KR100467712B1 - The Manufacturing of Bake Hardening Steels with High Formability - Google Patents

Abstract

The present invention relates to a method for manufacturing a cold rolled steel sheet, and more particularly, to a method for producing a high strength cold rolled steel sheet having excellent press formability and simultaneously having a hardening hardening characteristic.

The present invention is by weight% C content 0.004% or less, Mn: 0.6 ~ 1.0%, P: 0.04 ~ 0.08%, S: 0.006% or less, N: 0.003% or less, acid value Al: 0.06% or less, Ti: 0.026 ~ 0.034% balance: composed of Fe and other unavoidable impurities, wherein Ti is ((48/14 × N) + (48/12 × C)) ≦ Ti ≦ ((48/14 × N) + (48/12 × 1.5C)) steel slab finishing hot-rolled at temperatures above 910 ° C, hot-wound at temperatures above 700 ° C, cold-rolled at a cold reduction rate of 77% or more after pickling, and then 650 in a box annealing furnace. Provided is a method for producing a highly hardened bake hardened type high strength cold rolled steel sheet characterized by recrystallization annealing in a temperature range of ˜750 ° C.

Description

The Manufacturing of Bake Hardening Steels with High Formability

The present invention relates to a method for manufacturing a cold rolled steel sheet, and more particularly, to a method for producing a high strength cold rolled steel sheet having excellent press formability and simultaneously having a hardening hardening characteristic.

In recent years, as the shape of the automobile exterior becomes complicated and integrated, more excellent characteristics are required in formability of the steel sheet used in the automobile. To meet these demands, steel makers are working by minimizing the incorporation of elements such as solid carbon, solid solution nitrogen, and solid sulfur into steel, which impede the workability of steel sheets in steelmaking and refining processes. . However, since the above-mentioned solid elements are inevitably left in the slab in the process, carbon or nitrides such as titanium or niobium may be added alone or in combination to promote the formation of even small amounts of the solid elements in the hot rolling step. It uses IF steel (Interstitial Free Steel) manufacturing technology to secure formability by precipitating.

On the other hand, in the automotive industry, in order to reduce the cost through the simplification of the manufacturing process, the conventional molding parts technology that can be molded into a desired part at a time by eliminating the method of forming each of the conventional parts and then welding and assembling each other. The main application parts to which the integrated molding technology is applied are required to have better moldability than before in order to successfully process without forming cracks when molding as a side panel or a hood. As the steel sheet to be applied, a wide steel sheet having a width of 1700 mm or more is used.

However, in the past, the cold rolled steel sheet produced by the continuous annealing technology is mainly limited to a width of 1600 mm or less, and thus, it was difficult to use the main use parts to apply the integrated molding technology.

In addition, in recent years, the reduction of the thickness of the steel sheet applied as part of an active response to light weight and environmental problems for improving the fuel economy of the automobile is required. However, in the case of automotive exterior materials, the thickness reduction of steel sheet is accompanied by a decrease in dent resistance (which indicates the index of resistance to external pressure of steel sheet. have. In order to compensate for the decrease in dent resistance due to the reduction of the thickness of the steel sheet, high strength steel having a higher strength is required, but it is almost impossible to apply it to a part requiring deep processing as the workability is deteriorated when the high strength steel is applied. .

Therefore, in the case of automotive exterior materials, steel sheets with excellent quench hardening properties (hereinafter referred to as BH properties) are required, and quench hardened steel sheets have low yield strength before molding, and are excellent in processing and shape freezing, and high processing hardening ability after molding. Due to the increase in yield strength is a steel sheet having a characteristic that ensures excellent dent resistance.

