TWI698535B - Hot rolling thin steel strip and method for producing the same - Google Patents

Abstract

The present invention relates to a hot rolling thin steel strip and a method for producing the same. A steel billet is subjected to a rough rolling process and a finish rolling process to form a finish rolling steel material. An inlet temperature of the finish rolling process is higher than Ar3 temperature, and a departure temperature of that is lower than Ar3 temperature. Then, the finish rolling steel material is subjected to a winding process to obtain the hot rolling thin steel strip of the present invention. A thickness of the hot rolling thin steel strip is smaller than or equal to 1.5 mm, and a bulk thereof is a uniform grain structure.

Description

Hot rolled thin steel strip and manufacturing method thereof

The present invention relates to a hot-rolled steel strip, in particular to a hot-rolled thin steel strip with uniform grain structure at the edge and the core.

With the development of application fields and the reduction of processing procedures for back-end applications, the requirements for thin steel products have become more stringent. Among them, because the temperature drop rate of the thin steel strip is not uniform (the temperature drop rate at the edge is greater than the temperature drop rate at the core), during the production process, as the processing process progresses and the steel is converted between processing equipment, the The steel strip is easy to be processed in the dual phase region of the α zone (that is, the fertilizer grain iron phase) and the γ zone (that is, the austenitic iron phase), so that the coiled steel strip has a coarse-grained edge and a fine-grained core. The mixed crystal structure deteriorates its mechanical properties (for example, lower elongation), so it cannot meet the needs of the application. Moreover, due to the influence of the mixed crystal structure, the thin steel strip is prone to edge ripples and/or jagged defects during the subsequent cold rolling process.

In order to solve the surface defects of the aforementioned thin steel strips, these surface defects are generally removed by cutting, but this method will result in waste of products and increase labor costs. Secondly, since the grain structure of the thin steel strip is still a mixed crystal structure, it is still difficult to meet application requirements.

Another method is to roll in the single-phase region (in other words, roll in the α zone and complete the rolling in the α zone) to prevent the thin steel strip from forming a mixed crystal structure in the dual-phase zone. However, because the coiling temperature is too low, the thin steel strip is prone to produce an unrecrystallized structure, which reduces its elongation and is difficult to apply to subsequent processing.

In view of this, it is urgent to provide a hot-rolled thin steel strip and a manufacturing method thereof to improve the defects of the conventional hot-rolled thin steel strip.

Therefore, one aspect of the present invention is to provide a method for producing hot-rolled thin steel strips. This method uses a finishing rolling process and a coiling process with specific process parameters to produce hot-rolled thin steel strips to form a uniform The grain structure, and then improve the quality of hot-rolled thin steel strip.

Another aspect of the present invention is to provide a hot-rolled thin steel strip, which is manufactured by the aforementioned manufacturing method.

According to one aspect of the present invention, a method for manufacturing a hot-rolled thin steel strip is provided. In this production method, a steelmaking process is first performed to form a steel blank. Then, a heating process is performed on the steel billet to form a heated steel billet. Then, a rough rolling process is performed on the heated steel billet, and a finish rolling process is performed on the formed rough rolled steel material to form a finished rolled steel material. Among them, the inlet temperature of the finishing rolling process is greater than the Ar3 temperature of the steel, and the finishing temperature of the finishing rolling process is (Ar1 temperature of the steel -50°C) to (Ar1 temperature of the steel +50°C). Then, the finished rolled steel is subjected to a coiling process to obtain the hot rolled thin steel strip of the present invention. Among them, the coiling temperature of the coiling process is not less than 650℃.

According to an embodiment of the present invention, the aforementioned inlet temperature ranges from (Ar3 temperature + 10°C) to (Ar3 temperature + 200°C).

According to another embodiment of the present invention, the aforementioned finishing temperature ranges from (Ar1 temperature-30°C) to (Ar1 temperature + 30°C).

According to another embodiment of the present invention, before performing the aforementioned coiling process, this manufacturing method can selectively perform an air cooling process on the finished rolled steel.

According to still another embodiment of the present invention, after the aforementioned coiling process is performed, this manufacturing method can selectively perform a cold rolling process on the hot-rolled thin steel strip.

