KR100433248B1 - a cold-rolled steel with good formability and anti-dent property and the method of manufacturing the same - Google Patents

Abstract

The present invention has excellent workability and is easy to press processing, and the strength is increased in the press, painting and drying process during the manufacturing of automobile parts, and excellent in dent resistance in the final product. And it is an object to provide a high strength cold rolled steel sheet excellent in dent resistance and a method of manufacturing the same.

In the present invention, by weight%, carbon: 0.005% or less, nitrogen: 0.005% or less, sulfur: 0.02% or less, manganese: 0.2 to 2.0%, aluminum: 0.02 to 0.08%, phosphorus: 0.02 to 0.10%, Ti: 0.005 to It provides a high strength cold rolled steel sheet having a composition of 0.03%, controlling the ratio of Mn / Ti to 25 or more, and excellent workability and dent resistance including other Fe and unavoidable impurities, and a manufacturing method thereof.

Description

High-strength cold-rolled steel sheet for high processing, excellent in workability and vent resistance, and a method for manufacturing the same {a cold-rolled steel with good formability and anti-dent property and the method of manufacturing the same}

The present invention has excellent workability and is easy to press working, and excellent hardening hardenability, the strength is increased in the press, coating and drying process when manufacturing automobile parts, and excellent in dent resistance to the inner plate of the automobile and the inner side of the shock absorbing part. The present invention relates to a high strength cold rolled steel sheet and a method of manufacturing the same, and in particular, as a yield strength after painting and drying increases, it is used as an outer sheet steel sheet for automobiles requiring dent resistance and an inner sheet steel sheet for automobiles requiring impact absorbing properties, and is advantageous in plating property. The present invention relates to a high strength cold rolled steel sheet having excellent workability and dent resistance as well as a continuous annealing steel sheet, which can be applied to an inner plate of a hot dip galvanized steel sheet, and a method of manufacturing the same.

Conventional baking hardening steel sheet with excellent dent resistance is a method of controlling the carbon content of the ultra low carbon steel to 0.015% or less and securing carbon necessary for baking the hardening using an annealing furnace, and continuous carbon annealing of low carbon steel. Has been developed to secure carbon. In addition, recently, a method of controlling the amount of ultra low carbon steel carbonitride forming elements and using a continuous annealing process has been developed. The method of using an annealing furnace as an ultra low carbon steel has a disadvantage of lowering productivity. The method of using a continuous annealing furnace as a low carbon steel has low workability, and the amount of carbon required for hardening hardening is poor if the process variables of overaging are not managed well. Quality control is difficult because of lack. There are two types of hardened hardened steels containing carbonitride forming elements in ultra low carbon steels. One of them is Ti-added steel, which adds Ti to Ti / C equivalent or less in ultra-low carbon steel, and forms a solid hardening by using solid carbon. However, this method is difficult to control the ingredients. Another method of using ultra low carbon steels is to apply high temperature annealing and quenching by adding Nb. This also has a problem that the constraint of the annealing equipment.

The present invention has been made in order to solve the above problems, it is excellent in workability, easy to press processing, excellent baking hardenability increases the strength in the press, painting and drying process when manufacturing automotive parts, dent resistance It is an object of the present invention to provide a high strength cold rolled steel sheet having excellent workability and dent resistance applied to an outer plate of an automobile and an inner plate of an impact absorbing part, and a method of manufacturing the same.

In order to achieve the above object, the present invention provides a weight% of carbon: 0.005% or less, nitrogen: 0.005% or less, sulfur: 0.02% or less, manganese: 0.2 to 2.0%, aluminum: 0.02 to 0.08%, phosphorus: 0.02 It provides a high strength cold rolled steel sheet having a composition of ˜0.10%, Ti: 0.005 to 0.03%, adjusting the Mn / Ti ratio to be 25 or more, and having excellent workability and dent resistance including other Fe and unavoidable impurities.

