KR100729125B1 - High strength cold rolled steel sheet with excellent plastic strain ratio and its manufacturing method - Google Patents

Abstract

The present invention relates to a high-strength high strength cold-rolled steel sheet and a method for manufacturing the same, which are used as interior and exterior plate materials for automobiles. The purpose is to prepare.

Specifically, in weight percent, carbon (C): 0.001 to 0.03%, manganese (Mn): 0.2 to 0.5%, phosphorus (P): 0.04 to 0.07%, niobium (Nb): 0.005 to 0.009%, titanium (Ti) ): 0.003 ~ 0.05%, sulfur (S): 0.008% or less, nitrogen (N): 0.003% or less, aluminum (Al): 0.06% or less and consist of the remaining iron (Fe) and unavoidable impurities Characterized in that the content of titanium (Ti) is 48 × (C / 12 + N / 14 + S / 32-0.4 Nb / 93) + 0.01 <Ti ≤ 48 × (C / 12 + N / 14 + S /32-0.4Nb/93) + 0.03. The steel slab having the composition was wound at a temperature of 600 to 700 ° C. through a hot rolling process, then cold rolled at a reduction ratio of 70 to 80%, and then subjected to recrystallization annealing heat treatment at 800 ° C. to 850 ° C. for 20 to 100 seconds. The main content is cooling at a rate of 2 to 30 ° C./sec, maintaining the temperature at 350 ° C. to 450 ° C. for 2 to 5 minutes or less, and then performing final cooling.

Description

High strength cold rolled steel sheet with excellent plastic strain ratio and its manufacturing method {High Strength Steets which have good average plastic strain ratio and the method of developing those steels}

1 shows a change in the mechanical properties of the inventive steel 1 according to the annealing heat treatment temperature.

2 shows a change in the mechanical properties of the inventive steel 2 according to the annealing heat treatment temperature.

Figure 3 shows the change of the average plastic strain ratio (Rm) value of the inventive steels 1, 2 according to the annealing heat treatment temperature.

The present invention relates to a method for manufacturing a high strength high strength cold rolled steel sheet for use in automotive exterior parts, in particular, a tensile strength of 35kgf / mm2 or more, 38% or more elongation, 2.0 or more average plastic strain ratio (Rm = (R0 + 2R45) It relates to a high strength cold rolled steel sheet having a value of + R90) / 4) and a method of manufacturing the same.

Recently, as automobile molded products are complicated and integrated, demand for high strength steel sheets having excellent moldability is rapidly increasing. Accordingly, in order to remove elements such as solid solution carbon, solid solution nitrogen, and solid solution sulfur, which impair the formability of the cold rolled steel sheet from the steelmaking stage, carbon or nitride formation promoting elements such as titanium or niobium are added alone or in combination. A technique for manufacturing a cold rolled steel sheet for deep processing, in which solid solution elements are precipitated with carbon and nitride to improve formability.

Meanwhile, as interest in weight reduction and collision safety for improving fuel efficiency of automobiles is increasing, materials used for structural materials such as automobiles and exterior plates are required to secure strength as well as formability. The demand for the high strength high strength cold rolled steel sheet having the above strength is increasing.

Such high-strength cold rolled steel sheets for deep processing are applied to parts such as rear floors or hoods that simultaneously require high strength and high formability among interior and exterior materials. Since these parts are parts requiring high strength and high formability at the same time, steel sheets having a tensile strength of 35kgf / mm 2 or more, an elongation of 38% or more, and an average plastic anisotropy coefficient of 2.0 or more are essentially applied.

Techniques disclosed for the production of high strength high strength steel sheet include the following.

In CAMP-ISIJ, Vol. 9 (1996), p. 386, the component system of 0.002C-1.5Mn-0.6Si-0.06P-0.04Ti, in which Ti-added ultra low carbon steels were suitably added with Mn, P, Si, etc. It has been reported that the high strength cold rolled steel sheet having a tensile strength of 45kgf / mm2. This technique changes the precipitation pattern from fine Ti4C2S2 to coarse MnS precipitation as the Mn content is increased, and increases the content of C in ferrite by adding Si to promote the precipitation of TiC, resulting in high formability. To make steel. The steel sheet produced by this method is excellent in strength but has a problem in terms of formability since the Rm value remains at about 1.8.

