KR20050062194A - High formable and high strength cold rolled steel sheets with excellent weldability and paintability, and method for manufacturing the same - Google Patents

Abstract

The present invention relates to cold rolled steel sheets for structural members of automobiles, such as front side members.

In the present invention, by weight%, C: 0.05-0.15%, Si: 0.7-1.3%, S: 0.02% or less, N: 0.004% or less, Al: 0.02-0.06%, Mn: 1.2-1.8%, P: 0.05 ~ 0.10%, including the remaining Fe and other unavoidable impurities, the high workability and high-strength cold rolled steel sheet having excellent weldability and coating properties, including the Si and P satisfying Si + (50/8) P ≥ 1.2% And to a method for producing the same.

The present invention has the effect of providing a high-strength cold-rolled steel sheet that can reduce the cost of manufacturing automobiles due to excellent workability, there is no crack generation during press working, and also excellent weldability and paintability.

Description

High formable and high strength cold rolled steel sheets with excellent weldability and paintability, and method for manufacturing the same

The present invention relates to cold rolled steel sheets for structural members of automobiles such as front side members, and more particularly, has excellent workability, no cracking during press processing, and excellent weldability and paintability, resulting in cost. The present invention relates to a high strength cold rolled steel sheet and a method of manufacturing the same.

Cold rolled steel sheet for automobile structural members must be excellent in workability and surface treatment characteristics because it is processed into complex parts. In addition, weldability is also a characteristic that must be considered since it is finally assembled into a vehicle body by welding.

In general, a large amount of alloying elements, such as Mn and Si, is added to the cold rolled steel sheet for automobile structural members, whereby the alloying elements are concentrated to a surface layer during annealing to form a thin oxide layer. The oxide layer thus formed has a problem that not only deteriorates the weldability of the steel sheet but also adversely affects paintability and plating property.

The present invention is to solve the above-mentioned problems of the prior art, and to appropriately control the amount of Mn, Si, P, etc., and to perform a heat treatment to form a ferrite phase and residual austenite phase which minimizes the concentration of carbon, the surface of the steel sheet It is an object of the present invention to provide a highly workable high strength cold rolled steel sheet excellent in weldability and coating properties by electroless plating nickel on a steel plate after weakly oxidizing the oxide layer.

The present invention for achieving the above object by weight, C: 0.05 ~ 0.15%, Si: 0.7 ~ 1.3%, S: 0.02% or less, N: 0.004% or less, Al: 0.02 ~ 0.06%, Mn: 1.2 ˜1.8%, P: 0.05˜0.10%, remaining Fe and other unavoidable impurities, and comprises Si and P satisfying Si + (50/8) P ≧ 1.2%.

In addition, the present invention is a weight%, C: 0.05 ~ 0.15%, Si: 0.7 ~ 1.3%, S: 0.02% or less, N: 0.004% or less, Al: 0.02 ~ 0.06%, Mn: 1.2 ~ 1.8%, P : 0.05 to 0.10%, containing the remaining Fe and other unavoidable impurities, homogenizing the steel at a temperature of 1050-1300 ° C. wherein Si and P satisfy Si + (50/8) P ≧ 1.2%;

Hot-rolling the homogenized steel at 850-950 ° C. and then winding it at 400-600 ° C .;

Cold rolling the hot rolled steel sheet at a reduction ratio of 30 to 80%;

The cold rolled steel sheet is continuously annealed at 800 ~ 870 ℃ and then cooled to a cooling rate of 1 ~ 7 ℃ / second to 620 ~ 700 ℃, then cooled to a cooling rate of 10 ~ 100 ℃ / second to 350 ~ 450 ℃ Maintaining for 30 to 600 seconds in a temperature section of 350 ~ 450 ℃; And

Thereafter, the steel sheet is slightly pickled and nickel electroless plating.

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

According to the present invention, the amount of Mn, Si, P, etc. is appropriately controlled in the low carbon aluminum-kilted steel, and after continuous annealing heat treatment, finally, the residual austenite phase is distributed in an extremely low concentration of carbon to improve tensile strength and elongation. After the annealing heat treatment is finished, light pickling and electroless plating of nickel are used to remove the oxide layer on the surface of the steel sheet and prevent further oxidation to improve weldability and paintability.

