KR102020390B1 - High-strength steel sheet having excellent formability, and method for manufacturing thereof - Google Patents

Abstract

One aspect of the present invention is to provide a high-strength steel sheet excellent in formability by securing a high yield strength and ductility and a manufacturing method thereof.

Description

High strength steel sheet with excellent formability and manufacturing method thereof {HIGH-STRENGTH STEEL SHEET HAVING EXCELLENT FORMABILITY, AND METHOD FOR MANUFACTURING THEREOF}

The present invention relates to a high strength steel sheet used in a chassis structural member of an automobile, and more particularly, to a high strength steel sheet excellent in formability and a method of manufacturing the same.

Recently, according to the regulation of carbon dioxide to reduce global warming, it is strongly required to reduce the weight of the vehicle, and at the same time, ultra high strength of the steel sheet for automobiles is continuously made to improve the collision stability of the vehicle.

In general, hot rolled steel is pickled and applied to the chassis parts of a vehicle such as a lower arm and a wheel disc, and since these parts support a vehicle body, the strength of the parts must be excellent and at the same time due to fatigue during driving. To prevent breakage, the fatigue characteristics should be excellent.

In order to produce steel sheet for chassis parts of automobiles, most of them utilize low temperature transformation tissue. However, when the low temperature transformation structure is used to secure high strength and fatigue properties, it is difficult to secure an elongation of 30% or more at a tensile strength of 600 MPa or more. For this reason, since it is difficult to apply also to manufacture a complicated shape part by cold press molding, there exists a limit to the free part design suitable for a desired use.

On the other hand, as a way to secure the strength and molding of the steel at the same time, a large amount of austenite stabilizing elements such as carbon (C) and manganese (Mn) is added to form the structure of the steel in the austenite single phase, twins generated during deformation Has been proposed to secure strength and formability at the same time (Patent Document 1).

However, the conventional high manganese steel containing a large amount of C and Mn has a disadvantage in that the application is limited when used as a vehicle material due to its low yield strength and inferior collision characteristics.

Therefore, there is a need for development of an automotive material having excellent ductility so as to be able to cold molding with excellent yield strength.

Korean Patent Registration No. 10-1726093

One aspect of the present invention is to provide a high-strength steel sheet excellent in formability by securing a high yield strength and ductility and a manufacturing method thereof.

However, the subject of this invention is not limited to the content mentioned above. The problem of the present invention will be understood from the general contents of the present specification, those skilled in the art will have no difficulty understanding the additional problem of the present invention.

One aspect of the present invention, in weight%, carbon (C): 0.1-1.0%, manganese (Mn): 10-26%, aluminum (Al): 0.01-2.0%, chromium (Cr): 0.001-6.0% , Vanadium (V): 0.001 ~ 1.0%, molybdenum (Mo): 0.001 ~ 1.0%, the balance Fe and other unavoidable impurities, the Cr, V and Mo satisfy the following relation 1, austenitic as a microstructure It provides a high strength steel sheet comprising a phase with an area fraction of at least 90%, and excellent in moldability in which the fraction of unrecrystallized structure in the austenite phase is at least 10%.

[Relationship 1]

1.0% <Cr + Mo + V <6.0%

(Here, each element means a weight content.)

Another aspect of the present invention, the method comprising the steps of reheating the steel slab satisfying the above-described alloy composition and relation 1 to the temperature (RT) and time (Rt) satisfying the following relations 2 and 3; Manufacturing a hot rolled steel sheet by hot rolling after the reheating; And winding the hot rolled steel sheet to a temperature range of 650 ° C. or less at a cooling rate of 5 ° C./s or more, wherein the hot rolling is finish rolled to satisfy the following relational expression 4, and a high strength steel sheet having excellent formability. It provides a method of manufacturing.

