KR100363416B1 - Manufacturing method of high strength cold rolled steel sheet with excellent formability - Google Patents

Abstract

An object of the present invention is to provide a method for producing a high strength cold rolled steel sheet excellent in formability and hardening hardening in a method of manufacturing a high strength cold rolled steel sheet mainly used as a vehicle exterior material.

According to the present invention, in the method of manufacturing a high strength cold rolled steel sheet, the steel component composition step, the finish hot rolling step, the cold rolling step, and an annealing step of forming a recrystallized structure, the composition composition step of 0.0015 ~ 0.0025wt% C, 0.5 ~ 1.0wt% Mn, 0.04 ~ 0.06wt% P, 0.006wt% S, 0.003wt% N, 0.06wt% Acid Value Al, 0.007 ~ 0.013wt% The component ratio of Nb and Ti is (48/14 × N wt%) ≦ Ti ≦ (48/14 × N wt%) + (48/12 × C wt% × 0.6), balance Fe and other impurities In the finishing hot rolling step, the steel is finished hot rolling at 910 ° C. or more, in the cold rolling step, cold rolling is performed at a cold reduction rate of 77% or more, and in the annealing step, 840 to 860 ° C. Provided is a method for producing a high strength cold rolled steel sheet having excellent formability, characterized by performing recrystallization annealing at a temperature range of.

Description

Manufacturing method of high strength cold rolled steel sheet with excellent formability

The present invention relates to a method for manufacturing a high strength cold rolled steel sheet used as a vehicle exterior material, and more particularly, to a method for manufacturing a high strength cold rolled steel sheet having excellent formability and hardening hardening property by steel composition, rolling conditions, and heat treatment.

As part of an active response to weight reduction and environmental problems, such as to improve automobile fuel economy, a reduction in steel sheet thickness is required. However, in the case of the automotive exterior plate material, when the steel sheet thickness decreases, dents are easily generated, and thus there is a limit in reducing the thickness of the steel sheet.

Here, a dent is a groove | channel generate | occur | produced in a steel plate by external pressure.

In order to suppress the occurrence of dents due to the reduction of the steel sheet thickness, high strength steel having a higher strength is required. However, when the high strength steel is used as an automobile exterior material, the workability is reduced to parts requiring deep processing, and thus it cannot be used.

As a result, in the case of the steel plate hardening type steel sheet for automobiles, there is a limit to the increase in strength, and high strength steel in a range that does not cause problems during molding is applied. Therefore, the development of a steel sheet that can significantly improve the workability while securing a certain level or more of strength is necessary in terms of safety and light weight.

In general, the hardened hardened steel sheet is excellent in workability and shape freezing because of its low yield strength before molding, and is a steel sheet resistant to dents due to high work hardening ability after molding.

Conventional bake hardened steel sheets include iron and steel.

Iron and steel have been developed with a composite structured small hardened steel sheet with a tensile strength of 40Kgf / mm ² using low carbon, manganese, and chromium (C-Mn-Cr) system, yielding a very low yield ratio of 46% and a work hardening index of 0.28. In addition, the BH (Bake hardening) index, which is an evaluation index for hardening hardening, was also excellent at about 5 Kgf / mm ² , but the plastic anisotropy (r) coefficient, which is an evaluation index for workability, was very low at 1.2 level, requiring high formability. There is a problem that it can not be used for parts.

In order to solve the formability problem of low carbon composite steel, the plastic anisotropy value is 1.7 by using carbon or nitride forming element Ti or Nb alone or in combination using ultra low carbon steel with carbon content less than 0.005wt%. Steel sheet development technology with high formability is emerging, but there is a problem that can not secure the target strength and formability at the same time.

The present invention has been made to solve the problems of the prior art as described above, and the desired tensile strength, plastic anisotropy, and hardening hardening evaluation index by appropriate control of the composition of steel and hot rolling, cold rolling and continuous annealing An object of the present invention is to provide a method of manufacturing a high strength cold rolled steel sheet having excellent moldability to secure the moldability.

1 is a graph showing the r value and BH according to the contents of C and Ti,

2 is a block diagram showing a method of manufacturing a high strength cold rolled steel sheet according to the present invention.

According to the present invention for achieving the object as described above, in the method of manufacturing a high strength cold rolled steel sheet, comprising the steel component composition step, the finish hot rolling step, cold rolling step, and an annealing step to form a recrystallized structure In the composition step of the steel, 0.0015 ~ 0.0025wt% C, 0.5 ~ 1.0wt% Mn, 0.04 ~ 0.06wt% P, 0.006wt% or less S, 0.003wt% or less, 0.06wt% or less Acid value Al, 0.007-0.013wt% Nb, Ti component ratio is (wt% of 48 / 14xN) ≤Ti≤ (wt% of 48 / 14xN) + (wt% of 48 / 12xC x 0.6), to form a steel composed of the balance of Fe and other impurities, in the finishing hot rolling step the steel is finished hot rolling at 910 ℃ or more, in the cold rolling step is cold rolled to a cold reduction rate of more than 77% In the annealing step, high strength cold rolled steel sheet having excellent formability, characterized in that the recrystallization annealing at a temperature range of 840 ~ 860 ℃ A manufacturing method is provided.

