TWI841237B - Steel sheet and method of making the same - Google Patents

Abstract

A steel sheet is disclosed. The steel sheet comprises, based on a total weight of 100 wt%, 0.07 to 0.12 wt% carbon, 0.3 to 1 wt% silicon, 0.4 to 1 wt% manganese, 0.05 to 0.15 wt% phosphorus, 0.002 to 0.01 wt% sulfur, 0.01 to 0.1 wt% aluminum, 0.2 to 0.4 wt% copper, 0.2 to 1.5 wt% chromium, 0.002 to 0.02 wt% nitrogen, and 94 to 98 wt% iron.

Description

Steel material and method for manufacturing the same

The present invention relates to a steel material, in particular to a weather-resistant steel material and a manufacturing method thereof.

Weathering steel is mostly used in the manufacture of containers and exposed structural parts. In addition to emphasizing its weather resistance, processability and surface quality, in recent years, in order to improve the production efficiency of steel plate joining, the requirements for steel welding performance have become increasingly stringent. However, as the welding speed increases, problems such as burn-through and poor weld penetration often occur, and production must be slowed down. In addition, in order to meet the requirements of lightweight and energy saving and carbon reduction, weathering steel continues to develop towards high strength and thinness. In the past, in order to meet the best combination of strength and processability, the development of high-strength steel above 60 kg was mostly oriented towards a complex phase structure design with refined grains, that is, using ferrous iron plus corundum, or ferrous iron plus tantalum, or ferrous iron plus materia iron as the base structure, so that the steel has the dual characteristics of high ductility of ferrous iron and high tensile strength of corundum, tantalum or materia iron (hard phase).

However, as duplex steel develops towards higher strength, the proportion of hard phase in the composition needs to be increased; and the change in microstructure and the difference in hardness between the soft and hard phases will increase the problem of cracking at the interface of the two phases during subsequent cold forming. In addition, the increase in the proportion of hard phase will also lead to uneven distribution of the composition phase and banded structure, resulting in the steel strength exceeding the standard, but the elongation and machinability are far from sufficient.

In order to overcome the above problems, most of the current measures are to use ferrous iron as the main structure. In order to improve the strength of the ferrous iron phase, nigrum is often added to the alloy composition to refine the structure or perform multiple repeated cold working and annealing to achieve the effect of fine grain strengthening. However, steel with nigrum addition will cause the impedance to continue to increase due to grain refinement during the rolling process, which is not conducive to the development of thin plates. Furthermore, it is not easy to implement industrial mass production through multiple repeated processing.

In summary, it is known that weather-resistant steel and its manufacturing method need to be improved.

The main purpose of the present invention is to provide a steel material and a manufacturing method thereof, which has both high strength and high ductility.

To achieve the above-mentioned object, in one embodiment of the present invention, a steel material is provided, comprising: based on a total weight of 100 wt%, 0.07 to 0.12 wt% of carbon, 0.3 to 1 wt% of silicon, 0.4 to 1 wt% of manganese, 0.05 to 0.15 wt% of phosphorus, 0.002 to 0.01 wt% of sulfur, 0.01 to 0.1 wt% of aluminum, 0.2 to 0.4 wt% of copper, 0.2 to 1.5 wt% of chromium, 0.002 to 0.02 wt% of nitrogen, and 94 to 98 wt% of iron.

In one embodiment of the present invention, the steel material further comprises no more than 0.1 wt % of nickel.

In one embodiment of the present invention, the steel further comprises no more than 0.15 wt % of vanadium.

In one embodiment of the present invention, the steel further comprises no more than 0.3 wt % of titanium.

In one embodiment of the present invention, the weight ratio of the titanium to the nitrogen is between 3.42 and 14.

In one embodiment of the present invention, the steel contains 0.4 to 1 wt % silicon.

In one embodiment of the present invention, the ratio of the silicon to the aluminum is 9 to 13.

