recrystallized ferrite is 1 to 13%, a high yield ratio can be secured without deteriorating elongation and hole expansion. When the fraction of the non-recrystallized ferrite is less than 1% or greater than 13%, the yield ratio, elongation or hole expansion ratio is lowered. On the other hand, the non-recrystallized ferrite may be defined as ferrite formed by cooling the ferrite processed in the cold rolling process without being transformed into austenite during annealing. The non-recrystallized ferrite has an elongated form in the cold rolling direction.

Recrystallized ferrite: 67-98%

Recrystallized ferrite may be defined as ferrite formed by transforming ferrite processed in a cold rolling process into austenite during annealing and then transforming during cooling, and exhibits effects such as improving ductility and hole expandability of a steel sheet. When the fraction of the recrystallized ferrite is less than 67% or greater than 98%, the yield ratio, elongation or hole expansion ratio is lowered. The recrystallized ferrite is a normal polygonal ferrite.

Cementite: 1-20%

Cementite exerts an effect of increasing the strength and hardness of a steel sheet. When the fraction of the cementite is less than 1%, it may be difficult to secure strength. On the other hand, if it exceeds 20%, it may be difficult to secure mechanical properties because the precipitation of Ti, Nb, or V carbide is suppressed, and the ferrite fraction desired to be obtained by the present invention cannot be secured.

The steel sheet of the present invention provided as described above has tensile strength (TS): 610 MPa or more, yield ratio (YR): 0.8 to 0.95, tensile strength (TS) ² × √ elongation (EL) is 1.8 × 10 ⁶ ~ 2.3 × 10 ⁶ MPa ² % ^0.5 and tensile strength (TS) ² ×√hole expandability (HER): 2.5×10 ⁶ ~3.8×10 ⁶ MPa ² % ^0.5 , it has excellent strength, formability and high yield ratio.

Meanwhile, the steel sheet of the present invention may be a cold-rolled steel sheet or a coated steel sheet, and the coated steel sheet may be hot-dip galvanized, electro-galvanized, or hot-dip aluminum-plated.

Hereinafter, a method for manufacturing a high-strength steel sheet having excellent formability and a high yield ratio according to an embodiment of the present invention will be described.

First, a steel ingot or slab satisfying the above alloy composition is heated at 1000 to 1350 ° C. If the heating temperature is less than 1000 ° C., there is a possibility of hot rolling in a state out of the finish rolling temperature range. On the other hand, if it exceeds 1350 ℃, there is a possibility of melting by reaching the melting point of steel. The lower limit of the ingot or slab heating temperature is more preferably 1025°C, and even more preferably 1050°C. The upper limit of the steel ingot or slab heating temperature is more preferably 1325°C, and even more preferably 1300°C.

Thereafter, the heated steel ingot or slab is hot-rolled at a finish rolling temperature of 800 to 1000° C. to obtain a hot-rolled steel sheet. When the finish rolling temperature is less than 800° C., a large burden may be placed on the hot rolling mill due to the high strength of the steel. On the other hand, when the temperature exceeds 1000 ° C., crystal grains of the steel sheet become coarse after hot rolling, and mechanical properties may be deteriorated. The lower limit of the finish rolling temperature is more preferably 815°C, and still more preferably 830°C. The upper limit of the finish rolling temperature is more preferably 985°C, and still more preferably 970°C.

Meanwhile, after the finish rolling, the hot-rolled steel sheet may be cooled at an average cooling rate of 10° C./s or more to the following coiling temperature. The cooling is for refining crystal grains, and when the average cooling rate is less than 10° C./s, it may be difficult to sufficiently obtain the crystal grain refining effect. Since the average cooling rate is advantageous as it is faster, the upper limit of the average cooling rate is not particularly limited in the present invention, but considering the limitations of facilities and the like, it is difficult to exceed 500 ° C / s.

Thereafter, the hot-rolled steel sheet is wound at 300 to 600°C. When the coiling temperature is less than 300° C., the main phase of the hot-rolled steel sheet is composed of a high-strength low-temperature transformation phase, and thus, it may not be easy to coil the steel sheet. On the other hand, when the temperature exceeds 600 ° C., the scale generated on the surface of the hot-rolled steel sheet may be formed deep into the steel sheet, making pickling difficult. The lower limit of the coiling temperature is more preferably 315°C, and even more preferably 330°C. The upper limit of the coiling temperature is more preferably 585°C, and even more preferably 570°C.

