KR20090068990A - High strength steel sheet with good formability and method for producing the same - Google Patents

Abstract

A high-strength steel plate having high formability and the manufacturing method thereof are provided to use commonly for automobiles and electric home appliances with relatively low expenses by reducing the manufacturing cost, improving plating properties and securing high strength. A high-strength steel plate having high formability has the alloy composition of less than 0.02 weight percent carbon(C), less than 0.4 weight percent manganese(Mn), 0.005~0.08 weight percent sulfur(S), less than 0.1 weight percent aluminum(Al), 0.005~0.01 weight percent nitrogen(N), less than 0.08 weight percent phosphorus(P), 0.0005~0.002 weight percent boron(B), 0.05~0.15 weight percent niobium(Nb) and iron(Fe).

Description

High strength steel sheet with good formability and method for producing the same

The present invention relates to a high-strength steel sheet having a high formability and a method for manufacturing the same, and more particularly, to improve the plating properties by adjusting the amount of C, N and alloy elements in Nb-added IF (Interstitial free steel) steel It relates to a high strength steel sheet having a high formability capable of ensuring high strength and a method of manufacturing the same.

High strength steel sheet having high formability, which is used as a material for automobiles, home appliances, etc., requires age resistance to secure strength and formability. Aging is a kind of strain aging that causes a defect called stretcher strain as hardening occurs as carbon (C) and nitrogen (N), which are invasive solid solutions, adhere to potential over time.

The aging resistance of the cold rolled steel sheet can be secured by the normal annealing of the aluminum-kilted steel, but the annealing has a long annealing time, which leads to low productivity and severe material deviations for each part. Therefore, IF (Interstitial Free Steel) steel is mainly used to continuously anneal by adding strong carbon and nitride forming elements such as Ti and Nb. The IF steel completely removes dissolved carbon or dissolved nitrogen to ensure aging resistance.

However, after securing the aging resistance, the strength of the IF steel is reduced, and in order to solve this problem, the alloying elements of Mn, Si, and P are added to the IF steel to increase the strength of the IF steel.

Conventional methods for increasing the strength of IF steel are disclosed in Japanese Patent Application Laid-Open No. 57-0413349, Japanese Patent Application Laid-open No. Hei 10-158783, and the like. Japanese Laid-Open Patent Publication No. 57-0413349 secures strength by adding 0.04 to 0.12% of phosphorus (P) in titanium (Ti) -added IF steel, and lowers the content of P in Japanese Patent Laid-Open Publication No. H10-158783. In addition, the solid solution of manganese (Mn) and silicon (Si) is used together to secure strength.

However, when Mn and Si are added to IF steel in a large amount, Mn and Si are not completely dissolved in the IF steel and crystallized to the surface through grain boundaries at high temperature. Therefore, oxides of manganese oxide (MnO ₂ ) and silicon oxide (SiO ₂ ) are formed on the IF steel surface. The layer is formed preferentially. As described above, when the oxide layer is formed on the surface of the IF steel, molten zinc is not plated on the portion where the oxide layer is formed, which causes a decrease in the plating property of the IF steel.

In the case of P, it is not a problem when it is uniformly distributed in the steel, but it usually forms a compound of Fe ₃ P, which deteriorates the weldability of the IF steel and promotes temper brittleness during secondary processing.

As such, adding a large amount of alloying elements of Mn, Si, and P to the IF steel deteriorates the plating property and weldability of the IF steel, and reducing the addition amount of the alloying elements of Mn, Si, and P in order to prevent this decreases the strength of the IF steel. It is accompanied by a situation that needs to be improved.

The present invention is to solve the conventional problems as described above, an object of the present invention is a high-strength steel having a high formability that can secure high strength while improving the plating characteristics in Nb-added IF (Interstitial free steel) steel It is to provide a steel sheet and a method of manufacturing the same.

The present invention is to solve the conventional problems as described above, the object of the present invention, in weight%, carbon (C): 0.02% or less, manganese (Mn): 0.4% or less, sulfur (S): 0.005 -0.08%, aluminum (Al): 0.1% or less, nitrogen (N): 0.005-0.01%, phosphorus (P): 0.08% or less, boron (B): 0.0005-0.002%, niobium (Nb): 0.05-0.15 It contains%, and has an alloy composition of the remaining iron (Fe).