As a conventional hardening hardened steel sheet, it is known from iron and steel (68 (1982) No. 9 p.1276) that the composite structured hardening hardened steel sheet having a tensile strength of 40 Kgf / mm2 grade using a low carbon-manganese-chromium system The yield ratio was very low as 46% and the work hardening index value (n) was very high as about 0.28, and the BH (Bake Hardening), which is an evaluation index of the hardening hardening ability, was also very good at about 5Kgf / mm2. The plastic anisotropy (hereinafter referred to as r-value) coefficient value, which is an index, is very low at 1.2 level, and it has a big drawback that it cannot be used for parts requiring high formability.

In order to solve the formability problem of the low carbon type composite steel as described above, r value is obtained by using Ti or Nb, which is a carbon nitride forming element, alone or in combination, using ultra low carbon steel having a carbon content of less than 0.005% by weight. Steel sheet development technology with high formability of 1.7 or more is emerging, but this steel sheet also has a problem in that it is necessary to simultaneously secure the desired strength and formability. Representative examples of this technique include CAMP ISIJ Vol. 5 (1992) P.2051, CAMP ISIJ, 1991, P1934, etc., which are Mn, P, Si, Ti in ultra-low carbon steels of C: 0.003% or less by weight. , Nb is properly added, but the scale is formed on the surface of the steel sheet due to the high temperature annealing operation of 900 ° C. or higher, and it is difficult to apply the actual manufacturing industry because it has problems such as securing formability.

On the other hand, Japanese Patent Application No. 95-90386 as a previously known patent, "Mn, Nb, B, etc. are added to ultra low carbon steel as a" method of manufacturing a high strength cold rolled steel sheet having excellent deep workability and chemical conversion treatment " Α zone rolling, hot rolled crystal annealing, annealing temperature using alloy system designed to satisfy (3Si / 28 + 200P / 31) / (Mn / 55) = 15 ~ 40 and Nb (Nb / C = 5 ~ 25) By carburizing at ~ 950 ℃, r value of workability evaluation index is 2.2 or more and proper BH property is secured, but it is manufactured by continuous annealing due to equipment limitations due to α area rolling and lubrication rolling of hot rolling. There is a problem in manufacturing a cold rolled steel sheet having a width of more than 1700mm.

In addition, according to Japanese Patent Application No. 97-118955, which is excellent in press formability, according to Japanese Patent Application No. 97-118955, Si (1.0 or less) -Mn (2.0 or less) -P-Ti-Nb-Al-Ti (4C + 3.4N) + 1.5S ~ 0.1) -Nb, Ti (0.05 or more), B (25ppm or less) combination of additive elements and the size and distribution of its precipitate FeTiP can be controlled to secure r value of 1.8 ~ 2.3. The BH value, which is the hardening hardening characteristic, hardly appeared, which is not suitable for hardening hardening steel.

Accordingly, the present invention provides a conventional hot rolled steel sheet by using a component system in which Ti alone ultra low carbon IF steel is added with Mn, P and the like having excellent solid solution strengthening ability in order to manufacture a wide-strength cold rolled steel sheet having excellent moldability and baking hardening characteristics. It is an object of the present invention to provide a method for producing a high strength cold rolled steel sheet having excellent moldability and baking hardening characteristics after cold rolling.

1 is a graph showing the yield point stretching and baking hardening characteristics according to the annealing temperature.

The present invention, in order to achieve the above object, by weight% C: 0.004% or less, Mn: 0.6 ~ 1.0%, P: 0.04 ~ 0.08%, S: 0.006% or less, N: 0.003% or less, acid value Al : 0.06% or less, Ti: 0.026 to 0.034% balance: Fe and other inevitable impurities, wherein Ti is ((48/14 × N) + (48/12 × C)) ≦ Ti ≦ ((48/14 Finishing hot rolled steel slab that satisfies the relationship of × N) + (48/12 × 1.5C)) at over 910 ℃, hot rolled at a temperature above 700 ℃, and cold rolled at a cold reduction rate of 77% or more after pickling. Thereafter, there is provided a method for producing a high forming hardened type high strength cold rolled steel sheet characterized by recrystallization annealing at a temperature range of 650 to 750 ° C. in a box annealing furnace.