Thereafter, the coiled hot-rolled steel sheet is heat treated at 650 to 800° C. for 600 to 1700 seconds. The heat treatment is to improve the yield ratio of the final product by promoting the formation of precipitates in the hot-rolled steel sheet. When the heat treatment temperature is less than 650 °C or the heat treatment time is less than 600 seconds, it may not be easy to optimize the precipitates of the annealed hot-rolled steel sheet. On the other hand, when the heat treatment temperature exceeds 800 °C or the heat treatment time exceeds 1700 seconds, it may not be easy to form precipitates in the annealed hot-rolled steel sheet. The lower limit of the heat treatment temperature is more preferably 660°C, and even more preferably 670°C. The upper limit of the heat treatment temperature is more preferably 790°C, and even more preferably 780°C. The lower limit of the heat treatment time is more preferably 700 seconds, and even more preferably 800 seconds. The upper limit of the heat treatment time is more preferably 1600 seconds, and even more preferably 1500 seconds.

Meanwhile, after the heat treatment, a pickling process for removing scale formed on the surface of the steel sheet may be additionally performed. In the present invention, the pickling process is not particularly limited, and all pickling processes used in the art can be applied. can

Thereafter, the heat-treated hot-rolled steel sheet is cold-rolled at a cold rolling reduction ratio of 30 to 90% to obtain a cold-rolled steel sheet. If the cold rolling reduction ratio is less than 30%, it is difficult to secure an appropriate cold-rolled steel sheet shape, and if it exceeds 90%, it may be difficult to perform cold rolling in a short time due to the high strength of the steel sheet. The lower limit of the cold reduction ratio is more preferably 31%, and even more preferably 32%. The upper limit of the cold reduction ratio is more preferably 89%, and even more preferably 88%.

Thereafter, the cold-rolled steel sheet is reheated at 720 to 860° C. and maintained for 50 seconds or more. When the reheating temperature is less than 720° C., the unrecrystallized ferrite fraction exceeds 13%, making it difficult to obtain mechanical properties desired by the present invention. When the reheating temperature exceeds 860° C., the unrecrystallized ferrite fraction is not formed at 1% or more, making it difficult to obtain mechanical properties desired by the present invention. When the holding time is less than 50 seconds, it is difficult to obtain the mechanical properties desired by the present invention due to insufficient heat treatment time. The lower limit of the reheating temperature is more preferably 730°C, and even more preferably 740°C. The upper limit of the reheating temperature is more preferably 850°C, and even more preferably 840°C. The holding time is more preferably 55 seconds or more, and even more preferably 60 seconds or more. On the other hand, in the present invention, the longer the holding time is, the more advantageous it is, so the upper limit is not particularly limited. However, in terms of productivity, the holding time may be 600 seconds or less. Further, in the present invention, the average temperature increase rate during reheating is not particularly limited, and may be, for example, 1 to 100° C./s.

Thereafter, the reheated and maintained cold-rolled steel sheet is firstly cooled to 600 to 760° C. at an average cooling rate of 1° C./s or more. If the first cooling stop temperature is less than 600 ° C., the cementite fraction exceeds 20%, making it difficult to obtain the mechanical properties desired by the present invention. When the first cooling stop temperature exceeds 760 ° C., the cooling stop temperature is high, making it difficult to obtain the mechanical properties desired by the present invention. The lower limit of the first cooling stop temperature is more preferably 610°C, and even more preferably 620°C. The upper limit of the first cooling stop temperature is more preferably 750°C, and even more preferably 740°C. The first average cooling rate is more preferably 1.5 ℃ / s or more. Meanwhile, in the present invention, the upper limit of the primary average cooling rate is not particularly limited.

Thereafter, the primary cooled cold-rolled steel sheet is secondary cooled to 450 to 550 ° C at an average cooling rate of 2 ° C / s or more, and maintained for 50 seconds or more. If the secondary cooling stop temperature is less than 450 ° C., it is difficult to obtain the mechanical properties desired by the present invention due to the low heat treatment temperature. On the other hand, if the secondary cooling stop temperature exceeds 550 ° C., the fraction of non-recrystallized ferrite exceeds 13%, making it difficult to obtain the mechanical properties desired by the present invention. If the secondary cooling rate is less than 2 ° C / s, the cementite fraction exceeds 20%, making it difficult to obtain the mechanical properties desired by the present invention. When the holding time is less than 50 seconds, it is difficult to obtain the mechanical properties desired by the present invention due to insufficient holding time. The lower limit of the secondary cooling stop temperature is more preferably 460°C, and even more preferably 470°C. The upper limit of the secondary cooling stop temperature is more preferably 540°C, and even more preferably 530°C. It is more preferable that the second average cooling rate is 3° C./s or more. Meanwhile, in the present invention, the upper limit of the second average cooling rate is not particularly limited. Further, in the present invention, the longer the holding time is, the more advantageous it is, so the upper limit is not particularly limited. However, in terms of productivity, the holding time may be 1800 seconds or less.