[Relationship 2]

Reheating temperature (RT, ℃) ≤ 1500-207 [C]-5 [Mn]-3 [Cr]-34 [Mo]-44 [V]

[Relationship 3]

0.9 × (Slab Thickness, mm) <Reheat Time (Rt, min) <1.25 × (Slab Thickness, mm)

[Relationship 4]

Finish rolling start temperature (FST, ℃) <546 + 325 [C]-1.8 [Mn] + 17.4 [Cr] + 28.8 [Al] + 82.4 [Mo] + 213.2 [V]

(In the above relations 2 to 4, each element means a weight content.)

According to the present invention, it is possible to provide a steel sheet having high strength and high moldability from optimization of alloy composition and manufacturing conditions.

In particular, the high yield strength can improve the impact resistance, and due to the excellent ductility and bending performance not only advantageous to cold formability, but also excellent fatigue characteristics can be secured, such as parts requiring machining into complex shapes, etc. It can apply suitably.

Figure 1 shows a conceptual diagram of the precipitation strengthening effect by the addition of Cr, Mo and V in one aspect of the present invention. Among these, (a) shows a conceptual diagram in which the precipitation start temperature is reduced and the precipitate fraction increases at the same time when Cr, Mo, and V are added in combination, and the precipitation start temperature is reduced compared to the individual addition of each element, and (b) is Cr, Mo. And the concept of increasing the recrystallization temperature at the same rolling temperature by increasing the recrystallization temperature when the complex addition of V.
Figure 2 shows a slab shape in the case of leaving the relation 2 of the present invention in one embodiment of the present invention.
Figure 3 shows the results of observing the microstructure of the invention example in a transmission electron microscope in one embodiment of the present invention.

The inventors of the present invention can secure the strength and formability by maintaining the microstructure at austenite at room temperature with the addition of a large amount of carbon and manganese in the case of existing high manganese steel, while the yield strength is low inferior collision performance and fatigue performance problem It was recognized that there is. Therefore, in order to improve the yield strength and ductility of the steel as well as the strength and formability, in order to provide a steel sheet excellent in collision performance and fatigue performance was studied in depth.

As a result, by optimizing the alloy composition and manufacturing conditions to form a microstructure that is advantageous for securing the desired physical properties, it is possible to provide a high manganese steel sheet with excellent yield strength and high impact performance, while at the same time significantly improving fatigue performance. It was confirmed that the present invention was completed.

In particular, the present invention has a technical significance in controlling the content of elements advantageous for stabilizing austenite structure in the alloy composition and forming a large amount of unrecrystallized structure.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

High strength steel sheet excellent in formability according to an aspect of the present invention by weight% carbon (C): 0.1 ~ 1.0%, manganese (Mn): 10 ~ 26%, aluminum (Al): 0.01 ~ 2.0%, chromium (Cr ): 0.001 ~ 6.0%, vanadium (V): 0.001 ~ 1.0%, molybdenum (Mo): may include 0.001 ~ 1.0%.

Hereinafter, the reason for controlling the alloy composition of the high strength steel sheet of the present invention as described above will be described in detail. At this time, unless otherwise specified, the content of each alloy composition means weight%.

C: 0.1 ~ 1.0%

Carbon (C) is an element contributing to stabilization of the austenite phase, and it is advantageous to secure the austenite phase as its content increases. In addition, C increases the stacking fault energy (SFE) of the steel to improve strength and ductility at the same time.

If the content of C is less than 0.1%, the α '(alpha) -martensite phase is formed on the surface layer by decarburization during the high temperature processing of the steel sheet, resulting in inferior delay fracture and fatigue performance. In addition, it becomes difficult to secure strength and ductility to a target level. On the other hand, if the content exceeds 1.0%, there is a fear that the weldability is inferior because the electrical resistivity increases.

Therefore, in the present invention, C may be included in 0.1 to 1.0%, more preferably in 0.3 to 0.9%.

Mn: 10-26%

Manganese (Mn) is an element that stabilizes the austenite phase together with carbon (C).

