Steel plate for molten zinc bath equipment, its manufacturing method and molten zinc bath equipment

Abstract

To provide a steel sheet for a molten zinc bath facility having toughness and weld crack resistance required as a weld structure steel and excellent in molten zinc corrosion resistance and zinc crack resistance, a producing method thereof and a molten zinc bath facility.SOLUTION: Provided is a steel sheet for molten zinc bath facilities, which has a component composition in which specific amounts of C, Si, Mn are contained at mass%, P, S, Al, B are limited to a specific range, CEZ is limited to 0.44 or less, and the balance contains Fe and unavoidable impurities, wherein in a sheet thickness cross-section parallel to a rolling direction and vertical to a sheet surface, among a metallographic structure of a surface layer up to a t / 4 portion (t: sheet thickness) from a sheet surface in a sheet thickness direction, over 30% as an area rate is a ferritic composition, the balance is a hard second phase having a bainite structure of 90% or more by an area rate, and the absorption energy obtained in the Charpy impact test using a 2 mm V notch specimen taken from a position of 1 mm in the thickness direction from the plate surface is 27 J or more at -40°C.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a steel sheet for molten zinc bath equipment that constitutes a molten zinc bath kettle (bath) used when molten zinc treatment is performed on steel materials, etc. More specifically, the present invention relates to a steel sheet for molten zinc bath equipment that has excellent corrosion resistance against molten zinc, is less susceptible to cracking caused by molten zinc than conventional steels, and is economical because it can be gas cut or welded, and has excellent resistance to molten zinc corrosion and zinc cracking. [Background technology]

[0002] Hot-dip galvanizing has been widely used as an economical rust prevention method for steel materials, and is carried out by immersing the steel material to be plated in a steel bath containing molten zinc.

[0003] In the case of steel baths like those mentioned above, the molten zinc reacts with the iron in the bath at the interface where it comes into contact with the molten zinc, forming an iron-zinc alloy layer, which causes corrosion. The corrosion rate becomes extremely high when the temperature of the molten zinc is around 500°C, so if the bath temperature is not properly controlled, damage due to thinning or holes will occur in a short period of time, making the bath unusable.

[0004] To address the above-mentioned corrosion problem, Patent Document 1 discloses a technology for steel materials for zinc boilers that improves zinc corrosion resistance by controlling the C content to 0.12 to 0.30% and the P content to 0.015% or less in the chemical composition of the steel that forms the bathtub. Patent Document 2 discloses a technology for a steel material that has excellent resistance to molten zinc corrosion and molten zinc embrittlement by reducing cracking sensitivity in molten zinc by controlling the C content to 0.05 to 0.12 mass %. Patent Document 3 discloses a technique for ensuring not only molten zinc corrosion resistance but also high-temperature strength by appropriately controlling the amounts of C and Nb, thereby preventing deformation at high temperatures during use. Patent Document 4 discloses a technology for a steel material for a galvanizing tank in which the Cr and V contents are controlled to suppress the development of zinc cracks. Patent Document 5 discloses a technology in which the inclusion of 0.10% or more of Mo improves zinc corrosion resistance and prevents deformation at high temperatures during use, while Patent Document 6 discloses a technology in which the inclusion of 0.1% or more of Mo improves zinc corrosion resistance and prevents deformation at high temperatures during use. Patent Documents 7 and 8 disclose technologies for a steel material for a galvanizing pot and a plating tank in which the Al content is controlled in accordance with the P content to improve the corrosion resistance of molten zinc. Patent Document 9 discloses a technique for reducing the amount of erosion caused by corrosion by molten zinc by using a molten zinc-resistant steel material whose grain size is adjusted to No. 8 or more in terms of grain size number. Patent Document 10 discloses a technology for a steel sheet for molten zinc bath equipment that is excellent not only in molten zinc corrosion resistance but also in zinc cracking resistance by controlling the structure to one consisting of ferrite and pearlite and making the average aspect ratio of the ferrite structure 2 or more. Patent Document 11 discloses a technology for a molten zinc bath facility that uses a steel plate in which a structure is controlled to have a structure consisting of ferrite and pearlite, and the average aspect ratio of the ferrite structure is set to 2 or more. Furthermore, Patent Document 12 discloses a technology for a steel sheet for use in molten zinc bath equipment, which has excellent molten zinc corrosion resistance and zinc cracking resistance, in which, in a thickness cross section parallel to the rolling direction and perpendicular to the sheet surface, 70% or more of the metal structure of the surface layer from the sheet surface to the t / 4 part in the sheet thickness direction is a bainite structure in terms of area ratio. Patent Document 13 discloses a technology for a molten zinc bath equipment constructed using a steel sheet in which, in a thickness cross section parallel to the rolling direction and perpendicular to the sheet surface, the metal structure of the surface layer from the sheet surface to the t / 4 part in the sheet thickness direction has an area ratio of 70% or more of a bainite structure, and the remainder is a ferrite structure. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Unexamined Patent Publication No. 49-130310 [Patent Document 2] Japanese Patent Application Laid-Open No. 2003-231942 [Patent Document 3] Japanese Patent Publication No. 116746 / 1983 [Patent Document 4] Japanese Patent Application Publication No. 54-99031 [Patent Document 5] Japanese Patent Application Publication No. 49-107911 [Patent Document 6] Japanese Patent Application Laid-Open No. 2002-241888 [Patent Document 7] Japanese Patent Application Publication No. 53-8314 [Patent Document 8] Japanese Patent Application Laid-Open No. 2000-239816 [Patent Document 9] Japanese Patent Publication No. 55-31172 [Patent Document 10] Japanese Patent Application Laid-Open No. 2013-177682 [Patent Document 11] Japanese Patent Application Publication No. 2017-122280 [Patent Document 12] Japanese Patent Application Laid-Open No. 2013-177681 [Patent Document 13] Japanese Patent Application Laid-Open No. 2017-133106 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the steel materials described in Patent Documents 1 to 13 can suppress corrosion, but they do not have the resistance to cracks that initiate and propagate from grain boundaries on the surface of the steel material, and as a result, they do not have a sufficient lifespan as molten zinc bath equipment. That is, the steel materials described in Patent Documents 1 to 13 cannot be said to have sufficient zinc cracking resistance, including the ability to suppress cracking caused by molten zinc. Thus, there has been a demand for the establishment of technology for steel plates for molten zinc bath equipment that fully satisfy both corrosion resistance to corrosion by molten zinc (hereinafter also referred to as molten zinc corrosion resistance) and zinc cracking resistance while possessing the required toughness and weldability (hereinafter also referred to as weld cracking resistance) for welded structural steel. The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a steel plate for molten zinc bath equipment that is excellent in toughness, weld crack resistance, molten zinc corrosion resistance, and zinc cracking resistance, a manufacturing method thereof, and molten zinc bath equipment.