Hereinafter, look at from the reasons for the limitation of the present invention.

C: 0.05 to 0.15% by weight (hereinafter referred to simply as "%")

The C is a component concentrating on the austenite phase during two-phase annealing, slow cooling, and quenching, and concentrating on the austenite phase during ossampering at the bainite station, thereby contributing to lowering the martensite transformation temperature of the austenite phase to below room temperature. In addition, the solid solution strengthening effect of C itself is increased, and the amount of C added increases because the amount of retained austenite increases, and thus the amount of martensite increases, thereby improving strength and ductility.

When the content of C is less than 0.05%, grains grow and the effect of solid solution strengthening and precipitation strengthening by carbon is not sufficient, thus not enough tensile strength can be secured, and the amount of retained austenite formed in the continuous annealing process is low, resulting in strength and ductility. The degree of contribution to the improvement is also small. In addition, when the content of C exceeds 0.15%, not only problems such as deterioration of delayed fracture characteristics due to the formation of a large amount of residual austenite appear but also deterioration of weldability. Therefore, the content of C is preferably limited to 0.05 ~ 0.15%.

Si: 0.7 ~ 1.3%

The Si improves the strength of the steel by the solid solution strengthening effect, and removes the carbon from the ferrite phase to improve the ductility of the steel, and also inhibits the formation of carbides during the bainite transformation to contribute to the carbon concentration in the austenite phase. Ingredient. If the content of Si is less than 0.7%, the effect of the addition cannot be obtained. If the content of Si is more than 1.3%, there is a problem that Si oxide is present during hot rolling, which deteriorates pickling property, and also greatly deteriorates weldability of steel. It is desirable to limit the content to 0.7-1.3%.

S: 0.02% or less

S is an impurity that is inevitably contained in the production of steel, and if it is contained in excess of 0.02%, the strength is reduced by coarse MnS generation. Therefore, the content is preferably limited to 0.02% or less.

N: 0.004% or less

The N is also an impurity that is inevitably contained in the manufacture of the steel, and in order to contain more than 0.004%, a process for additionally adding N is required, and therefore it is preferable to limit the content to 0.004% or less.

Al: 0.02-0.06%

Al is a component added for deoxidation of steel. When less than 0.02% is added, oxygen is present in the steel to form manganese oxide, silicon oxide, and phosphorus oxide when oxide forming elements such as manganese, silicon, and phosphorus are added during steelmaking. Therefore, it is very difficult to control the composition of manganese, silicon, phosphorus, etc., and if it is added in excess of 0.06%, the amount of aluminum is more than necessary, which increases the manufacturing cost and causes a large amount of surface defects of the steel sheet. It is preferable to limit the addition amount to 0.02 to 0.06%.

Mn: 1.2 ~ 1.8%

Mn is an effective ingredient for delaying ferrite transformation of austenite phase formed by two-phase annealing and exhibiting a solid solution strengthening effect. When Mn is added less than 1.2%, Mn does not sufficiently inhibit the transformation of austenite to pearlite. When formed to reduce the elongation, and when added in excess of 1.8%, there is a problem that the hardenability is too high to reduce workability as well as deterioration of weldability, it is preferable to limit the content to 1.2 ~ 1.8%.

P: 0.05 ~ 0.010%

The P is often added as a job-enhancing element, but in the present invention, it is added to suppress the generation of carbides formed in the ostampering process and to increase the strength. In this case phosphorus plays the same role as silicon. This can be seen from the state of the case of adding 0.5% Si and 0.08% P on the basis of C-1.0Si-1.5Mn in Figure 1 has almost the same form.

If the amount of P added is less than 0.05%, the amount of carbon concentrated on the retained austenite is not sufficient, so that the stability of the retained austenite is lowered and the ductility is reduced. If the amount is more than 0.10%, the weldability is deteriorated. In addition, there is a problem that the material deviation of the steel becomes larger due to the center segregation, and the weldability is lowered. Therefore, the content is preferably limited to 0.05 to 0.10%.

In addition to the above components, it is composed of Fe and other unavoidable impurities.

