Hot-rolled steel plate

Abstract

The hot-rolled steel sheet of the present invention is a carbon (C) content of 0.005% by mass or more and 0.20% by mass or less; Silicon (Si) content of 0.004 mass% or more and 0.55 mass% or less, A manganese (Mn) content of 0.07 mass% or more and 1.65 mass% or less, Titanium (Ti) content is 0.005 mass% or less, The content of boron (B) is 0.0005% by mass or less, The balance consists of iron (Fe), aluminum (Al), nitrogen (N), and inevitable impurities. The nitrogen (N) content is 0.0065% by mass or more and 0.0120% by mass or less, The content of the aluminum (Al) is more than 0.070% by mass and 0.110% by mass or less, Since the content of dissolved nitrogen that is not combined with other elements is set to 0.0020 mass % or less, it is possible to provide an inexpensive hot-rolled steel sheet that has excellent aging resistance.

Description

[Technical Field]

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

[0002] There are two methods for producing hot-rolled steel sheets: one is to produce them in a blast furnace using iron ore, a natural resource, as the main raw material, and the other is to produce them in an electric furnace using iron scrap, a recycled resource, as the main raw material.

[0003] When producing hot-rolled steel sheets in a blast furnace, in addition to the energy required to melt the iron ore, a large amount of coke must be used to remove the oxygen contained in the iron ore, resulting in large carbon dioxide emissions.

[0004] In contrast, when manufacturing steel in an electric furnace, scrap iron is melted to make steel plate, so there is no need to use as much coke, and carbon dioxide emissions can be significantly reduced compared to manufacturing steel plate in a blast furnace.

[0005] However, electric furnace steel produced in the above electric furnace generally contains a higher amount of nitrogen (N) than blast furnace steel, and the nitrogen dissolved in the steel collects in the iron crystal lattice and blocks dislocations, which causes age hardening after production and tends to reduce workability.

[0006] Age hardening is a phenomenon in which carbon and nitrogen, which are interstitial solute atoms, gather around dislocations introduced by plastic deformation such as temper rolling, pinning the mobile dislocations and causing yield point elongation. This phenomenon becomes more pronounced as the content of solute carbon and solute nitrogen in the steel increases.

[0007] Patent Documents 1 and 2 disclose that the aging resistance of hot-rolled steel sheets can be improved by adding titanium (Ti) and boron (B) to electric furnace steel, precipitating the nitrogen in the steel as nitrides, thereby reducing the amount of dissolved nitrogen, and by keeping the contents of Cu and Sn, which are elements derived from iron scrap, within specified ranges. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 07-157845 [Patent Document 2] Japanese Patent Publication No. 07-197195 Summary of the Invention [Problem to be solved by the invention]

[0009] However, titanium (Ti) is expensive and increases the susceptibility of slab surface defects, and the addition of boron (B) causes side effects such as weld cracking, plating cracking, and deterioration of ductility and hole expandability.

[0010] The present invention has been made in view of the problems associated with the prior art, and an object of the present invention is to provide an inexpensive hot-rolled steel sheet having excellent aging resistance. [Means for solving the problem]

[0011] As a result of extensive research into achieving the above object, the present inventors have found that the above object can be achieved by fixing nitrogen in steel as aluminum nitride and reducing the amount of dissolved nitrogen that is not combined with other components, and have thus completed the present invention.

[0012] That is, the above problems are solved by the following (1) to (3) of the present invention. (1) The carbon (C) content is 0.005% by mass or more and 0.20% by mass or less; Silicon (Si) content of 0.004 mass% or more and 0.55 mass% or less, A manganese (Mn) content of 0.07 mass% or more and 1.65 mass% or less, Titanium (Ti) content is 0.005 mass% or less, The content of boron (B) is 0.0005% by mass or less, the balance consisting 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 content of the aluminum (Al) is more than 0.070% by mass and 0.110% by mass or less, A hot-rolled steel sheet characterized in that the content of dissolved nitrogen not combined with other elements is 0.0020 mass% or less. (2) The hot-rolled steel sheet according to the above item (1), characterized in that the content of aluminum (Al) is 0.000562 / nitrogen (N) content (mass%) or more. (3) The hot-rolled steel sheet according to the above item (1), wherein 80 mass % or more of the nitrogen (N) forms aluminum nitride (AlN). [Effects of the Invention]