[0007] In the present invention, "excellent toughness" means that the absorbed energy at -40°C is 27 J or more when a Charpy impact test is performed in accordance with JIS Z2242 using a 2 mm V-notch test piece obtained with the longitudinal direction of the test piece parallel to the rolling direction and 1 mm from the sheet surface in the sheet thickness direction.

[0008] In addition, in the present invention, excellent resistance to weld cracking means that in a y-type weld cracking test (JIS Z3158), a groove is prepared with a root spacing of 1.0 mm, and multi-layer welding is performed by MAG welding using a solid wire material conforming to the YGW15 (JIS Z3312) standard as the welding material, with a heat input of 30 to 36 kJ / cm to produce a y-type welded test piece, and no cross-sectional cracks are observed with the naked eye on the mirror-polished surfaces of each of the five cross sections of the y-type weld.

[0009] In the present invention, excellent resistance to molten zinc corrosion is defined as a corrosion amount of 100 mg / cm2, calculated by immersing a 40 mm x 25 mm x 4 mm test piece taken from the surface layer of a steel sheet in 99.99% pure zinc at a temperature of 500°C for 24 hours and dividing the mass change (mass loss) of the test piece before and after the test by the surface area of ​​the test piece (40 mm x 25 mm x 4 mm rectangular parallelepiped) before the test. 2 It means the following:

[0010] In the present invention, excellent zinc cracking resistance means that in an NBT test (notched round bar tensile test), a zinc wire is wrapped around a notch in a test piece, heated to allow molten zinc to adhere, the test temperature is set to 500°C, and the SLM value (SLM-400 value) at a fracture time of 400 seconds is 90% or more. [Means for solving the problem]

[0011] The present inventors conducted extensive research into the effect of metallurgical structure on zinc cracking resistance using steels with various chemical compositions. As a result, they found that by combining controlled rolling and accelerated cooling to control a microstructure in which more than 30% of the area is ferrite and the remainder is a hard second phase consisting mainly of bainite, and by ensuring that an absorbed energy of 27 J or more is obtained at −40°C in a Charpy impact test using a 2 mm V-notch test piece taken 1 mm from the sheet surface in the thickness direction with the longitudinal direction of the test piece parallel to the rolling direction, a steel material can be obtained that is excellent not only in molten zinc corrosion resistance but also in zinc cracking resistance. The present invention was completed based on these findings, and its gist is as follows.

[0012] [1] In mass%, C: More than 0.12% and less than 0.30% Si: 0.05% or less, Mn: 0.20 to 2.0% Contains P: 0.015% or less, S: 0.030% or less, Al: 0.070% or less, B: 0.0002% or less Limited to The CEZ shown in equation (1) is limited to 0.44 or less. The balance has a composition consisting of Fe and unavoidable impurities, In a plate thickness cross section parallel to the rolling direction and perpendicular to the plate surface, the metal structure of the surface layer from the plate surface to the t / 4 portion (t: plate thickness) in the plate thickness direction is a ferrite structure with an area ratio of more than 30%, and the remainder is a hard second phase having a bainite structure with an area ratio of 90% or more, A steel plate for molten zinc bath equipment that has an absorbed energy of 27J or more at -40°C in a Charpy impact test using a 2mm V-notch test piece taken 1mm from the plate surface in the plate thickness direction, with the longitudinal direction of the test piece parallel to the rolling direction. CEZ=C+Si / 17+Mn / 7.5+Cu / 13+Ni / 17+Cr / 4.5 +Mo / 3+V / 1.5+Nb / 2+Ti / 4.5+420×B...Formula (1) In formula (1), the element symbols represent the content (mass %) of the elements in the steel sheet. [2] The component composition further includes, in mass%, Cu: 0.1 to 0.5%, Ni: 0.1 to 2.0% Cr: 0.02% or more and less than 0.10% Mo: 0.02% or more and less than 0.10% Nb: 0.003 to 0.050%, V: 0.01 to 0.10%, Ti: 0.005 to 0.050%, Ca: 0.0002 to 0.0060%, Mg: 0.0002 to 0.0060%, REM: 0.0002 to 0.0060% The steel sheet for molten zinc bath equipment according to [1] above, containing one or more of the following: [3] A slab having the component composition described in [1] or [2] is heated to 1000 to 1250 ° C., After hot rolling with a rolling end temperature of 800°C or less, Start accelerated cooling at a steel sheet surface temperature in the range of Ar3 point -100°C to Ar3 point -20°C, A manufacturing method of steel sheet for molten zinc bath equipment, in which after accelerated cooling is completed, reheating is completed at a steel sheet surface temperature of 500 to 650°C. [4] A slab having the component composition described in [1] or [2] is heated to 1000 to 1250 ° C., After hot rolling with a rolling end temperature of 800°C or less, Start accelerated cooling at a steel sheet surface temperature in the range of Ar3 point -100°C to Ar3 point -20°C, After the accelerated cooling is completed, the steel plate surface temperature is stopped at less than 500°C. A manufacturing method for steel sheets for molten zinc bath equipment, tempered at a tempering temperature of 500 to 700°C. [5] A molten zinc bath facility using the steel plate for molten zinc bath facility according to [1] or [2] above. [Effects of the Invention]