By weight%, carbon (C): 0.02% or less, manganese (Mn): 0.4% or less, sulfur (S): 0.005-0.08%, aluminum (Al): 0.1% or less, nitrogen (N): 0.005-0.01% Phosphorus (P): 0.08% or less, boron (B): 0.0005-0.002%, niobium (Nb): 0.05-0.15%, has an alloy composition of the remaining iron (Fe), temperature after Ac3 point after continuous casting After reheating and holding for 3-4 hours, hot finish rolling is carried out at Ar3 point or more and Ar3 + 100 or less, and then cooled by forced cooling method, wound up to 600 ° C or less, cold rolled and continuously annealed.

The cold rolling is performed at a reduction ratio of 50 to 90%, and the continuous annealing is performed at 850 ° C. or lower. After the continuous annealing, precipitates such as CuS, MnS, NbC, and AlN are precipitated.

The average size of the precipitates is 0.2 μm or less, and the number of precipitates is 1 × 10 ⁵ / mm ² or more.

According to the present invention, in the method of manufacturing a high strength steel sheet having high formability, the content of alloying elements of Mn, Si, and P which deteriorate the plating property and weldability, and the content of C and N are increased, thereby plating characteristics of the high strength steel sheet having high formability. Improved.

In addition, the decrease in strength of the high strength steel sheet having high formability caused by lowering the content of the alloying elements of Mn, Si, P is added to Al and Nb, and the CuS, on the surface of the steel through hot rolling, cold rolling and continuous annealing. MnS, NbC and AlN to be finely precipitated to ensure high strength.

As such, the alloy design that increases the content of C and N and lowers the amount of alloying element can reduce the manufacturing cost, improve plating characteristics and secure high strength at the same time, so that it can be widely commercialized in automobiles and home appliances at relatively low cost. have.

Hereinafter, a preferred embodiment of the high strength steel sheet having a high formability according to the present invention and a manufacturing method thereof will be described in detail.

The high-strength steel sheet having high formability of the present invention is niobium (Nb) -based Interstitial Free Steel (IF) steel in weight%, carbon (C): 0.02% or less, manganese (Mn): 0.4% or less, sulfur (S) : 0.005-0.08%, aluminum (Al): 0.1% or less, nitrogen (N): 0.005-0.01%, phosphorus (P): 0.08% or less, boron (B): 0.0005-0.002%, niobium (Nb): 0.05-0.15% and consists of the remaining iron (Fe) and other unavoidable impurities.

In more detail, the contents of carbon (C) and nitrogen (N) are lowered by lowering the contents of manganese (Mn), silicon (Si), and phosphorus (P), which are alloying elements added to Nb-based IF steels, which inhibit plating properties. The strength reduction of Nb-based IF steel caused by lowering the content of alloying elements by increasing the alloying strength was increased by adding aluminum (Al) and niobium (Nb) to the IF steel through hot rolling, cold rolling, and continuous annealing. CuS, MnS, NbC and AlN to be precipitated in can be adjusted to be high strength.

Al and Nb added to the Nb-based IF steel combine with C or N to form precipitates of NbC and AlN, and the precipitates of NbC and AlN refine the grains of the Nb-based IF steel to enhance yield strength. Therefore, even if the Nb-based IF steel has a high content of C and N, aging resistance and high formability and strength can be secured by precipitation of Al and Nb.

Generally, sulfur (S) and nitrogen (N) react with titanium (Ti) preferentially and most of them are precipitated. However, in the present invention, since Ti is not added Nb alone added IF steel, S is naturally precipitated as MnS or CuS, N is AlN and C is precipitated as NbC.

The precipitates of CuS, MnS, NbC and AlN refine the grains, thereby improving the strength of the IF steel, and partially dissolved solid carbon remaining in the IF steel remains at the grain boundaries than in the grains, thereby ensuring room temperature non-aging characteristics. CuS and MnS are precipitates formed naturally when Ti is not added.

In addition, a large amount of NbC precipitates and AlN precipitates that are finely distributed in Nb-based IF steels have a positive effect on the increase in yield strength, improvement of strength-ductility balance characteristics, and in-plane anisotropy and plastic anisotropy. Get mad.