Hereinafter, the reason for numerical limitation of the composition and components of the inventive steel will be described.

First of all, C contained in steel serves to increase the baking hardness. In addition, as an invasive solid solution, it has a great influence on the formation of texture of the steel sheet during cold rolling and annealing. As the dissolved C content in the steel increases, the formation of the texture that favors the workability is delayed, thereby degrading the formability. Conventionally, in order to prevent formability deterioration, a method of producing solid solution C by Ti or Nb by adding Ti or Nb, which are strong carbon and nitride forming elements, alone or in combination has been mainstream. However, in order to have adequate BH property, the amount of employment C must be retained to some extent, and thus the problem of moldability deterioration was not actively solved. In other words, if the C content is increased, the BH characteristics are secured, but the machinability deteriorates, which acts as a major cause of cracking. However, in the case of the present invention, by controlling the C content in the weight% of 0.004% or less by controlling the operation characteristics of hot rolling / cold rolling / annealing, as well as securing the target BH property (≥3kgf / mm2), workability evaluation index It was possible to develop a technology that can secure the r value (≥2.0) at the same time.

Mn is a solid solution strengthening element and is essential for securing strength. In particular, it plays a role of precipitating all the steel S as MnS, which acts as a major factor of hot brittleness, while delaying the precipitation as TiS, which is a very advantageous element for setting the effective Ti content range required for C content management. In other words, since Ti is not required to precipitate S, Ti is set regardless of changes in S content, considering only C and N in steel, so the load of steelmaking operations is reduced and stable BH property is secured. There is this. However, when the Mn content is 0.6% or less, the above-mentioned effects are not obtained and there is a problem in securing the strength. On the other hand, in case of 1.0% or more, the target strength and MnS precipitation are perfectly achieved, but residual Mn acts as an impurity, resulting in material deterioration, and Mn thickening layer is generated on the surface of the steel sheet during annealing, thereby causing problems in oxidation resistance. It is preferable to manage the content at 0.6 to 1.0% by weight since it is more likely to occur.

Since P is an element which is very advantageous in securing strength without impairing formability, it is advantageous to secure as much strength as possible, but when added too much, secondary processing brittleness occurs during molding, which is the main cause of crack generation. In the present invention, the P content range for the purpose strength and crack prevention is limited to 0.04 to 0.08%.

In the present invention, Ti is an important element in terms of moldability and baking hardenability. Ti is added in consideration of the C and N content contained in steel, and the Ti content is ((48/14 × N) + ( 48/12 × C)) ≤ Ti ≤ ((48/14 × N) + (48/12 × 1.5C)) to be added within a range that satisfies the condition was limited to 0.026 ~ 0.034%. Ti content of less than 0.026% is advantageous in terms of BH property due to lack of effective Ti content to combine with nitrogen and carbon in the steel, but very disadvantageous in terms of formability, and Ti content above 0.034% is a small amount of solid carbon remaining in steel. Since the solid precipitates completely, the solid solution carbon is not re-dissolved even by proper annealing heat treatment, and thus BH property does not appear.

Al is an element added to refine the particle size and deoxidize the steel, and its content is preferably 0.06% or less, which is usually in the range of addition.

It is important to manage the S and N as low as possible as impurities in the steel, the content is preferably maintained to 0.006% or less, 0.003% or less, respectively.

Hereinafter, the manufacturing method according to the present invention will be described in detail.

After hot rolling the alloy component system formed to have the above characteristics under normal conditions and finishing hot rolling at 910 ° C. or higher, and then hot winding at a temperature of 700 ° C. or higher to almost completely form C in the TiC precipitate. By depositing so that the solid solution C does not remain, the (111) aggregate structure which is advantageous for workability at the time of recrystallization annealing is developed to secure the formation. On the other hand, low temperature winding below 700 ° C. results in a finer grain size, which is advantageous for secondary processing brittleness, but it does not completely precipitate solid solution C, which causes difficulty in securing moldability. Therefore, high temperature winding must be performed at the temperature of 700 degreeC or more.

