KR20010060397A - Manufacturing of bake hardning type cold rolled steel sheet having hight formability and softning - Google Patents

Abstract

PURPOSE: A method for manufacturing cold rolled steel sheets having superior bake hardenability and formability is provided to produce a soft bake-hardenable cold rolled steel sheet of a tensile strength of 30kgf/mm2 (bake hardenability 3.0-6.0kgf/mm2) for the application of automobile side panel and rear floor. CONSTITUTION: The cold rolled steel sheet having superior bake hardenability and formability is manufactured by homogenizing a super low nitrogen/carbon Al-killed steel containing C 0.0020-0.0025wt.%, Nb 0.008-0.012wt.%, B 0.0005-0.0015wt.% at 1200deg.C or higher; finish hot rolling in the temperature range of 900 to 950deg.C; coiling it in the temperature range of 700 to 750deg.C; cold rolling the hot rolled steel sheet at a reduction rate of 75 to 80% in a conventional temperature range; continuous annealing it in the temperature range of 850 to 860deg.C.; and then temper-rolling the cold rolled steel sheet at a reduction rate of 1.0 to 2.0%.

Description

MANUFACTURING OF BAKE HARDNING TYPE COLD ROLLED STEEL SHEET HAVING HIGHT FORMABILITY AND SOFTNING}

The present invention relates to a method for manufacturing a cold rolled steel sheet used in interior and exterior sheet materials of automobiles, and more particularly, to a method for manufacturing a high-molded soft-baked hardened steel sheet having a moldability of 2.0 or more based on an average r value.

Recently, the use of high-strength steel sheet in the vehicle body for the purpose of improving the fuel efficiency of the automobile and reducing the weight of the vehicle body has increased the demand for reducing the plate thickness and improving the dent resistance. The properties required for automotive cold rolled steel sheets include yield strength, tensile strength, good press formability, spot weldability, fatigue characteristics, and the like.

In general, steel sheets generally exhibit characteristics in which strength and workability are opposite to each other. The steel that can satisfy these two characteristics is largely a composite structured cold rolled steel sheet and a hardened hardened cold rolled steel sheet. In general, the composite tissue steel that can be easily manufactured has a tensile strength of 40kgf / mm2 or more, which is a material used in automobiles, but its elongation, which is a factor indicating stretchability, is higher than that of high tensile strength, The average r value is low and expensive alloying elements such as manganese and chromium are excessively added, resulting in an increase in manufacturing cost. However, the hardened hardened steel has a yield strength close to the soft steel sheet during press molding in steels with a tensile strength of 40kgf / mm2 or less, which is excellent in ductility, and increases the yield strength by itself when the coating is processed after press molding. It is attracting attention as a very ideal steel as compared with the conventional cold rolled steel sheet which is worsened.

Baking hardening is a kind of strain aging that occurs when carbon or nitrogen, an invasive element dissolved in steel, adheres to the potential generated during the deformation process. Because of this, deterioration of formability is accompanied by three-temperature aging, so it is very important to control the appropriate employment element.

In general, as a method for producing a cold-rolled steel sheet having a hardening hardenability, the low-carbon P-added Al-killed steel is simply wound at a low temperature, that is, the hot-rolled hardening by the annealing method using a cold winding having a temperature range of 400-500 ° C. Steel of about 4-5 kgf / mm2 was mainly used. This was because both of the moldability and the baking hardening were more easily achieved by the annealing. P-added Al-Killed steel by the continuous annealing method uses a relatively fast cooling rate, so it is easy to secure the hardening of the hardening, but the formability is deteriorated by rapid heating and short time annealing. Limited use. Thanks to the recent rapid development of steelmaking technology, it is possible to control the appropriate amount of solid solution in steel and to manufacture the hardened hardened cold rolled steel sheet with excellent moldability by using Al-Killed steel sheet containing strong carbonitride-forming elements such as Ti or Nb. It is increasingly used for automotive exterior materials that require dent resistance.

Japanese Patent Publication No. 61-026757 for C 0.0005-0.15%, S + N content ≤0.05% of Ti or Nb-added ultra low carbon cold rolled steel sheet, or Japanese Patent Publication No. 57-089437 The method for producing steel with a hardening hardening amount of about 4kgf / mm2 or more using Ti-added steel of 0.010% or less is introduced. This method is to control the addition amount of TI and Nb or the cooling rate at the time of annealing so that the amount of solid solution in the steel is appropriately applied to prevent hardening of the material while providing hardening hardening. However, in the case of Ti, Ti, and Nb composite additive steel, strict control of Ti, nitrogen, and sulfur is required in the steelmaking process in order to secure an appropriate hardening hardening amount, thereby causing a problem of cost increase. In addition, Nb-added steel is expected to deteriorate workability due to high temperature annealing and to increase manufacturing costs due to the addition of special elements.