For this purpose, CuS, MnS, NbC and AlN precipitates should be finely distributed in Nb-based IF steel. This is influenced by the content of Mn, S, Al, N, Nb, C contained in the Nb-based IF steel and their component ratio conditions, the cooling rate and the coiling temperature after hot rolling.

The function and content of the alloying elements which are the basic components of the present invention are as follows.

Carbon (C): 0.002 to 0.02%

Carbon is an indispensable element for imparting high strength to steel. When the carbon content is more than 0.02%, since the dissolved carbon greatly degrades the aging resistance, a large amount of expensive Nb must be added to remove the dissolved carbon. In this case, manufacturing costs rise and recrystallization temperatures rise. Therefore, the annealing temperature should be increased, otherwise the crystal grains of the annealed steel sheet become fine and ductility is greatly lowered. Therefore, the content of carbon is preferably 0.02% or less.

And, more preferably, the content of carbon (C) is at least 0.002%. If the content of carbon (C) is less than 0.002%, the amount of NbC precipitates decreases, so that the addition of solid solution strengthening elements increases the manufacturing cost, and may indicate weakness in plating characteristics or secondary brittleness. This is because the grains are easily coarsened to lower the strength and increase the in-plane anisotropy.

Manganese (Mn): 0.01-0.5%

Manganese (Mn) is known as a solid solution strengthening element that precipitates sulfur (S) dissolved in steel as MnS to prevent hot shortness by solid sulfur. Therefore, it is common to add a high content of manganese, but in the present invention, a small amount of copper is added while lowering the content of manganese, and MnS and CuS are precipitated very finely by appropriately adding sulfur. MnS and CuS precipitated as described above have been suggested to improve the tensile strength of the steel by grain refinement, and to improve the properties of yield strength by precipitation strengthening. Therefore, the content of manganese is preferably 0.5% or less based on the study results.

And in order to secure these characteristics, the content of manganese should be more than 0.01%. If the content of manganese is less than 0.01%, the content of sulfur remaining in solid solution in the steel is increased, so red heat brittleness may occur, and if the content of manganese is more than 0.3%, the content of manganese is high and coarse MnS This is because it is difficult to secure strength because precipitates are formed. Therefore, the content of manganese (Mn) is preferably 0.01-0.5%, more preferably the content of manganese (Mn) is 0.01-0.12%.

Sulfur (S): 0.005-0.02%

Sulfur (S) reacts with manganese (Mn) to form fine MnS precipitates. When the content of sulfur (S) is less than 0.005%, not only the amount of precipitates precipitated but also the number of precipitates precipitated is very small. When the content of sulfur is more than 0.02%, the content of solute sulfur is largely low, so the ductility and moldability are greatly lowered, and the sulfur content is preferably between 0.005-0.02% because of fear of red brittleness.

Aluminum (Al): 0.01 ~ 0.1% or less

Aluminum reacts with nitrogen (N) to form fine AlN precipitates, which have a grain refinement and strength enhancement effect by precipitation strengthening. If the content of aluminum is less than 0.01%, the amount of AlN precipitates is reduced and sufficient strength cannot be secured. If the content of aluminum is more than 0.1%, there is a difficulty in continuous casting, which may lower productivity and cause excessive yield strength. have. Therefore, the content of aluminum is preferably 0.1% or less, and more preferable content of aluminum (Al) is 0.01-0.1%.

Nitrogen (N): 0.004-0.02%

When the content of nitrogen (N) is less than 0.004%, the number of precipitated AlN is small, so that the effect of grain refinement and precipitation strengthening is small, and when the content of nitrogen exceeds 0.02%, aging guarantee by solid nitrogen is difficult. . Therefore, the content of nitrogen is preferably between 0.004 ~ 0.02%.

Phosphorus (P): 0.2% or less

Phosphorus (P) is an element having a high solid solution strengthening effect and a small decrease in r value, and is used for improving strength in steels that control precipitates. In general soft cold rolled steel sheet, the content of phosphorus (P) is preferably less than 0.015%, but P is added at 0.015 to 0.2% to improve strength. However, if P content is increased, secondary processing brittleness may occur. Therefore, by lowering the content of P to secure sufficient r value and elongation, to prevent secondary processing brittleness, to secure the strength of 340MPa or more by using precipitates such as AlN, NbC, MnS, CuS. In addition, when Sn and Cr are added in the present invention, the content of phosphorus (P) is preferably 0.2% or less, but more preferably 0.08% or less, since various strength designs are possible while the content of P is in the range of 0.1% or less. Is designed as.