[0013] The steel plate for molten zinc bath equipment according to the present invention has the toughness and weld crack resistance required for a steel for welded structures, and is excellent in molten zinc corrosion resistance and zinc cracking resistance. The steel plate for molten zinc bath equipment according to the present invention has excellent zinc cracking resistance, so that cracks caused by molten zinc are unlikely to occur, and the steel plate has sufficient resistance when cracks do occur. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, embodiments of the steel sheet for use in a molten zinc bath facility, the manufacturing method thereof, and the molten zinc bath facility according to the present invention will be described. It should be noted that this embodiment is described in detail to allow a better understanding of the gist of the invention, and does not limit the present invention unless otherwise specified. In the following description, "%" indicating the content of each component in the chemical composition indicates "% by mass" unless otherwise specified.

[0015] The steel sheet for molten zinc bath equipment of the present invention contains, by mass%, C: more than 0.12% and less than 0.30%, Si: 0.05% or less, Mn: 0.20 to 2.0%, P: 0.015% or less, S: 0.030% or less, Al: 0.070% or less, B: 0.0002% or less, and CEZ shown in formula (1) is limited to 0.44 or less, with the balance being Fe and unavoidable impurities. In the thickness section, the metal structure of the surface layer from the plate surface to the t / 4 part (t: plate thickness) in the plate thickness direction is a ferrite structure with an area ratio of more than 30%, and the remainder is a hard second phase with an area ratio of bainite structure of 90% or more.The longitudinal direction of the test piece is parallel to the rolling direction, and the absorbed energy obtained in a Charpy impact test using a 2mm V-notch test piece taken from a position 1mm from the plate surface in the plate thickness direction is 27J or more at -40°C. CEZ=C+Si / 17+Mn / 7.5+Cu / 13+Ni / 17+Cr / 4.5 +Mo / 3+V / 1.5+Nb / 2+Ti / 4.5+420×B...Formula (1) In formula (1), the element symbols represent the content (mass %) of the elements in the steel sheet.

[0016] <Component composition> First, the chemical composition of the steel plate for molten zinc bath equipment (hereinafter also simply referred to as steel plate) will be described.

[0017] C: More than 0.12% and less than 0.30% C is an important element for improving the strength of steel sheets and also contributes to the formation of bainite. If the C content is 0.12 mass% or less, the molten zinc corrosion resistance of the steel sheet is significantly reduced, so the C content must exceed 0.12%. On the other hand, if the C content is 0.30% or more, the weld crack resistance of the steel sheet deteriorates. Also, if the C content is 0.30% or more, the toughness decreases. For this reason, the C content is set to less than 0.30%, and preferably 0.15% or less.

[0018] Si:0.05% or less Although Si has a deoxidizing effect, it is not necessary if sufficient Al, a strong deoxidizing element, is added. Si also strengthens the base material, but this effect is relatively small compared to other elements. Furthermore, since Si significantly reduces the corrosion resistance of molten zinc, a low Si content is preferable, with the upper limit set at 0.05%, which is a stable reduction in operational efficiency. Furthermore, although there are limitations in steelmaking, it is preferable to keep the Si content at 0.02% or less.

[0019] Mn: 0.20 to 2.0% Mn is an element added from the viewpoint of ensuring base metal strength and bainite, and a content of 0.20% or more is necessary to contribute to the desired toughness and molten zinc cracking resistance (hereinafter also referred to as zinc cracking resistance). Therefore, the Mn content is set to 0.20% or more. The Mn content is preferably 0.25% or more, and more preferably 0.30% or more. On the other hand, a Mn content exceeding 2.0% significantly deteriorates weld crack resistance. Furthermore, if the Mn content exceeds 2.0%, toughness and zinc crack resistance decrease. Therefore, the Mn content is set to 2.0% or less. The Mn content is preferably 1.60% or less, and more preferably 1.30% or less.

[0020] P:0.015% or less P is an impurity element that is inevitably contained in steel plates, but since it deteriorates the toughness and zinc cracking resistance of steel plates, its content is preferably low, with the upper limit set at 0.015%, which is a low level that can be reduced in a stable manner from an operational standpoint. Furthermore, although there are steelmaking limitations, the P content is preferably 0.008% or less.

[0021] S: 0.030% or less Like P, S is an element that is inevitably contained in steel sheets, but since S reduces the toughness and zinc cracking resistance of the base material, the less S it contains, the better, so the upper limit is set to 0.030%, which is a lower limit that can be achieved in a stable manner from an operational standpoint. The S content is preferably 0.020% or less, and more preferably 0.010% or less.

