JP6302161B2 - Steel sheet for hot-dip zinc bath equipment with excellent hot-zinc corrosion resistance and hot-zinc cracking resistance - Google Patents

Description

The present invention relates to a steel sheet for a hot-dip zinc bath facility that constitutes a hot-dip zinc bath pot (tub) or the like used when hot-dip zinc treatment is performed on a steel material or the like. More specifically, it has excellent corrosion resistance against corrosion by molten zinc, is less likely to cause cracks due to molten zinc than conventional steel, and is economical and resistant to gas cutting and welding. those concerning the excellent molten zinc bath equipment for steel plate in the molten zinc corrosion resistance and zinc cracking.

  Conventionally, a hot dip galvanizing method has been widely used as an economical rust prevention treatment method applied to steel materials. Such a hot dip galvanizing method is performed by immersing a steel material to be plated into a steel bath in which hot dip zinc is put.

  In the steel bath as described above, the molten zinc reacts with the iron in the bath at the interface contacting with the molten zinc to form an iron-zinc alloy layer, so that corrosion proceeds. The corrosion rate at this time becomes very large when the temperature of the molten zinc is around 500 ° C. If the temperature control of the bathtub is not appropriate, damage due to thinning or perforation may occur in a short period of time. By doing so, there is a problem that the bathtub cannot be used.

  For the above-mentioned corrosion problem, in the chemical composition of the steel forming the bathtub, the temperature of molten zinc is around 500 ° C. by increasing the C content and suppressing the Si and P content. Steel materials with reduced corrosion rates have been proposed (see, for example, Patent Documents 1 to 4). According to the steel materials described in Patent Literatures 1 to 4, by having the above-described composition of the steel component, it has excellent corrosion resistance against the corrosive action caused by molten zinc, and suppresses the thickness reduction or perforation of the bathtub. Is possible.

JP 49-130310 A JP-A-53-8314 JP 54-99031 A JP 2002-241888 A

However, in the steel materials described in Patent Documents 1 to 4, while it is possible to suppress the thinning and perforation of the bathtub, cracks due to the action of zinc occur, and the life of the bathtub may be shortened. The present invention has been made in view of such circumstances, has the required strength, toughness and weldability as welded structural steel, and has excellent corrosion resistance against corrosion by molten zinc, and hardly cracked due to molten zinc, and to provide a resistance to molten zinc corrosion resistance and zinc cracking resistance excellent molten zinc bath equipment for steel plate.

  The inventors of the present invention diligently studied the influence of the metal structure on the zinc cracking property by using steel having a chemical composition excellent in hot zinc corrosion resistance. As a result, it was found that by making 70% or more (area ratio) of the metal structure a bainite structure, a steel material excellent in not only molten zinc corrosion resistance but also zinc crack resistance can be obtained. The present invention has been completed based on such findings, and the gist thereof is as follows.

[1] C: 0. More than 12 to 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. Limited to 070% or less, with the balance being Fe and inevitable impurities, in the plate thickness section parallel to the rolling direction and perpendicular to the plate surface, the surface layer metal from the plate surface to t / 4 part in the plate thickness direction A steel sheet for a hot dip zinc bath facility excellent in hot galvanic corrosion resistance and hot cracking resistance, wherein 70% or more of the structure is a bainite structure and the remainder is a ferrite structure.
[2] Further, by mass%, Nb: 0.003 to 0.050%, V: 0.01 to 0.10%, Ti: 0.005 to 0.050%, or one or more of them The steel sheet for molten zinc bath equipment excellent in molten zinc corrosion resistance and zinc cracking resistance according to the above-mentioned [1], comprising:
[3] Further, in terms of mass%, Cu: 0.1 to 0.5%, Ni: 0.1 to 2.0%, Cr: 0.1 to 2.0%, Mo: 0.02 to 1. The steel sheet for molten zinc bath equipment excellent in molten zinc corrosion resistance and zinc cracking resistance according to the above [1] or [2], comprising one or more of 0%.
[4] Further, by mass%, one or more of Ca: 0.0002 to 0.0030%, Mg: 0.0002 to 0.0030%, REM: 0.0002 to 0.0030% The steel sheet for hot-dip zinc bath equipment excellent in hot-zinc corrosion resistance and hot-zinc cracking resistance as described in any one of [1] to [3] above.
[5] The molten zinc corrosion resistance and resistance according to any one of the above [1] to [4], further comprising B: 0.0002 to 0.0010% by mass%. Steel sheet for molten zinc bath equipment with excellent zinc cracking properties.

