KR100544724B1 - Cold Rolled Steel Sheet with Superior Workability and Method for Manufacturing the Sheet - Google Patents

Abstract

The present invention relates to a cold rolled steel sheet and a method for manufacturing the same, which are used for materials such as automobile exterior panels, and to provide a cold rolled steel sheet having a high average plastic strain ratio and a small plastic strain ratio anisotropy coefficient and excellent workability, and a method of manufacturing the same. There is this.

In the present invention, C: 0.001 to 0.01%, Mn: 0.1 to 1.5%, P: 0.003 to 0.1%, S: 0.01% or less, Ti: 0.015 to 0.07%, acid soluble Al: 0.04 to 0.5%, N : 0.005% or less, balance Fe and other unavoidable impurities; And

A cold rolled steel sheet excellent in workability having an average plastic strain ratio r _m of 1.8 or more and a plastic strain ratio anisotropy coefficient Δr of 0.3 or less, and a method of manufacturing the same.

Plastic strain ratio, processability, plastic strain ratio anisotropy coefficient, aluminum, cold rolled steel sheet

Description

Cold rolled steel sheet with excellent workability and manufacturing method {Cold Rolled Steel Sheet with Superior Workability and Method for Manufacturing the Sheet}

1 is a result diagram showing ψ _{2 =} 45 ° cross section of the azimuth distribution function for the comparative steels and the inventive steels, where (a) represents comparative steel (1), (b) represents inventive steel (1), ( c) represents invention steel (2) and (d) represents invention steel (3).

The present invention relates to a cold-rolled steel sheet used in materials such as automotive shell plates and a method for manufacturing the same, and more particularly cold rolled steel having excellent workability with a relatively high average plastic strain ratio (r _m ) and a small plastic strain ratio anisotropy coefficient (Δr). It relates to a steel sheet and a method of manufacturing the same.

In order to perform a press molding without processing defects of the conventional automobile outer plate material and to smoothly manufacture a component having a desired shape after molding, a cold rolled steel sheet having excellent workability is required as a steel sheet for automobiles.

It is advantageous to manufacture parts of a desired shape by forming a car shell with a cold rolled steel sheet having a high elongation and a high plastic deformation ratio.

The elongation is a value representing the property of the steel sheet to be elongated without cracking when tensile, so when the elongation is large, the allowed deformation of the steel sheet is large.

Elongation is a mechanical property of steel that does not change significantly once the steel grade is determined.

The plastic strain ratio r value is a value defined by the ratio of the strain in the width direction to the strain in the thickness direction.

The steel sheet having a large plastic deformation ratio assumes that the deformation amount in the width direction is constant, and when the plate is stretched in a certain direction by a certain amount of deformation, the strain in the thickness direction is small, which means that the material can be processed without causing necking until a large deformation. do.

As a result, in order to improve the workability of the plate, the elongation and plastic strain must be increased.

On the other hand, the plastic deformation ratio anisotropy is an important mechanical property indicating the degree of workability. The plastic strain ratio has a different value depending on the tensile direction due to the anisotropic property of the sheet. The average plastic strain ratio, r _m and the plastic strain ratio anisotropy coefficient, Δr, represent the degree of change in the plastic strain ratio along the tensile direction.

In a manner that generally measures the plastic strain ratio in deulsu is a method of determining the r _m and Δr by the following measures the r _0, r _45, r ₉₀ of each plate member relational expression (1) and (2).

[Relationship 1]

r _m = (r ₀ + 2r ₄₅ + r ₉₀ ) / 4,

[Relationship 2]

Δr = (r ₀ -2r ₄₅ + r ₉₀ ) / 2

Here, r ₀ , r ₄₅ , r ₉₀ means the value of the plastic strain ratio in the tensile direction of 0 ° 45 ° 90 ° direction with respect to the rolling direction of the plate, respectively.

In order for Δr to be small, the difference in plastic strain ratio should be small when tensioned in each direction.

Small Δr values mean that the distribution of strains during press molding is uniform, which is advantageous for molding from deformation in stretching mode to large deformation.

Steels with high Δr and high r _m values improve machinability not only in stretching mode but also in deformation of deep drawing mode.