Next, after pickling, cold rolling is performed. At this time, the higher the cold reduction rate is, the better the formability is, but below 77%, the material properties required by the present invention do not come out.

Subsequently, recrystallization annealing of the cold rolled steel sheet is preferably performed in a box annealing furnace at a temperature range of 650 to 750 ° C. At temperatures below 650 ℃, hot-rolled unprecipitated solid solution carbon remains, which is advantageous in terms of BH properties, but there is a problem in securing formability due to formation of unrecrystallized crystal grains due to slow recrystallization growth, and the precipitates of precipitated TiC carbide are reused. Since it is not harmed, there is a problem in securing the desired hardening resistance. On the other hand, the temperature above 750 ℃ has the advantage of securing high BH properties due to the re-dissolution of solid carbon, but it is more likely to increase the load on the facility due to high temperature annealing and it is likely to cause problems in shape and scale formation. It is preferable to perform annealing in the temperature range of 750 degreeC.

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

Example

The slabs of the inventive steels (1,2,3) and the comparative steels (1,2) having the composition shown in Table 1 are hot rolled as shown in Table 2, and after winding hot finish rolling at 910 ° C or higher, pickling is carried out. In the cold rolling, which is the next process, a cold rolled steel sheet having a thickness of 0.8 mm was manufactured by applying a suitable cold rolling rate, and then, after annealing temperature was applied to each annealing temperature for recrystallization annealing, the annealing steel sheet was subjected to annealing. Material properties (BH, r value, tensile properties) and moldability were evaluated.

At this time, the comparative steel 1 is less Ti than the invention steel, and the high BH property is secured, but there is a problem in workability, and the comparative steel 2 is higher in the C content and Ti than the invention steel, and the workability is improved to some extent, but the appropriate BH It is expected that there will be many problems in securing the castle.

Table 1

division Chemical composition (wt%) Remarks C Mn P S N S.Al Ti Comparative Steel 1 0.0038 0.70 0.07 0.0062 0.0030 0.055 0.015 Low Ti Comparative Steel 2 0.0045 0.72 0.065 0.0081 0.0028 0.057 0.048 High Ti Inventive Steel 1 0.0036 0.68 0.055 0.0059 0.0027 0.056 0.028 Invention steel Inventive Steel 2 0.0038 0.85 0.054 0.0058 0.0026 0.048 0.031 Invention steel Invention Steel 3 0.0034 0.92 0.059 0.0058 0.0027 0.045 0.030 Invention steel

In addition, r values, which are the evaluation of formability, among the material properties of annealed steel sheets, were tested three times for each condition in the rolling direction, the 45 degree direction, and the 90 degree direction using ASTM standard size, and the average value was obtained. Taken as representative for. Formability evaluation was performed by a cupping test, in which the molding conditions were punch speed: 300 mm / min, punch diameter: 40 mm, die diameter: 43 mm, drawing ratio: 2.45, and specimen holding pressure (BHF): 0.5 ton. The moldability was determined with or without moldability.

As shown in Table 2, the inventive examples (1, 2, 3, 4) of the inventive steels (1, 2, 3) basically have a width of 1700 mm or more and have a tensile strength (TS) ≥ 35 kgf / mm2 and r value. It satisfies all the conditions of ≧ 2.0, and BH ≧ 3kgf / mm 2 and shows good molding characteristics without cracking during molding. In this case, the case of non-compliance with the above conditions in the comparative steel and the present invention was marked with *.