Boron (B): 0.0001-0.002%

Boron (B) is added to prevent secondary processing brittleness that may occur due to phosphorus (P) addition. Generally, boron is preferably added in an amount of 0.0001% or more, but if it is added in an amount of 0.002% or more, a material deviation may occur due to segregation.

Niobium (Nb): 0.01 to 0.15%

Niobium is added to steel to ensure aging resistance and to improve moldability. Niobium is a strong carbide-generating element, added to steel to precipitate NbC precipitates to precipitate carbon in solid solution, thereby securing steel age resistance. When the niobium content is less than 0.01%, the amount of precipitation of NbC precipitates is too small and the effect of strength improvement due to precipitation strengthening cannot be expected. In addition, when niobium is added in an amount exceeding 0.15%, Nb is dissolved so that the r value is lowered and the yield strength is increased, thereby weakening the formability. Therefore, the content of niobium is preferably between 0.01 and 0.15%.

In order to secure the above-mentioned CuS, MnS, NbC and AlN precipitates, the content of Cu, Mn, S, Nb, Al, N, C is managed as follows. In the relation below, each component is used in weight percent.

The finer the distribution of CuS, MnS, NbC and AlN precipitates, the better the strength, and the average size of the MnS precipitates and AlN precipitates is preferably 0.2 μm or less. According to the experimental results of the present invention, when the average size of the precipitate exceeds 0.2㎛, the strength is low, the in-plane anisotropy index is not good. Furthermore, although the precipitate of 0.2 micrometer or less is distributed in a large amount in the component system of this invention, the distribution number is not specifically limited. Preferably, the number of distribution of the precipitates is at least 1 × 10 ⁵ , more preferably at least 1 × 10 ⁶ per mm ² . The larger the distribution number of precipitates, the higher the plastic anisotropy index and the lower in-plane anisotropy index are.

In the present invention, when applied to a high-strength steel sheet of 340 MPa or more, one or two or more solid solution elements such as P, that is, Sn, Cr, Zr, and Mo, may be added. P, which is the alloying element of the present invention, has been described above, and the description of the contents of tin (Sn) and chromium (Cr) will be described without the redundant substrate.

Tin (Sn): 0.1-0.8%

Tin (Sn) is an element with high solid solution strengthening effect but low drop in elongation, which guarantees high strength in steels that control precipitates. Tin (Sn) content of more than 0.1% to ensure strength, in the case of more than 0.8% decreases the ductility of the steel. Therefore, the content of tin is preferably between 0.001 and 0.01%.

Chromium (Cr): 0.2-1.2%

Chromium (Cr) is an element that lowers the secondary work brittle temperature with high solid-solution strengthening effect and lowers the aging index by Cr carbide, which guarantees high strength in steels that control precipitates and lowers the in-plane anisotropy index. The content of chromium (Cr) should be 0.2% or more to ensure strength, and in the case of more than 1.2%, ductility is reduced. Therefore, the content of chromium is preferably between 0.2 and 1.2%.

Hereinafter, a method of manufacturing a high strength steel sheet having high formability having an alloy composition according to the present invention will be described in detail through one embodiment.

-Manufacturing method of high strength cold rolled steel sheet with high formability

By weight%, carbon (C): 0.02% or less, manganese (Mn): 0.4% or less, sulfur (S): 0.005-0.08%, aluminum (Al): 0.1% or less, nitrogen (N): 0.005-0.01% Phosphorus (P): 0.08% or less, boron (B): 0.0005-0.002%, niobium (Nb): 0.05-0.15%, steel slab composed of the remaining iron (Fe) and other inevitable impurities By hot rolling and cold rolling to ensure that the average size of CuS, MnS, NbC, and AlN precipitates less than 0.2㎛.

At this time, the average size of MnS precipitates and AlN precipitates of the cold rolled steel sheet is affected by the manufacturing process such as reheating temperature and winding temperature together with the design of the components, but in particular directly affected by the cooling rate after hot rolling.

