KR102209569B1 - High strength and ductility steel sheet, and method for manufacturing the same - Google Patents

Abstract

The present invention relates to a steel plate that can be used for automobile parts, etc., and to a steel plate having excellent ductility and strength, and a method of manufacturing the same.

Description

High strength, high ductility steel sheet and its manufacturing method {HIGH STRENGTH AND DUCTILITY STEEL SHEET, AND METHOD FOR MANUFACTURING THE SAME}

The present invention relates to a steel plate that can be used for automobile parts, etc., and to a steel plate having excellent ductility and strength, and a method of manufacturing the same.

In recent years, the automobile industry is paying attention to measures to reduce material weight and secure occupant stability in order to protect the global environment. In order to meet these requirements for stability and weight reduction, the application of high-strength steel sheets is rapidly increasing. In general, the higher the strength of the steel sheet is, the lower the ductility and workability are. Therefore, in the steel sheet for automobile members, a steel sheet having excellent strength, ductility and workability is required.

As a technique for improving the ductility of a steel sheet, a method of utilizing tempered martensite is disclosed in Patent Documents 1 and 2. Tempered martensite, which is made by tempering hard martensite, is a softened martensite, and shows a difference in strength from existing untempered martensite (fresh martensite). When fresh martensite is suppressed and tempered martensite is formed, ductility and workability increase.

However, with the techniques disclosed in Patent Documents 1 and 2, the product of tensile strength and elongation (TS×El) does not satisfy 22,000 MPa% or more, which means that it is difficult to secure a steel sheet having excellent strength and ductility.

On the other hand, in order to obtain both high strength and excellent ductility and workability, the steel sheet for automobile members has developed TRIP (Transformation Induced Plasticity) steel using the transformation organic plasticity of retained austenite. In Patent Documents 3 and 4, TRIP steel having excellent ductility and workability is disclosed.

Patent Document 3 tried to improve ductility and workability, including polygonal ferrite, retained austenite, and martensite, but it was not able to secure high strength because bainite was used as the main phase, and TS×El was also 22,000. It can be seen that the MPa% or more is not satisfied.

In Patent Document 4, ferrite is formed, residual austenite is refined, and a complex structure including tempered martensite is formed to improve ductility and workability, but it is difficult to secure high strength because a large amount of soft ferrite is included. There is.

It is a situation that has not yet met the demand for a steel sheet having high strength and excellent ductility.

Korean Patent Application Publication No. 10-2006-0118602 Japanese Patent Application Publication No. 2009-019258 Korean Patent Application Publication No. 10-2014-0012167 Korean Patent Application Publication No. 10-2010-0092503

An aspect of the present invention is to provide a steel sheet having excellent ductility and strength by optimizing the composition and microstructure of the steel sheet and a method of manufacturing the same.

The subject of the present invention is not limited to the above. Additional problems of the present invention are described in the general contents of the specification, and those of ordinary skill in the art to which the present invention pertains will not have any difficulty in understanding the additional problems of the present invention from the contents described in the specification of the present invention.

One aspect of the present invention is by weight %, C: more than 0.25 to 0.75%, Si: 4.0% or less, Mn: 0.9 to 5.0%, Al: 5.0% or less, P: 0.15% or less, S: 0.03% or less, N : 0.03% or less, the rest contains Fe and inevitable impurities,

The microstructure includes tampered martensite, bainite and retained austenite,

It relates to a high-strength high-ductility steel sheet that satisfies the following [Relational Formula 1].

[Relationship 1]

0.55 ≤ [Si]γ / [Si]av ≤ 0.85

(However, [Si]γ is the Si content (wt%) in the retained austenite, and [Si]av is the Si content (wt%) in the steel sheet)

Advantageous Effects of Invention According to the present invention, a steel sheet having high strength and excellent ductility and processing characteristics can be provided for automobile structures requiring weight reduction and stability at the same time.

The inventors of the present invention aim to stabilize the retained austenite in the Transformation Induced Plasticity (TRIP) steel comprising bainite and tempered martensite, and the retained austenite size and It was recognized that strength, ductility, and workability were affected through shape. By clarifying this, a method for improving the ductility and workability of high-strength steel was devised, and the present invention was reached.

Hereinafter, the present invention will be described in detail. First, the alloy composition of the steel sheet of the present invention will be described in detail.

The steel sheet of the present invention is in% by weight (hereinafter, %), C: greater than 0.25 to 0.75%, Si: 4.0% or less, Mn: 0.9 to 5.0%, Al: 5.0% or less, P: 0.15% or less, S: 0.03 % Or less, N: 0.03% or less, and the remainder contains Fe and unavoidable impurities. Additionally, Ti: 0-0.5%, Nb: 0-0.5%, V: 0-0.5%, Cr: 0-3.0%, Mo: 0-3.0%, Cu: 0-4.5%, Ni: 0-4.5% , B: 0~0.005%, Ca: 0~0.05%, REM excluding Y: 0~0.05%, Mg: 0~0.05%, W: 0~0.5%, Zr: 0~0.5%, Sb: 0 It may include ~0.5%, Sn: 0 ~ 0.5%, Y: 0 ~ 0.2%, Hf: 0 ~ 0.2%, and Co: 0 ~ 1.5%. Hereinafter, each alloy composition will be described in detail.

Carbon (C): more than 0.25 ~ 0.75%

C is an indispensable element for imparting the strength of the steel sheet and is a stabilizing element of retained austenite that increases the ductility of the steel sheet. When the C content is 0.25% or less, it is difficult to secure the necessary tensile strength, and when it exceeds 0.75%, cold rolling is difficult and the steel sheet cannot be manufactured. Therefore, the content of C is preferably more than 0.25 ~ 0.75% or less. The content of C is more preferably 0.31 to 0.75%.

