KR100685036B1 - Bake-hardenable cold rolled steel sheet with superior strength and aging resistance, galvannealed steel sheet using the cold rolled steel sheet and method for manufacturing the cold rolled steel sheet - Google Patents

Abstract

A high strength cold rolled steel sheet with excellent bake-hardenability, room temperature aging resistance and secondary work embrittlement resistance, a method for manufacturing the high strength cold rolled steel sheet, and a hot dip coated steel sheet using the high strength cold rolled steel sheet are provided. A bake-hardenable high strength cold rolled steel sheet with superior aging resistance has a composition comprising, by weight percent, 0.0025 to 0.0035% of C, 0.02% or less of Si, 0.2 to 1.2% of Mn, 0.05 to 0.11% of P, 0.01% or less of S, 0.08 to 0.12% of soluble Al, 0.0025% or less of N, 0.005 to 0.018% of Ti, 0.1 to 0.2% of Mo and 0.0005 to 0.0015% of B, a Ti content satisfying the relational expression of Ti^*(effective Ti)=total Ti-(48/14)N-(48/32)S<=0, with the balance being Fe and other inevitable impurities, and has 30 MPa or more of a bake-hardening amount, 30 MPa or less of an aging index, -30 deg.C or less of DBTT(Ductile Brittle Transition Temperature) at a working ratio of 2.0, and an ASTM grain size number of 9 or more.

Description

Bake-Hardenable Cold Rolled Steel Sheet with Superior Strength and Aging Resistance, Galvannealed Steel Sheet Using the Cold Rolled Steel Sheet and Method for Manufacturing the Cold Rolled Steel Sheet}

1 is a graph showing the effect of grain size on the quench hardening and aging index

2 is a graph showing the effect of Al content on the mechanical properties

3 is a graph showing the effect of Mo (statistical analysis) on baking hardening and aging index

Figure 4 is a cross-sectional microstructure photograph after annealing of an example of steel in accordance with the present invention

5 is a graph showing the effect of the drawing ratio for each material on the secondary work brittleness (DBTT)

The present invention relates to a method for manufacturing cold rolled steel sheets, hot-dip galvanized steel sheets and cold rolled steel sheets used in automobile exterior materials, and more particularly, to high-strength bake hardenable cold rolled steel sheets, hot-dip galvanized steel sheets, and cold-rolled steel sheets having excellent aging resistance. It relates to a manufacturing method of.

In recent years, the use of high-strength steel sheet in the vehicle body for the purpose of improving fuel efficiency of the automobile and reducing the weight of the vehicle body has further increased the demand for reducing the plate thickness and improving the dent resistance. Properties required for automotive cold rolled steel sheet include yield strength, tensile strength, good press formability, spot weldability, fatigue characteristics and corrosion resistance.

Double corrosion resistance is a required property for extending the life of auto parts in recent years. Such steel plates for improving corrosion resistance can be classified into two types, electroplating and hot dip plating. Electroplating steel sheet has better plating characteristics and better corrosion resistance than molten plating material, but steel sheet price is very expensive compared to molten plating material. Recently, it is reluctant to use it. . In recent years, automotive materials, mainly steel mills in various countries, have been producing hot-dip galvanizing materials and supplying them to automobile companies. Accordingly, even in hot-dip galvanizing materials, technologies for securing corrosion resistance far superior to the past levels have been developed. It is on the rise.

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 390 MPa or more. As a material used in automobiles, the elongation, which is a factor indicating stretchability, is higher than that of high tensile strength, but it exhibits press formability of automobiles. The average r value is low and expensive alloying elements such as manganese and chromium are added excessively, leading 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 less than 390 MPa. The ductility is excellent, and the yield strength naturally increases when the coating is processed after press molding. It is attracting attention as a very ideal steel compared to the conventional cold rolled steel.

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 aging at room temperature, so it is very important to control the appropriate employment element.

In general, as a method of manufacturing 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 winding method using a cold winding having a temperature range of 400-500 ° C. As a result, steel having a bake hardening amount of about 40-50 MPa was mainly used. This was because both of the moldability and the baking hardening were more easily achieved by the annealing. In case of P-added Al-Killed steel by continuous annealing method, it is easy to secure bake hardenability because it uses relatively fast cooling speed, but it has a problem of deterioration of formability due to rapid heating and short time annealing. It is limited. Thanks to the recent rapid development of steelmaking technology, it is possible to control the appropriate amount of solid solution in the steel and to manufacture the hardened hardened cold rolled steel sheet with excellent formability 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 Laid-Open No. 61-26757 discloses ultra low carbon cold rolled steel with Ti, Ti, and Nb composites having a C content of 0.0005-0.015% and an S + N content ≤005%, and Japanese Patent Publication No. 57089437 In the following, a method for producing steel having a hardening hardening amount of about 40 MPa or more using Ti-added steel of less than 0.010% C is presented.

This method is to control the amount of Ti and Nb added 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, in the case of the Nb-added steel in the patent, workability deterioration due to high temperature annealing and manufacturing cost increase due to the addition of special elements are expected.

Meanwhile, US Patent Nos. 5,556,485 and 5,656,102 to Bethlehem Steel of the United States have a C content of 0.0005-0.1%, Mn 0-2.5%, Al 0-0.5%, N 0-0.04% and Ti content of 0-0.5%. , A method for producing a hardened hardened cold rolled steel sheet using Ti-V type ultra low carbon steel in which the V content is controlled in the range of 0.005-0.6% is presented.

