KR101003221B1 - High strength Cold-rolled sheet with good formability, and method for producing the same - Google Patents

Abstract

The present invention relates to a high strength cold rolled steel sheet excellent in formability. The present invention is in weight%, carbon (C): 0.004-0.02%, manganese (Mn): 0.01-0.5%, sulfur (S): 0.005-0.01%, aluminum (Al): 0.01-0.1%, nitrogen (N ): 0.004-0.01%, phosphorus (P): 0.01 to 0.05%, boron (B): 0.0005-0.002%, titanium (Ti): 0.04-0.1%, niobium (Nb): 0.02-0.15%, copper (Cu ): 0.1-0.2%, nickel (Ni): 0.01-0.05%, chromium (Cr): 0.01-0.1%, tin (Sn): 0.02-0.1%, with the remaining iron (Fe) and other unavoidable impurities Alloy composition. In the present invention, since the strength of the cold rolled steel sheet using fine NbC and AlN precipitates and a small amount of Cu, Cr, Ni, and Sn instead of the alloying elements of Mn, Si, P, the coating properties as well as the plating properties can be improved and the manufacturing cost is also increased. There is a benefit of being saved.

Cold Rolled Sheet, High Formability, High Strength

Description

High strength cold-rolled sheet with good formability, and method for producing the same

The present invention relates to a cold rolled steel sheet, and more particularly, to a high strength cold rolled steel sheet excellent in formability capable of securing high strength while improving workability and a method of manufacturing the same.

Cold rolled steel sheet 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 non-aging property of the cold rolled steel sheet can be secured by the annealing of the aluminum-kilted steel, but the annealing has the disadvantage that the annealing time is long and the productivity is low and the material deviation is severe for each part. Therefore, in recent years, IF steel (Interstitial Free Steel) which is continuously annealed by adding strong carbon and nitride forming elements such as titanium (Ti) and niobium (Nb) is mainly used. The IF steel completely removes dissolved carbon or dissolved nitrogen to ensure aging resistance.

However, the strength of the IF steel decreases after securing the aging resistance, and the alloying elements of Mn, Si, and P are added to the IF steel to solve the above problems, thereby increasing the strength of the IF steel. It is added within 0.3% depending on the strength, in the case of high strength steel containing P, the content of P is generally 0.03 ~ 0.1%.

Such a conventional method of increasing the strength of IF steel is disclosed in JP-A-57-0413349, JP-A-10-158783, and the like. Japanese Laid-Open Patent Publication No. 57-0413349 adds 0.04 to 0.12% of phosphorus (P) to titanium (Ti) -added IF steel to secure the strength. While lowering the content, strength is secured by using a solid solution element of manganese (Mn) and silicon (Si) together.

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, so that oxides of manganese oxide (MnO ₂ ) and silicon oxide (SiO ₂ ) 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 addition, in the case of Nb-added IF steel, as the Mn content increases, the hot rolled tensile strength increases to increase the rolling load during cold rolling, which may cause the wear of the rolling rolls to become severe.

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, the object of the present invention is to C and N added to the Ti-Nb-based IF (Interstitial free steel) steel to satisfy the plating properties, high strength and formability at the same time And it is to provide a high strength cold rolled steel sheet excellent in formability to control the addition amount of the alloying element and its manufacturing method.

The present invention is to solve the conventional problems as described above, the present invention by weight, carbon (C): 0.004 ~ 0.02%, manganese (Mn): 0.01 ~ 0.5%, sulfur (S): 0.005- 0.01%, aluminum (Al): 0.01 to 0.1%, nitrogen (N): 0.004-0.01%, phosphorus (P): 0.01 to 0.05%, boron (B): 0.0005-0.002%, titanium (Ti): 0.04- 0.1%, niobium (Nb): 0.02-0.15%, copper (Cu): 0.1-0.2%, nickel (Ni): 0.01-0.05%, chromium (Cr): 0.01-0.1%, tin (Sn): 0.02- It contains 0.1%, has an alloy composition composed of the remaining iron (Fe) and other unavoidable impurities, and contains precipitates of NbC and AlN.

