KR101657800B1 - High strength cold rolled steel sheet having excellent stretch flange ability and method for manufacturing the same - Google Patents

Abstract

The present invention relates to a method for producing a steel sheet comprising 0.05 to 0.2% by weight of carbon (C), 0.05 to 0.1% by weight of silicon (Si), 1.5 to 2.0% by weight of manganese (Mn) : Not more than 0.015 wt%, aluminum (Al): not more than 0.02 wt% (excluding 0 wt%), niobium (Nb): not more than 0.1 wt% (%) Expressed by the following [relational expression 1], where D0 is the initial hole diameter (mm) and Dh is the hole diameter after the fracture (mm), and the remaining iron (Fe) and other unavoidable impurities, ) < / RTI > of 45% or more, and a method for manufacturing the same.
[Relation 1]
? = (Dh-Do) / Do * 100

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high strength cold rolled steel sheet having excellent stretch flangeability,

The present invention relates to a high-strength cold-rolled steel sheet mainly used for automobile impact and structural members, and a manufacturing method thereof.

Recently, cold rolled steel sheets used in automobiles, household appliances, etc. are required to have excellent strength and elongation flangeability. High-strength steel plates having high tensile strength have been actively employed to lighten the vehicle body and secure passenger stability. These high-strength steel plates have been developed in close relation with various legal regulations surrounding the automobile industry such as the automobile safety regulation law, the fuel consumption regulation law, the exhaust gas regulation law, and the fuel economy regulation by the high price has been strengthened, As a result, research and development have been accelerated, and many kinds of high strength steel sheets have been developed.

Generally, precipitation strengthened steel or ferrite / pearlite steel of a ferrite base has been used to increase the strength, and there has been a problem that the ductility and stretch flangeability of such steel decrease with increasing strength. Therefore, to solve this problem, there have been proposed techniques for securing a stretch flange and ductility by forming a mixed structure composed of an equiaxed ferrite or acicular ferrite and bainite.

For example, a method of improving stretch flangeability by suppressing segregation of P by performing low-temperature coiling while suppressing the amount of retained austenite as much as possible during winding, or a ferrite-bainite structure as the main body, wherein the ferrite ratio is 80% , And controlling the crystal grains having a ratio (ds / dl) of a short diameter (ds) to a long diameter (dl) of the crystal grains of 0.1 or more to 80% or more. There is a problem that the stretch flangeability deteriorates due to surface decarburization that may occur.

As another technique, there is a method of winding at a temperature of less than 400 DEG C in order to produce a high-strength hot-rolled steel sheet having good stretch flangeability and excellent molding processability. However, the heat transfer coefficient is rapidly changed at less than 400 DEG C, And it is difficult to control the microstructure. In addition, there is a method of controlling the fraction of bainite to 90% or more in order to improve elongation flangeability, but in this case, ductility is lowered and other formability except hole expandability is deteriorated.

Therefore, there is currently limited research on steel sheets that have excellent stretch flangeability and excellent strength, and it is necessary to conduct research.

Korean Patent Laid-Open Publication No. 2013-0058044 (published on March 23, 2013)

It is an object of the present invention to provide a high strength cold rolled steel sheet excellent in stretch flangeability and a method of manufacturing the same.

On the other hand, the object of the present invention is not limited to the above description. It will be understood by those of ordinary skill in the art that there is no difficulty in understanding the additional problems of the present invention.

In one aspect, the present invention provides a method for producing a carbon nanowire comprising: 0.05 to 0.2 wt% carbon; 0.05 to 0.1 wt% silicon; 1.5 to 2.0 wt% manganese; 0.03 wt% Sulfur (S): not more than 0.015 wt%, aluminum (Al): not more than 0.02 wt% (excluding 0 wt%), niobium (Nb): not more than 0.1 wt% (1) below, where Dh is the initial pore diameter (mm) and Dh is the pore diameter after the fracture (mm), and the balance of the iron (Fe) and other inevitable impurities, A high strength cold rolled steel sheet excellent in stretch flangeability with an expansion ratio (%)? Of 45% or more.