Thereafter, the secondly cooled and maintained cold-rolled steel sheet is thirdly cooled to room temperature. During the tertiary cooling, an average cooling rate may be 0.5 to 50° C./s.

Meanwhile, after the tertiary cooling, a plating process may be additionally performed. In the present invention, the plating process is not particularly limited, and all conventional processes used in the art can be used.

Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are only intended to illustrate the present invention in more detail and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and the matters reasonably inferred therefrom.

(Example)

After preparing a slab having a thickness of 100 mm having an alloy composition shown in Table 1, the slab was heated at 1200 ° C., and hot-rolled at a finish rolling temperature of 900 ° C. to prepare a hot-rolled steel sheet having a thickness of 3 mm. After cooling the hot-rolled steel sheet to the coiling temperature shown in Table 2 at an average cooling rate of 30° C./s, coiling was performed. Thereafter, the rolled hot-rolled steel sheet was heat-treated under the conditions shown in Table 2 below, pickled, and cold-rolled to prepare a cold-rolled steel sheet having a thickness of 1.5 mm. Thereafter, reheating, primary cooling, secondary cooling, and tertiary cooling were performed under the conditions described in Tables 2 and 3 below.

After measuring the microstructure and mechanical properties of the cold-rolled steel sheet prepared as described above, the results are shown in Table 4 below.

The microstructure was observed through SEM after polishing and nital etching of the cross section of the specimen taken from the cold-rolled steel sheet. After nital etching, the structure without irregularities on the surface of the specimen was found to be ferrite, and the structure having a spherical or lamellar structure was found to be cementite. In non-recrystallized ferrite containing many dislocations, a difference in crystal orientation occurs within the grain. Therefore, after measuring the crystal orientation of ferrite using FESEM-EBSD, the fraction was measured by distinguishing non-recrystallized ferrite and recrystallized ferrite among ferrite by Kernel Average Misorientation (KAM) method.

Mechanical properties were measured by tensile test and hole expansion test. For the tensile test, test pieces obtained according to the JIS5 standard were used based on the 0° direction with respect to the rolling direction of the cold-rolled steel sheet. The hole expansion test was measured by forming a conical punch with an apex angle of 60° into a 10 mm diameter punched hole (die inner diameter 10.3 mm, clearance 12.5%) by pressing and expanding at 20 mm/min in the direction where the burr of the punched hole is outward. .

Hole expansion ratio: HER (%) = {(D - D ₀ )/D ₀ } × 100

D: Hole diameter when a crack penetrates the steel plate (mm)

D ₀ : initial hole diameter (mm)

