KR101039984B1 - Batch annealing furnace heat trearment conditions for high strength steel - Google Patents

Abstract

The present invention relates to a box annealing furnace heat treatment method for improving the mechanical properties of cold rolled steel sheet, and more particularly to a box annealing furnace heat treatment method for improving the mechanical properties of 440MPa class high-tensile automotive steel sheet.

The present invention is to provide a high-tensile steel sheet for automotive grade 440MPa C: 1085ppm (hereinafter by weight), Si: 509ppm, Mn: 11000ppm, P: 186ppm, S: 34ppm, Cr: 166ppm, Mo: 29ppm, Al: 253ppm, N: 21ppm, O: 17ppm The remainder is a box annealing furnace heat treatment method consisting of a heating step, a heating holding step, a cooling step by using a cold rolled steel sheet having a chemical composition consisting of Fe and unavoidable impurities constituting a total of 100% by weight, The heating holding step is to maintain a heating holding time 10 ± 2 hours in the heating and holding temperature 760 ℃ ± 15 ℃ range to satisfy the yield strength 25 ~ 40 kg / ㎜, tensile strength 45 kg / ㎜ or more, elongation 34% or more It provides a box annealing furnace heat treatment method of a cold rolled steel sheet.

Box Annealing Furnace, Heat Treated, 440MPa

Description

BAN ANNEALING FURNACE HEAT TREARMENT CONDITIONS FOR HIGH STRENGTH STEEL}

The present invention relates to a box annealing furnace heat treatment method for improving the mechanical properties of cold rolled steel sheet, and more particularly to a box annealing furnace heat treatment method for improving the mechanical properties of 440MPa class high-tensile automotive steel sheet.

As global warming has risen, regulations on vehicle air emissions have been tightened. In the effort to reduce the vehicle's atmospheric gas, a variety of measures are being realized. Among them, attempts have been made to reduce the weight of the steel sheet used in the vehicle while simultaneously increasing the strength of the automobile steel sheet.

In order to improve the strength of automobile steel sheets, development of various high tensile steel sheets is being activated. Recently, the ratio of high tensile steel sheets required for one vehicle is over 50%. In order to satisfy the characteristics requiring the stability of the vehicle, the demand for high-strength automotive steel sheet is increasing more and more rapidly. These automotive steel sheets require excellent mechanical properties for the material properties of the steel sheet necessary for the production of the optimum automobile to meet the various needs of customers. The general manufacturing process of such cold rolled steel (CR) sheet for automotive is cold rolled hot rolled steel sheet (Hot Rolled Steel Sheet) at room temperature, and then subjected to heat treatment suitable for the required mechanical properties, predetermined cold rolled steel sheet (Cold Rolled Steel Sheet) It consists of obtaining.

The heat treatment of cold rolled steel is mainly divided into continuous annealing line (CAL) and batch annealing furnace (BAF). Recently, the heat treatment method of automotive cold rolled steel sheet is mostly used as continuous annealing heat treatment, and the results of mechanical characteristics research by heat treatment conditions are systematically performed by steel type. However, recent advances in BAF heat treatment technology and compact equipment improvements have increased the number of cases where automotive steel sheet manufacturing is processed in box annealing furnaces by taking advantage of low investment.

The present invention is to satisfy the mechanical properties (YP: 25 ~ 45 kg / mm2, TS: 45 kg / mm2, El .: 34% or more) required by 440MPa high-tensile automotive steel sheet using the hot-burned material produced in the blast furnace The purpose is to provide the optimum conditions of BAF heat treatment conditions, that is, soaking temperature and soaking time.

The present invention is to provide a high-tensile steel sheet for automotive grade 440MPa C: 1085ppm (hereinafter by weight), Si: 509ppm, Mn: 11000ppm, P: 186ppm, S: 34ppm, Cr: 166ppm, Mo: 29ppm, Al: 253ppm, N: 21ppm, O: 17ppm The remainder is a box annealing furnace heat treatment method consisting of a heating step, a heating holding step, a cooling step by using a cold rolled steel sheet having a chemical composition consisting of Fe and unavoidable impurities constituting a total of 100% by weight, The heating and holding step is to maintain a heating holding time 10 ± 2 hours in the heating and maintenance temperature range of 760 ℃ ± 15 ℃, to satisfy the yield strength 25 ~ 40 kg / ㎜, tensile strength 45 kg / ㎠ or more, elongation 34% or more It provides a box annealing furnace heat treatment method of a cold rolled steel sheet.

The present invention provides a box annealing furnace heat treatment method capable of providing a 440MPa class high strength steel sheet for automobiles.