[0022] Al: 0.070% or less Al is an element used for deoxidation, and in order to obtain the deoxidation effect, the Al content is preferably 0.010% or more, more preferably 0.015% or more, and even more preferably 0.020% or more. However, an Al content exceeding 0.070% causes many coarse inclusions to be present in the steel, reducing toughness, and therefore the upper limit is set to 0.070%. The Al content is preferably 0.060% or less, and more preferably 0.050% or less.

[0023] B: 0.0002% or less B is an element that segregates at grain boundaries and significantly deteriorates zinc cracking resistance. The lower the content, the better, with the upper limit set at 0.0002%, which is a level that can be reduced to ensure stable operation.

[0024] CEZ: 0.44 or less CEZ, as shown in the following formula (1), correlates with the molten zinc cracking resistance of the weld. If CEZ exceeds 0.44, the molten zinc cracking resistance deteriorates and cracks occur in the weld. Therefore, the upper limit of the CEZ value is set to 0.44. CEZ is preferably set to 0.42% or less, and more preferably 0.40% or less. CEZ=C+Si / 17+Mn / 7.5+Cu / 13+Ni / 17+Cr / 4.5 +Mo / 3+V / 1.5+Nb / 2+Ti / 4.5+420×B...Formula (1) In formula (1), the element symbol represents the content (mass%) of the element in the steel sheet, and when the element is not contained, the content of that element is set to zero.

[0025] The above is the basic chemical composition of the steel sheet in the present invention, with the balance being Fe and unavoidable impurities. In the present invention, the unavoidable impurities include 0.01% or less of Cu, Ni, Cr, and Mo, 0.002% or less of Nb, V, and Ti, and 0.0001% or less of B, Ca, Mg, and REM.

[0026] Furthermore, the steel sheet of the present invention may contain one or more of the following chemical compositions, as required.

[0027] Cu: 0.1 to 0.5% Cu is an optional element in the present invention and is added as needed to ensure the strength of the base material. Its effect is not realized unless the Cu content is 0.1% or more. Therefore, when Cu is contained, the lower limit of the Cu content is set to 0.1%. The Cu content is preferably 0.20% or more, and more preferably 0.30% or more. On the other hand, if Cu is contained in an amount exceeding 0.5%, not only does it increase costs but it also deteriorates toughness, so when Cu is contained, the upper limit of the Cu content is set to 0.5%.

[0028] Ni: 0.1 to 2.0% Ni is an optional element in the present invention, and is added as needed to ensure the strength of the base material. The effect is not realized unless the Ni content is 0.1% or more. Therefore, when Ni is contained, the lower limit of the Ni content is set to 0.1%. On the other hand, if the Ni content exceeds 2.0%, not only does it increase costs but it also causes slab cracking and surface defects on the steel sheet, so if Ni is contained, the upper limit of the Ni content is set to 2.0%.

[0029] Cr: 0.02% or more and less than 0.10% Cr is an optional element in the present invention and is added as needed to ensure the strength of the base metal. Since the effect is not realized unless the Cr content is 0.02% or more, when Cr is contained, the lower limit of the Cr content is set to 0.02%. The Cr content is preferably 0.03% or more, and more preferably 0.05% or more. On the other hand, if the Cr content is 0.10% or more, it promotes the reaction between zinc and the steel sheet surface, reducing the corrosion resistance of molten zinc. Therefore, if Cr is contained, the Cr content is set to less than 0.10%.

[0030] Mo: 0.02% or more and less than 0.10% Mo is an optional element in the present invention and is added as needed to ensure the strength of the base metal. This effect is not realized unless the Mo content is 0.02% or more. Therefore, when Mo is contained, the lower limit of the Mo content is set to 0.02%. The Mo content is preferably 0.03% or more, and more preferably 0.05% or more. On the other hand, if Mo is contained in an amount of 0.10% or more, not only does it increase costs but it also deteriorates toughness and zinc cracking resistance, so if Mo is contained, the Mo content is set to less than 0.10%.

[0031] Nb: 0.003 to 0.050% Nb is an optional element in the present invention, and is added when it is desired to improve the strength and toughness of the base material. This effect is not exhibited unless the Nb content is 0.003% or more, so when Nb is contained, the lower limit of the Nb content is set to 0.003%. The Nb content is preferably 0.010% or more, and more preferably 0.020% or more. On the other hand, if the Nb content exceeds 0.050%, not only will the cost increase but also the weld crack resistance will deteriorate. Therefore, when Nb is contained, the upper limit of the Nb content is set to 0.050%.

[0032] V: 0.01 to 0.10% V is an optional element in the present invention and is added when it is desired to improve the strength of the base material. This effect is not exhibited unless the V content is 0.01% or more, so the lower limit of the V content is set to 0.01%. The V content is preferably 0.030% or more, and more preferably 0.040% or more. On the other hand, if the V content exceeds 0.10%, not only will the cost increase but also the toughness of the base material and the weld will deteriorate, so the upper limit of the V content is set to 0.10%.

[0033] Ti: 0.005 to 0.050% Ti is an optional element in the present invention, and is added when it is desired to improve the toughness of the base material and the toughness of the weld. This effect is not exhibited unless the Ti content is 0.005% or more, so the lower limit of the Ti content is set to 0.005%. On the other hand, if the Ti content exceeds 0.050%, the toughness of the base metal and the welded joint will be deteriorated, so the upper limit of the Ti content is set to 0.050%.