Silicon (Si): 4.0% or less (excluding 0)

Si is an element having an effect of improving strength by solid solution strengthening, and is an element that strengthens ferrite, uniformizes the structure, and improves workability. In addition, it is an element that contributes to the formation of retained austenite by suppressing the precipitation of cementite. When the Si content exceeds 4.0%, plating defects such as non-plating in the plating process and weldability of the steel sheet are deteriorated, so the content of Si is preferably 4.0% or less.

Aluminum (Al): 5.0% or less (excluding 0)

The Al is an element that acts on deoxidation by bonding with oxygen in steel. In addition, it is an element that stabilizes retained austenite by suppressing precipitation of cementite like Si. When the Al content exceeds 5.0%, the workability of the steel sheet is deteriorated and inclusions are increased. Therefore, the content of Al is preferably 0 to 5.0%.

On the other hand, the sum of Si and Al (Si+Al) is preferably 1.0 to 6.0%. The Si and Al are components that affect microstructure formation in the present invention and affect ductility and bending workability. Therefore, in order to have excellent ductility and bending workability, the sum of Si and Al is preferably 1.0 to 6.0%. More preferably, it contains 1.5 to 4.0%.

Manganese (Mn): 0.9~5.0%

The Mn is an element useful for enhancing both strength and ductility. The above effect can be obtained at 0.9% or more, but when it exceeds 5.0%, the weldability and impact toughness of the steel sheet are deteriorated. In addition, when Mn is included in an amount exceeding 5.0%, the bainite transformation time increases and the concentration of C in the austenite is insufficient, and thus the required residual austenite fraction cannot be secured. Therefore, the content of Mn is preferably 0.9 to 5.0%.

Phosphorus (P): 0.15% or less

P is an element that is contained as an impurity and deteriorates the impact toughness. Therefore, it is preferable to manage the P content to 0.15% or less.

Sulfur (S): 0.03% or less

S is an element that is contained as an impurity, makes MnS in the steel sheet, and deteriorates ductility. Therefore, the content of S is preferably 0.03% or less.

Nitrogen (N): 0.03% or less

The N is an element that is contained as an impurity and causes a crack in the slab by making nitride during continuous casting. Accordingly, the content of N is preferably 0.03% or less.

The rest contains Fe and impurities that are inevitably included. Meanwhile, the steel sheet of the present invention has an alloy composition that may be additionally included in addition to the above-described alloy components, which will be described in detail below.

Titanium (Ti): 0-0.5%, niobium (Nb): 0-0.5%, and vanadium (V): 0-0.5% at least one of

Ti, Nb, and V are elements that make precipitates and refine crystal grains. It may be contained in order to improve the strength and impact toughness of the steel sheet. When the content of each of Ti, Nb and V exceeds 0.5%, not only the impact toughness is lowered by formation of excessive precipitates, but also causes an increase in manufacturing cost, so the content is preferably 0.5% or less.

Chrome (Cr): 0-3.0% and molybdenum (Mo): 0-3.0% at least one of

Cr and Mo are elements that inhibit austenite decomposition during alloying treatment and stabilize austenite in the same manner as Mn. When the respective contents of Cr and Mo exceed 3.0%, the bainite transformation time increases, and the concentration of C in the austenite is insufficient, so that the required residual austenite fraction cannot be secured. Therefore, it is preferable that each content of Cr and Mo is 3.0% or less.

Copper (Cu): 0-4.5% and Nickel (Ni): 0-4.5% at least one of

The Cu and Ni are elements that stabilize austenite and inhibit corrosion. The Cu and Ni are concentrated on the surface of the steel plate to prevent hydrogen from moving into the steel plate, thereby suppressing the delayed hydrogen destruction. When each content of Cu and Ni exceeds 4.5%, it causes not only an excessive characteristic effect, but also an increase in manufacturing cost. Therefore, it is preferable that each content of Cu and Ni is 4.5% or less.

Boron (B): 0~0.005%

B is an element that improves hardenability to increase strength and suppresses nucleation of grain boundaries. When the content of B exceeds 0.005%, it causes an increase in manufacturing cost as well as an excessive characteristic effect. Therefore, the content of B is preferably 0.005% or less.

Calcium (Ca): 0 to 0.05%, magnesium (Mg): 0 to 0.05% and rare earth elements (REM) excluding yttrium (Y): 0 to 0.05% at least one of

The REM refers to a total of 17 elements of Sc, Y and lanthanoids. REM except for Ca, Mg, and Y can improve the ductility of the steel sheet by making the sulfide spherical. When each content of REM excluding Ca, Mg and Y exceeds 0.05%, it causes not only an excessive characteristic effect but also an increase in manufacturing cost. Therefore, it is preferable that each content of REM excluding Ca, Mg and Y is 0.05% or less.

Tungsten (W): 0 to 0.5% and zirconium (Zr): 0 to 0.5% at least one of

The W and Zr are elements that increase the strength of the steel sheet by improving the hardenability. When each content of W and Zr exceeds 0.5%, it causes not only an excessive characteristic effect but also an increase in manufacturing cost. Therefore, it is preferable that each content of W and Zr is 0.5% or less.

Antimony (Sb): 0-0.5% and tin (Sn): 0-0.5% at least one of

The Sb and Sn are elements that improve plating wettability and plating adhesion of the steel sheet. When each content of Sb and Sn exceeds 0.5%, the brittleness of the steel sheet increases, and cracks may occur during hot or cold working. Therefore, it is preferable that each content of Sb and Sn is 0.5% or less.

