KR20010064846A - A method for high strength cold rolled steel sheet having superior impact absorption property and formability and a method for manufacturing it - Google Patents

Abstract

PURPOSE: A method for manufacturing high tensile strength cold rolled steel sheets (80-90kgf/mm2) having superior impact absorbability and formability is provided to form an adequate quantity of residual austenite so that elongation rate as well as yield strength is improved, thereby it is applicable to automobile bumper requiring superior impact absorbability. CONSTITUTION: The high tensile strength cold rolled steel sheet is manufactured by (i) hot rolling a steel slab comprising C 0.10wt.%, Si 0.2-0.8wt.%, Mn 2.0-2.5wt.%, S 0.015wt.% or less, P 0.020wt.% or less, Al 0.04wt.% or less, N 0.004wt.% or less, Nb 0.01-0.05wt.%, a balance of Fe, and other inevitable impurities wherein the steel slab satisfies the following an inequality: (58.2-145.5C£%|-8.2Si£%|-14.5Mn£%|)/222< Nb£%|<(68.2-145.5C£%|-8.2Si£%|-14.5Mn£%|)/222; (ii) cold rolling the hot rolled steel sheet; (iii) continuous annealing the cold rolled steel sheet wherein the temperature range satisfies the following an inequality: 563+651C£%|+42Si£%|+18Mn £%|< annealing temperature£°C|< 870°C; (iv) rapid cooling to an austempering start temperature of 420-480deg.C at a cooling rate of 20-100deg.C/sec; (v) and then austempering to 380 to 430deg.C for 300-500deg.C.

Description

High strength cold rolled steel with excellent shock absorption and formability and its manufacturing method {A METHOD FOR HIGH STRENGTH COLD ROLLED STEEL SHEET HAVING SUPERIOR IMPACT ABSORPTION PROPERTY AND FORMABILITY AND A METHOD FOR MANUFACTURING IT}

The present invention relates to a cold rolled steel sheet applied to an automobile bumper and the like, and more particularly, to a high strength cold rolled steel sheet having a tensile strength of 80 to 90 kgf / mm 2, which improves the stability of a vehicle during an automobile crash due to its excellent energy absorption. It is about.

As the environment emerges as a socially important issue, regulations on automobile emissions and fuel economy are tightening around the world. In addition, as the number of automobile accidents increases with the increase of automobiles, the safety of passengers is emphasized, and regulations on collision stability continue to be strengthened. Offset crash assessment test was added.

In response, automakers around the world are developing vehicles that are lightweight and have excellent crash safety, which requires the use of materials of high strength.

On the other hand, in order to ensure the safety of the vehicle, the bumper should have excellent energy absorption. To this end, it is necessary to increase the strength of the bumper material, and it is essential to develop a cold rolled steel sheet having excellent shock absorption and formability for the bumper rail.

The conventional high strength cold rolled steel sheet was manufactured by the following method.

First, by using the work hardening by cold rolling, by performing recovery annealing after cold rolling to increase the strength to the recrystallized structure, to reduce the amount of the additive alloy and to provide excellent weldability. However, this method has a disadvantage in that elongation is lowered and moldability is deteriorated.

Secondly, there is a method of manufacturing a composite tissue steel in which austenite is formed at an annealing point of A ₁ or more after cold rolling, and then quenched to transform austenite into martensite or bainite to increase the strength of the material. However, this method has a disadvantage in that it is difficult to manufacture and the elongation is low because the cooling rate must be fast in the heat treatment process.

Third, there is a manufacturing method of processed organic transformation steel which increases the elongation by remaining residual austenite in the steel. This method adds a large amount of elements of C: 0.2% by weight or more (hereinafter referred to as "%"), Si: 1.5-2.5%, and Al: 1.0% or more, and maintains the heat treatment temperature at or above the martensite transformation temperature. Carbon is concentrated to form residual austenite. In other words, when the residual austenite is formed, the elongation of the material increases, so that the moldability is improved. However, since a large amount of Si and Al are added, the weldability is poor and the yield strength is low.

Accordingly, the present inventors have repeatedly conducted research and experiments to solve the above problems and propose the present invention based on the results. The present invention forms an appropriate amount of retained austenite and increases the elongation. To increase the yield strength by precipitation hardening, to provide a high-strength cold-rolled steel sheet and a method of manufacturing the same that can provide excellent impact absorption and formability, the object.

1 is a graph showing the material change of the steel according to the Nb content

Figure 2 is a graph comparing the tensile strength measured by the strength change equation and the experimentally measured tensile strength

3 is a graph showing the austenitic fraction according to the annealing temperature

In the present invention, C: 0.10 to 0.20%, Si: 0.2 to 0.8%, Mn: 2.0 to 2.5%, S: 0.015% or less, P: 0.020% or less, Al: 0.04% or less, N: 0.004% or less , Nb: 0.01 ~ 0.05%, the remaining Fe and other unavoidable impurities, satisfy the following equation 1, and the residual austenite content of 4 ~ 6%, characterized in that the high strength cold rolled steel sheet with excellent shock absorption and formability It is about.

