Hot-rolled steel sheet

Abstract

In the hot-rolled steel sheet of the present invention, the content of carbon (C) is 0.005% by mass or more and 0.20% by mass or less, the content of silicon (Si) is 0.004% by mass or more and 0.55% by mass or less, the content of manganese (Mn) is 0.07% by mass or more and 1.65% by mass or less, the content of titanium (Ti) is 0.005% by mass or less, the content of boron (B) is 0.0005% by mass or less, and the remainder consists of iron (Fe), aluminum (Al), nitrogen (N), and unavoidable impurities. Furthermore, the content of nitrogen (N) is 0.0065% by mass or more and 0.0120% by mass or less, the content of aluminum (Al) is greater than 0.070% by mass or less and 0.110% by mass or less, and the content of dissolved nitrogen not combined with other components is 0.0020% by mass or less. Therefore, a hot-rolled steel sheet with excellent aging resistance and low cost can be provided.

Description

Technical Field

[0001] This invention relates to a hot-rolled steel plate, and more specifically, to a hot-rolled steel plate manufactured in an electric furnace using scrap steel as the main raw material. Background Technology

[0002] There are two main methods for manufacturing hot-rolled steel plates. One method uses natural iron ore as the main raw material and is carried out in a blast furnace. The other method uses recyclable scrap steel as the main raw material and is carried out in an electric furnace.

[0003] When hot-rolled steel plates are manufactured in blast furnaces, in addition to the energy required to melt iron ore, a large amount of coke is also needed to remove the oxygen contained in the iron ore, resulting in a large amount of carbon dioxide emissions.

[0004] In contrast, when manufacturing steel plates in an electric arc furnace, scrap steel is melted into steel plates, so a large amount of coke is not needed, which can significantly reduce carbon dioxide emissions compared to manufacturing steel plates in a blast furnace.

[0005] However, electric furnace materials manufactured by electric furnaces typically contain more nitrogen (N) than blast furnace materials. Nitrogen dissolved in steel accumulates in the iron lattice and hinders dislocations, thus making it prone to age hardening after manufacturing, resulting in reduced machinability.

[0006] It should be noted that age hardening is a phenomenon in which carbon and nitrogen, as intrusive solid solution atoms, accumulate around dislocations introduced by plastic deformation such as leveling and rolling, fixing mobile dislocations and thus producing yield elongation. This phenomenon becomes more pronounced with increasing solid solution carbon and nitrogen content in the steel.

[0007] Patent documents 1 and 2 disclose the following: by adding titanium (Ti) or boron (B) to the electric furnace material, nitrogen in the steel is precipitated in the form of nitrides, thereby reducing dissolved nitrogen, while controlling the content of elements Cu and Sn from scrap steel within a specified range, the aging resistance of hot-rolled steel plates can be improved.

[0008] Existing technical documents Patent documents Patent Document 1: Japanese Patent Application Publication No. 07-157845 Patent Document 2: Japanese Patent Application Publication No. 07-197195 Summary of the Invention

[0009] The problem that the invention aims to solve However, titanium (Ti) is expensive and increases the slab’s surface susceptibility to defects. In addition, the addition of boron (B) can lead to side effects such as welding cracks, coating cracks, and reduced ductility and porosity.

[0010] The present invention was made in view of the above-mentioned problems existing in the prior art, and its purpose is to provide a hot-rolled steel sheet with excellent aging resistance and low price.

[0011] Methods for solving problems The inventors conducted in-depth research to achieve the above objectives and found that by fixing the nitrogen in steel into aluminum nitride and reducing the dissolved nitrogen that has not combined with other components, the above objectives can be achieved, thus completing the present invention.

[0012] That is, the above-mentioned problems are solved by the following (1) to (3) of the present invention.

[0013] (1) A hot-rolled steel plate, characterized in that, The carbon (C) content is 0.005% by mass or more and 0.20% by mass or less. The silicon (Si) content is 0.004% by mass or more and 0.55% by mass or less. The manganese (Mn) content is 0.07% by mass or more and 1.65% by mass or less. The titanium (Ti) content is less than 0.005% by mass. The boron (B) content is less than 0.0005% by mass. The remaining portion consists of iron (Fe), aluminum (Al), nitrogen (N), and unavoidable impurities. The nitrogen (N) content mentioned above is 0.0065% by mass or more and 0.0120% by mass or less. The aluminum (Al) content mentioned above is greater than 0.070% by mass and less than 0.110% by mass. The content of dissolved nitrogen that is not combined with other components is less than 0.0020% by mass.