steel grade Alloy composition (% by weight) C Si Mn P S Al N Ti Nb V Ti+Nb+V etc A 0.14 0.45 1.34 0.009 0.0011 0.28 0.0032 0.15 0.01 0.02 0.18 B 0.12 0.38 1.25 0.010 0.0010 0.24 0.0029 0.01 0.12 0.01 0.14 C 0.17 0.34 1.27 0.008 0.0008 0.29 0.0025 0.02 0.03 0.11 0.16 D 0.16 0.39 1.19 0.012 0.0010 0.31 0.0028 0.08 0.07 0.05 0.20 E 0.15 0.43 1.31 0.008 0.0007 0.27 0.0030 0.11 0.06 0.01 0.18 CR: 0.43 F 0.07 0.68 1.77 0.007 0.0011 0.66 0.0031 0.09 0.02 0.08 0.19 Mo: 0.38 G 0.23 0.64 0.47 0.010 0.0013 0.62 0.0027 0.01 0.09 0.07 0.17 Ni: 0.35 H 0.14 0.42 1.18 0.011 0.0012 0.23 0.0036 0.10 0.01 0.02 0.13 Cu: 0.44 I 0.16 0.39 1.22 0.009 0.0009 0.25 0.0039 0.04 0.01 0 0.05 B: 0.0021 J 0.12 0.37 1.28 0.008 0.0010 0.28 0.0028 0.09 0.02 0.01 0.12 Ca: 0.003 K 0.20 0.61 0.50 0.011 0.0011 0.59 0.0027 0.11 0.01 0.02 0.14 REM (without Y): 0.001 L 0.15 0.36 1.32 0.009 0.0009 0.33 0.0032 0.01 0.08 0.02 0.11 Mg: 0.002 M 0.16 0.43 1.31 0.007 0.0011 0.30 0.0028 0.01 0.04 0 0.05 W: 0.16 N 0.13 0.41 1.35 0.011 0.0008 0.32 0.0025 0.02 0.11 0.01 0.14 Zr: 0.14 O 0.21 0.58 0.49 0.010 0.0007 0.60 0.0032 0.01 0.09 0.02 0.12 Sb: 0.11 P 0.14 0.37 1.28 0.009 0.0009 0.27 0.0030 0.02 0.01 0.11 0.14 Sn: 0.08 Q 0.15 0.34 1.25 0.011 0.0012 0.25 0.0031 0.01 0.01 0.12 0.14 Y: 0.03 R 0.17 0.40 1.27 0.007 0.0010 0.29 0.0028 0.01 0.02 0.08 0.11 Hf: 0.04 XA 0.03 0.36 1.16 0.011 0.0009 0.33 0.0026 0.12 0.01 0.01 0.14 XB 0.28 0.39 1.19 0.009 0.0010 0.31 0.0032 0.15 0.01 0.02 0.18 XC 0.11 0.74 1.28 0.010 0.0012 0.25 0.0035 0.13 0.02 0.01 0.16 XD 0.13 0.42 0.45 0.012 0.0008 0.27 0.0028 0.14 0.02 0.01 0.17 XE 0.14 0.41 1.82 0.008 0.0007 0.28 0.0025 0.02 0.11 0.02 0.15 XF 0.12 0.37 1.33 0.007 0.0011 0.73 0.0027 0.01 0.13 0.01 0.15 XG 0.15 0.35 1.30 0.011 0.0010 0.31 0.0031 0.24 0.01 0.01 0.26 XH 0.13 0.46 1.34 0.009 0.0008 0.33 0.0028 0.02 0.23 0.02 0.27 XI 0.12 0.43 1.26 0.008 0.0010 0.26 0.0032 0.01 0.01 0.25 0.27 XJ 0.14 0.34 1.27 0.012 0.0009 0.32 0.0034 0.06 0.09 0.08 0.23

division steel grade Hot-rolled steel winding temperature
(℃) Hot-rolled steel sheet heat treatment temperature
(℃) Hot-rolled steel sheet heat treatment time
(candle) Cold-rolled steel sheet heating rate
(℃/s) Cold rolled steel sheet reheating temperature
(℃) Reheating Cold Rolled Steel
holding time
(candle) Invention example 1 A 550 700 1200 10 750 120 Comparative Example 1 A 500 830 1300 10 750 120 Comparative Example 2 A 500 620 1500 10 780 90 Comparative Example 3 A 550 750 1800 10 780 120 Comparative Example 4 A 450 750 500 10 780 90 Comparative Example 5 A 400 700 1100 10 880 120 Comparative Example 6 A 400 750 900 10 700 100 Comparative Example 7 A 500 700 1200 10 830 30 Comparative Example 8 A 450 750 1000 10 830 120 Comparative Example 9 A 400 750 1400 10 750 120 Comparative Example 10 A 450 700 1200 10 750 120 Comparative Example 11 A 550 750 1100 10 750 90 Comparative Example 12 A 550 700 1400 10 750 120 Comparative Example 13 A 450 700 1200 10 750 100 Invention example 2 B 500 680 1400 10 750 100 Invention Example 3 C 350 700 1600 10 750 120 Invention example 4 D 550 770 700 10 750 90 Invention example 5 E 350 700 1100 10 750 90 Example 6 F 450 780 1000 10 750 100 Example 7 G 400 670 1300 10 840 120 Invention Example 8 H 500 700 1300 10 750 120 Inventive Example 9 I 550 750 1500 10 750 90 Inventive Example 10 J 550 750 1600 10 800 100 Inventive Example 11 K 550 700 1100 10 750 90 Inventive Example 12 L 450 700 1300 10 820 120 Inventive Example 13 M 400 700 700 10 750 100 Inventive Example 14 N 400 750 1200 10 750 90 Inventive Example 15 O 500 750 1100 10 840 90 Inventive Example 16 P 500 700 1300 10 750 120 Inventive Example 17 Q 450 750 1400 10 740 120 Inventive Example 18 R 450 700 1200 10 750 100 Comparative Example 14 XA 450 750 15000 10 780 100 Comparative Example 15 XB 500 750 1200 10 780 90 Comparative Example 16 XC 550 700 1200 10 780 120 Comparative Example 17 XD 550 700 1300 10 750 90 Comparative Example 18 XE 550 700 1000 10 750 120 Comparative Example 19 XF 550 750 1200 10 780 100 Comparative Example 20 XG 500 750 1400 10 750 120 Comparative Example 21 XH 500 700 1100 10 780 100 Comparative Example 22 XI 500 700 1200 10 750 90 Comparative Example 23 XJ 450 700 1300 10 750 120