According to another aspect of the present invention, a hot-rolled thin steel strip is provided, which is produced by the aforementioned method, wherein the hot-rolled thin steel strip is a ferrous iron phase.

According to an embodiment of the present invention, the thickness of the aforementioned hot-rolled thin steel strip is less than or equal to 1.5 mm.

According to another embodiment of the present invention, the difference in grain size between the edge and the core of the aforementioned hot-rolled thin steel strip is less than 3.

Applying the hot-rolled thin steel strip and its manufacturing method of the present invention, the grain structure of the hot-rolled thin steel strip is adjusted by the temperature parameters of the finishing rolling process and the coiling process, so that the hot-rolled thin steel strip produced The whole has a uniform ferrous iron phase grain structure. Secondly, the difference in the grain size of the hot-rolled thin steel strip is less than 3, so it does not have a mixed crystal structure with extremely different grain sizes, but has a good elongation. In addition, the produced hot-rolled thin steel strip can be directly coiled after the hot-rolling process and does not need to be processed by the cold-rolling process.

100‧‧‧Method

110/120/130/140/150/160‧‧‧Operation

In order to have a more complete understanding of the embodiments of the present invention and its advantages, please refer to the following description and 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 the related drawings are described as follows: [FIG. 1] is a flowchart showing a method of manufacturing a hot-rolled thin steel strip according to an embodiment of the present invention.

[Figure 2] is an optical micrograph showing the hot rolled thin steel strip according to Example 1 of the present invention.

[Figure 3] is an optical microscope photograph showing the hot rolled thin steel strip according to Example 2 of the present invention.

[Figure 4] is an optical micrograph showing the hot rolled thin steel strip according to Example 3 of the present invention.

[Figure 5A] is an optical microscope photograph showing the edge of the hot rolled thin steel strip according to Example 4 of the present invention.

[Figure 5B] is an optical microscope photograph showing the central position of the hot rolled thin steel strip according to Example 4 of the present invention.

[FIG. 6A] is an optical microscope photograph showing the edge of the hot rolled thin steel strip according to Example 5 of the present invention.

[Figure 6B] is an optical microscope photograph showing the central position of the hot rolled thin steel strip according to Example 5 of the present invention.

[FIG. 7A] is an optical microscope photograph showing the edge of the hot-rolled thin steel strip according to Comparative Example 1 of the present invention.

[Figure 7B] is an optical microscope photograph showing the central position of the hot rolled thin steel strip according to Comparative Example 1 of the present invention.

[Figure 8A] is an optical microscope photograph showing the edge of the hot rolled thin steel strip according to Comparative Example 2 of the present invention.

[Figure 8B] is an optical microscope photograph showing the central position of the hot rolled thin steel strip according to Comparative Example 2 of the present invention.

The manufacture and use of the embodiments of the present invention are discussed in detail below. However, it can be understood that the embodiments provide many applicable inventive concepts, which can be implemented in various specific contents. The specific embodiments discussed are for illustration only, and are not intended to limit the scope of the present invention.

The "inlet temperature of the rolling process" referred to in the present invention refers to the temperature at which the steel material starts the rolling process. Secondly, "Ar3 temperature" refers to the temperature at which the grain structure of the steel begins to transform from austenite into ferrite during the cooling process, and "Ar1 temperature" refers to the temperature at which the steel is During the cooling process, the final temperature at which the grain structure of the steel is completely transformed from the austenitic iron phase to the fat iron phase. Among them, Ar1 temperature and Ar3 temperature can be measured by dilatometer or calculated by formula.

Please refer to FIG. 1, which shows a flow chart of a method of manufacturing a hot-rolled thin steel strip according to an embodiment of the present invention. In the method 100, the steelmaking process is performed first to form a steel blank, as shown in operation 110. The prepared steel blank contains 0.01 wt% to 0.07 wt% of carbon, 0.1 wt% to 0.5 wt% of manganese, no more than 0.03 wt% of phosphorus, not much Less than 0.02 weight percent of sulfur, no more than 0.05 weight percent of silicon, no more than 0.3 weight percent of aluminum, no more than 0.01 weight percent of nitrogen, no more than 0.002 weight percent of boron, the balance of iron and unavoidable impurities. Among them, molten steel does not intentionally add chromium, molybdenum and niobium. In other words, the formed steel blank does not substantially contain chromium, molybdenum and niobium.