[Relation 1]

? = (Dh-Do) / Do * 100

On the other hand, the cold-rolled steel sheet may have a tensile strength of 590 MPa or more.

On the other hand, the microstructure of the cold-rolled steel sheet may contain at least 80% of martensite in an area fraction.

In another aspect, the present invention provides a method of manufacturing a semiconductor device, comprising 0.05 to 0.2% by weight of carbon (C), 0.05 to 0.1% by weight of silicon (Si), 1.5 to 2.0% by weight of manganese (Mn) Sulfur (S): not more than 0.015 wt%, aluminum (Al): not more than 0.02 wt% (excluding 0 wt%), niobium (Nb): not more than 0.1 wt% % By weight, and the balance of Fe (Fe) and other unavoidable impurities; Cold rolling the hot rolled steel sheet; Annealing annealing after cold rolling; A quenching heat treatment after annealing; And performing post heat treatment after the quenching heat treatment; Wherein the content (% by weight) of the carbon (C) is set to [C], the content (wt%) of manganese (Mn) is set to [Mn], the annealing heat treatment temperature (° C) is defined as [T2], a method of producing a high strength cold rolled steel sheet excellent in stretch flangeability satisfying the following [Relation 2] is provided.

[Relation 2]

3.4 [T1] / ° C + 4041 [C] / weight% + 350 [Mn] / weight% - 0.48 [T2] / ° C.

The step of preparing the hot-rolled steel sheet may include the steps of: 0.05 to 0.2% by weight of carbon (C), 0.05 to 0.1% by weight of silicon (Si), 1.5 to 2.0% by weight of manganese (Mn) (Al): 0.02 wt% or less (excluding 0 wt%), niobium (Nb): 0.1 wt% or less (excluding 0 wt%), nitrogen N): not more than 0.01% by weight, and remainder iron (Fe) and other unavoidable impurities to a temperature of 1100 to 1300 캜; Subjecting the reheated steel slab to finishing hot rolling so that the final hot rolling temperature is equal to or higher than Ar3; And winding the hot-rolled steel sheet at a temperature of 720 占 폚 or lower; . ≪ / RTI >

On the other hand, the annealing heat treatment step may be performed by annealing annealing in a temperature range of 710 to 790 ° C.

Meanwhile, the step of post-heat-treating may be post-heat-treated at a temperature in the range of 25 to 310 ° C.

In addition, the solution of the above-mentioned problems does not list all the features of the present invention. The various features of the present invention and the advantages and effects thereof will be more fully understood by reference to the following specific embodiments.

According to the present invention, it is possible to provide a cold-rolled steel sheet having excellent ductility and excellent strength.

1 is a graph showing the relationship between the value of the relational expression 2 of the present invention and the tensile strength.

Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be modified into various other forms, and the scope of the present invention is not limited to the embodiments described below. Further, the embodiments of the present invention are provided to more fully explain the present invention to those skilled in the art.

The inventors of the present invention have conducted extensive studies to solve the above problems and have found that the present inventors have found that when carbon, silicon, manganese, phosphorus, sulfur, aluminum, niobium, , Nitrogen (N), iron (Fe), and the like in a specific composition ratio, a high strength cold rolled steel sheet excellent in stretch flangeability can be obtained when a cold rolled steel sheet having a high hole expanding ratio is produced. Thereby completing the invention.

Specifically, the high-strength cold-rolled steel sheet having excellent stretch flangeability according to the present invention comprises 0.05 to 0.2 wt% of carbon (C), 0.05 to 0.1 wt% of silicon (Si), 1.5 to 2.0 wt% of manganese (Mn) P: not more than 0.03 wt%, sulfur (S): not more than 0.015 wt%, aluminum (Al): not more than 0.02 wt% (excluding 0 wt%), niobium (Nb) ), Nitrogen (N): 0.01 wt% or less, and the balance of iron (Fe) and other inevitable impurities, Do is the initial pore diameter (mm) and Dh is the pore diameter after fracture (mm) The hole expanding rate (%) [lambda] expressed by [Relation 1] is 45% or more.

The reason for adding each component constituting the steel composition of the present invention and the appropriate content range thereof are as follows.