In the case of Korean Patent Application 95-66473, C 0.0015-0.0030%, N 0.003% or less, Nb 0.002-0.01% and boron (B) were added 0.0003-0.001%, while limiting the Nb / C atomic ratio to 0.5 or less It introduces a method for producing an organic coated cold rolled steel sheet having excellent baking hardenability. However, as a result of the analysis by the present inventors through more precise experiments, the above method is excellent in corrosion resistance as an organic coated steel sheet, but the Nb / C atomic ratio is 0.5 or less to control the annealing temperature to 830 ° C. and to secure an appropriate amount of hardening hardening. Since it contains a very large amount of solid solution carbon from the hot rolling step, the material after the continuous annealing is somewhat deteriorated. The average r value, which is a criterion of moldability, is about 1.9 among the material properties shown in the above-described known invention steels, such as side panels and rear floors, which are deep processing parts of automobile exterior materials. There is a problem in that it is difficult to apply to egg-shaped parts requiring an average r value of 2.0 or more.

In order to overcome the above-mentioned problems, the present invention has an effect of improving the aging effect of BN precipitation of solid solution nitrogen by adding Nb, which is a strong carbonitride-forming element, to a proper level or adding a small amount of B to a father-bearing system having an appropriate C content of the present invention. In addition, in order to manufacture a high-temperature hardened hardened cold-rolled steel sheet having an average r value of 2.0 or more, it is rolled at an appropriate cold rolling rate to secure an average r value of the present invention and to increase annealing temperature to secure an appropriate amount of solid carbon. To provide a manufacturing method to obtain a soft high-molded hardened cold-rolled cold rolled steel sheet having a tensile strength of 30kgf / mm2 with a bake hardening amount of 3.0-6.0kgf / mm2 by adjusting the rolling slightly higher than the rolling reduction rate. do.

In order to achieve the above object, in the present invention, by weight% C: 0.0020-0.0025%, Si: 0.02% or less, Mn: 0.07-0.4%, P: 0.008-0.04%, S: 0.01% or less, soluble (Soluble) Extremely low nitrogen and extremes in which B is added in an amount ranging from 0.0005 to 0.0015% so as to satisfy an atomic ratio of 0.5 to 1.0 of B / N while containing Al: 0.02-0.06%, N: 0.0020% or less, and Nb: 0.008-0.012%. After homogenizing and heat-treating low carbon Al-Killed steel at 1200 ℃ or higher, it is wound at the temperature range of 900-950 ℃ and wound at the temperature range of 700-750 ℃, and then cold rolled at 75-80% cold rolling rate. The present invention provides a method for producing a high-molded bake hardened cold-rolled steel sheet having an average r value of 2.0 or more by performing continuous annealing at a temperature range of 850-870 ° C. and temper rolling at a rolling rate of 1.0-2.0%.

Hereinafter, the technology of the present invention will be described in detail.

Carbon (C) is an element showing solid solution hardening and baking hardening. When the carbon content is less than 0.0020%, the tensile strength is insufficient due to the very low carbon content, and the absolute carbon content is low, so that sufficient baking hardening is not obtained. In addition, when it is more than 0.0025% in the range of Nb amount required by the present invention, when the high temperature annealing such as 850-870 ℃ has a lot of carbon redissolved in the NbC precipitates, the amount of solid solution in the steel is excessive, so the hardening hardening Since high and normal temperature aging resistance is not secured, a stretcher strain occurs during press molding, thereby degrading moldability and ductility.

Silicon (Si) is an element that increases the strength, but the strength increases as the amount added increases, but ductility deterioration is remarkable, so it is preferable to limit the amount to 0.02% or less.

Manganese (Mn) refines the particles without ductile damage, and precipitates the strong sulfur completely with MnS to prevent hot brittleness due to the formation of FeS. More than 0.07% of Mn is required. The decrease in ductility is remarkable, while the increase is sharp, so the amount of addition is preferably limited to 0.07-0.4%.