Then, a heating process is performed on the steel billet, and the formed heated steel billet is subjected to a rough rolling process to form a rough rolled steel, as shown in operation 120 and operation 130. The heating process is used to increase the temperature of the steel billet to facilitate the subsequent rough rolling process. Accordingly, the rough rolling process is a hot rolling process. It should be noted that there are no special restrictions on the rolling parameters of the rough rolling process (for example: inlet temperature, cutting rate and finishing temperature, etc.). They are mainly used to cut the thickness of the steel blank to make the rough rolled steel produced After the subsequent finishing rolling process, the material can meet the back-end application requirements.

In some embodiments, the temperature of the aforementioned heating process may be greater than 1000°C. In other embodiments, the temperature of the heating process may be 1100°C to 1300°C. It is understandable that, in some embodiments, in order to reduce the energy cost of the process, the finishing temperature of the rough rolling process may not be less than the inlet temperature of the subsequent finishing rolling process.

After the operation 130 is performed, a finish rolling process is performed on the rough-rolled steel material to form a finished-rolled steel product, as shown in operation 140. When the finishing rolling process is performed, the temperature of the rough rolled steel is greater than the Ar3 temperature (that is, the inlet temperature of the finishing rolling process is greater than the Ar3 temperature). In other words, before the finish rolling process, the grain structure of the rough rolled steel is austenitic iron phase. After finishing the rolling process, the finishing temperature of the finishing rolling process can range from (Ar1 temperature -50°C) to (Ar1 temperature +50°C).

When the finishing rolling process is carried out, since the inlet temperature is greater than the Ar3 temperature, and the finishing temperature is (Ar1 temperature-50°C) to (Ar1 temperature + 50°C), the grain structure of the steel is transformed from austenitic iron phase It is the fat iron phase, and the grain structure of the finished rolled steel formed based on the finishing temperature can be a mixed crystal structure of austenitic iron phase and fat iron phase, or a single structure of fat iron phase. It should be noted that when the finishing temperature is higher than Ar1 temperature to (Ar1 temperature +50℃), although the grain structure of the finished rolled steel is a mixed grain structure of austenitic iron phase and fat iron phase, The difference between the finishing temperature and the Ar1 temperature is not more than 50°C, so the austenitic iron phase content is small. Accordingly, when the finished rolled steel is subjected to the continuous coiling process, these small amounts of austenitic iron phase are not likely to cause the produced hot-rolled thin steel strip to produce mixed crystals with coarse grains at the edges but fine grains at the core. organization. In other words, the hot-rolled thin steel strip produced can still have a uniform structure of the ferrous iron phase.

If the inlet temperature of the finishing rolling process is not greater than the Ar3 temperature, because the temperature drop rate of the thin steel sheet is relatively fast, the finishing temperature of the finishing rolling process cannot easily meet the aforementioned range, and when the subsequent coiling process is performed, the obtained The grain structure of the finished rolled steel is not easy to recrystallize, and it is difficult to grow into a coarser grain structure, thereby reducing the elongation of the hot rolled thin steel strip.

If the finishing temperature of the finishing rolling process is greater than (Ar1 temperature + 50℃), the finished rolled steel produced will have too much austenitic iron phase, and it is easy for the subsequent coiling process to have a grain structure at the edge. Recrystallization grows into coarse crystals, but the grain structure of the core changes from austenitic iron phase to fine-grained ferrous iron phase, which cannot meet the application requirements. If the finishing temperature of the finishing rolling process is less than (Ar1 temperature-50℃), the too low finishing temperature will easily cause the grain structure of the finished rolled steel During the coiling process, the recrystallized structure cannot be formed, and the elongation of the hot-rolled thin steel strip is reduced.

