TWI912203B - Formed steel and method of fabricating the same - Google Patents

Abstract

A formed steel and a method of fabricating the same are provided. The method of fabricating the formed steel includes providing a slab with a specific composition; performing a thermal operation on the slab to obtain a thermal slab; performing a hot rolling operation on the thermal slab to obtain a hot-rolled steel, where a finishing temperature is between 800ºC and 900ºC; and performing a cold working operation on the hot-rolled steel to obtain the formed steel. Consequently, the resulting hot-rolled steel exhibits enhanced cold-work formability and toughness, enabling the formed steel to achieve mechanical properties that meet application requirements.

Description

Formed steel and its manufacturing method

This invention relates to a shaped steel product and a method for manufacturing the same, and more particularly to a shaped steel product comprising niobium and a method for manufacturing the same.

The conventional hot-rolled steel production process involves rapid cooling after hot rolling to obtain the hot-rolled steel. However, the hot-rolled steel produced by the aforementioned method contains a greater amount of ferrite or ductile iron microstructure, resulting in poor cold workability and making it unsuitable for subsequent large-scale reduction (e.g., more than 70%) cold working operations. Therefore, the microstructure and mechanical properties of the hot-rolled steel produced by this conventional method cannot meet the processing requirements of downstream manufacturers.

Conventional steelmaking processes that exclude spheroidizing annealing involve adding high-valence elements such as chromium and/or molybdenum to the steel billet and then rapidly cooling it (e.g., at a cooling rate of 3°C/s to 8°C/s) after hot rolling. However, the hot-rolled steel produced by conventional methods is mainly ferrochrome and ferrocarbon, with a low proportion of ferromagnesia (e.g., no more than 40%) and high hardness or strength. Consequently, it has poor cold workability and cannot be subjected to subsequent large-scale reduction (e.g., more than 70%) cold working operations.

Another known wire rod process that excludes spheroidizing annealing also involves rapid cooling after three hot rolling operations to reduce the temperature to 620°C to 800°C. The resulting hot-rolled steel has higher mechanical strength, so its cold workability is limited, making it impossible to perform subsequent large reduction (e.g., more than 70%) cold working operations. This can easily lead to a high rate of cold cracking defects in subsequent processes.

Another conventional method is to achieve the effect of direct softening heat treatment by additionally configuring online annealing furnaces and/or induction heating equipment. However, this method requires additional equipment costs, so the production efficiency is lower.

In view of this, there is an urgent need to provide a shaped steel and its manufacturing method to produce steel with better cold workability under a simplified process.

One aspect of this invention is to provide a method for manufacturing shaped steel, which involves heating and hot rolling steel with specific composition and controlling the rolling temperature to produce hot-rolled steel with better cold workability.

Another aspect of the present invention is to provide a shaped steel material, which is produced using the method described above.

According to one aspect of the present invention, a method for manufacturing shaped steel is provided. The method includes providing a steel billet, wherein the steel billet comprises, based on 100 wt%, 0.14 wt% to 0.16 wt% carbon; 0.01 wt% to 0.10 wt% silicon; 0.35 wt% to 0.95 wt% manganese; 0.015 wt% to 0.05 wt% aluminum; 0.020 wt% to 0.045 wt% niobium; and the balance iron and unavoidable impurities. The method also includes heating the billet to obtain a heated billet; hot rolling the heated billet to obtain hot-rolled steel, wherein the finishing temperature of the hot rolling operation is 800°C to 900°C; and cold working the hot-rolled steel to obtain shaped steel.

According to one embodiment of the present invention, the heating temperature of the above heating operation is 1100°C to 1200°C.

According to one embodiment of the present invention, the above method further includes a cooling step of the hot-rolled steel after the hot rolling operation, wherein the cooling rate of the cooling step is 0.3℃/s to 0.5℃/s.

According to one embodiment of the present invention, the reduction rate of the above-mentioned cold working operation is 70% to 85%.

According to one embodiment of the present invention, the spheroidizing annealing step is excluded before the above-mentioned cold working operation.

According to one embodiment of the present invention, the above-mentioned cold working operation includes wire drawing, rolling, forging, extrusion, stamping or any combination thereof.

According to one embodiment of the present invention, the hot-rolled steel has a tensile strength of 361 MPa to 403 MPa, a yield strength of 242 MPa to 264 MPa, and an elongation of 36.7% to 42.3%.

