KR101153696B1 - Steel Sheet having Excellent Yield Strength and Stretch Flange Ability and Manufacturing Method Thereof - Google Patents

Abstract

The present invention is in weight percent, carbon (C): 0.02 to 0.1%, silicon (Si): 0.1 to 2.5%, manganese (Mn): 0.1 to 2.3%, chromium (Cr): 0.01 to 1.5%, aluminum (Al) ): 0.02 to 0.1%, niobium (Nb): 0.01 to 0.08%, titanium (Ti): 0.5 * 48/14 * [nitrogen content]% to 0.1%, phosphorus (P): 0.05% or less, sulfur (S) : 0.015% or less, nitrogen (N): 0.015% or less, containing residual iron (Fe) and other unavoidable impurities, and the microstructure yields 30-95% of bainite, 10% or less of phase martensite, and residual ferrite The present invention relates to a steel sheet excellent in strength and elongation flangeability.

Description

Steel Sheet having Excellent Yield Strength and Stretch Flange Ability and Manufacturing Method Thereof}

The present invention relates to a steel sheet and a method for manufacturing the same, which can be applied to automobile parts molded by various molding methods such as press forming or roll forming, and more particularly, to a steel sheet having excellent elongation flangeability and high yield strength It is about a method.

In recent years, the automobile industry has been required to reduce the thickness of components according to stricter environmental regulations and safety evaluations, and it is essential to increase the strength of the steel sheet to support this.

In general, precipitation-reinforced steel or ferrite / pearlite steel of ferrite base has been used in order to increase the strength, and such steel has a problem in that ductility and elongation flangeability decrease as strength increases. Therefore, in order to solve this problem, a technique for securing stretch flangeability and ductility has been proposed by forming a mixed structure composed of equiaxed ferrite or acicular ferrite and bainite.

This technique includes Japanese Patent Application Laid-Open No. 1996-269538, which is a method for improving elongation flangeability by suppressing segregation of P by winding at low temperature while suppressing the amount of retained austenite during winding. In addition, Korean Laid-Open Patent Publication No. 2003-55339, which relates to a hot-rolled steel sheet having strength of 690 MPa or more and excellent elongation and elongation flange, mainly based on ferrite-bainite structure. At this time, the ferrite ratio is 80% or more, and the ratio of the short diameter (ds) and the long diameter (dl) of the crystal grains (ds / dl) of 0.1 or more suggests a method of controlling so that more than 80% of the crystal grains, but The same manufacturing method is very effective for improving the extension flange, but the importance of weldability in the application of automotive parts has been overlooked. There is a problem in that the extension flange is deteriorated by decarburization.

Another technique is Japanese Laid-Open Patent Publication No. 2008-001984, which is mainly wound up at a temperature of less than 400 ° C. in order to produce a high strength hot rolled steel sheet having good elongation flangeability and excellent molding processability. Although it is a technology, the heat transfer coefficient suddenly changes below 400 ° C., and thus the temperature hit ratio decreases during the winding operation, making it difficult to control the microstructure.

Another technique is Japanese Laid-Open Patent Publication No. 2008-069425. Although the fraction of bainite is controlled to 90% or more in order to improve the extension flange, in this case, the ductility decreases, so that other than hole expansion There is a disadvantage that the moldability deteriorates.

The present invention is to solve the problems of the known technology, to provide a high-strength steel sheet and a method for manufacturing the same that can be manufactured more economically and excellent stretch flangeability and high yield strength.

In one embodiment, the present invention provides, in weight percent, carbon (C): 0.02 to 0.1%, silicon (Si): 0.1 to 2.5%, manganese (Mn): 0.1 to 2.3%, chromium (Cr): 0.01 to 1.5 %, Aluminum (Al): 0.02 to 0.1%, niobium (Nb): 0.01 to 0.08%, titanium (Ti): 0.5 * 48/14 * [nitrogen content]% to 0.1%, phosphorus (P): 0.05% or less , Sulfur (S): 0.015% or less, nitrogen (N): 0.015% or less, balance iron (Fe) and other unavoidable impurities, the microstructure of bainite 30-95%, phase martensite 10% and residual Provided is a steel sheet having excellent yield strength and stretch flangeability including ferrite.