In general, V is more stable than carbonitride-forming elements such as Ti and Nb, so that the annealing temperature can be lowered. Thus, VC, which is a carbide produced by V during hot rolling, can be re-dissolved even if the annealing temperature is lower than that of the Nb system. It is possible to secure bake hardening.

However, since V forms carbides such as VC, but the re-dissolution temperature is very low and does not substantially improve moldability, the U.S. Patent adds about 0.02% or more of Ti to achieve moldability. .

Therefore, the U.S. patent has a problem in that it is disadvantageous in terms of aging resistance because of a large grain size as well as an increase in manufacturing cost due to a large amount of Ti.

On the other hand, Japanese Patent Application Laid-Open No. 5-93502 discloses a technique for increasing BH property by adding Sn, and Japanese Patent Laid-Open No. 9-249936 adds V and Nb in combination to reduce stress concentration at grain boundaries. A technique for improving ductility is proposed, Japanese Patent Laid-Open Publication No. Hei 8-49038 discloses a technique for improving moldability by addition of Zr, and Japanese Patent Laid-Open Publication No. Hei 7-278654 adds Cr. The technique for improving the formability by minimizing the deterioration of the high strength and work hardening index (N value) is proposed.

However, the above-mentioned techniques are focused only on improving the quench hardening property or improving the moldability, and the deterioration of aging resistance due to the increase of quench hardening property and the increase of P content which is inevitably added due to the high strength of quench hardening steel. It does not mention problems such as secondary brittleness.

However, in general, as the hardening hardening property increases, the room temperature aging resistance deteriorates. In particular, according to the research results of the present inventors, secondary hardening of brittle hardened steel in which solid solution carbon is present as the P content added for higher strength increases. This deterioration, which was found to be more severe as the P content increases.

For example, when the P content added to produce 340 MPa grade hardened steel is 0.07%, the DBTT (Ductile Brittle Transition Temperature), the criterion for determining secondary brittleness, is 1.9 to -20 in drawing ratio. When P content is added about 0.09% to manufacture high strength steel of 390MPa grade, DBTT is very deteriorated as 0 ~ 10 ℃.

These steels are all about 5ppm of B is added to the steel is generally known to improve the secondary brittleness when B is added, but due to excessive P content, it seems that there was a limit in improving DBTT by B.

On the other hand, if the addition of excessive B than the current level to improve the secondary processing brittleness causes a material degradation by B, the amount of addition is also limited.

Therefore, the DBTT for preventing secondary work brittleness should be at least -20 ° C. or more, so it is necessary to examine new components or manufacturing conditions other than B in the hardened hardened steel.

An object of the present invention is to provide a high tensile cold rolled steel sheet having excellent baking hardening, room temperature aging resistance, and secondary processing brittleness, and a method of manufacturing the same.

In addition, the present invention is to provide a hot-dip galvanized steel sheet using the high-tensile cold-rolled steel sheet of the present invention, an object thereof.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated.

In the present invention, by weight% C: 0.0025-0.0035%, Si: 0.02% or less, Mn: 0.2-1.2%, P: 0.05-0.11%, S: 0.01% or less, Soluble Al: 0.08-0.12%, N: 0.0025% or less, Ti: 0.005-0.018%, Mo: 0.1-0.2%, and B: 0.0005-0.0015%, and the Ti content satisfies the following relation (1), and is composed of the balance Fe and other unavoidable impurities Become,

[Relationship 1]

Ti ^* (Effective Ti) = Total Ti-(48/14) N-(48/32) S ≤ 0

And the baking hardening amount (BH) of 30 MPa or more, the aging index (AI) of 30 MPa or less, DBTT and ASTM No. The present invention relates to a high tensile strength hardenable hardened cold rolled steel sheet having excellent aging resistance having a grain size of 9 or more and a hot dip coated steel sheet using the cold rolled steel sheet.

In addition, the present invention by weight% C: 0.0025-0.0035%, Si: 0.02% or less, Mn: 0.2-1.2%, P: 0.05-0.11%, S: 0.01% or less, Soluble Al: 0.08-0.12 %, N: 0.0025% or less, Ti: 0.005-0.018%, Mo: 0.1-0.2% and B: 0.0005-0.0015%, and the Ti content satisfies the following relation (1),

[Relationship 1]

Ti ^* (Effective Ti) = Total Ti-(48/14) N-(48/32) S ≤ 0

Al-killed steel composed of the balance Fe and other unavoidable impurities is subjected to homogenization heat treatment at 1200 ° C. or higher, and then hot-rolled at a temperature range of 900-950 ° C. and wound at a temperature range of 600-650 ° C., followed by 75- Cold rolling at 80% reduction rate, continuous annealing at the temperature range of 760-790 ° C, and temper rolling at 1.2-1.5% reduction rate to produce high tensile hardening hardened cold rolled steel sheet with excellent aging resistance. It is about.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

In general, when carbon or nitrogen is added to steel, carbonitrides such as TiN, AlN, TiC, Ti ₄ C ₂ S ₂ and NbC are formed by combining with precipitate forming elements such as Al, Ti or Nb in the hot rolling step. Carbon or nitrogen, which cannot be combined with these carbonitride-forming elements, is present in solid solution in the steel, which affects the hardening or aging properties.