When the content of copper (Cu) satisfies 0.1 to 0.14%, the content of copper (Cu), nickel (Ni), chromium (Cr), and tin (Sn) is (Cu + Ni + Cr + Sn) ≤ 0.18 It is a range that satisfies the expression of%.

The average size of the precipitates is more than 0 and less than 0.1㎛.

By weight%, carbon (C): 0.004-0.02%, manganese (Mn): 0.01-0.5%, sulfur (S): 0.005-0.01%, aluminum (Al): 0.01-0.1%, nitrogen (N): 0.004 -0.01%, phosphorus (P): 0.01 to 0.05%, boron (B): 0.0005-0.002%, titanium (Ti): 0.04-0.1%, niobium (Nb): 0.02-0.15%, copper (Cu): 0.1 -0.2%, nickel (Ni): 0.01-0.05%, chromium (Cr): 0.01-0.1%, tin (Sn): 0.02-0.1%, the alloy composition consisting of the remaining iron (Fe) and other unavoidable impurities After continuous casting, it is re-heated to a temperature of at least Ac3 point and maintained for 3-4 hours. After hot finishing rolling at Ar3 point or more and Ar3 + 100 or less, it is cooled, wound up at room temperature or more and 600 ° C or less, cold rolled, and continuously annealed. It is characterized by.

When the content of copper (Cu) satisfies 0.1 to 0.14%, the content of copper (Cu), nickel (Ni), chromium (Cr), and tin (Sn) is (Cu + Ni + Cr + Sn) ≤ 0.18 It is a range that satisfies the expression of%.

Cold rolled steel sheet of the present invention is to reduce the content of the alloying elements of Mn, Si, P deteriorating the plating property and weldability, improve the strength and plating properties by increasing the content of C and N.

In addition, the decrease in strength of the steel sheet caused by lowering the content of the alloying elements of Mn, Si, and P is added by a small amount of tramp elements (Cu, Cr, Ni, Sn), and the grains are subjected to hot rolling, cold rolling, and continuous annealing. NbC and AlN inside the fine precipitate to ensure high strength.

As such, the alloy design that increases the content of C and N and decreases the addition amount of alloying elements (Mn, Si, P) can reduce the manufacturing cost and simultaneously improve the plating characteristics and high strength, so it is widely used in automobiles and home appliances at relatively low cost. There is an effect that can be commercialized.

In particular, tramp elements (Cu, Cr, Ni, Sn) are elements that should be actively added in blast furnaces, or are included in scrap in electric furnace operations, which are inevitable impurities that do not need to be added. have.

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

The cold rolled steel sheet of the present invention is, by weight%, carbon (C): 0.004-0.02%, manganese (Mn): 0.01-0.5%, sulfur (S): 0.005-0.01%, aluminum (Al): 0.01-0.1% , Nitrogen (N): 0.004-0.01%, phosphorus (P): 0.01-0.05%, boron (B): 0.0005-0.002%, titanium (Ti): 0.04-0.1%, niobium (Nb): 0.02-0.15% , Copper (Cu): 0.1-0.2%, nickel (Ni): 0.01-0.05%, chromium (Cr): 0.01-0.1%, tin (Sn): 0.02-0.1%, and the remaining iron (Fe) and It consists of other unavoidable impurities.

In more detail, the cold rolled steel sheet of the present invention is a Ti-Nb-based IF (Interstitial free steel) steel, which lowers the content of manganese (Mn), silicon (Si), and phosphorus (P), which deteriorates plating property and weldability. Increasing the content of carbon (C) and nitrogen (N) to improve the plating characteristics and lowering the strength of the cold rolled steel sheet produced by lowering the content of alloying elements, copper (Cu), chromium (Cr) ), Nickel (Ni) and tin (Sn) are added to ensure high strength.