In TMS-AIME (1989), p161, which was published in 1994, as Mn and P content in Ti-Mn-P composite additive steel increased, not only the tensile strength but also the Rm value increased, and the Rm value> 2.2, TS≥40kg / It is reported that it is possible to manufacture high-strength steel of mm2 grade. To this end, strict manufacturing conditions such as cryogenic winding of 350 ° C. or less after hot rolling (maintaining 350 ° C. for 1 hr after quenching) and rapid heating when annealing materials are heated are required, and an aggregate structure that absolutely affects formability is required. To develop, the surface layer must be surface ground after hot rolling.

However, these manufacturing techniques have a lot of problems in actual operation. In other words, at present facilities, cryogenic winding and rapid heating below 350 ° C. are almost impossible, and surface grinding of hot-rolled sheets will in particular result in the inconvenience of work and decrease in error rate.

In Korea Patent Application 2001-0012538, C: 0.002% or less, Mn: 0.6 ~ 1.0%, P: 0.06 ~ 0.1%, S: 0.006% or less, N: 0.003% or less, Al: 0.04% or less, Nb: 0.0078 ~ 0.015%, B: The content of Ti in a component of 0.001% or less is represented by the following relation (48/14 × N) + (48/12 × C **) ≤ Ti ≤ ((48/14 × N) + (48/32 × S) + (48/12 × C **)), C ** = C-0.4C *, C * = (12/93) after adjusting to a range satisfying Nb, winding temperature 680 ~ 720 ℃, It has been disclosed that ultra-high strength steel sheets having a cold rolling reduction of 77% or more, annealing temperature of 850 ° C. or more, a tensile strength of 35 kg / mm 2 or more, and an Rm value of 2.5 or more can be manufactured. This technique is a method of improving the planar anisotropy by slightly increasing the content of P in the Republic of Korea Patent Application 1999-0035104 can produce a high strength steel sheet with very excellent formability. However, this technique requires a high annealing temperature of more than 850 ℃ has the disadvantage of causing an increase in the manufacturing cost.

In the present invention, in order to solve the above problems, by controlling the alloy components and the content of the steel appropriately, a high-strength high strength steel sheet having excellent properties such as tensile strength of 35kg / mm 2 or more, elongation of 38% or more, Rm value of 2.0 or more The purpose is to prepare.

The present invention limits the chemical composition of the base iron in order to achieve the above object as follows. In weight% (hereinafter all weight%), carbon (C): 0.001 to 0.03%, manganese (Mn): 0.2 to 0.5%, phosphorus (P): 0.04 to 0.07%, niobium (Nb): 0.005 to 0.009%, Titanium (Ti): 0.003 ~ 0.05%, sulfur (S): 0.008% or less, nitrogen (N): 0.003% or less, aluminum (Al): 0.06% or less The main feature is that the content of titanium (Ti) is 48 × (C / 12 + N / 14 + S / 32-0.4Nb / 93) + 0.01 ≦ Ti ≦ 48 × (C / 12 + N / 14 + S / 32-0.4Nb / 93) + 0.03 is a major feature. The steel slab having the composition was wound at a temperature of 600 to 700 ° C. through a hot rolling process, then cold rolled at a reduction ratio of 70 to 80%, and then subjected to recrystallization annealing heat treatment at 800 ° C. to 850 ° C. for 20 to 100 seconds. The main content is cooling at a rate of 2 to 30 ° C./sec, maintaining the temperature at 350 ° C. to 450 ° C. for 2 to 5 minutes or less, and then performing final cooling.

Hereinafter, the present invention will be described in detail.

The present invention adds the Ti content in the steel within the range of 0.003 to 0.05%, especially 0.03 to 0.05%, and also the relation 48 × (C / 12 + N / 14 + S / 32-0.4Nb / 93) +0.01 ≤ Ti ≤ 48 × (C / 12 + N / 14 + S / 32-0.4Nb / 93) + 0.03 to control the content to satisfy the tensile strength of 35kg / mm2 or more, elongation 38% even in the hot and cold rolling process In the above, there is a feature to develop a high strength high strength steel sheet that satisfies the characteristics of Rm value 2.0 or more.

Main chemical composition

Carbon (C) is an invasive solid solution element in steel, and has a great influence on strength and texture formation of steel sheets during cold rolling and annealing heat treatment. When the amount of solid solution carbon contained in steel is high, the strength is increased, and 110 and 100 textures, which are disadvantageous to processability, tend to develop rather than 111 texture which is advantageous for processability. In addition, as the amount of carbon increases, the Ti and Nb contents for precipitation as carbides are relatively high, leading to an increase in manufacturing cost, and deterioration of moldability due to excessive increase in TiC precipitates. Therefore, the greater the amount of carbon is not good in terms of manufacturing cost and formability, so the upper limit was limited to 0.03%. On the contrary, when the amount of carbon is not too small, and the unit price increases due to difficulties in the decarburization manufacturing process, the lower limit thereof is limited to 0.001%. In the present invention, it was confirmed that keeping the carbon (C) content in particular between 0.001 and 0.003% can bring about stable strength.