The steel sheet may additionally include 0.005 to 0.05% by weight of antimony (Sb).

The yield strength of the steel sheet is preferably 600 MPa or more, and the tensile strength is 780 MPa or more.

The steel sheet may be a hot rolled steel sheet or a pickling steel sheet.

As another embodiment of the present invention, in weight%, carbon (C): 0.02 to 0.1%, silicon (Si): 0.1 to 2.5%, manganese (Mn): 0.1 to 2.3%, chromium (Cr): 0.01 to 1.5 %, Aluminum (Al): 0.02 to 0.1%, niobium (Nb): 0.01 to 0.08%, titanium (Ti): 0.5 * 48/14 * [nitrogen content]% to 0.1%, phosphorus (P): 0.05% or less Hot rolling at an Ar3 transformation point after heating the slab containing sulfur (S): 0.015% or less, nitrogen (N): 0.015% or less, residual iron (Fe) and other unavoidable impurities; Cooling the hot rolled steel sheet at a cooling rate of 20 ° C./s or more, and yield strength and stretching plan for winding the cooled steel sheet at a temperature of Ms (martensite transformation start temperature) or more below a value of the following formula (1): Provided is a method for producing a steel sheet having excellent oil resistance.

Equation (1): 791.3-3454.3 (carbon content) -27.2 (silicon content) +4.3 (manganese content)

The steel sheet may additionally include 0.005 to 0.05% by weight of antimony (Sb).

The manufacturing method may further include a step of pickling after the winding step.

The present invention provides a steel sheet that is excellent in weldability and high in yield strength, and which is easy to be applied to structural members.

According to the present invention, the component system and the coiling temperature of the steel sheet are properly controlled, and bainite is used as the main phase, and the island martensite fraction is formed to 10% or less, thereby improving the elongation flangeability and the yield strength. Weldability can be improved by appropriately controlling components having excellent hardenability.

Hereinafter, the component system of the steel sheet of the present invention will be described.

Carbon (C): 0.02 to 0.1% by weight

Carbon is a useful element that combines with a carbide forming element to precipitate as a carbide or is dissolved in ferrite to improve strength. If the carbon content is less than 0.02% by weight, it is difficult to secure strength as a material for automobile parts. On the other hand, when it exceeds 0.1% by weight, the strength is excessively increased due to martensite formed during quenching after welding, thereby degrading weldability.

Silicon (Si): 0.1-2.5 wt%

Silicon improves the strength of ferrite by solid solution strengthening and inhibits carbide precipitation to prevent cluster formation of pearlite. When the content of silicon is less than 0.1% by weight, the above effects are insignificant. On the other hand, when it exceeds 2.5 weight%, rolling property falls very much.

Manganese (Mn): 0.1-2.3 wt%

Manganese prevents redness brittleness due to FeS formation in which sulfur and iron are inevitably contained in the steel manufacturing process, and improves the strength of the steel by solid solution strengthening. When the content of manganese is less than 0.1% by weight, the above effects are insignificant. On the other hand, if the content exceeds 2.3% by weight, the bainite transformation is very delayed, resulting in a problem that the fraction of the martensite phase is finally increased.

Chromium (Cr): 0.01-1.5 wt%

Chromium is a hardenability-improving element that prevents surface decarburization of steels and contributes to securing low temperature transformation structure during cooling. In addition, precipitates formed by precipitate forming elements such as titanium and niobium are not easily re-precipitated after melting due to high temperature rapid heating and rapid cooling during welding, so that the hardness of the weld heat affected zone deteriorates. Can be. When the content of chromium is less than 0.01% by weight, the above effects are insignificant. On the other hand, when it exceeds 1.5 weight%, ductility falls and manufacturing cost rises.

Aluminum (Al): 0.02 to 0.1 wt%

Aluminum serves as a deoxidizer and prevents the formation of non-metallic inclusions upon solidification. When the aluminum content is less than 0.02% by weight, the above effects are insignificant. On the other hand, when it exceeds 0.1 weight%, manufacturing cost rises.