Yttrium (Y): 0 to 0.2% and hafnium (Hf): 0 to 0.2% at least one of

The Y and Hf are elements that improve the corrosion resistance of the steel sheet. When the respective contents of Y and Hf exceed 0.2%, the ductility of the steel sheet may be deteriorated. Therefore, it is preferable that each content of Y and Hf is 0.2% or less.

Cobalt (Co): 0~1.5%

The Co is an element that increases the TRIP effect by promoting bainite transformation. When the Co content exceeds 1.5%, weldability and ductility of the steel sheet may be deteriorated. Therefore, the content of Co is preferably 1.5% or less.

The microstructure of the steel sheet of the present invention includes tempered martensite, bainite and retained austenite. As a preferred example, by volume fraction, 30 to 75% of tempered martensite, 10 to 50% of bainite, 10 to 40% of residual austenite are included, and 5% or less of ferrite and other inevitable tissues are included. . The inevitable structure includes fresh martensite, pearlite, martensite austenite constituent (M-A), and the like. If the fresh martensite or pearlite is excessively formed, the ductility and workability of the steel sheet may be inferior, or the fraction of retained austenite may be reduced.

As shown in the following relational formula 1, the value obtained by dividing the Si content ([Si] γ, wt%) contained in the retained austenite by the Si content ([Si]av, wt%) contained in the steel sheet is 0.55 to 0.85 It is desirable.

[Relationship 1]

0.55 ≤ [Si]γ / [Si]av ≤ 0.85

The steel sheet of the present invention has a product of tensile strength and elongation (TS×El) of 22,000 MPa% or more, R/t (R is the minimum bending radius (mm) that does not cause cracks after a 90° bending test, and t is the steel sheet The thickness (mm)) is 0.5 to 3.0, and the balance between strength and ductility is excellent.

In the present invention, in order to secure not only high strength but also excellent ductility and workability, it is important to stabilize the retained austenite of the steel sheet. In order to stabilize retained austenite, it is necessary to concentrate C and Mn into austenite in ferrite, bainite, and tempered martensite of the steel sheet. However, if C is concentrated in austenite using ferrite, the strength of the steel sheet may be insufficient due to the low strength characteristics of ferrite. Therefore, it is preferable to use bainite and tempered martensite to concentrate C and Mn in austenite. In addition, by controlling the Si content ([Si]?) in the retained austenite, it is possible to enrich C and Mn in a large amount from bainite and tempered martensite into the retained austenite. Therefore, it is possible to stabilize the retained austenite by controlling Si in retained austenite. Accordingly, in the present invention, the retained austenite is stabilized by setting [Si]γ/[Si]av to 0.55 or more. However, if [Si]γ / [Si]av exceeds 0.85, the concentration of C and Mn in the retained austenite is insufficient and the residual austenite becomes unstable to tensile deformation, resulting in a decrease in ductility and workability, resulting in TS×El. It is less than 22,000 MPa%, or R/t exceeds 3.0, which is not preferable.

The steel sheet containing retained austenite has excellent ductility and workability due to the transformation organic plasticity that occurs during the transformation from austenite to martensite during processing. When the residual austenite of the steel sheet is less than 10%, TS×El may be less than 22,000 MPa% or R/t may exceed 3.0. On the other hand, when the residual austenite fraction exceeds 40%, local elingation may decrease. Therefore, in order to obtain a steel sheet excellent in both the balance of strength and ductility and workability, the fraction of retained austenite is preferably 10 to 40%.

On the other hand, untempered martensite (fresh martensite) and tempered martensite are both microstructures that improve the strength of the steel sheet. However, compared with tempered martensite, fresh martensite has a property of greatly reducing the ductility of the steel sheet. This is because the microstructure of the tempered martensite is not softened by the tempering heat treatment. Therefore, in order to provide a steel sheet having excellent strength and ductility balance and workability of the present invention, it is preferable to use tempered martensite. If the fraction (volume fraction) of the tempered martensite is less than 30%, it is difficult to secure more than 22,000 MPa% of TS×El, and if it exceeds 75%, ductility and workability are deteriorated, and TS×El is less than 22,000 MPa%. Or R/t is more than 3.0, which is not preferable.

In order to improve the balance of strength and ductility and workability of the steel sheet, it is preferable to appropriately contain bainite. When the bainite fraction (volume fraction) is 10% or more, TS×El may be 22,000 MPa% or more, and R/t may be 0.5 to 3.0. However, more than 50% of bainite relatively decreases the tempered martensite fraction, resulting in TS×El being less than 22,000 MPa%, which is undesirable.

Hereinafter, an example of a method of manufacturing the steel sheet of the present invention will be described in detail. In the steel sheet manufacturing method of the present invention, first, a steel ingot or steel slab having the above-described alloy composition is prepared, and the steel ingot or steel slab is heated to hot-rolled, and then annealing, winding, pickling, and cold rolling to prepare a cold-rolled steel sheet. .