-Hot rolling process

The steel slab to which the alloying element is added is reheated at a sufficiently high temperature and hot rolled. At this time, if the reheating temperature is low, the coarse precipitates generated during continuous casting remain in a completely insoluble state, and many coarse precipitates remain even after hot rolling, and thus must be reheated at a sufficiently high temperature.

That is, the steel slab to which the alloying element is added is reheated to a temperature of at least Ac 3 point and maintained for 3-4 hours, and hot-rolled at a temperature of at least Ar 3 to Ar 3 +100 and then cooled by forced cooling. And it is wound at less than 600 ℃ to produce a fine hot rolled steel sheet structure.

Thus, hot finishing based on the Ar3 transformation temperature is because when the hot finishing rolling temperature is less than the Ar3 transformation temperature, the workability is lowered due to the formation of the rolled grain and the strength is lowered. Therefore, it is preferable to perform hot finishing rolling at an Ar3 point or more and an Ar3 + 100 or less.

And after the hot rolling of the hot rolled steel sheet to increase the final reduction rate so as to enter the steel sheet into the cooling line as soon as possible to prevent the growth of crystal grains, the cooling rate is preferably at least 5 ℃ / sec. And even if the component ratio is controlled to obtain a fine precipitate according to the present invention, if the cooling rate is less than 5 ℃ / sec, the average size of the precipitate may exceed 0.2㎛.

-Winding condition

As mentioned above, although hot rolling is performed, it is preferable that a coiling temperature is 600 degrees C or less. This is because when the coiling temperature exceeds 600 ° C., the precipitate grows too coarsely and the grain refining effect is reduced, making it difficult to secure the strength.

-Cold rolling process

As a step of processing the hot rolled steel sheet into a final desired shape, cold rolling is preferably performed at a reduction ratio of 50 to 90%. If the cold reduction rate is less than 50%, the amount of nucleation is small at the time of annealing recrystallization, and grains grow too much at the time of annealing, and thus the strength decreases due to coarsening of the annealing recrystallization grain. In the case where the cold reduction rate is greater than 90%, the amount of nucleation is so large that the annealing recrystallized grain becomes too fine, so that the ductility decreases, thereby degrading the formability.

-Continuous annealing

After cold rolling, the structure formed by annealing at a temperature of 850 ° C. or less is fine and the average size of NbC precipitates and AlN precipitates is 0.1 μm or less. At this time, the continuous annealing temperature plays an important role in determining the material of the product. In other words, if the annealing temperature is too low, recrystallization is not completed and the target ductility value cannot be secured. If the annealing temperature is higher than 850 ° C, the strength decreases due to coarsening of the recrystallized grain. And the annealing temperature holding time is maintained so that recrystallization is completed, the recrystallization is completed when about 10 seconds or more.

Hereinafter, a method of manufacturing a high strength steel sheet having high formability according to the present invention will be described in detail in comparison with the prior art. To aid the understanding of the invention, the data values according to the examples according to the present invention and the conventional comparative examples are shown as a table.

Table 1 shows the present invention and the conventional alloy design is divided into Examples and Comparative Examples, Table 2 shows the results of the mechanical properties according to the alloy design of Table 1.

(Final alloy component wt% of steel sheet: balance Fe) C Si Mn P S Al N Ti Nb B Comparative Example 1 0.0020 0.2 0.99 0.068 0.0068 0.04 0.0043 0.022 0.059 0.0006 Comparative Example 2 0.0030 0.2 1.01 0.067 0.0063 0.049 0.0055 0.022 0.057 0.0007 Comparative Example 3 0.0030 0.2 1.58 0.051 0.0057 0.041 0.0054 0.08 Comparative Example 4 0.0040 0.2 0.98 0.050 0.0061 0.022 0.0061 0.78 Comparative Example 5 0.0035 0.3 0.59 0.072 0.0060 0.37 0.0044 0.08 Example 1 0.0100 0.1 0.1 0.047 0.0065 0.052 0.0047 0.057 0.02 0.0005 Example 2 0.0070 0.1 0.14 0.050 0.0059 0.1 0.0053 0.058 0.0006 Example 3 0.0070 0.1 0.11 0.045 0.0056 0.062 0.0080 0.061 0.0010 Example 4 0.0060 0.1 0.3 0.050 0.0063 0.063 0.0090 0.063 0.009 Example 5 0.0080 0.1 0.11 0.040 0.0057 0.052 0.0064 0.076 0.006