In addition, the present invention is a method of manufacturing a high strength cold rolled steel sheet,

By weight% C: 0.10 to 0.20%, Si: 0.2 to 0.8%, Mn: 2.0 to 2.5%, S: 0.015% or less, P: 0.020% or less, Al: 0.04% or less, N: 0.004% or less, Nb: A steel slab containing 0.01 to 0.05%, the remaining Fe and other unavoidable impurities, and hot rolling the steel slab that satisfies the following relation 1, and then cold-rolled, and then continuously annealed in the temperature range of the following relation 2, and subjected to the ostempering It relates to a method for producing a high strength cold rolled steel sheet excellent in shock absorption and moldability.

[Relationship 1]

(58.2-145.5C [%]-8.2Si [%]-14.5Mn [%]) / 222 <Nb [%]

<(68.2-145.5C [%]-8.2Si [%]-14.5Mn [%]) / 222

[Relationship 2]

563 + 651C [%] + 42Si [%] + 18Mn [%] <Annealing Temperature [℃] <870 ℃

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

C is an element that stabilizes austenite by lowering the martensite transformation temperature. If the amount is large, residual austenite is formed at room temperature to increase the elongation of the material. However, if the content is too high, the weldability is deteriorated, which is disadvantageous to the welding process of the steel sheet and the welding of automobile parts during the production of cold rolled steel sheet. Therefore, it is preferable to add 0.10 to 0.20%.

The Si is an element that suppresses carbide formation in the process of austempering to increase the amount of solid solution C, thereby increasing the content of C diffused into the residual austenite and stabilizing the residual austenite. However, the higher the content, the lower the weldability, especially when it is added in excess of 1.0%, the residual austenite is excessively formed, thereby reducing the yield strength and lowering the shock absorbency. Therefore, the Si content is preferably set to 0.2 to 0.8%. Do.

Mn is an element that increases the strength of the material and stabilizes austenite by decreasing the transformation temperature of martensite, such as C. When the content is increased, the deformation resistance of the hot rolled steel sheet is increased, and thus the cold rolling property is deteriorated. Therefore, the Mn content is preferably set to 2.0 to 2.5%.

S is an element which combines with Mn to form a MnS precipitate. By the way, since MnS is formed as inclusions and can act as a starting point of cracking, the less it is advantageous, the S content is preferably limited to 0.015% or less.

Since P is segregated at grain boundaries and causes brittle fracture in the playing and rolling processes, the P content is preferably limited to 0.020% or less.

Al is an element for depositing O in steel, and after precipitation of O, it is preferable to limit the content of residual Al to 0.04% or less.

It is preferable to limit the N to 0.004% or less, because the content of N increases to form AlN to increase the strength and lower the elongation.

The Nb is a precipitation hardening element that increases the yield strength together with the tensile strength of the steel, and reacts with C to precipitate NbC, thereby increasing the strength of the steel, as shown in FIG. However, when a large amount is added, the elongation is reduced, so it is preferable to set the content at 0.01 to 0.05% in terms of elongation.

However, in the present invention, since the steel sheet having an excellent yield strength and elongation at the same time by appropriately adjusting the content of Nb, it is necessary to set the desired content of Nb in terms of strength. Accordingly, the content of Nb is determined according to the content of C, Si, Mn, the range is preferably set according to the following relational formula (1).

[Relationship 1]

(58.2-145.5C [%]-8.2Si [%]-14.5Mn [%]) / 222 <Nb [%]

<(68.2-145.5C [%]-8.2Si [%]-14.5Mn [%]) / 222

Equation 1 is obtained by using the strength change equation according to the change of the content of C, Si, Mn obtained by measuring the tensile strength while changing the C, Si, Mn content, and the tensile strength measurement experiment results according to the Nb content, According to the formula, it is possible to set the content of Nb, the tensile strength of 80 ~ 90kgf / mm2 when adding C, Si, Mn and Nb.

On the other hand, Figure 2 is a graph comparing the tensile strength measured by the experimental results and the tensile strength obtained by the strength change equation according to the change in the content of C, Si, Mn, shows that the calculation equation is in good agreement with the experimental results.

The cold rolled steel sheet of the present invention is composed of the above composition, and in order to obtain a high elongation, 4 to 6% of retained austenite must be contained.