In order to improve the formability of conventional steel sheet for automobiles, Ti or Nb is added to the cold rolled steel sheet to precipitate solid solution C and N as carbides or nitrides (Japanese Patent Laid-Open No. 9-296226, Japanese Patent Laid-Open No. 9-296226), or The texture was controlled by controlling the amount of Ti and N in the carbon steel (DE3843732).

However, the Ti-added steel alone has a problem of poor stretching property due to a relatively high Δr value, and has a problem of using BAF as a aging problem in the case of low and medium carbon steels.

The present inventors have conducted research and experiments to solve the above problems of the prior art, and based on the results, the present invention proposes the present invention, which has a relatively high average plastic strain ratio and a small plastic strain ratio anisotropy coefficient. To provide an excellent cold rolled steel sheet and a method of manufacturing the same, the purpose is.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated.

In the present invention, C: 0.001 to 0.01%, Mn: 0.1 to 1.5%, P: 0.003 to 0.1%, S: 0.01% or less, Ti: 0.015 to 0.07%, acid soluble Al: 0.04 to 0.5%, N : 0.005% or less, balance Fe and other unavoidable impurities; And

It is related with the cold rolled steel sheet excellent in workability whose average plastic deformation ratio (r _m ) is 1.8 or more and plastic strain ratio anisotropy coefficient ((DELTA) r) is 0.3 or less.

In the present invention, C: 0.001 to 0.01%, Mn: 0.1 to 1.5%, P: 0.003 to 0.1%, S: 0.01% or less, Ti: 0.015 to 0.07%, and acid soluble Al: 0.04 to 0.5% , N: 0.005% or less, steel with remainder Fe and other unavoidable impurities is reheated at 1050-1300 ℃, hot-rolled at finishing rolling temperature of 890 ~ 950 ℃ and winding temperature of 600 ~ 750 ℃, and then 65 ~ Cold rolling at 90% reduction rate, continuous annealing at 750 ~ 870 ℃ crack zone temperature, 20 ~ 100 ℃ / s cooling zone cooling rate and 350 ~ 500 ℃ overaging zone temperature, 0.5 ~ 2% The present invention relates to a method of manufacturing a cold rolled steel sheet having excellent workability by rough rolling at a reduction ratio of.

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

In the present invention, steel C is an invasive solid solution, and has a great influence on the texture of the steel sheet in the cold rolling and annealing process.

The higher the amount of solid solution carbon contained in the steel, the weaker the component of the {111} texture structure, which is advantageous for the increase of the r _m value and the decrease of the Δr value, thereby lowering the formability of the annealing plate.

When the C content in the steel exceeds 0.003% by weight, the Ti and Nb contents for precipitation as carbides are relatively high, which is disadvantageous in terms of material cost increase, but the precipitated fine TiC improves the strength of the steel to secure yield strength and improve tensile strength. Effective in

The reason for limiting the lower limit of the C content to 0.001% by weight is to limit the composition to produceable by conventional steelmaking techniques, and the reason for limiting the upper limit to 0.01% by weight is the case where the content of C is 0.01% by weight or more. This is because the problem of room temperature aging occurs due to the solid solution C during manufacturing by the continuous annealing method.

Mn in steel is an effective element for the solid solution strengthening effect, in particular, precipitates S in steel with MnS, thereby suppressing plate breakage and high temperature embrittlement caused by S during hot rolling.

However, according to the experiments related to the present invention, when the Mn content is less than 0.1% by weight, a strength synergistic effect cannot be obtained, and there is a problem in securing moldability because the steel S cannot be precipitated completely into Mn.

Although it is advantageous in terms of strength to contain a large amount for solid solution strengthening, it is also possible to lower the elongation due to grain boundary segregation of Mn, so the upper limit is preferably set to 1.5% by weight.

The higher the content of P in the steel, the more favorable for the increase in strength, but the excessive addition of P increases the possibility of brittle fracture, which increases the probability of slab breakage during hot rolling, and facilitates diffusion and segregation to the grain boundary after annealing is completed. In accordance with this, the problem of secondary brittleness is increased during molding by increasing the DBTT (Ductile Brittle Transition Temperature). Therefore, the upper and lower sides are preferably set to 0.1% by weight, and the lower limit is 30kgf / mm ² or more. It is preferable to limit the amount to 0.003% by weight.