[0013] According to the present invention, nitrogen in steel is fixed as aluminum nitride and the amount of dissolved nitrogen that is not combined with other components is reduced, so that an inexpensive hot-rolled steel sheet with excellent aging resistance can be provided. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a graph showing the changes over time in an aging test of Example 1 and Comparative Example 6. [Figure 2] 1 is a graph showing the changes over time in an aging test of Example 4 and Comparative Example 7. [Figure 3] 1 is a graph showing the particle analysis results of Example 1 and Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0015] The hot-rolled steel sheet of the present invention will be described in detail below. 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, Silicon (Si) content of 0.004 mass% or more and 0.55 mass% or less, A manganese (Mn) content of 0.07 mass% or more and 1.65 mass% or less, Titanium (Ti) content is 0.005 mass% or less, The content of boron (B) is 0.0005% by mass or less, The balance consists of iron (Fe), aluminum (Al), nitrogen (N), and inevitable impurities. The nitrogen (N) content is 0.0065% by mass or more and 0.0120% by mass or less, The content of the aluminum (Al) is more than 0.070% by mass and 0.110% by mass or less, The content of dissolved nitrogen that is not combined with other components is 0.0020 mass % or less.

[0016] Nitrogen contained in the slab of steel produced by melting combines with aluminum in the slab when the slab is cooled, and is fixed as aluminum nitride.

[0017] However, hot-rolled steel sheets are produced by heating a slab (steel billet) to approximately 1150°C and rolling it in the subsequent hot-rolling process. During the hot-rolling process, the temperature of the slab exceeds the melting temperature of aluminum nitride for a long period of time, causing the aluminum nitride to decompose and dissolve in the hot-rolled steel sheet as nitrogen that is not combined with other elements, thereby reducing the aging resistance.

[0018] The present inventors have noticed that the melting temperature of aluminum nitride changes depending on the concentrations of nitrogen and aluminum contained in a steel slab.

[0019] That is, by adjusting the aluminum content in accordance with the amount of nitrogen contained, the melting temperature of aluminum nitride can be increased, which suppresses the decomposition of aluminum nitride in the hot rolling process, reduces the amount of nitrogen that is not combined with other components that are solid-solved in the hot-rolled steel sheet, and improves the aging resistance of the hot-rolled steel sheet.

[0020] The melting temperature of the aluminum nitride in the face-centered cubic lattice structure can be calculated by the following formula (1). log(Al×N)=-7400 / T+1.95...Equation (1) In the formula (1), Al represents the mass % of aluminum, N represents the mass % of nitrogen, and T represents the absolute temperature.

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

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

[0023] The aluminum nitride precipitated in the steel slab is not decomposed during the hot rolling process, or the precipitation of aluminum nitride is promoted, and 80 mass% or more of the nitrogen in the hot-rolled steel sheet remains as aluminum nitride, which reduces the amount of dissolved nitrogen that is not combined with other components in the hot-rolled steel sheet and improves aging resistance.

[0024] Next, the chemical composition of the hot-rolled steel sheet of the present invention will be described. Unless otherwise specified, "%" means "mass %" which indicates the concentration of the target additive element in the steel.

[0025] Carbon (C) content: 0.005% to 0.20% Carbon is a basic element that increases the strength of steel, and if the carbon content exceeds 0.20%, the ductility, deep drawability, and stretch flangeability of the hot-rolled steel sheet decrease. Furthermore, in the case of low carbonization through normal refining, the target is 0.005% or more, and reducing it to less than 0.005% will result in a deterioration in refining costs and yield.