To achieve the above-mentioned purpose, in another embodiment of the present invention, a method for manufacturing the steel material as described above is also provided, comprising: reheating a steel billet, wherein an air-fuel ratio is 0.85 to 1.2, the heating temperature is 1050°C to 1300°C, and the temperature is maintained for 1 to 3 hours; and the reheated steel billet is derusted by high-pressure water, and then rolled at a speed of 8 to 15 m/s, and the final rolling temperature is (Ar3+10)°C to (Ar3+50)°C, so as to obtain a finished rolled steel material.

In one embodiment of the present invention, the method further comprises: laminar cooling the rolled steel at a cooling rate of more than -30°C/s, and controlling the intermediate temperature to be between (Ar1-20)°C and (Ar1-80)°C.

In one embodiment of the present invention, the water pressure of the high-pressure water is 170 to 350 bar.

In one embodiment of the present invention, the steel is rolled including hot rough rolling and hot finish rolling in sequence. In the hot rough rolling stage, the average reduction per pass is less than 40 mm, and in the hot finish rolling stage, the average reduction per pass is less than 20 mm, so as to obtain the finished steel.

The beneficial effect of the present invention is that the microstructure of the obtained steel is a ferrous iron phase, the ferrous iron grain size is between 2 and 8 μm, and there are a plurality of precipitates dispersed in the ferrous iron phase, the precipitates contain carbides with a particle size of less than 20 nm, and nitrides with a particle size of less than 4 μm. The steel of the present invention can have a yield strength of more than 550 MPa, a yield ratio of between 0.85 and 0.95, and a total elongation of more than 19%.

The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. In addition, in order to better illustrate the present invention, many specific details are given in the specific implementation methods below. Those skilled in the art should understand that the present invention can also be implemented without certain specific details.

A steel material according to one embodiment of the present invention comprises: based on a total weight of 100 wt%, 0.07 to 0.12 wt% of carbon (e.g., 0.07, 0.08, 0.09, 0.1, 0.11, 0.12 wt%), 0.3 to 1 wt% (e.g., 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1 wt%) of silicon, 0.4 to 1 wt% (e.g., 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1 wt%) of manganese, 0.05 to 0.15 wt% (e.g., 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.11, 0.12, 0.13, 0.14, 0.15 wt%) of phosphorus, 0.002 to 0.01 wt% (e.g. 0.002, 0.003, 0.004, 0.005, 0.006, 0.007, 0.008, 0.009, 0.01 wt%) of sulfur, 0.01 to 0.1 wt% (e.g. 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1 wt%) of aluminum, 0.2 to 0.4 wt% (e.g. 0.2, 0.3, 0.4 wt%) of copper, 0.2 to 1.5 wt% (e.g., 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.1, 1.2, 1.3, 1.4, 1.5 wt%) of chromium, 0.002 to 0.02 wt% (e.g., 0.002, 0.003, 0.004, 0.005, 0.006, 0.007, 0.008, 0.009, 0.01, 0.02 wt%) of nitrogen, and 94 to 98 wt% (e.g., 94, 95, 96, 97, 98 wt%) of iron.

Specifically, silicon can form iron silicate with iron to form a protective layer between the rust scale and the base material. In addition, the addition of titanium can form titanium nitride, titanium carbide and/or titanium carbonitride with nitrogen/carbon, thereby reducing carbon to form ferrite, thereby achieving the effect of grain refinement. Preferably, the ratio of silicon to aluminum is 9 to 13, such as 9, 10, 11, 12, 13, so that the steel has better welding performance; the silicon content is greater than/equal to 0.4 wt%, which can improve the surface quality and weather resistance of the steel, and when the content ratio of titanium to nitrogen is above 3.42, it can improve the coarsening of austenite grains and improve the strength of the steel.

