TWI703220B - Automobile steel and method of manufacturing the same - Google Patents

Abstract

The present invention relates to an automobile steel and a method of manufacturing the same. Slab with specific composition is subjected to rolling process and heat treatment process with specific condition for changing the metallographic structure of an automobile steel. The obtained automobile steel has specific amounts and proportion of metallurgical structure, so that the automobile steel has excellent tensile strength, elongation and product of strength and ductility.

Description

Automobile steel and manufacturing method thereof

The present invention relates to a steel for automobiles and a manufacturing method thereof, in particular to a steel for automobiles with a specific metallographic structure and a manufacturing method thereof.

Considering safety and energy efficiency, automotive steel is required to have high strength, light weight and high formability. Generally speaking, high-strength steel with a tensile strength higher than 340MPa can be used, and ultra-high-strength steel with a tensile strength higher than 780MPa.

Currently, dual-phase steel and quenching and partitioning (Quenching and Partitioning, QP) steel are used as automotive steel. The dual-phase steel has a high work hardening rate and can prevent local deformation during the stamping process. The low yield ratio and good bake hardenability of the dual-phase steel are conducive to improving the formability, dent resistance and impact absorption of the steel. In order to make the dual-phase steel have both high strength and good formability, generally the ratio of the loose iron in the metallographic structure of the dual-phase steel is greatly increased, and the ratio of the ferrite iron is reduced, but the high-strength dual-phase steel obtained Steel has low elongation. For example, a dual-phase steel with a tensile strength of 980 MPa or more may have an elongation of 14% or less. Therefore, high-strength dual-phase steel can only be subjected to simple bending processing, and cannot be cold-formed, and its application will be limited.

Secondly, the metallurgical structure of QP steel contains more than 65% of matian loose iron and tempered matian loose iron, more than 25% austenitic iron, and less than 10% fat grain iron and toughened iron. Therefore, the tensile strength of QP steel can be as high as 980MPa to 1862MPa, and the strong plastic product can be as high as 20GPa% to 32.5GPa%. However, QP steel is made through a quenching and distribution process. Among them, the quenching and distribution process is to directly quench the annealed steel sheet. In addition, the manufacturing process of QP steel includes a tempering step. Accordingly, the manufacturing process of QP steel is cumbersome and the equipment is relatively expensive, making it difficult to meet application requirements.

In view of this, it is urgent to provide a steel for automobiles and a manufacturing method thereof to solve the above-mentioned problems.

Therefore, one aspect of the present invention is to provide a method for manufacturing automobile steel, which involves subjecting a steel blank with a specific composition ratio to a rolling process and a heat treatment process with specific conditions to change the quality of the automobile steel obtained. Microstructure.

Another aspect of the present invention is to provide a steel for automobiles, which is produced by the aforementioned method. The obtained automobile steel has a metallographic structure with a specific content and proportion, so that the automobile steel has both high tensile strength, high elongation and high strength plastic product.

According to the above aspect of the present invention, a method for manufacturing automobile steel is proposed. First, the steel blank is heated to obtain a heated steel blank, wherein the heating temperature of the heating step is 1150°C to 1300°C. The steel billet includes 0.10% to 0.25% by weight of carbon, 0.5% to 2.0% by weight of silicon, 1.5% to 3.0% by weight of manganese, 0.04% to 1.0% by weight of aluminum, 0.02% to 0.7% by weight Chromium, not more than 0.02% by weight of molybdenum, not more than 0.02% by weight of phosphorus, not more than 0.015% by weight of sulfur, and the balance is iron and unavoidable impurities. Then, a rolling process is performed on the heated steel blank to form a rolled steel sheet. Then, the rolled steel sheet is subjected to an annealing step to form an annealed steel sheet, wherein the annealing step is performed at an annealing temperature of 740°C to 860°C for 90 seconds to 600 seconds.

Then, a cooling step is performed to cool the annealed steel sheet to 350°C to 500°C to form a cooled steel sheet. Afterwards, the cooling steel plate is subjected to an over-aging process to obtain automotive steel, wherein the over-aging process is treated at an over-aging temperature of 350°C to 500°C for 2 minutes to 25 minutes. The manufacturing method excludes the tempering step, and the automotive steel contains fat iron, toughened iron and austenitic iron, of which austenitic iron is at least 10%.

According to an embodiment of the present invention, the heating time of the above heating step is 2 hours to 4 hours.

