KR101657784B1 - High ductility and strength cold rolled steel sheet with reduced cracking in hot-rolling and method for manufacturing the same - Google Patents

Abstract

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold-rolled steel sheet suitably used for automobile structural members, building materials, and the like, and more particularly, to a cold-rolled steel sheet having high ductility and high strength without cracking during hot rolling and a manufacturing method thereof.

Description

TECHNICAL FIELD [0001] The present invention relates to a high-ductility high-strength cold-rolled steel sheet having reduced cracking during hot rolling and a method for manufacturing the same. BACKGROUND OF THE INVENTION 1. Field of the Invention [0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold-rolled steel sheet suitably used for automotive structural members, building materials, and the like, and more particularly to a cold-rolled steel sheet having high ductility and high strength without cracking during hot rolling and a method for manufacturing the same.

In recent years, Transformation Induced Plasticity Steel (TRIP Steel) has been developed and used, which is excellent in workability compared to conventional precipitation strengthening or solid solution strengthening steels.

The transformed organo-plastic steel can improve the strength and ductility by controlling a cooling rate and a cooling end temperature in a cooling process after austenite is formed in the annealing process, thereby partially retaining austenite at room temperature. The metastable retained austenite is transformed into martensite by deformation to increase elongation by delaying local stress concentration relaxation and necking with increasing strength. Therefore, it is important that the above-mentioned transformationally-induced plasticity steel maintain austenite at a certain rate or more at room temperature.

In order to provide the above-mentioned transformationally-induced plasticity steel, a method of securing a large amount of retained austenite at room temperature by controlling the annealing condition by adding manganese (Mn) as an austenite stabilizing element at 4 wt% or more is proposed have. However, the method described above is disadvantageous in that it is not suitable for the continuous annealing process because annealing must be performed at a temperature of 650 ° C or less for 5 hours or more in order to secure retained austenite and good physical properties.

In order to solve such a problem, in Patent Document 2, silicon (Si) and aluminum (Al), which are ferrite stabilizing elements, are added to ensure the annealing temperature and time suitable for the continuous annealing process. However, when the content of Al is increased, a certain fraction of the delta ferrite remains at a temperature below the freezing point, so that an abnormal reverse rolling of the austenite and the delta ferrite is inevitable at the time of hot rolling. In the fraction of the specific delta ferrite, Which may cause plate breakage during cold rolling.

Therefore, it is required to develop a technique capable of reducing edge cracks in hot rolling in a high ductile high strength steel containing a large amount of Mn and Al.

Korean Patent Publication No. 1998-0045322 Korean Patent Publication No. 2009-0120759

One aspect of the present invention is to provide a high ductility high-strength cold-rolled steel sheet which not only secures strength and ductility at the same time, but also does not cause an edge crack at the time of hot rolling, and a method of manufacturing the same.

An aspect of the present invention provides a method of manufacturing a semiconductor device, comprising: 0.05 to 0.3% of carbon (C), 3.0 to 8.0% of manganese (Mn), 1.0 to 4.0% of aluminum (Al) 0.03% or less (excluding 0), sulfur (S): 0.015% or less (excluding 0), nitrogen (N): 0.02% or less And other impurities, and the C, Mn, Si and Al satisfy the following relational expression (1): " (1) "

[Relation 1]

(-55.8 [C] - 6.2 [Mn] + 17.3 [Si] + 28.4 [Al] - 16.8 <20 or> 49

(In the above relational expression 1, C, Mn, Si, and Al mean the content by weight.)

According to another aspect of the present invention, there is provided a method of manufacturing a steel slab, comprising: reheating a steel slab satisfying the above-described composition and relationship to a temperature of 1100 to 1300 캜; Subjecting the reheated slab to finish hot rolling at a temperature of 800 to 950 캜 to produce a hot-rolled steel sheet; Winding the hot-rolled steel sheet at a temperature of 750 ° C or lower; Hot rolling the cold rolled steel sheet to obtain a cold rolled steel sheet; And a step of subjecting the cold-rolled steel sheet to annealing at 700 to 830 ° C, wherein the occurrence of cracks during hot-rolling is reduced, thereby providing a method of manufacturing a high-ductility high-strength cold-rolled steel sheet.

According to the present invention, it is possible to fundamentally solve the problem that an edge crack is generated when a large amount of Mn and Al is contained for securing strength and ductility.

Further, the cold-rolled steel sheet according to the present invention can be suitably used for applications requiring both strength and ductility, such as automobile structural members and building materials.