In other embodiments of the present invention, the steel may contain no more than 0.1 wt % nickel. In other embodiments of the present invention, the steel may contain no more than 0.15 wt % vanadium. In other embodiments of the present invention, the steel may contain no more than 0.3 wt% of titanium; selectively, the steel may contain 0.01 to 0.3 wt% of titanium, and the weight ratio of the titanium to the nitrogen is between 3.42 and 14 (for example: 3.42, 3.5, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14), and is a fertile iron phase, the grain size of the fertile iron is 2 to 8 µm, and has a plurality of analytes dispersed in the fertile iron phase, the plurality of precipitates contain carbides with a particle size of less than 20 nm, and nitrides with a particle size of less than 4 µm, the steel has a yield strength of more than 550 MPa, a yield ratio of between 0.85 and 0.95, and a total elongation of more than 19%. Of course, the present invention is not limited thereto. In other embodiments of the steel material that does not contain titanium, the microstructure of the steel material is a ferrous iron phase and a pulverized iron structure, the ferrous iron grain size is 4 to 8 μm, the yield strength of the steel material can reach more than 355 MPa, the yield ratio is between 0.65 and 0.8, and the total elongation is more than 36%.

According to another embodiment of the present invention, a method for manufacturing the steel material as described above mainly includes the steps of reheating the steel billet, hot rolling, hot rolling cooling, coiling and pickling (reprocessing step); specifically, the method includes: reheating the steel billet, wherein an air-fuel ratio is 0.85 to 1.2 (for example: 0.85, 0.9, 1, 1.1, 1.2), a heating temperature is 1050°C to 1300°C (for example: 1050, 1100, 1150, 1200, 1250, 1300°C), and the temperature is maintained for 1 to 3 hours (for example: 1, 1.5, 2, 2.5, 3 hours); and derusting the reheated steel billet with high-pressure water, wherein the water pressure is 170 to 350 bar (e.g. 170, 180, 190, 200, 250, 300, 350 bar), and then rolled at a speed of 8 to 15 m/s (e.g. 8, 9, 10, 11, 12, 13, 14, 15 m/s), and the final rolling temperature is (Ar3+10)°C to (Ar3+50)°C, so as to obtain a finished rolled steel. The rolling of the steel includes sequentially performing hot rough rolling and hot finish rolling, in the hot rough rolling stage, the average reduction per pass is less than 40 mm, and in the hot finish rolling stage, the average reduction per pass is less than 20 mm.

Further, the rolled steel is subjected to laminar cooling at a cooling rate of more than -30 °C/s, and optionally, to laminar cooling at a cooling rate of more than -60 °C/s; and the intermediate temperature is controlled to be between (Ar1-20) °C and (Ar1-80) °C. The cooled steel is coiled at (Ar1-40) °C to (Ar1-160) °C to obtain a steel coil; finally, it can be cleaned with acid to remove the rust scale on the surface, and then reprocessed.

In order to further demonstrate the technical means and technical effects of the present invention for steel and its manufacturing method, various tests are used to illustrate the present invention.

Testing method:

Microstructure and precipitate analysis: The steel structure was electrolytically polished, and then the microstructure, precipitates and nano-carbides were observed and the grain size was measured using a scanning electron microscope, a transmission electron microscope and an electron backscatter diffraction (EBSD). The results are shown in Table 3 below.

Surface quality analysis of copper hot cracking of steel: Use optical and electron microscopes to perform metallographic observations on the surface and cross-section of hot-rolled steel to confirm whether there is copper enrichment and copper hot cracking at the grain boundaries of the parent material. The results are shown in Table 3 below, where "◯" indicates that there is no hot cracking of the steel and the surface quality is good; "Δ" indicates that the copper hot cracking is slight and the surface quality is acceptable; "×" indicates that the parent steel hot cracking is severe and the surface quality is poor.

Weathering resistance evaluation of cyclic corrosion test: Bare steel was subjected to cyclic corrosion test according to ASTM G85.A5. The results are shown in Table 3 below, where "◎" indicates good weathering resistance; "◯" indicates good weathering resistance; and "×" indicates poor weathering resistance.

