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

Abstract

To provide a steel sheet for molten zinc bath facilities having excellent corrosion resistance to corrosion due to molten zinc in both a host material and a weld heat influence section, and difficult to generate crack caused by the molten zinc, and excellent 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 0.36 or less, Pcmy is limited to 0.15 or less, and the balance contains Fe and unavoidable impurities, wherein in a sheet thickness cross section parallel to a rolling direction, and perpendicular to the plate surface. a metallographic structure of a surface layer from a sheet surface to a t / 4 portion (t: sheet thickness) in a plate thickness direction has ferrite and pearlite, the balance is a structure where the area rate is 3% or less and the tensile strength is less than 400 MPa.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 amount of C to 0.12 to 0.30% and the amount of P 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 for improving zinc corrosion resistance and preventing deformation at high temperatures during use by adding 0.10% or more of Mo, and Patent Document 6 discloses a technology for improving zinc corrosion resistance and preventing deformation at high temperatures during use by adding 0.1% or more of Mo. 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 sheet in which the structure is controlled to be composed of ferrite and pearlite and the average aspect ratio of the ferrite structure is 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 in the thickness direction from the sheet surface to the t / 4 part 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 Document 1 and Patent Documents 3 to 9 have high C and Mo contents, and hard heat-affected zones are formed in the steel plates by welding during the manufacture of molten zinc bath equipment, which deteriorates the zinc cracking resistance of the welded heat-affected zones and leads to a shortened lifespan of the molten zinc bath equipment. The zinc cracking resistance of the steel material described in Patent Document 2 is evaluated by a high-temperature tensile test without a notch, but this evaluation is insufficient, and it is difficult to say that the steel material is sufficiently effective for application to a galvanizing tank. Furthermore, methods for optimizing the rolling conditions during steel plate production, such as those described in Patent Documents 10 to 13, have also been disclosed, but there have been problems such as the need for special equipment and temperature control techniques. The present invention has been made in view of the above circumstances, and aims to provide a steel plate for molten zinc bath equipment that has excellent corrosion resistance against corrosion by molten zinc in both the base material and the weld heat affected zone, is less susceptible to cracking caused by molten zinc, and is excellent in molten zinc corrosion resistance and zinc cracking resistance, a manufacturing method thereof, and molten zinc bath equipment.

[0007] In the present invention, excellent corrosion resistance to corrosion by molten zinc (molten zinc corrosion resistance) in both the base metal and the weld heat affected zone means that 40 mm × 25 mm × 4 mm test pieces taken from the surface layer of a steel plate (base metal) and a steel plate that has been given a thermal history equivalent to that of a weld heat affected zone are immersed in zinc with a purity of 99.99% at a temperature of 500°C for 24 hours, and the amount of corrosion calculated by dividing the amount of change in mass (amount of mass loss) of the test piece before and after the test by the surface area of ​​the test piece (40 mm × 25 mm × 4 mm rectangular parallelepiped) before the test is 200 mg / cm or more. 2 It means the following:

[0008] In the present invention, the expression "resistance to cracking due to molten zinc in both the base material and the weld heat-affected zone and 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 specimen taken from a steel plate (base material) and a steel plate that has been given a thermal history equivalent to that of a weld heat-affected zone, the test specimen is 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 80% or more. The thermal history corresponding to the weld heat-affected zone is as follows: the time from room temperature to 1350°C is 50 seconds, the holding time at 1350°C is 10 seconds, and the cooling rate from 1350°C to 300°C is 40°C / second. Below 300°C, it is allowed to cool in the air. [Means for solving the problem]

[0009] The present inventors have conducted extensive research into improving the molten zinc corrosion resistance and zinc cracking resistance of steel sheets and their weld heat-affected zones for steel sheets manufactured solely by hot rolling without heating from slabs or special temperature control. They have found that reducing the strength of steel sheets is effective in preventing molten zinc cracking in the non-weld heat-affected zones (base metal) of steel sheets when they are repeatedly used in molten zinc bath equipment, and that excellent zinc cracking resistance can be ensured by controlling the tensile strength to less than 400 MPa. Next, similar studies were carried out on the weld heat affected zone of steel plates, and as a result, it was found that by setting CEZ to 0.36 or less and Pcmy to 0.15 or less, hardening of the weld heat affected zone can be suppressed and excellent zinc cracking resistance can be ensured. The present invention was completed based on these findings, and the gist of the present invention is as follows.