FIG. 1 shows the result of visual observation of occurrence of edge cracks after hot rolling of Comparative Steel 1 (A) and Bill Steel 1 (B).

The inventors of the present invention have conducted intensive studies to solve the problem of edge cracking when a large amount of Mn and Al is contained in order to secure strength and ductility. As a result, it has been found that C, Mn, It has been found that a cold rolled steel sheet free from edge cracks in hot rolling while retaining high ductility and high strength can be provided when the content relationship between Si and Al is closely controlled and the present invention has been accomplished.

Hereinafter, the present invention will be described in detail.

A high ductility high strength cold rolled steel sheet with reduced cracking during hot rolling according to one aspect of the present invention comprises 0.05 to 0.3% of carbon (C), 3.0 to 8.0% of manganese (Mn) 1.0% or less of silicon (Si): not more than 2.0% (excluding 0), phosphorus (P): not more than 0.03% ): Not more than 0.02% (excluding 0).

Hereinafter, the reason for controlling the alloy components of the cold-rolled steel sheet provided in the present invention will be described in detail. Herein, unless otherwise specified, the content of each component means weight%.

C: 0.05 to 0.3%

Carbon (C) is an essential element for stabilizing the retained austenite and securing strength, and in order to obtain the above-mentioned effect, it is preferable to add carbon (C) in an amount of 0.05% or more. However, if the content exceeds 0.3%, there is a problem that the weldability is poor.

Therefore, the content of C in the present invention is preferably limited to 0.05 to 0.3%.

Mn: 3.0 to 8.0%

Manganese (Mn) is an element which inhibits the formation of ferrite and facilitates the formation of austenite. When the content of Mn is less than 3%, it is difficult to ensure the stability of retained austenite. Therefore, Mn is preferably contained in an amount of 3% . However, if the content exceeds 8%, not only is there a problem in that the process cost due to the excessive formation of the band due to segregation and the excessive amount of alloying during the conversion operation is increased, but also the coarse annealing agglomerates are formed, There is a problem causing surface defects.

Therefore, in the present invention, the content of Mn is preferably limited to 3.0 to 8.0%, more preferably 4.0 to 7.0%.

Al: 1.0 to 4.0%

Aluminum (Al) is an element which is useful for expanding the ferrite phase. When continuous annealing is applied as in the present invention, it has an effect of increasing the annealing temperature, which is advantageous in connection work with other steel types in continuous annealing furnace operation.

If the content of Al is less than 1.0%, the effect of increasing the annealing temperature is small and the two-phase microstructure is difficult to obtain. On the other hand, when the content exceeds 4.0%, the strength is rapidly lowered I can not.

Therefore, in the present invention, the content of Al is preferably limited to 1.0 to 4.0%, more preferably 1.5 to 4.0%.

Si: 2.0% or less (excluding 0)

Silicon (Si) is an element that inhibits the precipitation of carbide in ferrite and stabilizes austenite. However, in a steel containing a large amount of Mn as in the present invention, Mn is distributed to austenite in a large amount to stabilize the retained austenite. Therefore, Si is preferably contained in an amount of 2.0% or less, It is acceptable. On the other hand, when the content of Si exceeds 2.0%, it is not preferable because an oxide is formed to deteriorate the plating property.

Therefore, the content of Si in the present invention is preferably limited to 2.0% or less (excluding 0).

P: 0.03% or less (excluding 0)

When phosphorus (P) is an impurity element and its content exceeds 0.03%, there is a problem that the weldability decreases and the risk of brittleness of steel increases.

Therefore, the content of P in the present invention is preferably limited to 0.03% or less.

S: 0.015% or less (excluding 0)

Sulfur (S) is an impurity element like P, and when it exceeds 0.015%, there is a problem that ductility and weldability of the steel sheet are impaired.

Therefore, in the present invention, it is preferable to limit the content of S to 0.015% or less.

N: 0.02% or less (excluding 0)

Nitrogen (N) is an element effective for stabilizing austenite. However, if the content exceeds 0.02%, there is a problem that the risk of brittleness is greatly increased.

Therefore, in the present invention, it is preferable to limit the content of N to 0.02% or less.

Among the above-mentioned components, C, Mn, Si, and Al preferably satisfy the relation 1 defined as follows.

[Relation 1]

(-55.8 [C] - 6.2 [Mn] + 17.3 [Si] + 28.4 [Al] - 16.8) <20 or> 49

(In the above relational expression 1, C, Mn, Si, and Al mean the content by weight.)