C or N added higher than the conventional steel sheet is combined with Ti, Al, and Nb added to the Ti-Nb-based IF steel to form precipitates of TiN, TiC, NbC, and AlN. In general, the precipitates of NbC and AlN enhance the yield strength by making grains fine. The NbC and AlN precipitates finely distributed in Ti-Nb-based IF steel increase the yield strength and balance the strength-ductility due to precipitation strengthening. It has a positive effect on the improvement of properties and in-plane anisotropy and plastic anisotropy.

Therefore, even if the content of C and N is added higher than the conventional, it is possible to secure the aging resistance and high formability and strength by the precipitation of Ti, Al and Nb. TiN, TiC, NbC and AlN precipitates must be finely distributed to increase yield strength due to precipitation strengthening. For this purpose, the contents of Ti, Al, N, Nb, and C in Ti-Nb-based IF steel and their component ratios Conditions It is important to set the conditions of the cooling rate and the coiling temperature after hot rolling.

Cu, Cr, Ni, and Sn are solid solution strengthening elements and compensate for the decrease in strength of the steel sheet by lowering the addition amount of alloying elements together with C and N. Since Cu, Cr, Ni, and Sn are inevitably contained in molten steel in an electric furnace operation, it is possible to secure a solid solution strengthening effect only by controlling the components without additional addition. Of course, blast furnace operation is an element that must be actively added, but the addition amount is much smaller than the conventional alloying elements, which is advantageous in terms of cost reduction.

Hereinafter, the cold rolled steel sheet of the present invention and a manufacturing method thereof will be described in detail below.

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

Carbon (C): 0.004-0.02 wt%

Carbon is an indispensable element for imparting high strength to steel. If the carbon content exceeds 0.02%, the dissolved carbon will greatly degrade the aging resistance, so 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 must be increased, otherwise the grains of the annealed steel become fine and ductility is greatly lowered.

And, if the content of carbon (C) is added less than 0.004%, the amount of NbC precipitates are reduced, so that a solid solution strengthening element must be added, thereby increasing the manufacturing cost. In addition, there may be a weakness in the plating properties and secondary processing brittleness, there is a problem that the strength is lowered and the in-plane anisotropy is increased by easily coarse grains of the hot rolled sheet. Therefore, the carbon (C) content is preferably adjusted within the range of 0.004 to 0.02%.

Manganese (Mn): 0.01-0.5 wt%

Manganese (Mn) is known as a solid solution strengthening element that precipitates sulfur (S) dissolved in steel as MnS to prevent hot shortness caused by solid solution 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. As a result, MnS and CuS precipitated have been suggested to improve the tensile strength of steel by grain refinement and to improve the properties of yield strength by precipitation strengthening. Therefore, the content of manganese is preferably limited to less than 0.5% 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 increases, so that red heat brittleness may occur. If the content of manganese is more than 0.3%, the content of manganese is high, resulting in coarse MnS precipitates. This is because it is difficult to secure the strength. 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): 0.005-0.01 wt%

Sulfur (S) reacts with manganese (Mn) to form fine MnS precipitates. When the sulfur (S) content is less than 0.005%, not only the amount of precipitates precipitated but also the number of precipitates precipitated is very small. In the present invention, since the Mn content is small, when the sulfur content is more than 0.01%, the content of the solid solution is high, so the ductility and moldability are greatly lowered, and there is a fear of red brittleness. Therefore, the content of sulfur is preferably between 0.005-0.01%.

Aluminum (Al): 0.01 ~ 0.1 wt%

Aluminum reacts with nitrogen to form fine AlN precipitates, which have grain-fineness 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 leads to low productivity and excessive yield strength. have. Therefore, the content of aluminum is preferably between 0.01 and 0.1%.

Nitrogen (N): 0.004-0.01 wt%

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 it exceeds 0.01%, it is difficult to guarantee the aging by solid nitrogen. Therefore, the content of nitrogen is preferably between 0.004 ~ 0.01%.

Phosphorus (P): 0.03-0.1 wt%

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 phosphorus content should be limited to 0.015% or less, but 0.015 ~ 0.2% P is added for strength improvement. However, if P content is increased, secondary processing brittleness may occur. Therefore, by lowering the content of P to ensure a sufficient r value and elongation to prevent secondary processing brittleness.