Carbon (C): 0.05 to 0.2 wt%

The content of C is preferably 0.05 to 0.2% by weight. C is required for securing the strength of martensite, and therefore, it should be added in an amount of 0.05 wt% or more. However, if the content exceeds 0.2% by weight, the weldability may be poor.

Silicon (Si): 0.05 to 0.1 wt%

The content of Si is preferably 0.05 to 0.1% by weight. Si is a ferrite stabilizing element and has a disadvantage in that it weakens the strength by promoting the formation of cold ferrite after annealing in a conventional continuous annealing furnace in which a cooling section exists and also has a disadvantage in that the strength is weakened by surface enrichment due to Si and oxidation caused dent defects It is preferable to limit the upper limit to 0.1% by weight.

Manganese (Mn): 1.5 to 2.0 wt%

The content of Mn is preferably 1.5 to 2.0% by weight. When Mn is less than 1.5% by weight, ferrite is easy to form upon cooling. When Mn is more than 2% by weight, Mn is precipitated in segregation , There is a problem in that the amount of alloy iron is increased due to an excessive amount of alloying material in the band formation and transfer operation.

Phosphorus (P): 0.03% by weight or less

The content of P is preferably 0.03% or less. The steel P may not be contained as an impurity element, but if it is inevitably included, if the content exceeds 0.03 wt%, the weldability decreases and the risk of brittleness of the steel increases, and the possibility of occurrence of the dent defect becomes high. It is preferred to limit it to weight%.

Sulfur (S): 0.015 wt% or less

The content of S is preferably 0.015 wt% or less. S, like P, does not need to be included as an impurity element in the steel, but if it is inevitably contained, it inhibits ductility and weldability of the steel sheet. If the content exceeds 0.015% by weight, there is a possibility of deteriorating the ductility and weldability of the steel sheet It is preferable to limit the upper limit to 0.015% by weight.

Aluminum (Al): not more than 0.02% by weight

The content of Al is preferably 0.02% by weight or less. Al is an alloying element that expands the ferrite phase. When the continuous annealing process in which the cooling is present is utilized as in the present invention, there is a disadvantage that ferrite formation is accelerated. Since hot Al rolling can be deteriorated by AlN formation, It is preferable to limit it to 0.2% by weight.

Niobium (Nb): not more than 0.1% by weight

The content of Nb is preferably 0.1 wt% or less. Nb is an element that segregates in the austenite grain boundaries and inhibits the coarsening of the austenite grains during the annealing heat treatment. Therefore, when Nb is added in an amount of more than 0.1% by weight, there is a problem in that the amount of alloy iron is increased due to excessive alloying amount The upper limit is preferably limited to 0.1% by weight.

Nitrogen (N): not more than 0.01% by weight

The content of N is preferably 0.01% by weight or less. N is not necessarily included. If it is inevitably included, if it exceeds 0.01% by weight, the risk of occurrence of cracks during performance through AlN formation or the like is greatly increased, so that the upper limit is preferably limited to 0.01% by weight.

The remainder of the present invention is iron (Fe). However, in the ordinary steel manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of steel making.

On the other hand, the cold-rolled steel sheet of the present invention has a hole expansion ratio (%)? Of 45% or more as expressed by the following [Relation 1], where Do is the initial hole diameter (mm) and Dh is the hole diameter For example, about 50 to 80%. The hole expandability is an index for evaluating elongation flangeability. When a circular hole is formed in a specimen and then expanded by using a conical punch, the hole is expanded until the crack generated at the edge of the hole penetrates at least one direction in the thickness direction The amount of enlargement can be expressed as a ratio to the size of the initial hole. In this case, an excellent stretch flangeability can be secured.

 [Relation 1]

? = (Dh-Do) / Do * 100

On the other hand, the cold-rolled steel sheet of the present invention preferably has a tensile strength (TS) of about 590 MPa or more, for example, about 600 to 1200 MPa, and in this case, excellent strength can be secured.