As described above, when a large amount of Mn and Al is contained in order to secure strength and ductility, as the content of Al increases, delta ferrite remains at a constant fraction of the temperature below the freezing point, and austenite and delta Anomalous reverse rolling of the ferrite becomes inevitable, which may cause cracking of the hot rolling edge when delta ferrite is produced in a specific fraction range.

Accordingly, in the present invention, the relation 1 is set in consideration of this, and in the relation 1, the fraction of the expected delta ferrite in the hot rolling is expressed as the component content. Here, Si and Al are elements that increase the fraction of delta ferrite, and C and Mn are elements that decrease the fraction of delta ferrite.

Therefore, when the calculated value of the left side of the relational expression 1 in the composition range of each component proposed in the present invention is less than 20 or exceeds 49, that is, the area fraction of the delta ferrite determined by the composition of the relational expression 1 is less than 20% Or more than 49%, the hot crack is not generated, and therefore, it is preferable to control it as the above range. In this case, if the value is negative, it means that the area fraction of the delta ferrite is 0%.

The cold-rolled steel sheet of the present invention may further include at least one of titanium (Ti), niobium (Nb), and vanadium (V) in addition to the above-described component composition.

Ti, Nb and V are effective elements for increasing the strength of the steel and making the grain size finer. When the content of each of these elements is less than 0.005%, it is difficult to secure the above-mentioned effect. On the other hand, And it is undesirable because precipitates are formed excessively and the ductility of the steel can be greatly lowered.

Except for the above-mentioned components, the remainder consists of Fe and unavoidable impurities.

The cold-rolled steel sheet of the present invention, which satisfies the above-described compositional and component relations, is composed of a two-phase microstructure, and is preferably composed of austenite having an area fraction of 10 to 40% and the remainder ferrite.

If the percentage of austenite is less than 10%, the ductility due to transformation and organic firing can not be sufficiently secured, and the intended properties can not be obtained. On the other hand, the fraction of austenite Exceeds 40%, the concentration of stabilizing elements C and Mn in the austenite is lowered, so that the mechanical stability of the austenite is decreased, and a balance between strength and ductility can not be secured.

The cold-rolled steel sheet of the present invention having such a microstructure can secure a balance (TS x El) of tensile strength and elongation of 20,000 MPa ·% or more.

Hereinafter, a method of manufacturing a cold-rolled steel sheet according to another aspect of the present invention will be described in detail.

First, the steel slab satisfying the above-described composition of components and the composition relation is reheated.

If the reheating temperature is less than 1100 ° C, the load increases rapidly during the subsequent hot rolling. On the other hand, if the reheating temperature is higher than 1300 ° C, the reheating temperature There is a problem that the amount of the surface scale is increased as well as the surface roughness increases.

Therefore, in the present invention, the reheating process is preferably performed at a temperature in the range of 1100 to 1300 ° C.

Preferably, the reheated steel slab is hot-rolled to produce a hot-rolled steel sheet, wherein the hot-rolling is preferably performed at 800 to 950 ° C.

When the rolling temperature is lower than 800 ° C., the rolling load is increased significantly and the rolling becomes difficult. On the other hand, when the temperature exceeds 950 ° C., the thermal fatigue of the rolling roll is greatly increased, do.

Therefore, in the present invention, the hot rolling is preferably carried out at a temperature of 800 to 950 캜.

However, the present invention can perform hot rolling in a single phase of austenite or hot rolling in austenite and delta ferrite anomalies depending on the steel composition range.

It is preferable to wind the hot-rolled steel sheet produced in accordance with the above.

At this time, it is preferable that the winding process is performed at a temperature of 750 ° C or lower. If the coiling temperature exceeds 750 ° C, if the coiling temperature is too high, a scale is excessively generated on the hot-rolled steel sheet surface to cause surface defects, .

Therefore, in the present invention, the winding process is preferably performed at 750 DEG C or lower. At this time, although the lower limit of the coiling temperature is not particularly limited, the plate shape due to the uneven cooling and the transformation of the martensite may be inferior at a low coiling temperature. Therefore, Is more preferable.

It is preferable that the rolled steel sheet is pickled to remove the oxide layer and then subjected to cold rolling in order to match the shape and thickness of the steel sheet to produce a cold rolled steel sheet.

Normally, cold rolling is carried out in order to secure the thickness required by the customer. At this time, there is no restriction on the reduction rate, but in order to suppress generation of coarse ferrite grains during recrystallization in the subsequent annealing process, . However, if the reduction rate exceeds 80%, there is a problem that the rolling load increases greatly.