[0010] [1] In mass%, C: More than 0.02% and less than 0.12% 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 Limit to Limiting CEZ shown in formula (1) to 0.36 or less, and limiting Pcmy shown in formula (2) to 0.15 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 has ferrite and pearlite, and the remainder is a structure in which the area ratio is 3% or less, Steel plate for molten zinc bath equipment, with a tensile strength of less than 400 MPa. 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) Pcmy=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20 +Mo / 7.5+V / 10+5×B... Equation (2) In formulas (1) and (2), 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 0.5% 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 0.05%, 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] Heating a slab having the component composition described in [1] or [2] above, Hot rolling is performed with the rolling end temperature at 730°C or higher, This is a manufacturing method for steel sheets for molten zinc bath equipment, which are then air-cooled. [4] A molten zinc bath facility using the steel plate for molten zinc bath facility according to [1] or [2] above. [Effects of the Invention]

[0011] According to the present invention, a steel plate for molten zinc bath equipment can be obtained that has excellent corrosion resistance against corrosion by molten zinc in both the base material and the weld heat-affected zone, and is less susceptible to cracking caused by molten zinc, and has excellent molten zinc corrosion resistance and zinc cracking resistance. DETAILED DESCRIPTION OF THE INVENTION

[0012] 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.

[0013] The steel sheet for molten zinc bath equipment of the present invention contains, by mass%, C: more than 0.02% but less than 0.12%, Si: 0.05% or less, Mn: 0.20 to 2.0%, with P: 0.015% or less, S: 0.030% or less, Al: 0.070% or less, and B: 0.0002% or less, with CEZ shown in formula (1) limited to 0.36 or less, and Pcmy shown in formula (2) limited to 0.15 or less, with the balance being Fe and unavoidable impurities. In a thickness cross section parallel to the rolling direction and perpendicular to the sheet surface, the metal structure of the surface layer in the thickness direction from the sheet surface to the t / 4 part (t: sheet thickness) has ferrite and pearlite, with the balance being 3% or less in terms of area fraction, and has a tensile strength of less than 400 MPa. 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) Pcmy=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20 +Mo / 7.5+V / 10+5×B... Equation (2) In formulas (1) and (2), the element symbols represent the content (mass %) of the elements in the steel sheet.

[0014] <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.

[0015] C: More than 0.02% and less than 0.12% C is an important element for improving the strength of steel sheets, but it also hardens the steel sheets and their weld heat-affected zones, promoting the occurrence of molten zinc cracking. If the C content is 0.02 mass% or less, the grain boundaries in the weld heat-affected zones deteriorate significantly, significantly reducing zinc cracking resistance, so it is necessary to contain more than 0.02% C. On the other hand, if the C content is 0.12% or more, the weld heat affected zone of the steel sheet hardens and the zinc cracking resistance deteriorates, so the C content is set to less than 0.12%, preferably less than 0.05%.

[0016] Si:0.05% or less Si has a deoxidizing effect, but is not necessary when Al, a strong deoxidizing element, is added in sufficient amounts. Si also strengthens the base material, but this effect is relatively small compared to other elements. Furthermore, since Si significantly reduces molten zinc corrosion resistance, a low Si content is preferable, with the upper limit set at 0.05 mass%, which is a stable reduction possible from an operational standpoint. Furthermore, although there are steelmaking limitations, it is preferable to limit the Si content to 0.02 mass% or less.

[0017] Mn: 0.20 to 2.0% Mn is an element that is added from the viewpoint of improving the strength and toughness of the base material, and a content of 0.20% or more is necessary in order to contribute to the strength and toughness of the base material. On the other hand, since a Mn content exceeding 2.0% hardens the base material and the weld heat-affected zone and significantly deteriorates zinc cracking resistance, the Mn content is set to 2.0% or less, preferably 1.6% or less, and more preferably 1.3% or less.

[0018] P:0.015% or less P is an impurity element that is inevitably contained in steel sheets, but because it has a negative effect on weldability and zinc cracking resistance, 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. Also, although there are steelmaking limitations, the P content is preferably 0.008% or less.

[0019] S: 0.030% or less Like P, S is an element that is inevitably contained in steel sheets. However, since S reduces the toughness and weldability of the base material, a lower S content is preferable. Therefore, the upper limit is set to 0.030%, which is a lower limit that can be achieved in a stable manner in operation. Furthermore, if the S content exceeds 0.030%, the zinc cracking resistance of both the base material and the heat-affected zone decreases. For this reason, the S content is set to 0.030% or less. The S content is preferably 0.020% or less, and more preferably 0.010% or less.