If the content of such Mn is less than 10%, the α '(alpha) -martensite phase is formed during deformation, making it difficult to secure a stable austenite phase. On the other hand, if the content exceeds 26%, the strength improving effect is saturated, rather there is a problem that the manufacturing cost rises.

Therefore, in the present invention, Mn may be contained in 10 to 26%, more advantageously 13 to 24%.

Al: 0.01 ~ 2.0%

Aluminum (Al) is usually added for deoxidation of steel, but in one aspect of the present invention, the stack defect energy (SFE) of the steel is increased to suppress the formation of ε (epsilon) -martensite, thereby improving the ductility and delayed fracture resistance of the steel. Play a role.

If the Al content is less than 0.01%, the ductility of the steel is rather deteriorated due to the rapid work hardening phenomenon, resulting in inferior delayed fracture characteristics. On the other hand, if the content exceeds 2.0%, the strength is lowered, the castability is inferior, and the surface quality is deteriorated by forming an oxide on the steel surface during hot rolling.

Therefore, in the present invention, Al may be included in an amount of 0.01% to 2.0%, more preferably 0.051% to 2.0%, and even more advantageously 0.1% to 1.9%.

Cr: 0.001-6.0%

Chromium (Cr) is an effective element for improving the surface properties and the strength of steel. In one aspect of the present invention there is an effect of lowering the temperature at the start of precipitation of carbonitride, which has a major effect on precipitation strengthening. More specifically, it is possible to further increase the precipitation strengthening effect compared to steel that does not contain Cr by decreasing the size of the precipitate by lowering the temperature at which precipitation starts by Cr, while increasing the area fraction of the precipitate.

In order to obtain the above-mentioned effect, it is preferable to include Cr in an amount of 0.001% or more, but when the content exceeds 6.0%, coarse carbides are formed at grain boundaries during hot rolling, which impairs hot workability and increases manufacturing costs. .

Therefore, in the present invention, Cr may be included in 0.001 ~ 6.0%, more preferably in 0.05 ~ 4.0%.

V: 0.001-1.0%

Vanadium (V) is an element that reacts with carbon or nitrogen to form carbonitrides, and is advantageous in improving yield strength due to refinement of grain size and precipitation strengthening.

In order to sufficiently obtain the above-mentioned effect, it is preferable to include V in an amount of 0.001% or more. However, when the content exceeds 1.0%, coarse carbonitride is formed at a high temperature, thereby deteriorating hot workability.

Therefore, in the present invention, V may be included in 0.001% to 1.0%, more preferably in 0.1% to 0.8%.

Mo: 0.001-1.0%

Molybdenum (Mo) is an element that reacts with carbon or nitrogen to form carbonitrides, and plays an advantageous role in improving yield strength by miniaturization and precipitation strengthening.

In order to sufficiently obtain the above-mentioned effect, it is preferable to include Mo in 0.001% or more, but when the content exceeds 1.0%, coarse carbonitride is formed at a high temperature, and there is a problem in that hot workability is lowered.

Therefore, in the present invention, Mo may be included in 0.001% to 1.0%, more preferably 0.05% to 0.8%.

In the high strength steel sheet of the present invention that satisfies the above-mentioned alloy composition, the sum of the contents of Cr, Mo, and V is preferably more than 1.0% (Equation 1 below). When the sum of the contents of Cr, Mo and V exceeds 1.0%, a precipitation strengthening effect can be sufficiently obtained, thereby improving yield strength and tensile strength. However, if the sum of the contents is 6.0% or more, the unrecrystallized temperature may increase rapidly, resulting in excessive material anisotropy.

[Relationship 1]

1.0% <Cr + Mo + V <6.0%

(Here, each element means a weight content.)

In the present invention, it is possible to lower the starting temperature of the precipitates by simultaneously adding the Cr, Mo and V, thereby increasing the formation and fraction of fine precipitates (see FIG. 1A). In addition, the recrystallization temperature is increased by the simultaneous addition of Cr, Mo, and V to obtain an effect of inducing the improvement of the unrecrystallized fraction (see FIG. 1B).