Manganese (Mn) increases the strength as a solid solution strengthening element, precipitates sulfur (S) in the steel to MnS to suppress the occurrence of plate breakage and high temperature embrittlement by S during hot rolling. When the concentration of manganese in the steel is lower than 0.2%, it is not sufficient to secure the strength and removal of S, and when the concentration of manganese in the steel is higher than 0.5%, the moldability is deteriorated, so the range is limited to 0.2 to 0.5%.

Phosphorus (P) is also a solid solution element that can increase the strength without significantly degrading the formability, which is advantageous for securing strength, but when excessively added, it is segregated at grain boundaries and causes brittle fracture, so its content is limited to 0.04 to 0.07%. .

Niobium (Nb) is an element that precipitates solid solution carbon into coarse (Ti, Nb) C during hot rolling to improve moldability. If the content is less than 0.005%, the amount of precipitation to (Ti, Nb) C is reduced, and the effect of improving moldability is reduced.If the content is more than 0.009%, the possibility of precipitation to NbC also increases to (Ti, Nb) C. Since the precipitation probability is relatively small, the content is limited to 0.005 to 0.009%.

Titanium (Ti) is the most important element in the present invention, while undergoing hot rolling processes such as steelmaking and continuous casting, precipitated solid nitrogen and solid carbon in the steel with a compound such as TiN or TiC to greatly improve the formability of the steel sheet. It plays a role. In particular, when titanium in the content range satisfying the following relation (1) is added, the dissolved nitrogen and dissolved carbon in the steel are effectively removed, so that the average plastic strain ratio (Rm) 2.0 does not occur even if the high temperature annealing heat treatment of 850 ° C. or higher is not performed during the cold rolling process. The high strength steel plate which has the above high moldability can be manufactured.

48 × (C / 12 + N / 14 + S / 32-0.4Nb / 93) + 0.01 ≦ Ti ≦ 48 × (C / 12 + N / 14 + S / 32-0.4Nb / 93) +0.03 ·······(One)

48 (C / 12 + N / 14 + S / 32-0.4Nb / 93) in the above relation (1) is the theoretical value needed to precipitate carbon and nitrogen sulfur except for the amount of carbon precipitated by niobium. Titanium is required, and by adding more titanium than this, the solid solution of carbon and nitrogen is more completely precipitated to maximize moldability. In this case, the titanium content is limited to 0.003 to 0.05%. If the content is less than 0.003%, FeTiP precipitates are known to deteriorate formability due to insufficient precipitation of solid solution carbon and nitrogen. Because it increases. In the case of the present invention, when the Ti content is in the low range of 0.001% to 0.003%, it was found that keeping it at 0.03% to 0.05% is the most stable and desirable result.

Since sulfur (S) and nitrogen (N) are elements that are inevitably added as impurities during the steelmaking process, it is desirable to manage them as low as possible. In consideration of the realistic technical level and the increase in manufacturing cost, the content was limited to 0.008% or less of sulfur and 0.003% or less of nitrogen, which are manageable ranges under normal operating conditions.

Aluminum (Al) is an element added to refine the particle size of steel and deoxidize, and its content is limited to 0.06% or less, which is usually added.

The slabs formed as described above are obtained by ingot or continuous casting after obtaining molten steel through a steelmaking process. The slab is produced by hot rolling, winding, cold rolling, and annealing to produce a steel sheet having a target mechanical property. The manufacturing conditions for each process will be described in detail below.

Hot rolling process

The hot rolled slab is hot rolled at a temperature above the Ar3 transformation temperature to obtain a fine hot rolled structure. After hot rolling, it is desirable to descale the surface by using a high pressure descaling device or by applying strong pickling.

Winding process

The hot rolled steel sheet is wound at a temperature of 600 ~ 700 ℃, this winding temperature is a temperature for obtaining a structure for obtaining the optimum mechanical properties after cold rolling and recrystallization heat treatment, when the winding temperature is less than 600 ℃ (Ti, This is because it is difficult to form Nb) C precipitates, and when the temperature exceeds 700 ° C., the final microstructure becomes coarse, making it difficult to manufacture a steel sheet having sufficient strength and increasing the surface scale, and thus poor surface characteristics.