As an example, it is preferable to heat the steel ingot or steel slab to a temperature of 1000 to 1350°C, and finish hot rolling at a temperature of 800 to 1000°C. When the heating temperature is less than 1000° C., there is a possibility to be hot-rolled below the finish hot-rolling temperature range. In addition, when the heating temperature exceeds 1350°C, it may reach the melting point of the steel and melt. On the other hand, when the finish hot rolling temperature is less than 800°C, a large burden may be placed on the rolling mill due to the high strength of the steel. In addition, when the finish hot rolling temperature exceeds 1000°C, the crystal grains of the steel sheet are coarse after hot rolling, and thus the physical properties of the high-strength steel sheet may be reduced. In order to refine the crystal grains of the hot-rolled steel sheet, it is preferable to cool at a cooling rate of 10°C/s or more after finishing hot rolling, and wind up at a temperature of 400 to 600°C. When the winding temperature is less than 400°C, winding is not easy, and when the winding temperature exceeds 600°C, a scale generated on the surface of the hot-rolled steel sheet is formed up to the inside of the steel sheet, making pickling difficult.

It is preferable to perform a hot rolling annealing heat treatment step in order to facilitate pickling and cold rolling after the winding. The hot rolling annealing heat treatment is preferably performed for 600 to 1700 seconds at a temperature range of 650 to 850°C. When the hot-rolling annealing heat treatment temperature is less than 650°C or less than 600 seconds, the strength of the hot-rolled annealing heat-treated steel sheet is high, so cold rolling may not be easy. On the other hand, if the hot rolling annealing heat treatment temperature exceeds 850°C or exceeds 1700 seconds, pickling may not be easy due to a scale deeply formed inside the steel sheet.

Meanwhile, in order to remove scale generated on the surface of the steel sheet after the winding, it is pickled and cold-rolled. The pickling and cold rolling conditions are not particularly limited, and the cold rolling is preferably set to a cumulative reduction ratio of 30 to 90%. If the cold rolling cumulative reduction ratio exceeds 90%, it may be difficult to perform cold rolling in a short time due to the high strength of the steel sheet.

The cold-rolled steel sheet may be produced as an unplated cold-rolled steel sheet through an annealing heat treatment process, or may be produced as a plated steel sheet through a plating process to impart corrosion resistance. For plating, a plating method such as hot-dip galvanizing, electro-zinc plating, or hot-dip aluminum plating can be applied, and the method and type are not particularly limited.

In order to secure high strength and excellent ductility and workability according to the present invention, an annealing heat treatment step is performed. Hereinafter, an example will be described in detail.

The cold-rolled steel sheet is heated over Ac3 (primary heating) and maintained for at least 50 seconds (primary holding).

When the primary heating or primary holding temperature is less than Ac3, ferrite may be formed, and bainite, retained austenite, and tempered martensite are not sufficiently formed, so that [Si]γ / [Si]av, TS of the steel sheet ×El can be reduced. In addition, when the primary holding time is less than 50 seconds, the structure cannot be sufficiently uniformed, and the physical properties of the steel sheet are deteriorated. The upper limit of the primary heating temperature and the upper limit of the primary oiling time are not particularly limited, but in order to suppress the decrease in toughness due to grain coarsening, the primary heating temperature is 950°C or less, and the primary holding time is 1200 seconds or less. It is preferable to do it.

After the first maintenance, it is preferable to cool (first cooling) to a temperature range of 100 to 300°C at an average cooling rate of 1°C/s or more. The upper limit of the primary cooling rate does not need to be specifically defined, and is preferably set to 100°C/s or less. When the primary cooling stop temperature is less than 100°C, the tempered martensite is excessively formed, and the residual austenite is insufficient, thereby reducing the [Si]γ / [Si]av, TS×El and bending workability of the steel sheet. I can. On the other hand, when the primary cooling stop temperature exceeds 300°C, bainite becomes excessive and tempered martensite is insufficient, so that TS×El of the steel sheet may be reduced.

After the first cooling, it is preferable to heat (secondary heating) to a temperature range of 300 to 500°C at a temperature increase rate of 5°C/s or more, and maintain (secondary maintenance) in this temperature range for 50 seconds or more. The upper limit of the heating rate does not need to be specifically defined, and is preferably 100°C/s or less. If the secondary heating or secondary holding temperature is less than 300°C or the holding time is less than 50 seconds, the tempered martensite becomes excessive and the Si and Al content control in the residual austenite is insufficient, so as to secure the residual austenite fraction. It is difficult. As a result, the [Si]γ/[Si]av, TS×El and bending workability of the steel sheet can be reduced. On the other hand, when the secondary heating or holding temperature exceeds 500°C or the holding time exceeds 172,000 seconds, control of the Si content in the residual austenite is insufficient, and it is difficult to secure a fraction of the residual austenite. As a result, the [Si]γ / [Si]av and TS×El of the steel sheet are reduced.

After the secondary maintenance, it is preferable to cool to room temperature (secondary cooling) at an average cooling rate of 1°C/s or more to room temperature.

Hereinafter, embodiments of the present invention will be described in detail. It should be noted that the following examples are only for understanding the present invention and are not intended to specify the scope of the present invention. The scope of the present invention is determined by the matters described in the claims and the matters reasonably inferred therefrom.

(Example)

A steel slab with a thickness of 100 mm having the alloy composition according to Table 1 (the rest are Fe and inevitable impurities) was prepared, heated at 1200°C, and then finished hot-rolled at 900°C, with an average of 30°C/s. It cooled at a cooling rate and wound at 500°C to prepare a hot-rolled steel sheet having a thickness of 3 mm. The hot rolled steel sheet was subjected to hot rolling annealing heat treatment under the conditions of Tables 2 and 3. Thereafter, after pickling to remove the surface scale, cold rolling was performed to a thickness of 1.5 mm.

Thereafter, heat treatment was performed under the annealing heat treatment conditions disclosed in Tables 2 to 5 to prepare a steel sheet.

