Hot-rolled steel sheet

Abstract

A hot-rolled steel sheet of the present invention has a carbon (C) content of 0.005 mass% to 0.20 mass%, a silicon (Si) content of 0.004 mass% to 0.55 mass%, a manganese (Mn) content of 0.07 mass% to 1.65 mass%, a titanium (Ti) content of 0.005 mass% or less, a boron (B) content of 0.0005 mass% or less, and the remainder consists of iron (Fe), aluminum (Al), nitrogen (N), and unavoidable impurities.　Furthermore, by setting the nitrogen (N) content to be 0.0065 mass% to 0.0120 mass%, the aluminum (Al) content to be more than 0.070 mass% but at most 0.110 mass%, and the content of solid-solution nitrogen not combined with other components to be 0.0020 mass% or less, it is possible to provide an inexpensive hot-rolled steel sheet with excellent aging resistance.

Description

Hot-rolled steel sheet

[0001] The present invention relates to a hot-rolled steel sheet, and more particularly to a hot-rolled steel sheet manufactured in an electric furnace using iron scrap as a main raw material.

[0002] As methods for manufacturing a hot-rolled steel sheet, there are two methods: a method of manufacturing in a blast furnace using iron ore, which is a natural resource, as a main raw material, and a method of manufacturing in an electric furnace using iron scrap, which is a recycled resource, as a main raw material.

[0003] When manufacturing a hot-rolled steel sheet in a blast furnace, in addition to the energy for melting iron ore, a large amount of coke needs to be used to remove oxygen contained in the iron ore, resulting in a large amount of carbon dioxide emissions.

[0004] On the other hand, when manufacturing in an electric furnace, since iron scrap is melted into a steel sheet, there is no need to use a large amount of coke, and the carbon dioxide emissions can be significantly reduced compared to the manufacturing of steel sheets in a blast furnace.

[0005] However, the electric furnace materials manufactured in the above electric furnace generally have a higher nitrogen (N) content compared to blast furnace materials. Nitrogen dissolved in the steel accumulates in the iron crystal lattice and hinders dislocations, so age hardening progresses after manufacturing and the workability is likely to deteriorate.

[0006] Incidentally, age hardening is a phenomenon in which carbon and nitrogen, which are interstitial solute atoms, accumulate around dislocations introduced by plastic deformation such as quenching and tempering rolling, and the mobile dislocations are pinned, resulting in yield point elongation. The phenomenon becomes more prominent as the content of dissolved carbon and dissolved nitrogen in the steel increases.

[0007] In Patent Documents 1 and 2, it is disclosed that by adding titanium (Ti) or boron (B) to electric furnace materials and precipitating nitrogen in the steel as nitrides to reduce the dissolved nitrogen, and by setting the contents of Cu and Sn, which are elements derived from iron scrap, within a predetermined range, the age resistance of the hot-rolled steel sheet is improved.

[0008] Japanese Patent Application Laid-Open No. 07-157845, Japanese Patent Application Laid-Open No. 07-197195

[0009] However, titanium (Ti) is expensive, increases susceptibility to surface defects in slabs, and the addition of boron (B) leads to side effects such as weld cracking, plating cracking, and deterioration of ductility and hole-expanding properties.

[0010] This invention has been made in view of the problems of the prior art, and its objective is to provide an inexpensive hot-rolled steel sheet with excellent aging resistance.

[0011] The inventors of this invention, after conducting extensive research to achieve the above objective, discovered that the objective can be achieved by fixing nitrogen in steel as aluminum nitride and reducing the amount of dissolved nitrogen that has not combined with other components, and thus completed the present invention.