On the other hand, the high strength steel sheet of the present invention is one or more selected from the group consisting of silicon (Si): 0.001 ~ 0.8%, titanium (Ti): 0.01 ~ 1.0% and boron (B): 0.0005 ~ 0.01% in addition to the alloy composition described above It may further include.

However, even if the high strength steel sheet of the present invention does not contain these elements, there is no problem in securing the intended physical properties.

Si: 0.001-0.8%

Silicon (Si) may be added to improve the yield strength and tensile strength of the steel by solid solution strengthening. In addition, Si can be used as a deoxidizer.

For the above-mentioned effects, Si may be included in an amount of 0.001% or more. However, when the content exceeds 0.8%, a large amount of Si oxide is formed on the surface during hot rolling, which lowers pickling properties and increases electrical resistivity, thereby inferring weldability. There is a problem.

Therefore, in one aspect of the present invention, the addition of Si may include 0.001 to 0.8%. More advantageously 0.6% or less, even more advantageously 0.5% or less.

Ti: 0.01 ~ 1.0%

Titanium (Ti) reacts with nitrogen in the steel to precipitate as nitride to improve hot rolling. In addition, some of them combine with carbon to form a precipitated phase of the carbide serves to improve the strength.

For the above-described effects, Ti may be included in an amount of 0.01% or more, but when the content thereof exceeds 1.0%, there is a problem in that the precipitate is excessively formed and the fatigue characteristics are inferior.

Therefore, in one aspect of the present invention, the addition of Ti may include 0.01 to 1.0%.

B: 0.0005 ~ 0.01%

Boron (B) is an effective element for improving the hot rolling property by strengthening the grain boundaries of the cast steel even with a small amount of addition.

For the above-described effects, but may be included in 0.0005% or more, but if the content exceeds 0.01% the effect is saturated, rather there is a problem that causes an increase in the manufacturing cost.

Therefore, in one aspect of the present invention, the addition of B may include 0.0005 to 0.01%.

The remaining component of the present invention is iron (Fe). However, in the conventional manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably mixed, and thus cannot be excluded. Since these impurities are known to those skilled in the art, all of them are not specifically mentioned in the present specification.

For example, it may contain nitrogen, where nitrogen (N) may comprise 0.015% or less.

In order to improve the collision characteristics and fatigue characteristics by securing high yield strength and ductility together with the high strength targeted in the present invention, the microstructure of the steel sheet satisfying the alloy composition described above needs to be configured as follows.

Specifically, the high-strength steel sheet of the present invention may include an austenitic phase in an area fraction of 90% or more, and may include 10% or more of unrecrystallized structure in the austenite phase.

Since the recrystallized structure has a high hardness and prevents dislocation movement, an effect of improving yield strength of steel can be obtained. In addition, since the high cycle fatigue characteristics of the steel sheet increases in proportion to the yield strength, it is possible to obtain an effect of improving the fatigue characteristics by securing the non-recrystallized structure. To this end, the non-recrystallized tissue may be included in more than 10%. As the fraction of the unrecrystallized structure increases, the yield strength increases to increase the fatigue strength, and therefore the upper limit thereof is not particularly limited. However, if the fraction of the non-recrystallized structure exceeds 70% material anisotropy increases, it is preferable to properly control the fraction of the non-recrystallized structure in consideration of the part characteristics.

In the present invention, unrecrystallized tissue refers to a tissue having an intraoral orientation distribution of 1 degree or more.

In addition, the high strength steel sheet of the present invention can secure excellent strength and ductility at the same time by securing an austenite phase of 90% or more.

Except for the austenite phase, ferrite, epsilon martensite, It may include one or more of alpha prime (α ') martensite.

