KR950007784B1 - Making method of cold rolling steel sheet - Google Patents

Abstract

soaking an Al-killed steel comprising (by wt.) 0.10-0.20% C, 0.6-1.6% Si, 1.0-2.0% Mn, 0.10-0.30% Mo, 0.02-0.1% Nb, less than 0.007% N, balance Fe and inevitable impurities with less than 0.35 carbon equivalent (Ceq=C+Si/30 + Mn/20+2p+4S) at 1200-1250 deg.C; coiling at 400-600 deg.C after conventional hot-rolling; cold-rolling with reduction of 30-80%; soaking at 740-860 deg.C for 20-120 secs. by continuous annealing; and over-aging at 500-300 deg.C. The obtained steel plate has good spot-weldability and press-workability and super-high strength.

Description

Manufacturing method of ultra-high strength cold rolled steel sheet with excellent spot weldability and press workability

1 is a microstructure photograph according to the thermal eye rolling conditions of the comparative steel and the invention steel.

2 is a graph showing the relationship between tensile strength and tensile strength x elongation of inventive and comparative steels.

The present invention relates to a method for manufacturing an ultra high strength cold rolled steel sheet used for reinforcing materials such as automobiles, and more particularly, to a method for producing an ultra high strength cold rolled steel sheet excellent in spot weldability and press formability.

The development of high strength cold rolled steel sheets with a tensile strength of 35kgf / mm ² or more, which began to respond to the automobile safety regulation law established in the United States in the late 1960s, was accelerated by the 1st and 2nd oil waves of '73 and '79. . The high strength cold rolled steel sheet developed so far is required for high strength cold rolled steel sheet with excellent workability of 35-45kgf / mm ² grade suitable for internal and external steel sheet formed into a somewhat complicated shape and the length workability for automobile reinforcement material However, it can be largely divided into high strength cold rolled steel sheet having a tensile strength of 45-100kgf / mm ² and the like that require high strength level. As a manufacturing method of the high strength cold rolled steel sheet belonging to the latter, solid solution hardened steel, niobium (Nb), titanium (Ti), etc., strengthened by addition of solid solution strengthening elements such as silicon (Si), manganese (Mn) and phosphorus (P) Precipitated hardened steels and steels precipitated and strengthened by the addition of carbonitride-forming elements of heat treatment at the two-phase zone temperature of austenite and ferrite, followed by quenching to form a hard martensite phase in the final structure. There is a composite structure reinforcement method to strengthen by forming, in particular, high strength cold rolled steel sheet having a tensile strength of 80kgf / mm ² or more is specifically called ultra high strength cold rolled steel sheet. Meanwhile, the material level of the ultra-high strength cold rolled steel sheet is not only tensile strength but also a combination of strength and ductility, that is, a combination of tensile strength and elongation, which is one of important material properties. In general, in the case of ultra high strength of 100kgf / mm ² tensile strength, the combination of strength x elongation of 2,500kgf / mm ² ..% Or more is considered to be excellent in press workability.

In the case of the door guard bar, which is used to minimize the deformation of the door by absorbing the impact during the side collision of the car, the side collision safety is regulated in the United States, etc. / mm ² grade high strength cold rolled steel sheet is used, but these parts are not always stressed, so it deforms only at the time of collision and absorbs collision energy. This weight reduction is possible by using the existing 60kgf / mm grade ² high strength cold rolled steel sheet of ultra high strength cold rolled steel sheet of tensile strength 100kgf / mm ² .

In the manufacturing method of ultra-high strength cold rolled steel sheet, by adding titanium (Ti) which delays recrystallization, precipitation of fine precipitates is performed. After a short period of time in the recovery structure strengthening method, austenitic single-phase zone or two-phase zone of ferrite and austenite, a part or all of the austenitic martensite is rapidly cooled by using gas jet cooling or water cooling. The metamorphic tissue strengthening method of transforming into a site or transforming into some bainite, and the recovery annealing transformation tissue strengthening method produced by continuous annealing by combining the above two methods. However, in any of the above methods, it is difficult to secure an elongation of 25% or more at a tensile strength of 100 kgf / mm ² or more, and thus ultra-high strength cold rolled steel sheet is formed by using a roll forming method, which is mainly formed by bending. According to this situation, productivity is extremely bad. In addition, as a large amount of alloying elements such as carbon and manganese are added to secure the strength, there is a problem of greatly deteriorating the spot weldability in the automobile assembly process by increasing the carbon equivalent of the steel.