In manufacturing a cold rolled steel sheet using the steel slab formed as described above, after reheating in the temperature range of 1050 ~ 1300 ℃ usually, the hot rolling is finished at the finish rolling temperature of 890 ~ 940 ℃, at 600 ~ 700 ℃ After winding, it is preferable to cold-roll at a cold reduction rate of 40 to 70%.

Then, the continuous annealing, the present invention is characterized in that the temperature at the time of the continuous annealing is carried out in the range of the formula (2).

[Relationship 2]

563 + 651C [%] + 42Si [%] + 18Mn [%] <Annealing Temperature [℃] <870 ℃

The reason for this is as follows.

In order to increase the elongation using steel containing C, Si, and Mn, the content of residual austenite must be increased.

In order to make residual austenite when annealed using a continuous annealing furnace, annealing is made to form austenite, cooled to an ostempering temperature, and then austem tempered to form bainite, while confining solid solution C to residual austenite to stabilize austenite. You have to. At this time, it is important to set the annealing temperature optimally according to the steel of each component.

In other words, when the annealing temperature is too low, the austenite is not dissolved in the hot rolled steel sheet so much austenite is formed and the ferrite is reduced so that a lot of bainite is finally formed, thereby increasing strength and decreasing elongation.

On the other hand, the elements that greatly influence the annealing temperature determination are C, Si, Mn, and the result of measuring the amount of austenite when the steel produced by changing each component is maintained at each annealing temperature for 51 seconds is shown in FIG. Is shown. As shown in Figure 3, the amount of austenite changes depending on the annealing temperature, which can be seen that the annealing temperature is related to the content of C, Si, Mn.

That is, in the case of using steel of the same component, in order to increase the amount of retained austenite, it is necessary to form the maximum ferrite and diffuse the maximum solid solution C in the austenite. At this time, the ferrite must be dissolved while maximally increasing the ferrite fraction, since the presence of a large amount of carbide in the ferrite reduces the amount of solid solution C diffused into austenite when the ferrite is present. In order to completely dissolve the ferrite, the amount of austenite should be 19, 28 and 38% for each steel with a C content of 0.1, 0.15 and 0.20%. If the austenite is formed more than this, the amount of ferrite is reduced, so it is difficult to form a large amount of retained austenite.

Therefore, annealing temperature of 563 + 651C [%] + 42Si [%] + 18Mn [%] or more is preferable to completely dissolve the pearlite under such conditions. On the other hand, when the annealing temperature is higher than 870 ° C, too much austenite is formed, so the upper limit is preferably set to 870 ° C.

On the other hand, in order to form the appropriate austenite in the above temperature range, it is preferable to anneal for 50 seconds or more.

Then, annealing as described above to austenite the material formed austenite, usually quenched to the oscillating start temperature 420 ~ 480 ℃ at a cooling rate of 20 ~ 100 ℃ / s so that the pearlite is not formed, annealing at the time of It is preferable to cool for 300-500 seconds to 380-430 degreeC so that a part of the formed austenite may be formed into bainite and solid solution C diffuses into residual austenite to stabilize the residual austenite.

Hereinafter, the present invention will be described in detail through examples.

(Example)

The steel slab, as shown in Table 1 below, was reheated to 1250 ° C. and the final rolling temperature was 910 ° C. to perform hot rolling. Thereafter, the sheet was wound at 620 ° C., and the surface oxide layer of the hot rolled sheet was removed by pickling, followed by 50% cold rolling to prepare a 1.4 mm thick cold rolled steel sheet. Then, the cold rolled plate was heat-treated in a continuous annealing furnace under the conditions shown in Table 2 below, the annealing time was 51 seconds, the ostempering time was 300 seconds.

Then, the material properties for the specimens were measured and the results are shown in Table 2 below.

Steel grade C Si Mn P S Sol.Al Nb Mo Invention steel 0.14 0.48 2.20 0.010 0.009 0.038 0.018 - Comparative Steel 1 0.14 0.98 2.10 0.010 0.009 0.038 0.018 - Comparative Steel 2 0.14 0.47 2.40 0.048 0.011 0.031 Comparative Steel 3 0.14 0.93 2.11 0.013 0.010 0.045 Comparative Steel 4 0.15 0.99 2.39 0.011 0.009 0.035 - - Comparative Steel 5 0.14 0.98 2.11 0.011 0.010 0.044 - 0.10 Comparative Steel 6 0.14 1.00 2.05 0.046 0.01 0.010 Comparative Steel 7 0.14 1.00 2.05 0.040 0.009 0.044 0.009 - Comparative Steel 8 0.17 1.01 2.10 0.043 0.01 0.010 - - Comparative Steel 9 0.14 1.43 1.77 0.013 0.010 0.039 - - Comparative Steel 10 0.14 1.43 1.79 0.013 0.011 0.039