Phosphorus (P) is a substitution type alloy element having the greatest solid solution strengthening effect, and serves to improve in-plane anisotropy and strength. However, when the total amount of phosphorus is less than 0.008%, the above-mentioned effect cannot be obtained. If the addition of 0.04% or more, the soft steel sheet presented in the present invention cannot be manufactured due to the rapid increase in strength, and P is excessively added due to the excessive addition of P. Deterioration of ductility is inevitable, such as segregation and withdrawal of materials. Therefore, the amount of addition needs to be limited to 0.008-0.04%.

Sulfur (S) precipitates as a sulfide of MnS at high temperature. However, if the S content is excessive, the precipitated MnS and the remaining S embrittles the grain boundary, causing hot brittleness. In addition, even if the amount of S added to completely precipitate the MnS precipitate, when the S content is large, it is difficult to secure an average r value of 2.0 or more required by the present invention due to material deterioration due to excessive MnS precipitate. It is preferable to limit to the following.

Aluminum (Al) is added for the deoxidation of steel, but if it is added below 0.02%, it is disadvantageous to mechanical properties such as oxidative inclusions in the steel, resulting in deterioration of workability. Especially, it does not completely precipitate solid solution nitrogen into AIN. Causing age degradation. In addition, when excessively added at more than 0.06%, the solid solution of nitrogen can be completely precipitated with AIN to prevent age degradation due to nitrogen, but it causes hardening of the material and an increase in manufacturing cost. Therefore, in order to solve the above problem, it is preferable to limit the AI content to 0.02-0.06%.

Nitrogen (N) is present in solid solution before or after annealing, thereby degrading the formability of the steel. Aging deterioration is much larger than that of other invasive elements, so it is necessary to fix with Al or B. However, if the content is more than 0.0020%, the content of Al or B to remove the solid solution nitrogen increases to harden the material and increase the manufacturing cost, so the content should be strictly limited to less than 0.0020%. There is.

Nb combines with carbon to precipitate NbC precipitates to increase strength and improve moldability. However, when added below 0.008%, Nb does not sufficiently fix carbon in the range of carbon components presented in the present invention, and thus contains a large amount of solid solution carbon in the steel from the hot rolling step, so that general ultralow carbon Al- without special elements is added. As with the same steel as the Killed steel, the hardening hardenability is increased, but it is difficult to manufacture high hardenable harden hardening steel having an average r value of 2.0 or more required by the present invention, and it is impossible to secure room temperature aging resistance due to excessive solid solution carbon. In addition, if the amount of Nb is added at 0.012% or more, the formation of excessive NbC precipitates and the re-dissolution temperature of the NbC precipitates increase, so that the NbC precipitates cannot be re-dissolved even at the annealing temperature of the present invention, 850-870 ° C. It is very difficult to find the amount of carbon employed. On the other hand, in order to secure an appropriate amount of dissolved carbon by re-dissolving NbC precipitates, the annealing temperature should be increased to 870 ° C. or higher, thereby causing work deterioration such as buckling during annealing. Therefore, in order to secure a stable level of hardening hardening through an appropriate amount of solid solution carbon at an appropriate annealing temperature of 850-870 ° C, it is necessary to limit the amount of Nb to the range of 0.008-0.012% as described above.

Boron (B) improves the hardenability during heat treatment and increases the hardening hardening through interaction with carbon. In particular, the precipitation of BN also serves to prevent aging deterioration due to solid nitrogen. However, when the B content is less than 0.0005%, the above-described effects cannot be obtained, and in particular, sufficient BN precipitation does not occur, causing room temperature aging deterioration by solid solution nitrogen. However, if B content is added more than 0.0015%, there is no aging deterioration effect of solid solution nitrogen due to BN precipitation, but the addition of B content is 0.0005-0.0015 because ductility deterioration is greater than the increase in strength due to remarkable grain refinement. It is desirable to limit to%.

After dissolving in the converter with the above composition, the continuous cast slab is heated at 1200 ° C. or higher where the austenite structure before hot rolling can be sufficiently homogenized, and hot rolling is carried out at a temperature range of 900-950 ° C., directly above the Ar ₃ temperature. To finish.

If the slab temperature is lower than 1200 ℃, if the steel structure is not uniform austenite grains, a mixture is generated, resulting in deterioration of the material.

When the hot rolling temperature is 900 ° C. or less, the top, tail, 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 occur and defects such as orange peel are likely to occur on the surface after processing.