Niobium (Nb): 0.01 to 0.08 wt%

Niobium bonds with carbon to precipitate as fine carbide, thereby improving the strength of the steel. When the content of niobium is less than 0.01% by weight, the above effects are insignificant. On the other hand, when it exceeds 0.08 weight%, ductility falls and manufacturing cost rises.

Titanium (Ti): 0.5 * 48/14 * [nitrogen content] weight%-0.1 weight%

Titanium combines with nitrogen to precipitate TiN to improve strength. In order to exhibit such an effect, 50% or more of 48/14 * [nitrogen content] weight% which is a stoichiometric expectation value is added preferably. But. If it exceeds 0.1% by weight, a large amount of TiC is formed, rather the ductility is lowered and the manufacturing cost is increased.

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

However, since nitrogen, phosphorus, and sulfur are generally mentioned impurities, they will be briefly described as follows.

Nitrogen (P): 0.015% by weight or less

Nitrogen is an inevitable impurity, and it is desirable to control it as low as possible. In theory, the nitrogen content is advantageously limited to 0%, but inevitably contained in the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the nitrogen content is preferably limited to 0.015% by weight.

Phosphorus (P): 0.05 wt% or less

Phosphorus is an impurity contained inevitably, and is contained in steel to reduce weldability. Therefore, it is preferable to control phosphorus as low as possible. In theory, the phosphorus content is advantageously limited to 0%, but inevitably contained in the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the phosphorus content is preferably limited to 0.05% by weight.

Sulfur (S): 0.015 wt% or less

Sulfur is an inevitable impurity, and reacts with manganese to form MnS to increase the content of precipitates, so it is desirable to suppress the content as much as possible. In theory, the sulfur content is advantageously limited to 0%, but inevitably contained in the manufacturing process. Therefore, it is important to manage the upper limit, the upper limit of the sulfur content in the present invention is preferably limited to 0.015% by weight.

In addition, the steel of the present invention may further improve the effect of the present invention when additionally adding antimony (Sb) described below.

Antimony (Sb): 0.005 to 0.05 wt%

Antimony is an element useful for improving surface quality, such as suppressing decarburization and improving scale peelability. If it is less than 0.005% by weight, the above effect is insignificant. On the other hand, in the case of exceeding 0.05% by weight, the effect increase is not large compared to the increase in manufacturing cost.

As the steel sheet having the above-described component system, it is necessary to limit the microstructure of the steel sheet to preferable conditions for becoming a steel sheet excellent in yield strength and stretch flangeability.

The columnar phase of the microstructure of the steel sheet of the present invention is bainite, and is contained in an area fraction of 30 to 95%. By the bainite structure of the said range, the strength, ductility, etc. of a steel plate can be improved. If bainite is less than 30%, the strength drops. In addition, island martensite is contained in less than 10% by area fraction. When it contains more than 10%, yield strength falls and elongation improves, but hole expandability falls. By appropriate implementation of the winding process described below, the fraction of martensite and retained austenite can be limited to 10% or less. In addition, the balance other than the above structure is made of ferrite.

The steel sheet satisfying the above component system has a tensile strength of 780 MPa or more, and a yield strength of 600 MPa or more.

The most preferred method derived by the present inventors for producing the steel sheet which satisfies the object of the present invention as described above will be described below.

In the manufacturing method of the present invention, after heating the slab having the steel composition of the present invention, the heated slab is hot rolled above the Ar3 transformation point, and then cooled and wound up. At this time, the step of heating the slab corresponds to a conventional heating process.

Hereinafter, detailed conditions of each step will be described.

Hot Rolling: Finish finishing at temperatures above Ar3

When the two-phase reverse rolling is carried out in the present invention, since the mixed structure is formed and the microstructure is not uniform, workability may be reduced, it is preferable to finish the finishing rolling at a temperature of Ar3 or higher.

Cooling: Cooling rate above 20 ℃ / s

Cooling is started at the cooling rate of 20 degree-C / s or more, and finishing cooling is carried out at the coiling temperature demonstrated below. When cooling is carried out at a rate of less than 20 ° C / s ferrite transformation occurs largely there is a problem that the fraction of the ferrite increases to decrease the strength.