Accordingly, the present inventors have proposed the present invention to solve the above problems of the conventional methods, the present invention not only improves the spot weldability while lowering the carbon equivalent at the same strength level as compared to the conventional high strength cold rolled steel sheet In order to provide a method of manufacturing an ultra-high strength cold rolled steel sheet having improved press formability by having a high ductility, an object thereof is provided.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated.

The present invention is a method for producing an ultra-high strength cold rolled steel sheet, in weight%, carbon (C): 0.10 to 0.20%, silicon (Si): 0.6 to 1.6%, manganese (Mn): 1.0 to 2.0%, molybdenum ( Mo): 0.10 to 0.30%, niobium (Nb): 0.02 to 0.1%, nitrogen (N): 0.007% or less, remainder Fe and other unavoidable impurities, carbon equivalent (Ceq) (Ceq = C + Si / 30) Al-Killed steel with + Mn / 20 + 2P + 4S) not more than 0.35 was cracked at 1,200-1,250 ° C, hot rolled in the usual manner and wound at 400-600 ° C, and then 30- After cold rolling at 80% cold rolling rate, it is cracked for 20-120 seconds at 740-860 ℃ by continuous annealing method, and then overaged at 500-300 ℃ temperature range to provide excellent spot weldability and press workability. It relates to a method for producing a high strength cold rolled steel sheet.

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

When the content of C is 0.10% or less, residual austenite formation for securing a predetermined strength level and ductility is insufficient, and when the content of C is 0.20% or more, the carbon equivalent is increased to significantly deteriorate the weldability, so the content of C is 0.10. It is desirable to limit it to -0.20%.

The Si is annealed in the two-phase region of austenite and ferrite to promote the formation of cornerstone ferrite during cooling, and to inhibit the precipitation of carbide during overaging treatment to delay the bainite transformation to promote carbon enrichment in the austenite phase. It is an element that promotes residual austenite formation as an element, but if it is added more than 1.6%, it not only degrades paintability and corrosion resistance, but also significantly deteriorates weldability by forming an oxide in the weld during flash butt welding. In the case of the addition below, the amount of residual austenite formed is small and the combination of strength and ductility worsens. Therefore, the amount of Si added is preferably limited to 0.6 to 1.6%.

The Mn prevents red heat brittleness by sulfur (S) and exhibits a solid solution effect. The Mn promotes the formation of residual austenite by inhibiting the deposition of cementite by affinity with carbon in the steel, so that the strength and ductility are appropriate. In order to secure a combination, more than 1.0% is required, but if more than 2.0% causes micro segregation after cold rolling and recrystallization annealing, thereby impairing the workability of the annealing plate, the Mn content is preferably limited to 1.0 to 2.0%. .

Al is added to prevent deterioration of the material due to deoxidizer and aging in aluminum-kilted steel, but the deoxidizing power is insufficient at 0.03% or less and 0.07% or more, indicating that the deoxidizing effect is saturated. It is preferable to limit the amount added.

Since N has a problem of deterioration of aging upon excessive addition, the content of N is preferably limited to 0.007% or less.

Mo is a ferrite stabilizing element such as silicon (Si), which is very advantageous for the formation of residual austenite, and also exhibits precipitation strengthening effect due to molybdenum precipitate, and therefore, addition of 0.10% or more is required for combination of strength and ductility, but 0.30% When the additive is added, the fraction of bainite phase increases and the strength increases, but the ductility decreases rapidly, which not only worsens the combination of strength and ductility, but also cracks occur during cold rolling. The amount of addition is preferably limited to 0.1-0.3%.

The Nb is an element exhibiting a precipitation strengthening effect by forming a fine precipitate in the steel, and niobium, a reinforcing carbonitride-forming element, forms a precipitate in the form of carbonitride (NbCN, Niobium Carbo nitride) to refine ferrite grains and thereby As the amount of transformation increases and the carbon is concentrated on the austenite phase, the residual austenite content is increased. When it is added below 0.02%, the effect of precipitation of fine precipitates cannot be obtained significantly. It is preferable to limit the addition amount to 0.02 to 0.1% because there is a problem of increasing the recrystallization temperature.