In some embodiments, when the finishing rolling process is performed, the temperature of the rough-rolled steel material may range from (Ar3 temperature + 10°C) to (Ar3 temperature + 200°C). In some embodiments, the finishing temperature of the finishing rolling process may range from (Ar1 temperature -30°C) to (Ar1 temperature + 30°C), and preferably (Ar1 temperature -20°C) to (Ar1 temperature + 20°C) .

After the finishing rolling process is performed, the finished rolled steel is subjected to the coiling process to obtain the hot rolled thin steel strip of the present invention, as shown in operation 150 and operation 160. The coiling temperature of the coiling process is not less than 650℃. When the coiling process is performed, the grain structure in the finished rolled steel can form a recrystallized structure to form larger-scale grains, so the elongation of the hot-rolled thin steel strip can be further improved.

If the coil temperature is less than 650°C, the grain structure in the finished rolled steel will not be able to form a recrystallized structure, but will have finer grains, thereby reducing the elongation of the hot rolled thin steel strip.

It should be noted that because the temperature of the hot-rolled thin steel strip is extremely fast, the finished rolled steel produced by the finishing rolling process is preferably directly coiled to ensure that the temperature of the finished rolled steel is not less than 650°C. However, based on the configuration of the existing steelmaking plant, the steel strip formed by the hot rolling equipment generally must be cooled by the cooling equipment first to reduce its temperature, and then coiled by the coiling equipment. Accordingly, in some embodiments, in order to avoid drastic changes in the configuration of the equipment, the existing cooling equipment can be used to selectively perform an air cooling process on the finished rolled steel before the coiling process is performed. It should be understood that this cooling process (ie, air cooling process) cannot be avoided Yes, so it is not used to cool steel after finishing rolling. Therefore, in these embodiments, no other cooling medium (such as water) is used in the cooling process to avoid the temperature of the finished rolled steel material from being lower than the aforementioned coil temperature (ie, less than 650°C).

In some embodiments, after the coiling process is performed, the hot-rolled thin steel strip can be selectively cold-rolled, so that the thickness of the steel strip can be further reduced to meet application requirements. Among them, because the hot-rolled thin steel strip has a uniform coarse-grained structure of the ferrous iron phase and has a good elongation, the cold-rolled steel strip does not have surface defects such as edge ripples and/or serrations.

In a specific example, the thickness of the hot-rolled thin steel strip produced by the present invention is less than or equal to 1.5 mm. Among them, the whole grain structure of the hot rolled thin steel strip is a uniform ferrous iron phase, and the difference in grain size between the edge and the core is less than 3. Accordingly, the hot-rolled thin steel strip of the present invention has good elongation.

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

Preparation of hot rolled thin steel strip Example 1

First, the steelmaking process is carried out to obtain the steel blank of Example 1. The steel billet contains 0.014 weight percent of carbon, 0.15 weight percent of manganese, 0.008 weight percent of phosphorus, 0.01 weight percent of silicon, 0.035 weight percent of aluminum, 0.001 weight percent of nitrogen, 0.003 weight percent of sulfur, 0.0001 weight percent Of boron, the total amount is less than 0.01 weight percent Compared with chromium, molybdenum and niobium, and the balance of iron, the Ar3 temperature of the steel blank is 890℃, and the Ar1 temperature is 820℃.

Then, after a heating process and a rough rolling process, the rough rolled steel material of Example 1 is formed. Then, a finishing rolling process is performed on the rough rolled steel to form a finishing rolled steel, wherein the inlet temperature of the finishing rolling process is greater than 890°C, and the finishing temperature is 850°C. Afterwards, the finished rolled steel was subjected to a coiling process to obtain the hot-rolled thin steel strip of Example 1, wherein the temperature of the coiling process was 750°C.

The obtained hot-rolled thin steel strips were evaluated in yield strength, tensile strength, and elongation, respectively, and the evaluation results are shown in Table 1. Among them, the yield strength, tensile strength, elongation and other evaluation methods are measured using instruments and methods well known to those with ordinary knowledge in the technical field to which the present invention belongs, so they will not be repeated here.