High strength steel sheet of the present invention can improve the yield strength from the precipitation strengthening effect, for this purpose V-Mo-Cr-based carbide having an average size of 3 ~ 90nm based on the equivalent circle diameter of 10 ¹² pieces / cm ² or more It may include.

If the size of the V-Mo-Cr-based carbide is too small or too large, the precipitation strengthening effect intended in the present invention cannot be sufficiently obtained, and even if the number of carbides is too small, the precipitation strengthening effect cannot be sufficiently obtained.

In particular, the above-mentioned carbide is advantageously present mainly in the non-recrystallized tissue, and preferably, the fraction of carbide in the unrecrystallized tissue may be 1.5 times or more of the carbide fraction in the recrystallized tissue.

The high strength steel sheet of the present invention has the above-described alloy composition and microstructure, thereby securing a yield strength of 650 MPa or more, a tensile strength of 900 MPa or more, and an elongation of 35% or more.

That is, the high strength steel sheet of the present invention can secure excellent impact characteristics and fatigue characteristics due to high yield strength and high elongation, and such high strength steel sheet can be advantageously applied to automobile chassis parts requiring cold forming. .

Hereinafter, the method of manufacturing the high strength steel plate excellent in the moldability provided by this invention which is another aspect of this invention is demonstrated in detail.

Briefly, the present invention can produce the target high-strength steel sheet through the [steel slab reheating-hot rolling-cooling-winding] process, each step will be described in detail below.

[Reheating Steel Slabs]

First, it is preferable to prepare a steel slab having an alloy composition proposed in the present invention, and then reheat it. This process is performed in order to perform the following hot rolling process smoothly, and to fully acquire the physical property of the target steel plate.

In one aspect of the present invention, when reheating the steel slab, the reheating may be performed at a temperature Rt satisfying the following relational expression 2 at a time Rt satisfying the following relational formula 3.

[Relationship 2]

Reheating temperature (RT, ℃) ≤ 1500-207 [C]-5 [Mn]-3 [Cr]-34 [Mo]-44 [V]

[Relationship 3]

0.9 × (Slab Thickness, mm) <Reheat Time (Rt, min) <1.25 × (Slab Thickness, mm)

(In the above relations 2 and 3, each element means a weight content.)

If the temperature of the reheating exceeds the value derived by Equation 2, partial melting occurs in the segregation zone in the slab, causing embrittlement of the core, and in severe cases, the slab spreading occurs during the subsequent hot rolling, and the hot rolling progresses. This problem becomes difficult.

In addition, if the time of reheating is less than 0.9 × (slab thickness, mm), complete re-dissolution of the precipitation elements may not be achieved, and thus a target level of strength may not be obtained in the final product. On the other hand, when the time is 1.25 × (slab thickness, mm) or more, there is a problem in that the phenomenon of bending under the subsequent hot rolling occurs due to the slab load, which makes the progress of hot rolling difficult.

On the other hand, in the present invention, the target slab thickness may be 100 ~ 250t (mm).

[Hot rolled]

As described above, the hot-rolled steel slab may be hot rolled to manufacture a hot rolled steel sheet.

Specifically, the reheated slab may be manufactured to a bar (roll) rolled by more than 80% of the initial slab thickness after the initial several times of rolling, in which case when the finish rolling under the conditions satisfying the following relational formula 4, the area fraction 10 A hot rolled steel sheet containing unrecrystallized structure of more than% can be obtained.

More specifically, a large amount of dislocations can be formed in the unrecrystallized structure by controlling the starting temperature during the finish rolling to a value derived by the following Equation 4 below, and the dislocations yield by providing a precipitate formation site. It promotes the formation of carbides, which are major in improving strength, that is, V-Mo-Cr based carbides having an average grain size of 3 to 70 nm. As a result, the fraction of carbide in the uncrystallized structure can be secured to 1.5 times or more of the carbide fraction in the recrystallized structure.

In the following relation 4, the finish rolling start temperature means a temperature immediately after the final rolling after the first several rolling.