In order to prevent moldability deterioration due to the solid solution carbon remaining on the hot rolled steel sheet after the hot rolling, a high temperature winding of 700 to 750 ° C. is required. However, if the coiling temperature exceeds 750 ℃ abnormal grain growth occurs to obtain a good material, and if the temperature is less than 700 ℃ resulting in the increase in yield strength and deterioration of the moldability due to the fine grain of the hot-rolled structure.

After hot rolling, the steel is pickled in the usual manner and cold rolled at a cold rolling rate of 75-80%. Cold rolling ratio of 75% or more is a prior manufacturing condition for securing an average r value of 2.0 or more required by the present invention. In other words, when the cold rolling rate is 75% or less, the driving force required to form the aggregate structure affecting the average r value in the steel layer is small and sufficient moldability of 2.0 or more is not secured. If the cold rolling rate is more than 80%, the driving force for forming the aggregate structure increases, but the grain size becomes very fine due to excessive rolling rate, which causes hardening of the material.

Cold-rolled steel is to be worked in the continuous annealing conditions of the conventional method, but in the present invention, the annealing temperature is limited to 850-870 ℃. In other words, when the annealing temperature is less than 850 ℃, the annealing temperature is rather low, it is difficult to secure the appropriate solid solution carbon content by re-dissolution of NbC precipitates in the component system proposed in the present invention, so that the stable annealing hardenability cannot be obtained. Re-dissolution of precipitates is very active, so the amount of solid solution carbon in the steel increases in the component system of the present invention, but the quench hardening property is increased, but the room temperature aging resistance is deteriorated due to excessive increase of the quench hardening property. In particular, when the annealing temperature is higher than 870 ℃, it is difficult to control the tension during continuous annealing due to high temperature annealing, and workability is deteriorated, such as the burner life is reduced.

By using the calcined hardened type cold rolled steel sheet prepared as described above, temper rolling of 1.0-2.0% is performed slightly higher than the normal rolling rate for the purpose of securing appropriate calcined hardenability and room temperature aging resistance. When the temper rolling ratio is 1.0% or less in producing hardened hardened steel, aging occurs in a short time when maintained at room temperature, thereby increasing yield strength and causing a strainer strain that is fatal to press working. However, if the temper rolling ratio is 2.0% or more, the room temperature aging resistance can be sufficiently secured, but the work hardening caused by excessive temper rolling causes the material to deteriorate, thereby obtaining a soft small hardened steel having excellent moldability required by the present invention. none.

Hereinafter, the present invention will be described in more detail with reference to Examples.

Example

Table 1 below shows the chemical composition of the inventive steel and the comparative steel with strict control of the amount of carbon and Nb or the addition of a small amount of B. Steel 1-5 is the inventive steel, and steel 6-10 is the comparative steel.

Hot rolling was carried out using the steel of Table 1, and then rolled at a cold rolling rate of 78%, which is the scope of the present invention, followed by continuous annealing at high temperature annealing at 850 ° C., followed by a temper reduction rate of about 1.5% to measure mechanical properties. One result is shown in Table 2.

Table 1

Steel grade Alloy component (wt%) Remarks C Si Mn P S Sol.Al N Nb B Ti One 0.0022 0.005 0.15 0.0095 0.0086 0.045 0.0020 0.0120 - - Invention steel 2 0.0020 0.007 0.15 0.010 0.0084 0.044 0.0018 0.0100 - - Invention steel 3 0.0023 0.008 0.18 0.010 0.0036 0.048 0.0015 0.0097 - - Invention steel 4 0.0021 0.010 0.18 0.010 0.0090 0.048 0.0018 0.0097 - - Invention steel 5 0.0025 0.010 0.15 0.0095 0.010 0.040 0.0013 0.012 0.0008 - Invention steel 6 0.0021 0.005 0.140 0.010 0.011 0.058 0.0020 0.0097 - - Comparative steel 7 0.0012 0.010 0.180 0.009 0.012 0.048 0.0019 0.01 - - Comparative steel 8 0.0025 0.010 0.120 0.010 0.009 0.035 0.0025 0.020 0.0020 - Comparative steel 9 0.0030 0.010 0.150 0.010 0.010 0.040 0.0025 - - - Comparative steel 10 0.0030 0.007 0.150 0.012 0.010 0.047 0.0023 - - 0.012 Comparative steel

Steel No. 1-5 of the present invention strictly restricting carbon and Nb as described above has a bake hardening amount of 3.2-5.5 kgf / mm 2, an average r value of 2.07-2.51, elongation of 45% or more, and tensile strength of 28.8-30.5, as shown in Table 2 below. kgf / mm 2, yield strength of 19.1-20.1kgf / mm 2 indicates that the present invention can produce a small hardening type cold rolled steel sheet having excellent moldability of an average r value of 2.0 or more and very excellent at room temperature aging resistance. On the other hand, the yield strength of the present invention is somewhat higher than the tensile strength because the temper rolling ratio of about 1.5% higher than the normal level was applied in order to prevent deterioration of room temperature aging resistance, which is a unique problem of the hardened hardened steel.