[0020] 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 0.015% or more, more preferably 0.018% 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.

[0021] B: 0.0002% or less B is an element that segregates at grain boundaries and significantly deteriorates the zinc cracking resistance of the weld heat affected zone. The lower the content, the better, with the upper limit set at 0.0002%, which is a stable level for operational reasons.

[0022] CEZ: 0.36 or less CEZ, shown in the following formula (1), correlates with the zinc cracking resistance of steel welds. If CEZ exceeds 0.36, the zinc cracking resistance of the weld heat affected zone of steel plates used in molten zinc bath equipment during repeated use deteriorates. For this reason, the upper limit of CEZ is set to 0.36. CEZ is preferably set to 0.35% or less, more preferably 0.34% 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.

[0023] Pcmy: 0.15 or less Pcmy, shown in the following formula (2), correlates with the hardness of the weld heat-affected zone of steel. If Pcmy exceeds 0.15, the hardness of the weld heat-affected zone of steel plates used in molten zinc bath equipment during repeated use increases, and zinc cracking resistance deteriorates. For this reason, the upper limit of Pcmy is set to 0.15. Pcmy is preferably set to 0.14% or less, and more preferably 0.13% or less. Pcmy=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20 +Mo / 7.5+V / 10+5×B... Equation (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.

[0024] The above is the basic chemical composition of the steel sheet in the present invention, with the balance consisting of Fe and unavoidable impurities. In the present invention, impurities are defined as follows: Si, Cu, Ni, Cr, Mo, and V are less than 0.01%, B is less than 0.0001%, Nb is less than 0.003%, Ti is less than 0.005%, and Ca, Mg, and REM are less than 0.0002%.

[0025] Furthermore, the steel sheet of the present invention may contain one or more of the following chemical compositions, as required.

[0026] 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%.

[0027] Ni: 0.1 to 0.5% Ni 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 Ni content is 0.1% or more. Therefore, when Ni is contained, the lower limit of the Ni content is set to 0.1%. The Ni content is preferably 0.30% or more, and more preferably 0.40% or more. On the other hand, if the Ni content exceeds 0.5%, 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 0.5%.

[0028] 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 molten zinc corrosion resistance. Therefore, if Cr is contained, the Cr content is set to less than 0.10%.

[0029] 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. Its effect is not realized unless the content is 0.02% or more. Therefore, when Mo is contained, the lower limit of Mo 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 also reduces toughness. Furthermore, if Mo is contained in an amount of 0.10% or more, the tensile strength becomes 400 MPa or more, and the zinc cracking resistance of the base material decreases. Therefore, if Mo is contained, the Mo content is set to less than 0.10%.

[0030] 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 does it increase costs but also reduces the toughness of the weld. Furthermore, if the Nb content exceeds 0.050%, the tensile strength becomes 400 MPa or more, and the zinc cracking resistance of the base material decreases. Therefore, if Nb is contained, the upper limit of the Nb content is set to 0.050%.

[0031] V: 0.01 to 0.05% V is an optional element in the present invention and is added when it is desired to improve the strength of the base metal. This effect is not exhibited unless the V content is 0.01% or more, so the lower limit of V 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 0.05%, 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 0.05%.

[0032] 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 manifested unless the Ti content is 0.005% or more, so the lower limit of the Ti content is set to 0.005%. The Ti content is preferably 0.008% or more, and more preferably 0.010% or more. 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%. The Ti content is preferably 0.040% or less, and more preferably 0.030% or less.

[0033] 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 toughness of the weld heat-affected zone or the internal quality evaluated by ultrasonic flaw detection, etc. This effect is not exhibited unless the Ca content is 0.0002% or more, so 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 oxides formed will cluster and rather deteriorate the internal quality, so the upper limit of the Ca content is set to 0.0060%.

[0034] 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 0.0010% or more, and more preferably 0.0015% or more. On the other hand, if the Mg content exceeds 0.0060%, the oxides formed 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.

[0035] 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 oxides formed will cluster and rather deteriorate the internal quality, so the upper limit of the REM content is set to 0.0060%. 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.