TS (MPa) YS (MPa) EL (%) r-value Comparative Example 1 443 314 33 1.2 Comparative Example 2 450 282 35 1.5 Comparative Example 3 440 344 36 1.2 Comparative Example 4 459 278 38 1.8 Comparative Example 5 427 290 37 1.4 Example 1 440 316 34 1.4 Example 2 463 329 41 1.6 Example 3 441 296 42 1.8 Example 4 446 293 39 1.7 Example 5 462 311 36 1.5

[TS (MPa): Tensile strength, YS (MPa): Yield strength, EL (%): Elongation, r-value: Index indicating drawing property]

Table 2 shows the steel slab having the alloy design shown in Table 1, heated at 1250 ℃ for 1 hour, hot rolled at 900 ℃ for finishing, then quenched to 560 ℃, rolled at 70% of cold rolling rate and annealed at 800 ℃. Tensile strength, yield strength, elongation, etc. of the manufactured steel sheet were measured.

Through the examples and comparative examples of Table 1 and Table 2, even if the alloying elements (Si, Mn, P) is reduced, the strength is improved and the formability is improved by the addition of Al and Nb according to the increase in the content of C and N. have.

Here, the reference value of high strength steel sheet having high formability satisfying the mechanical properties is tensile strength: 440 or more, elongation: 34% or more, and r-value is 1.4 or more. Able to know.

In addition, when the content of Nb is lowered in Example 1 and Ti is added in place of Nb, the strength and formability may be satisfied, but the strength and formability may not be improved compared to the case where Nb alone is added. .

As described above, the high strength steel sheet having high formability of the present invention may have characteristics of high strength cold rolled steel sheet of 400 MPa or more according to the component design. By controlling the addition of solid solution element to the steel sheet to ensure high strength properties. That is, in the case of high strength steel containing P, 1.0% of Mn and 0.3% of Si are added, and the content of P is generally 0.03% to 0.2%, but in the present invention, the content of such alloying elements is lowered and the content of C and N is lower. As the content of Al and Nb is increased by adding 0.03 to 0.05% and 0.01 to 0.03%, respectively, it is possible to secure strength by precipitates of AlN and NbC and maintain favorable properties for plating.

Within the scope of the basic technical idea of the present invention, many other modifications are possible to those skilled in the art, and the scope of the present invention should be interpreted based on the appended claims. will be.

Claims (4)

By weight%, carbon (C): 0.02% or less, manganese (Mn): 0.4% or less, sulfur (S): 0.005-0.08%, aluminum (Al): 0.1% or less, nitrogen (N): 0.005-0.01% Phosphorus (P): 0.08% or less, boron (B): 0.0005-0.002%, niobium (Nb): 0.05-0.15%, and has a high formability, characterized in that the alloy composition of the remaining iron (Fe) High strength steel sheet. By weight%, carbon (C): 0.02% or less, manganese (Mn): 0.4% or less, sulfur (S): 0.005-0.08%, aluminum (Al): 0.1% or less, nitrogen (N): 0.005-0.01% Phosphorus (P): 0.08% or less, boron (B): 0.0005-0.002%, niobium (Nb): 0.05-0.15%, containing the alloy composition of the remaining iron (Fe), After continuous casting, it is reheated to a temperature of Ac3 or more and maintained for 3-4 hours. After hot finishing rolling at Ar3 or more and Ar3 + 100 or less, it is cooled by forced cooling and wound up to 600 ° C or less, cold rolled, and continuously annealed. Method for producing a high strength steel sheet having a high formability, characterized in that. The method of claim 2, The cold rolling is carried out at a reduction ratio of 50 to 90%, and the continuous annealing is performed at 850 ° C. or lower, and after the continuous annealing, precipitates such as CuS, MnS, NbC, and AlN are precipitated. Method of manufacturing steel sheet. The method of claim 3, wherein The average size of the precipitate is 0.2㎛ or less, the number of precipitates is 1X10 ⁵ / mm ² or more method of manufacturing a high strength steel sheet having high formability, characterized in that.