In particular, since nitrogen has a much higher diffusion rate than carbon, deterioration of aging resistance is very fatal compared to an increase in BH. Therefore, in general, nitrogen is to be removed from the steel as much as possible, and in particular, Al or Ti is precipitated with nitrogen rather than carbon at high temperature, so there is no big problem even if it is determined that there is little BH property or aging due to nitrogen in the steel.

However, carbon is an element that is indispensable to steel and its properties are determined by its content.

The small hardened steel proposed by the present invention is very important in the role of carbon, and the small hardened carbon remains in the steel to achieve both hard hardening and aging resistance.

However, the effect on the hardening hardening and aging may vary depending on the location where the dissolved carbons in the steel are also present, that is, at grain boundaries or in grains.

In other words, solid carbon, which can be measured through internal friction tests, is mainly solid carbon in the grains, and because it is relatively free to move, combined with the operating potential, the aging characteristics are affected. The item evaluating the aging characteristics is the aging index, or AI (Aging Index).

In general, when the aging index (AI) is 30MPa or more, aging occurs before the maintenance of 6 months at room temperature, which may cause serious defects in press working.

However, the solid solution carbons present in the grain boundaries exist in grain boundaries, which are relatively stable regions, making it difficult to detect vibrations such as internal friction.

Since the solid carbons present in the grain boundary are in a relatively stable position, they have little effect on low temperature aging such as AI, and are activated and influence under high temperature baking conditions such as baking hardening.

Therefore, it can be said that the dissolved carbon in grains affects both aging and hardening hardenable at the same time, but the dissolved carbons in grain boundary only affect hardening hardening.

However, since the grain boundary is a relatively stable region, it is reported that all of the dissolved carbon present in the grain boundary does not affect the hard hardening, and about 50% of the solid carbon present in the grain boundary affects the hard hardening.

Therefore, when appropriately controlling the presence of such dissolved carbon, that is, to control the addition of the added dissolved carbon in the grain boundary rather than possible in the grain boundary will be able to secure both aging resistance and baking hardening.

To this end, it is important to control the grain size as well as to properly manage the amount of carbon added to the steel.

This is because when the amount of carbon added is very large or small, it is difficult to secure proper baking hardening and aging resistance even when controlling the position of solid solution carbon.

Figure 1 shows the relationship between the bake hardening amount (BH) value and the aging index (AI) value according to the change in grain size of the results of the study performed by the present inventors.

As shown in FIG. 1, grain size number (No); ASTM No.)] increases, that is, the finer the grains, the lower the AI value relative to the BH value is remarkable, and this can be seen that the BH-AI value gradually increases to excellent aging resistance.

Based on the results of FIG. 1, the present inventors attempted to refine the annealing grain size below an appropriate level in order to distribute as much of the solid solution carbon present in the steel as possible.

As a result of the research of the present inventors, ASTM No. It was found that controlling to 9 or more is preferable.

On the other hand, it is necessary to strictly control the total amount of carbon added to the steel even if a large amount of solid solution carbon is distributed in the grain boundary.

This is because even if the added carbon content increases excessively, even if the grain size becomes fine, the dissolved carbon present in the grain increases in proportion to the total amount of added carbon, thereby deteriorating room temperature aging resistance as the amount of dissolved carbon in the steel increases.

In the present invention, the total amount of carbon added to satisfy these conditions was set to 25-35 ppm.

However, even if the carbon content is controlled, when Ti is added in an amount of more than Ti to form precipitates such as TiN or TiS in Ti-added ultra low carbon steel, Ti is combined with carbon to form carbides such as TiC.

In addition, under such conditions, since the amount of solid solution carbon remaining in the steel changes as the Ti content changes, it is difficult to control the appropriate amount of solid solution carbon.

Therefore, in order to overcome this problem, the present invention intends to control all carbon added by adding less than the amount Ti is combined with S and N, as shown in the following relation (1).

[Relationship 1]

Ti ^* (Effective Ti) = Total Ti-(48/14) N-(48/32) S ≤ 0

On the other hand, in the present invention, in addition to Ti, the effect of AlN precipitates through the addition of Al was considered in order to more secure the baking hardening and aging resistance.

In general, in Ti-added steels with low Al content, nitrogen is mostly coarse precipitated as TiN or AlN at a high temperature of 1300 ° C. or higher, and thus does not have a significant effect on the solid solution effect or grain refinement.

Therefore, such AlN has only an effect of removing solid solution nitrogen in the steel like TiN precipitates.

As a result of various experiments using the inventive steel, the carbon content is very narrowly limited to 25-35 ppm, thereby producing small minor hardened steel having BH and age resistance within a narrow range.

In the case of customers, the higher BH value and the aging resistance of 6 months or more are required. Therefore, there is a need for a technology for increasing the hardening resistance within the range that does not impair possible aging resistance.

Al is very effective in this respect. In other words, when Sol.Al is added in the range of 0.02-0.06%, which is a normal level, it simply plays a role of fixing nitrogen solution. However, when it is added more than 0.08%, precipitates of AlN become very fine, resulting in crystal grain recrystallization. Since it acts as a kind of barrier to prevent growth, grains become finer than Ti-added steel without Sol.Al, which results in an increase in hardening hardness without changing AI values.