The microstructure of the thus prepared steel sheet was observed and the results are shown in Tables 6 and 7. Among the microstructures, ferrite (F), bainite (B), tempered martensite (TM), and pearlite (P) were observed through SEM after nital etching of the polished specimen cross section. Among them, the fractions of bainite and tempered martensite, which are difficult to distinguish, were calculated using the expansion curve after dilation evaluation. On the other hand, since fresh martensite (FM) and retained austenite (residual γ) are also difficult to distinguish, the fraction of martensite and retained austenite observed by the SEM was subtracted from the fraction of retained austenite calculated by the X-ray diffraction method. The value was determined as the fresh martensite fraction.

Meanwhile, [Si]γ / [Si]av, TS×El, and R/t of the prepared steel sheet were observed, and the results are shown in Tables 8 and 9.

The Si content ([Si]γ) contained in the retained austenite was determined in the Si content measured in the retained austenite phase using an Electron Probe MicroAnalyser (EPMA). The [Si]av means the average Si content of the entire steel sheet.

The TS×El and R/t were evaluated by a tensile test and a V-bending test. Tensile test was carried out with a test piece taken in accordance with JIS No. 5 standard based on the direction of 90° to the rolling direction of the rolled sheet, and the TS×El was determined. R/t was determined as the value obtained by dividing the minimum bending radius R at which no crack occurs after a 90° bending test by taking a specimen based on the 90° direction with respect to the rolling direction of the rolled plate by the thickness t of the plate.

Steel grade Chemical composition (% by weight) C Si Mn P S Al N Cr Mo Etc A 0.39 1.98 2.13 0.011 0.0008 0.02 0.0032 0.51 B 0.38 2.03 2.21 0.010 0.0013 0.02 0.0028 0.23 0.18 C 0.37 1.95 1.88 0.010 0.0010 0.02 0.0029 0.47 D 0.33 2.31 3.95 0.009 0.0012 0.03 0.0030 0.49 E 0.41 1.85 2.06 0.008 0.0009 0.03 0.0031 F 0.52 1.68 2.33 0.009 0.0008 0.02 0.0027 G 0.72 1.64 2.41 0.012 0.0011 0.02 0.0034 H 0.38 0.87 2.11 0.011 0.0010 1.93 0.0033 I 0.36 1.08 2.07 0.011 0.0013 2.35 0.0031 J 0.35 0.02 1.95 0.010 0.0010 4.67 0.0030 Ti: 0.05 K 0.43 1.74 1.93 0.008 0.0011 0.02 0.0035 Nb: 0.05 L 0.41 1.89 1.88 0.009 0.0011 0.02 0.0028 V: 0.05 M 0.39 1.75 1.92 0.011 0.0012 0.02 0.0027 Ni: 0.36 N 0.38 1.89 2.18 0.012 0.0013 0.03 0.0024 Cu: 0.35 O 0.38 1.68 2.22 0.013 0.0007 0.03 0.0028 B: 0.003 P 0.36 1.88 2.26 0.012 0.0008 0.02 0.0026 Ca: 0.002 Q 0.37 1.84 2.37 0.008 0.0009 0.02 0.0031 REM: 0.001 R 0.44 1.73 2.45 0.009 0.0009 0.02 0.0031 Mg: 0.001 S 0.42 1.77 2.38 0.010 0.0010 0.02 0.0034 W: 0.11 T 0.31 1.95 2.19 0.010 0.0011 0.02 0.0033 Zr: 0.10 U 0.32 1.98 2.03 0.009 0.0013 0.03 0.0032 Sb: 0.02 V 0.39 1.82 2.41 0.008 0.0012 0.02 0.0030 Sn: 0.02 W 0.36 1.78 2.26 0.009 0.0012 0.02 0.0027 Y: 0.01 X 0.37 3.64 2.14 0.009 0.0007 0.03 0.0029 Hf: 0.01 Y 0.37 2.27 2.18 0.011 0.0007 0.03 0.0028 Co: 0.35 XA 0.21 1.92 2.05 0.011 0.0008 0.03 0.0024 XB 0.78 1.94 2.11 0.008 0.0011 0.02 0.0031 XC 0.39 0.02 2.16 0.012 0.0012 0.03 0.0027 XD 0.38 4.26 2.07 0.012 0.0009 0.02 0.0032 XE 0.40 0.03 2.31 0.008 0.0010 5.31 0.0026 XF 0.41 1.84 0.75 0.009 0.0010 0.02 0.0033 XG 0.38 1.88 5.64 0.011 0.0012 0.02 0.0031 XH 0.38 1.96 2.20 0.010 0.0011 0.02 0.0030 3.38 XI 0.36 1.89 2.08 0.009 0.0010 0.02 0.0027 3.41

division number Steel grade Hot rolled steel sheet
Winding temperature
(℃) Hot rolled steel sheet
Annealing temperature
(℃) Hot rolled steel sheet
Annealing time
(s) 1st average
Heating rate
(℃/s) Primary maintenance
Temperature
(℃) Primary maintenance
time
(s) Invention example One A 500 750 1500 10 880 120 Comparative example 2 A 500 900 1300 Poor pickling Comparative example 3 A 550 600 1600 Breakage occurs during cold rolling Invention example 4 B 500 750 1200 10 880 120 Comparative example 5 B 500 750 1800 Poor pickling Comparative example 6 B 450 700 500 Breakage occurs during cold rolling Comparative example 7 B 500 750 1600 10 730 120 Comparative example 8 B 450 750 1400 10 880 One Comparative example 9 B 500 700 1200 10 880 120 Invention example 10 B 450 750 1100 10 880 120 Invention example 11 B 500 750 1400 10 880 120 Invention example 12 C 500 700 1300 10 880 120 Comparative example 13 C 550 750 1200 10 880 120 Comparative example 14 C 450 700 800 10 880 120 Comparative example 15 C 500 750 900 10 880 120 Comparative example 16 C 500 750 1500 10 880 120 Comparative example 17 C 500 800 1600 10 880 120 Comparative example 18 C 450 750 1500 10 880 120 Invention example 19 D 500 750 1200 10 880 120 Invention example 20 E 450 700 1700 10 880 120 Invention example 21 F 500 700 600 10 880 120 Invention example 22 G 500 750 1500 10 880 120 Invention example 23 H 550 750 1500 10 880 120 Invention example 24 I 500 850 1300 10 880 120 Invention example 25 J 500 650 1200 10 880 120