Thereafter, the cold-rolled steel sheet produced in the above-described manner is preferably annealed in a continuous annealing process.

At this time, the annealing heat treatment is preferably performed in a temperature range of 700 to 830 ° C. If the annealing temperature is less than 700 ° C., reverse transformation into austenite is insufficient, recrystallization is insufficient, On the other hand, if the annealing temperature exceeds 830 占 폚, the fraction of austenite becomes excessive and it becomes difficult to secure the stability of retained austenite.

Therefore, the annealing heat treatment in the present invention is preferably carried out in a temperature range of 700 to 830 ° C.

After the annealing heat treatment, hot-dip galvanized steel sheet can be produced by performing hot-dip galvanizing, and galvannealed hot-dip galvanizing can be performed to produce a galvannealed steel sheet.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the following examples are only illustrative of the present invention in more detail and do not limit the scope of the present invention.

( Example )

A steel having the composition shown in the following Table 1 was vacuum-melted with a 34 Kg ingot, and then a hot-rolled slab was produced through sizing rolling. Thereafter, the slab was reheated at 1,200 DEG C for one hour, finishing hot-rolled at 900 DEG C, and then charged into a preheated furnace, maintained for one hour, and then subjected to hot rolling to simulate hot-rolled coiling. Table 2 shows whether or not the steel composition of the hot-rolled steel sheet thus prepared satisfies the relational expression 1 and whether edge cracking occurs during hot rolling.

Thereafter, cold rolled steel sheets were pickled and cold-rolled at a cold rolling reduction rate of 50% for the specimens having no edge cracks among the hot rolled steel sheets, and then annealed at 700 to 830 ° C for annealing. The thus obtained cold-rolled steel sheet was measured for tensile strength and elongation, and is shown in Table 2 below.

division Component composition (% by weight) C Mn Si Al P S N Ti Nb V Inventive Steel 1 0.10 6.0 0.01 1.5 0.011 0.005 0.003 - - - Invention river 2 0.15 6.0 0.5 1.5 0.012 0.004 0.004 - - - Invention steel 3 0.15 7.0 1.0 1.5 0.010 0.003 0.004 0.02 - - Inventive Steel 4 0.15 6.0 1.5 1.0 0.013 0.005 0.004 - 0.02 - Invention steel 5 0.12 7.0 1.0 1.5 0.011 0.004 0.004 - - 0.02 Invention steel 6 0.25 4.0 1.2 1.0 0.012 0.004 0.004 - - - Invention steel 7 0.15 6.0 1.0 1.5 0.011 0.005 0.003 - - - Inventive Steel 8 0.12 6.0 1.0 1.5 0.010 0.005 0.004 - - - Invention river 9 0.15 5.0 1.0 1.5 0.011 0.003 0.004 - - - Invented Steel 10 0.15 6.0 1.5 1.5 0.010 0.003 0.003 - - - Invention steel 11 0.10 6.0 1.5 1.5 0.013 0.005 0.003 - - - Invention steel 12 0.15 6.0 1.5 1.8 0.011 0.004 0.004 - - - Invention steel 13 0.15 6.0 2.0 1.5 0.012 0.004 0.003 - - - Invented Steel 14 0.12 7.0 1.0 3.5 0.011 0.005 0.003 - - - Comparative River 1 0.15 6.0 1.5 2.1 0.012 0.004 0.004 0.02 - - Comparative River 2 0.10 6.0 0 3.0 0.012 0.004 0.004 - 0.02 - Comparative Steel 3 0.15 7.0 1.0 2.8 0.011 0.004 0.004 - - 0.02 Comparative Steel 4 0.15 7.0 1.0 3.5 0.012 0.004 0.004 - - -

division Relationship 1
Calculated value Whether hot-rolled edge cracks occur Mechanical properties Tensile Strength (MPa) Elongation (%) TS 占 El (MPa 占%) Inventory 1 -17 radish 815 31.1 25347 Inventory 2 -11 radish 894 32.9 29413 Inventory 3 -9 radish 1025 27.4 28085 Honorable 4 -8 radish 975 26.9 26228 Inventory 5 -7 radish 1019 27.1 27615 Inventory 6 -6 radish 1186 20.6 24432 Honorable 7 -2 radish 995 31.1 30945 Honors 8 -One radish 897 33.1 29691 Proposition 9 4 radish 1037 21.6 22399 Inventory 10 6 radish 1008 25.4 25603 Exhibit 11 9 radish 809 36.0 29124 Inventory 12 15 radish 1134 22.1 25061 Inventory 13 15 radish 1035 24.1 24944 Inventory 14 50 radish 905 26.5 23983 Comparative Example 1 23 U - - - Comparative Example 2 26 U - - - Comparative Example 3 28 U - - - Comparative Example 4 48 U - - -

(In Table 2, Comparative Examples 1 to 4 did not measure the tensile strength and elongation as the hot-edge cracking occurred.)