Weldability evaluation: Gas Metal Arc Welding (GMAW) was used, with the shielding gas being Ar-CO ₂ /80-20%, welding current being 120 to 145A, welding voltage being 18 to 20V, and welding speed being 110 to 120 cm/min. The results are shown in Table 3 below, where “◯” indicates good welding, “Δ” indicates fair welding, and “×” indicates poor welding.

Tensile test: The steel is processed into JIS No. 5 test pieces and subjected to tensile test to measure its yield strength (YS), tensile strength (TS) and elongation (EI), etc.

Table 1 shows examples 1 to 4 of steel blanks with different compositions, wherein example 1 does not contain Ni, V, and Ti; and example 3 does not contain Ni. Table 2 shows heating furnace and hot rolling process control conditions 1 to 4. The examples in Table 1 are combined with the conditions in Table 1 to perform reheating and hot rolling treatments to obtain various steel materials. The detailed test results are as follows.

Test results:

When the air-fuel ratio is greater than 1.05 (condition 1 in Table 2), the obtained steel has a thick decarburized layer and coarsened grains, wherein the formation of the decarburized layer is mainly related to the heating furnace temperature and the high air-fuel ratio. When the air-fuel ratio is less than 1.05 (conditions 2 to 4 in Table 2), and the lower the reheating temperature of the steel billet, the above situation can be significantly improved. Preferably, the temperature of the heating furnace is 1060 to 1100°C, and the air-fuel ratio is 0.9 to 0.98.

After the steel pieces of Examples 1 to 4 were reheated and hot rolled in a heating furnace under Conditions 1 to 4 of Table 2, a rust scale was formed on the surface of the steel pieces. The composition of the rust scale was, from the surface of the rust scale to the surface of the steel piece, ferrous oxide (Fe ₂ O ₃ ), ferrous oxide (Fe ₃ O ₄ ), ferrous oxide (FeO), and a combination of ferrous oxide and ferrous silicate (Fe ₂ SiO ₄ ), wherein when the reheating temperature was below 1100°C and the furnace atmosphere air-fuel ratio was below 1.05, the rust scale included spherical or irregular copper.

When the silicon content is greater than/equal to 0.4 wt% (Examples 1, 2, and 4), good surface quality and weather resistance of the steel can be obtained under the conditions of Table 2. On the contrary, when the silicon content is less than 0.4 wt% (Example 3), good surface quality of the steel cannot be obtained regardless of whether the steel is subjected to high roughing and finishing reduction rates during hot rolling at a high reheating temperature (e.g., Conditions 1 and 3 of Table 2) or low roughing and finishing reduction rates during hot rolling at a low reheating temperature (e.g., Condition 2 of Table 2).

When the steel blank does not contain titanium (Example 1), and the laminar cooling is performed at a relatively low cooling rate of less than -50 ° C / s, the microstructure of the obtained steel is a ferrous iron phase and a pulverized iron structure, wherein the pulverized iron ratio is 15 to 20%; and as the laminar cooling rate is increased to more than -60 ° C / s, in addition to the obvious refinement of the steel structure, the pulverized iron ratio is also reduced to less than 15%, and the overall ferrous iron grain size is between 4 and 10 μm. The yield strength of this steel can reach more than 355 MPa, the yield ratio is 0.65 to 0.8, and the total elongation is more than 36%.