Psalter Manufacture conditions Material characteristics Annealing Temperature (℃) Ostempering start temperature (℃) Yield strength (kgf / ㎡) Tensile strength (kgf / ㎡) Elongation (%) Invention Invention steel 800 450 58.0 83.0 19.1 830 450 54.8 82.4 20.5 860 450 54.0 81.7 22.5 Comparative material Comparative Steel 1 800 450 50.3 81.4 23.0 830 450 47.2 78.3 25.2 860 450 44.2 78.0 24.6 830 500 45.6 89.1 16.3 830 400 46.0 81.3 19.7 Comparative Steel 2 800 450 49.2 84 20.9 830 450 50.5 84.5 16.8 860 450 50.9 84.4 17.9 Comparative Steel 3 800 450 40.5 80.7 23.5 830 450 39.6 79.1 25.1 860 450 45.0 78.8 24.2 Comparative Steel 4 800 450 46.8 83.5 23.5 830 450 43.6 81.7 24.6 860 450 38.0 82.4 22.6 830 500 46.7 89.1 16.6 830 400 48.7 85.7 20.9 Comparative Steel 5 800 450 50.0 87.6 19.6 830 450 48.2 83.9 22.3 860 450 42.7 82.7 22.2 830 500 49.2 74.7 17.8 830 400 43.0 79.4 23.6 Comparative Steel 6 800 450 40.9 86.9 22.1 830 450 44.0 83.4 22.9 860 450 45.0 84.3 21.1 830 500 39.9 85.3 16.9 830 400 43.3 81.5 22.6 Comparative Steel 7 800 450 45.0 87.8 23.6 830 450 42.0 85.2 23.4 860 450 42.5 85.3 21.1 830 500 44.1 86.5 14.2 830 400 48.7 82.7 19.8 Comparative Steel 8 800 450 40.1 76.2 22.5 830 450 41.9 76.4 21.5 860 450 44.7 76.7 17.7 830 500 41.7 92.7 15.8 830 400 42.5 87.0 22.3 Comparative Steel 9 800 450 35.2 83.6 19.7 830 450 36.2 84.4 18.8 600 450 40.0 84.8 17.5 Comparative Steel 10 800 450 37.4 73.7 24.8 830 450 35.1 74.7 25.1 860 450 34.7 75.1 25.1

As shown in Table 2, the invention material of the present invention can be seen that the tensile strength and elongation are equivalent to the comparative material, the yield strength is superior to the comparative material.

Therefore, the present invention can be judged to be excellent in moldability when manufacturing an automobile bumper by stretching and foaming, respectively.

As described above, the present invention has a high yield strength, excellent impact absorption, high elongation, excellent moldability, and has an effect of being applied to an automobile bumper rail to improve stability of an automobile.

Claims (3)

By weight% C: 0.10 to 0.20%, Si: 0.2 to 0.8%, Mn: 2.0 to 2.5%, S: 0.015% or less, P: 0.020% or less, Al: 0.04% or less, N: 0.004% or less, Nb: High strength cold rolled steel sheet having excellent shock absorption and formability, containing 0.01 to 0.05%, remaining Fe and other unavoidable impurities, satisfying the following relational formula 1, and having 4 to 6% of retained austenite [Relationship 1] (58.2-145.5C [%]-8.2Si [%]-14.5Mn [%]) / 222 <Nb [%] <(68.2-145.5C [%]-8.2Si [%]-14.5Mn [%]) / 222 In the manufacturing method of high strength cold rolled steel sheet, By weight% C: 0.10 to 0.20%, Si: 0.2 to 0.8%, Mn: 2.0 to 2.5%, S: 0.015% or less, P: 0.020% or less, Al: 0.04% or less, N: 0.004% or less, Nb: A steel slab containing 0.01 to 0.05%, the remaining Fe and other unavoidable impurities, and hot rolling the steel slab that satisfies the following relation 1, and then cold-rolled, and then continuously annealed in the temperature range of the following relation 2, and subjected to the ostempering Method for producing high strength cold rolled steel sheet with excellent shock absorption and formability [Relationship 1] (58.2-145.5C [%]-8.2Si [%]-14.5Mn [%]) / 222 <Nb [%] <(68.2-145.5C [%]-8.2Si [%]-14.5Mn [%]) / 222 [Relationship 2] 563 + 651C [%] + 42Si [%] + 18Mn [%] <Annealing Temperature [℃] <870 ℃ The method according to claim 2, wherein after the continuous annealing at a cooling rate of 20 ~ 100 ℃ / sec quenched to the osmosis start temperature 420 ~ 480 ℃ and then subjected to ostempering to cool to 300 to 500 seconds to 380 ~ 430 ℃ Method for producing high strength cold rolled steel sheet with excellent shock absorption and formability