TWI808779B - Automobile steel material and method of manufacturing the same - Google Patents

Abstract

The present invention relates to automobile steel material and a method of manufacturing the same. In the method of manufacturing the automobile steel material, a specific heating process and specific cooling processes are performed on a slab with a specific allay composition to manufacture the automobile steel material with a specific metallographic structure such that the resulted automobile steel material has high strength and good processability.

Description

Steel material for automobile and manufacturing method thereof

The present invention relates to a steel for automobiles and a manufacturing method thereof, and in particular to a steel for automobiles with high strength and high processability and a manufacturing method thereof.

In response to the needs of energy saving, environmental protection and safety in the modern automobile industry, steel materials for automobiles must have high deformation resistance and high formability, so that when the purpose of energy saving and environmental protection is achieved by reducing the weight of the steel, the workability and strength of the steel can not be lost, so as to ensure driving safety.

The conventional manufacturing method of steel for automobiles is to manufacture dual-phase steel, and its metallographic structure may include a main phase of ferrite phase (such as about 80 volume percent to about 95 volume percent) and a second phase of hemp iron phase (such as about 5 volume percent to about 20 volume percent). While this dual phase steel offers good formability, it suffers from poor deformation strength.

Another conventional manufacturing method of steel for automobiles is to manufacture multi-phase steel, and its metallographic structure may include a first phase of ferrite phase (such as about 35 volume percent to about 65 volume percent), a second phase of ductile iron phase (such as about 30 volume percent to about 51 volume percent), and a third phase of hemp iron phase (such as about 5 volume percent to about 15 volume percent). However, the manufacturing method of this multi-phase steel can only improve the strength and processability of the steel to a limited extent, and cannot make it have both high deformation resistance and high formability.

In view of this, there is an urgent need to develop a new steel for automobiles and a manufacturing method thereof to improve the above-mentioned shortcomings.

In view of the above problems, one aspect of the present invention is to provide a method for manufacturing steel for automobiles. This manufacturing method is to use a steel billet with a specific alloy composition, a specific heating treatment and a specific cooling treatment to produce a steel for automobiles with a specific metallographic structure, so that it has both high strength and good processability.

Another aspect of the present invention is to provide a steel for automobiles, which is produced by the aforementioned manufacturing method.

According to an aspect of the present invention, a method for manufacturing steel for automobiles is proposed. In this manufacturing method, the steel billet is heated to obtain a heated steel billet, wherein the heating temperature of the heat treatment is 1150°C to 1300°C. The steel billet contains 0.08 to 0.20 weight percent of carbon, 0.2 to 1.25 weight percent of silicon, 1.0 to 2.5 weight percent of manganese, 0.02 to 0.70 weight percent of chromium, 0.15 to 0.30 weight percent of molybdenum, 0.02 to 0.045 weight percent of aluminum, not more than 0.02 weight percent of phosphorus, not more than 0.015 weight percent of sulfur, etc. Amount of iron and unavoidable impurities. Then, hot rolling is carried out on the heated steel billet to obtain finished steel products, wherein the finishing temperature of the hot rolling treatment is 880° C. to 950° C. Coil processing is then performed on the rolled steel to obtain hot-rolled steel coils. Cold rolling is performed on hot rolled steel coils to obtain cold rolled steel products. Annealing the cold-rolled steel to obtain the annealed steel, wherein the annealing temperature of the annealing is greater than 860°C. Cool the annealed steel to 550°C to 750°C at a pre-cooling rate of 3°C/sec to 20°C/sec. Cool the pre-cooled annealed steel to 350°C to 400°C at a recooling rate of 20°C/sec to 100°C/sec to obtain cooled steel. Cooled steel is overaged to obtain steel for automobiles. Steel materials for automobiles contain not less than 80 volume percent of Matian loose iron phase and not more than 20 volume percent of ferrite phase and ductile iron phase.

According to an embodiment of the present invention, the coiling temperature of the coiling treatment is 500°C to 700°C.

According to another embodiment of the present invention, after the coiling process, the manufacturing method optionally includes pickling.

According to yet another embodiment of the present invention, the cold rolling ratio of the cold rolling treatment is not less than 40%.