[0026] Silicon (Si) content: 0.004% by mass or more and 0.55% by mass or less Silicon is an inexpensive element with high solid solution strengthening ability and contributes to increasing the strength of steel, so the content is 0.004 mass % or more. Furthermore, if the content exceeds 0.55 mass %, strong scale with an uneven thickness is formed during hot rolling, and scale remains or dents appear on the surface of the steel sheet even after pickling, deteriorating the surface quality of the final product.

[0027] Manganese (Mn) content: 0.07% by mass or more and 1.65% by mass or less Manganese is an element that not only strengthens the hot-rolled steel sheet, but also acts to fix S, an impurity element that causes high-temperature embrittlement of steel, as MnS, so the manganese content is 0.07 mass % or more. However, if added in excess of 1.65 mass %, formability and plating ability are impaired.

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

[0029] Boron (B) content of 0.0005% by mass or less Like titanium, boron combines with nitrogen to precipitate as BN, reducing the amount of solute nitrogen in the steel and improving aging resistance. In the present invention, the solute nitrogen in the steel is precipitated as aluminum nitride, so there is no need to add boron. Furthermore, boron increases hot deformation resistance, impairing productivity, and increases the finishing temperature, leading to an increase in scale defects. Therefore, the boron content is set to 0.0005% by mass or less.

[0030] The balance other than the above elements consists of iron (Fe), nitrogen (N), aluminum (Al), and inevitable impurities.

[0031] Nitrogen (N) content: 0.0065% by mass or more and 0.0120% by mass or less Nitrogen is an unavoidable impurity in steel. In particular, when steel is smelted in an electric furnace, the N content is higher than when steel is smelted using the blast furnace-converter method because atomic active nitrogen generated during arc discharge penetrates into the molten steel.

[0032] A low nitrogen (N) content is desirable in order to improve strain aging resistance and workability; a high content of N leads to deterioration of formability and the occurrence of stretcher strain due to age hardening. In the present invention, the nitrogen solid solution in the steel is fixed by precipitating aluminum nitride, so there is no need to reduce the nitrogen content by degassing, and the nitrogen content is 0.0065 mass % or more and 0.0120 mass % or less. The increase in tensile strength due to dissolved nitrogen is eliminated by the nitrogen fixation, so that it is necessary to adjust the carbon equivalent to compensate for the tensile strength, if necessary.

[0033] Aluminum (Al) content is greater than 0.070 mass% and less than or equal to 0.110 mass% Aluminum is an element that is actively added because it causes dissolved nitrogen in steel to precipitate as aluminum nitride and is also effective in deoxidizing steel. However, like Si, excessive Al increases scale defects, reduces platability, and further deteriorates weldability, so the Al content is more than 0.070 mass % and not more than 0.110 mass %.

[0034] The inevitable impurities include phosphorus (P), sulfur (S), and other 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] Like Mn, phosphorus causes uneven concentration during solidification, impairing the surface quality after pressing. Adding a large amount of P also has significant drawbacks, such as reducing resistance to secondary work embrittlement and degrading the galvanizability of the steel sheet. To avoid these drawbacks, the phosphorus content should be kept below 0.050%.

[0036] Sulfur is inevitably contained in steel, and segregation at grain boundaries can cause red embrittlement, cracking of slabs and hot-rolled sheets, and a deterioration in resistance to secondary work embrittlement. For this reason, it is desirable for the sulfur content to be 0.050% or less.

[0037] Copper can be contained in an amount of 0.01% or more because it improves the corrosion resistance and strength of steel sheets, but excessive addition can lead to cracks in slabs and hot-rolled sheets due to a decrease in hot ductility, so it is desirable to add 1% or less.

[0038] Tin has the effect of suppressing surface oxidation of steel sheets, and is effective in maintaining surface quality by reducing scale defects, surface nitriding, and decarburization. However, since excessive tin content deteriorates formability, it is desirable for the content to be 0.05% or less.

[0039] Nickel has the effect of improving the corrosion resistance and low-temperature toughness of the steel sheet, but if it is added in excess, the strength increases and workability deteriorates, so the content is preferably 0.20% or less.

[0040] Chromium has the effect of improving the corrosion resistance of steel sheets and improving strength by forming carbides, but if it is added in excess, the strength increases and workability deteriorates, so the content is preferably 0.30% or less.