According to the steel sheet for hot dip zinc bath equipment excellent in hot galvanic corrosion resistance and zinc cracking resistance of the present invention, it has a predetermined steel component, in a plate thickness section parallel to the rolling direction and perpendicular to the plate surface. In the thickness direction, of the surface layer metal structure from the plate surface to t / 4 part, the area ratio is 70% or more bainite structure, so it has excellent corrosion resistance against corrosion by molten zinc, and In addition, cracks due to molten zinc are unlikely to occur, and the required strength, toughness and weldability as welded structural steel are achieved, and an economical steel sheet for molten zinc bath equipment that enables gas cutting and welding is realized. it can. Therefore, by constructing a hot-dip zinc bath kettle (tub) using the steel sheet for hot-dip zinc bath equipment excellent in hot-zinc corrosion resistance and zinc cracking resistance of the present invention, corrosion, cracking, etc. are unlikely to occur, and long life Therefore, the industrial effect is extremely large, and the contribution to society is immeasurable from the viewpoint of the safety of the structure.

Further, according to the method for manufacturing a hot-dip zinc bath steel sheet having excellent resistance to hot zinc corrosion and zinc cracking according to the present invention, a steel having a predetermined steel component is cast into a slab and heated to 950 ° C. or higher. The surface temperature of the steel sheet measured with a radiation thermometer is Ar ₃ point + 150 ° C or lower, and finish rolling is performed in a temperature range of Ar ₃ point or higher, and immediately water-cooled. It is possible to efficiently produce a steel sheet for hot dip zinc bath equipment that has high corrosion resistance, is less likely to crack due to molten zinc, and has the required strength, toughness and weldability as a welded structural steel. It becomes possible. That is, according to the present invention, it is possible to manufacture a steel sheet for a hot dip zinc bath facility while omitting tempering heat treatment (quenching, tempering, etc.) after hot rolling and reducing manufacturing costs.

It is a figure for demonstrating typically an example of the steel sheet for hot-dip zinc bath equipment excellent in the hot-dip zinc corrosion resistance and hot-zinc cracking resistance concerning the present invention, and shows the relation between the form of metal structure, and hot zinc corrosion amount. It is a graph to show. It is a figure for demonstrating typically an example of the steel sheet for hot-dip zinc bath equipment excellent in the hot-zinc corrosion resistance and hot-zinc cracking resistance which concerns on this invention, and the relationship between the form of a metal structure, and _SLM-400 value It is a graph which shows.

Hereinafter, embodiments of a steel sheet for hot dip zinc bath equipment (hereinafter, simply referred to as “steel board for hot dip zinc bath equipment”) excellent in molten zinc corrosion resistance and zinc cracking resistance according to the present invention and its manufacturing method will be described. This will be described with reference to the drawings as appropriate.
In addition, since this embodiment is described in detail for better understanding of the gist of the invention, the present invention is not limited unless otherwise specified.
In the following description, “%” indicating the content of each component in the chemical component composition indicates “mass%” unless otherwise specified.

[Steel plate for molten zinc bath equipment]
The steel sheet for molten zinc bath equipment excellent in molten zinc corrosion resistance and zinc cracking resistance of the present invention is C: 0.10 to 0.30% by mass%, Si: 0.05% or less, Mn: 0.00%. 20 to 2.0%, P: 0.015% or less, S: 0.030% or less, Al: 0.070% or less, the balance is made of Fe and inevitable impurities, in the rolling direction In the plate thickness cross section parallel and perpendicular to the plate surface, 70% or more of the metal structure of the surface layer from the plate surface to t / 4 part in the plate thickness direction is a bainite structure and is roughly configured. .