Even if the above components are satisfied, too much silicon addition causes a problem of greatly deteriorating the weldability of the steel sheet. In the present invention, the same effect as in the case where a large amount of silicon is added can be obtained by reducing the amount of silicon added and adding phosphorus, which is a carbide forming inhibitory element. Therefore, in the present invention, the addition amount of silicon and phosphorus has an important effect on the material. In the present invention, when Si + (50/8) P is less than 1.2%, it is difficult to reduce the carbon content of the ferrite and the formation of residual austenite is difficult. Therefore, Si + (50/8) P is limited to 1.2% or more. It is preferable.

Method for producing a cold rolled steel sheet of the present invention is as follows.

First, the steel of the said composition is homogenized at 1050-1300 degreeC. If the homogenization treatment temperature is less than 1050 ℃, the rolling load increases during hot rolling, the rolling is difficult, and if it exceeds 1300 ℃ scale is formed thick on the surface of the slab, the real rate is lowered and internal oxidation occurs due to silicon, manganese, etc. Since there is a problem that the surface of the final product is poor, the homogenization treatment temperature is preferably limited to 1050 ~ 1300 ℃.

Thereafter, the homogenized steel is finished hot rolled at 850 to 950 ° C. and then wound at 400 to 600 ° C. If the finishing hot rolling temperature is less than 850 ℃, the rolling load increases during hot rolling, which is difficult to roll. If the finishing hot rolling temperature exceeds 950 ℃, a thick scale is formed on the surface of the hot rolled sheet, making pickling difficult and the adhesion of the electroplated steel sheet. Since there is a problem to lower, the finish hot rolling temperature is preferably limited to 850 ~ 950 ℃. In addition, when the coiling temperature is less than 400 ℃ a second phase of high strength is generated in the hot rolled steel sheet not only increases the strength of the hot rolled steel sheet, but also has a problem that the shape of the steel sheet worsens, which makes it difficult to later cold rolling, exceeds 600 ℃ When coarse pearlite is formed on the hot rolled steel sheet, the coarse pearlite does not easily dissolve during the annealing process, so that an annealing steel sheet having a uniform structure cannot be obtained, thereby degrading the workability of the cold rolled steel sheet. Therefore, the winding temperature is preferably limited to 400 ~ 600 ℃.

When hot rolling is completed as described above, cold rolling is performed to manufacture the steel sheet in the shape and thickness of the desired steel sheet. In the present invention, it is preferable to limit the reduction rate during cold rolling to 30 to 80%. The reason is that when the reduction rate during cold rolling is less than 30%, sufficient energy necessary for recrystallization does not accumulate. This is because when it deteriorates and exceeds 80%, the rolling load increases during cold rolling, which reduces productivity.

The cold rolled steel sheet as described above is continuously annealed at 800 ~ 870 ℃. Then it is cooled to a cooling rate of 1 ~ 7 ℃ / second to 620 ~ 700 ℃, then cooled to a cooling rate of 10 ~ 100 ℃ / second to 350 ~ 450 ℃ and maintained for 30 ~ 600 seconds in the temperature range of 350 ~ 450 ℃ .

The continuous annealing is a process for uniformly dispersing the second phase during cooling by completely re-dissolving the pearlite formed in the hot rolling step, if the annealing temperature is less than 800 ℃ it is difficult to ensure sufficient processability and also to form the austenite phase at low temperature There is a problem that the austenite transformation does not occur enough to maintain, and if it exceeds 870 ℃, the transformation is completely transformed into austenite and then transformed to ferrite again during the cooling process. Therefore, there is a problem that the elongation is reduced. Therefore, the continuous annealing temperature is preferably limited to 800 ~ 870 ℃.

In the cooling to 620 ~ 700 ℃ after the annealing, if the cooling rate is less than 1 ℃ / second, the austenite phase formed during annealing is transformed into a pearlite phase, if it exceeds 7 ℃ / second can ensure a sufficient amount of ferrite There is a problem that the workability is deteriorated. In addition, when the cooling end temperature of the cooling is less than 620 ℃ austenite is transformed to pearlite is difficult to produce residual austenite, which is disadvantageous to secure the workability of the steel, if it exceeds 700 ℃, the cornerstone ferrite is not produced sufficiently, the residual austenite There is a problem of reducing the ductility of the steel by reducing the stability.