division number Steel grade Hot rolled steel sheet
Winding temperature
(℃) Hot rolled steel sheet
Annealing temperature
(℃) Hot rolled steel sheet
Annealing time
(s) 1st average
Heating rate
(℃/s) Primary maintenance
Temperature
(℃) Primary maintenance
time
(s) Invention example 26 K 500 750 1400 10 880 120 Invention example 27 L 450 800 1600 10 880 120 Invention example 28 M 500 700 1200 10 880 120 Invention example 29 N 500 750 900 10 880 120 Invention example 30 O 550 700 1300 10 880 120 Invention example 31 P 450 750 1000 10 880 120 Invention example 32 Q 500 700 1200 10 880 120 Invention example 33 R 500 750 1500 10 880 120 Invention example 34 S 450 750 1300 10 880 120 Invention example 35 T 500 700 1100 10 880 120 Invention example 36 U 500 800 1400 10 880 120 Invention example 37 V 550 750 800 10 880 120 Invention example 38 W 450 750 1200 10 880 120 Invention example 39 X 500 700 900 10 880 120 Invention example 40 Y 550 800 1400 10 880 120 Comparative example 41 XA 500 750 1000 10 880 120 Comparative example 42 XB 500 750 1100 10 880 120 Comparative example 43 XC 450 750 1500 10 880 120 Comparative example 44 XD 500 750 1400 10 880 120 Comparative example 45 XE 500 700 1200 10 880 120 Comparative example 46 XF 550 800 1100 10 880 120 Comparative example 47 XG 500 750 1500 10 880 120 Comparative example 48 XH 450 700 1500 10 880 120 Comparative example 49 XI 500 750 1300 10 880 120

division number Steel grade 1st average
Cooling rate
(℃/s) 1st cooling
Stop temperature
(℃) Quadratic mean
Heating rate
(℃/s) 2nd maintenance
Temperature
(℃) 2nd maintenance
time
(s) Quadratic mean
Cooling rate
(℃/s) Invention example One A 20 200 15 400 300 10 Comparative example 2 A Poor pickling Comparative example 3 A Breakage occurs during cold rolling Invention example 4 B 20 220 15 400 300 10 Comparative example 5 B Poor pickling Comparative example 6 B Breakage occurs during cold rolling Comparative example 7 B 20 200 15 400 300 10 Comparative example 8 B 20 210 15 400 600 10 Comparative example 9 B 0.5 230 15 350 300 10 Invention example 10 B 20 200 15 400 300 10 Invention example 11 B 20 220 15 450 300 10 Invention example 12 C 20 200 15 400 300 10 Comparative example 13 C 20 70 15 400 300 10 Comparative example 14 C 20 330 15 400 600 10 Comparative example 15 C 20 180 15 270 300 10 Comparative example 16 C 20 190 15 530 300 10 Comparative example 17 C 20 200 15 400 40 10 Comparative example 18 C 20 220 15 400 172,800 10 Invention example 19 D 20 200 15 400 300 10 Invention example 20 E 20 280 15 450 600 10 Invention example 21 F 20 200 15 400 300 10 Invention example 22 G 20 130 15 340 300 10 Invention example 23 H 20 260 15 470 300 10 Invention example 24 I 20 210 15 400 300 10 Invention example 25 J 20 180 15 400 300 10

division number Steel grade 1st average
Cooling rate
(℃/s) 1st cooling
Stop temperature
(℃) Quadratic mean
Heating rate
(℃/s) 2nd maintenance
Temperature
(℃) 2nd maintenance
time
(s) Quadratic mean
Cooling rate
(℃/s) Invention example 26 K 20 200 15 400 300 10 Invention example 27 L 20 200 15 400 300 10 Invention example 28 M 20 210 15 400 300 10 Invention example 29 N 20 220 15 400 600 10 Invention example 30 O 20 200 15 450 300 10 Invention example 31 P 20 200 15 400 300 10 Invention example 32 Q 20 180 15 400 300 10 Invention example 33 R 20 220 15 400 300 10 Invention example 34 S 20 210 15 400 300 10 Invention example 35 T 20 200 15 350 300 10 Invention example 36 U 20 190 15 450 300 10 Invention example 37 V 20 200 15 400 600 10 Invention example 38 W 20 220 15 400 300 10 Invention example 39 X 20 180 15 400 300 10 Invention example 40 Y 20 200 15 350 300 10 Comparative example 41 XA 20 200 15 400 300 10 Comparative example 42 XB 20 200 15 400 300 10 Comparative example 43 XC 20 190 15 400 300 10 Comparative example 44 XD 20 180 15 400 300 10 Comparative example 45 XE 20 200 15 450 600 10 Comparative example 46 XF 20 220 15 400 300 10 Comparative example 47 XG 20 200 15 400 300 10 Comparative example 48 XH 20 200 15 450 300 10 Comparative example 49 XI 20 210 15 400 300 10