If the carbon equivalent is 0.35 or more, there is a problem that the hardness of the welded portion is increased and the spot weldability of the automobile assembly line is degraded. Therefore, the carbon equivalent is preferably limited to 0.35 or less in the present invention.

The steel slab formed as described above is reheated to hot roll, wherein the reheating temperature is a temperature range of 1,200-1,250 ° C., which is an austenite single phase region, and is homogenized for 1 hour in this temperature region and hot After rolling, the hot rolled coil is wound at 400-600 ° C. to make the hot rolled structure into a ferrite and bainite state. If the coiling temperature is higher than 600 ℃, the elongation increases slightly but the tensile strength does not reach the target level. If the coiling temperature is lower than 400 ℃, it is good to secure the strength, but it is difficult to coil during work. Therefore, it is preferable to limit the winding temperature to 400-600 ° C. for the strength and ductility combination of the final annealing plate.

Next, the hot rolled sheet treated as described above is cold rolled. In this case, when the cold reduction ratio is 30% or less, internal defects such as dislocations are insufficient, and cold crystallization is difficult, so that recrystallization is difficult and 80% or more. In the case of rolling, the load is large, which deteriorates the rolling property, so the cold reduction rate is preferably limited to the range of 30-80%.

Next, the cold rolled steel sheet is subjected to annealing by a continuous annealing method, in which the continuous annealing is cracked for 20-120 seconds at a temperature of 740-860 ° C., followed by a cooling process and then bainite at 500-300 ° C. By carrying out the transformation treatment, the structure of the final annealing plate consists of ferrite, residual austenite and bainite. When the continuous annealing temperature is below 740 ° C., some deformed grains remain in the annealing plate structure, making it difficult to secure ductility. When the annealing temperature is above 860 ° C., the amount of solid carbon in the austenite phase immediately after heating decreases. It is preferable to limit the annealing temperature to 740-860 ° C. because the stability of the austenite remaining in the solution is weakened to deteriorate the elongation, strength and ductility. In addition, if the annealing time is too short, recrystallization does not occur, so it is advantageous to increase the fraction of austenite obtained during annealing by increasing the fraction of austenite obtained during annealing as long as 20 seconds or more. It is preferable to limit it to within 2 minutes in consideration of the above.

In the case of the overaging treatment, that is, the bainite transformation temperature is 500 ° C or more, the fraction of the retained austenite phase rapidly decreases due to precipitation of the cementite phase, and the combination of strength and ductility is drastically reduced. The strength rises rapidly due to the formation on the site, but there is a problem in that the ductility decreases. Therefore, the treatment temperature is preferably limited to 300-500 ° C.

In the case of annealing as described above, the reason why the ductility of the final annealing plate is improved by maintaining the austenite at room temperature by maintaining the bainite transformation temperature is as follows. That is, the residual austenite maintained at room temperature transforms the processed organic into martensite with deformation, and at this time, volume expansion occurs. As a result, the compressive stress is generated in the surrounding ferrite due to the expansion. Is formed. The dislocations formed at this time remove the yield point phenomena due to the discontinuous yield behavior in the early stage of deformation since it not only increases the yield strength by hardening the ferrite but also increases the processing potential density. In addition, in the later deformation, dislocation propagation occurs around the martensite transformed from residual austenite, which increases the work hardening rate and the necking point moves toward the high strain rate. It will contribute to improving the ductility.

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

Example 1

After hot rolling the steel slab formed as shown in Table 1, followed by winding at the winding temperature as shown in Table 2, and then performing cold rolling, annealing and overaging under the same conditions as in Table 2, mechanical properties, and tensile strength. Elongation and spot weldability were measured and the results are shown in Table 2 below.

On the other hand, the microstructure of the hot rolled steel sheet wound the invention steel (a) at 550 ℃ and the hot rolled steel sheet wound the invention steel (a) at 620 ℃ in the steel of Table 1 and the results are shown in Figure 1 It was.

In Figure 1 (a) shows a microstructure photograph of the steel wound at 550 ℃, Figure 1 (b) shows a microstructure photograph of the steel wound at 620 ℃.