Figure 2 shows the change in mechanical properties of the molten plated material with the change in Sol.Al content.

As shown in FIG. 2, the BH value increases and then decreases with increasing Al content, and the content of Sol.Al exhibiting the BH effect is about 0.08-0.12%. If the Sol.Al content is further increased, the r value and elongation (El), which are indexes of formability, are lowered, and the oxidation inclusions increase during steelmaking due to excessive addition of Sol.Al, resulting in deterioration of surface quality. Can be.

Through this study, the inventor suggested that the Sol.Al content is 0.08-0.12%.

The following relation (2) shows the effect of Sol.Al addition on the improvement of baking hardening within the range of Sol.Al presented by the present inventors in a statistical manner.

[Relationship 2]

Bake Hardening (BH) = 50- (885 × Ti) + (62 × Al)

In the steel sheet of the present invention, the Ti and Al contents are preferably controlled such that the baking hardening amount in the relation (2) is 30 MPa or more.

Even though the carbon content, the Sol.Al and the Ti content are controlled in this way, the role of the hot rolled coiling temperature is very important in the Ti-added ultra low carbon steel.

In other words, even if the BH property improvement and room temperature aging resistance improvement by using the grain refinement effect using Ti as in the present invention, the grain size increases in the hot rolling step because the coiling temperature is very increased, the grain size of the later recrystallization annealing Grain coarsening of 9 or less occurs, and the AI value exceeds 30 MPa or more required by the present invention steel.

On the other hand, if the coiling temperature is lowered below a certain level, the room temperature aging resistance is improved, but the grain refinement is very large, and in the case of Ti-added steel, the solid solution carbon in steel is increased by low temperature winding, so that the yield strength is increased and the elongation and r value are reduced. Not only the deterioration of sex but also the aging deterioration.

Therefore, in the present invention, in order to solve this problem, the hot rolled coiling temperature is narrowly limited to 600-650 ° C.

On the other hand, in terms of secondary work brittleness, molding of parts generally performed in automobile companies can obtain a desired shape by several repeated press processing. In other words, the secondary brittleness means that a crack occurs in the processing performed after the primary press working. Such cracks cause phosphorus (P) in the steel to exist as a grain boundary and thus weaken the binding force of the grains, and thus fracture occurs around the grain boundaries.

Basically, it is preferable not to add phosphorus (P) element in order to remove secondary processing brittleness, but in general, the employment element having the smallest elongation decrease compared to the increase in strength is P, and above all, the cost is low. There is this.

Therefore, in order to achieve high strength in steel materials, it is basically added. However, in recent years, research is being conducted to enhance the reinforcing effect through other employment elements instead of ligament to remove such secondary processing brittleness even if the manufacturing cost goes up a little.

However, the results to date indicate that P will continue to be used as a fortification element for the foreseeable future.

In order to improve the secondary work brittleness in P-added steels, it is possible to increase the site competition effect or the binding strength of grain boundary by adding the B or the like by remaining the element in the steel, such as hard hardened steel. In order to minimize the grain boundary diffusion of P by lowering the coiling temperature below a certain temperature at the stage, studies to prevent secondary processing brittleness are being conducted, but there is no complete solution.

Therefore, in the present invention steel, Mo was to be considered for more stable secondary workability improvement.

According to the results of the present inventors, Mo is very advantageous for improving the secondary work brittleness because it improves the binding force of the grain boundary.

In addition, since Mo has affinity with solid solution carbon in steel, it is advantageous in aging resistance because it suppresses diffusion of dissolving carbon into dislocation after long-term maintenance at room temperature.

Figure 3 shows the results of the statistical analysis of the effect of improving the aging resistance by the addition of Mo of the present inventors.

As shown in FIG. 3, as the Mo content is increased, there is no significant difference in the BH property, but the AI value is lowered, indicating that the aging resistance is improved.

However, as a result of the research of the present inventors, in the Nb-added steel, it was expected to improve the aging property with only 0.1% or less of Mo. However, in the case of Ti-added steel like the present invention, the grain size is somewhat larger than that of the Nb-added steel, and the added carbon content is somewhat higher. In order to improve the efficiency, it was necessary to increase the Mo content.

To this end, the evaluation of aging resistance according to Mo addition amount in Ti-added steel showed that Mo addition of 0.1 ~ 0.2% was very effective for aging resistance and secondary processing brittleness.

Equation (3) below shows the effect of improving the aging resistance of Mo in Ti-added steel by a statistical method.

[Relationship 3]

Aging Index (AI) = 44-(423 × Ti)-(125 × Mo)

In the steel sheet of the present invention, the Ti and Mo contents are preferably controlled so that the aging index in the above relation (3) is 30 MPa or less.

Hereinafter, the component and manufacturing conditions of this invention steel are demonstrated.

Carbon (C) is an element showing solid solution hardening and baking hardening.

If the carbon content is less than 0.0025%, the tensile strength is insufficient due to the very low carbon content, and even if the Ti content is added as in the relation (1), the sufficient hardening hardening is not obtained because the absolute carbon content in the steel is low.

On the other hand, when the content exceeds 0.0025%, the grain refining effect of Nb additive steel is greatly increased, so that the hardening hardening property is very high and the secondary work brittleness is improved, but the room temperature aging resistance is not secured due to excessive dissolved carbon content. Stretcher strain occurs during molding, resulting in poor moldability and ductility.

