KR100368220B1 - A method for manufacturing a soft cold rolling steel sheet having high formability and annealed hardenability - Google Patents

Abstract

The present invention relates to a method for manufacturing a cold rolled steel sheet used in the interior and exterior of automobiles, etc. The object of the present invention is to strictly control the content of carbon, nitrogen, and sulfur, and then to add a trace amount of Ti, which is a strong carbonitride-forming element, Low-carbon Al-kilted steel at low temperature winding partially produces metastable TiC precipitates, and improves formability due to the solid solution reduction effect due to TiC precipitation and secures proper employment carbon by re-dissolving TiC precipitates during continuous annealing. to 3.0kgf / mm has a tensile strength 30kfg / mm ^2, the flexible process for producing a cold-rolled steel sheet having less than a ^second amount greater than the appropriate bake hardening to provide addition result by molding.

In order to achieve the above object, the present invention is a weight% of C: 0.0015 ~ 0.0025%, Si: 0.2% or less, Mn: 0.07 ~ 0.4%, P: 0.008 ~ 0.04%, S: 0.008 ~ 0.015%, Soluble ) Al: 0.02 to 0.08%, N: 0.0025% or less, Ti: 0.008 to 0.018%, and Ti content is (48/14) N≤Ti≤ (48/14) N + (48/32) S The ultra-low nitrogen and ultra-low carbon Al-kilted steel with Ti added to satisfy the above requirements are subjected to homogenization heat treatment at 1200 ° C or higher, and then hot-rolled at a temperature range of 900 to 950 ° C. The present invention relates to a method for producing a hardened hardened cold rolled steel sheet having excellent formability, which is formed by forming a TiC precipitate and performing cold rolling, continuous annealing at a temperature range of 800 to 860 ° C., and rough rolling at 1.0 to 2.0%. Shall be.

Description

A method for manufacturing soft cold rolled steel with excellent formability and hardening hardening {A METHOD FOR MANUFACTURING A SOFT COLD ROLLING STEEL SHEET HAVING HIGH FORMABILITY AND ANNEALED HARDENABILITY}

The present invention relates to a method for manufacturing cold rolled steel sheets used in automobile interior and exterior plates, and more particularly, to soft winding at low temperature using ultra-low carbon Al-kilde steel with a small amount of Ti, a powerful carbonitride-forming element. It relates to a cold rolled steel sheet manufacturing method.

In recent years, in order to improve fuel efficiency and reduce the weight of the vehicle body by reducing the weight of automobiles, the demand for improving the dent resistance as well as the reduction of the thickness of the cold rolled steel sheet for automobiles has increased. The properties required for automotive cold rolled steel sheet include yield strength, tensile strength, good press formability, spot weldability, fatigue characteristics and paint corrosion resistance.

In general, steel sheets generally have characteristics in which strength and formability are opposite to each other, and steels capable of satisfying such characteristics include a composite structure steel sheet and a hardened hardened steel sheet. In general, composite tissue steel that can be easily manufactured has a tensile strength of 40kgf / mm ² or more, and as a material used in automobiles, moldability is deteriorated compared to high tensile strength, making it difficult to process as an automobile exterior material and expensive manganese. Excessive addition of chromium, etc., leads to an increase in manufacturing cost. However, the hardened hardened steel has a yield strength close to that of a soft steel sheet during press molding in steels with a tensile strength of 40kgf / mm ² or less, and thus has excellent ductility.The yield strength of the hardened steel is 3 ~ It is a steel that rises by about 6 kgf / mm ² , and has attracted attention as a very ideal steel as compared to the conventional cold rolled steel sheet, which is deteriorated in formability when the strength increases.

Baking hardening is a kind of strain aging that occurs when carbon or nitrogen, an invasive element employed in steel, is fixed by the potential generated during the deformation process. Aging occurs in a short time when the room temperature is maintained, and since it causes serious defects in molding, it is very important to control the appropriate amount of solid elements.

As a method of manufacturing a cold rolled steel sheet having a bake hardenability, a low carbon P-added Al-kilted steel is simply annealed after hot rolling at a low temperature in the temperature range of 400 to 500 ° C., and the bake hardening amount is about 4 to 5 kgf / mm ² . The method of preparation was mainly used. This is because both moldability and baking hardenability are more easily achieved by prolonged annealing. However, the annealing method uses the continuous annealing method that can meet the demands of the times of mass production due to the disadvantage that productivity and the material deviation of coil length are very large due to long time annealing. Manufacturing is mainstream. On the other hand, when the hardened hardened steel is manufactured by continuous annealing using P-added low carbon Al-kilted steel, it is easy to secure the hardened hardenability because it uses a relatively fast cooling rate, but the moldability is deteriorated by rapid heating and short time annealing. There is a problem, and it is limitedly used only in the site where workability is not required.