Then, in cooling to 350 ~ 450 ℃, if the cooling rate is less than 10 ℃ / sec, the austenite phase formed during annealing is transformed into a pearlite phase or the fraction of austenite phase present in the bainite section is greatly reduced, 100 ℃ / If it exceeds the second, a large amount of nitrogen is added during cooling, there is a problem that the manufacturing cost is increased. In addition, if the cooling end temperature of the cooling is less than 350 ℃ the austenite phase is transformed to the martensite phase to reduce the fraction of the retained austenite, and if it exceeds 450 ℃ the transformation from the austenite phase to the bainite phase occurs too quickly There is a problem that the austenite fraction is sharply reduced.

After the cooling, while maintaining for 30 to 600 seconds in the temperature section of 350 ~ 450 ℃ again, if the holding time is less than 30 seconds to convert the retained austenite to bainite to secure the residual austenite sufficiently enriched carbon Difficult to exceed 600 seconds, there is a problem that the stability of the residual austenite is too high or the bainite transformation occurs so that the ductility of the steel is sharply reduced.

As described above, the cold rolled steel sheet that has been annealed should be removed because oxides of elements such as silicon and manganese are formed on the surface. To this end, cold rolled steel sheets are weakened. Representative acid solutions that can be used at this time include a hydrochloric acid solution of a low concentration (6% or less).

Thereafter, nickel is electroless plated to prevent secondary oxidation of the pickled surface. In this case, it is possible to use a nickel solution containing 6% or less of nickel. Fig. 2 shows the component distribution of the surface of the steel plate plated with weak pickling and nickel electroless plating. As can be seen in FIG. 2, it can be seen that almost all oxide layers of silicon and manganese existing on the surface of the steel sheet are removed by weak pickling and nickel electroless plating.

Cold rolled steel sheet of the present invention prepared as described above can be produced by electroplating steel sheet by electroplating in the usual conditions.

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

EXAMPLE

Steels having the components of Table 1 below were homogenized at 1250 ° C. and then hot rolled. At this time, finishing hot rolling temperature was 900 degreeC, and the winding temperature was 550 degreeC.

Thereafter, cold rolling was performed at a reduction ratio of 50%, and the cold rolled specimens were continuously annealed at 830 ° C, and then cooled to a cooling rate of 3 ° C / sec to 680 ° C. It was then cooled to 400 ° C. at a cooling rate of 30 ° C./sec and held at this temperature for 370 seconds.

The mechanical properties of the specimens prepared as described above were measured, and the results are shown in Table 2 below.

division Ingredient (% by weight) C Si S N Al Mn P Ti Si + (50/8) P Inventive Steel 1 0.08 1.02 0.011 0.0034 0.045 1.54 0.05 - 1.33 Inventive Steel 2 0.101 0.99 0.011 0.0040 0.045 1.44 0.051 - 1.31 Invention Steel 3 0.078 1.03 0.010 0.0029 0.035 1.50 0.081 - 1.54 Comparative Steel 1 0.081 0.41 0.012 0.0033 0.043 1.50 0.06 - 0.79 Comparative Steel 2 0.104 0.80 0.008 0.0022 0.044 0.80 0.012 - 0.88 Comparative Steel 3 0.080 1.01 0.008 0.046 0.050 1.54 0.008 - 1.06 Conventional Steel 1 0.084 0.41 0.010 0.0033 0.044 0.51 0.011 0.034 0.48

division Mechanical properties Yield strength (kgf / ㎡) Tensile strength (kgf / ㎡) Elongation (%) Work Hardening Index (n) Inventive Steel 1 41.9 60.0 35.6 0.25 Inventive Steel 2 42.9 61.3 35.9 0.25 Invention Steel 3 43.0 62.9 36.1 0.25 Comparative Steel 1 33.3 50.9 30.6 0.17 Comparative Steel 2 39.5 54.4 31.5 0.19 Comparative Steel 3 40.5 57.1 34.9 0.20 Conventional Steel 1 45.5 64.5 22.3 0.18

As can be seen in Table 2, the invention steels 1 to 3 satisfying the scope of the present invention exhibits excellent strength and workability with a tensile strength of 60 kgf / mm 2 or more, an elongation of 35% or more, and a work hardening index of 0.25 or more. there was. Therefore, the invention steel which satisfies the scope of the present invention has excellent press workability because of high elongation and work hardening index despite high tensile strength.