Therefore, in the present invention, it is preferable to limit the content of carbon to 0.0025 to 0.0035%.

Silicon (Si) is an element that increases the strength, but as the amount added increases, the strength increases, but ductility deterioration is remarkable. In particular, Si is an element that degrades the hot-plating property.

In the present invention, the addition amount is limited to 0.02% or less in order to prevent material deterioration and plating characteristic deterioration by Si.

Manganese (Mn) is an element that refines particles without damaging ductility, and precipitates sulfur in steel completely with MnS to prevent hot brittleness due to the formation of FeS and to strengthen steel. In the present invention, when the Mn content is less than 0.2%, proper tensile strength cannot be secured, and when it is added more than 1.2%, the strength increases and the formability deteriorates due to solid solution strengthening. In the annealing process during manufacture, a large amount of oxides such as MnO are formed on the surface, thereby degrading the plating adhesion. Also, a large amount of plating defects, such as stripes, are generated to deteriorate the product quality. Therefore, the addition amount is preferably limited to 0.2-1.2%. .

Phosphorus (P) is a substitution type alloy element having the greatest solid solution strengthening effect, and serves to improve in-plane anisotropy and improve strength.

In addition, the results of the study of the present inventors P serves to promote the development of (111) aggregates, which are advantageous for improving the average r value in the future annealing step by miniaturizing the hot-rolled sheet grains, and in particular, the position with carbon in terms of the effect of the small hardenability. As the phosphorus content increased due to the competitive effect, the baking hardening tended to increase.

However, there is a problem in that secondary work brittleness deteriorates due to a weakening of the binding force of grain boundaries when P is increased.

However, when the phosphorus content is less than 0.05%, the secondary work brittleness is improved due to the small amount of phosphorus present in the grain boundary, but the material improvement effect by the grain refinement effect is insignificant, and when it exceeds 0.11%, the strength is sharp compared to the improvement in formability. An increase occurs, and the excessive addition of P amount causes the secondary work brittleness, which causes P to segregate at grain boundaries and withdraw the material, to be greatly degraded.

Therefore, the content of P is limited to 0.05-0.11%.

Sulfur (S) is an element that should be precipitated as sulfide of MnS at high temperature to prevent hot brittleness by FeS.

However, when the content of S is excessive, there is a possibility that S remaining after precipitation into MnS embrittles grain boundaries and causes hot brittleness.

In addition, even if the amount of S added to completely precipitate the MnS precipitate, when the S content is large, material degradation due to excessive precipitate occurs, so it is preferable to limit the added amount to 0.01% or less.

Aluminum (Al) is usually added for deoxidation of steel, but in the present invention, it exhibits an effect of doubling grain refining effect and calcining hardenability by AlN precipitation.

As also shown in the above relation (2), the greater the amount of Al added, the better the BH property. However, considering the material and the like, it is necessary to control the appropriate amount of addition.

In order to achieve the effect of adding Al in the present invention, the Al content should be added at least 0.08% or more.

However, when Al is added in excess of 0.12%, the surface quality decreases due to deterioration of formability and the increase of oxidation inclusions during steelmaking, and also increases the manufacturing cost due to excessive addition of Al, so the amount of Al added is 0.08. It is desirable to limit it to -0.12%.

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, and thus it is necessary to fix it with Ti or Al.

In general, nitrogen has a very fast diffusion rate compared to carbon, and therefore, when present as solid nitrogen, the deterioration of room temperature aging resistance is very serious compared to that of solid carbon.

In addition, since the yield strength increases and elongation and r value deteriorate due to the remaining of solid solution nitrogen, the content of N is limited to 0.0025% or less in the present invention.

Ti is a carbonitride forming element and forms nitrides such as TiN, sulfides such as TiS or Ti ₄ C ₂ S ₂ and carbides such as TiC in the steel.

However, the present invention steel is a steel grade in which solid carbon remains in steel, and the Ti content needs to be controlled as in the relation (1).

In addition, when the Ti content is less than 0.005%, the relation (1) is satisfied, but the Ti content is so small that the grain size increases, so that the grain refinement effect disappears.

That is, this violates the effect of improving the aging resistance due to the grain refining effect pursued by the present invention steel, thereby deteriorating the aging resistance, and also accompanied by the deterioration of formability such as elongation and r value by solid carbon in the steel.

On the other hand, when the Ti content is more than 0.018% does not satisfy the condition of the relation (1), it causes a decrease in the hardening hardening due to the reduction of solid solution carbon in the steel.

As such, in the present invention, the content of Ti is 0.005 to 0.018%, and the above relation (1) must be satisfied.

Mo is one of the very important elements considered in the present invention.

Mo is dissolved in steel to improve strength or to form Mo-based carbides.

However, the most important role of Mo is to increase the binding strength of grain boundaries in the presence of solid solution, thereby improving grain breakdown caused by phosphorus, that is, secondary processing brittleness, and suppressing the diffusion of carbon by affinity with solid carbon. It is to improve the aging resistance. The relationship (3) shows the aging effect by Mo in a quantitative manner. For this purpose, an appropriate range of Mo addition is required. In other words, when Mo is less than 0.1%, the above effect could not be obtained in Ti-added steel.