TABLE 1

TABLE 2

○: Excellent spot welding ×: Poor spot welding

As shown in Table 2, the steel component is included in the scope of the present invention, but in the case of the comparative material (1-3) whose hot rolling temperature is outside the range of the present invention, the combination of elongation x tensile strength is 2,500 kgf / mm ^2. %. By showing the following values, it can be seen that the strength-yarn combination is inappropriate.

In addition, in the case of the comparative material (4) containing less than the content of C in the present invention, it can be seen that the spot weldability is excellent but the strength cannot be secured, and the comparative material having more C than the present invention (5). ), It can be seen that the target level cannot be achieved in terms of material and spot weldability.

In addition, in the case of the comparative material 6 to which Nb was not added, it was found that the spot weldability was excellent but not only the strength was secured, but the combination value of tensile strength x elongation was low.

In addition, in the case of comparative materials (8 to 13) reinforced by adding Nb or Ti alone or in combination, tensile strength and ductility (tensile strength x elongation) did not exceed 2,500 kgf / mm ^2. %. have.

In addition, it can be seen that the comparative materials (5, 9, 11, and 12) having a carbon equivalent of more than 0.35 show poor results in spot weldability.

In addition, in the case of the comparative material 13, it was difficult to ensure ductility even at the time of annealing by normal ordinary annealing, and it was difficult to press work.

On the other hand, the present invention (ad) exhibits excellent properties with a tensile strength of 100-110kgf / mm ² , elongation of 25-30%, and a combination of tensile strength and elongation of more than 2,500kgf / mm ^2.0 %. Therefore, problems such as workability problems and spot weldability, which have emerged as disadvantages of the conventional super high strength cold rolled steel sheet, can be greatly improved.

As shown in FIG. 1, when the invention steel (a) is wound at 620 ° C., in FIG. 1 (b), the microstructure of the hot rolled sheet is mainly composed of ferrite and pearlite phases. When a) is wound at 550 ° C., it can be seen that the microstructure is composed of ferrite, bainite, and the like in FIG. 1 (a).

Example 2

C: 0.1-0.17%, Si: 1.0%, Mn: 1.5%, Mo: 0.15-0.25%, Al: 0.034%, N: 0.0041%, Nb: 0.082%, remainder Fe and other unavoidable impurities Comparative steel (1-10) shown in Table 1 of Example 1 was hot-rolled at 550 ℃, cold-rolled at a cold reduction rate of 60%, maintained at 810 ℃ for 1 minute and overaged at 400 ℃ to 350 ℃ After the treatment, the combination relationship between tensile strength and tensile strength x elongation was examined, and the results are shown in FIG.

As shown in FIG. 2, in the case of the comparative steel, the combined value of elongation x tensile strength is 2,500 kgf / mm ^2. % Or less, whereas in the present invention, 2,500 kgf / mm ² . It can be seen that the strength combination is improved.

As described above, the present invention is to control the carbon equivalent to less than 0.35 by using a low carbon aluminium-kilted steel added with silicon (Si), manganese (Mn), niobium (Nb), molybdenum (Mo), etc. as a strengthening element The weldability is improved, and the hot rolled winding temperature is 400-600 ° C. so that the hot rolled structure is composed of ferrite and bainite phase, followed by cold rolling and continuous annealing to contain some residual austenite phase in the structure of the annealing plate. Compared with the conventional high strength cold rolled steel sheet, the ductility is remarkably improved at the same strength level, thereby producing an ultra high strength cold rolled steel sheet having excellent press workability.

Claims (1)

In the method for producing an ultra-high strength cold rolled steel sheet, C: 0.10 to 0.20%, Si: 0.6 to 1.6%, Mn: 1.0 to 2.0%, Mo: 0.10 to 0.30%, Nb: 0.02 to 0.1%, N : 0.007% or less, remainder Fe and other inevitable impurities, carbon equivalent (Ceq) (Ceq = C + Si / 30 + M n-20 + 2P + 4S) was cracked at 1,200-1,250 ℃ for aluminum-killed steel with 0.35 or less, then hot-rolled in the usual way and wound at 400-600 ℃, then 30-80 After cold rolling at a cold reduction rate of%, cracking for 20-120 seconds at 740-860 ° C. by continuous annealing, and then overaging at 500-300 ° C. temperature weldability and press formability. This excellent ultra high strength cold rolled steel sheet production method.