In the following, a preferred embodiment of the method for producing a high-molded hardened hardened high strength cold rolled steel sheet according to the present invention will be described in detail.

1 is a graph showing the r value and the BH value according to the contents of C and Ti, Figure 2 is a block diagram showing a method of manufacturing a high strength cold rolled steel sheet according to the present invention.

As shown in FIG. 1 or FIG. 2, the steel composition includes a steel component forming step, a finish hot rolling step, a cold rolling step, and an annealing step of recrystallization annealing in a continuous annealing furnace.

In the composition composition of the steel, the steel component is 0.0015 ~ 0.0025wt% C, 0.5 ~ 1.0wt% Mn, 0.04 ~ 0.06wt% P, 0.006wt% S, 0.003wt% N, 0.06wt% It is composed of the following acid value Al, 0.01 to 0.016wt% Ti, 0.007 to 0.013wt% Nb, the balance of Fe and other unavoidable impurities, wherein the composition ratio of Ti is (48/14 × N wt% ) ≤ Ti ≤ (wt% of 48 / 14xN) + (wt% of 48 / 12xC x 0.6) must be satisfied (S1).

In the finishing hot rolling step, the steel slab (Slab) formed in the composition composition step of the steel is finished hot rolling at 910 ℃ or more (S2).

In the cold rolling, hot rolling is carried out at a temperature of 700 ° C. or higher (S3), and after pickling (S4), cold rolling is performed at a reduction ratio of 77% or more (S5).

In the annealing step, annealing is performed at a temperature range of 840 ° C. to 860 ° C. in the continuous annealing furnace so that the structure in the steel is recrystallized (S6).

C contained in the steel is an invasive solid solution element, and has a great influence on the formation of texture of the steel sheet during cold rolling and annealing. The larger the amount of C remaining in the steel, the more delayed the formation of an aggregate structure favorable for workability and the lower the moldability.

Conventionally, Ti, a carbide and nitride forming element, was prepared by precipitation of Ti (Nb) C by solid solution C by adding Nb alone or in combination. The steel on which Ti (Nb) C is deposited is referred to as ultra low carbon aluminum killed steel.

However, in order to have the characteristics of baking hardening, the amount of solid solution C is retained to some extent to secure the desired baking hardening characteristic. However, when the C content is increased, the BH characteristics are secured, but workability is deteriorated, which causes cracks during processing.

Therefore, in the present invention, when controlling the C content to 0.0025wt% or less, not only can secure the desired BH of 3Kg / mm ² or more, but also the workability evaluation index (r) value is very high to secure BH and formability at the same time. It is possible.

However, when the C content is 0.0015 wt% or less, workability is advantageous, but it is difficult to secure BH. Therefore, the appropriate level of C content is managed at 0.0015 to 0.0025 wt%.

Mn in the steel is a solid solution strengthening element and is an essential element for securing strength. In particular, Mn precipitates all S remaining in the steel as MnS during hot rolling, and delays the precipitation of TiS, which is advantageous for setting an effective Ti content range required for C content management. That is, since the effective Ti content required to precipitate S is not necessary, Ti sets the appropriate range considering the C and N in the steel regardless of the change in the S content, thereby reducing the load of the steelmaking industry and ensuring stable BH. It has the advantage of being.

If the Mn content is 0.5% or less, not only the remaining S in the steel cannot be precipitated completely by MnS, but also there is a problem in securing the strength. If it is more than 1.0%, the target strength and MnS precipitation can be achieved perfectly, but the residual Mn acts as an impurity, resulting in material change, and the Mn enrichment layer is generated on the surface of the steel sheet during annealing, which may cause oxidation problems in the steel. have. Therefore, the content of Mn is 0.5 to 1.0wt%.

Since P in the steel does not impair the formability and the strength pictorial is advantageous, it is advantageous to add as much as possible to secure the target strength.However, when excessively added, the secondary work brittleness occurs during molding, causing a crack. By doing so, the content range of P is limited to 0.04 to 0.06%.

Ti in steel is an important element in terms of formability and hardening hardening. The added Ti is added considering the C and N content contained in the steel, and the Ti content is the atomic equivalent ratio (wt% of 48/14 x N) ≤ Ti ≤ (wt% of 48/14 x N) + (48 / It is limited to 0.01 to 0.016% depending on the condition of the relationship of 12 x C wt% x 0.6).

Nb in the steel is an element that precipitates a part of the solid solution C to NbC during hot rolling, and plays an advantageous role in forming the aggregate structure during annealing, thereby improving formability. When the content of Nb is less than 0.007%, NbC hardly precipitates, so that the formation of aggregates is difficult. When the content of Nb is more than 0.013%, the residual amount of solid solution C is insufficient to obtain sufficient BH. Therefore, the Nb titration content is limited to 0.007 to 0.013%.

Al in the steel is an element added for the refinement and deoxidation of the particle size of the steel, and its content is preferably 0.06% or less, which is a range usually added.

It is important to manage S and N in the steel as low as the grade of steel grade impurities.