According to one embodiment of the present invention, the hot-rolled steel comprises a ferrite microstructure and a ferrous microstructure.

According to one embodiment of the present invention, the hot-rolled steel comprises a granular iron microstructure with a volume fraction of not less than 85%.

According to another aspect of the invention, a shaped steel material is provided, which is produced using the method described above.

The shaped steel and its manufacturing method of this invention involve heating and hot rolling steel with a specific composition, and controlling the final rolling temperature to increase the precipitation of niobium, thereby obtaining a greater amount of soft-phase ferrous granular structure. This results in hot-rolled steel with better cold workability and toughness. Therefore, the shaped steel obtained after cold working can possess mechanical properties that meet application requirements.

In this disclosure, component symbols and/or letters are repeated in various specific instances. This repetition is intended to simplify and clarify the description and does not imply any relationship between the various embodiments and/or configurations discussed.

The manufacture and use of embodiments of the present invention will now be discussed in detail. However, it is understood that the embodiments provide many applicable inventive concepts that can be implemented in a wide variety of specific contexts. The specific embodiments discussed are for illustrative purposes only and are not intended to limit the scope of the present invention.

As used in this invention, “around,” “about,” “approximately,” or “substantially” generally mean within 20 percent, 10 percent, or 5 percent of the stated value or range.

As described above, this invention provides a shaped steel product and its manufacturing method. This method involves heating and hot rolling steel with a specific composition, controlling the final rolling temperature to increase the precipitation of niobium, thereby obtaining a greater amount of soft-phase ferrous granular structure. This results in hot-rolled steel with better cold workability and toughness. Therefore, the shaped steel obtained after cold working can possess mechanical properties that meet application requirements.

Please refer to Figure 1, which is a flowchart illustrating a method 100 for manufacturing shaped steel according to some embodiments of the present invention. First, operation 110 is performed, providing a steel billet. In some embodiments, based on a steel billet of 100 wt%, the billet contains 0.14 wt% to 0.16 wt% carbon, 0.01 wt% to 0.10 wt% silicon, 0.35 wt% to 0.95 wt% manganese, 0.015 wt% to 0.05 wt% aluminum, 0.020 wt% to 0.045 wt% niobium, the balance iron, and unavoidable impurities. In some embodiments, the steel billet excludes chromium, boron, and titanium.

The present invention adds a small amount of niobium to the steel billet to precipitate niobium compounds in subsequent processes, and has a grain-refining effect. This helps the hot-rolled steel to have a larger volumetric ratio of ferroalloys, thereby exhibiting better toughness and formability. If the niobium content in the steel billet is too low (e.g., less than 0.020 wt%), the grain-refining effect is poor; conversely, if the niobium content in the steel billet is too high (e.g., greater than 0.045 wt%), the niobium compounds cannot precipitate uniformly, resulting in inconsistent properties of the shaped steel and increased material costs. When the steel billet contains silicon and aluminum within the above-mentioned content range, it helps to remove oxygen and other impurities in subsequent processes, thereby improving the purity of the steel. When the steel billet contains manganese within the above-mentioned range, it helps to improve the mechanical strength of the resulting shaped steel. When the steel billet contains carbon within the above-mentioned range, it helps to improve the formability and hardness of the resulting shaped steel.

Next, operation 120 is performed to heat the steel billet to obtain a heated steel billet. In some embodiments, the heating temperature is approximately 1100°C to approximately 1200°C. Utilizing the aforementioned higher heating temperature increases the amount of niobium dissolved, thereby increasing the amount of niobium precipitated during subsequent hot rolling operations.

Then, operation 130 is performed to hot roll the heated billet to obtain hot-rolled steel. In some embodiments, the hot rolling operation involves hot rolling using multiple rolling mills. In some embodiments, the finishing temperature of the hot rolling operation is approximately 800°C to approximately 900°C. If the finishing temperature is too high (e.g., above 900°C), since this temperature range is the temperature range where the maximum amount of niobium precipitates, finishing temperatures above 900°C will not allow for complete precipitation of niobium precipitates; conversely, if the finishing temperature is too low (e.g., below 800°C), the rolling resistance will be too high, and it may result in excessive hardness in the subsequently hot-rolled steel.