However, in the case of Comparative Steel 1 in which the amount of Si added was less than the range of the present invention, not only was it difficult to reduce the ferritic carbon in the annealing step, but also the austenite remained difficult, resulting in poor workability.

In the case of Comparative Steel 2 having an added amount of Mn less than the range of the present invention, in the annealing process, the austenite phase easily transformed to pearlite during the quenching after the two-phase reverse heat treatment, and thus the ductility was greatly reduced.

In the case of Comparative Steel 3 lacking the amount of P added, tensile strength and elongation were low.

Conventional steel 1 is a precipitation hardening type high strength steel having a typical tensile strength of 60kgf / mm2, by adding titanium to form titanium carbonitride, the tensile strength is high, but the workability was very poor due to the low elongation and work hardening index.

Inventive steel 3 in the specimen subjected to continuous annealing was slightly pickled in a hydrochloric acid solution (concentration: 5%), and then plated with nickel electroless plating in a solution containing 3% nickel. Using the pickled and electroless plated specimens and the pickled and electroless plated specimens, the weldability (continuous stroke score: lifetime of the weld tip that can be welded until a good welding result in spot welding) was evaluated. Is shown in Table 3 below.

division Light pickling and nickel electroless plating Weldability Continuous RBI Inventive Example practice 2600 Comparative example Do not conduct 2100

As can be seen in Table 3, in the invention example subjected to the weak pickling and nickel electroless plating, weldability was superior to that of the comparative example without the weak pickling and nickel electroless plating.

Figure 3 is a comparison of the phosphate structure after the phosphate treatment of the invention example and the comparative example, the invention example was confirmed that the coating properties are greatly improved by a weak pickling and nickel electroless plating because the phosphate structure is uniformly distributed.

As described above, according to the present invention, there is an effect of providing a high-strength cold rolled steel sheet which is excellent in workability and does not generate cracks during press working, and also has excellent weldability and paintability, which can reduce costs in automobile manufacturing.

1 is a diagram showing a change in the state diagram by the addition.

2 is a graph showing surface component distributions of Comparative Examples and Inventive Examples.

Figure 3 is a photograph comparing the phosphate tissue after the phosphate treatment of the comparative example and the invention example.

Claims (2)

By weight%, C: 0.05-0.15%, Si: 0.7-1.3%, S: 0.02% or less, N: 0.004% or less, Al: 0.02-0.06%, Mn: 1.2-1.8%, P: 0.05-0.10% , High Fe-formed high-strength cold-rolled steel sheet is formed including the remaining Fe and other unavoidable impurities, the Si and P satisfying Si + (50/8) P ≥ 1.2%. By weight%, C: 0.05-0.15%, Si: 0.7-1.3%, S: 0.02% or less, N: 0.004% or less, Al: 0.02-0.06%, Mn: 1.2-1.8%, P: 0.05-0.10% , Comprising the remaining Fe and other unavoidable impurities, and homogenizing the steel at which Si and P satisfy Si + (50/8) P ≥ 1.2% at 1050 to 1300 ° C .; Hot-rolling the homogenized steel at 850-950 ° C. and then winding it at 400-600 ° C .; Cold rolling the hot rolled steel sheet at a reduction ratio of 30 to 80%; The cold rolled steel sheet is continuously annealed at 800 ~ 870 ℃ and then cooled to a cooling rate of 1 ~ 7 ℃ / second to 620 ~ 700 ℃, then cooled to a cooling rate of 10 ~ 100 ℃ / second to 350 ~ 450 ℃ Maintaining for 30 to 600 seconds in a temperature section of 350 ~ 450 ℃; And Thereafter, the steel sheet is slightly pickled and electroless plated with nickel. The manufacturing method of the high-strength cold rolled steel sheet having excellent weldability and coating properties, comprising: a.