In addition, the present invention comprises the steps of homogenizing the ultra-low carbon aluminum-kilted steel made of the composition at a temperature range of 1050 to 1300 ℃; Finishing hot rolling at a temperature in the range of 850 ° C. to 950 ° C. directly above the Ar ₃ transformation point; Hot rolling in a temperature range of 450 to 650 ° C .; Rolling at a cold reduction rate of 40 to 80%; It provides a process for producing a high strength cold rolled steel sheet excellent in workability and dent resistance consisting of; performing continuous annealing in the temperature range of 750 ~ 900 ℃.

Hereinafter, a high strength cold rolled steel sheet excellent in workability and dent resistance according to the present invention and a manufacturing method thereof will be described in detail.

First, the reason for limiting the composition range of the high strength cold rolled steel sheet excellent in workability and dent resistance according to the present invention is as follows.

When the amount of carbon (C) is more than 0.005% by weight (hereinafter, referred to as%), wrinkles or stretcher strains are generated during press processing due to an increase in yield strength due to aging before processing. In addition, if the amount of carbon is too high, it does not sufficiently fix the carbon in the winding step after hot rolling, thereby inhibiting the development of a texture that is advantageous for workability during annealing. Therefore, the upper limit of carbon addition was limited to 0.005%.

If the amount of nitrogen is more than 0.005%, the amount of titanium required to completely fix the nitrogen increases, and as a result, the cost is increased. Nitrogen, which is not completely fixed, is precipitated as aluminum nitrite during the hot rolling phase, but the winding temperature is If it is low, it will not be completely precipitated, thus inhibiting the development of aggregates that are advantageous for processability during annealing. In addition, when the amount of nitrogen is increased, all of the titanium added in a small amount is consumed, and thus the amount of titanium to fix carbon is insufficient, resulting in deterioration of workability.

Manganese (Mn) is less than 0.2% it is difficult to secure the tensile strength. Manganese also reduces the Ar ₃ temperature because it is an element that stabilizes austenite. When the temperature of Ar ₃ is decreased, the precipitation of titanium carbide is reduced in the winding step after hot rolling, thereby inhibiting the precipitation of FeTiP and promoting the precipitation of TiC during cold winding. Therefore, by reducing the amount of dissolved carbon as much as possible by the precipitation of TiC in the hot rolling step, it is possible to obtain a texture in the annealing step that is advantageous for workability. Therefore, the minimum addition amount of manganese was restrict | limited to 0.2%. When 2.0% or more of manganese was added, the upper limit was added to 2.0% because it inhibited the development of a processability favorable texture. In addition, the Mn / Ti ratio of 25 or more does not completely precipitate sulfur by using manganese when the Mn / Ti ratio is 25 or less, thereby producing TiS and Ti ₄ C ₂ S ₂ compounds in the steel. Since the compound is only soluble at high temperatures, it is not dissolved in the annealing step. Therefore, it is not possible to secure sufficient amount of carbon used for baking. Therefore, the ratio of Mn / Ti was limited to 25 or more.

Aluminum (Al) is added to the steel for deoxidation. When the amount of aluminum is less than 0.02%, oxygen is present in the steel, which forms manganese oxide and titanium oxide when oxide forming elements such as manganese and titanium are added during steelmaking. It is difficult to control components such as manganese and titanium. In addition, when the amount of aluminum is more than 0.08%, the amount of aluminum is added more than necessary to increase the manufacturing cost and generate a large amount of surface defects of the steel sheet, so the upper limit of aluminum is limited to 0.08%.