In addition, when the Mo content exceeds 0.2%, the effect of improving the secondary processing brittleness or age resistance is insignificant compared to the addition of Mo, and there is a problem in that the manufacturing cost is significantly increased by adding a large amount of Mo. Therefore, considering the production cost and the effect compared to the amount, the Mo content is preferably limited to the range of 0.1-0.2%.

B exists in steel as an invasive element and is dissolved in grain boundaries or combines with nitrogen to form nitrides of BN.

B is an element having a great influence of the material compared to the added amount, and it is necessary to strictly limit the added amount. In other words, when a small amount of B is added to the steel, it segregates at grain boundaries and improves the secondary brittleness.

However, when the amount is added above a certain amount, it is necessary to add an appropriate range because material degradation occurs that causes an increase in strength and a significant decrease in ductility.

In the present invention, it is set to 0.0005-0.0015% in consideration of such characteristics and the ability of steelmaking for the current B addition.

The steel slab (Slab) formed as described above is heated at 1200 ° C. or more, where the austenite structure before hot rolling can be sufficiently homogenized, to finish hot rolling in the temperature range of 900-950 ° C., which is directly above the Ar ₃ temperature.

If the slab temperature is less than 1200 ℃, the structure of the steel does not become uniform austenite grains, and since the mixing occurs, the material is degraded.

If the hot finishing temperature is less than 900 ℃, the top, tail and edges of the hot-rolled coil become single-phase areas, which increases the in-plane anisotropy and deteriorates the formability. After processing, defects such as orange peel are likely to occur on the surface.

The grain size after the hot rolling process is ASTM No. Winding at 600-650 ° C. is necessary to prevent deterioration of formability due to excessive grain refinement with an appropriate grain refinement effect of 9 or more. If the coiling temperature exceeds 650 ℃, the grain size after annealing increases, even if the carbon and Ti content satisfies the constituent conditions presented in the present invention, sufficient grain refining effect is not obtained, and the grain boundary segregation of phosphorus increases. As a result, secondary work brittleness deteriorates.

On the other hand, when the coiling temperature is less than 600 ℃ grain size becomes fine but the degree is too severe to improve the aging resistance and secondary processing brittleness, but excessive rise in yield strength and deterioration of moldability is caused.

After hot rolling, the steel is pickled in the usual manner, followed by cold rolling at a cold rolling rate of 75-80%.

The reason why the cold rolling ratio is higher than 75% is to improve the moldability, in particular, the r value, as well as improving the aging resistance due to the grain refinement effect pursued by the present invention steel.

On the other hand, when the cold rolling ratio exceeds 80%, the grain refining effect is great, but the degree of refining of the grain size becomes very large due to excessive rolling rate, which causes hardening of the material, and also due to excessive cold rolling increase. The r value gradually decreases.

Next, the cold rolled steel sheet as described above is continuously annealed by a conventional method in a temperature range of 760-790 ° C.

If the annealing temperature is less than 760 ° C, the yield strength increases and elongation and r value deteriorate due to the presence of unrecrystallized grains.

If the annealing temperature exceeds 790 ℃ moldability is improved but the grain size is ASTM No. Less than 9, the AI value is 30 MPa or less, resulting in deterioration of aging resistance.

The temper rolling is carried out at a rolling rate of 1.2 to 1.5% which is somewhat higher than the usual temper rolling ratio in order to secure proper bake hardening resistance and room temperature aging resistance by using the bake hardening type cold rolled steel sheet manufactured by the above method.

The reason why the temper rolling ratio is set higher than 1.2% is to prevent the deterioration of room temperature aging due to solid carbon in steel.

However, when the temper rolling ratio is excessively increased to more than 1.5%, even though the room temperature aging resistance is improved, the temper rolling rate is high and work hardening occurs, so that the material is degraded. In this case, plating adhesion is degraded due to excessive temper rolling and peeling of the plating layer occurs. Therefore, temper rolling is preferably performed at a rolling rate of 1.2 to 1.5%, which is an appropriate condition to solve these problems.

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

As shown in Table 1, the steel formed as shown in Table 1 was hot rolled, cold rolled, and continuously annealed, followed by temper rolling at a temper rolling rate of about 1.5% after alloy plating at a melt plating temperature of 450 ° C. DBTT was measured at a processing ratio of 2.0 as an item for evaluating values, AI values, grain size and secondary processing brittleness, and the results are shown in Table 2 below.

Moreover, 200 times the cross-sectional photograph was observed about the invention steel 4 after annealing, and the result is shown in FIG.

In addition, the invention steel (6), comparative steel (12) and 0.0019C-0.63Mn-0.056P-0.03Sol.Al-0.005Ti-0.006Nb-0.0014N-based steel materials (NSC Co., Ltd.) according to the processing ratio change The change of DBTT was observed and the result is shown in FIG.