[0036] <Metal structure> In a 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 part (t: plate thickness) in the plate thickness direction has ferrite and pearlite, and the remainder is 3% or less in area ratio. 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 metallographic structure of the surface layer in the thickness direction from the sheet surface to the t / 4 portion is composed of ferrite and pearlite. By controlling the structure to be composed of ferrite and pearlite, the tensile strength of the steel sheet can be reduced and zinc cracking resistance can be ensured. If bainite or martensite is generated in large amounts in parts other than ferrite, the strength of the steel sheet increases and the zinc cracking resistance deteriorates. The area ratios of ferrite and pearlite are not particularly limited, but it is preferable that the area ratio of ferrite is 70 to 90% and the area ratio of pearlite is 10 to 30%. The presence of a small amount of one or more of bainite, martensite, island martensite, and cementite in the remainder other than ferrite and pearlite is permitted, but the total area ratio of these structures must be 3% or less.

[0037] In the present invention, etching is performed with 5 vol% nital, and the metal structure is observed with an optical microscope (magnification: 100x) and an electron microscope (magnification: 1000x). When observed with the optical microscope, white, round structures are considered to be ferrite structures, and when observed with the optical microscope, black structures that appear as layers when magnified with the electron microscope are considered to be pearlite. The remaining parts include bainite, martensite, island martensite, and cementite. There is no particular method for measuring the area fraction of parts other than ferrite and pearlite. The metallographic structure is etched with 5 vol% nital, and photographs of the metallographic structure are taken with an optical microscope and an electron microscope. The photographs are then loaded into a drawing software such as Photoshop, and the white, round structures (ferrite structures) observed with an optical microscope and the black, layered structures (pearlite) observed with an optical microscope are colored. The area of ​​these colored parts is calculated using image analysis software (ImageJ Ver. 1.53, National Institutes of Health), and this is divided by the overall size of the photograph to determine the ratio (area fraction). This area fraction is then subtracted from 100 to obtain the area fraction.

[0038] <Tensile test> The tensile strength (tensile strength in the width direction of the steel plate) obtained from a tensile test specimen taken so that the longitudinal direction of the test specimen is perpendicular to the rolling direction is less than 400 MPa The tensile properties of the steel sheet of the present invention are evaluated by the tensile strength tested using tensile test pieces taken perpendicular to the rolling direction from the full thickness or from 1 / 4 thickness from the surface. If the tensile strength of the steel sheet is 400 MPa or more, the zinc cracking resistance of the steel sheet will deteriorate, so the tensile strength is set to be less than 400 MPa. The tensile test is carried out in accordance with JIS Z2241.

[0039] <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 above-described composition is heated, hot-rolled to a rolling finish temperature of 730° C. or higher, and then air-cooled to produce a steel sheet.

[0040] Slab heating The slab heating temperature has almost no effect on the strength, molten zinc corrosion resistance, or zinc cracking resistance, so the conditions are not particularly specified. From the viewpoint of manufacturability, the preferred range of the heating temperature is 1000°C or higher and 1300°C or lower.

[0041] Rolling end temperature: slab surface temperature 730°C or higher If the rolling end temperature is lower than 730°C at the slab surface, the structure may become fine-grained and the tensile strength may become 400 MPa or more, so the lower limit of the rolling end temperature is set to 730°C, and more preferably, the lower limit is set to 750°C. The upper limit of the rolling end temperature in hot rolling is not particularly limited, but from the viewpoint of suppressing the occurrence of surface defects after rolling, the rolling end temperature is preferably 970°C or less, and more preferably 950°C or less. The temperature is determined by measuring the surface of the slab immediately after rolling with a radiation thermometer.

[0042] Air cooling after rolling If accelerated cooling by water cooling is performed after rolling, the microstructure will become mainly bainite or mainly ferrite and bainite, and the strength will increase, resulting in a deterioration of zinc cracking resistance. Therefore, air cooling is performed immediately after rolling without accelerated cooling. Note that cases where the cooling rate is slower than air cooling in the atmosphere due to stacking of steel sheets, etc., are also included in air cooling. The average cooling rate of the air-cooling is not particularly limited, but may be 0.01 to 1.50°C / s. The cooling stop temperature during the air-cooling is not particularly limited, but is preferably -20 to 50°C. The above-mentioned accelerated cooling refers to cooling at an average cooling rate of more than 1.50°C / s, with a cooling stop temperature of -20 to 700°C.