On the other hand, it is preferable that the microstructure of the cold-rolled steel sheet according to the present invention includes at least 80% of martensite in an area fraction, and the remainder may include ferrite and bainite. When the final microstructure contains more than 80% of martensite, it is easy to realize a high strength cold rolled steel sheet excellent in tensile strength.

The high-strength cold-rolled steel sheet having excellent stretch flangeability of the present invention described above can be manufactured by various methods, and the production method thereof is not particularly limited. However, as an embodiment thereof, it can be produced by the following method.

More specifically, the present invention relates to a method for producing a high strength cold rolled steel sheet excellent in stretch flangeability, comprising 0.05 to 0.2% by weight of carbon (C), 0.05 to 0.1% by weight of silicon (Si) 0.03 wt% or less (excluding 0 wt%), sulfur (S): 0.015 wt% or less (excluding 0 wt%), aluminum (Al): 0.02 wt% or less Preparing a hot-rolled steel sheet containing 0.1 wt% or less of niobium (Nb) (excluding 0 wt%), 0.01 wt% or less of nitrogen (N), and the balance of iron (Fe) and other unavoidable impurities; Cold rolling the hot rolled steel sheet; Annealing annealing after cold rolling; A quenching heat treatment after annealing; And performing post heat treatment after the quenching heat treatment; Wherein the content (% by weight) of the carbon (C) is set to [C], the content (wt%) of manganese (Mn) is set to [Mn], the annealing heat treatment temperature ([Deg.] C) is [T2], it may be that it satisfies the following [Relation 2].

[Relation 2]

3.4 [T1] / ° C + 4041 [C] / weight% + 350 [Mn] / weight% - 0.48 [T2] / ° C.

First, a steel ingot or a slab satisfying the above composition (hereinafter, referred to as a slab) is prepared, and the slab is reheated to 1100 to 1300 ° C. The reheating temperature is preferably 1100 to 1300 占 폚 so as to ensure a normal hot rolling temperature. If the reheating temperature is less than 1100 ° C, the hot rolling load may rapidly increase. If the reheating temperature is higher than 1300 ° C, the surface scale amount may increase, leading to a loss of the material.

Next, after the reheating, it is preferable to carry out hot rolling by a usual method and finish-finish at a temperature higher than Ar3. This is because, under Ar 3, ferrite + austenite bifunctional or ferrite reverse rolling may be performed to form a composite grain structure, and a malfunction due to fluctuations in hot rolling load may be caused.

Next, the hot-rolled steel sheet is rolled at a temperature of 720 DEG C or less. If the coiling temperature exceeds 720 占 폚, the oxide film on the surface of the steel sheet may be excessively generated and cause defects.

Next, the hot-rolled steel sheet produced as described above is annealed in a temperature range of 710 to 790 ° C in a continuous annealing furnace having an ordinary continuous cooling section after pickling and cold rolling. If the annealing temperature is lower than 710 占 폚, the risk of non-recrystallization may increase, and it may be difficult to form a sufficient austenite, and it may be difficult to ensure the desired strength in the present invention. If the annealing temperature is higher than 790 DEG C, the amount of bainite increases suddenly due to the formation of excessive austenite, which may lead to an excessive increase in yield strength and deterioration of ductility.

Next, after quenching heat treatment at a cooling rate of 160 DEG C / s or more after the annealing heat treatment, the steel sheet is subjected to a post heat treatment (tempering) in a temperature range of 25 to 310 DEG C, for example, a temperature range of 25 to 250 DEG C, The excellent high-strength cold-rolled steel sheet can be produced. If the post-heat treatment temperature is lower than 25 ° C, there is a problem that the tempering effect for carbon diffusion can not be sufficiently obtained. If the post-heat treatment temperature is higher than 310 ° C, the strength of the martensite phase is greatly reduced .

The present invention also provides a method for producing a high strength cold rolled steel sheet excellent in stretch flangeability, wherein the content (wt%) of carbon (C) and the content (wt%) of manganese (Mn) , And the annealing heat treatment temperature (° C.) is set to [T1] and the post-heat treatment temperature (° C.) is set to [T2], for example, 3.4 [T1] / ° C. + 4041 [C] / weight% + 350 [Mn] / weight% - 0.48 [T2] / ° C may be about 3350 ~ 4200.