According to an embodiment of the present invention, the finishing temperature of the aforementioned rolling process is 880°C to 950°C.

According to an embodiment of the present invention, the aforementioned cooling step is to cool the annealed steel sheet to 550°C to 750°C at a cooling rate of 5°C/sec to 20°C/sec, and at a rate of 20°C/sec to 100°C/sec Another cooling rate is to cool the annealed steel sheet from 550°C to 750°C to 350°C to 500°C.

According to an embodiment of the present invention, after the rolling process, the above-mentioned manufacturing method may optionally include a coiling step at a coiling temperature of 500°C to 700°C.

According to an embodiment of the present invention, after the coiling step, the above manufacturing method may optionally include a cold rolling step, and the cold rolling step has a cold rolling rate of at least 50% to form a rolled steel sheet.

According to another aspect of the present invention, an automobile steel is provided, which is obtained by the above-mentioned manufacturing method of automobile steel. Automobile steel contains 0.10% to 0.25% by weight of carbon, 0.5% to 2.0% by weight of silicon, 1.5% to 3.0% by weight of manganese, 0.04% to 1.0% by weight of aluminum, and 0.02% to 0.7% by weight % Of chromium, not more than 0.02% by weight of molybdenum, not more than 0.02% by weight of phosphorus, not more than 0.015% by weight of sulfur, and the balance is iron and unavoidable impurities. Automobile steel contains fat iron, toughened iron and austenitic iron, of which austenitic iron is at least 10%.

According to an embodiment of the present invention, the above-mentioned automobile steel may optionally contain no more than 10% of Astian loose iron, 30% to 60% of ferrite iron, and 30% to 60% of the toughened iron.

According to an embodiment of the present invention, the tensile strength of the aforementioned automobile steel is at least 780Mpa, and the elongation of the automobile steel is at least 20%.

According to an embodiment of the present invention, the strong plastic product of the aforementioned automobile steel is at least 20 GPa%.

The automobile steel and the manufacturing method thereof of the present invention can change the metallographic structure of the obtained automobile steel by subjecting a steel blank with a specific composition ratio to a rolling process and a heat treatment process with specific conditions. The obtained automobile steel has a metallographic structure with a specific content and proportion, so that the automobile steel has both high tensile strength, high elongation and high strength plastic product.

Based on the foregoing, the present invention provides a steel for automobiles and a method for manufacturing the same. The steel blanks with specific composition ratios are subjected to rolling and heat treatment processes with specific conditions to change the gold of the automobile steel obtained. Phase organization. The obtained automobile steel has a metallographic structure with a specific content and ratio, so that the automobile steel has both high tensile strength, high elongation and high strength plastic product without using more complicated steps and more expensive equipment.

Please refer to FIG. 1, which shows a flowchart of a method 100 for manufacturing automobile steel according to an embodiment of the present invention. First, as shown in step 110 of the method 100, a steel blank is provided. The steel billet is obtained through agglomeration or continuous casting, and includes 0.10% to 0.25% by weight of carbon, 0.5% to 2.0% by weight of silicon, 1.5% to 3.0% by weight of manganese, and 0.04% to 1.0% by weight. 0.02% to 0.7% by weight of chromium, not more than 0.02% by weight of molybdenum, not more than 0.02% by weight of phosphorus, not more than 0.015% by weight of sulfur, and the remainder is iron and unavoidable impurities.

When the carbon content in the aforementioned raw materials is less than 0.10% by weight, the hardness of the steel will be insufficient. If the carbon content is more than 0.25% by weight, it will lead to poor elongation of the steel and cracking workability.

When the silicon content in the aforementioned raw materials is less than 0.5% by weight, the strength of the steel cannot be effectively improved. If the silicon content is more than 2.0% by weight, the temper brittleness of the steel will increase and the ductility will decrease.

When the manganese content in the aforementioned raw materials is less than 1.5% by weight, effective desulfurization and deoxidation cannot be achieved, resulting in failure to increase the wear resistance and strength of the steel. If the manganese content is more than 3.0% by weight, the elongation of the steel will be reduced, and the cracking workability will be reduced.

When the aluminum content in the aforementioned raw materials is less than 0.04% by weight, the steel cannot have sufficient elongation and impact toughness. If the aluminum content is more than 1.0% by weight, it will affect welding performance and cracking processability.