And, to compensate for this, by using a solid solution strengthening by a small amount of Cu, Ni, Cr, Sn and precipitates such as AlN, NbC to secure the strength of 340MPa or more. In the present invention, it is possible to design a variety of strength while keeping the content of P in the range of 0.1% or less. Therefore, the content of phosphorus is preferably between 0.03 ~ 0.1%.

Boron (B): 0.0001 to 0.002 wt%

Boron (B) is added to prevent secondary processing brittleness that may occur due to the addition of phosphorus (P). 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.

Titanium [Ti]: 0.04 to 0.1 wt%

Titanium (Ti) is an element that is basically added to Ti-Nb-based IF steel and combines with C or N to form fine precipitates of TiN and TiC at grain boundaries. However, when the oxidizing property is strong and added in an appropriate amount or more, there are problems in that many non-metallic inclusions are formed during steelmaking, causing surface defects on the steel sheet and increasing the recrystallization temperature. Accordingly, the content of titanium is preferably regulated in the range of 0.04 ~ 0.1%.

Niobium (Nb): 0.02 to 0.15 wt%

Niobium is added to ensure aging resistance of steel sheet 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.

Niobium cannot be expected to improve the strength due to precipitation hardening when the addition amount is less than 0.02%, and when the addition amount is more than 0.15%, Nb is dissolved to decrease the r value and increase the yield strength. This weakens the moldability. Therefore, the content of niobium is preferably between 0.02 and 0.15%.

Copper (Cu): 0.1-0.2 wt%

Copper (Cu) exists as an impurity, that is, a tramp element, which cannot be removed in a steelmaking process using scrap as a raw material. If the content of copper is more than 0.1% to form a fine CuS precipitate to increase the strength, but if the content exceeds 0.2%, not only decreases the elongation, r value and surface quality of the steel but also generates heat brittle at high temperature. Therefore, the copper content is preferably regulated between 0.1-0.2%.
More preferably, copper (Cu) does not exceed 0.14%. This is because, when the copper content exceeds 0.14%, the r value, which is a plastic workability index, is lowered.

Nickel (Ni): 0.01-0.05 wt%

Nickel (Ni) is a Tramp element like copper (Cu). Nickel should be added at least 0.01% in order to strengthen the solid solution, and it is desirable to regulate it at 0.05% or less in order to prevent a decrease in r value and elongation.

Tin (Sn): 0.002-0.01 wt%

Tin (Sn), like copper and nickel, also exists as impurities, i.e. tramp elements, which cannot be removed in the steelmaking process using scrap as a raw material. Tin has a high solute strengthening effect, but unlike other tramp elements, it plays a decisive role in reducing the mechanical properties of steel.

In more detail, when the content of Sn (Sn) is 0.01% or more, not only the strength of the steel is rapidly increased, but also the elongation and r value are decreased to adversely affect the formability. However, more than 0.002% must be added to increase the strength, so tin is preferably added in the range of 0.002-0.01%.

Chromium (Cr): 0.01 ~ 0.1%

Chromium (Cr) is also a Tramp element. Chromium has a high solids strengthening effect and a low secondary brittleness temperature. In addition, chromium lowers the aging index by forming chromium carbide, and in particular, guarantees high strength and lowers the in-plane anisotropy index in steels for controlling precipitates as in the present invention. Such chromium content may be secured when the content is 0.01% or more, and in the case of more than 0.1%, the ductility is lowered, so it is preferably added in the range of 0.01 to 0.1%.

Here, the sum of copper (Cu), nickel (Ni), chromium (Cr) and tin (Sn) components is preferably not more than 0.18%. This is because when the sum of the above components exceeds 0.18%, the ductility decreases and the moldability deteriorates. At this time, the copper content should satisfy the range of 0.1 to 0.14%, because the r value, which is a plastic workability index, decreases when the copper content exceeds 0.14%.

 In addition, the finer the precipitates in the component system, the better the strength. In particular, the average size of the NbC precipitates and AlN precipitates is preferably more than 0 and 0.1 µm or less.