Since S and N are elements that are inevitably added as impurities in steel, it is important to manage them as low as possible. However, the less they manage, the higher the refining cost of the steel.

Therefore, in consideration of operating conditions, the S content is preferably 0.01% by weight or less, and the N content is preferably limited to 0.005% by weight or less.

The Ti is an element for forming C and N in carbide and nitride to inhibit yield point formation and to promote the development of {111} assembly structure components, and to improve processability, when the content is 0.015% by weight or less. And N is insufficient to bind carbide and nitride, and when it is 0.07% by weight or more, the elongation is reduced by excessive solid solution Ti, and the content thereof is preferably set to 0.015 to 0.07% by weight.

The acid-soluble Al is an element that is conventionally added to refine the particle size and deoxidation of the steel, in the present invention is added to develop a steel having an r _m value of 1.8 or more and a Δr value of 0.3 or less, so as to secure an acceptable Δr value. It is preferable to limit a lower limit to 0.04 weight%.

In general, the recrystallized aggregates developed in Ti-added cold rolled steel sheets are γ-fiber aggregates including {111} <112> and {111} <110>. These textures have the effect of reducing plastic strain ratio anisotropy. Have.

When the content of the acid-soluble Al is added more than 0.04%, the γ-fiber aggregate structure including {111} <112> and {111} <110> becomes stronger, thereby reducing plastic deformation ratio anisotropy.

However, when too much acid value Al is contained, since the elongation and r _m value are reduced, it is preferable to limit the upper limit to 0.5 weight%.

That is, the content of the acid-soluble Al is preferably set to 0.04 to 0.5% by weight.

The steel formed as described above is wound up after the steel structure is hot rolled in the austenite state at an Ar3 temperature or higher.

After hot rolling, the coiling temperature of the coil is set to a high temperature to form TiC smoothly in the wound state to minimize the solid solution carbon.

The hot rolled steel sheet manufactured as described above is subjected to cold rolling, continuous annealing, and temper rolling.

The manufacturing conditions of the present invention are not particularly limited, and any of them can be used under ordinary conditions, and an example of a preferable manufacturing method is as follows.

That is, after reheating the steel composition as described above at 1050 ~ 1300 ℃ hot rolled to hot finish temperature conditions of 890 ~ 950 ℃, then wound at a temperature of 600 ~ 750 ℃, with a reduction ratio of 65 ~ 90% After cold rolling and continuous annealing, temper rolling is carried out at a reduction ratio of 0.5 to 2% to remove the yield point phenomenon.

The continuous annealing is preferably carried out in a manner that the cracking zone temperature is 750 ~ 870 ° C and the quenching zone cooling rate is quenched at 20 ~ 100 ° C / s and then heat treated at an overaging temperature of 350 ~ 500 ° C.

The steel produced as described above has an r _m value of 1.8 or more and a Δr value of 0.3 or less.

Therefore, the steel produced according to the present invention has excellent workability in deformation of the stretching mode and the deep drawing mode.

As described above, according to the present invention, a cold rolled steel sheet having excellent workability in deformation of the stretching mode and the deep drawing mode in which the r _m value is 1.8 or more and the Δr value is 0.3 or less while the elongation is maintained despite having a solid solution Al is produced.

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

(Example)

Steels having the components shown in Table 1 below were dissolved and hot rolled.

In the hot rolling, the reheating temperature was 1200 ° C., the finishing temperature was 920 ° C., and the winding temperature was 630 ° C. 74% cold rolling was performed after pickling off the surface oxide layer of the hot rolled plate manufactured as described above.

The cold rolled steel sheet was heat treated in a continuous annealing furnace. The temperature of the cracks during the heat treatment was 830 ° C., and the holding time of the cracks was 35 seconds.

The overaging temperature was 430 ° C. After heat treatment, temper rolling was carried out at a reduction ratio of 0.7%.