River bell Chemical composition (wt%) Remarks C Mn P S Sol.Al Ti Nb N Mo B One 0.0023 0.58 0.060 0.0082 0.087 0.009 - 0.0022 0.134 0.0005 Invention steel 2 0.0027 0.25 0.068 0.0081 0.098 0.014 - 0.0017 0.148 0.0005 Invention steel 3 0.0033 0.35 0.058 0.0058 0.105 0.015 - 0.0019 0.162 0.0007 Invention steel 4 0.0029 0.61 0.071 0.0083 0.118 0.013 - 0.0015 0.159 0.0005 Invention steel 5 0.0030 0.98 0.091 0.0057 0.104 0.010 - 0.0013 0.188 0.0007 Invention steel 6 0.0029 1,11 0.10 0.0073 0.089 0.011 - 0.0021 0.162 0.0009 Invention steel 7 0.0064 0.64 0.069 0.0071 0.082 0.001 - 0.0017 0.121 0.0007 Comparative steel 8 0.0022 0.63 0.066 0.0085 0.040 0.025 - 0.0015 0.115 0.0005 Comparative steel 9 0.0012 0.65 0.070 0.0072 0.095 0.011 - 0.0019 0.159 0.0008 Comparative steel 10 0.0021 0.93 0.096 0.0089 0.043 0.010 0.022 0.0017 0.121 0.0006 Comparative steel 11 0.0022 0.59 0.062 0.0066 0.071 0.012 - 0.0022 0.034 0 Comparative steel 12 0.0029 0.99 0.099 0.0078 0.041 0.017 - 0.0021 0 0.0007 Comparative steel 13 0.0030 0.62 0.047 0.0085 0.021 0 - 0.0019 0 0 Comparative steel 14 0.0023 0.98 0.120 0.0078 0.098 0.014 - 0.0023 0.031 0 Comparative steel

Steel grade Winding temperature CT (℃) Cold Rolling Rate (%) Annealing Temperature (℃) BH (MPa) AI (MPa) Grain Size (ASTM No.) DBTT (℃) Remarks One 620 78 780 47.4 23.4 9.7 -40 Invention steel 2 620 77 790 43.7 19.6 9.9 -50 Invention steel 3 620 78 775 43.2 17.4 10.1 -50 Invention steel 4 610 76 790 45.8 18.6 9.5 -40 Invention steel 5 620 78 790 47.6 16.3 10.3 -40 Invention steel 6 620 78 790 45.8 19.1 11.1 -40 Invention steel 7 620 78 780 68.0 55.2 10.2 -50 Comparative steel 8 640 78 770 25.8 21.1 8.2 10 Comparative steel 9 620 78 790 0 0 8.1 20 Comparative steel 10 630 76 790 0 0 9.1 20 Comparative steel 11 620 78 780 43.8 34.6 10.9 0 Comparative steel 12 630 77 790 35.0 36.8 9.2 -20 Comparative steel 13 620 76 790 44.1 22.8 9.5 -10 Comparative steel 14 640 78 790 43.7 20.6 9.8 0 Comparative steel

As shown in Table 2, carbon 0.0025-0.0033%, manganese 0.25-1.11%, phosphorus 0.058-0.10%, sulfur 0.0057-0.0083%, soluble Al 0.087-0.118%, nitrogen 0.0013-0.0022%, Ti 0.01 Inventive steel (1-6) with strict control of the contents of carbon, TI, Sol.Al and Mo to satisfy the range of -0.015%, Mo 0.134-0.188 and B 0.0005-0.0009% has a grain size of 9.5 as ASTM No. -11.1 (average grain size 7.7-13.4mm), which is ASTM No. It satisfies the condition of 9 or more.

On the other hand, as shown in Figure 4, in the case of the invention steel (4) it can be seen that it has a very uniform crystal grain distribution in the entire cross-section as well as very fine grains.

In addition, as shown in Table 2, the grains of the inventive steels are fine because fine AlN precipitates are formed in the steel due to the addition of an Al content higher than the normal level, which prevents the growth of grains during annealing with NbC precipitates. Therefore, it can be seen that the grain hardening effect has a range of 43.2-47.6 MPa and the AI value, which is an index indicating room temperature aging resistance, is 16.3-23.4 MPa, which is very excellent in balance between BH and room temperature aging resistance.

In addition, the invented steels having a lower AI value than the high calcined hardening amount appeared to have a grain refining effect by AlN precipitates and a delayed effect of solid carbon in steel by addition of Mo.

In addition, as shown in Figure 5, the invention steel (6) can be seen that the overall DBTT properties compared to the comparative steel 12 and the NSC material by the increase in the bonding strength between grains due to the addition of Mo.

On the other hand, the comparative steel (7) was added 0.0064% higher than the carbon content of 0.0025-0.0035% proposed in the present invention, the hot rolling temperature and the annealing temperature satisfies the scope of the present invention.

Comparative steel (7) has a recrystallized grain size of ASTM No. of 10.2, which is very fine.However, the carbon content is very high, and the DBTT property is excellent due to the increase of solid solution carbon in steel, but the BH value is very high and the AI value is over 30MPa. It can be seen that the deterioration.

Comparative steel (8) is a steel added with 0.025% of Sol.Al content of 0.04%, which is lower than 0.08-0.12% of the present invention and higher than Ti content of Ti.

Therefore, the comparative steel 8 had no effect of grain refinement and BH value increase by AlN, and all the carbon added in the steel was precipitated by TiC due to the addition of high Ti content, and hardly hardened hardening appeared. It can be seen that the site competion effect with phosphorus (P) is lowered due to the reduction of carbon, thereby deteriorating DBTT characteristics.

Comparative steel (9) is a steel other components satisfy the present invention range, but the carbon content is 0.0012%, which is lower than the present invention range.

Therefore, the comparative steel 9 had coarse grains and did not obtain BH and AI properties due to such a decrease in absolute carbon content, and also deteriorated as much as 20 ° C in DBTT.