According to the experimental results, when the average size of the precipitate exceeds 0.1㎛, the strength is low, the in-plane anisotropy index is not good. Furthermore, in the component system of the present invention, a large amount of precipitates of 0.1 µm or less are distributed, but the number of distribution thereof is not particularly limited. Preferably the number of distribution of precipitates is 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.

The present invention contains the above-described component system, and the rest are substantially iron (Fe) and unavoidable elements, and fine amounts of unavoidable impurities are also allowed as elements contained according to the situation of raw materials, materials, manufacturing facilities, and the like.

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

-Manufacturing method of cold rolled steel sheet

By weight%, carbon (C): 0.004-0.02%, manganese (Mn): 0.01-0.5%, sulfur (S): 0.005-0.01%, aluminum (Al): 0.01-0.1%, nitrogen (N): 0.004 -0.01%, phosphorus (P): 0.01 to 0.05%, boron (B): 0.0005-0.002%, titanium (Ti): 0.04-0.1%, niobium (Nb): 0.02-0.15%, copper (Cu): 0.1 Steel containing -0.2%, nickel (Ni): 0.01-0.05%, chromium (Cr): 0.01-0.1%, tin (Sn): 0.02-0.1% and having a composition of the remaining iron (Fe) and other unavoidable impurities The slab is hot rolled and cold rolled to satisfy an average size of 0.1 μm or less of NbC and AlN precipitates.

In this case, the average size of NbC precipitates and AlN precipitates of the cold rolled steel sheet is influenced by the manufacturing process such as reheating temperature and winding temperature together with the component design, 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 finished rolling at a temperature of Ar 3 or more 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.

In addition, 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 grains, the cooling rate is preferably at least 5 ℃ / sec. And even if the component ratio is controlled in order 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 be more than 0.1㎛.

-Winding condition

Although it winds up after hot rolling as mentioned above, it is preferable that winding temperature is more than normal temperature and 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. Here, room temperature means the temperature of the ordinary atmosphere.

-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 when the annealing material is crystallized, and the grains grow too much when the annealing is performed. Coarsening of the annealing recrystallized grains lowers the strength. In the case where the cold reduction ratio is greater than 90%, the amount of nucleation is so large that the annealing recrystallized grains become too fine, so the ductility decreases and the moldability decreases.

-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, if the recrystallization is completed in about 10 seconds or more.

Hereinafter, a high strength cold rolled steel sheet excellent in formability according to the present invention and a manufacturing method thereof will be described in detail in comparison with the prior art. To help understand 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) division
C Si Mn P S Al N Ti Nb B Cu Ni Cr Sn × 10000 × 100 × 1000 × 10000 × 1000 × 10000 × 100 × 1000 Comparative Example 1 20 - 50 50 68 400 43 22 20 6 - - - - Comparative Example 2 30 10 70 50 63 490 55 22 42 7 - - - - Comparative Example 3 30 10 100 51 57 410 54 - 65 - - - - - Comparative Example 4 40 20 59 50 61 220 61 - 78 - - - - - Comparative Example 5 35 30 98 72 60 370 44 - 80 - - - - - Example 1 60 - 10 30 65 520 47 57 20 5 10 3 2 9 Example 2 55 - 14 39 59 400 53 40 20 6 10 3 2 8 Example 3 70 - 11 45 56 620 80 40 34 10 10 4 3 8 Example 4 80 - 29 60 63 630 90 - 63 9 11 4 2 10 Example 5 80 - 38 70 57 520 64 - 76 6 10 4 3 10

division TS (MPa) YS (MPa) EL (%) r-value Remarks Comparative Example 1 341 260 41 1.7 340 MPa class Comparative Example 2 350 282 40 1.6 340 MPa class Comparative Example 3 392 344 38 1.8 390 MPa class Comparative Example 4 412 278 36 1.6 390 MPa class Comparative Example 5 459 310 35 1.4 440 MPa class Example 1 358 316 42 1.4 340 MPa class Example 2 360 329 40 1.6 340 MPa class Example 3 400 296 39 1.8 390 MPa class Example 4 446 325 37 1.7 440 MPa class Example 5 462 341 35 1.5 440 MPa class

[TS (MPa): Tensile Strength, YS (MPa): Yield Strength, EL (%): Elongation, r-value: Index indicating Formability]

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.