However, 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 formability by using Al-kilted steel added with strong carbonitride-forming elements such as Ti or Nb. Increasing use is being made for automotive exterior materials requiring dent resistance.

Japanese Patent Laid-Open No. 61026757 discloses a method for producing ultra low carbon cold rolled steel with Ti, Ti, and Nb composites having a C content of 0.0005 to 0.015% and an S + N content of ≤0.005%. 57089437 introduces a method for manufacturing cold rolled steel sheets with a hardening hardening amount of 4kgf / mm ² or more, using Ti-added steel of less than 0.01% C. In the above methods, by controlling the addition amount or cooling rate of Ti and Nb, the solid solution element in the steel is appropriately maintained to prevent material degradation while maintaining the hardening hardenability.

However, in the case of Ti, Ti, and Nb composite additive steels, strict control of Ti, nitrogen, and sulfur is required in the steelmaking process to secure the appropriate hardening hardening amount, which puts a heavy burden on the steelmaking process. In particular, in the case of Nb additive steel, There were problems causing workability deterioration.

The present invention has been made in order to solve the above problems, it was not possible to process at a tensile strength of 35kgf / mm ² or more by cold winding using ultra-low carbon Al-kilde steel containing a small amount of Ti, a strong carbonitride forming element It can be applied to deep processing parts such as quarter panel among automotive exterior materials, and to provide soft baking hardening type cold rolled steel sheet with a tensile strength of 30kgf / mm ² or less having a baking hardening amount of 3kgf / mm ² or more. There is this.

In order to achieve the above object, the present invention is a weight% of C: 0.0015 ~ 0.0025%, Si: 0.2% or less, Mn: 0.07 ~ 0.4%, P: 0.008 ~ 0.04%, S: 0.008 ~ 0.015%, soluble ( Soluble) Al: 0.02 to 0.08%, N: 0.0025% or less, Ti: 0.008 to 0.018%, and the Ti addition amount is (48/14) N≤Ti≤ (48/14) N + (48/32) S The ultra low nitrogen and ultra low carbon Al-kilted steel with Ti added to satisfy the formula are homogenized and heat-treated at 1200 ° C or higher, and then hot-rolled at a temperature range of 900 to 950 ° C, and wound at a low temperature in a temperature range of 500 to 600 ° C. After forming the TiC precipitates of the present invention is cold-rolled, continuous annealing and temper rolling of 1.0 ~ 2.0% in the temperature range of 800 ~ 860 ℃ provides a method for producing a small hardening type soft cold rolled steel sheet excellent in formability. .

One of the important features of the present invention configured as described above is to perform low temperature winding in the temperature range of 500 ~ 600 ℃.

Generally, in the case of ultra low carbon cold rolled steel sheet containing Ti, precipitates generated in steel include TiN, TiS, Ti ₄ C ₂ S ₂ , TiFeP, and TiC. Among the precipitates, TiFeP is usually a precipitate formed when the P content is added higher than 0.04%. Ti ₄ C ₂ S ₂ is a precipitate produced at a low temperature of slab homogenization heat treatment temperature of 1200 ° C. or lower, and a P content of 0.04% or less, which is not produced in the steel of the present invention which performs a homogenization heat treatment of 1200 ° C. or higher. In addition, precipitates formed when Ti is added to satisfy the formula of more than stoichiometric, that is, Ti≥ (48/14) N + (48/32) S, include TiN, TiS, and TiC. .

On the other hand, it is known that TiC precipitates are not produced when Ti amount is added below the chemosugar, but many researchers including the present inventors can confirm that a small amount of TiC precipitates are produced even under Ti equivalent weight. TiC precipitates are stabilized at high temperatures of more than 700 ℃, re-dissolved in continuous annealing, and high temperature annealing is required above 860 ℃ to secure solid carbon, resulting in problems such as buckling and workability during annealing. However, the present inventors maintain the TiC precipitate as a metastable precipitate when cold wound below 600 ° C., so that the solid carbon of the solid carbon may be dissolved by re-dissolving the TiC precipitate even in the continuous annealing operation at a temperature range of 800 to 860 ° C. It could be confirmed that it can be secured.