According to yet another embodiment of the present invention, the annealing time of the annealing treatment is 90 seconds to 10 minutes.

According to yet another embodiment of the present invention, the overaging treatment is performed at an overaging temperature of 150° C. to 400° C. for 2 minutes to 25 minutes.

According to another embodiment of the present invention, after the aging treatment, the manufacturing method optionally includes cooling the automotive steel to room temperature.

According to another aspect of the present invention, a steel for automobiles is proposed, which is produced by the above-mentioned manufacturing method of steel for automobiles, wherein the steel for automobiles comprises a matian loose iron phase, a fertile iron phase, and a toughened iron phase, and the ratio of the volume percentage of the matian loose iron phase and the total volume percentage of the fertile iron phase and the toughened iron phase is 4 to 5.

According to an embodiment of the present invention, the volume percentage of the Worth field iron phase in the steel for automobiles is not more than 5%.

According to one embodiment of the present invention, the steel for automobiles has a tensile strength of not less than 1300MPa, a yield strength of not less than 1030MPa and an elongation of not less than 3%, and the 90-degree bending radius of the steel for automobiles is less than 4 times the thickness of the steel for automobiles.

The method of manufacturing steel for automobiles according to the present invention includes performing specific heating and cooling treatments on steel billets with specific alloy compositions to produce steel for automobiles with a specific metallographic structure, thereby making it both high-strength and good workability.

The making and using of embodiments of the invention are discussed in detail below. It should be appreciated, however, that the embodiments provide many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are illustrative only and do not limit the scope of the invention.

The manufacturing method of the steel for automobile of the present invention is to carry out specific heating and cooling treatment on the billet with specific alloy composition, so that the single-phase steel for automobile with ultra-high deformation resistance strength and good formability can be produced, and then the energy saving, environmental protection and safety requirements of the automobile industry can be achieved. Among them, the so-called "single-phase steel" refers to the metallographic structure of the steel with not less than 80 volume percent of Matian loose iron phase and not more than 20 volume percent of ferrite phase and ductile iron phase.

The "anti-deformation strength" referred to in the present invention refers to the evaluation of tensile strength and yield strength. When the tensile strength is not less than 1300MPa and the yield strength is not less than 1030MPa, the steel has ultra-high deformation resistance (hereinafter referred to as strength). The "formability" referred to in the present invention refers to evaluation by elongation and bendability. When the elongation is not less than 3%, and the 90-degree bending radius is less than 4 times the thickness, the steel has good formability (hereinafter referred to as formability).

Please refer to FIG. 1 , the manufacturing method 100 of steel for automobiles first heats the steel billet to obtain the heated steel billet, as shown in operation 110 . The steel billet contains 0.08 to 0.20 weight percent of carbon, 0.2 to 1.25 weight percent of silicon, 1.0 to 2.5 weight percent of manganese, 0.02 to 0.70 weight percent of chromium, 0.15 to 0.30 weight percent of molybdenum, 0.02 to 0.045 weight percent of aluminum, not more than 0.02 weight percent of phosphorus, not more than 0.015 weight percent of sulfur, etc. amount of iron, and unavoidable impurities.

If the carbon content is less than 0.08% by weight, the tensile strength and/or yield strength of the produced steel is too low, reducing its strength. Conversely, if the carbon content is greater than 0.20% by weight, the resulting steel does not have proper elongation and proper bendability, thereby reducing its workability. Additionally, in some embodiments, the carbon content may be 0.14 to 0.18 weight percent.

If the silicon content is less than 0.2% by weight, the tensile strength and/or yield strength of the produced steel is too low, reducing its strength. Conversely, if the silicon content is greater than 1.25% by weight, the resulting steel does not have proper elongation and proper bendability, which reduces its workability. Additionally, in some embodiments, the silicon content may range from 0.50% by weight to 1.25% by weight.

If at least one of the manganese content, chromium content and molybdenum content is less than the aforementioned corresponding content, excessive manganese iron and/or ductile iron may be formed in the subsequent pre-cooling treatment, re-cooling treatment and post-cooling treatment. Too much gritty iron and too much ductile iron can unduly reduce the tensile strength and/or yield strength of the steel, or unduly increase the elongation and bendability. Conversely, if at least one of the manganese content, chromium content and molybdenum content is greater than the above-mentioned corresponding content, excessive moss iron will be formed during these cooling treatments, and the tensile strength and/or yield strength of the steel will be excessively increased, thus reducing its workability.