[0014] (2) The hot-rolled steel sheet according to item (1) above, characterized in that the content of aluminum (Al) is 0.000562 / the content of nitrogen (N) (mass%) or more.

[0015] (3) The hot-rolled steel sheet according to item (1) above, characterized in that, of the nitrogen (N), at least 80% by mass is aluminum nitride (AlN).

[0016] The effects of the invention According to the present invention, since nitrogen in steel is fixed as aluminum nitride, the amount of dissolved nitrogen that has not combined with other components is reduced, thus enabling the provision of hot-rolled steel sheets with excellent aging resistance and low cost. Attached Figure Description

[0017] Figure 1 This is a graph showing the changes in aging tests of Example 1 and Comparative Example 6 over time.

[0018] Figure 2 This is a graph showing the changes in aging tests of Example 4 and Comparative Example 7 over time.

[0019] Figure 3 These are graphs showing the particle analysis results of Example 1 and Comparative Example 1. Detailed Implementation

[0020] The hot-rolled steel sheet of the present invention will be described in detail.

[0021] In the hot-rolled steel sheet of the present invention, the carbon (C) content is 0.005% by mass or more and 0.20% by mass or less. The silicon (Si) content is 0.004% by mass or more and 0.55% by mass or less. The manganese (Mn) content is 0.07% by mass or more and 1.65% by mass or less. The titanium (Ti) content is less than 0.005% by mass. The boron (B) content is less than 0.0005% by mass. The remainder consists of iron (Fe), aluminum (Al), nitrogen (N), and unavoidable impurities.

[0022] Furthermore, the nitrogen (N) content is 0.0065% by mass or more and 0.0120% by mass or less. The aluminum (Al) content mentioned above is greater than 0.070% by mass and less than 0.110% by mass. The content of dissolved nitrogen that is not combined with other components is less than 0.0020% by mass.

[0023] The nitrogen contained in the steel billet (slab) obtained by smelting will combine with the aluminum in the steel billet during the cooling process and be fixed as aluminum nitride.

[0024] However, hot-rolled steel sheets are made by heating the slab (steel billet) to about 1150°C and rolling it through a subsequent hot rolling process. Therefore, in the above-mentioned hot rolling process, the temperature of the slab exceeds the melting temperature of aluminum nitride for a long time, and aluminum nitride will decompose and become nitrogen solid solution in the hot-rolled steel sheet that has not been combined with other components, thereby reducing its resistance to aging.

[0025] The inventors have noted that the melting temperature of aluminum nitride varies depending on the concentrations of nitrogen and aluminum contained in the steel billet (slab).

[0026] That is, by adjusting the aluminum content according to the amount of nitrogen it contains, the melting temperature of aluminum nitride can be increased, the decomposition of aluminum nitride in the hot rolling process can be suppressed, the amount of nitrogen dissolved in hot-rolled steel plate that has not combined with other components can be reduced, thereby improving the aging resistance of hot-rolled steel plate.

[0027] The melting temperature of the aluminum nitride in the face-centered cubic lattice structure can be obtained by the following equation (1).

[0028] log(Al×N)=-7400 / T+1.95 ･･･Equation (1) In equation (1), Al represents the mass percentage of aluminum, N represents the mass percentage of nitrogen, and T represents the absolute temperature.

[0029] The aluminum content relative to total nitrogen in hot-rolled steel plate depends on the temperature of the billet in the hot rolling process. However, according to the above formula (1), if it is 0.000562 / N (mass%) or higher, the melting temperature of aluminum nitride can be maintained above 1150°C. Therefore, it can suppress the melting of nitrogen that is fixed as aluminum nitride during billet (slab) manufacturing in the heating furnace of the hot rolling process, or promote its fixation.

[0030] Furthermore, nitrogen (N) in hot-rolled steel sheets preferably forms aluminum nitride (AlN) at a rate of 80% or more by mass.