Winding: Ms (Martensitic transformation start temperature) ~ formula (1)

Equation (1): 791.3-3454.3 (carbon content) -27.2 (silicon content) +4.3 (manganese content)

In the present invention, the coiling temperature is an important factor for controlling the microstructure of the steel sheet. If the winding is carried out below the start temperature of martensite transformation, martensite is formed at the beginning of the curling process, thereby reducing the ductility of the steel. The upper limit of the coiling temperature was derived using a thermocalk (THERMOCALC® TCFE5) data to determine the diffusion-free transformation temperature according to the composition change of carbon, silicon, manganese.

Thereafter, the hot rolled steel sheet may be manufactured as a pickling steel sheet through a pickling process.

Hereinafter, the present invention will be described in detail through examples.

(Example)

Steel slabs satisfying the compositions shown in Table 1 were prepared by vacuum induction melting to a thickness of 30 mm and a width of 175 mm, and re-heated at 1200 ° C. for 1 hour, followed by hot rolling to obtain a hot rolled thickness of 2.5 mm. Here, hot rolling finish temperature is more than Ar3 transformation point. After the rolling, it is cooled at the cooling rate shown in Table 2 below, and wound at a winding temperature. After winding, yield strength (YS), tensile strength (TS), elongation (EL) and hole expansion ratio (HER: hole expansion ratio) were measured together and shown in Table 2 below.

Hole extensibility, an index for evaluating elongation flangeability, when a circular hole is punched into a specimen and then expanded using a conical punch, the hole expands until at least one crack in the edge of the hole penetrates in the thickness direction. The amount may be expressed as a ratio with respect to the size of the initial hole, and specifically, the hole expansion rate may be derived by the following Equation 2.

Λ = (Dh-Do) / Do * 100

(λ is the hole expansion rate (%), Do is the initial hole diameter (mm), Dh is the hole diameter after fracture (mm))

The definition of clearance at the time of punching the initial hole is also necessary to evaluate the hole expandability, which is expressed as the ratio of die and punch to the thickness of the test piece. In the embodiment of the present invention, a clearance of 10% was used.

(Equation 3) C = 0.5 * (d _d -d _p ) / t * 100 (%)

(C is clearance (%), d _d is inner diameter of punching die (mm), d _p is diameter of punching punch (d _p = 10mm), t is thickness of test piece (mm))

In addition, the microstructure picture of Inventive Example 2 is shown in Figure 1, the microstructure picture of Comparative Example 1 is shown in Figure 2.

division C Si Mn P S Al Cr Ti Nb N Sb Inventive Steel 1 0.04 1.01 1.68 0.006 0.0021 0.035 0.71 0.06 0.033 0.0052 Inventive Steel 2 0.07 1.06 2.00 0.007 0.0026 0.021 0.864 0.038 0.034 0.0047 0.023 Invention Steel 3 0.07 1.51 1.99 0.009 0.0029 0.029 0.681 0.041 0.034 0.0046 0.023 Inventive Steel 4 0.07 1.08 1.97 0.008 0.0013 0.031 0.85 0.06 0.028 0.0049 0.018 Comparative Steel 1 0.14 1.50 2.01 0.007 0.0031 0.028 - - 0.032 0.0056 0.022 Comparative Steel 2 0.14 1.49 2.00 0.009 0.0028 0.029 - - - 0.0056 0.020

Psalter
number Steel grade Cooling rate
(℃) Winding temperature upper limit
(℃) Winding temperature (℃) YS
(MPa) TS
(MPa) Hand
(%) HER
(%) Inventory 1 Inventive Steel 1 40 633 400 697 795 19.2 96 Comparative Example 1 Inventive Steel 1 15 633 400 497 650 24.0 105 Inventive Example 2 Inventive Steel 2 40 529 400 842 959 14.3 86 Inventory 3 Inventive Steel 2 40 529 520 743 937 15.4 58 Comparative Example 2 Inventive Steel 2 40 529 580 682 968 16.9 38 Honorable 4 Invention Steel 3 40 517 460 697 947 16.1 53 Comparative Example 3 Inventive Steel 4 40 529 300 951 1125 8.5 60 Inventory 5 Inventive Steel 4 40 529 400 887 984 11.7 72 Inventory 6 Inventive Steel 4 40 529 460 827 1006 13 63 Honorable 7 Inventive Steel 4 40 529 520 771 1021 12.8 53 Comparative Example 4 Comparative Steel 1 40 275 520 627 837 18.6 37 Comparative Example 5 Comparative Steel 2 40 276 520 547 713 21.8 46