[0012] In other words, the above problems are solved by the following (1) to (3) of the present invention. (1) A hot-rolled steel sheet characterized by having a carbon (C) content of 0.005% by mass or more and 0.20% by mass or less, a silicon (Si) content of 0.004% by mass or more and 0.55% by mass or less, a manganese (Mn) content of 0.07% by mass or more and 1.65% by mass or less, a titanium (Ti) content of 0.005% by mass or less, a boron (B) content of 0.0005% by mass or less, the remainder consisting of iron (Fe), aluminum (Al), nitrogen (N), and unavoidable impurities, the nitrogen (N) content of 0.0065% by mass or more and 0.0120% by mass or less, the aluminum (Al) content exceeding 0.070% by mass and 0.110% by mass or less, and the content of solid-solution nitrogen not combined with other components being 0.0020% by mass or less. (2) The hot-rolled steel sheet according to item (1) above, characterized in that the aluminum (Al) content is 0.000562 / nitrogen (N) content (mass%) or more. (3) The hot-rolled steel sheet according to item (1) above, characterized in that 80% by mass or more of the nitrogen (N) forms aluminum nitride (AlN).

[0013] According to the present invention, by fixing nitrogen in the steel as aluminum nitride and reducing the amount of dissolved nitrogen that has not combined with other components, it is possible to provide an inexpensive hot-rolled steel sheet with excellent aging resistance.

[0014] This graph shows the changes over time in the aging tests of Example 1 and Comparative Example 6. This graph shows the changes over time in the aging tests of Example 4 and Comparative Example 7. This graph shows the particle analysis results of Example 1 and Comparative Example 1.

[0015] The hot-rolled steel sheet of the present invention will now be described in detail. The hot-rolled steel sheet of the present invention has a carbon (C) content of 0.005% by mass or more and 0.20% by mass or less, a silicon (Si) content of 0.004% by mass or more and 0.55% by mass or less, a manganese (Mn) content of 0.07% by mass or more and 1.65% by mass or less, a titanium (Ti) content of 0.005% by mass or less, a boron (B) content of 0.0005% by mass or less, and the remainder consists of iron (Fe), aluminum (Al), nitrogen (N), and unavoidable impurities. The nitrogen (N) content is 0.0065% by mass or more and 0.0120% by mass or less, the aluminum (Al) content is greater than 0.070% by mass and 0.110% by mass or less, and the content of solid-solution nitrogen that has not combined with other components is 0.0020% by mass or less.

[0016] The nitrogen contained in the molten steel slab combines with the aluminum in the slab as it cools, and is fixed as aluminum nitride.

[0017] However, since hot-rolled steel sheets are produced by heating and rolling a slab (steel billet) to approximately 1150°C in the subsequent hot-rolling process, the slab temperature remains above the melting point of aluminum nitride for a long period during the hot-rolling process. This causes the aluminum nitride to decompose and dissolve as nitrogen in the hot-rolled steel sheet without being combined with other components, thereby reducing its aging resistance.

[0018] The inventors focused on the fact that the melting temperature of aluminum nitride changes depending on the nitrogen and aluminum concentrations contained in the steel billet (slab).

[0019] In other words, by adjusting the aluminum content in accordance with the amount of nitrogen contained, the melting temperature of aluminum nitride can be increased, suppressing the decomposition of aluminum nitride during the hot rolling process, reducing the amount of nitrogen that is not combined with other components dissolved in the hot-rolled steel sheet, and improving the aging resistance of the hot-rolled steel sheet.

[0020] The melting temperature of the above aluminum nitride in a face-centered cubic lattice structure can be determined by the following formula (1): log(Al×N) = -7400 / T + 1.95 ...Formula (1) where Al is the mass percentage of aluminum, N is the mass percentage of nitrogen, and T is the absolute temperature.

[0021] The ratio of aluminum to total nitrogen in a hot-rolled steel sheet depends on the temperature of the steel billet during the hot-rolling process, but according to formula (1) above, if it is 0.000562 / N (mass%) or higher, the melting temperature of aluminum nitride can be maintained at 1150°C or higher. This suppresses the melting of nitrogen fixed as aluminum nitride during steel billet (slab) production in the heating furnace during the hot-rolling process, or promotes its fixation.

[0022] Furthermore, it is preferable that 80% or more of the nitrogen (N) in the hot-rolled steel sheet forms aluminum nitride (AlN).