[0043] The above manufacturing method makes it possible to efficiently manufacture steel plates for molten zinc bath equipment that are less susceptible to cracking due to molten zinc and that have the weldability required for welded structural steel without using accelerated cooling equipment or heat treatment equipment. The steel plate for molten zinc bath equipment of the present invention is preferably a thick steel plate, preferably having a thickness of 20 mm or more. There is no particular upper limit, but 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 therefore 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.

[0044] The present invention also provides a molten zinc bath facility using the above-mentioned steel plate for molten zinc bath facility. [Example]

[0045] 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.

[0046] [Steel plate manufacturing] In the steelmaking process, the deoxidation and desulfurization of molten steel and its chemical composition were controlled, and slabs with the chemical composition shown in Table 1 were produced by continuous casting. In Table 1, blank columns for elements are treated as having a content of zero in the calculation of CEZ and Pcmy. Next, these slabs were heated under the conditions in Table 2, hot-rolled, and then immediately air-cooled without accelerated cooling (0.01 to 1.5°C / s) down to 10 to 50°C to obtain steel sheets. For the comparative steel sheet No. 12, accelerated cooling (10°C / s) was applied immediately after rolling, and then air cooling (0.05°C / s) was performed to 20°C.

[0047] [Evaluation test] The following evaluation tests were conducted on the steel plates for molten zinc bath equipment manufactured by the above method. It was. For the evaluation of the metal structure, a test piece covering the t / 4 part (t: plate thickness) from the plate surface in the plate thickness direction was taken, the surface was mirror polished and then etched with 5 vol% nital, and the area from the plate surface to the t / 4 part was observed under a microscope, and photographs were taken continuously at 100x magnification and a position 1 mm from the surface were taken. The results were judged from photographs taken at 1000x magnification at the t / 4 position, and those that were mainly ferrite + pearlite or ferrite + pearlite were marked with an "O", while those that were otherwise (mainly ferrite + bainite or mainly bainite in this comparative example) were marked with an "X".

[0048] Tensile tests are conducted in accordance with JIS Z 2241. The direction perpendicular to the rolling direction is the longitudinal direction of the test piece, and No. 5 full-thickness test pieces are used for plate thicknesses of 50 mm or less, while No. 4 round bar tensile test pieces are taken from the 1 / 4 thickness position for plate thicknesses over 50 mm. Tensile tests are conducted to evaluate the tensile strength, with a value of less than 400 MPa considered to pass.

[0049] To evaluate molten zinc corrosion resistance, 40mm x 25mm x 4mm test pieces taken from the surface of steel plate (base material) and steel plate that had been given a thermal history equivalent to that of a weld heat-affected zone were 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 200mg / cm 2 Those with a value of 200 mg / cm or less are considered "○" (pass). 2 Those exceeding this limit were evaluated as "x" (failure). 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). Test pieces were taken from steel plate (base material) and steel plate that had been given a thermal history equivalent to that of a weld heat-affected zone. A zinc wire was wrapped around the notch of the test piece and heated to deposit molten zinc. The test temperature was set at 500°C, and samples with an SLM value (SLM-400 value) of 80% or more at a fracture time of 400 seconds were evaluated as "Good," and samples with an SLM value of less than 80% were evaluated as "Poor." The thermal history corresponding to the weld heat-affected zone was as follows: the time from room temperature to 1350°C was 50 seconds, the holding time at 1350°C was 10 seconds, and the cooling rate from 1350°C to 300°C was 40°C / second. Below 300°C, the specimen was allowed to cool in the air.

[0050] [Evaluation results] The results of each evaluation are listed in Table 2. 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.

[0051] [Table 1]

[0052] [Table 2]

Claims

1. In mass%, C: more than 0.02% and less than 0.12%; 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 Limit to Limiting CEZ shown in formula (1) to 0.36 or less, and limiting Pcmy shown in formula (2) to 0.15 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 has ferrite and pearlite, and the remainder is a structure in which the area ratio is 3% or less, A steel plate for molten zinc bath equipment, having a tensile strength of less than 400 MPa. 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) Pcmy=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20 +Mo / 7.5+V / 10+5×B...Formula (2) In formulas (1) and (2), 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 0.5%, 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: 0.01-0.05%, 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. Heating a slab having the component composition according to claim 1 or 2, Hot rolling is performed with a rolling end temperature of 730°C or higher, Then air cool, 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 has ferrite and pearlite, and the remainder is a structure in which the area ratio is 3% or less, A method for manufacturing a steel sheet for molten zinc bath equipment having a tensile strength of less than 400 MPa.

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

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