[0031] Because the aluminum nitride precipitated in the billet (slab) is not decomposed during the hot rolling process, or the precipitation of aluminum nitride is promoted, more than 80% by mass of nitrogen in the hot-rolled steel plate remains in the form of aluminum nitride, thereby reducing the dissolved nitrogen in the hot-rolled steel plate that has not combined with other components and improving the aging resistance.

[0032] Next, the composition of the hot-rolled steel sheet of the present invention will be described. Unless otherwise specified, "%" refers to the concentration of the desired added element in the steel, i.e., "mass %".

[0033] The carbon (C) content is 0.005% or more and 0.20% or less. Carbon is a fundamental element for improving the strength of steel. When the carbon content is greater than 0.20%, the ductility, deep drawing and stretching and flanging formability of hot-rolled steel sheets will decrease.

[0034] Furthermore, the target for achieving low carbon emissions through conventional refining is usually above 0.005%, while controlling it to below 0.005% will lead to increased refining costs and a lower yield.

[0035] The silicon (Si) content is 0.004% by mass or more and 0.55% by mass or less. Silicon is an element with high solid solution strengthening ability and low price, which helps to improve the strength of steel, and therefore accounts for more than 0.004% by mass.

[0036] Furthermore, if the amount exceeds 0.55% by mass, a solid and uneven iron oxide scale will be formed during hot rolling. Even after pickling, iron oxide scale residue or pits will still appear on the surface of the steel plate, resulting in a deterioration in the surface quality of the final product.

[0037] The manganese (Mn) content is 0.07% by mass or more and 1.65% by mass or less. Manganese is not only an element that strengthens hot-rolled steel plates, but it also has the function of fixing the impurity element S that causes high-temperature embrittlement of steel into MnS, so it is above 0.07% by mass.

[0038] However, if added in excess, exceeding 1.65% by mass, it will impair formability and plating properties.

[0039] The titanium (Ti) content is less than 0.005% by mass. Titanium combines with nitrogen to precipitate as TiN, reducing dissolved nitrogen in the steel and thus improving aging resistance. In this invention, since dissolved nitrogen in the steel precipitates as aluminum nitride, there is no need to add titanium. Furthermore, excessive addition of titanium leads to hardening of the microstructure and the formation of coarse carbonitrides, resulting in reduced formability. Therefore, the scrap steel may contain less than 0.005% by mass.

[0040] The boron (B) content is less than 0.0005% by mass. Like titanium, boron combines with nitrogen to precipitate as BN, reducing dissolved nitrogen in the steel and thus improving aging resistance. In this invention, since dissolved nitrogen in the steel precipitates as aluminum nitride, there is no need to add boron. Furthermore, boron increases resistance to hot deformation, hindering production efficiency, and also increases oxide scale defects due to increased final rolling temperature; therefore, its content is less than 0.0005% by mass.

[0041] The remaining portion, besides the elements mentioned above, consists of iron (Fe), nitrogen (N), aluminum (Al), and unavoidable impurities.

[0042] The nitrogen (N) content is ≥0.0065% by mass and ≤0.0120% by mass. Nitrogen is an unavoidable impurity in steel. Especially when steel is smelted in an electric furnace, the atomic reactive nitrogen generated during the electric arc discharge will penetrate into the molten steel, resulting in a higher N content compared to steel smelted in a blast furnace-converter process.

[0043] Regarding nitrogen (N) content, the lower the content, the better, in order to improve strain resistance and processability. If the content is too high, it will lead to poor formability and tensile strain marks caused by aging hardening.

[0044] In this invention, nitrogen dissolved in steel is precipitated in the form of aluminum nitride to fix nitrogen, so there is no need to reduce the nitrogen content through degassing treatment, and the nitrogen content is 0.0065% by mass or more and 0.0120% by mass or less.

[0045] Since the nitrogen fixation effect eliminates the portion of the increase in tensile strength caused by dissolved nitrogen, it is necessary to compensate for the increase in tensile strength by adjusting the carbon equivalent when necessary.

[0046] The aluminum (Al) content is greater than 0.070% by mass and less than 0.110% by mass. Aluminum can cause dissolved nitrogen in steel to precipitate as aluminum nitride and is effective in deoxidizing steel, so it is an element that is actively added.