Tensile strength, total elongation, average plastic strain and plastic strain ratio anisotropy coefficients were examined for the specimens prepared as described above, and the results are shown in Table 2 below.

The comparative steel (1) and the inventive steel (1-3) were examined for the cross section of ψ ₂ = 45 ° of the azimuth distribution function, and the results are shown in FIG.

C Mn P S Sol-Al Ti N Inventive Steel 1 0.0037 0.34 0.03 0.011 0.11 0.052 0.0021 Inventive Steel 2 0.0039 0.34 0.03 0.011 0.21 0.054 0.0016 Invention Steel 3 0.0039 0.33 0.03 0.011 0.40 0.053 0.0015 Comparative Steel 1 0.0032 0.36 0.03 0.011 0.015 0.05 0.0019 Comparative Steel 2 0.0032 0.35 0.03 0.011 0.022 0.058 0.0012 Comparative Steel 3 0.0034 0.34 0.03 0.011 0.022 0.047 0.0018 Comparative Steel 4 0.0030 0.35 0.03 0.011 0.035 0.053 0.0019 Comparative Steel 5 0.0032 0.35 0.03 0.011 0.022 0.058 0.0024

Tensile strength (kgf / mm ² ) % Total elongation r _m Δr Inventive Steel 1 32.97 43.7 1.82 0.28 Inventive Steel 2 33.55 43.5 1.88 0.26 Invention Steel 3 33.48 43.1 1.83 0.29 Comparative Steel 1 31.19 45.1 1.73 0.56 Comparative Steel 2 32.26 44.9 1.74 0.34 Comparative Steel 3 31.92 44.6 1.75 0.41 Comparative Steel 4 31.43 45.5 1.72 0.57 Comparative Steel 5 30.81 46.0 1.61 0.70

As shown in Table 2, in the case of the inventive steel (1-3) manufactured according to the present invention, it can be seen that the Δr value is lower than 0.3 and the r _m value is higher than 1.8.

On the other hand, in the case of the comparative steel (1-5) outside the scope of the present invention it can be seen that the Δr value is higher than 0.3, the r _m value is lower than 1.8.

As described above, the steel conforming to the present invention has a smaller Δr value and a higher r _m value than the comparative steel outside the present invention, thereby obtaining uniform deformation distribution during press molding, thereby stably forming parts.

As shown in Figure 1, in the case of the invention steel (1-3) prepared according to the present invention it can be seen that the γ-fibre aggregate tissue is relatively strong development.

As described above, the present invention has an effect that can be effectively applied to the field of automobile parts by providing a cold rolled steel sheet having excellent workability with an r _m value of 1.8 or more and a Δr value of 0.3 or less, and particularly, the automobiles using the steel according to the present invention. When forming a part, the part can be easily machined due to excellent workability in deformation of the stretching mode and the deep drawing mode.

Claims (2)

By weight% C: 0.001-0.01%, Mn: 0.1-1.5%, P: 0.003-0.1%, S: 0.01% or less, Ti: 0.015-0.07%, acid soluble Al: 0.04-0.5%, N: 0.005% Hereinafter consisting of balance Fe and other unavoidable impurities; And The average plastic strain ratio (r _m ) is 1.8 or more, and the plastic strain ratio anisotropy coefficient (Δr) is 0.3 or less. Cold rolled steel sheet with excellent workability By weight% C: 0.001-0.01%, Mn: 0.1-1.5%, P: 0.003-0.1%, S: 0.01% or less, Ti: 0.015-0.07%, acid-soluble Al: 0.04-0.5%, N: 0.005% Hereinafter, a steel having a balance of Fe and other unavoidable impurities is reheated at 1050 to 1300 ° C., hot rolled at a finish rolling temperature of 890 to 950 ° C. and a winding temperature of 600 to 750 ° C., and then a reduction ratio of 65 to 90%. Cold-rolled and then continuously annealed at 750 ~ 870 ℃ crack zone temperature, 20 ~ 100 ℃ / s quench zone cooling rate and 350 ~ 500 ℃ overaging band condition, and then adjusted to 0.5 ~ 2% reduction rate. Manufacturing method of cold rolled steel sheet excellent in workability, characterized in that rolling

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