In some embodiments, after the hot rolling operation described above, a cooling step can be performed on the hot-rolled steel to allow complete precipitation of niobium. In this embodiment, the cooling rate of the cooling step is approximately 0.3°C/s to approximately 0.5°C/s. Controlling the cooling rate within the aforementioned range avoids the retention of niobium in the hot-rolled steel and prevents unnecessary energy consumption.

In some embodiments, the resulting hot-rolled steel comprises a ferroic microstructure and a ferrous microstructure. In this embodiment, the hot-rolled steel comprises a ferroic microstructure with a volume fraction of not less than 85%, preferably about 85% to about 91%. Through hot-rolling operations and cooling steps under specific conditions, a large amount of niobium precipitation and grain refinement can be promoted, thereby shifting the continuous cooling transformation (CCT) curve to the left. The abundant precipitated niobium can serve as nucleation sites for ferroic granules, thus contributing to the acquisition of a greater amount of soft-phase ferroic microstructure.

Furthermore, the numerous fine niobium particles effectively hinder and anchor grain boundary movement, thus suppressing grain growth and maintaining the original fine-grained structure. This enhances the toughening effect of the fine grains. This grain refinement also results in a more uniform distribution and better dispersion of ferroferrite and ferrite. Consequently, the resulting hot-rolled steel exhibits excellent toughness and cold workability. In some embodiments, the hot-rolled steel possesses tensile strengths of 361 MPa to 403 MPa, yield strengths of 242 MPa to 264 MPa, and elongation of 36.7% to 42.3%.

Then, operation 140 is performed to cold-work the hot-rolled steel to obtain shaped steel. In some embodiments, the cold-working operation includes drawing, rolling, forging, extrusion, stamping, or any combination thereof. In some embodiments, method 100 excludes the spheroidizing annealing step before performing the cold-working operation. Since the hot-rolled steel already possesses suitable mechanical strength and elongation, the spheroidizing annealing step can be omitted, and the cold-working operation can be performed directly.

By means of the above method 100, the formed steel produced can meet the application requirements of fasteners, such as meeting the mechanical strength of grade 5.8 screws (i.e., tensile strength greater than 520 MPa and yield strength greater than 420 MPa). In some embodiments, the formed steel produced has a tensile strength of 533 MPa to 585 MPa, a yield strength of 501 MPa to 546 MPa, and an elongation of 10.0% to 15.3%.

The following examples illustrate the application of this invention, but they are not intended to limit the invention. Those skilled in the art can make various modifications and refinements without departing from the spirit and scope of this invention. Examples 1 to 4

Figure 2A is a flowchart illustrating the process 200A performed in Examples 1 to 4. First, operation 201 is performed to provide a steel billet, wherein the steel billet composition of Examples 1 to 4 is shown in Table 1 below.

Table 1 Ingredients (wt%) C Si Mn Cr Al B Ti Nb Implementation Example 1 0.14 0.01 0.95 - 0.05 - - 0.03 Implementation Example 2 0.15 0.02 0.36 - 0.038 - - 0.02 Implementation Example 3 0.15 0.1 0.37 - 0.015 - - 0.045 Implementation Example 4 0.16 0.01 0.35 - 0.045 - - 0.02

The manufacturing process of this embodiment will be explained using Example 1. Operation 203 involves heating at a temperature of 1150°C. Then, operation 205 involves hot rolling and cooling, with a final rolling temperature of 900°C and a cooling rate of 0.5°C/s. The hot-rolled steel produced in operation 205 has a ferrite volume fraction of 91%, a tensile strength of 361 MPa, a yield strength of 242 MPa, and an elongation of 42.3%. Next, operation 207 involves wire drawing and cold forging. Finally, the obtained shaped steel is subjected to mechanical property analysis as described in operation 209. The formed steel has a tensile strength of 533 MPa, a yield strength of 501 MPa, and an elongation of 15.3%.

The process steps of Examples 2 to 4 are the same as those of Example 1, all utilizing process 200A as shown in Figure 2A. The differences lie in the heating temperature of operation 203, the rolling temperature of operation 205, and the cooling rate, except for the different composition of the steel billet. In Example 2, the heating temperature is 1100℃, the rolling temperature is 850℃, and the cooling rate is 0.5℃/s; in Example 3, the heating temperature is 1200℃, the rolling temperature is 900℃, and the cooling rate is 0.4℃/s; and in Example 4, the heating temperature is 1100℃, the rolling temperature is 800℃, and the cooling rate is 0.3℃/s. The granular iron volume ratio and mechanical properties of the hot-rolled steel products of Examples 2 to 4, as well as the mechanical properties of the shaped steel products, are shown in Table 2 below.