Phosphorus (P) is added as a solid solution strengthening element, but in the present invention, FeTiP was added in order to secure carbon necessary for hardening annealing by producing FeTiP in the solid solution strengthening effect and annealing step. If too little phosphorus is added, there is no solid solution strengthening effect, so the lower limit of phosphorus is limited to 0.02%. When phosphorus is added to steel, FeTiP is produced by combining with titanium, which is produced when TiC is decomposed during annealing. At this time, since the dissolved carbon continues to exist after annealing, it is possible to secure the carbon used for hardening the baking. Normally, FeTiP and TiC are produced competitively, but by performing cold winding during hot rolling, the precipitation of FeTiP is suppressed to fix as much solid carbon as possible, thereby forming a bonded structure that is advantageous for workability. The annealing promotes the precipitation of FeTiP by controlling the amount of titanium and phosphorus. If the amount of phosphorus is too high, the weldability deteriorates, there is a possibility that plate breakage occurs in the hot rolling and cold rolling stages, and secondary work brittleness occurs. Therefore, the upper limit of phosphorus addition was limited to 0.1%.

Titanium (Ti) is added as an element to fix carbon, but if too much is added, Ti ₄ C ₂ S ₂ is formed at high temperature, so it is difficult to secure carbon necessary for baking. This is because once Ti ₄ C ₂ S ₂ is formed, it is not decomposed even in the annealing step. In addition, since the dissolution temperature of TiC rises, it becomes difficult to decompose TiC during annealing. Therefore, the upper limit of the addition amount of titanium was limited to 0.03%. If too little titanium is added, the titanium will combine with nitrogen to form TiN at high temperatures, thus preventing the solid solution of carbon in the hot rolling phase. Therefore, it is not possible to obtain an aggregate structure favorable to workability in the annealing step. Therefore, the lower limit of addition of titanium was limited to 0.005%.

First, the steel melt | dissolved with the said composition is performed homogenizing in the temperature range of 1050-1300 degreeC. And subjected to finish hot rolling at a temperature of not less than Ar ₃ transformation point 850~950 ℃, and the hot rolling coiling temperature in the range of 450~650 ℃. If the homogenization treatment temperature is less than 1050 ℃, the structure of the steel is not homogenized and the mixture is generated, causing the deterioration of the material, and when the homogenization treatment temperature is 1300 ℃ or more, mainly precipitates fine precipitates worse workability. Therefore, the homogenization temperature range is preferably limited to 1050-1300 ℃. In addition, if the finishing temperature during hot rolling is 850 ° C. or lower, the upper, lower and edges of the hot rolled coil become single phase regions, thereby increasing in-plane anisotropy and degrading formability. In addition, when the temperature is higher than 950 ° C, remarkable coarse grains may occur and surface defects such as orange peel may occur on the surface after processing. Therefore, the finish temperature during hot rolling is preferably limited to 850 ~ 950 ℃. In addition, when the hot rolling temperature is above 650 ° C, FeTiP precipitates in the hot rolling step, so that the amount of titanium for fixing solid solution carbon is insufficient. If the temperature is below 450 ° C, rapid cooling must be performed after finishing rolling. Therefore, it is preferable to limit hot rolling coil temperature to 450-650 degreeC.

Although the reduction rate does not significantly affect the material, it is preferable to limit the reduction rate to 40 to 80% because the workability is low when the reduction rate is 40% or less, and the load is greatly applied during cold rolling when the reduction ratio is 80% or more. . .

After cold rolling, annealing is performed. At this time, the annealing temperature is continuously performed in a temperature range of 750 to 900 ° C. The temperature range of the continuous annealing is a normal condition, even after adopting the annealing method passing through the slow cooling step and the aging band after hot rolling treatment can ensure the hardening hardening.

The present invention will be described in detail through the following examples.

(Example)

Table 1 shows the chemical components of the inventive steel and the comparative steel. The molten steel was maintained for 1 hour in a 1250 ° C. heating furnace, followed by hot rolling. At this time, the hot rolling finish temperature was 900 ℃, the coiling temperature was 540 ℃, cold rolling with a cold reduction rate of 75% and then subjected to continuous annealing at an annealing temperature of 830 ℃. During continuous annealing, quenching was performed at −5 ° C./sec in the slow cooling zone and at −30 ° C./sec in the quench cooling zone. The specimens after continuous annealing were subjected to a tensile test using a universal tensile tester.