Cold rolling process

After the pickled hot rolled sheet is pickled, it is cold rolled, and the cold reduction rate is preferably 70 to 80%. Cold rolling deforms the hot-rolled structure and its strain energy becomes the energy of the recrystallization process. If the cold reduction rate is less than 70%, it is difficult to form an aggregate structure favorable for formability, and the moldability is deteriorated. Since the probability of cracking and plate breakage at the edge of the steel sheet increases, the range is preferably 70 to 80%.

Annealing Heat Treatment  fair

After cold rolling, the rolled steel sheet is subjected to recrystallization annealing heat treatment at 800 ° C to 850 ° C for 20 to 100 seconds. The recrystallization annealing heat treatment is a process of recovering the deformed structure after cold rolling and forming an aggregate structure favorable to formability. As shown in FIG. 1, when the temperature is lower than 800 ° C., the elongation, which is an index of formability, is sharply lowered. The Rm value of 2 also falls below 2.0. In addition, when the temperature exceeds 850 ℃, the tensile strength is likely to be less than 35kgf / mm2, and the manufacturing cost is increased by high-temperature work, so the range is limited to 800 ~ 850 ℃. If the annealing heat treatment time is less than 20 seconds, recrystallization recovery does not occur sufficiently, and if it exceeds 100 seconds, the productivity is reduced at a slow production rate, so the range is limited to 20 to 100 seconds. After annealing heat treatment, slow cooling and quenching are carried out in a speed range of 2 to 30 ° C./sec to a temperature of 350 to 450 ° C., and a cooling zone is required if the speed is too slow as a cooling capacity range of a conventional gas cooling method. The longer the above, the higher the cost of the device, and in the case of more than 30 ° C., the cost of the device also increases due to the need for equipment supplementation and water cooling facilities. After the quenching process, an overaging heat treatment is performed for 2 to 5 minutes at a temperature of 350 ° C. to 450 ° C., which is a section for depositing the dissolved carbon as much as possible. If it exceeds 450 ℃ to increase the manufacturing cost according to the high temperature operation was limited to the range 350 ℃ ~ 450 ℃. The reason for setting the heat treatment holding period of 2 to 5 minutes is the same. After overaging heat treatment, final cooling is performed at a normal cooling rate, and surface roughness is imparted through temper rolling as necessary.

The mechanical properties of the product thus produced are measured. In the present invention, the measurement and average plastic strain ratio (Rm) were calculated as follows. That is, the mechanical property measured the tensile strength and elongation using the tensile tester after processing the JIS No. 5 standard test piece to the rolling 90 degree direction. The average plastic strain ratio (Rm) value is also JIS 5 standard, after processing the specimen in the rolling direction, 45 ° direction, 90 ° direction, respectively, taking the average value of R0, R45, R90, in the range of 2-15%, Rm value was calculated using the following relation (2).

Rm = (R0 + 2R45 + R90) / 4) (2)

Hereinafter, the present invention will be described in more detail with reference to one embodiment. However, the present invention is not limited by the following examples.

EXAMPLE

Hot rolled finish rolling was performed at 950 ° C for the inventive steels (steels 1 to 2) and comparative steels (steels 3 to 7) having the chemical components shown in Table 1, and the hot rolled steel sheets wound at 600 to 700 ° C. Cold rolling and annealing heat treatment were performed under the process conditions indicated in.

As can be seen in Table 1, in the case of the invention steel No. 1 and No. 2 Ti content was added within the range of the relational formula 1. From Comparative Steel No. 3 to No. 7, Ti was added under the shortage of the relational formula (1).

Inventive steel Nos. 1 and 2 were cold rolled to about 78% by cold rolling a hot rolled steel sheet of about 3.6mm thickness to 0.8mm thickness, and the final thickness of 0.7 ~ 0.8 in comparison steel Nos. 3 to 7 with 70 ~ 76% reduction ratio. It was rolled about mm.

After cold rolling, the annealing heat treatment was performed at various temperatures as shown in Table 2, and then slowly cooled to 3 ° C./sec up to about 670 ° C., followed by quenching at about 22 ° C./sec and performing overaging heat treatment at a temperature of 400 ° C., followed by final cooling. It was.

The mechanical properties of the test samples were measured as described above, and the average plastic deformation ratio (Rm) was calculated to obtain the results shown in Table 2. This is as follows.