division number Steel grade ferrite
(vol.%) Bainite
(vol.%) Tempord
Martensite
(vol.%) Fresh
Martensite (vol.%) Residual
Austenite
(vol.%) Pearlite
(vol.%) Invention example One A 0 23 58 0 19 0 Comparative example 2 A Poor pickling Comparative example 3 A Breakage occurs during cold rolling Invention example 4 B 0 22 57 0 21 0 Comparative example 5 B Poor pickling Comparative example 6 B Breakage occurs during cold rolling Comparative example 7 B 28 7 5 0 3 57 Comparative example 8 B 19 5 62 6 8 0 Comparative example 9 B 17 13 57 0 5 8 Invention example 10 B 0 20 56 One 23 0 Invention example 11 B 0 19 59 0 22 0 Invention example 12 C 0 25 54 0 21 0 Comparative example 13 C 0 6 86 0 8 0 Comparative example 14 C 0 71 8 One 20 0 Comparative example 15 C 0 14 80 0 6 0 Comparative example 16 C 0 22 69 One 8 0 Comparative example 17 C 0 13 81 One 5 0 Comparative example 18 C 0 26 65 2 7 0 Invention example 19 D 0 18 59 0 23 0 Invention example 20 E 0 21 60 0 19 0 Invention example 21 F 0 22 57 One 20 0 Invention example 22 G 0 24 51 One 24 0 Invention example 23 H 0 19 61 0 20 0 Invention example 24 I 0 42 37 2 19 0 Invention example 25 J 0 17 48 0 35 0

division number Steel grade ferrite
(vol.%) Bainite
(vol.%) Tempord
Martensite
(vol.%) Fresh
Martensite (vol.%) Residual
Austenite
(vol.%) Pearlite
(vol.%) Invention example 26 K 0 21 57 0 22 0 Invention example 27 L 0 19 58 0 23 0 Invention example 28 M 0 22 60 0 18 0 Invention example 29 N 0 17 61 One 21 0 Invention example 30 O 0 25 56 One 18 0 Invention example 31 P 0 28 60 0 12 0 Invention example 32 Q 0 23 58 0 19 0 Invention example 33 R 0 18 57 0 25 0 Invention example 34 S 0 22 59 One 18 0 Invention example 35 T 0 24 56 0 20 0 Invention example 36 U 0 20 58 0 22 0 Invention example 37 V 0 23 57 One 19 0 Invention example 38 W 0 18 47 0 35 0 Invention example 39 X 0 21 56 One 22 0 Invention example 40 Y 0 15 61 0 24 0 Comparative example 41 XA 0 19 67 0 14 0 Comparative example 42 XB 0 23 22 12 43 0 Comparative example 43 XC 0 24 71 One 4 0 Comparative example 44 XD 0 19 43 19 19 0 Comparative example 45 XE 0 20 46 16 18 0 Comparative example 46 XF 0 21 67 0 7 5 Comparative example 47 XG 0 15 51 13 21 0 Comparative example 48 XH 0 16 49 18 17 0 Comparative example 49 XI 0 19 52 14 15 0

division number Steel grade [Si]γ/[Si]av TSХEL (MPa%) R/t Invention example One A 0.69 32148 1.86 Comparative example 2 A Poor pickling Comparative example 3 A Breakage occurs during cold rolling Invention example 4 B 0.75 28965 1.94 Comparative example 5 B Poor pickling Comparative example 6 B Breakage occurs during cold rolling Comparative example 7 B 0.92 15615 1.57 Comparative example 8 B 0.94 26582 4.45 Comparative example 9 B 0.91 20371 1.48 Invention example 10 B 0.76 31105 2.68 Invention example 11 B 0.73 29022 1.27 Invention example 12 C 0.57 31505 1.84 Comparative example 13 C 0.90 15740 5.62 Comparative example 14 C 0.82 20583 2.35 Comparative example 15 C 0.93 13481 7.81 Comparative example 16 C 0.97 21547 2.73 Comparative example 17 C 0.92 17554 9.34 Comparative example 18 C 0.94 20968 2.27 Invention example 19 D 0.71 27840 1.61 Invention example 20 E 0.77 30287 1.94 Invention example 21 F 0.79 32158 2.17 Invention example 22 G 0.80 29623 2.08 Invention example 23 H 0.76 28074 2.36 Invention example 24 I 0.72 29336 1.83 Invention example 25 J 0.73 31824 1.50

division number Steel grade [Si]γ/[Si]av TSХEL (MPa%) R/t Invention example 26 K 0.77 30546 1.34 Invention example 27 L 0.72 31058 1.81 Invention example 28 M 0.70 28145 2.09 Invention example 29 N 0.75 29652 1.47 Invention example 30 O 0.68 30082 1.65 Invention example 31 P 0.71 32147 1.02 Invention example 32 Q 0.74 28638 2.64 Invention example 33 R 0.67 30186 1.82 Invention example 34 S 0.73 30625 2.38 Invention example 35 T 0.76 29310 2.19 Invention example 36 U 0.65 27851 2.47 Invention example 37 V 0.72 29507 2.35 Invention example 38 W 0.81 28653 1.84 Invention example 39 X 0.74 31068 2.08 Invention example 40 Y 0.78 30241 1.92 Comparative example 41 XA 0.80 16723 2.21 Comparative example 42 XB 0.76 21105 9.25 Comparative example 43 XC 0.93 12638 5.38 Comparative example 44 XD 0.73 29525 6.33 Comparative example 45 XE 0.68 28036 5.94 Comparative example 46 XF 0.91 16254 2.76 Comparative example 47 XG 0.78 24852 4.52 Comparative example 48 XH 0.64 23086 6.61 Comparative example 49 XI 0.72 25128 7.08

As shown in Tables 1 to 9, in the case of the invention examples satisfying the conditions presented in the present invention, the values of [Si]γ / [Si]av are included in the range of 0.55 to 0.85, and TS × El is 22,000 MPa % Or more, and R/t is included in the range of 0.5 to 3.0, and it can be seen that while having excellent strength, excellent ductility and workability.