Aluminum is a deoxidizer for steelmaking. If the aluminum content is less than 0.02 weight percent, the deoxidation effect will be poor. If the aluminum content is greater than 0.045% by weight, it will not help to improve the strength, but will only increase unnecessary costs.

Phosphorus and sulfur are derived from impurities in the raw materials of casting billets or inevitable impurities produced in the process. Both phosphorus and sulfur will reduce the strength of steel, so the content of the two must be controlled within the corresponding content range mentioned above.

In some embodiments, at least one of niobium, titanium, vanadium and copper can be substantially excluded from the constituent metal elements of the steel billet of the present invention, that is, the manufacturing method of the steel for automobiles of the present invention can exclude the use of steel billets containing niobium, titanium, vanadium and/or copper, so as to reduce costs.

The temperature of heat treatment is 1150°C to 1300°C. This heat treatment is used to solidify the alloy components of the steel billet to form the Wostian iron phase. If the heat treatment temperature is lower than 1150°C, the steel billet cannot be completely solidified, and the full-Worth field iron phase cannot be formed, so a single-phase steel with a specific metallographic structure cannot be produced. On the contrary, if the heat treatment temperature is higher than 1300°C, although the aforementioned single-phase steel can be produced, it is a waste of energy and does not conform to the economic effect. In some embodiments, the heating time of the heat treatment may be 2 hours to 4 hours, so as to completely solidify the steel billet, thereby facilitating the formation of the Worth field iron phase.

After operation 110 , hot rolling is performed on the heated billet to obtain finished rolled steel, as shown in operation 120 . The finishing temperature of the hot rolling treatment is 880°C to 950°C. If the finishing temperature is lower than 880°C, the rolling force of hot rolling will be increased, making it difficult to carry out hot rolling. Conversely, if the finish rolling temperature is higher than 950°C, although the hot-rolling elongation force will be reduced, it is easy to form a hot-rolled structure of a fertile iron phase and a grainy iron phase during subsequent cooling and coiling, so that rolling cracks may occur during subsequent cold rolling. In addition, the hot-rolling rate of the aforementioned hot-rolling treatment may be a hot-rolling rate commonly used by those with common knowledge.

After operation 120 , the rolled steel is coiled to obtain a hot rolled steel coil, as shown in operation 130 . When the coiling temperature is 500°C to 700°C, it is more beneficial to control the Matian loose iron phase, fertilized iron phase and toughened iron phase within the above-mentioned specific range, so the strength and workability of the steel are further improved.

In some embodiments, after the coiling treatment, the manufacturing method may optionally include pickling treatment, wherein an acid solution is used to remove surface defects of the hot-rolled steel coil to improve its surface quality.

After operation 130 , the hot-rolled steel coil is subjected to cold-rolling treatment to obtain cold-rolled steel, as shown in operation 140 . The cold rolling rate of cold rolling treatment is not less than 40% to refine the metallographic structure, so the strength and processability of the steel can be improved. Preferably, the cold rolling rate can be 40% to 60%.

After operation 140 , the cold-rolled steel is annealed to obtain annealed steel, as shown in operation 150 . If the temperature of the annealing treatment is not higher than 860°C, it is easy to generate more ferrite phases and reduce the strength of the steel. Preferably, the temperature of the annealing treatment is greater than 860° C. and not greater than 880° C., and the temperature is maintained for 90 seconds to 10 minutes at a temperature within this range. When the temperature holding time is within the aforementioned range, it is beneficial to control the volume percentages of ferrite phase, ductile iron and hemp iron to meet the aforementioned specific range of the present invention.

After operation 150 , the annealed steel is cooled to 550° C. to 750° C. at a pre-cooling rate of 3° C./s to 20° C./s, that is, pre-cooling treatment, as shown in operation 160 . If the pre-cooling rate is lower than 3°C/sec, too much fertilizer granular phase will be formed, which will reduce the strength of the steel. On the contrary, if the pre-cooling rate is higher than 20° C./sec, the operation capacity of the cooling equipment is exceeded, and the pre-cooling treatment cannot be performed.