Example 2 to Example 5 and Comparative Example 1 and Comparative Example 2

The hot-rolled thin steel strips of Example 2 to Example 5 and Comparative Example 1 and Comparative Example 2 use the same process steps as the method of manufacturing the hot-rolled thin steel strip of Example 1, except that the difference lies in Example 2 to implementation. Example 5 and Comparative Example 1 and Comparative Example 2 use different compositions of steel billets and process parameters, and their conditions are shown in Table 1. Among them, the Ar3 temperature of the steel billet of Example 2 is 890°C, and the Ar1 temperature is 820°C; the Ar3 temperature of the steel billet of Example 3 is 890°C, and the Ar1 temperature is 820°C; the Ar3 of the steel billet of Example 4 The temperature is 873°C, and the Ar1 temperature is 800°C; the Ar3 temperature of the steel billet of Example 5 is 873°C, and the Ar1 temperature is 800°C. In addition, both Comparative Example 1 and Comparative Example 2 were finished rolling in the γ domain (in other words, the finishing temperature of the finishing rolling process of Comparative Example 1 and Comparative Example 2 were both higher than their Ar3 temperature).

Similarly, the hot-rolled thin steel strips prepared in Example 2 to Example 5 and Comparative Example 1 and Comparative Example 2 were evaluated by yield strength, tensile strength, and elongation, respectively, and the results are as in first As shown in the table.

Please refer to Figures 2 to 8B, in which Figures 2 to 4 respectively show optical micrographs of the hot-rolled thin steel strips according to Examples 1 to 3 of the present invention, Figure 5A, Figure 6A, Figure 7A and Figure 8A The optical microscope photographs showing the edges of the hot-rolled thin steel strips of Example 4, Example 5, Comparative Example 1 and Comparative Example 2 according to the present invention, respectively, and Figure 5B, Figure 6B, Figure 7B and Figure 8B respectively show Optical microscope photographs of the central position of the hot-rolled thin steel strips of Example 4, Example 5, Comparative Example 1 and Comparative Example 2 according to the present invention.

According to FIG. 2 to FIG. 4, the grain size of the hot-rolled thin steel strips of Examples 1 to 3 are all 7. According to FIG. 5A and FIG. 5B, in Embodiment 4, the grain size of the hot-rolled thin steel strip at a distance of 50 mm from the edge is 8, and the grain size of the hot-rolled thin steel strip at the center is 8. According to FIGS. 6A and 6B, it can be seen that in Embodiment 5, the grain size of the hot-rolled thin steel strip 50 mm from the edge is 8, and the grain size of the hot-rolled thin steel strip at the center is 8.

Since the finishing temperature of the finishing rolling process of the present invention is the Ar1 temperature ±50°C, before the subsequent coiling process, the whole finished rolled steel is a fine-grained ferrous iron phase. Then, after coiling at a temperature of not less than 650°C, the fine-grained structure of the steel can form a recrystallized structure to form a coarser grain size. Therefore, the hot-rolled thin steel strip of the present invention has a uniform The grain structure (the difference in grain size is less than 3), and the uniform coarse grain structure is easy to help improve the elongation of the hot rolled thin steel strip.

According to FIG. 7A and FIG. 7B, in Comparative Example 1, the hot-rolled thin steel strip at a distance of 20 mm from the edge has a grain structure with grain sizes of 9 and 12, and the grain size at the center is 9. At the edge of the hot-rolled thin steel strip, the content of the grain structure with a grain size of 9 is 10%, and the content of the grain structure with a grain size of 12 is 90%. As shown in FIGS. 7A and 7B, the edge position (FIG. 7A) of Comparative Example 1 is mixed crystal, and the center position (FIG. 7B) is coarse crystal. Among them, since the temperature of the sampling position in FIG. 7B is higher, the size of the ferrite phase after the phase change is also larger.

According to FIGS. 8A and 8B, it can be seen that in Comparative Example 2, the hot-rolled thin steel strip at a distance of 75 mm from the edge and at the center has a grain structure with a grain size of 9 and 12. At the edge, the grain structure with a grain size of 9 The content is 20% and the content of the grain structure with a grain size of 12 is 80%; in the center, the content of the grain structure with a grain size of 9 is 10% and the grain structure with a grain size of 12 The content is 90%.