[0034] Ca: 0.0002 to 0.0060% Ca is an optional element in the present invention, and is added when it is desired to improve the internal quality evaluated by the toughness of the weld heat-affected zone or ultrasonic flaw detection. Since this effect is not exhibited unless the Ca content is 0.0002% or more, the lower limit of the Ca content is set to 0.0002%. The Ca content is preferably 0.0010% or more, and more preferably 0.0015% or more. On the other hand, if the Ca content exceeds 0.0060%, the formed oxides will cluster and actually deteriorate the internal quality, so the upper limit of the Ca content is set to 0.0060%. The Ca content is preferably 0.0050% or less, and more preferably 0.0040% or less.

[0035] Mg: 0.0002 to 0.0060% Mg is an optional element in the present invention and is added when it is desired to improve the toughness of the weld heat-affected zone or the internal quality as evaluated by ultrasonic flaw detection. This effect is not realized unless the Mg content is 0.0002% or more, so the lower limit of the Mg content is set to 0.0002%. The Mg content is preferably 0.0005% or more, and more preferably 0.0010% or more. On the other hand, if the Mg content exceeds 0.0060%, the formed oxides will cluster and actually deteriorate the internal quality, so the upper limit of the Mg content is set to 0.0060%. The Mg content is preferably 0.0055% or less, and more preferably 0.0050% or less.

[0036] REM: 0.0002 to 0.0060% REM is an optional element in the present invention, and is added when it is desired to improve the toughness of the weld heat-affected zone or the internal quality evaluated by ultrasonic flaw detection, etc. This effect is not exhibited unless the REM content is 0.0002% or more, so the lower limit of the REM content is set to 0.0002%. The REM content is preferably 0.0010% or more, and more preferably 0.0015% or more. On the other hand, if the REM content exceeds 0.0060%, the formed oxides will cluster and actually deteriorate the internal quality, so the upper limit of the REM content is set to 0.0060%. The REM content is preferably 0.0050% or less, and more preferably 0.0040% or less. Here, REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanoid elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM content is the total content of one or more elements selected from the above REM elements.

[0037] <Metal structure> In a thickness cross section parallel to the rolling direction and perpendicular to the plate surface, the surface metal structure from the plate surface to the t / 4 part (t: plate thickness) in the plate thickness direction has an area ratio of more than 30% ferrite structure, and the remainder is a hard second phase mainly consisting of bainite structure. The metallographic structure of the steel sheet of the present invention is characterized in that, in a thickness cross section parallel to the rolling direction and perpendicular to the sheet surface, the surface metallographic structure in the thickness direction from the sheet surface to the quarter thickness portion is a ferrite structure in an area ratio of more than 30%. If the area ratio of the ferrite structure is 30% or less, cracks are likely to occur at the grain boundaries of the hard second phase portion, which is mainly composed of bainite due to the molten zinc, and the crack propagation rate increases, resulting in deterioration of zinc cracking resistance. Although no upper limit is specified, in the present invention, since the C content is controlled to more than 0.12%, it does not actually exceed 80%, so if specifically specified, it is set to 80% or less.

[0038] The remainder of the microstructure is a hard second phase consisting mainly of bainite. The hard second phase accounts for less than 70% of the total microstructure, and the hard second phase is considered to be "mainly bainite" when its area ratio is 90% or more. In addition to bainite, other hard second phases include pearlite, martensite, island martensite, and cementite.

[0039] In the present invention, 5 vol% nital etching is performed, and the metal structure is observed under an optical microscope (magnification: 100x). The round white structures are identified as ferrite structures, and the remaining parts (mainly consisting of bainite structures, which are lath-shaped structures) are identified as the hard second phase.

[0040] <Impact test> The absorbed energy obtained in a Charpy impact test using a 2mm V-notch test piece with the longitudinal direction of the test piece parallel to the rolling direction and taken 1mm from the plate surface in the plate thickness direction is 27J or more at -40°C. The impact test properties of the steel sheet of the present invention are evaluated by the absorbed energy obtained in a Charpy impact test using a 2 mm V-notch test piece, the longitudinal direction of which is parallel to the rolling direction and taken from a position 1 mm from the sheet surface in the sheet thickness direction. If the absorbed energy at -40°C is less than 27 J, the rate of crack propagation in the case of grain boundary cracking caused by molten zinc increases, resulting in a deterioration of zinc cracking resistance, so the lower limit is set to 27 J. The Charpy impact test is carried out in accordance with JIS Z2242.

[0041] <Steel plate manufacturing conditions> Next, a method for producing a steel sheet according to the present invention will be described. In the method for producing a steel sheet of the present invention, a slab having the aforementioned chemical composition is heated to 1000 to 1250°C and hot rolled to a rolling end temperature of 800°C or less. After that, accelerated cooling is started at the steel sheet surface temperature with an accelerated cooling start temperature in the range of Ar3 point -100°C to Ar3 point -20°C, and after the accelerated cooling is completed, reheating is completed at the steel sheet surface temperature of 500 to 650°C. Alternatively, in the method for producing a steel sheet of the present invention, a slab having the aforementioned chemical composition is heated to 1000 to 1250°C and hot rolled to a rolling end temperature of 800°C or less, and then accelerated cooling is started at a steel sheet surface temperature in a range of Ar3 point -100°C to Ar3 point -20°C, and after accelerated cooling is completed, reheating is completed when the steel sheet surface temperature is below 500°C, and the steel sheet is tempered at a tempering temperature of 500 to 700°C.