Table 2 Hot-rolled steel Formed steel Ferrous iron content Tensile strength Subjugation strength elongation Tensile strength Subjugation strength elongation Implementation Example 1 91% 361 MPa 242 MPa 42.30% 533 MPa 501 MPa 15.30% Implementation Example 2 90% 382 MPa 253 MPa 40.60% 569 MPa 528 MPa 13.50% Implementation Example 3 85% 395 MPa 256 MPa 38.70% 577 MPa 537 MPa 11.20% Implementation Example 4 89% 403 MPa 264 MPa 36.70% 585 MPa 546 MPa 9.96% Comparative Examples 1 to 4

Figure 2B is a flowchart illustrating process 200B performed in Comparative Examples 1 to 4. First, operation 211 is performed to provide steel billets, wherein the steel billet compositions of Comparative Examples 1 to 4 are shown in Table 3 below.

Table 3 Ingredients (wt%) C Si Mn Cr Al B Ti Nb Comparative example one 0.19 0.03 0.83 0.1 0.027 0.0015 0.029 - Comparative Example 2 0.2 0.02 0.9 0.15 0.03 0.002 0.025 - Comparative example three 0.21 0.01 0.85 - 0.045 0.0015 0.015 - Comparative Example 4 0.22 0.25 0.9 - 0.033 - - -

The process of the comparative example will be explained below using Comparative Example 1. Operation 213, a heating operation, is performed at a temperature of 1100°C. Then, operation 215, a hot rolling and cooling process, is performed, with a final rolling temperature of 1000°C and a cooling rate of 2.0°C/s. The hot-rolled steel produced in operation 215 has a ferrite volume fraction of 70%, a tensile strength of 502 MPa, a yield strength of 317 MPa, and an elongation of 28.1%. Next, the wire drawing and spheroidizing annealing steps (operation 217) are performed. The spheroidizing annealing step involves annealing at 735°C to 745°C for 3 to 6 hours after the wire drawing step, followed by cooling at a rate of 8°C/s to 12°C/s. After the material softens and its mechanical strength decreases, the fine drawing and cold forging steps (operation 218) are performed. Finally, the mechanical properties of the obtained formed steel are analyzed in step 219. The formed steel has a tensile strength of 568 MPa, a yield strength of 524 MPa, and an elongation of 14.3%.

The process steps of Comparative Examples 2 to 4 are the same as those of Comparative Example 1, all using process 200B as shown in Figure 2B. The differences lie in the heating temperature of operation 213, the rolling temperature of operation 215, and the cooling rate, except for the different steel billet composition. The heating temperature of Comparative Example 2 was 1100℃, the rolling temperature was 1000℃, and the cooling rate was 2.0℃/s; the heating temperature of Comparative Example 3 was 1050℃, the rolling temperature was 950℃, and the cooling rate was 1.5℃/s; the heating temperature of Comparative Example 4 was 1050℃, the rolling temperature was 950℃, and the cooling rate was 1.5℃/s. The granular volume ratio and mechanical properties of the hot-rolled steels of Comparative Examples 2 to 4, as well as the mechanical properties of the formed steels, are shown in Table 4 below.

Table 4 Hot-rolled steel Formed steel Ferrous iron content Tensile strength Subjugation strength elongation Tensile strength Subjugation strength elongation Comparative example one 70% 502 MPa 317 MPa 28.10% 568 MPa 524 MPa 14.30% Comparative Example 2 68% 513 MPa 322 MPa 27.20% 575 MPa 533 MPa 12.30% Comparative example three 65% 519 MPa 334 MPa 25.90% 582 MPa 539 MPa 11.30% Comparative Example 4 61% 533 MPa 348 MPa 22.30% 593 MPa 544 MPa 10.30%

According to Tables 2 and 4, compared with Comparative Examples 1 to 4, the hot-rolled steel produced in Examples 1 to 4 has a higher proportion of ferrous iron microstructure, lower mechanical strength, and higher elongation. Therefore, Examples 1 to 4 can produce hot-rolled steel that is easier to perform subsequent cold working operations. The main reason is that the composition of the billets in Examples 1 to 4 differs from that in Comparative Examples 1 to 4 (Examples 1 to 4 contain niobium, while Comparative Examples 1 to 4 do not contain niobium, and Comparative Examples 1 to 3 also contain chromium, boron, and titanium), and the finishing rolling temperature of the hot rolling operation in Comparative Examples 1 to 4 is higher, and the cooling rate of the subsequent cooling step is higher.