<Chemical component composition>
Steel sheets for hot dip zinc bath equipment of the present invention are: C: 0.10 to 0.30%, Si: 0.05% or less, Mn: 0.2 to 2.0%, P: 0.015% or less, S : 0.030% or less, Al: 0.070% or less of each component is contained as an essential element (or unavoidable element), the balance consists of Fe and unavoidable impurities.
Moreover, in the steel sheet for hot dip zinc bath equipment of the present invention, Nb: 0.003 to 0.050%, V: 0.01 to 0.10%, Ti: 0.005 to 0.050 in terms of mass%. %, Cu: 0.1-0.5%, Ni: 0.1-2.0%, Cr: 0.1-2.0%, Mo: 0.02-1.0%, Ca: 0.0. One or more of 0002 to 0.0030%, Mg: 0.0002 to 0.0030%, REM: 0.0002 to 0.0030%, B: 0.0002 to 0.0010%, It can be set as the structure which selects suitably and contains.
Below, the reason for limitation of the chemical component composition of the steel material in this invention is demonstrated.

“C: Carbon” 0.10 to 0.30 mass%
C is an important element for improving the strength of the steel sheet and contributes to the formation of bainite. When the C content is less than 0.10% by mass, the molten zinc corrosion resistance of the steel sheet is greatly reduced, the formation of bainite is insufficient, and the molten zinc cracking resistance is also reduced. % Or more of C must be added. Moreover, addition of more than 0.12 mass% is more preferable from a viewpoint of ensuring favorable hardenability. However, when 0.30 mass% or more of C is added, the weldability of the steel sheet deteriorates, so 0.30 mass% was made the upper limit.

“Si: silicon” 0.05 mass% or less Si has a deoxidizing action, but is unnecessary when Al, which is a strong deoxidizing element, is sufficiently added. Si also has the effect of strengthening the base material, but its effect is relatively small compared to other elements. In addition, since Si greatly reduces the resistance to molten zinc corrosion, its content is preferably small, and the upper limit is 0.05% by mass that can be stably reduced in operation. Moreover, although there is a restriction | limiting on steel manufacture, it is more preferable to make content of Si 0.02 mass% or less.

“Mn: Manganese” 0.20 to 2.0 mass%
Mn is an element added from the viewpoint of securing the base material strength and bainite, and 0.20% by mass or more is necessary to contribute to the base material strength and the molten zinc cracking resistance. Moreover, when the addition amount of C added simultaneously is as low as 0.12 mass% or less, it is preferable to add Mn more than 0.5 mass% in order to stably obtain a target structure. However, the addition of 2.0% by mass or more of Mn greatly deteriorates the weldability, so the Mn addition range was set to 0.20 to 2.0% by mass.

“P: Phosphorus” 0.015% by mass or less P is an impurity element and is inevitably contained in the steel sheet, but, like Si, greatly reduces the resistance to molten zinc corrosion and adversely affects weldability. Therefore, the content is preferably as small as possible, and the upper limit is set to 0.015% by mass that can be stably reduced in operation. Moreover, although there is a restriction | limiting on steel manufacture, it is more preferable that content of P shall be 0.008 mass% or less.

“S: Sulfur” 0.030% by mass or less S is an element inevitably contained in the steel plate as in the case of P. However, since S decreases the base metal toughness and weldability, it is preferable that S be less. For this reason, the upper limit was set to 0.030% by mass that can be stably reduced in operation.

“Al: Aluminum” 0.070% by mass or less Al is an element used for deoxidation, and in order to obtain the deoxidation effect, addition of 0.015% by mass or more is preferable. However, addition of 0.070% by mass or more of Al causes many coarse inclusions to exist in the steel and lowers the toughness, so the upper limit was made 0.070% by mass.