Table 2 shows the change in mechanical properties according to the annealing temperature of the inventive steel and the comparative steel. Steel 1-2 of the present invention exhibits high strength and high workability with a tensile strength of 35 kg / mm ² or more, yield strength of 30 kg / mm ² or less, elongation of 35% or more, r value of 1/6 or more, and at the same time hardening hardening value of 3 kg. It shows excellent bake hardening characteristics over / mm ² .

In the case of Comparative Steel 4, since the amount of titanium is too high, the workability is excellent, but the hardening hardening value is very low. In the case of Comparative Steel 5, since the amount of manganese is too small and the amount of Ti is high, sufficient bake hardening value cannot be obtained.

Chemical Compositions and Manufacturing Conditions of Invented and Comparative Steels River Chemical composition (% by weight) Manufacture conditions Remarks C N S Mn Ti P Al CT AT One 0.0028 0.0022 0.013 0.74 0.022 0.080 0.040 540 830 Invention steel 2 0.0032 0.0020 0.012 1.52 0.025 0.078 0.045 540 830 Invention steel 4 0.0033 0.0020 0.012 1.50 0.050 0.080 0.039 540 830 Comparative steel 5 0.0022 0.0022 0.013 0.015 0.032 0.080 0.043 540 830 Comparative steel

Mechanical Properties of Invented and Comparative Steels River Mechanical property Remarks Yield strength (kgf / mm2) Tensile strength (kgf / mm2) Elongation (%) r Curing hardening amount (kgf / mm2) One 25 39 38 1.7 3.2 Invention steel 2 28 43 37 1.6 3.3 Invention steel 4 29 45 36 1.7 0.2 Comparative steel 5 20 35 40 1.8 1.4 Comparative steel

(* Hot Rolling Condition: Slab Reheating Temperature: 1250 ℃

Hot Rolled Finish Temperature: 900 ℃

Winding Temperature: 540 ℃

* Cold rolling condition: Cold rolling reduction 75%

* Annealing condition: annealing temperature 830 ℃, annealing time: 30 seconds, slow cooling cooling rate: -3 ℃ / sec, cooling cooling temperature: -30 ℃ / sec)

As described above, according to the present invention, since cold-rolled steel sheets or hot-dip galvanized steel sheets are manufactured under normal hot rolling and continuous annealing or hot-dip plating conditions, there are few constraints on process variables during steel sheet manufacturing. Not only is it excellent in processability, but also its yield strength is greatly increased after drying, so it can be used as a high-strength cold rolled steel sheet for high processing, which is applied to automobile's outer plate and inner plate requiring shock absorption. In addition, in comparison with the inventive steel and the comparative steel according to the present invention, there is an effect of improving the quench hardening property that increases the strength in the press, coating and drying process during the manufacturing of automobile parts. It is effective in providing excellent high strength cold rolled steel sheet.

Claims (2)

By weight% of carbon: 0.005% or less, nitrogen: 0.005% or less, sulfur: 0.02% or less, manganese 0.2 to 2.0%, aluminum: 0.02 to 0.08%, phosphorus: 0.02 to 0.10%, Ti: 0.005 to 0.03% A high-strength cold rolled steel sheet having excellent workability and tensile resistance, which is configured to adjust the Mn / Ti ratio to 25 or more, and includes other Fe and unavoidable impurities. First homogenizing the ultra low carbon aluminum guild steel having the composition of claim 1 in a temperature range of 1050 to 1300 ° C .; Finishing hot rolling at a temperature range of 850 ° C. to 950 ° C. directly above the Ar 3 transformation point; Hot rolling in a temperature range of 450 to 650 ° C .; Rolling at a cold reduction rate of 40 to 80%; Performing continuous annealing in the temperature range of 750 ~ 900 ° C; The manufacturing method of the high strength cold rolled steel sheet for high processing excellent in workability and vent resistance.