Hereinafter, the present invention will be described in detail.

First, the reason for limiting the composition range of the present invention is as follows.

Carbon (C) has solid solution hardening and hardening hardening. If the carbon content is less than 0.0015%, the tensile strength is insufficient. Therefore, the sufficient amount of hardening hardenability is obtained because the absolute amount of added carbon is very small even if solid carbon is secured by re-dissolving a large amount of TiC precipitate produced by cold winding in continuous annealing operation. Not obtained. In addition, if the content exceeds 0.0025%, the amount of solid solution carbon in the steel after continuous annealing is high, and the hardening hardening property is high, but the room temperature aging resistance is not secured. The deterioration is great. Therefore, the amount of carbon added is limited to the range of 0.0015 ~ 0.0025%.

Silicon (Si) is an element that increases the strength, the strength increases as the amount added, but the ductility deterioration is remarkable, so the addition amount is limited to 0.2% or less.

Manganese (Mn) finely particles without damaging ductility, and precipitates sulfur in the steel with MnS to prevent hot brittleness due to FeS generation, in the case of Ti-added steel like the present invention TiS precipitation by MnS precipitation By suppressing it somewhat, the amount of expensive Ti addition can be reduced somewhat. However, in order to obtain such an effect, addition of more than 0.07% is required, but when it is added in excess of 0.4%, the ductility deterioration is remarkable as compared with the sudden increase in strength due to solid solution strengthening, so it is desirable to limit the amount to 0.07 to 0.4%. Do.

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 content of phosphorus is less than 0.008%, the above-described effects cannot be obtained. When the content of phosphorus is more than 0.04%, the soft cold rolled steel sheet required by the present invention is manufactured because the material is embrittled by segregation at the grain boundary with the decrease in the ductility. it's difficult. Therefore, the addition amount is limited to the range of 0.008 ~ 0.04%.

Sulfur (S) is precipitated as MnS or TiS-based sulfide at high temperature, so if the amount of sulfur added is largely changed, the amount of Mn and Ti necessary for precipitation into sulfide is increased. It is very difficult to control the amount of Ti. Therefore, the more narrowly managed the range of sulfur addition as possible to obtain a stable baking hardenability, and in order to lower the expensive Ti amount, the upper limit of the content is preferably 0.015%.

When aluminum (Al) is added as 0.02% or less as the deoxidizer of steel, the mechanical properties deteriorate, such as oxidative inclusions increase in steel, resulting in poor workability. In addition, since excessive addition of 0.08% or more causes hardening of the material and an increase in manufacturing cost, the addition amount is preferably limited to 0.02 to 0.08%.

Since nitrogen (N) is precipitated as nitride of TiN at a high temperature in the steel of the present invention, it is necessary to manage the range of nitrogen as low as possible in order to control the amount of Ti to obtain appropriate baking hardening property. Therefore, the steelmaking process is preferably limited to 0.0025% or less, which is easy to manufacture practically.

Since titanium (Ti) is precipitated as Ti-based nitrides and sulfides at high temperatures, it is very important to add Ti in an appropriate range to obtain proper baking hardening. If the amount of Ti is too small, the sulfur and nitrogen in the steel cannot be fixed, so the hardening hardenability is increased, but the moldability and room temperature aging resistance are deteriorated. However, if the amount of Ti is too high, all solid carbon, as well as nitrogen and sulfur in the steel, cannot be precipitated by TiC, so that the hardening hardenability cannot be obtained. Also, due to the increase of the melting temperature due to the formation of excessive TiC precipitates, the dissolved carbon of the TiC precipitates is dissolved again. To ensure the very high temperature annealing of 860 ℃ or more is required. Therefore, in order to obtain stable bake hardenability, the amount of Ti needs to be added so as to satisfy the formula (48/14) N ≦ Ti ≦ (48/14) N + (48/32) S. The content range of satisfying Ti is limited to 0.008 ~ 0.018%.

After dissolving the ultra low nitrogen and ultra low carbon Al-kilde steel of the above composition in a converter, the continuously cast slab is reheated at a temperature of 1200 ° C. or higher at which the austenite structure before hot rolling can be sufficiently homogenized to be 900 directly above the Ar ₃ temperature. Finish hot rolling in the temperature range of 950 ℃.