[Relationship 4]

Finish rolling start temperature (FST, ℃) <546 + 325 [C]-1.8 [Mn] + 17.4 [Cr] + 28.8 [Al] + 82.4 [Mo] + 213.2 [V]

(In the above relation 4, each element means a weight content.)

[Cooling and winding]

The hot rolled steel sheet manufactured as described above may be cooled and then wound up.

At this time, after cooling to a temperature range of 650 ° C. or less at an average cooling rate of 5 ° C./s or more, it may be wound up at that temperature.

If the average cooling rate is less than 5 ° C / s, coarse carbides of Cr-based is formed, which is undesirable because it inhibits the formability of steel. The upper limit of the average cooling rate is not particularly limited, and may be appropriately selected according to equipment specifications. For example, it may be performed at 100 ° C / s or less.

In addition, when the cooling end temperature, that is, the coiling temperature exceeds 650 ℃ there is a problem that Cr-based coarse carbide is formed during cooling to room temperature after the coiling. The lower limit of the winding temperature is not particularly limited, and there is no problem even if it is performed at room temperature. Here, room temperature means a temperature of about 15 ~ 35 ℃.

On the other hand, pickling and temper rolling after winding may be carried out as necessary, and the conditions at this time will be based on the usual conditions, and are not particularly limited.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, it is necessary to note that the following examples are only for illustrating 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 producing a steel slab having the alloy composition shown in Table 1, it was produced to each hot rolled steel sheet under the conditions shown in Table 2.

Thereafter, the mechanical properties of each hot rolled steel sheet were evaluated and the structure was observed, and the results are shown in Table 3 below.

In this case, a tensile test was performed to measure yield strength (YS), tensile strength (TS), and elongation (El), and elongation means total elongation.

Also, for the evaluation of hole expandability (HER), a hole with a diameter of 10 mm was punched out with a clearance of 12%, and the hole was fixed using a conical jig to measure the expansion rate until the crack penetrated the punching surface. It is shown.

In addition, the material anisotropy was evaluated in the L direction and the C direction for the bendability, and the bendability L is the radius (r) of the bending punch when the test piece made perpendicular to the rolling direction is subjected to a 90 degree bending test parallel to the rolling direction. It is shown by calculating the thickness (t) of the specimen. Bendability C showed the result of a 90 degree bending test perpendicular to the rolling direction of a plate-like specimen prepared horizontally in the rolling direction. In this case, the smaller the bendability (r / t) value, the better the bend characteristic was evaluated.

Steel grade Alloy composition (% by weight) Relationship 1 C Mn Cr Al Mo V Si Ti B N One 0.82 16.9 2.63 1.81 0.31 0.51 0.029 0.07 0.0020 0.007 3.45 2 0.75 15.5 1.00 1.80 0.31 0.55 0.060 0.03 0.0015 0.006 1.86 3 0.80 15.8 2.50 1.72 0.31 0.30 0.026 0.06 0.0018 0.008 3.11 4 0.55 17.6 3.18 0.10 0.31 0.31 0.002 0.10 0.0020 0.012 3.8 5 0.60 17.4 0 0.08 0.31 0.31 0.040 0.05 0.0040 0.014 0.62 6 0.40 22.0 0.05 0.02 0.10 0.30 0.060 0.06 0.0030 0.010 0.45 7 0.85 18.0 0.5 1.80 0.10 0.30 0.051 0.05 0.0020 0.009 0.9

(Equation 1 in Table 1 shows the sum of the contents of Cr, Mo and V.)