First, in case of steel No. 1 and No. 2 satisfying relation (1), when the annealing temperature of 810 ~ 850 ° C. within the process conditions of the present invention, the tensile strength is more than 35kg / mm2, the elongation is more than 38%, the Rm value is 2.0 or more. You can see it. However, in case of steel Nos. 1 and 2, when the temperature was less than 800 ° C outside the process conditions of the present invention, the Rm value was less than 2.0 due to insufficient development of an advantageous formability, and the tensile strength was higher than the temperature above 850 ° C. The phenomenon of falling below 35kg / mm2 can be confirmed. Mechanical properties change according to the annealing temperature of the invention steels 1 and 2 are shown in FIGS. 1 and 2 and 3. 1 is a graph showing a change in mechanical properties of the invention steel No. 1 when the annealing temperature is changed, the yield strength and tensile strength gradually decreases as the annealing temperature increases, the elongation can be seen to increase. 2 is a graph showing a change in the mechanical properties of the invention steel 2 when the annealing temperature is changed, the change pattern is similar to steel No. 1, but the annealing heat treatment is performed at a slightly higher temperature than steel No. 1 to ensure stable physical properties I can see. Figure 3 is a graph showing the change of the Rm value according to the annealing temperature Rm value tends to increase as the annealing temperature of the invention steel 1, 2 times increased, the annealing heat treatment temperature is about 800 to obtain an Rm value of 2.0 or more It can be seen that the temperature should be controlled at or above ℃.

As shown in Table 1, the steels No. 3 to No. 7 contain Ti which is less than the Ti content in the relation (1), so the tensile strength and elongation are excellent in the temperature range of about 810 ~ 850 ° C. It can be seen that the Rm value is less than 2.0.

As described above, according to the present invention, carbon (C): 0.001 to 0.03%, manganese (Mn): 0.2 to 0.5%, phosphorus (P): 0.04 to 0.07%, niobium (Nb): 0.005 to 0.009 %, Titanium (Ti): 0.003 ~ 0.05%, sulfur (S): 0.008% or less, nitrogen (N): 0.003% or less, aluminum (Al): 0.06% or less, remaining iron (Fe) and unavoidable impurities It is characterized in that the main composition, the content of titanium (Ti) is 48 × (C / 12 + N / 14 + S / 32-0.4 Nb / 93) + 0.01 ≤ Ti ≤ 48 × (C / 12 + A steel slab having a composition satisfying N / 14 + S / 32-0.4Nb / 93) +0.03 is wound at a temperature of 600 to 700 ° C. through a hot rolling process and then cold rolled to a rolling reduction of 70 to 80%. After that, recrystallization annealing heat treatment was performed at 800 ° C. to 850 ° C. for 20 to 100 seconds, cooled at a rate of 2 to 30 ° C./sec, held at 350 ° C. to 450 ° C. for 2 to 5 minutes or less, and finally cooled. Through the manufacturing process, the tensile strength of 35kgf / mm2 or more, 38% or more elongation, 2.0 or more Having a high strain plastic strain ratio (Rm) value, it was possible to prepare a molded high strength cold rolled steel sheet.

Claims (3)

Tensile strength 35kgf / mm2 or more, elongation of 38% or more, average plastic strain ratio (Rm) 2.0 or more in order to produce a high strength high strength cold rolled steel sheet, carbon (C): 0.001 to 0.03%, manganese (Mn): 0.2 0.5%, phosphorus (P): 0.04% to 0.07%, niobium (Nb): 0.005% to 0.009%, titanium (Ti): 0.003% to 0.05%, sulfur (S): 0.008% or less, nitrogen (N): 0.003% Below, aluminum (Al): 0.06% or less, steel slab composed of the remaining iron (Fe) and inevitable impurities are wound at a temperature of 600 ~ 700 ℃ through a hot rolling process, and then reduced to 70 ~ 80% After cold rolling at a rate, a recrystallization annealing heat treatment was performed at 800 ° C. to 850 ° C. for 20 to 100 seconds, cooled at a rate of 2 to 30 ° C./sec, and reached a temperature of 350 ° C. to 450 ° C. at a temperature of 2 ° to this temperature range. In cold rolled steel sheet which is held for 5 minutes and then finally cooled, The content of titanium (Ti) is in relation to the C, N, S, Nb 48 × (C / 12 + N / 14 + S / 32-0.4 Nb / 93) + 0.01 <Ti ≤ 48 × ( C / 12 + N / 14 + S / 32-0.4Nb / 93) + 0.03 to form a high sensitivity cold rolled steel sheet characterized by satisfying an average plastic deformation ratio (Rm) of 2.0 or more delete delete

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