If the slab reheating temperature is less than 1200 ° C, the steel structure is not uniformly austenite grains, and the grains are generated, resulting in deterioration of the material. Therefore, the reheating temperature is preferably 1200 ° C or higher.

If the finish hot rolling temperature is below 900 ℃, the top, tail and edges of the hot rolled coil become single-phase areas, resulting in an increase in in-plane anisotropy and deterioration of moldability. It is preferable that the temperature range is 900 to 950 ° C. since defects such as orange peel may easily occur on the surface after processing.

After the above hot rolling, low temperature winding is performed in a temperature range of 500 to 600 ° C., which is one of the main features of the present invention.

When the coiling temperature is higher than 600 ℃, the processability is improved by increasing the grain size. However, TiC precipitates that are precipitated in small amounts are stabilized and sufficient hardening hardenability cannot be obtained. In order to ensure high temperature annealing of 860 ℃ or more, workability deterioration occurs during annealing.

On the other hand, if the coiling temperature is below 500 ° C, proper baking hardening property is ensured by re-dissolving TiC precipitate after continuous annealing, but the coiling temperature is very low, resulting in remarkably fine grains and deterioration of moldability. Since the hot workability for carrying out is deteriorated, it is preferable to make the appropriate temperature range into 500-600 degreeC.

It is preferable to perform the temper rolling of 1.0-2.0% which is somewhat higher than the general temper rolling ratio as a means for securing the appropriate calcination hardening resistance and aging resistance by using the bake hardening type cold rolled steel sheet manufactured by the above method. If the temper rolling ratio is less than 1.0%, it is very disadvantageous in terms of aging resistance such that aging occurs at room temperature for a long time, yield strength is increased and yield point elongation, which is fatal to press processing, is reproduced. However, when the temper rolling ratio is more than 2.0%, work hardening occurs due to excessive temper rolling, which increases the strength and deteriorates the ductility.

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

Example 1

Table 1 below shows the strict control of the contents of carbon, nitrogen, and sulfur, in particular, the amount of Ti added to satisfy the formula of (48/14) N ≦ Ti ≦ (48/14) N + (48/32) S. It shows chemical composition of steel and comparative steel, steel 1 ~ 3 is invention steel and the rest steel is comparative steel.

Steel grade Alloy component (wt%) CT (℃) Remarks C Mn P S Sol.Al N Ti (48/14) N (48/14) N + (48/32) S One 0.0017 0.13 0.0094 0.012 0.039 0.0025 0.009 0.0086 0.027 540 Invention steel 1-1 700 Comparative steel 2 0.0020 0.15 0.0080 0.010 0.035 0.0025 0.012 0.0086 0.024 540 Invention steel 3 0.0022 0.12 0.0094 0.011 0.039 0.0024 0.016 0.0082 0.025 540 Invention steel 3-1 710 Comparative steel 4 0.0018 0.11 0.0090 0.012 0.034 0.0023 0.028 0.0079 0.026 540 Comparative steel 4-1 700 Comparative steel 5 0.0018 0.12 0.0095 0.011 0.041 0.0034 0.039 0.012 0.028 540 Comparative steel 5-1 700 Comparative steel 6 0.0040 0.15 0.0120 0.015 0.045 0.0020 0.015 0.0069 0.029 700 Comparative steel

Steel grade Yield strength (kgf / mm ² ) Tensile Strength (kgf / mm ² ) Elongation (%) r _m Curing hardening (kgf / mm ² ) Remarks One 20.9 30.6 48.6 1.80 4.8 Invention steel 1-1 20.5 29.4 49.0 1.80 2.9 Comparative steel 2 20.0 29.8 46.1 1.81 3.9 Invention steel 3 19.8 29.7 46.7 1.85 4.6 Invention steel 3-1 20.5 29.5 47.4 1.87 2.8 Comparative steel 4 19.6 29.2 45.5 1.87 2.1 Comparative steel 4-1 18.7 29.4 47.7 1.90 0.4 Comparative steel 5 17.5 29.4 46.4 1.93 0.1 Comparative steel 5-1 18.8 29.1 46.6 1.95 0 Comparative steel 6 23.4 33.5 29.6 1.52 5.5 Comparative steel

In Table 2, hot rolling is performed using the steel having the composition of Table 1, and in particular, the hot rolling temperature is controlled to 700 ° C. and 540 ° C. for the remaining steels except for No. 2 and No. 6, respectively. After cold annealing and continuous annealing at 830 ° C., a controlled rolling rate of about 1.5% was performed, and the mechanical properties and baking hardening amount were measured.