[Relation 2]

3.4 [T1] / ° C + 4041 [C] / weight% + 350 [Mn] / weight% - 0.48 [T2] / ° C.

According to the research conducted by the inventors of the present invention, when the relation [2] is satisfied, specifically, when the annealing heat treatment temperature and post heat treatment temperature together with the content of carbon and manganese are controlled within a specific range, Strength, and had a low level of tensile strength when it was not satisfactory.

Hereinafter, the present invention will be described more specifically by way of examples.

Example

The steel having the composition (unit: wt%) shown in the following Table 1 was vacuum-melted with a 34 kg ingot, followed by sizing rolling to prepare a hot-rolled slab. The hot rolled coils were maintained at a temperature of 1200 ° C. for 1 hour, after finishing rolling at 900 ° C., and then charged into a preheated furnace at 680 ° C. for 1 hour. After pickling and 50% cold rolling, the specimens annealed at 710-790 ° C were water-cooled at a cooling rate of 160 ° C / s and then subjected to post-heat treatment at 25 ° C to 310 ° C to measure the mechanical properties of the steel sheet. Are shown in Table 2.

Remarks C Si Mn P S Al Nb N Comparative River 1 0.2 0.1 0.8 0.01 0.003 0.015 0.02 0.004 Comparative River 2 0.2 0.1 0.2 0.01 0.003 0.015 0.02 0.004 Inventive Steel 1 0.06 0.1 1.7 0.01 0.003 0.015 0.02 0.004 Invention river 2 0.2 0.1 1.5 0.01 0.003 0.015 0.02 0.004 Invention steel 3 0.09 0.1 1.7 0.01 0.003 0.015 0.02 0.004

Remarks Heat treatment condition Mechanical properties Relationship 1 Relation 2 Steel grade Annealing temperature Post heat treatment temperature Yield strength The tensile strength Elongation Yield ratio (° C) (° C) (MPa) (MPa) (%) Comparative Example 1 710 25 450 542 20.4 0.83 41 3240 Comparative River 1 Comparative Example 2 220 452 486 26.5 0.93 42 3147 Comparative Example 3 240 442 473 27.2 0.93 42 3137 Comparative Example 4 310 390 443 32.4 0.88 47 3104 Comparative Example 5 710 25 424 490 24.9 0.87 19 2881 Comparative River 2 Comparative Example 6 220 505 410 23.6 1.23 31 2787 Comparative Example 7 240 475 382 27.6 1.24 49 2778 Comparative Example 8 310 414 337 34.9 1.23 48 2744 Comparative Example 9 750 25 351 565 12.1 0.62 5 3017 Comparative Example 10 220 383 547 13.1 0.7 17 2923 Comparative Example 11 240 395 536 15.1 0.74 36 2914 Comparative Example 12 310 379 482 15.6 0.79 29 2880 Inventory 1 750 220 436 626 23.2 0.7 58 3412 Inventive Steel 1 Inventory 2 240 431 604 25.6 0.71 58 3402 Inventory 3 790 220 450 654 21.9 0.69 53 3548 Honorable 4 240 444 626 17.4 0.71 59 3538 Inventory 5 310 467 598 19.6 0.78 76 3505 Inventory 6 710 25 384 684 19.7 0.56 51 3662 Invention river 2 Honorable 7 220 530 609 18.2 0.87 50 3568 Honors 8 750 310 457 635 21.3 0.72 54 3350 Invention steel 3 Proposition 9 790 310 534 692 16.1 0.77 47 3486

1. Specimen size and shape: Specification JIS-5, specimen processing in the tensile direction perpendicular to the rolling direction, Cross Head Speed: 10 mm / min

2. Yield strength (MPa): (load corresponding to 0.2% elongation) (N) / specimen cross section (mm ² )

3. Tensile strength (MPa): Maximum tensile load (N) / Specimen cross section (mm ² )

4. Elongation (%): (distance between the points after fracture-gauge distance) / gauge distance

As can be seen from Tables 1 and 2, when the value of the relational expression 1 proposed by the present invention is 45% or more, it has excellent hole expandability. In the case of Inventive Examples 1 to 9, it can be seen that the stretch flangeability is excellent .