The present invention provides the optimum temperature conditions and holding time of the box annealing furnace heat treatment that can provide a high tensile steel sheet.

The present invention brings the effect of providing a heat treatment condition of high tensile strength steel sheet to satisfy the yield strength of 25 ~ 40 kg / mm 2, tensile strength of 45 kg / mm 2 or more, elongation of 34% or more.

Hereinafter, with reference to the accompanying drawings will be described an embodiment of the box annealing furnace heat treatment method of a high tension steel sheet for automobiles according to the present invention.

In this process, the thickness of the lines or the size of the components shown in the drawings may be exaggerated for clarity and convenience of description.

In addition, terms to be described later are terms defined in consideration of functions in the present invention, which may vary according to a user's or operator's intention or custom.

Therefore, definitions of these terms should be made based on the contents throughout the specification.

First, the box annealing furnace heat treatment process will be briefly described.

The box annealing furnace process consists of ① steel sheet loading → ② atmosphere homogenization → ③ heating → ④ soaking → ⑤ cooling → ⑥ take-out. The characteristics of the material aspect of the steel structure in this process are composed of the first stage of recovery, the second stage of recrystallization, and the third stage of grain growth. In the present invention, an annealing cycle scheme for the optimization of the temperature and time of the cracking process that can satisfy the mechanical properties of the 440MPa high-tensile automotive steel sheet during the annealing process will be highlighted.

Hereinafter, each process of the box annealing furnace heat treatment process will be described in detail. First, the steel sheet is loaded in accordance with the processing capacity, steel type, thickness, and annealing cycle in the furnace, and the outside of the furnace is shielded. Then, nitrogen (N2) atmosphere gas is furnace-furnished. Purging is carried out by drawing in a cover.

Purging the atmospheric gas is a pretreatment step performed to remove the oxidative atmosphere in the treatment space to prevent oxide formation and decarburization of the steel sheet after the annealing treatment, and after purging, the surface of the steel sheet after annealing can be beautiful and clean characteristics can be obtained.

After purging, hydrogen (H2) is introduced to heat the loaded internal material and form a reducing atmosphere, and heated to a target holding temperature at an appropriate heating rate. The heating holding temperature and the heating holding time are set and maintained respectively.

After the appropriate cooling time, the heating hood is removed and the cooling unit with cooling fan and the cooling unit with cooling water are mounted according to the cooling temperature and then operated. The annealing heat treatment process is terminated by cooling and finally separating the material.

The cold rolled steel sheet produced through the heat treatment process of the box annealing furnace is different from the material properties required according to the heating holding temperature and the heating holding time. In the present invention, the annealing holding time and temperature conditions suitable for the use of the high tension cold rolled steel sheet for automobiles are shown. With the main focus on the material A and B steel, the heating and holding temperature and heating holding time conditions are 440MPa class target value (YP: 25 ~ 40kg / mm2, TS: 45kg / mm2 or more, El.:47% In order to enable the above), the following tests were conducted to find the optimum conditions for the heat treatment process conditions of the box annealing furnace such as the holding temperature range and the holding time range.

<Experiment Method>

The present invention was tested using material A and material B.

The materials are blast furnace material (hereinafter A material) of 0.1938% and blast furnace material (hereinafter B material) of 0.1938% as shown in Table 1.

Prior to the experiment of the box annealing furnace heat treatment, in order to reproduce the pickling and cold rolling process, the hot rolled material was pickled with hydrochloric acid to remove the oxide layer on the surface of the hot rolled steel sheet and cold rolled at room temperature to be used as a test material. Cold reduction rate was 70%, and the thickness of the steel plate was set to 1.0 mm, respectively.

Box annealing furnace heat treatment simulation test (BAF Simulation Test) using the Electric Furnace BAF Simulator (Max. Heating temperature: 1000 ℃) was used as the atmosphere gas 100% hydrogen (H ₂ ).

The specimen size was 240mm x 47mm and then tested by setting the holding temperature and the holding time, respectively, based on the basic heat treatment cycle of FIG.

Table 2 shows the temperature of each step of the basic heat treatment cycle of FIG.