division steel grade 1st average
cooling rate
(℃/s) 1st cooling
stop temperature
(℃) 2nd average
cooling rate
(℃/s) 2nd cooling
stop temperature
(℃) Secondary
holding time
(℃) 3rd average
cooling rate
(℃/s) Invention example 1 A 10 700 20 500 200 10 Comparative Example 1 A 10 650 20 500 200 10 Comparative Example 2 A 10 650 20 500 150 10 Comparative Example 3 A 10 700 20 500 200 10 Comparative Example 4 A 10 650 20 500 100 10 Comparative Example 5 A 10 700 20 500 200 10 Comparative Example 6 A 10 650 20 500 150 10 Comparative Example 7 A 10 700 20 500 100 10 Comparative Example 8 A 10 780 20 500 200 10 Comparative Example 9 A 10 580 20 500 150 10 Comparative Example 10 A 10 700 0.5 500 200 10 Comparative Example 11 A 10 700 20 570 200 10 Comparative Example 12 A 10 650 20 430 200 10 Comparative Example 13 A 10 650 20 500 30 10 Invention example 2 B 10 700 20 500 100 10 Invention example 3 C 10 650 20 500 200 10 Invention Example 4 D 10 740 20 500 200 10 Invention Example 5 E 10 700 20 500 150 10 Example 6 F 10 740 20 500 200 10 Example 7 G 10 700 20 530 100 10 Invention Example 8 H 10 620 20 500 200 10 Inventive Example 9 I 10 700 20 500 150 10 Inventive Example 10 J 10 650 20 530 200 10 Inventive Example 11 K 10 630 20 500 100 10 Inventive Example 12 L 10 700 20 500 200 10 Inventive Example 13 M 10 650 20 480 150 10 Inventive Example 14 N 10 700 20 480 200 10 Inventive Example 15 O 10 700 20 500 200 10 Inventive example 16 P 10 650 20 500 100 10 Inventive Example 17 Q 10 700 20 500 100 10 Inventive Example 18 R 10 700 20 500 200 10 Comparative Example 14 XA 10 700 20 500 150 10 Comparative Example 15 XB 10 650 20 500 200 10 Comparative Example 16 XC 10 650 20 500 200 10 Comparative Example 17 XD 10 700 20 500 200 10 Comparative Example 18 XE 10 700 20 500 150 10 Comparative Example 19 XF 10 650 20 500 200 10 Comparative Example 20 XG 10 700 20 500 200 10 Comparative Example 21 XH 10 700 20 500 100 10 Comparative Example 22 XI 10 650 20 500 200 10 Comparative Example 23 XJ 10 700 20 500 200 10