Furthermore, if it is cooled to an excessively low temperature (ie lower than 550° C.), too much ductile iron phase will be formed, thereby reducing the strength of the steel. Conversely, if it is cooled to an excessively high temperature (that is, higher than 750°C), it will exceed the temperature resistance of the cooling equipment.

After operation 160 , the pre-cooled annealed steel material is cooled to 350° C. to 400° C. at a recooling rate of 20° C./s to 100° C./s, that is, recooled, as shown in operation 170 . If the recooling rate is lower than 20°C/sec, too much ductile iron phase will be formed, which will reduce the strength of the steel. Conversely, if the recooling rate is higher than 100°C/sec, the operation capacity of the cooling equipment is exceeded, and recooling cannot be performed. Furthermore, if it is cooled to too low a temperature (that is, lower than 350°C), too much loose iron will be formed, and the workability of the steel will be reduced. Conversely, if it is cooled to an excessively high temperature (that is, higher than 400°C), too much ductile iron phase will be formed, which will reduce the strength of the steel.

In some embodiments, the re-cooling treatment is directly followed by the pre-cooling treatment, that is, after the pre-cooling treatment is completed, the re-cooling treatment is performed immediately without holding the temperature, so as to facilitate the control of the volume percentages of the ferrite phase, ductile iron and hematite loose iron within the specific range of the aforementioned invention.

After operation 170 , the cooled steel is subjected to an aging treatment to obtain steel for automobiles, as shown in operation 180 . In some embodiments, the overaging treatment is to treat the cooled steel material at an overaging temperature of 150° C. to 400° C. for 2 minutes to 25 minutes. When the temperature and time of the overaging treatment are under the aforementioned conditions, it is not only beneficial to control the volume percentages of ferrite phase, ductile iron, and mottled iron within the specific range of the aforementioned invention, but also enables the formed hard and brittle mottled iron to obtain the softening effect of tempered carbide precipitation, thereby improving the workability of steel. Preferably, the temperature of the overaging treatment may be 150°C to 350°C, so that the average particle size of the carbides is 10nm to 100nm, thus further improving the workability of the steel.

In some embodiments, after performing operation 180 , the manufacturing method 100 may optionally include a post-cooling treatment to cool the automotive steel to room temperature.

In some embodiments, after the pre-cooling and/or re-cooling, the manufacturing method 100 can eliminate the use of tempering, so as to shorten the process time and prevent the quenching-tempering process from cooling before heating, thereby saving energy costs.

The steel material for automobiles of the present invention is produced by the aforementioned manufacturing method of the present invention. Steel materials for automobiles contain not less than 80 volume percent of the Matian loose iron phase, and not more than 20 volume percent of the ferrite phase and the ductile iron phase. In some preferred embodiments, the ratio of the volume percentage of the mosaic iron phase and the total volume percentage of the fertile iron phase to the ductile iron phase of the steel for automobiles is 4 to 5. When the metallographic structure of automotive steel meets the aforementioned conditions, its strength and workability can be improved.

In some embodiments, the steel for automobiles does not contain the snow-white carbon-iron phase, so as to improve its strength and workability.

In terms of strength, the tensile strength of the automotive steel can be not less than 1300MPa, and preferably 1300MPa to 1400MPa. In addition, the yield strength of steel for automobiles may be not less than 1030 MPa, and preferably 1030 MPa to 1200 MPa.

In terms of workability, the elongation of steel for automobiles is greater than 3%, preferably 3% to 10%, and more preferably 7% to 8%. In addition, the 90-degree bending radius of the automotive steel material is less than 4 times the thickness of the automotive steel material, preferably greater than 2.5 times and less than 4 times the thickness of the automotive steel material, and more preferably 2 to 3 times the thickness of the automotive steel material.

When the tensile strength, yield strength, elongation and 90-degree bending radius of the steel for automobiles are within the above-mentioned corresponding ranges, the steel can have both high deformation resistance and good formability, so as to meet the energy-saving, environmental protection and safety requirements of the automobile industry.