Comparative steel 10 is a steel in which the Sol.Al content is outside the scope of the present invention and Nb is added.

The comparative steel (10) had a low Sol.Al content of 0.043%, which could not be expected to improve the grain refining effect by Al and the improvement of BH value, and the Nb content was also 0.022%. Although the grain size was ASTM No. 9.1, the BH value was not obtained at all due to the lack of solid solution carbon in the steel due to excessive NbC precipitation, and the DBTT characteristics were also degraded due to the disappearance of the solid solution carbon in the steel.

Comparative steel 11 is a steel that Mo content is lower than the scope of the present invention, B is not added, the AI value is 30MPa or more, it can be seen that the DBTT characteristic is very deteriorated due to the addition of Mo, B.

Comparative steel (12) is a steel with less Slo.Al added than the present invention and no Mo at all, and deterioration of aging resistance and DBTT characteristics due to the decrease of the bonding strength between grains due to the addition of Mo to the high P content Deteriorated.

Comparative steel 13 is a steel without the addition of Sol.Al, Ti, Mo and B is not added, there is no room for further improvement in grain refining effect and small hardenability due to the lack of the addition of Sol.Al and Ti In addition, DBTT characteristics deteriorated due to the absence of Mo and B.

Comparative steel 14 is 0.12% of P, exceeding 0.05-0.11% of the component range of the present invention, and B is not added. Although the DBTT characteristic is improved by Mo, the addition of P is very high. There was a limit to the improvement effect. Especially, the addition of B lost the improvement effect of DBTT characteristics, and the DBTT was 0 ° C.

According to the present invention, a high tensile cold rolled steel sheet and a hot-dip galvanized steel sheet excellent in bake hardening resistance, room temperature aging resistance, and secondary work brittleness can be provided.

Claims (5)

By weight, C: 0.0025-0.0035%, Si: 0.02% or less, Mn: 0.2-1.2%, P: 0.05-0.11%, S: 0.01% or less, Soluble Al: 0.08-0.12%, N: 0.0025% or less, Ti: 0.005-0.018%, Mo: 0.1-0.2% and B: 0.0005-0.0015%, and the Ti content satisfies the following relation (1), and is composed of the balance Fe and other unavoidable impurities, [Relationship 1] Ti ^* (Effective Ti) = Total Ti-(48/14) N-(48/32) S ≤ 0 And the baking hardening amount (BH) of 30 MPa or more, the aging index (AI) of 30 MPa or less, DBTT and ASTM No. High tensile strength hardenable hardenable cold rolled steel sheet characterized by having a grain size of 9 or more The Ti content and Al content are controlled by the following relation (2) so that the bake hardening amount (BH) is 30 Mpa or more, and the Ti content is set to 30 Mpa or less by the following relation (3). And high tensile strength hardening hardening cold rolled steel sheet, characterized in that the Mo content is controlled [Relationship 2] Bake Hardening (BH) = 50- (885 × Ti) + (62 × Al) [Relationship 3] Aging Index (AI) = 44-(423 × Ti)-(125 × Mo) By weight, C: 0.0025-0.0035%, Si: 0.02% or less, Mn: 0.2-1.2%, P: 0.05-0.11%, S: 0.01% or less, Soluble Al: 0.08-0.12%, N: 0.0025% or less, Ti: 0.005-0.018%, Mo: 0.1-0.2% and B: 0.0005-0.0015%, and the Ti content satisfies the following relation (1), and is composed of the balance Fe and other unavoidable impurities, [Relationship 1] Ti ^* (Effective Ti) = Total Ti-(48/14) N-(48/32) S ≤ 0 And the baking hardening amount (BH) of 30 MPa or more, the aging index (AI) of 30 MPa or less, DBTT and ASTM No. High tensile strength bake hardenable hot-dip galvanized steel sheet characterized by having a grain size of 9 or more 4. The Ti content according to claim 3, wherein the Ti content and the Al content are controlled such that the baking hardening amount (BH) is 30 Mpa or more by the following relation (2), and the Ti content is 30 Mpa or less by the following relation (3). And high tensile resistance bake hardenable hot-dip galvanized steel sheet characterized in that the Mo content is controlled [Relationship 2] Bake Hardening (BH) = 50- (885 × Ti) + (62 × Al) [Relationship 3] Aging Index (AI) = 44-(423 × Ti)-(125 × Mo) By weight, C: 0.0025-0.0035%, Si: 0.02% or less, Mn: 0.2-1.2%, P: 0.05-0.11%, S: 0.01% or less, Soluble Al: 0.08-0.12%, N: 0.0025% or less, Ti: 0.005-0.018%, Mo: 0.1-0.2%, and B: 0.0005-0.0015%, and the Ti content satisfies the following relational formula (1), [Relationship 1] Ti ^* (Effective Ti) = Total Ti-(48/14) N-(48/32) S ≤ 0 Al-killed steel composed of the balance Fe and other unavoidable impurities is subjected to homogenization heat treatment at 1200 ° C. or higher, and then hot-rolled at a temperature range of 900-950 ° C. and wound at a temperature range of 600-650 ° C., followed by 75- Cold rolling at 80% reduction rate, continuous annealing at a temperature range of 760-790 ° C., and temper rolling at 1.2-1.5% reduction ratio. Manufacturing Method

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