Steel grade Slab
thickness
(mm) RT
(℃) Rt
(minute) Relation
2
(RT) Relation
3
(Rt) FST
(℃) Relation
4 (FST) Winding
Temperature
(℃) Cooling
speed
(℃ / s) division One 250 1200 270 1204.9 225 <Rt <313 964 1014 470 25.7 Inventive Example 1 2 250 1220 280 1229.5 225 <Rt <313 950 974 600 18.5 Inventive Example 2 3 250 1221 260 1224.2 225 <Rt <313 980 960 450 27.5 Comparative Example 1 4 250 1280 240 1264.4 225 <Rt <313 995 843 520 24.8 Comparative Example 2 5 250 1250 253 1264.6 225 <Rt <313 992 804 510 25.1 Comparative Example 3 6 250 1220 258 1290.5 225 <Rt <313 960 710 300 34.0 Comparative Example 4 2 250 1208 240 1229.5 225 <Rt <313 1001 974 650 18.6 Comparative Example 5 2 140 1220 153 1229.5 126 <Rt <175 951 974 430 27.1 Inventive Example 3 2 140 1280 153 1229.5 126 <Rt <175 Impossible to roll - - - Comparative Example 6 2 140 1200 260 1229.5 126 <Rt <175 Impossible to roll - - - Comparative Example 7 One 250 1201 269 1204.9 225 <Rt <313 1005 1014 430 29.8 Comparative Example 8 7 250 1210 251 1216.0 225 <Rt <313 920 923 470 23.0 Comparative example
9

division Microstructure Mechanical properties γ fraction
(%) Unresolved
Fraction (%) Precipitation statue
Count (10 ¹² ) Precipitation rate YS
(MPa) TS
(MPa) El
(%) HER
(%) Bendability L
(r / t) Bendability C
(r / t) Inventive Example 1 96 50 2.10 2.1 839 1077 46 27 0.5 0.0 Inventive Example 2 98 25 1.60 2.0 750 1015 50 32 0.3 0.0 Comparative Example 1 97 0 0.30 0 644 1032 57 39 0.5 0.0 Comparative Example 2 99 0 0.20 0 557 1049 48 21 0.3 0.0 Comparative Example 3 99 0 0.05 0 475 1047 51 19 0.2 0.0 Comparative Example 4 100 0 0.05 0 430 959 53 26 0.3 0.0 Comparative Example 5 98 0 0.08 0 630 1015 51 32 0.7 0.2 Inventive Example 3 98 30 1.50 1.7 740 1020 47 32 0.3 0.0 Comparative Example 6 Evaluation as non-rolling X (melting of slab part) Comparative Example 7 Evaluation as non-rolling X (Bending under slab during rolling) Comparative Example 8 96 5 0.60 0 614 1059 43 29 0.5 0.2 Comparative Example 9 99 8 0.05 0.3 606 955 45 28 0.5 0.3

(Table 3 shows the ratio of the number of precipitated phases in the uncrystallized tissue and the number of precipitated phases in the recrystallized tissue (the number of precipitated phases in the uncrystallized tissue / the number of precipitated phases in the recrystallized tissue).

In addition, in Table 3, γ means an austenite phase, except for the γ fraction, at least one of ferrite, epsilon martensite, and alpha prime martensite.)

As shown in Tables 1 to 3, Inventive Examples 1 to 3 satisfying all of the alloy composition, component relations, and manufacturing conditions proposed by the present invention have a tensile strength of 980 MPa or more and an elongation of 40% while securing a yield strength of 650 MPa or more. As described above, the bendability (HER) can be ensured at 25% or more.

That is, the high strength steel sheet provided by the present invention is not only excellent in formability, but also can improve collision characteristics and fatigue characteristics.

On the other hand, Comparative Examples 1 to 5 and Comparative Examples 8 to 9, in which at least one of alloy composition, component relations, and manufacturing conditions deviate from the present invention, are not sufficiently formed or excessively recrystallized, resulting in high yield strength and high ductility. Could not be secured.

In addition, in Comparative Example 6, where the reheating temperature was too high, partial melting of the slab occurred, and subsequent rolling processes were impossible (FIG. 2 (a)). In the case of Comparative Example 7, which was conducted for an excessively long time of reheating, hot rolling was performed. The slab was bent downwards, which also could not continue the rolling process (Fig. 2 (b)).