[0023] The aluminum nitride precipitated in the steel billet (slab) is not decomposed during the hot rolling process, or its precipitation is accelerated, resulting in more than 80% by mass of nitrogen remaining as aluminum nitride in the hot-rolled steel sheet. This reduces the amount of dissolved nitrogen that has not combined with other components in the hot-rolled steel sheet, thereby improving its aging resistance.

[0024] Next, the component composition of the hot-rolled steel sheet of the present invention will be described. Unless otherwise specified, "%" refers to "mass%" indicating the concentration of the target additive element in the steel.

[0025] The carbon (C) content is between 0.005% and 0.20%. Carbon is a fundamental element that increases the strength of steel, and if the carbon content exceeds 0.20%, the ductility, deep drawability, and elongation flangeability of hot-rolled steel sheets decrease. Furthermore, in normal refining processes, a carbon content of 0.005% or higher is the guideline, and reducing it to below 0.005% worsens refining costs and yield.

[0026] The silicon (Si) content should be between 0.004% by mass and 0.55% by mass. Silicon is an inexpensive element with high solid solution strengthening ability and contributes to increasing the strength of steel, hence the recommended content is 0.004% by mass or more. If the content exceeds 0.55% by mass, a strong and unevenly thick scale forms during hot rolling, resulting in scale residue and indentations on the steel sheet surface even after pickling, degrading the surface quality of the final product.

[0027] The manganese (Mn) content should be between 0.07% by mass and 1.65% by mass. Manganese is not only an element that strengthens hot-rolled steel sheets, but it also has the effect of fixing sulfur (S), an impurity element that causes high-temperature embrittlement of steel, as MnS, hence the content should be 0.07% by mass or more. However, if added in excess, exceeding 1.65% by mass, formability and plating properties will be impaired.

[0028] Titanium (Ti) content of 0.005% by mass or less. Titanium combines with nitrogen and precipitates as TiN, reducing the amount of dissolved nitrogen in the steel and improving its aging resistance. In this invention, the dissolved nitrogen in the steel is precipitated as aluminum nitride, so there is no need to add titanium. Furthermore, excessive addition of titanium leads to hardening of the structure and formation of coarse carbonitrides, resulting in a decrease in formability. Therefore, the amount of titanium that can be contained in iron scrap is 0.005% by mass or less.

[0029] Boron (B) content is 0.0005% by mass or less. Boron, like titanium, combines with nitrogen and precipitates as BN, reducing the amount of dissolved nitrogen in the steel and improving aging resistance. In this invention, the dissolved nitrogen in the steel is precipitated as aluminum nitride, so there is no need to add boron. Furthermore, boron increases the resistance to hot deformation, hindering productivity, and increases the amount of scale defects due to the rise in finishing temperature, so the content is 0.0005% by mass or less.

[0030] The remainder of the material, other than the elements mentioned above, consists of iron (Fe), nitrogen (N), aluminum (Al), and unavoidable impurities.

[0031] The nitrogen (N) content is between 0.0065% by mass and 0.0120% by mass. Nitrogen is an unavoidable impurity in steel. In particular, when steel is melted in an electric furnace, atomically active nitrogen generated during arc discharge enters the molten steel, resulting in a higher N content compared to when steel is melted in a blast furnace-converter method.

[0032] The nitrogen (N) content is desirable to be low in order to improve strain aging resistance and workability, as high levels can lead to deterioration of formability and the occurrence of stretcher strain due to age hardening. In this invention, the nitrogen is fixed by precipitating the dissolved nitrogen in the steel as aluminum nitride, so there is no need to reduce the nitrogen content by degassing, and the nitrogen content is 0.0065% by mass or more and 0.0120% by mass or less. Since the increase in tensile strength caused by the dissolved nitrogen is eliminated by the above nitrogen fixation, it is necessary to adjust the carbon equivalent to compensate for the tensile strength as needed.

[0033] Aluminum (Al) content is between 0.070% by mass and 0.110% by mass. Aluminum is an element that is actively added because it precipitates dissolved nitrogen in steel as aluminum nitride and is effective in deoxidizing steel. However, excessive Al, like Si, leads to an increase in scale defects, a decrease in plating properties, and a deterioration of weldability, so its content is between 0.070% by mass and 0.110% by mass.