Because the hot-rolled steel produced in Comparative Examples 1 to 4 has higher mechanical strength and lower elongation, an additional spheroidizing annealing step is required before subsequent cold working to achieve the mechanical strength requirements of grade 5.8 screws. Therefore, the conventional processes used in Comparative Examples 1 to 4 require more time and process steps, resulting in longer post-processing time and increased production costs. In addition to low production efficiency, this also leads to significant energy consumption and carbon emissions.

According to the above embodiments, the shaped steel and its manufacturing method provided by this invention involve heating and hot rolling steel with a specific composition, and controlling the finishing rolling temperature to increase the precipitation of niobium, thereby obtaining a greater amount of soft-phase ferrous granular structure. This results in hot-rolled steel with better cold workability and toughness. Therefore, the shaped steel obtained after cold working can possess mechanical properties that meet application requirements.

Although the present invention has been disclosed above with several embodiments, it is not intended to limit the present invention. Anyone with ordinary knowledge in the art to which the present invention pertains may make various modifications and alterations without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention shall be determined by the appended patent application scope.

100: Method; 110, 120, 130, 140: Operation; 200A, 200B: Process; 201, 203, 205, 207, 209, 211, 213, 215, 217, 218, 219: Operation

A better understanding of the subject matter of this disclosure will be achieved by referring to the following detailed description and accompanying figures. [Figure 1] is a flowchart illustrating a method for manufacturing shaped steel according to some embodiments of the present invention. [Figure 2A] is a flowchart illustrating the processes performed in embodiments one through four. [Figure 2B] is a flowchart illustrating the processes performed in comparative examples one through four.

Domestic storage information (please record in the order of storage institution, date, and number) No overseas storage information (please record in the order of storage country, institution, date, and number) None

100: Method

110, 120, 130, 140: Operations

Claims (10)

A method for manufacturing a shaped steel product includes: providing a steel billet, wherein the steel billet comprises, based on 100 wt%, 0.14 wt% to 0.16 wt% carbon; 0.01 wt% to 0.10 wt% silicon; 0.35 wt% to 0.95 wt% manganese; 0.015 wt% to 0.05 wt% aluminum; and 0.020 wt% to 0.045 wt% aluminum. The billet contains wt% niobium; and the balance of iron and unavoidable impurities; a heating operation is performed on the billet to obtain a heated billet; a hot rolling operation is performed on the heated billet to obtain a hot-rolled steel product, wherein the final rolling temperature of the hot rolling operation is 800°C to 900°C; and a cold working operation is performed on the hot-rolled steel product to obtain the shaped steel product. The method for manufacturing shaped steel as described in claim 1, wherein a heating temperature of the heating operation is 1100°C to 1200°C. The method for manufacturing the shaped steel as described in claim 1 further includes: after the hot rolling operation, performing a cooling step on the hot-rolled steel, wherein the cooling rate of the cooling step is from 0.3°C/s to 0.5°C/s. The method for manufacturing shaped steel as described in claim 1, wherein the reduction rate of the cold working operation is 70% to 85%. The method for manufacturing shaped steel as described in claim 1, wherein a spheroidizing annealing step is excluded before the cold working operation. The method for manufacturing shaped steel as described in claim 1, wherein the cold working operation includes drawing, rolling, forging, extrusion, stamping or any combination thereof. The method for manufacturing shaped steel as described in claim 1, wherein the hot-rolled steel has a tensile strength of 361 MPa to 403 MPa, a yield strength of 242 MPa to 264 MPa, and an elongation of 36.7% to 42.3%. The method for manufacturing shaped steel as described in claim 1, wherein the hot-rolled steel comprises a ferrite microstructure and a ferrous microstructure. The method for manufacturing shaped steel as described in claim 8, wherein the hot-rolled steel comprises a granular iron structure with a volume fraction of not less than 85%. A shaped steel product manufactured using the method described in any one of claims 1 to 9.