[0041] If molybdenum is added in excess, the strength increases and workability deteriorates, so the content is preferably 0.05% or less.

[0042] Vanadium improves workability and weld toughness, but excessive addition 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 an excessive content may reduce the toughness of the hot-rolled steel sheet, so the content is preferably 0.005% or less. [Example]

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

[0045] Steel materials having the chemical compositions shown in Table 1 were melted in an electric furnace to prepare slabs. In Comparative Examples 1 to 3, the slabs were produced after degassing, while in Comparative Examples 4 to 10 and Examples 1 to 10, the slabs were produced without degassing.

[0046] Next, this slab was heated in a heating furnace so that the maximum atmospheric temperature did not exceed 1230°C, and extracted and hot-rolled at a slab temperature of about 1150°C. 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 produced.

[0047] [Table 1]

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

[0049] The total nitrogen content of the hot-rolled steel sheets of Examples 1 to 10 and Comparative Examples 1 to 10 was measured according to JIS G 1228-3 Iron and steel - Nitrogen determination method - Part 3: inert gas fusion - thermal conductivity method. Furthermore, nitrides precipitated in the hot-rolled steel sheet were extracted by 10 mass% AA electrolytic extraction (20 A / cm 2The nitrogen was collected on a filter (0.2 μm mesh) and decomposed with (H2SO4 + K2SO4). The mixture was then distilled with 50% NaOH vapor and the amount of nitrogen fixed as nitride was measured using bis-pyrazolone spectrophotometry. The amount of dissolved nitrogen was calculated as the difference between the total nitrogen content and the amount of nitrogen fixed as nitrides.

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

[0051] [Table 2]

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

[0053] The results of real-time aging tests for Example 1 and Comparative Example 6 (SPHC) and Example 4 and Comparative Example 7 (SS400) are shown in Figures 1 and 2. Compared to the Comparative Examples, the Examples showed a smaller increase in yield point and a smaller decrease in elongation. This confirmed that the hot-rolled steel sheets of the present invention have excellent aging resistance even in real-time tests.

[0054] <Particle analysis> Particle analysis was carried out as follows. A backscattered electron image is acquired with the contrast adjusted so that metal parts appear white and non-metal parts appear black. Each non-metallic part in the backscattered electron image is irradiated with an electron beam, and the resulting X-rays are analyzed to obtain information on the composition of the non-metallic parts. The data obtained for the non-metallic parts was checked, and the locations where Al and N were detected simultaneously, and the locations 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 energy dispersive X-ray analyzer (AZtec Energy) Observation conditions: Measurement magnification 1000x, WD 10 mm, total measurement area 1 mm 2 , accelerating voltage 15 kV

[0056] As shown in Fig. 3, in the examples in which the contents of both Al and N were high, a large amount of Al+N was detected, and the equivalent circle diameter was 1 µm or less. In the comparative examples, no AlN was detected.

[0057] <Mechanical properties / processability> Tensile test specimens (JIS No. 5 test specimens for the SPHC and SPHT3 steel types, and JIS No. 1A test specimens for the SS400 steel type) 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 except for those with a plate thickness of 16 mm, and the workability was evaluated. The evaluation results are shown in Table 3.

[0058] [Table 3]

[0059] Although the hot-rolled steel sheet of the present invention contains Al, no decrease in strength or workability was observed.

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

[0061] [Table 4]

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

[0063] The metal structures of the examples and comparative examples were all ferrite+pearlite structures, and the grain size numbers according to the JIS G0551 comparative method were 9.6 to 10.4.

Claims

1. 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 mass% or more and 1.65 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, the balance consisting of iron (Fe), nitrogen (N), aluminum (Al), and inevitable impurities; The nitrogen (N) content is 0.0065% by mass or more and 0.0120% by mass or less, The content of the aluminum (Al) is more than 0.070 mass% and 0.110 mass% or less, A hot-rolled steel sheet characterized in that the content of dissolved nitrogen not combined with other elements is 0.0020 mass% or less.

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

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

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