As shown in Table 2, in Comparative Example 1, the cooling rate was too low to lower the yield strength and tensile strength. In Comparative Example 2, the yield strength was lowered because the coiling temperature exceeded the upper limit of the coiling temperature of the present invention and the martensite fraction exceeded 10%, the elongation was increased, but the hole expansion ratio was lowered. As shown in Figure 2, it can be confirmed that more than 10% of the martensite phase. In Comparative Example 3, martensite was formed to increase strength, but elongation was decreased. In Comparative Examples 4 and 5, the content of carbon was high and the hole expansion ratio was lowered.

On the contrary, Inventive Examples 1 to 7 satisfy the yield strength of 600 MPa or more, the tensile strength of 780 MPa or more, the elongation of 10% or more, and the hole expansion rate of 50% or more. In addition, it can be seen from FIG. 1 that in the case of the invention example 2, the island martensite was formed to 10% or less.

1 is a microstructure photograph of Inventive Example 2. FIG.

Figure 2 is a microstructure photograph of Comparative Example 2.

Claims (8)

By weight, carbon (C): 0.02 to 0.1%, silicon (Si): 0.1 to 2.5%, manganese (Mn): 0.1 to 2.3%, chromium (Cr): 0.01 to 1.5%, aluminum (Al): 0.02 ~ 0.1%, niobium (Nb): 0.01 to 0.08%, titanium (Ti): 0.5 * 48/14 * [nitrogen content]% to 0.1%, phosphorus (P): 0.05% or less, sulfur (S): 0.015% Nitrogen (N): 0.015% or less, including residual iron (Fe) and other unavoidable impurities, and the microstructure includes yield strength and elongation including 30-95% bainite, 10% or less martensite phase, and residual ferrite Steel plate with excellent flangeability. The steel sheet of claim 1, wherein the steel sheet further includes antimony (Sb): 0.005 to 0.05 wt%. The steel sheet having excellent yield strength and stretch flangeability according to claim 1, wherein the yield strength of the steel sheet is 600 MPa or more. The steel sheet having excellent yield strength and stretch flangeability according to claim 1, wherein the tensile strength of the steel sheet is 780 MPa or more. The steel sheet having excellent yield strength and stretch flangeability according to claim 1, wherein the steel sheet is a hot rolled steel sheet or a pickled steel sheet. By weight, carbon (C): 0.02 to 0.1%, silicon (Si): 0.1 to 2.5%, manganese (Mn): 0.1 to 2.3%, chromium (Cr): 0.01 to 1.5%, aluminum (Al): 0.02 ~ 0.1%, niobium (Nb): 0.01 to 0.08%, titanium (Ti): 0.5 * 48/14 * [nitrogen content]% to 0.1%, phosphorus (P): 0.05% or less, sulfur (S): 0.015% Nitrogen (N): 0.015% or less, hot rolling the slab including the remaining iron (Fe) and other unavoidable impurities after heating at an Ar3 transformation point or more; Cooling the hot rolled steel sheet at a cooling rate of 20 ° C./s or more; And A method for producing a steel sheet having excellent yield strength and elongation flangeability, wherein the cooled steel sheet is wound at a temperature of Ms (martensite transformation start temperature) or more and below a value of the following formula (1). Equation (1): 791.3-3454.3 (carbon content) -27.2 (silicon content) +4.3 (manganese content) The method of claim 6, wherein the steel sheet further comprises antimony (Sb): 0.005 to 0.05 wt%. The method of claim 6, wherein the manufacturing method further comprises pickling after the winding step.

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