In some embodiments, the yield ratio of the steel may be 0.75 to 0.90, and preferably 0.80 to 0.85. When the yield ratio of the steel is in the aforementioned range, the steel can have better deformation resistance.

The following examples are used to illustrate the application of the present invention, but they are not intended to limit the present invention. Anyone skilled in this art can make various changes and modifications without departing from the spirit and scope of the present invention.

Preparation of steel

Example 1

The steel in Example 1 is prepared by heating the billet at 1150°C to 1300°C for 2 hours to 4 hours, then hot rolling the heated billet at a finishing temperature of 880°C to 950°C, and then coiling the rolled steel at a coiling temperature of 500°C to 700°C. Then the cold-rolled steel is subjected to annealing treatment, pre-cooling treatment, re-cooling treatment, over-aging treatment and post-cooling treatment in sequence, wherein the over-aging treatment is at 150°C to 350°C, the temperature is maintained for more than 2 minutes to 25 minutes, and the post-cooling treatment is to cool the steel to room temperature, so as to obtain the steel of Example 1. Then, the evaluation was performed by the evaluation method described later.

Examples 2 to 3 and Comparative Examples 1 to 3

The steel materials of Examples 2 to 3 and Comparative Examples 1 to 3 were produced in a method similar to that of Example 1. The difference is that the composition of the steel billet was changed in Examples 2 to 3, and the composition and recooling treatment of the steel billet were changed in Comparative Examples 1 to 2, and the composition and heat treatment of the steel billet were changed in Comparative Example 3. The specific conditions and evaluation results of the aforementioned Examples 1 to 3 and Comparative Examples 1 to 3 are shown in Table 1 and Figures 2 to 3 below.

Evaluation method

1. Test of tensile strength

The test of tensile strength is carried out according to the standard method CNS 2112, G2014, and the conditions are those used by those with ordinary knowledge.

2. Test of yield strength

The test of yield strength is carried out according to the standard method CNS 2112, G2014, and the conditions are those used by those with ordinary knowledge.

3. Elongation test

The elongation test is carried out according to the standard method CNS 2112, G2014, and the gauge length is 50mm, and the conditions are those used by those with common knowledge.

4. Bending test

The bending test is based on the V block method specified in the standard method JIS Z 2248 to evaluate the 90-degree bending radius, and the conditions are those that are used by those with ordinary knowledge.

5. Test of metallographic structure

The test of the metallographic structure is to use a scanning electron microscope to capture images of the automobile steel materials of each embodiment and each comparative example, and measure the area percentages of the ferrite phase, toughened iron phase, and hemp iron phase in the image. The conditions are those that are commonly used by those with ordinary knowledge. The results of the metallographic structure are shown in Table 1, Figure 2 and Figure 3 below. The symbol "F" in the column of Table 1 indicates the ferrite phase, the symbol "B" indicates the ductile iron phase, and the symbol "M" indicates the Ma Tian loose iron phase. For example, "F+B+M" means that the metallographic structure has a ferrite phase, a ductile iron phase, and a loose iron phase, and so on. In addition, the symbol "O" in the judgment column indicates that the steel contains no less than 80% volume percentage of Ma Tian iron phase, the sum of the volume percentages of fertile iron phase and ductile iron phase is no more than 20%, and the volume percentage of Wast iron phase is no more than 5%. On the contrary, the symbol "X" indicates that the steel does not meet the aforementioned conditions. "α", "B" and "M" in Fig. 2 and Fig. 3 indicate ferrite phase, ductile iron phase and hematian loose iron phase respectively.

Table 1

Please refer to Table 1, Figure 2 and Figure 3. Examples 1 to 3 control the alloy composition of the steel billet and the conditions of heat treatment and recooling treatment to obtain a single-phase steel with a specific metallographic structure (that is, no less than 80 volume percent of the grain iron phase and no more than 20 volume percent of the ferrite phase and ductile iron phase).

However, comparative examples 1 and 2 used too high temperature for recooling treatment, and the steel produced was multiphase steel, and its metallographic structure contained ferrite phase, ductile iron phase and hemp iron, and the volume percentages of the three were relatively close, so the strength of the steel was reduced.