[0047] However, like Si, excessive Al will lead to increased iron oxide scale defects, decreased plating properties, and consequently poor weldability. Therefore, the limit is greater than 0.070% by mass and less than 0.110% by mass.

[0048] As unavoidable impurities, phosphorus (P), sulfur (S), and elements derived from scrap steel such as copper (Cu), tin (Sn), nickel (Ni), chromium (Cr), molybdenum (Mo), vanadium (V), and niobium (Nb) can be listed. The total content of these elements is less than 3% by mass.

[0049] Similar to manganese (Mn), phosphorus can cause uneven concentration during solidification, thus impairing the surface quality after stamping. Furthermore, adding large amounts of phosphorus has significant drawbacks, such as reducing resistance to secondary processing brittleness and worsening the plating properties of the steel sheet. To avoid these problems, the phosphorus content is preferably below 0.050%.

[0050] Sulfur is inevitably present in steel, and its segregation at grain boundaries can lead to cracking in slabs and hot-rolled plates due to hot brittleness, as well as decreased resistance to secondary processing brittleness. Therefore, the sulfur content is preferably below 0.050%.

[0051] Copper can improve the corrosion resistance and strength of steel plates, so it can be contained in more than 0.01% of the material. However, excessive addition can lead to a decrease in heat ductility and cause cracks in slabs and hot-rolled plates. Therefore, it is preferable to contain less than 1%.

[0052] Tin has the effect of inhibiting oxidation on the surface of steel plates, which helps to reduce iron oxide scale defects, surface nitriding and decarburization, thereby maintaining surface quality. However, excessive amounts will worsen the formability. Therefore, its content is preferably below 0.05%.

[0053] Nickel can improve the corrosion resistance and low-temperature toughness of steel plates, but excessive amounts will lead to increased strength and poor machinability. Therefore, its content is preferably below 0.20%.

[0054] Chromium can improve the corrosion resistance of steel plates and increase strength by forming carbides, but excessive amounts will lead to increased strength but poor workability. Therefore, its content is preferably below 0.30%.

[0055] Excessive molybdenum content can lead to increased strength but decreased processability; therefore, its content is preferably below 0.05%.

[0056] Vanadium can improve processability and weld toughness, but excessive addition may lead to a decrease in the toughness of hot-rolled steel sheets. Therefore, its content is preferably below 0.005%.

[0057] Niobium is effective in improving oxidation resistance at high temperatures, but excessive content may reduce the toughness of hot-rolled steel sheets. Therefore, its content is preferably below 0.005%. Example

[0058] The present invention will be described in detail below through embodiments, but the present invention is not limited to the following embodiments.

[0059] Slabs were prepared by electric furnace melting using steel with the chemical composition shown in Table 1.

[0060] Among them, Comparative Examples 1 to 3 were melted after degassing treatment, while Comparative Examples 4 to 10 and Examples 1 to 10 were melted without degassing treatment.

[0061] The slab was then heated in a furnace, with the atmosphere temperature not exceeding 1230°C. It was then extracted and hot-rolled at approximately 1150°C. Under these conditions, hot-rolled steel sheets with thicknesses of 1.5mm, 2.0mm, 2.1mm, 2.3mm, 4.15mm, and 16mm were manufactured.

[0062] Table 1 The Al content of the hot-rolled steel plates of Examples 1-10 and Comparative Examples 1-10 was determined according to the method of iron and steel-spark discharge emission spectroscopy analysis in JIS G 1253.

[0063] The total nitrogen content of the hot-rolled steel plates of Examples 1-10 and Comparative Examples 1-10 was determined according to JIS G 1228-3 Iron and Steel - Quantitative Methods for Nitrogen - Part 3: Inert Gas Melting-Thermal Conductivity Method.

[0064] In addition, the nitrides precipitated in the hot-rolled steel sheet were subjected to 10% by mass AA electrolytic extraction (20A / cm² constant current), and collected using a filter (0.2μm sieve). After decomposition, the nitrogen fixed in the form of nitrides was determined by steam distillation with 50% NaOH and the method of bispyrazolone spectrophotometry.

[0065] The difference between the total nitrogen content and the amount of nitrogen fixed in the form of nitrides is calculated as the amount of dissolved nitrogen.

[0066] JIS No. 5 tensile test specimens were prepared from the above steel plates, and their aging index (AI) was determined.