[0034] Inevitable impurities include phosphorus (P), sulfur (S), and elements derived from iron scrap, such as copper (Cu), tin (Sn), nickel (Ni), chromium (Cr), molybdenum (Mo), vanadium (V), and niobium (Nb). The total content of these elements is 3% by mass or less.

[0035] Phosphorus, like Mn, causes concentration non-uniformity during solidification, damaging the surface quality after pressing. Also, adding a large amount of P has significant demerits such as reducing the resistance to secondary processing brittleness and deteriorating the plating property of the steel sheet. To avoid these, the phosphorus content is preferably 0.050% or less.

[0036] Sulfur is inevitably contained in steel and segregates at grain boundaries, leading to cracking of slabs and hot-rolled sheets due to red-hot embrittlement and deterioration of the resistance to secondary processing brittleness. Therefore, the sulfur content is preferably 0.050% or less.

[0037] Copper can improve the corrosion resistance and strength of the steel sheet, so it may be contained at 0.01% or more. However, excessive addition causes cracking of slabs and hot-rolled sheets due to a decrease in hot ductility, so it is preferably 1% or less.

[0038] Tin has the effect of suppressing surface oxidation of the steel sheet and is effective in maintaining surface quality by reducing scale defects, surface nitriding, and decarburization. However, if it becomes excessive, the formability deteriorates, so the content is preferably 0.05% or less.

[0039] Nickel can improve the corrosion resistance and low-temperature toughness of the steel sheet. However, if it becomes excessive, the strength increases and the workability deteriorates, so the content is preferably 0.20% or less.

[0040] Chromium can improve the corrosion resistance of the steel sheet and the strength by forming carbides. However, if it becomes excessive, the strength increases and the workability deteriorates, so the content is preferably 0.30% or less.

[0041] Molybdenum, if excessive, increases the strength and deteriorates the workability, so the content is preferably 0.05% or less.

[0042] Vanadium improves workability and the toughness of the welded part. However, if added excessively, it may deteriorate the toughness of the hot-rolled steel sheet, so the content is preferably 0.005% or less.

[0043] Niobium is effective in improving oxidation resistance at high temperatures, but if it is contained excessively, it may reduce the toughness of the hot-rolled steel sheet. Therefore, the content is preferably 0.005% or less.

[0044] Hereinafter, the present invention will be described in detail with reference to examples, but the present invention is not limited to the following examples.

[0045] Steel materials having the chemical component compositions shown in Table 1 were melted in an electric furnace to prepare slabs. Note that Comparative Examples 1 to 3 were degassed to melt the slabs, and Comparative Examples 4 to 10 and Examples 1 to 10 were melted without degassing.

[0046] Next, this slab was heated in a heating furnace so that the ambient temperature did not exceed 1230 °C at maximum, and the slab temperature was extracted at about 1150 °C and hot-rolled. Under the above conditions, hot-rolled steel sheets with thicknesses of 1.5 mm, 2.0 mm, 2.1 mm, 2.3 mm, 4.15 mm, and 16 mm were manufactured.

[0047]

[0048] The Al contents of the hot-rolled steel sheets of Examples 1 to 10 and Comparative Examples 1 to 10 were measured by the spark discharge optical emission spectrometry method of JIS G 1253, Iron and Steel.

[0049] The total nitrogen contents of the hot-rolled steel sheets of Examples 1 to 10 and Comparative Examples 1 to 10 were measured by the method of JIS G 1228-3, Iron and Steel - Nitrogen Determination Method - Part 3: Inert Gas Fusion - Thermal Conductivity Method. Further, nitrides precipitated in the hot-rolled steel sheet were electrolytically extracted at 10 mass% AA (20 A / cm ２ constant current), collected with a filter (0.2 μm mesh), and decomposed with (H ２ SO ４ +K ２ SO ４ ), and then distilled with 50 mass% NaOH steam, and the amount of nitrogen fixed as nitrides was measured by the bis-pyrazolone absorptiometry method. The amount of dissolved nitrogen was calculated as the difference between the total nitrogen content and the amount of nitrogen fixed as nitrides.