When the chromium content in the aforementioned raw materials is less than 0.02% by weight, the hardness, strength and plasticity of the steel cannot be improved. If the chromium is 0.7% by weight, the steel has reached the required mechanical properties.

When the molybdenum content in the aforementioned raw materials is greater than 0.02% by weight, the hardness of the steel is too high and the elongation is insufficient, resulting in cracking workability. When the phosphorus content in the aforementioned raw materials is greater than 0.02% by weight, the steel will be brittle at room temperature and reduce impact resistance.

Next, as shown in step 120 of the method 100, the steel blank is heated to obtain a heated steel blank. The heating temperature of the heating step is 1150°C to 1300°C. If the heating temperature falls outside the aforementioned range, the content and proportion of the metallographic structure obtained later will be affected, and the desired mechanical properties will not be achieved. In one embodiment, the heating time of the heating step is 2 hours to 4 hours.

Next, as shown in step 130 of the method 100, the heated steel blank is subjected to a rolling process to form a rolled steel sheet. In one embodiment, the finishing temperature of the rolling process is 880°C to 950°C.

In one embodiment, after performing the rolling process, the manufacturing method of the present invention may optionally include a coiling step at a coiling temperature of 500°C to 700°C. In still other embodiments, the manufacturing method of the present invention may optionally include a pickling step, wherein the pickling step is performed after the coiling step. In yet another embodiment, a cold rolling step can be optionally performed after the pickling step to form a rolled steel sheet, wherein the cold rolling rate of the cold rolling step is at least 50%.

Then, as shown in step 140 of method 100, an annealing step is performed on the rolled steel sheet to form an annealed steel sheet, wherein the annealing step is performed at an annealing temperature of 740°C to 860°C for 90 seconds to 600 seconds, and then at 740°C. C to less than 840°C is preferred. If the annealing temperature is lower than 740°C, the automobile steel will contain too much Asada scattered iron structure. If the annealing temperature is higher than 860°C, the automobile steel will contain too much metamorphic iron structure, so the present invention cannot be obtained. The required specific content and ratio of the metallographic structure will lead to the failure to achieve the required mechanical properties.

Next, as shown in step 150 of the method 100, the annealed steel sheet is cooled to form a cooled steel sheet. In one embodiment, the cooling step of the present invention is to first cool the annealed steel sheet to 550°C to 750°C at a cooling rate of 5°C/sec to 20°C/sec, and then cool the annealed steel sheet at a cooling rate of 20°C/sec to 100°C/sec. Another cooling rate is from 550°C to 750°C to 350°C to 500°C.

After that, as shown in step 160 and step 170 of the method 100, an over-aging process is performed on the cooled steel plate to obtain the automobile steel of the present invention. The over-aging process can transform the high-temperature structure in the steel (ie, austenitic iron) into a medium-temperature structure (ie, toughened iron). In this embodiment, the over-aging process is performed at an over-aging temperature of 350°C to 500°C for 2 minutes to 25 minutes. If the temperature of the over-aging process is lower than 350°C, the automobile steel will contain too much hemp iron structure. If the temperature of the over-aging process is higher than 500°C, the automobile steel will contain too much fat-grained iron structure. Obtaining the specific content and ratio of the metallographic structure required by the present invention, resulting in failure to achieve the required mechanical properties.

The obtained automobile steel is based on fat grain iron, toughened iron and residual austenitic iron, and is doped with a small amount of Asada iron phase. Among them, the content of fertilizer grain iron is 30% to 60%, the content of toughened iron is 30% to 60%, the content of austenitic iron is at least 10%, and the content of Madian loose iron is not more than 10%.

The present invention also discloses a steel for automobiles, which is obtained by the above-mentioned manufacturing method. This automobile steel contains 0.10% to 0.25% by weight of carbon, 0.5% to 2.0% by weight of silicon, 1.5% to 3.0% by weight of manganese, 0.04% to 1.0% by weight of aluminum, and 0.02% to 0.7% by weight. % By weight of chromium, not more than 0.02% by weight of molybdenum, not more than 0.02% by weight of phosphorus, not more than 0.015% by weight of sulfur, and the balance is iron and unavoidable impurities.

The obtained automobile steel has a tensile strength of at least 780Mpa, an elongation of at least 20%, and a strong plastic product of at least 20GPa%.