[0042] Slab heating temperature: 1000~1250℃ Slabs produced by continuous casting or ingot casting are heated to reduce deformation resistance during hot rolling and to adjust the metal structure. If the heating temperature is below 1000°C, the strength of the base material decreases, the reduction effect during hot rolling becomes insufficient, and the internal quality, as evaluated by ultrasonic testing, deteriorates. Furthermore, hot rolling cannot be completed by the desired rolling end temperature and accelerated cooling start temperature. A steel sheet obtained at a heating temperature of less than 1000°C has reduced toughness and zinc cracking resistance, so the lower limit of the heating temperature is set to 1000°C. On the other hand, if the heating temperature exceeds 1250°C, the toughness also deteriorates and the desired Charpy impact test characteristics and zinc cracking resistance cannot be obtained, so the upper limit of the heating temperature is set to 1250°C. The heating temperature may be determined by heat transfer calculations based on the furnace temperature of the heating furnace, etc.

[0043] Rolling end temperature: 800℃ or less Within the range of about 700 to 900°C, the lower the rolling finish temperature in hot rolling, the more the toughness and zinc cracking resistance improve. There is no particular lower limit to the rolling end temperature in hot rolling, but from the viewpoint of ensuring the shape after rolling, the rolling end temperature is preferably 650°C or higher, and more preferably 680°C or higher. On the other hand, if the rolling end temperature in hot rolling exceeds 800°C as the surface temperature of the steel sheet, the desired toughness cannot be obtained and zinc cracking resistance deteriorates, so the upper limit of the rolling end temperature in hot rolling is set to 800°C. The rolling end temperature is preferably set to 790°C or less, and more preferably to 780°C or less. The temperature is determined by measuring the surface of the steel sheet immediately after rolling with a radiation thermometer.

[0044] Accelerated cooling start temperature: Steel plate surface temperature in the range of Ar3 point - 100°C to Ar3 point - 20°C The accelerated cooling start temperature is adjusted to control the metallographic structure. If the temperature falls below Ar3 point - 100°C, using the steel sheet surface temperature as a reference, the second phase of the metallographic structure will not be mainly bainite, and pearlite and cementite will form, reducing toughness and zinc cracking resistance. Therefore, the lower limit is set to Ar3 point - 100°C. The accelerated cooling start temperature is preferably Ar3 point - 80°C or higher, and more preferably Ar3 point - 70°C or higher. If the accelerated cooling start temperature exceeds Ar3 point - 20°C using the steel sheet surface temperature as a reference, the ferrite area ratio of the metal structure will be 30% or less, making it easier for cracks to occur at grain boundaries due to molten zinc, and the rate of crack propagation will increase, deteriorating zinc cracking resistance, so the upper limit is set to Ar3 point - 20°C. The accelerated cooling start temperature is preferably Ar3 point - 25°C or less, and more preferably Ar3 point - 30°C or less. The Ar3 point can be measured actually, but it can also be calculated using formula (2). The surface temperature is determined by measuring the steel plate with a radiation thermometer immediately after it enters the accelerated cooling equipment. Ar3(℃)=910-310×C-80×Mn-20×Cu-15×Cr-55×Ni-80×Mo...(2) In formula (2), the element symbol represents the content (mass%) of the element in the steel sheet, and when the element is not contained, the content of that element is set to zero.

[0045] After the accelerated cooling is started at the accelerated cooling start temperature, either of the following treatments (1) or (2) is carried out. (1) After the accelerated cooling is completed, the steel plate surface temperature (cooling stop temperature) is 500 to 650°C and the reheating is completed. (2) After the accelerated cooling is completed, the steel plate surface temperature (cooling stop temperature) is below 500°C, and the plate is tempered at a tempering temperature of 500-700°C.

[0046] (1) Cooling stop temperature: 500 to 650°C at the surface temperature of the steel plate after reheating In the present invention, accelerated cooling is applied to control the metal structure to a desired level and ensure zinc cracking resistance. If the cooling stop temperature is less than 500°C, a significant decrease in strength occurs during hot-dip galvanizing, which is usually performed at about 450 to 500°C, and deformation occurs, and zinc cracking resistance decreases as this occurs repeatedly. Therefore, the lower limit of the cooling stop temperature is set to 500°C. The cooling stop temperature is preferably 520°C or higher, and more preferably 550°C or higher. On the other hand, if the cooling stop temperature exceeds 650°C, the hard second phase will not be mainly bainite, and the zinc cracking resistance will decrease due to a decrease in toughness caused by brittle pearlite and cementite, so the upper limit of the cooling stop temperature is set to 650°C. The cooling stop temperature is preferably set to 640°C or less, and more preferably to 630°C or less. In the present invention, the accelerated cooling is not particularly limited, but may be cooling from the surface of the steel sheet with water. The cooling rate during accelerated cooling is higher than that of air cooling, and can be, for example, 1°C / sec or more. In the present invention, reheating refers to an air-cooling treatment that is carried out after the completion of accelerated cooling in order to reduce the temperature difference between the steel sheet surface, which has been excessively cooled during accelerated cooling, and the interior of the steel sheet, which is at a relatively high temperature compared to the steel sheet surface.