As can be seen from Tables 1 and 2, in Examples 1 to 9, which satisfy the alloy composition and heat treatment conditions proposed by the present invention, the tensile strength is 590 MPa or more, which indicates that excellent strength is secured. At this time, as a result of the electron microscope, the microstructure of the cold-rolled steel sheets of Inventive Examples 1 to 9 contained at least 80% of martensite and the remaining ferrite and bainite in an area fraction.

However, in the case of Comparative Examples 1 to 12, stretch flangeability is generally lower than that of Examples 1 to 9, and it is found that the strength is low because the desired microstructure can not be secured beyond the alloy composition proposed by the present invention.

1 is a graph showing the relationship between the value of the relational expression 2 of the present invention and the tensile strength. As can be seen from FIG. 1, when the value of the relational expression 2 proposed by the present invention is 3254 or more, an excellent tensile strength of 590 MPa or more can be secured. On the other hand, when the value is less than 3254, the tensile strength is low.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed exemplary embodiments, but, on the contrary, It will be obvious to those of ordinary skill in the art.

Claims (7)

0.05 to 0.2 wt% of carbon (C), 0.05 to 0.1 wt% of silicon (Si), 1.5 to 2.0 wt% of manganese (Mn), 0.03 wt% or less of phosphorus (P) (Excluding 0% by weight), niobium (Nb): 0.1% by weight or less (excluding 0% by weight), nitrogen (N): 0.01% (Fe) and other unavoidable impurities,
(%) Lambda expressed by the following [Relation 1] is 45% or more when Do is the initial hole diameter (mm) and Dh is the hole diameter after fracture (mm)
High-strength cold-rolled steel sheet excellent in elongation flangeability including 80% or more of martensite in an area fraction of microstructure.
[Relation 1]
? = (Dh-Do) / Do * 100
The method according to claim 1,
Wherein the cold-rolled steel sheet has a tensile strength of 590 MPa or more.
delete 0.05 to 0.2 wt% of carbon (C), 0.05 to 0.1 wt% of silicon (Si), 1.5 to 2.0 wt% of manganese (Mn), 0.03 wt% or less of phosphorus (P) (Excluding 0% by weight), niobium (Nb): 0.1% by weight or less (excluding 0% by weight), nitrogen (N): 0.01% (Fe) and other unavoidable impurities;
Cold rolling the hot rolled steel sheet;
Annealing the steel sheet in a temperature range of 710 to 790 ° C after the cold rolling;
A quenching heat treatment from the annealing heat treatment temperature after the annealing heat treatment; And
Performing post-heat treatment after the quenching heat treatment;
Wherein the content (% by weight) of the carbon (C) is set to [C], the content (wt%) of manganese (Mn) is set to [Mn], the annealing heat treatment temperature (T2), and satisfies the following relational expression (2): " (2) "
[Relation 2]
3.4 [T1] / ° C + 4041 [C] / weight% + 350 [Mn] / weight% - 0.48 [T2] / ° C.
5. The method of claim 4,
The step of preparing the hot-
0.05 to 0.2 wt% of carbon (C), 0.05 to 0.1 wt% of silicon (Si), 1.5 to 2.0 wt% of manganese (Mn), 0.03 wt% or less of phosphorus (P) (Excluding 0% by weight), niobium (Nb): 0.1% by weight or less (excluding 0% by weight), nitrogen (N): 0.01% (Fe) and other unavoidable impurities to a temperature of 1100 to 1300 占 폚;
Subjecting the reheated steel slab to finishing hot rolling so that the final hot rolling temperature is equal to or higher than Ar3; And
Winding the hot-rolled steel sheet at a temperature of 720 占 폚 or lower;
Wherein the high-strength cold-rolled steel sheet has excellent stretch flangeability.
delete 5. The method of claim 4,
Wherein the post-heat treatment step is post-heat treatment in a temperature range of 25 to 310 DEG C, and the stretch flangeability is excellent.

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