As shown, steps 1 to 3 are made of a heating step, step 4 is a holding step, and steps 5 to 8 are cooling steps.
First, as shown in Table 2 and FIG. 1, the heating step of the box annealing furnace heat treatment method is heated to 300 ° C. in a range of 0 to 3 hours, to 720 ° C. in a range of 3 to 8 hours, and to 8 to 12 hours. In the manner of heating to 760 ℃.
Next, the heating maintenance step is maintained at 760 ℃ in the range of 12 to 22 hours.
Next, the cooling step cools down to 680 ° C in the range of 22 to 26 hours, to 550 ° C in the range of 26 to 30 hours, to 350 ° C in the range of 30 to 34 hours, and 80 in the range of 34 to 45 hours. It is carried out by cooling to ℃.
In addition, it is preferable that the total running time of the heating step, the heating holding step and the cooling step has a cycle of 45 hours when the experimental method is synthesized.

Then, using the A material and B material, the test was carried out by changing the holding temperature of step 4 to 720 ℃, 740 ℃, 760 ℃. In addition, the temperature of the cold spot at each holding temperature was tested by setting the temperature 10 ℃ lower than the holding temperature.

Here, the hot spot defines the maximum heating portion of the cold rolled steel sheet, the cold spot refers to the minimum heating portion of the cold rolled steel sheet, the cold point is lower than the holding temperature of the cold rolled steel sheet when entering the box annealing furnace This means that the temperature difference across the entire surface is within 10 ° C.
At this time, the number of test pieces was prepared in the rolling direction 2EA, rolling 45 degree direction 1EA, and rolling right angle direction 3EA, respectively.Thermocouple was attached to the edge of the specimen to check the furnace setting temperature and the actual specimen temperature. After the operation, the temperature distribution (ΔT) was confirmed.

In order to evaluate the mechanical properties, specimens of ASTM 370 Subsize were processed and subjected to tensile test using a universal tensile tester (Zwick, Z100 / SN5A), yield strength (YS), tensile strength (TS), elongation (El.), The work hardening index (n) was measured, respectively, and the hardness was measured using a Rockwell B-scale hardness tester (Matsuzawa, DXT-3).

And, in order to consider the difference between the carbide distribution and the microstructure of the material, the optical microscope (Nikon, EPIPHOT200) was observed at low and high magnification, respectively.

<Experimental Results>

Figure 2 shows the BAF heat treatment microstructure of the A material.

The upper part is the structure which passed through holding temperature of 720 degreeC, 740 degreeC, and 760 degreeC, respectively, and the lower part reproduces the temperature condition of the cold spot which generate | occur | produces at the holding temperature of upper part.

BAF heat treatment conditions in the experiment were microstructure after changing the holding temperature condition from 720 ° C to 760 ° C based on the same treatment time (45H cycle, 10H holding time). In contrast, carbides are uniformly distributed within grain boundaries and within grain boundaries, and typical Pan-cake textures appear after BAF heat treatment.

Overall uniform grain size is shown. At a high temperature of 760 ° C, a carbide was dissolved in austenite, and then a pearlite transformation appeared around the grain boundary during cooling, and the rest was distributed as ferrite.

Figure 3 shows the BAF heat treatment microstructure of the B material.

As shown in Fig. 2, the upper part is a structure that has undergone holding temperatures of 720 ° C., 740 ° C. and 760 ° C., respectively, and the lower part reproduces a cold spot temperature condition generated at the upper holding temperature.

Compared with material A, the carbon content is relatively low (1085ppm), resulting in less carbides and typical Pan-cake texture characteristics after BAF heat treatment. In material A, which has a high carbon content, the pearlite fraction is higher than that in material B, and the form of carbide distribution is somewhat complicated.

The lower microstructures of FIGS. 2 and 3 are microstructures that appear after heat treatment by inferring cold spot conditions of upper conditions, and are generally hot spot conditions during heat treatment to ensure uniformity of materials after BAF heat treatment. In order to reduce the difference between the cold spot condition and the maximum difference (ΔT) is given.

4 is a graph showing the yield strength (YP) of the A material and B material heat treated under each condition, Figure 5 is a graph showing the tensile strength (TS) of the material A and B heat treated under each condition 6 is a graph showing the elongation (El) of A and B materials subjected to heat treatment under respective conditions, and FIG. 7 is a work hardening index (n) of A and B materials heat treated under respective conditions. 8 is a graph showing Rockwell hardness (HRB) of A and B materials subjected to heat treatment under respective conditions.

Referring to FIG. 4, the yield strength (YP) value of material A is generally lower than that of material B. In material B, since the ratio of Si and P components is relatively higher than that of material A, yield strength ( YP) is high.

Looking at the results according to the temperature conditions in Figure 4, it can be seen that the YP value is not satisfied below 730 ℃, the YP value shows a tendency to increase to a constant value as the temperature increases. In addition, it can be seen that the yield strength value is rather high under the hot spot condition.