When the titanium content is above 0.01 wt% and the nitrogen content is above 0.002 wt% (Example 2-4), if the ratio of the titanium to nitrogen content is below 3.5 and the laminar cooling rate is below -50 °C/s, the microstructure of the obtained steel is a ferrous iron phase and a pulverized iron structure. As the cooling rate increases, the microstructure of the steel turns into a ferrous iron phase, wherein since the ratio of the titanium to nitrogen content is below 3.42, the titanium nitride cannot be effectively precipitated during the reheating stage of the casting and the steel, resulting in a significant coarsening of the austenite grains. In addition, titanium cannot produce a large amount of supersaturated carbide precipitation during the hot rolling stage, resulting in the steel strength of the composition of Example 2 in Table 1 being low after being processed under the various process conditions in Table 2 (see Table 3). However, if the ratio of titanium to nitrogen is below 3.5 and the laminar cooling rate is below -50 °C/s, the microstructure of the obtained steel is a ferrous iron phase and a pulverized iron structure. As the cooling rate is increased to above -60 °C/s, the microstructure of the obtained steel is a ferrous iron phase with a grain size of 2 to 8 µm, and has a plurality of precipitates dispersed in the ferrous iron phase, and the plurality of precipitates contain carbides with a particle size of less than 20 nm and nitrides with a particle size of less than 4 µm. The resulting steel has a yield strength of over 550 MPa, a yield ratio of 0.85 to 0.95, a total elongation of over 19%, and has excellent quality characteristics.

When the sulfur content is below 50 ppm, or the ratio of silicon to aluminum content is above 13, at the same welding voltage, the molten steel will flow toward the edge of the molten pool, resulting in poor weldability. When the sulfur content is 60 to 80 ppm, and the ratio to aluminum content is 9 to 13, at the same welding voltage, the molten steel will flow toward the center of the molten pool and in the penetration direction, with good weldability.

When the average shearing amount of the steel billet in the hot rolling rough rolling stage is larger, combined with a lower average shearing amount in the finishing rolling, a finer structure and better plate shape control can be obtained. Preferably, the average shearing amount of the hot rolling rough rolling is 25 to 35 mm, and the average shearing amount of the finishing rolling is less than 20 mm.

[Table 1] Differentiation Alloy composition and content (wt%) C Si Mn P S Cr Ni Cu V Al Ti N Ti/N Embodiment 1 0.07 0.47 0.42 0.09 0.004 0.68 - 0.27 - 0.04 - 0.006 0 Embodiment 2 0.07 0.52 0.58 0.09 0.007 1.1 0.08 0.32 0.06 0.04 0.07 0.023 3 Embodiment 3 0.09 0.35 0.48 0.12 0.008 0.81 - 0.28 0.07 0.03 0.18 0.015 12 Embodiment 4 0.11 0.88 0.82 0.14 0.008 1.12 0.06 0.2 0.09 0.07 0.21 0.015 14 [Table 2] Process conditions Heating furnace Hot rolling process Furnace temperature (°C) Furnace air-fuel ratio Average reduction of rough rolling per pass (mm) Average reduction of finishing rolling per pass (mm) Finishing temperature (°C) Laminar cooling cooling rate (°C/s) Steel belt speed (m/s) Median temperature (°C) Coil temperature (°C) Condition 1 1160~1250 1.07~ 1.12 22~32 17~12 Ar3+30~Ar3+50 -(30~50) 8~10 Ar1-20~Ar1-80 Ar1-80~Ar1-120 Condition 2 1080~1150 0.92~1.03 18~25 15~10 Ar3+30~Ar3+50 -(60~80) 8~10 Ar1+10~Ar1-40 Ar1-40~Ar1-60 Condition 3 1080~1150 0.9~ 0.95 20~30 16~13 Ar3+30~Ar3+50 -(80~120) 9~13 Ar1-20~Ar1-60 Ar1-80~Ar1-140 Condition 4 1050~1080 0.89~ 0.92 25~35 16~10 Ar3+10~Ar3+30 -(80~100) 8~11 Ar1-40~Ar1-80 Ar1-100~Ar1-160 [table 3] Steel Process conditions Microstructure Average mechanical properties Quality characteristics Phase composition*1 Average particle size of granular iron (µm) Average size of titanium carbide (nm) Average particle size of titanium nitride (µm) Yield strength (MPa) Yield ratio Elongation (%) Surface quality*2 Weather resistance*3 Weldability*4 Embodiment 1 Condition 1 F+P 5~8 - - 387 0.72 37.2 ◯ ◯ × Condition 2 F+P 7~10 - - 363 0.75 38.1 ◯ ◯ × Condition 3 F+P 4~6 - - 398 0.76 36.5 ◯ ◯ × Embodiment 2 Condition 1 F+P 4~6 11~15 1~3 582 0.88 24.3 ◯ ◯ ◯ Condition 2 F 5~7 16~18 2~4 560 0.91 25.5 ◯ ◎ ◯ Condition 4 F 2~4 ＜10 0.5~1.5 610 0.92 24.1 ◯ ◎ ◯ Embodiment 3 Condition 1 F+P 4~6 11~15 1~3 655 0.91 23.2 Δ ◯ ◯ Condition 2 F 6~8 16~18 2~4 682 0.9 21.5 × ◯ ◯ Condition 3 F 3~5 12~14 1~3 672 0.89 20.7 Δ ◎ ◯ Embodiment 4 Condition 2 F 5~7 16~18 2~4 595 0.9 23.1 ◯ ◎ ◯ Condition 3 F 3~5 11~14 1~3 694 0.89 19.5 ◯ ◎ ◯ Condition 4 F 2~4 ＜10 0.5~1.5 680 0.92 21.4 ◯ ◎ ◯