[0067] The aging index was obtained by applying a 7.5% pre-strain, heating at 100°C for 3600 seconds to promote aging, followed by a tensile test, and dividing the difference between the lower yield load after aging and the maximum load when the pre-strain was applied by the cross-sectional area of ​​the parallel part of the test piece before pre-strain.

[0068] The measurement results are shown in Table 2.

[0069] Table 2 As can be seen from the results in Table 2, the hot-rolled steel plate of the present invention has a low aging index and excellent aging resistance.

[0070] The real-time aging test results for Examples 1 and 6 (SPHC) and Examples 4 and 7 (SS400) are shown below. Figure 1 , Figure 2 Compared to the comparative example, the results show that the increase in yield point and the decrease in elongation are smaller in the embodiment. Therefore, the hot-rolled steel sheet of the present invention has also been confirmed to have excellent aging resistance in real-time testing.

[0071] <Particle Analysis> Particle analysis is performed according to the following steps.

[0072] • Acquire reflected electron images while adjusting the contrast so that metallic parts appear white and non-metallic parts appear black.

[0073] • Irradiate each non-metallic part in the acquired reflected electron image with an electron beam, and obtain the composition information of the non-metallic part by analyzing the obtained X-rays.

[0074] • Confirm the obtained data of the non-metallic part, and count the locations where Al and N are detected simultaneously, and the locations where Al and O are detected simultaneously.

[0075] The results are as follows Figure 3 As shown.

[0076] Equipment used: JSM-7100F electron microscope manufactured by NEC Corporation and AZtec Energy spectrometer manufactured by Oxford Instruments. Observation conditions: 1000x magnification, WD 10mm, total measurement area 1mm², accelerating voltage 15kV like Figure 3 As shown, in the examples with high Al and N contents, a large amount of Al+N was detected, with a circular equivalent diameter of less than 1 μm. AlN was not detected in the comparative examples.

[0077] <Mechanical Properties & Machinability> Tensile test specimens were collected for the hot-rolled steel sheets of Examples 1-10 and Comparative Examples 1-10 (JIS No. 5 test specimens were used for SPHC and SPHT3 steel grades, and JIS No. 1A test specimens were used for SS400 steel grade), and tensile tests (yield point, tensile strength, and elongation) were performed. Simultaneously, hole expansion tests and Eriksen tests were conducted on hot-rolled steel sheets other than those with a thickness of 16 mm to evaluate processability. The evaluation results are shown in Table 3.

[0078] Table 3 Despite the addition of Al, no decrease in strength or reduced processability was observed in the hot-rolled steel sheet of the present invention.

[0079] <Plane fatigue bending> To evaluate the effect of adding Al on fatigue properties, a planar fatigue bending test was conducted on SPHC material. The results are shown in Table 4.

[0080] Table 4 The fatigue limit ratios of Example 1 and Comparative Example 5 were the same, and no effect of adding Al on fatigue properties was observed.

[0081] The microstructures of the metals in the examples and comparative examples are ferrite + pearlite, and the grain size grades measured according to the JIS G0551 comparative method are 9.6 to 10.4.

Claims

1. A hot-rolled steel plate, characterized in that, The carbon (C) content is 0.005% by mass or more and 0.20% by mass or less. The silicon (Si) content is 0.004% by mass or more and 0.55% by mass or less. The manganese (Mn) content is 0.07% by mass or more and 1.65% by mass or less. The titanium (Ti) content is less than 0.005% by mass. The boron (B) content is less than 0.0005% by mass. The remaining portion consists of iron (Fe), nitrogen (N), aluminum (Al), and unavoidable impurities. The nitrogen (N) content mentioned above is 0.0065% by mass or more and 0.0120% by mass or less. The aluminum (Al) content mentioned above is greater than 0.070% by mass and less than 0.110% by mass. The content of dissolved nitrogen that is not combined with other components is less than 0.0020% by mass.

2. Hot-rolled steel sheet according to claim 1, characterized in that, The aluminum (Al) content is 0.000562 / nitrogen (N) content (mass%) or higher.

3. The hot-rolled steel sheet according to claim 1, characterized by, Of the nitrogen (N) mentioned above, more than 80% by mass form aluminum nitride (AlN).

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