It should be noted that the manufacturing method of the present invention excludes other additional processes after the over-aging process, to obtain automobile steel with the above-mentioned mechanical properties. In a specific example, the manufacturing method of the present invention does not include a tempering step. Therefore, when the manufacturing method of the present invention is used, cumbersome steps and expensive equipment can be avoided, and automobile steel with both high tensile strength, high elongation and high plastic product can be obtained.

Several embodiments are used below to illustrate the application of the present invention, but they are not used to limit the present invention. Those 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. Retouch. Example 1

First, the steel billet is heated at a temperature of 1150°C to 1300°C (please refer to the composition of the steel billet shown in Table 1) for 2 hours to 4 hours, and then rolled at 880°C to 950°C. Then, the coil is coiled at 500°C to 700°C, and after pickling, the cold rolling process is performed, wherein the rolling rate of the cold rolling process is more than 50%. Next, annealing is performed at 740°C to 860°C for 90 seconds to 600 seconds to form an annealed steel sheet. Then, first cool the annealed steel sheet at a cooling rate of 5°C/sec to 20°C/sec to 550°C to 750°C, and then cool the annealed steel sheet at a cooling rate of 20°C/sec to 100°C/sec to 350°C to 500°C, or directly cool the annealed steel sheet to 350°C to 500°C to form a cooled steel sheet.

Then, the cooling steel plate is subjected to an over-aging process at a temperature of 350°C to 500°C. After 2 minutes to 25 minutes, cool to room temperature to obtain the automobile steel of Example 1. The metallographic structure, tensile strength, elongation and strong plastic product of the produced automobile steel were measured by the following evaluation methods. The results are shown in Table 1, respectively. Examples 2 to 6

Examples 2 to 6 used the same method as Example 1 to produce automotive steel, except that Examples 2 to 6 used different steel blanks from Example 1 (the steel blanks of Examples 2 to 6 had a composition such as Table 1). Comparative examples 1 to 3

Comparative Examples 1 and 3 are commercially available QP steels, and Comparative Example 2 is a commercially available dual-phase steel (the steel blank compositions of Comparative Examples 1 to 3 are shown in Table 1). Evaluation method

The metallographic structure, tensile strength, elongation, and strong plastic product of steel were measured using conventional instruments and methods. The evaluation results are shown in Table 1. In Table 1, the "B" of the metallographic structure represents toughened iron, "F" represents fat iron, "M" represents Asada loose iron, "RA" represents residual austenitic iron, and "TM" represents tempered iron. Matian scattered iron.

Table 1 shows that Examples 1 to 6 and Comparative Example 1 all contain fat iron, toughened iron and austenitic iron. However, after measurement, the ferrous iron content in the steel materials of Examples 1 to 6 is 30% to 60% and the austenitic iron content is at least 10%, while the ferrous iron content in the steel of Comparative Example 1 is less It is 30% and austenitic iron is less than 10%, so the metallographic structure of Comparative Example 1 is different from that of Examples 1 to 6.

Secondly, please refer to FIGS. 2A and 2B, which are respectively the metallographic structure of the steel materials of Example 1 and Comparative Example 3 of the present invention, and the scale ruler of FIGS. 2A and 2B represents 10 μm.

It can be seen from FIG. 2A that the metallurgical structure of Example 1 contains fat grain iron, toughened iron, austenitic iron and doped with a small amount of Asada iron. It can be seen from FIG. 2B that the metallurgical structure of Comparative Example 3 contains tempered Asada iron.

It can be seen from Table 1 that Examples 1 to 6 have excellent tensile strength, elongation and strong plastic product. In contrast, although the tensile strength of Comparative Examples 1 to 2 can reach more than 780MPa, the elongation of Comparative Example 1 is less than 20%, the elongation of Comparative Example 2 is less than 20% and the strong plastic product is less than 20 GPa %. Although Comparative Example 3 can achieve the mechanical properties required by the present invention, the quenching and distribution process of Comparative Example 3 is relatively complicated and the equipment is relatively expensive.

It can be seen from the above-mentioned embodiments that the automotive steel and the manufacturing method thereof of the present invention have the advantage of subjecting a steel blank with a specific composition ratio to a rolling process and a heat treatment process with specific conditions to change the quality of the automotive steel obtained. Microstructure. The obtained automobile steel has a metallographic structure with a specific content and proportion, so that the automobile steel has both high tensile strength, high elongation and high strength plastic product.