Steel No. 6 meets the range of components suggested by the present invention, but the S content is somewhat high as 0.011%, especially the cold rolling rate is low as 70%, which is excellent in hardening hardening but the average r value is 1.91. The target was not achieved.

Steel No. 7 has a carbon content of 0.0012%, which is much lower than the lower limit of carbon, 0.002%, so the content of S exceeds the standard value, and the cold rolling rate is 70%, even if it is lower than the scope of the present invention. However, the hardening hardening was not obtained at all due to the lack of absolute carbon content.

Steel No. 8 was not obtained because the content of Nb and B is 0.02% and 0.002%, respectively, which is outside the range of the composition of the present invention steel. In addition, due to segregation at the grain boundary due to excessive B content, deterioration of the material with high tensile strength, very low elongation and average r value occurred, and nitrogen was 0.0025%, which is outside the range required by the present invention steel.

In the 9th steel, all the components except for nitrogen satisfy the regulatory ranges required by the steel of the present invention, but all the added carbon is dissolved carbon because Nb is not added at all. The presence of cure hardening of 6.4kgf / mm2 was higher than 3.0-6.0kgf / mm2 of steel material regulated by the present invention, and the moldability such as average r value was slightly deteriorated. Severe aging defects may occur when supplied.

Steel 10 is out of the composition limit of the steel of the present invention, and also added Ti instead of Nb, but the amount precipitates nitride and sulfide, so all the added carbon remains in strong strength from the hot rolling stage and all exist as solid carbon. Because of this, the hardening hardening properties were very high, which was beyond the level required by the present invention steel, and in particular, the average r value was deteriorated as 1.77 due to the dissolved carbon present from the hot rolling stage.

[Table 2]

River bell Cold rolling rate Material performance Yield strength (kgf / ㎡) Tensile strength (kgf / ㎡) Elongation (%) r Curing hardening rate (kgf / mm2) Remarks One 78% 20.0 30.3 46.9 2.43 3.9 Invention steel 2 78% 20.0 30.5 46.7 2.51 4.2 Invention steel 3 78% 19.1 28.8 51.3 2.33 3.2 Invention steel 4 78% 20.1 29.9 45.5 2.16 5.5 Invention steel 5 78% 19.9 29.7 47.1 2.07 4.7 Invention steel 6 75% 20.5 31.1 47.5 1.91 4.0 Comparative steel 7 75% 18.0 28.6 49.5 2.09 0 Comparative steel 8 78% 22.1 33.7 40.2 1.81 0 Comparative steel 9 78% 20.8 30.0 42.6 1.79 6.4 Comparative steel 10 78% 21.9 30.3 42.4 1.77 5.6 Comparative steel

Therefore, according to the present invention, by forming a high-molded soft-baking hardened cold-rolled steel sheet having a moldability of 2.0 or more on the basis of an average r value, it has an excellent effect that can be applied to difficult molded products such as side panels, rear layers, etc. among automotive exterior panels.

Claims (3)

Ultra low nitrogen and ultra low carbon Al-Killed steels containing B in the range of 0.0005-0.0015% and containing Nb: 0.008-0.012% in a basic component system having C: 0.0020-0.0025% by weight and above 1200 ° C Finishing hot rolling at a temperature range of 900-950 ° C. after the homogenization heat treatment, and winding in a temperature range of 700-750 ° C .; Continuously annealing the hot rolled plate after cold rolling at a normal temperature range; A method of manufacturing a high-molded soft-baking hardened cold-rolled steel sheet comprising the step of rough rolling the cold rolled sheet. The method of claim 1, wherein the cold rolling of the hot rolled sheet is carried out at a cold rolling rate of 75-80%, the continuous annealing is carried out at a temperature range of 850-860 ℃ a high-molded soft-baking hardened cold-rolled steel sheet How to manufacture. The method of claim 1, wherein the cold rolling of the cold rolled sheet is performed at a rolling rate of 1.0-2.0%.