Therefore, less than 0.006% and less than 0.003% respectively.

EXAMPLE

Table 1 shows the compositional components of the steel, Table 2 is a table comparing the material properties appearing according to the heat treatment temperature and cold rolling conditions for the steel composition shown in Table 1.

And, Figure 1 is a graph showing the r value and BH according to the content of C and Ti.

The steel prepared as shown in Table 1 is wound by finishing hot rolling at 910 ° C or higher, pickled, and cold-rolled steel sheets having a thickness of 0.6 mm are fabricated by applying an appropriate cold reduction rate. do.

And the moldability is evaluated by the material properties (BH, r value, tensile properties) and the cupping test (Cupping Test) of the annealed steel sheet. At this time, Comparative steel 1 is a case where the C content is higher than the invention steel, Comparative Steel 2 is a steel that is expected to show little BH characteristics because the Ti content is higher than the invention steel.

The characteristics of the annealed steel sheet were tested three times for each condition in the rolling direction, 45 degree direction, and 90 degree direction using the standard size of the American Society for Testing and Testing (ASTM). Taken as a representative value.

Moldability evaluation was carried out cupping test, wherein the molding conditions were formed under the conditions of punch speed 300mm / min, punch diameter 40mm, die diameter 43mm, drawing ratio 2.45, specimen holding pressure (BHF) 0.5 tons The moldability is judged as possible.

division Component (wt%) Remarks C Mn P Ti Nb S N Comparative Steel 1 0.0034 0.6 0.053 0.015 0.013 0.008 0.003 High C Steel Comparative Steel 2 0.0013 0.38 0.045 0.045 0 0.01 0.0028 High Ti Steel Inventive Steel 1 0.0018 0.6 0.055 0.012 0.008 0.006 0.0028 Invention steel Inventive Steel 2 0.0020 0.8 0.052 0.016 0.012 0.0053 0.0027 Invention steel Inventive Steel 3 0.0024 0.7 0.056 0.014 0.013 0.0055 0.0029 Invention steel

division Rolling Condition (℃) Cold Rolling Condition Material characteristics Formability status Remarks FT CT Rolling reduction Annealed (℃) TS r value BH Comparative Steel 1 Comparative Example 1 913 723 77% 813 36.2 1.72 4.5 × crack r value Comparative Steel 2 Comparative Example 2 920 700 77% 840 35.3 2.12 0 O Molding BH Castle Inventive Steel 1 Comparative Example 3 915 630 77% 853 35.9 1.95 3.2 △ crack r value Inventive Example 1 917 706 77% 846 36.1 2.18 3.5 O Molding Good Inventive Steel 2 Comparative Example 4 917 705 68% 847 35.6 2.02 3.6 △ crack r value Comparative Example 5 918 723 77% 800 36.8 1.95 3.4 × crack r value Inventive Example 2 923 709 80% 852 35.2 2.21 3.2 O Molding Good Inventive Steel 3 Inventive Example 3 923 713 80% 849 35.7 2.18 3.6 O Molding Good Inventive Example 4 912 706 77% 856 36.3 2.13 3.1 O Molding Good

FT: hot finishing temperature, CT: winding temperature, TS: tensile strength (Kgf / mm ² ), BH: baking hardening capacity (Kgf / mm ² ), O: good, △: normal, ×: bad.

As a result of the test, the invention example has a basic strength of 35 kgf / mm ² or more, an r value of 2.1 or more, and a BH of ³ kgf / mm ² or more, and no crack occurred during molding.

As described in detail above, the method of manufacturing a high strength cold rolled steel sheet having excellent formability according to the present invention manufactures a small hardening type internal flame resistant steel sheet having both high moldability and moldability at the same time, thereby requiring a high formability of an automobile exterior member. It has the advantage of being suitable for use.

Although the technical idea of the method for producing a small hardening type high strength cold rolled steel sheet excellent in formability of the present invention has been described with the accompanying drawings, this is illustrative of the best embodiments of the present invention and is not intended to limit the present invention. In addition, it is obvious that any person skilled in the art can make various modifications and imitations without departing from the scope of the technical idea of the present invention.

Claims (1)

In the manufacturing method of high strength cold rolled steel sheet, The composition of steel, finishing hot rolling, cold rolling, and annealing to form a recrystallized structure, In the steel composition step, 0.0015 ~ 0.0025wt% C, 0.5 ~ 1.0wt% Mn, 0.04 ~ 0.06wt% P, 0.006wt% S, 0.003wt% N, 0.06wt% acid The soluble Al, 0.007-0.013 wt% Nb, Ti component ratio is (wt% of 48/14 x N) ≤ Ti ≤ (wt% of 48/14 x N) + (wt% of 48/12 x C x 0.6 ), The remainder is formed of steel composed of Fe and other impurities, In the finishing hot rolling step, the steel is finished hot rolling at 910 ℃ or more, In the cold rolling step, cold rolling at a cold reduction rate of 77% or more, The annealing step is a high strength cold rolled steel sheet manufacturing method characterized in that the recrystallization annealing in a temperature range of 840 ~ 860 ℃.