TABLE 2

division Hot rolled Cold rolled Material characteristics (kg / mm2) Molding characteristics Poor characteristics FT (℃) CT (℃) Rolling reduction Annealed (℃) TS R value BH o: Good x: Bad Comparative Steel 1 Comparative Example 1 912 700 78 680 35.2 1.7 ※ 4.5 X ※ crack Machinability Comparative Steel 2 Comparative Example 2 920 700 77 710 37.2 2.2 1.2 * O Molding Comparative Example 3 917 650 * 78 700 37.5 1.6 1.3 * X ※ crack BH, Machinability Inventive Steel 1 Inventive Example 1 921 700 78.5 710 36.2 2.0 3.4 O Molding Inventive Steel 2 Inventive Example 2 922 700 77.8 680 36.4 2.1 3.1 O Molding Inventive Example 3 923 700 77.5 710 36.7 2.06 3.2 O Molding Comparative Example 4 918 700 77.6 810 ※ 36.4 1.96 3.8 X ※ Molding Annealing Temperature Invention Steel 3 Inventive Example 4 920 700 77 700 37.1 2.11 3.3 O Molding Comparative Example 5 920 630 * 78.2 610 ※ 37.5 1.72 2.8 ※ X ※ crack Machinability

On the other hand, the comparative examples (1, 2, 3) of the comparative steel (1, 2) is one of the cases where the Ti content is low or high, the winding temperature is low, it can be seen that it is not suitable for the above conditions, in particular Ti In Comparative Steel 2 having a large content, it can be seen that the target BH property is not obtained in any case. In addition, when the coiling temperature is low (Inventive Steel 3 / Comparative Example 5), when the annealing temperature is high or low (Inventive Steel 2 / Comparative Example 4, Inventive Steel 3 / Comparative Example 5), the r value is less than the target when molding There is a high possibility of cracking.

1 is a diagram showing the yield point stretching characteristics and BH characteristics according to the box annealing heat treatment temperature after cold rolling, the yield point stretching and BH properties increase with decreasing annealing temperature and then decrease and reincrease again. As the annealing temperature increases, the dislocation is partially lost and thus the inhibitory effect of the dissolved carbon disappears temporarily. As the temperature increases, the dissolved carbon starts to precipitate as carbide, and the BH decreases.

However, at annealing temperatures above 650 ° C, carbides are re-dissolved into solid carbon, which not only yields yield points but also increases BH properties, thus securing a target BH of 3kgf / mm2 or more. As the temperature increases, the BH property also increases, but at temperatures above 750 ℃, the adverse effect of quality such as scale formation on the surface of the steel sheet becomes apparent, so it is understood that proper annealing temperature management is very important.

That is, the reason why it is possible to manufacture high-forming BH steel by controlling the components and the annealing temperature is that after the hot rolling, the solid solution carbon is almost completely precipitated out of carbide and then removed, followed by annealing. This is because it is possible to secure moldability by forming, and then, when high temperature annealing is performed, carbides can be re-dissolved into solid solution carbon to secure BH properties.

As described above, the present invention can provide a high-strength cold rolled steel sheet having excellent formability and BH property by appropriately combining steel components, and appropriately controlling hot rolling, cold rolling operating conditions, and box annealing operating conditions. Particularly suitable for use as a material for parts requiring high formability of automotive exterior materials.

Claims (2)

In the present invention, by weight% C: 0.004% or less, Mn: 0.6-1.0%, P: 0.04-0.08%, S: 0.006% or less, N: 0.003% or less, acid value Al: 0.06% or less, Ti: 0.026- 0.034% balance: composed of Fe and other unavoidable impurities, wherein Ti is ((48/14 × N) + (48/12 × C)) ≦ Ti ≦ ((48/14 × N) + (48/12 × 1.5C)) The slab of steel which satisfies the relationship of hot-rolled and cold-rolled under normal operating conditions, and then recrystallized annealing in a box annealing furnace, characterized in that the production method of high-forming hardened hardened cold-rolled high strength steel sheet. 2. The method for producing a high forming hardened type high strength cold rolled steel sheet according to claim 1, wherein the recrystallization annealing in the box annealing furnace is performed at a temperature range of 650 to 750 ° C.