Although the alloying elements (Si, Mn, P) are reduced through the examples and comparative examples of Tables 1 and 2, the addition of Al and Nb and the trace amount of Tramp elements (Cu, Ni, Cr, Sn) it can be seen that the strength is improved and the moldability is improved.

As described above, the cold rolled steel sheet of the present invention may have characteristics of a high forming high strength cold rolled steel sheet of 340 MPa or more, depending on the component design. By controlling the amount of the solid solution strengthening element added to the steel sheet to ensure high strength characteristics.

That is, in the case of high strength steel containing P, 1% Mn and 0.3% Si are added, and the content of P is generally 0.03% to 0.2%, but in the present invention, the content of alloying elements that inhibit such plating properties and weldability is increased. The trace amount of Tramp elements (Cu, Ni, Cr, Sn) was added, and as the contents of C and N were increased, the Al and Nb contents were added 0.03 to 0.05% and 0.01 to 0.03%, respectively. The strength by the precipitate was secured. In addition, it is possible to maintain advantageous properties for plating by reducing the content of conventional alloying elements (Si, Mn, P).

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 (5)

By weight%, carbon (C): 0.004-0.02%, manganese (Mn): 0.01-0.5%, sulfur (S): 0.005-0.01%, aluminum (Al): 0.01-0.1%, nitrogen (N): 0.004 -0.01%, phosphorus (P): 0.01 to 0.05%, boron (B): 0.0005-0.002%, titanium (Ti): 0.04-0.1%, niobium (Nb): 0.02-0.15%, copper (Cu): 0.1 -0.2%, nickel (Ni): 0.01-0.05%, chromium (Cr): 0.01-0.1%, tin (Sn): 0.02-0.1%, the alloy composition consisting of the remaining iron (Fe) and other unavoidable impurities It has a high strength cold rolled steel sheet having excellent moldability, characterized in that it comprises a precipitate of NbC, AlN. The method of claim 1, When the content of copper (Cu) satisfies 0.1 to 0.14%, the content of copper (Cu), nickel (Ni), chromium (Cr), and tin (Sn) is (Cu + Ni + Cr + Sn) ≤ 0.18 High strength cold rolled steel sheet with excellent formability, characterized in that it satisfies the formula of%. 3. The method of claim 2, High strength cold rolled steel sheet having excellent moldability, characterized in that the average size of the precipitate is more than 0 0.1㎛. By weight%, carbon (C): 0.004-0.02%, manganese (Mn): 0.01-0.5%, sulfur (S): 0.005-0.01%, aluminum (Al): 0.01-0.1%, nitrogen (N): 0.004 -0.01%, phosphorus (P): 0.01 to 0.05%, boron (B): 0.0005-0.002%, titanium (Ti): 0.04-0.1%, niobium (Nb): 0.02-0.15%, copper (Cu): 0.1 -0.2%, nickel (Ni): 0.01-0.05%, chromium (Cr): 0.01-0.1%, tin (Sn): 0.02-0.1%, the alloy composition consisting of the remaining iron (Fe) and other unavoidable impurities Have, After continuous casting, it is reheated to the temperature of Ac3 point or more and maintained for 3-4 hours. After hot finish rolling at Ar3 point or more and Ar3 + 100 or less, it is cooled, coiled at room temperature or more and 600 ℃ or less, cold rolled, and continuously annealed. The manufacturing method of high strength cold rolled steel sheet excellent in moldability. The method of claim 4, wherein When the content of copper (Cu) satisfies 0.1 to 0.14%, the content of copper (Cu), nickel (Ni), chromium (Cr), and tin (Sn) is (Cu + Ni + Cr + Sn) ≤ 0.18 Method for producing a high strength cold rolled steel sheet having excellent formability, characterized in that the range of satisfying the formula of%.