Carbon (C) 0.0017 ~ 0.0022%, Manganese (Mn) 0.1 ~ 0.2%, Phosphorus (P) 0.008 ~ 0.010, Nitrogen (N) 0.002 ~ 0.0025%, Titanium (Ti) 0.008 ~ 0.016% / 14) In the steels 1 to 3 of the invention steel controlled to satisfy the formula of N≤Ti≤ (48/14) N + (48/32) S, the bake hardening amount is 3.9 ~ 4.8kgf / mm ² , and the tensile strength is 29 ~ 30kgf / mm ² , yield strength about 20 kgf / mm ² , elongation 46-49% and average r-value (r _m ) 1.8-1.85, indicating that the cold rolled steel sheet having excellent moldability and hardening hardenability can be produced by the present invention. Could.

However, as in 1-1 and 3-1, the composition is the same as that of the present invention, but its hot rolled coiling temperature is over 700 ℃, so it is difficult to secure solid carbon by re-dissolving TiC precipitate during continuous annealing. The amount was very low, ³ kgf / mm ² .

In the 4 and 5 steels, the amount of Ti is out of the component regulation range of the present invention steel, and the amount of Ti is added more than the formula of (48/14) N ≦ Ti ≦ (48/14) N + (48/32) S. Therefore, all the added carbon components are precipitated as TiC precipitates, so the annealing temperature for re-dissolution of TiC precipitates becomes very high. kgf / mm ² and a very low 0.1kgf / mm ^2. On the other hand, in order to re-dissolve the TiC precipitates precipitated in the 4 and 5 rivers, high temperature annealing of 860 ° C. or higher is required, and deterioration of continuous annealing occurs when such high temperature annealing is performed.

In addition, the 4-1 and 5-1 steels have the same components as the above 4 and 5 steel, out of the regulatory range of the Ti component presented in the present invention, in particular, the hot-rolled winding temperature is 700 ℃ presented in the present invention steel since one does not have a very high re-dissolution of a small amount of TiC precipitates almost achieved in the 830 ℃ conventional annealing operation temperature above the hot rolling coiling temperature than the amount of bake hardening each 0.4kgf / mm ^2, and 4 and 5 as beongang 0kgf / mm ² Very low.

On the other hand, steel No. 6 has a carbon content of 0.004%, which is outside the range of component regulation of the present invention steel. Especially, the hot rolling temperature is 700 ° C., which is very high, and it is difficult to secure solid carbon by re-dissolving a small amount of TiC precipitate. However, the Ti content was added to satisfy the formula of 48/14) N≤Ti≤ (48/14) N + (48/32) S, and the carbon content was excessively added as 0.004% to prevent TiC precipitates. The amount of hard carbon in steel is very high, and the hardening hardening rate is 5.5kgf / mm ² , but the strength is high and the ductility is deteriorated due to excessive amount of solid carbon. Especially, the average r value is 1.52, which is very low. Couldn't get it.

The present invention configured as described above partially produces the metastable TiC precipitate by low temperature winding, thereby improving the formability due to the solid solution reduction effect due to the TiC precipitation and securing the appropriate employment carbon by re-dissolving the TiC precipitate in the continuous annealing operation. In addition to the formability, there is a useful effect in the production of soft cold rolled steel sheet having an appropriate baking hardening amount of 3 ~ 6kgf / mm ² level.

Claims (1)

By weight% C: 0.0015 ~ 0.0025%, Si: 0.2% or less, Mn: 0.07 ~ 0.4%, P: 0.008 ~ 0.04%, S: 0.008 ~ 0.015%, Soluble Al: 0.02 ~ 0.08%, N: Ultra low nitrogen containing Ti to 0.0025% or less, Ti: 0.008 to 0.018%, and Ti addition amount satisfies the formula (48/14) N≤Ti≤ (48/14) N + (48/32) S After ultra-low carbon Al-kilted steel is homogenized heat treatment at 1200 ℃ or higher, finish hot rolling at temperature range of 900 ~ 950 ℃, cold winding at temperature range of 500 ~ 590 ℃ to produce small amount of TiC precipitate, then cold rolling, 800 A method for producing a hardened hardened soft cold rolled sheet having excellent moldability, characterized by performing continuous annealing and temper rolling of 1.0 to 2.0% in the temperature range of ˜860 ° C.