In Comparative Example 3, the heat treatment was performed at a temperature that was too low, and the steel produced was a dual-phase steel, and its metallographic structure contained ferrite phase and hemp iron, and the ratio of the volume percentage of the two was close to 1, so the strength of the steel was reduced.

To sum up, the manufacturing method of the automotive steel of the present invention is to perform specific heating and cooling treatments on the steel billet with a specific alloy composition, so as to obtain the automotive steel with a specific metallographic structure, and then make it have both high strength and good processability.

Although the present invention has been disclosed above in terms of implementation, it is not intended to limit the present invention. Anyone with ordinary knowledge in the technical field of the present invention can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention should be defined by the scope of the appended patent application.

100: method 110,120,130,140,150,160,170,180: Operation

In order to have a more complete understanding of the embodiments of the present invention and their advantages, please refer to the following descriptions together with the corresponding drawings. It must be emphasized that the various features are not drawn to scale and are for illustration purposes only. The contents of relevant diagrams are explained as follows: FIG. 1 is a flowchart illustrating a method for manufacturing steel for automobiles according to an embodiment of the present invention. FIG. 2 is a scanning electron microscope image of a steel material for automobiles according to an embodiment of the present invention. FIG. 3 is a scanning electron microscope image of a steel material for automobiles according to a comparative example of the present invention.

100: method

110,120,130,140,150,160,170,180: Operation

Claims (6)

A method for manufacturing steel for automobiles, comprising: performing a heating treatment on a steel billet to obtain a heated steel billet, wherein a heating temperature of the heating treatment is 1150° C. to 1300° C., a heating time of the heating treatment is 2 hours to 4 hours, and the steel billet contains: 0.08 weight percent to 0.20 weight percent carbon; 0.2 weight percent to 1.25 weight percent silicon; 1.0 weight percent to 2.5 weight percent manganese; 0.02 weight percent to 0.70 Chromium by weight; 0.15 to 0.30 weight percent molybdenum; 0.02 to 0.045 weight percent aluminum; not more than 0.02 weight percent phosphorus; not more than 0.015 weight percent sulfur; the balance of iron; Steel coil, wherein the coiling temperature of the coiling treatment is 500°C to 700°C; performing a cold rolling treatment on the hot-rolled steel coil to obtain a cold-rolled steel material, wherein a cold rolling rate of the cold-rolling treatment is 40% to 60%; performing an annealing treatment on the cold-rolled steel material to obtain an annealed steel material, wherein an annealing temperature of the annealing treatment is greater than 860°C and not greater than 880°C, and an annealing time of the annealing treatment is 90 seconds to 10 minutes; The annealed steel is cooled to 550°C to 750°C at a pre-cooling rate of 3°C/s to 20°C/s; the pre-cooled annealed steel is cooled to 350°C to 400°C at a recooling rate of 20°C/s to 100°C/s to obtain a cooled steel; to 25 minutes, and the steel for automobiles contains: not less than 80% by volume of a Matian iron phase; and not more than 20% by volume of a ferrite phase and a ductile iron phase. The manufacturing method of steel for automobiles as described in claim 1, wherein after the coiling treatment, the manufacturing method further includes a pickling treatment. The manufacturing method of the steel for automobile according to claim 1, wherein after the overaging treatment, the manufacturing method further includes cooling the steel for automobile to a room temperature. A steel for automobiles is produced by the manufacturing method as described in any one of claims 1 to 3, wherein the steel for automobiles comprises a grainy iron phase, a fertile iron phase, and a ductile iron phase, and a ratio of a volume percentage of the grainy iron phase and a total volume percentage of a fertile iron phase to a ductile iron phase is 4 to 5. The steel for automobiles as described in claim 4, wherein the volume percentage of the ferrite phase of the steel for automobiles is not more than 5%. The steel for automobiles as described in Claim 4, wherein the steel for automobiles has a tensile strength of 1300MPa to 1400MPa, a yield strength of 1030MPa to 1200MPa, and an elongation of 3% to 10%, and a 90-degree bending radius of the steel for automobiles is greater than 2.5 times and less than 4 times the thickness of the steel for automobiles.