As shown in Table 2, in Examples 1 to 14, which satisfied the component composition and component relation controlled by the present invention, cracks were not generated at the edge portion in hot rolling, and strength and ductility were excellent. (TS El) value of 20,000 MPa ·% or more.

On the other hand, in the case of Comparative Examples 1 to 4 which do not satisfy the component relational expression even in the case of satisfying the composition composition proposed in the present invention, edge cracks occurred in all cases during hot rolling.

FIG. 1 is a visual check of occurrence of edge cracks after the hot rolling of the comparative steel 1 and the inventive steel 1. In the case of invention steel 1 (B), edge cracks do not occur, whereas in comparative steel 1 (A) Then, it can be confirmed that an edge crack occurred.

Claims (7)

(C): 0.05 to 0.3%, manganese (Mn): 3.0 to 8.0%, aluminum (Al): 1.0 to 4.0%, silicon (Si): not more than 2.0% P): not more than 0.03% (excluding 0), sulfur (S): not more than 0.015% (excluding 0), nitrogen (N): not more than 0.02% (excluding 0), the balance Fe and other impurities C, Mn, Si and Al satisfy the following relational expression 1 and the area fraction of delta ferrite formed in hot rolling is less than 20% or more than 49%, the occurrence of cracks during hot rolling is reduced.

[Relation 1]
(-55.8 [C] - 6.2 [Mn] + 17.3 [Si] + 28.4 [Al] - 16.8) <20 or> 49
(In the above relational expression 1, C, Mn, Si, and Al mean the content by weight.)
The method according to claim 1,
The cold-rolled steel sheet further comprises at least one selected from the group consisting of 0.005 to 0.3% of titanium (Ti), 0.005 to 0.3% of niobium (Nb), and 0.005 to 0.3% of vanadium (V) High ductility high strength cold rolled steel sheet with reduced cracking during hot rolling.
The method according to claim 1,
Wherein the cold-rolled steel sheet has a microstructure and contains 10 to 40% of austenite and the remainder ferrite in an area fraction, and the generation of cracks during hot rolling is reduced.
The method according to claim 1,
Wherein the cold-rolled steel sheet has a balance of tensile strength and elongation (TS x El) of 20,000 MPa ·% or more, and reduces the occurrence of cracks during hot rolling.
(C): 0.05 to 0.3%, manganese (Mn): 3.0 to 8.0%, aluminum (Al): 1.0 to 4.0%, silicon (Si): not more than 2.0% P): not more than 0.03% (excluding 0), sulfur (S): not more than 0.015% (excluding 0), nitrogen (N): not more than 0.02% (excluding 0), the balance Fe and other impurities C, Mn, Si and Al are reheated to a temperature of 1100 to 1300 占 폚 in a steel slab satisfying the following relational expression 1;
Subjecting the reheated slab to finish hot rolling at a temperature of 800 to 950 캜 to produce a hot-rolled steel sheet;
Winding the hot-rolled steel sheet at a temperature of 750 ° C or lower;
Hot rolling the cold rolled steel sheet to obtain a cold rolled steel sheet; And
Annealing the cold-rolled steel sheet at 700 to 830 ° C,
Wherein the area fraction of the delta ferrite formed in the hot rolling is less than 20% or more than 49%, and cracking is reduced during hot rolling.

[Relation 1]
(-55.8 [C] - 6.2 [Mn] + 17.3 [Si] + 28.4 [Al] - 16.8) <20 or> 49
(In the above relational expression 1, C, Mn, Si, and Al mean the content by weight.)
6. The method of claim 5,
The steel slab may further comprise at least one selected from the group consisting of 0.005 to 0.3% of titanium (Ti), 0.005 to 0.3% of niobium (Nb), and 0.005 to 0.3% of vanadium (V) A method for producing a high ductility high strength cold rolled steel sheet with reduced cracking during hot rolling.
6. The method of claim 5,
Wherein the hot-rolled steel sheet is further subjected to hot-dip galvanizing or galvannealing after the annealing heat treatment.

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