3 shows a tissue photograph of Inventive Example 1, it is possible to distinguish between recrystallized tissue and unrecrystallized tissue, and it can be confirmed that the unrecrystallized tissue is sufficiently formed.

Claims (9)

By weight%, carbon (C): 0.1-1.0%, manganese (Mn): 10-26%, aluminum (Al): 0.01-2.0%, chromium (Cr): 0.001-6.0%, vanadium (V): 0.001 -1.0%, molybdenum (Mo): 0.001-1.0%, the balance Fe and other unavoidable impurities, the Cr, V and Mo satisfy the following relation 1,
The microstructure includes an austenite phase in an area fraction of 90% or more, and the fraction of unrecrystallized tissue in the austenite phase is 10% or more,
High strength steel sheet with excellent formability, comprising V-Mo-Cr-based carbide having an average size of 3 to 90 nm with a circle equivalent diameter of 10 ¹² pieces / cm ² or more.

[Relationship 1]
1.0% <Cr + Mo + V <6.0%
(Here, each element means a weight content.)
The method of claim 1,
The steel sheet is by weight, formability further comprises at least one selected from the group consisting of silicon (Si): 0.001 ~ 0.8%, titanium (Ti): 0.01 ~ 1.0% and boron (B): 0.0005 ~ 0.01%. This excellent high strength steel sheet.
The method of claim 1,
The steel sheet is a high strength steel sheet having excellent moldability, containing aluminum (Al) in 0.051 ~ 2.0%.
delete The method of claim 1,
A high strength steel sheet having excellent formability, wherein the fraction of carbide in the uncrystallized structure is 1.5 times or more than the fraction of carbide in the recrystallized structure.
The method of claim 1,
The steel sheet is a high strength steel sheet having excellent formability having a yield strength of 650 MPa or more, a tensile strength of 900 MPa or more, and an elongation of 35% or more.
By weight%, carbon (C): 0.1-1.0%, manganese (Mn): 10-26%, aluminum (Al): 0.01-2.0%, chromium (Cr): 0.001-6.0%, vanadium (V): 0.001 ~ 1.0%, molybdenum (Mo): 0.001 ~ 1.0%, the balance Fe and other unavoidable impurities, the Cr, V and Mo is a steel slab that satisfies the following relation 1, the temperature satisfying the relations 2 and 3 Reheating at RT) and time (Rt);
Manufacturing a hot rolled steel sheet by hot rolling after the reheating; And
And winding the hot rolled steel sheet in a temperature range of 650 ° C. or less at a cooling rate of 5 ° C./s or more,
The hot rolling is a method of producing a high strength steel sheet excellent in formability to finish rolling to satisfy the following relational formula 4.

[Relationship 1]
1.0% <Cr + Mo + V <6.0%
[Relationship 2]
Reheating temperature (RT, ℃) ≤ 1500-207 [C]-5 [Mn]-3 [Cr]-34 [Mo]-44 [V]
[Relationship 3]
0.9 × (Slab Thickness, mm) <Reheat Time (Rt, min) <1.25 × (Slab Thickness, mm)
[Relationship 4]
Finish rolling start temperature (FST, ℃) <546 + 325 [C]-1.8 [Mn] + 17.4 [Cr] + 28.8 [Al] + 82.4 [Mo] + 213.2 [V]
(In the above relations 2 to 4, each element means a weight content.)
The method of claim 7, wherein
The steel slab is formed by weight, further comprising one or more selected from the group consisting of silicon (Si): 0.001 to 0.8%, titanium (Ti): 0.01 to 1.0% and boron (B): 0.0005 to 0.01%. Method for producing high strength steel sheet with excellent properties.
The method of claim 7, wherein
The steel slab is a method for producing a high strength steel sheet having excellent moldability, containing aluminum (Al) in 0.051 ~ 2.0%.

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