Accordingly, since the finishing temperature of the finishing rolling process of the hot-rolled thin steel strip of Comparative Example 1 and Comparative Example 2 is greater than the Ar3 temperature, the grain structure of the finishing rolled steel formed by the finishing rolling process is still austenitic iron phase. However, the temperature drop rate of the thin steel strip is faster, so the edge position of the thinner finished rolled steel will cool down faster, and the metamorphosis is the ferrite phase, but the temperature of the core is still higher than the Ar3 temperature, so the finished rolled steel The heart is still austrian iron phase. Therefore, during the coiling process, the grain structure of the edge can continue to grow, but the structure of the heart is transformed from the austenitic iron phase to the fat iron phase, and has a smaller grain size. As a result, the hot-rolled thin steel strip produced has a coarse-grained surface but a fine-grained mixed-crystal structure at the core, which is prone to surface defects such as edge waves and/or jagged.

Accordingly, the hot-rolled thin steel strip manufacturing method of the present invention can obtain a hot-rolled thin steel strip with a uniform structure and grain size as a whole through a finishing rolling process and a coiling process with specific parameters, and With good elongation, it can be directly used in back-end applications. In addition, if the hot-rolled thin steel strip of the present invention is further subjected to a cold rolling process, the formed steel strip does not have defects such as edge waves and/or sawtooth, and therefore has good quality.

Although the present invention has been disclosed in the above embodiments, 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 changes without departing from the spirit and scope of the present invention. Retouching, therefore, the scope of protection of the present invention shall be subject to the scope of the attached patent application.

100‧‧‧Method

110/120/130/140/150/160‧‧‧Operation

Claims (8)

A method for manufacturing a hot-rolled thin steel strip includes: performing a steelmaking process to form a steel billet, wherein the steel billet contains: 0.01 wt% to 0.07 wt% carbon; 0.1 wt% to 0.5 wt% manganese; Not more than 0.03 weight percent of phosphorus; not more than 0.02 weight percent of sulfur; not more than 0.05 weight percent of silicon; not more than 0.3 weight percent of aluminum; not more than 0.01 weight percent of nitrogen; not more than 0.002 weight percent of boron; and the rest A large amount of iron and inevitable impurities; and the steel blank does not substantially contain chromium, molybdenum and niobium; a heating process is performed on the steel blank to form a heated steel blank; the heated steel blank is subjected to a rough rolling process, To form a rough-rolled steel; perform a finishing rolling process on the rough-rolled steel to form a finishing-rolled steel, wherein an inlet temperature of the finishing rolling process is greater than the Ar3 temperature of the rough-rolling steel, and the finishing rolling process A finishing temperature ranges from 50°C lower than the Ar1 temperature of the rough rolled steel to 50°C higher than the Ar1 temperature; and a coiling process is performed on the finished rolled steel to form the hot-rolled thin steel strip, wherein the coil The coil temperature in one of the processes is not less than 650℃. According to the method for manufacturing the hot-rolled thin steel strip described in the first item of the scope of patent application, the inlet temperature is 10°C to 200°C higher than the Ar3 temperature. The method for manufacturing the hot-rolled thin steel strip as described in item 1 of the scope of patent application, wherein the finishing temperature ranges from 30°C lower than the Ar1 temperature to 30°C higher than the Ar1 temperature For example, the method for manufacturing the hot-rolled thin steel strip described in item 1 of the scope of patent application, before performing the coiling process, further includes: performing an air cooling process on the finished rolled steel. The method for manufacturing the hot-rolled thin steel strip as described in item 1 of the scope of the patent application, after the coiling process is performed, further includes: performing a cold rolling process on the hot-rolled thin steel strip. A hot-rolled thin steel strip is prepared by the method as described in items 1 to 5 of the scope of patent application, wherein the hot-rolled thin steel strip is a ferrous iron phase. The hot-rolled thin steel strip described in item 6 of the scope of patent application, wherein the thickness of the hot-rolled thin steel strip is less than or equal to 1.5 mm. The hot-rolled thin steel strip described in item 6 of the scope of patent application, wherein the difference in grain size between one edge and one core of the hot-rolled thin steel strip is less than 3.

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