division microstructure (% area) mechanical properties recrystallization
ferrite non-redetermination
ferrite cementite YR TS ² ×√EL (MPa ² % ^0.5 ) TS ² ×√HER
(MPa ² % ^0.5 ) Invention example 1 81 7 12 0.87 2,135,084 3,024,325 Comparative Example 1 82.5 0.5 17 0.82 1,724,634 2,406,501 Comparative Example 2 83.7 0.3 16 0.84 1,588,125 2,274,842 Comparative Example 3 84.3 0.7 15 0.83 1,608,532 2,387,638 Comparative Example 4 85.4 0.6 14 0.82 1,713,228 2,269,006 Comparative Example 5 83 0 17 0.79 1,671,539 2,445,219 Comparative Example 6 75 14 11 0.96 2,430,054 3,926,502 Comparative Example 7 74 17 9 0.97 2,536,248 3,835,426 Comparative Example 8 86.4 0.6 13 0.83 1,730,150 2,323,005 Comparative Example 9 70 8 22 0.81 1,554,426 2,268,514 Comparative Example 10 72 5 23 0.82 1,696,522 2,434,269 Comparative Example 11 74 15 11 0.97 2,638,145 4,053,387 Comparative Example 12 85.7 0.3 14 0.83 1,569,897 2,332,060 Comparative Example 13 87.2 0.8 12 0.82 1,756,458 2,455,831 Invention example 2 80 9 11 0.85 2,037,548 3,196,147 Invention example 3 81 6 13 0.94 2,294,432 3,785,165 Invention example 4 97 2 One 0.89 2,183,055 3,432,584 Invention Example 5 84 7 9 0.81 1,811,246 2,520,614 Example 6 68 12 20 0.86 1,902,750 3,048,620 Example 7 95 3 2 0.90 2,063,284 2,601,423 Invention Example 8 86 6 8 0.82 2,139,551 3,174,562 Inventive Example 9 70 11 19 0.84 2,288,405 3,724,357 Inventive Example 10 81 7 12 0.87 1,936,524 2,836,520 Inventive Example 11 76 10 14 0.89 1,923,658 2,912,548 Inventive Example 12 82 8 10 0.93 1,823,042 2,528,027 Inventive Example 13 72 10 18 0.85 2,046,954 3,703,248 Inventive Example 14 82 6 12 0.88 2,135,745 3,203,865 Inventive Example 15 85 7 8 0.92 2,065,483 3,095,684 Inventive example 16 80 9 11 0.90 2,126,841 3,186,405 Inventive Example 17 83 8 9 0.87 1,935,687 2,889,421 Inventive Example 18 73 10 17 0.86 2,025,342 2,932,586 Comparative Example 14 94 2 4 0.76 1,452,301 2,096,328 Comparative Example 15 70 16 14 0.94 2,769,523 4,256,214 Comparative Example 16 81 7 12 0.89 1,635,204 2,352,735 Comparative Example 17 83 9 8 0.83 1,535,218 2,464,524 Comparative Example 18 74 15 11 0.92 2,589,245 4,095,168 Comparative Example 19 79 8 13 0.88 1,762,147 2,230,650 Comparative Example 20 73 15 12 0.96 2,466,325 3,968,057 Comparative Example 21 74 16 10 0.97 2,623,024 4,146,885 Comparative Example 22 70 19 11 0.96 2,500,362 3,813,402 Comparative Example 23 72 20 8 0.96 2,714,268 4,035,964

As can be seen from Tables 1 to 4, in the case of Inventive Examples 1 to 18, appropriate microstructures were secured according to the alloy composition and manufacturing conditions proposed by the present invention, and through this, excellent strength, formability and high yield ratio It can be seen that it has

On the other hand, in the case of Comparative Examples 1 to 23, since the alloy composition or manufacturing conditions proposed by the present invention were not satisfied, appropriate microstructures were not secured, and thus, mechanical properties were inferior.

Claims (18)