Referring to FIG. 5, it can be seen that the tensile strength (TS) value of A and B materials does not satisfy the quality assurance condition under the condition of less than 760 ° C., which contains a large amount of carbon, which is an element of strength improvement, close to about 2000 ppm. In spite of high carbon such as A material, it can be inferred that under BAF heat treatment conditions, there is little effect of precipitation strengthening and solid solution strengthening obtained from general continuous annealing heat treatment, and thus the strength improvement is limited.

Referring to FIG. 6, the elongation (El) value indicates that the material A is not satisfactory at all heat treatment conditions, and the material B is satisfactory at 750 ° C. or higher. As shown in the microstructure of FIG. 3, it is considered that carbides in austenite under BAF heat treatment conditions are improved as the elongation of the material is improved due to the increase in the ferrite fraction due to the perlite transformation during cooling after solid solution.

Referring to FIG. 7, the work hardening index (n) value tends to be somewhat higher than that of material A, which is determined by the difference in carbon content.

Referring to FIG. 8, it can be seen that Rockwell hardness (HRB) also shows a result of B material having a somewhat higher value like n.

As a result of the test, B material is used to satisfy the mechanical characteristics (YP: 25 ~ 45 kg / mm2, TS: 45 kg / mm2, El .: 34% or more) required for 440MPa class high strength automotive steel sheet. It can be seen that it should be maintained for 10 Hr at a holding temperature of 760 ° C.

Although the present invention has been described with reference to the embodiments shown in the drawings, this is merely exemplary, and those skilled in the art to which the art belongs can make various modifications and other equivalent embodiments therefrom. I will understand.

Therefore, the true technical protection scope of the present invention will be defined by the claims below.

1 is a graph showing the time and temperature of the box annealing furnace heat treatment cycle,

Figure 2 is a photograph showing the microstructures heat-treated by the annealing of the material A,

Figure 3 is a photograph showing the microstructure heat-treated by the box annealing of B material,

Figure 4 is a graph showing the yield strength (YP) of the A material and B material heat-treated under each condition,

Figure 5 is a graph showing the tensile strength (TS) of the A material and B material heat-treated under each condition,

Figure 6 is a graph showing the elongation (El) of the A material and B material heat-treated under each condition,

7 is a graph showing the work hardening index (n) of the A material and the B material heat-treated under each condition;

8 is a graph showing Rockwell hardness (HRB) of A and B materials subjected to heat treatment under respective conditions.

Claims (6)

C: 1085 ppm (hereinafter by weight), Si: 509 ppm, Mn: 11000 ppm, P: 186 ppm, S: 34 ppm, Cr: 166 ppm, Mo: 29 ppm, Al: 253 ppm, N: 21 ppm, O: 17 ppm, the remainder is Fe and inevitable impurities In the box annealing furnace heat treatment method consisting of a heating step, a heating holding step, a cooling step using a cold rolled steel sheet having a chemical composition constituting a total amount of 100% by weight, The heating and maintaining step is maintained by heating maintenance time 10 ± 2 hours in the heating and maintenance temperature range of 760 ℃ ± 15 ℃, Box annealing furnace heat treatment method of the cold rolled steel sheet characterized in that it satisfies the yield strength 25 ~ 40 kg / ㎠, tensile strength 45 kg / ㎠ or more, elongation 34% or more. The method of claim 1, The heating step of the box annealing furnace heat treatment method is made by heating to 300 ℃ in the range of 0 to 3 hours, to 720 ℃ in the range of 3 to 8 hours, to 760 ℃ in the range of 8 to 12 hours, Cooling step of the box annealing furnace heat treatment method is cooled to 680 ℃ in the range of 22 to 26 hours, to 550 ℃ in the range of 26 to 30 hours, to 350 ℃ in the range of 30 to 34 hours, 34 to 45 hours In the range, the cooling is performed to 80 ° C. Box annealing furnace heat treatment method of the cold rolling steel sheet, characterized in that the total running time of the heating step, the heating holding step and the cooling step has a cycle of 45 hours. The method of claim 1, The heating maintenance step Box temperature annealing furnace heat treatment method of the cold-rolled steel sheet, characterized in that the temperature difference between the temperature of the hot spot of the maximum heating portion of the cold rolled steel sheet and the cold spot of the minimum heating portion of the cold rolled steel sheet within 10 ℃. delete The method of claim 1, Box annealing furnace heat-treatment method characterized in that the microstructure is composed of the structure and the ferrite transformed into pearlite. The method of claim 1, The box annealing furnace heat treatment method is a box annealing furnace heat treatment method, characterized in that using the atmosphere gas 100% hydrogen (H ₂ ).

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