[0047] (2) Cooling stop temperature: Steel plate surface temperature after reheating is less than 500°C, and tempering temperature: 500 to 700°C Assuming that tempering is applied after accelerated cooling, zinc cracking resistance can be ensured even with a cooling stop temperature of less than 500°C as described above. In this case, if the tempering temperature is below 500°C, a significant decrease in strength occurs during hot-dip galvanizing, which is usually performed at about 450 to 500°C, and deformation occurs, and zinc cracking resistance decreases as this process is repeated, so the lower limit of the tempering temperature is set to 500°C. The tempering temperature is preferably 520°C or higher, and more preferably 540°C or higher. On the other hand, if the tempering temperature exceeds 700°C, the strength and toughness will decrease significantly, resulting in a decrease in zinc cracking resistance, so the upper limit of the tempering temperature is set to 700°C. The tempering temperature is preferably 680°C or less, and more preferably 670°C or less. Tempering is performed using a heating furnace or high-frequency induction heating furnace. The tempering temperature can be measured using the furnace temperature if the heating furnace is sufficiently uniformly heated. In other cases, the temperature at the center of the plate thickness determined by heat transfer calculations should be used.

[0048] The above manufacturing method makes it possible to efficiently manufacture steel plates for molten zinc bath equipment that have excellent corrosion resistance against corrosion by molten zinc, are less susceptible to cracking caused by molten zinc, and have the toughness and weldability required for welded structural steel. According to the present invention, by applying accelerated cooling, it is possible to omit the thermal refining heat treatment after hot rolling, thereby enabling the manufacture of steel plates for molten zinc bath equipment while reducing manufacturing costs. The steel plate for molten zinc bath equipment of the present invention is preferably a thick steel plate, preferably having a thickness of 30 mm or more. Although there is no particular upper limit, from the viewpoint of reducing the weight of the bath equipment, the thickness is preferably 100 mm or less. Furthermore, the steel plate for molten zinc bath equipment of the present invention can be used without applying a coating or the like to the surface, and does not require surface treatment. The steel sheet for molten zinc bath equipment of the present invention is suitable for application to molten zinc bath equipment such as a molten zinc bath kettle (bath) etc. Specifically, the molten zinc bath equipment is one in which the steel sheet for molten zinc bath equipment of the present invention is formed into a bath by bending and welding, and liquid zinc at 500°C or less is filled therein to immerse an object to be plated, and the bath may be equipped with a heating device.

[0049] The present invention also provides a molten zinc bath facility using the above-mentioned steel plate for molten zinc bath facility. [Example]

[0050] The present invention will be explained in more detail below by giving examples of the steel sheet for molten zinc bath equipment and the manufacturing method thereof according to the present invention. However, the present invention is not limited to the following examples, and can be practiced by making appropriate modifications within the scope of the above and below-mentioned aims, and all such modifications are included in the technical scope of the present invention.

[0051] [Steel plate manufacturing] In the steelmaking process, the deoxidation and desulfurization of molten steel and the chemical composition were controlled, and slabs with the chemical composition shown in Table 1 below were produced by continuous casting. In Table 1, Ar3 was calculated using the above-mentioned formula (2), but if the amount of a component is blank, it is treated as zero in the calculation of Ar3. Also, if the amount of a component is blank in Table 1, the content is treated as zero in the calculation of CEZ. Next, these slabs were heated and hot-rolled under the conditions shown in Table 2, and then subjected to accelerated cooling to obtain steel plates. Accelerated cooling was not applied to steel plate No. 15. Steel plates Nos. 13, 14, and 22 were tempered after accelerated cooling.

[0052] [Evaluation test] The following evaluation tests were conducted on the steel plates for molten zinc bath equipment manufactured by the above method. It was. To evaluate the metallographic structure, test specimens were taken from the steel plate surface to the thickness-quarter portion (t: plate thickness). The surface was mirror-polished and then etched with 5 vol% nital. The entire surface was observed under a microscope, and the area ratio of ferrite in the surface metallographic structure was investigated using image analysis. Images were taken continuously at 100x magnification, and the percentage (area ratio) of ferrite in the surface metallographic structure was investigated using image analysis software such as Photoshop. The area of ​​the colored portion was calculated using image analysis software (Image J Ver. 1.53, manufactured by the National Institutes of Health), and this was divided by the overall size of the photograph to determine the ratio (area ratio). The steel sheets with an area ratio of ferrite in the metal structure exceeding 30% were rated as passing. The second phase (hard second phase) was determined by observing the surface layer of the steel sheet from the sheet surface to the quarter thickness portion with an optical microscope and determining the second phase from photographs taken at 100x magnification. Those mainly composed of bainite were rated as "Good", and those other than that (mainly composed of pearlite in the comparative examples (steel sheets No. 15 and No. 20)) were rated as "Poor".

[0053] The Charpy impact test was conducted in accordance with JIS Z2242. The longitudinal direction of the test piece was parallel to the rolling direction, and a 2 mm V-notch test piece was taken from a position 1 mm from the plate surface in the plate thickness direction. The Charpy impact test was conducted using the absorbed energy obtained. An absorbed energy of 27 J or more at -40°C was considered to be a pass.

[0054] To evaluate molten zinc corrosion resistance, a 40mm x 25mm x 4mm test piece taken from the surface of the steel plate was immersed in 99.99% pure zinc at a temperature of 500°C for 24 hours. The amount of corrosion was calculated by dividing the mass change (mass loss) of the test piece before and after the test by the surface area of ​​the test piece (40mm x 25mm x 4mm rectangular parallelepiped) before the test. And the corrosion amount after 24 hours of immersion is 100mg / cm 2 Those with a value of 100 mg / cm or less are considered "○" (pass). 2 Those exceeding this limit were evaluated as "x" (failure).