However, the comparative examples of Nos. 2 to 6 overlapped with the alloy composition range of the present invention, but the hot rolling annealing temperature and time after hot rolling were outside the ranges suggested in the present invention, so that pickling failure occurred or fracture occurred during cold rolling. I could confirm.

On the other hand, Comparative Example No. 7 had low primary heating or holding temperature in the annealing heat treatment process after cold rolling, and ferrite was excessively formed, and the bainite and tempered martensite fractions were insufficient, so that [Si]γ / [Si ]av exceeded 0.85 and TS×El was less than 22,000 MPa%. In Comparative Example No. 8, the first holding time was short and the structure became uneven, the ferrite fraction was excessively formed, and the bainite and retained austenite fractions were insufficient. As a result, [Sil]γ / [Si]av exceeded 0.85, and R/t exceeded 3.0. No. In Comparative Example 9, since the primary cooling rate was low, ferrite was excessively formed, and the residual austenite fraction was insufficient, so that [Si]γ / [Si]av exceeded 0.85 and TS×El was less than 22,000 MPa%. .

In addition, the comparative example of No. 13 was formed excessively with tempered martensite due to the low primary cooling stop temperature, and the residual austenite fraction was insufficient, so that [Si]γ / [Si]av exceeded 0.85, and TS× El is less than 22,000 MPa%, and R/t exceeds 3.0. In Comparative Example No. 14, the primary cooling stop temperature was higher than that suggested in the present invention, so that bainite was excessively formed, and the formation of tempered martensite was insufficient. As a result, TS x El was less than 22,000 MPa%.

Comparative Examples of Nos. 15 and 16 are cases where the secondary heating or holding temperature is low or high, and residual austenite is not formed in an appropriate range, and [Si] γ / [Si] av exceeds 0.85, and TS It can be seen that x El is less than 22,000 MPa%. In particular, in the case of No. 15, the tempered martensite was also excessively formed, and the R/t exceeded 3.0.

Comparative examples of Nos. 17 and 18 are cases in which the secondary holding time is insufficient or excessive. In the comparative example of No. 17, the tempered martensite is excessively formed and the residual austenite is insufficient, so that [Si]γ/ [Si]av exceeded 0.85, TS×El became less than 22,000 MPa%, and R/t exceeded 3.0. In the case of No. 18, residual austenite was insufficient, and [Si]γ/[Si]av exceeded 0.85, and TS×El was found to be less than 22,000 MPa%.

The comparative examples of Nos. 41 to 49 are when the manufacturing conditions suggested by the present invention are satisfied or out of the alloy composition range. In these cases, it can be seen that the conditions of [Si]γ / [Si]av, TS×El, and R/t of the present invention are not satisfied. Meanwhile, No. Comparative Example 43 is a case where the sum of Si and Al (Al+Si) is less than 1.0% in the alloy composition of the present invention, and satisfies all the conditions of [Si]γ / [Si]av, TS×El, and R/t. You can see what you can't do.

Claims (5)

In% by weight, C: more than 0.25 to 0.75%, Si: 4.0% or less, Mn: 0.9 to 5.0%, Al: 5.0% or less, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, the rest Contains Fe and unavoidable impurities,
The microstructure, by volume fraction, contains 30 to 61% of tempered martensite, 10 to 50% of bainite, 12 to 40% of retained austenite, 5% or less of ferrite, and inevitable structure,
Satisfying the following [Relational Expression 1]
The product of tensile strength and elongation (TS×El) is 22,000 MPa% or more, R/t (R is the minimum bending radius (mm) that does not cause cracks after 90° bending test, and t is the thickness of the steel plate (mm) Is a high-strength, high-ductility steel sheet of 0.5 to 3.0.
[Relationship 1]
0.55 ≤ [Si]γ / [Si]av ≤ 0.85
(However, [Si]γ is the Si content (wt%) in the retained austenite, and [Si]av is the Si content (wt%) in the steel sheet)
The method according to claim 1,
The steel sheet is a high strength, high ductility steel sheet further comprising any one or more of the following (1) to (9).
(1) At least one of Ti: 0~0.5%, Nb: 0~0.5%, and V: 0~0.5%
(2) Cr: 0 to 3.0% and Mo: at least one of 0 to 3.0%
(3) Cu: 0-4.5% and Ni: 0-4.5% at least one of
(4) B: 0~0.005%
(5) Ca: 0~0.05%, REM excluding Y: 0~0.05% and Mg: at least one of 0~0.05%
(6) One or more of W: 0~0.5% and Zr: 0~0.5%
(7) Sb: 0 to 0.5% and Sn: at least one of 0 to 0.5%
(8) One or more of Y: 0~0.2% and Hf: 0~0.2%
(9) Co: 0~1.5%
The method according to claim 1,
The total amount of Si and Al (Si+Al) is 1.0 to 6.0% of a high strength, high ductility steel sheet.
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