In summary, the microstructure of the steel obtained by the present invention can be a ferrous iron phase, the grain size of the ferrous iron is 2 to 8 μm, and there are a plurality of precipitates dispersed in the ferrous iron phase, the plurality of precipitates contain carbides with a particle size of less than 20 nm, and nitrides with a particle size of less than 4 μm. The steel of the present invention can have a yield strength of more than 550 MPa, a yield ratio of between 0.85 and 0.95, and a total elongation of more than 19%.

Although the present invention has been disclosed with preferred embodiments, they are not intended to limit the present invention. Any person skilled in the art may make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the scope defined in the attached patent application.

without

without

Claims (9)

A steel material comprising: based on a total weight of 100wt%, 0.07 to 0.12wt% carbon, 0.3 to 1wt% silicon, 0.4 to 1wt% manganese, 0.05 to 0.15wt% phosphorus, 0.002 to 0.01wt% sulfur, 0.01 to 0.1wt% aluminum, 0.2 to 0.4wt% copper, 0.2 to 1.5wt% chromium, 0.002 to 0.02wt% nitrogen, 94 to 98wt% iron, and no more than 0.15wt% vanadium. The steel material as described in claim 1, wherein the steel material also contains no more than 0.1wt% nickel. The steel material as described in claim 1, wherein the steel material also contains no more than 0.3wt% of titanium. The steel material as described in claim 3, wherein the weight ratio of the titanium to the nitrogen is between 3.42 and 14. The steel material as described in claim 1, wherein the steel material contains 0.4 to 1 wt% silicon. The steel material as described in claim 1, wherein the ratio of the silicon to the aluminum is 9 to 13. A method for manufacturing a steel material as described in claim 1, comprising: reheating a steel billet, wherein an air-fuel ratio is 0.85 to 1.2, the heating temperature is 1050°C to 1300°C, and the temperature is maintained for 1 to 3 hours; and the reheated steel billet is derusted by high-pressure water, and then rolled at a speed of 8 to 15 m/s, and the final rolling temperature is (Ar3+10)°C to (Ar3+50)°C, so as to obtain a finished rolled steel material. The method as described in claim 7, wherein the method further comprises: laminar cooling the rolled steel at a cooling rate of more than -30°C/s, and controlling the intermediate temperature at (Ar1-20)°C to (Ar1-80)°C. The method as described in claim 7, wherein the rolling of the steel material includes sequentially performing hot rough rolling and hot finish rolling, wherein in the hot rough rolling stage, the average reduction per pass is less than 40 mm, and in the hot finish rolling stage, the average reduction per pass is less than 20 mm.