In % by weight, C: 0.05 to 0.25%, Si: 0.7% or less (excluding 0%), Mn: 0.46 to 1.8%, Al: 0.7% or less (excluding 0%), P: 0.05% or less (excluding 0%) excluding), S: 0.03% or less (excluding 0%), N: 0.03% or less (excluding 0%), the total amount of one or more of Ti, Nb, and V: 0.22% or less, the balance Fe and other unavoidable impurities include,
Microstructure is a high-strength steel sheet with excellent formability and high yield ratio, including non-recrystallized ferrite: 1-13%, recrystallized ferrite: 67-98%, and cementite: 1-20%, by area%.
The method of claim 1,
The steel sheet is a high-strength steel sheet having excellent formability and a high yield ratio, further comprising a total amount of at least one of Cr and Mo: 0.8% or less.
The method of claim 1,
The steel sheet is a high-strength steel sheet having excellent formability and a high yield ratio, further comprising a total amount of at least one of Cu and Ni: 0.8% or less.
The method of claim 1,
The steel sheet is a high-strength steel sheet having excellent formability and a high yield ratio further comprising B: 0.005% or less.
The method of claim 1,
The steel sheet is a high-strength steel sheet having excellent formability and a high yield ratio, further comprising a total amount of at least one of Ca, REM (excluding Y) and Mg: 0.05% or less.
The method of claim 1,
The steel sheet is a high-strength steel sheet having excellent formability and a high yield ratio, further comprising a total amount of at least one of W and Zr: 0.5% or less.
The method of claim 1,
The steel sheet is a high-strength steel sheet having excellent formability and a high yield ratio, further comprising a total amount of at least one of Sb and Sn: 0.5% or less.
The method of claim 1,
The steel sheet is a high-strength steel sheet having excellent formability and a high yield ratio, further comprising a total amount of at least one of Y and Hf: 0.2% or less.
The method of claim 1,
The steel sheet has tensile strength (TS): 610 MPa or more, yield ratio (YR): 0.8 to 0.95, tensile strength (TS) ² ×√ elongation (EL) is 1.8 × 10 ⁶ ~2.3 × 10 ⁶ MPa ² % ^0.5 and tensile Strength (TS) ² ×√hole expandability (HER): 2.5×10 ⁶ ~3.8×10 ⁶ MPa ² % ^0.5 High-strength steel sheet with excellent formability and high yield ratio.
In % by weight, C: 0.05 to 0.25%, Si: 0.7% or less (excluding 0%), Mn: 0.46 to 1.8%, Al: 0.7% or less (excluding 0%), P: 0.05% or less (excluding 0%) excluding), S: 0.03% or less (excluding 0%), N: 0.03% or less (excluding 0%), the total amount of one or more of Ti, Nb, and V: 0.22% or less, the balance Fe and other unavoidable impurities Heating the steel ingot or slab containing at 1000 ~ 1350 ℃;
Obtaining a hot-rolled steel sheet by hot-rolling the heated steel ingot or slab at a finish rolling temperature of 800 to 1000° C.;
winding the hot-rolled steel sheet at 300 to 600° C.;
heat-treating the rolled hot-rolled steel sheet at 650-800° C. for 600-1700 seconds;
Obtaining a cold-rolled steel sheet by cold-rolling the heat-treated hot-rolled steel sheet at a cold rolling reduction ratio of 30 to 90%;
Reheating the cold-rolled steel sheet at 720 to 860° C. and maintaining the cold-rolled steel sheet for 50 seconds or more;
firstly cooling the reheated and maintained cold-rolled steel sheet to 600 to 760° C. at an average cooling rate of 1° C./s or more;
Secondarily cooling the primary cooled cold-rolled steel sheet to 450 to 550 ° C at an average cooling rate of 2 ° C / s or more, and holding it for 50 seconds or more; and
Method for producing a high-strength steel sheet having excellent formability and a high yield ratio, comprising the step of tertiary cooling the secondary cooling and maintained cold-rolled steel sheet to room temperature.
The method of claim 10,
The steel ingot or slab is a method for producing a high-strength steel sheet having excellent formability and a high yield ratio further comprising a total amount of at least one of Cr and Mo: 0.8% or less.
The method of claim 10,
The steel ingot or slab is a method for producing a high-strength steel sheet having excellent formability and a high yield ratio further comprising a total amount of one or more of Cu and Ni: 0.8% or less.
The method of claim 10,
The steel ingot or slab is a method for producing a high-strength steel sheet having excellent formability and a high yield ratio further comprising B: 0.005% or less.
The method of claim 10,
The steel ingot or slab is a method for producing a high-strength steel sheet having excellent formability and a high yield ratio, further comprising a total amount of one or more of Ca, REM (excluding Y) and Mg: 0.05% or less.
The method of claim 10,
The steel ingot or slab is a method for producing a high-strength steel sheet having excellent formability and a high yield ratio further comprising a total amount of one or more of W and Zr: 0.5% or less.
The method of claim 10,
The steel ingot or slab is a method for producing a high-strength steel sheet having excellent formability and a high yield ratio further comprising a total amount of one or more of Sb and Sn: 0.5% or less.
The method of claim 10,
The steel ingot or slab is a method for producing a high-strength steel sheet having excellent formability and a high yield ratio, further comprising a total amount of at least one of Y and Hf: 0.2% or less.
The method of claim 10,
After the finish rolling, a method for producing a high-strength steel sheet having excellent formability and a high yield ratio, further comprising the step of cooling the hot-rolled steel sheet to the coiling temperature at an average cooling rate of 10 ° C / s or more.