[0055] Additionally, zinc cracking resistance was evaluated using an NBT test (notched round bar tensile test) (see Nippon Steel Technical Report No. 348, 1993, pp. 63-70). A zinc wire rod was wrapped around the notch of the test piece and heated to deposit molten zinc. The test temperature was set at 500°C, and specimens with an SLM value (SLM-400 value) of 90% or more at a fracture time of 400 seconds were evaluated as "Good," and specimens with an SLM value of less than 90% were evaluated as "Poor."

[0056] Weld crack resistance was evaluated by the presence or absence of cross-sectional cracks using the y-type weld cracking test (JIS Z3158). Specifically, a groove was prepared with a root gap of 1.0 mm for each final plate thickness, and multi-pass welding was performed by MAG welding with a heat input of 30 to 36 kJ / cm using solid wire material conforming to the YGW15 (JIS Z3312) standard. The presence or absence of cross-sectional cracks was visually inspected on the polished surfaces of five cross sections of the y-type weld. Tests in which no cross-sectional cracks were observed with the naked eye in any of the five cross sections were evaluated as "○" (pass), and tests in which cross-sectional cracks were observed with the naked eye in one or more cross sections were evaluated as "×" (fail).

[0057] [Evaluation results] The results of each evaluation are listed in Table 3. The steel sheets of the present invention satisfy the target performance, whereas the steel sheets of the comparative examples do not satisfy any of the target performance.

[0058] [Table 1]

[0059] [Table 2]

[0060] [Table 3]

Claims

1. In mass%, C: more than 0.12% and less than 0.30%; Si: 0.05% or less, Mn: 0.20-2.0% Contains P: 0.015% or less, S: 0.030% or less, Al: 0.070% or less, B: 0.0002% or less Limited to The CEZ shown in formula (1) is limited to 0.44 or less, The balance has a composition consisting of Fe and unavoidable impurities, In a plate thickness cross section parallel to the rolling direction and perpendicular to the plate surface, the metal structure of the surface layer from the plate surface to the t / 4 portion (t: plate thickness) in the plate thickness direction is a ferrite structure in an area ratio of more than 30%, and the remainder is a hard second phase having a bainite structure in an area ratio of 90% or more, A steel sheet for molten zinc bath equipment, in which the absorbed energy obtained in a Charpy impact test using a 2 mm V-notch test piece taken from a position 1 mm from the sheet surface in the sheet thickness direction, with the longitudinal direction of the test piece parallel to the rolling direction, is 27 J or more at -40°C. CEZ=C+Si / 17+Mn / 7.5+Cu / 13+Ni / 17+Cr / 4.5 +Mo / 3+V / 1.5+Nb / 2+Ti / 4.5+420×B...Formula (1) In formula (1), the element symbols represent the content (mass %) of the elements in the steel sheet.

2. The component composition further includes, in mass%, Cu: 0.1 to 0.5%, Ni: 0.1-2.0%, Cr: 0.02% or more and less than 0.10% Mo: 0.02% or more and less than 0.10% Nb: 0.003 to 0.050%, V: 0.01-0.10%, Ti: 0.005 to 0.050%, Ca: 0.0002-0.0060%, Mg: 0.0002-0.0060%, REM: 0.0002-0.0060% The steel sheet for molten zinc bath equipment according to claim 1, comprising one or more of the following:

3. A slab having the component composition according to claim 1 or 2 is heated to 1000 to 1250°C, After hot rolling with a rolling end temperature of 800°C or less, The accelerated cooling start temperature is Ar at the steel plate surface temperature. ３ Point -100°C to Ar ３ Start accelerated cooling in the range of -20°C. After the accelerated cooling is completed, the steel plate surface temperature is 500 to 650°C and the reheating is completed. In a plate thickness cross section parallel to the rolling direction and perpendicular to the plate surface, the metal structure of the surface layer from the plate surface to the t / 4 portion (t: plate thickness) in the plate thickness direction is a ferrite structure in an area ratio of more than 30%, and the remainder is a hard second phase having a bainite structure in an area ratio of 90% or more, A method for manufacturing a steel sheet for molten zinc bath equipment, in which the absorbed energy obtained in a Charpy impact test using a 2 mm V-notch test piece taken from a position 1 mm from the plate surface in the plate thickness direction, with the longitudinal direction of the test piece parallel to the rolling direction, is 27 J or more at -40°C.

4. A slab having the component composition according to claim 1 or 2 is heated to 1000 to 1250°C, After hot rolling with a rolling end temperature of 800°C or less, The accelerated cooling start temperature is Ar at the steel plate surface temperature. ３ Point -100°C to Ar ３ Start accelerated cooling in the range of -20°C. After the accelerated cooling is completed, the reheating is completed when the steel plate surface temperature is less than 500°C. Tempering at a tempering temperature of 500 to 700°C. In a plate thickness cross section parallel to the rolling direction and perpendicular to the plate surface, the metal structure of the surface layer from the plate surface to the t / 4 portion (t: plate thickness) in the plate thickness direction is a ferrite structure in an area ratio of more than 30%, and the remainder is a hard second phase having a bainite structure in an area ratio of 90% or more, A method for manufacturing a steel sheet for molten zinc bath equipment, in which the absorbed energy obtained in a Charpy impact test using a 2 mm V-notch test piece taken from a position 1 mm from the plate surface in the plate thickness direction, with the longitudinal direction of the test piece parallel to the rolling direction, is 27 J or more at -40°C.

5. A molten zinc bath facility using the steel plate for molten zinc bath facility according to claim 1 or 2.

Images