Although the present invention has been disclosed in several embodiments as above, it is not intended to limit the present invention. Anyone with ordinary knowledge in the technical field to which the present invention pertains can make various modifications without departing from the spirit and scope of the present invention. Modifications and modifications, therefore, the scope of protection of the present invention shall be subject to the scope of the attached patent application.

100: method

10/120/130/140/150/160/170: steps

In order to make the above and other objectives, features, advantages and embodiments of the present invention more comprehensible, the detailed description of the accompanying drawings is as follows [Figure 1] is a flow chart showing a method for manufacturing automobile steel according to an embodiment of the present invention. [Fig. 2A] and [Fig. 2B] respectively show the metallurgical structure of the automobile steel prepared in Example 1 and Comparative Example 3 of the present invention.

100: method

110/120/130/140/150/160/170: steps

Claims (10)

A method for manufacturing automobile steel includes: performing a heating step on a steel blank to obtain a heated steel blank, wherein a heating temperature of the heating step is 1150°C to 1300°C, and the steel blank includes: 0.10 weight % To 0.25% by weight of carbon; 0.5% to 2.0% by weight of silicon; 1.5% to 3.0% by weight of manganese; 0.04% to 1.0% by weight of aluminum; 0.02% to 0.7% by weight of chromium; not more than 0.02% by weight of molybdenum; not more than 0.02% by weight of phosphorus; not more than 0.015% by weight of sulfur; and the remaining amount is iron and unavoidable impurities; the heated steel blank is subjected to a rolling process to form a rolled steel plate Perform an annealing step on the rolled steel sheet to form an annealed steel sheet, wherein the annealing step is performed at an annealing temperature of 740°C to 860°C for 90 seconds to 600 seconds; and a cooling step is performed to cool the annealed steel sheet to 350°C to 500°C to form a cooling steel plate; and subjecting the cooling steel plate to an over-aging process to obtain the automotive steel, wherein the over-aging process is treated at an over-aging temperature of 350 to 500°C for 2 minutes To 25 minutes, where the manufacturing method excludes a tempering step, and the automotive steel is packaged in Contains fertilized iron, toughened iron and austenitic iron, of which the austenitic iron is at least 10%. According to the manufacturing method of automobile steel described in item 1 of the scope of patent application, a heating time of the heating step is 2 hours to 4 hours. According to the manufacturing method of automobile steel described in item 1 of the scope of patent application, a finishing temperature of the rolling process is 880°C to 950°C. The method for manufacturing automobile steel as described in item 1 of the scope of patent application, wherein the cooling step is to cool the annealed steel sheet to 550°C to 750°C at a cooling rate of 5°C/sec to 20°C/sec, and to Another cooling rate of °C/sec to 100°C/sec cools the annealed steel sheet from 550°C to 750°C to 350°C to 500°C. As for the manufacturing method of automobile steel described in item 1 of the scope of patent application, after the rolling process, the manufacturing method further includes a coiling step at a coil temperature of 500°C to 700°C. According to the method of manufacturing automobile steel described in item 5 of the scope of patent application, after the coiling step, the manufacturing method further includes a cold rolling step, and the cold rolling step has at least 50% cold rolling Rate to form the rolled steel sheet. An automobile steel, which is obtained by the method for manufacturing automobile steel as described in any one of items 1 to 6 of the scope of the patent application, wherein the automobile steel contains: 0.10 wt% to 0.25% by weight carbon 0.5 wt% to 2.0 wt% silicon; 1.5 wt% to 3.0 wt% manganese; 0.04 wt% to 1.0 wt% aluminum; 0.02 wt% to 0.7 wt% chromium; not more than 0.02 wt% molybdenum; no Phosphorus of more than 0.02% by weight; sulfur of not more than 0.015% by weight; and the remaining amount is iron and unavoidable impurities, and the steel for automobiles includes fat iron, toughened iron and austenitic iron, wherein the Voss Tiantie is at least 10%. The automobile steel described in item 7 of the scope of patent application, wherein the automobile steel further contains not more than 10% of hemp iron, the fertilizer grain iron is 30% to 60%, and the toughened iron is 30% To 60%. For the automotive steel described in item 7 of the scope of patent application, the tensile strength of the automotive steel is at least 780Mpa, and the elongation of the automotive steel is at least 20%. For the automotive steel described in item 7 of the scope of patent application, the strong plastic product of the automotive steel is at least 20 GPa%.

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