[0050] A JIS No. 5 tensile test specimen was prepared from the above steel plate, and its aging index (AI) was measured. The aging index was determined by applying a 7.5% pre-strain, then heating at 100°C for 3600 seconds to accelerate aging, followed by a tensile test. The difference between the lower yield load after accelerated aging and the maximum load when pre-strain was applied was divided by the cross-sectional area of ​​the parallel section of the specimen before pre-strain. The measurement results are shown in Table 2.

[0051]

[0052] The results in Table 2 show that the hot-rolled steel sheet of the present invention has a low aging index and excellent aging resistance.

[0053] Figures 1 and 2 show the results of the real-time aging tests for Example 1 and Comparative Example 6 (SPHC) and Example 4 and Comparative Example 7 (SS400). Compared to the comparative examples, the examples showed a smaller increase in yield point and a smaller decrease in elongation. This confirms that the hot-rolled steel sheet of the present invention exhibits excellent aging resistance even in real-time tests.

[0054] <Particle Analysis> Particle analysis was performed in the following manner: - A backscattered electron image was acquired with the contrast adjusted so that metallic parts appeared white and non-metallic parts appeared black. - Composition information of the non-metallic parts was obtained by irradiating each non-metallic part of the acquired backscattered electron image with an electron beam and analyzing the resulting X-rays. - The obtained data of the non-metallic parts was checked, and the locations where Al and N were detected simultaneously, and where Al and O were detected simultaneously were counted. The results are shown in Figure 3.

[0055] Equipment used: JEOL JSM-7100F electron microscope and Oxford Instruments AZtec Energy energy-dispersive X-ray spectrometer. Observation conditions: Magnification 1000x, WD 10mm, total measurement area 1mm. ２ Acceleration voltage 15kV

[0056] As shown in Figure 3, in the examples with high levels of both Al and N, numerous Al+N molecules were detected, with an equivalent circle diameter of 1 μm or less. In the comparative example, no AlN was detected.

[0057] <Mechanical Properties and Workability> Tensile test specimens (JIS No. 5 specimens for SPHC and SPHT3 steel grades, and JIS No. 1A specimens for SS400 steel grade) were taken from the hot-rolled steel sheets of Examples 1 to 10 and Comparative Examples 1 to 10, and tensile tests (yield point, tensile strength, elongation) were performed. In addition, hole expansion tests and Erichsen tests were performed on the hot-rolled steel sheets, excluding the 16 mm thick sheet, to evaluate workability. The evaluation results are shown in Table 3.

[0058]

[0059] Despite the addition of Al, the hot-rolled steel sheet of the present invention did not show any decrease in strength or workability.

[0060] <Plane Fatigue Bending> To evaluate the effect of Al addition on fatigue properties, a plane fatigue bending test was conducted on SPHC material. The results are shown in Table 4.

[0061]

[0062] The fatigue limit ratios of Example 1 and Comparative Example 5 were equivalent, and no effect of Al addition on fatigue characteristics was observed.

[0063] The metallographic structures of both the examples and comparative examples were ferrite + pearlite structures, and the particle size numbers according to the JIS G0551 comparative method ranged from 9.6 to 10.4.

Claims

1. A hot-rolled steel sheet characterized by having a carbon (C) content of 0.005% by mass or more and 0.20% by mass or less, a silicon (Si) content of 0.004% by mass or more and 0.55% by mass or less, a manganese (Mn) content of 0.07% by mass or more and 1.65% by mass or less, a titanium (Ti) content of 0.005% by mass or less, a boron (B) content of 0.0005% by mass or less, the remainder consisting of iron (Fe), nitrogen (N), aluminum (Al), and unavoidable impurities, with a nitrogen (N) content of 0.0065% by mass or more and 0.0120% by mass or less, an aluminum (Al) content exceeding 0.070% by mass and 0.110% by mass or less, and a solid-solution nitrogen content not combined with other components of 0.0020% by mass or less.

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

3. The hot-rolled steel sheet according to claim 1, characterized in that 80% or more by mass of the nitrogen (N) forms aluminum nitride (AlN).

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