“Nb: Niobium” 0.003 to 0.050 mass%
“V: Vanadium” 0.01 to 0.10% by mass
“Ti: Titanium” 0.005 to 0.050 mass%
Nb, V, and Ti are selective elements in the present invention, and are added as necessary to ensure the strength of the base material. However, if both are added in large amounts, the base material toughness and weldability are deteriorated. The addition amount was 0.003 to 0.050 mass%, V was 0.01 to 0.10 mass%, and Ti was 0.005 to 0.050 mass%. These elements may be added alone or in combination from the viewpoint of securing the strength of the base material.

“Cu: Copper” 0.1 to 0.5 mass%
"Ni: nickel" 0.1-2.0 mass%
“Cr: Chromium” 0.1 to 2.0 mass%
“Mo: Molybdenum” 0.02 to 1.0 mass%
Cu, Ni, Cr, and Mo are also selective elements in the present invention. Like Nb, V, and Ti described above, Cu, Ni, Cr, and Mo are added as necessary to ensure the strength of the base material. Since the toughness and weldability are deteriorated, Cu is 0.1 to 0.5% by mass, Ni is 0.1 to 2.0% by mass, Cr is 0.1 to 2.0% by mass, and Mo is 0%. The added amount was 0.02 to 1.0% by mass. These elements may be added alone or in combination from the viewpoint of securing the strength of the base material.

“Ca: calcium” 0.0002 to 0.0030 mass%
“Mg: Magnesium” 0.0002 to 0.0030 mass%
“REM: rare earth element (lanthanoid element)” 0.0002 to 0.0030 mass%
Ca, Mg, and REM are also selective elements in the present invention, and are added as necessary to ensure the toughness of the high heat input welding heat-affected zone, but when added in large quantities, coarse inclusions remain in the steel. As a result, the toughness of the base metal and the weld heat-affected zone is reduced, so Ca is 0.0002 to 0.0030 mass%, Mg is 0.0002 to 0.0030 mass%, and REM is 0.0002 to 0.0030. The amount added was mass%. These elements may be added alone or in combination from the viewpoint of securing the strength of the base material.

“B: Boron” 0.0002 to 0.0010 mass%
B is also a selective element in the present invention, and is added as necessary to ensure the strength of the base material of the steel sheet. The base material strength effect is manifested when 0.0002% or more of B is added, but when added in a large amount, the base material toughness and weldability may be deteriorated, so 0.0002 to 0.0010% by mass. The amount added.

<Metallic structure>
The steel sheet for hot dip zinc bath equipment of the present invention has an area ratio in the surface layer metal structure from the plate surface to t / 4 part in the plate thickness direction in the plate thickness section parallel to the rolling direction and perpendicular to the plate surface. 70% or more is constituted as a bainite structure. In the present invention, a metal structure is observed with an optical microscope, and a lath-like structure is determined as a bainite structure (also referred to as a bainite phase). The balance of the bainite structure is a ferrite structure (also referred to as a ferrite phase).

  The steel having the chemical composition as described above can be obtained, for example, by melting a steel ingot having the steel composition in a vacuum melting furnace. In addition, the metal structure can be changed by changing the rolling temperature and cooling conditions when manufacturing the steel plate from the steel ingot in the manufacturing method described in detail later. This makes it possible to produce a steel sheet in which the metal structure of the surface layer is a bainite phase alone or a mixture of a ferrite phase and a bainite phase.

The present inventors conducted molten zinc corrosion and zinc cracking tests using the steel sheet for molten zinc bath equipment having the above composition, and the results are shown in the graphs of FIGS. Here, molten zinc corrosion is a test in which a test piece taken from a steel sheet is immersed in molten pure zinc and the resistance to molten zinc corrosion is evaluated by a change in weight per unit area. The zinc cracking test is a test in which molten zinc is adhered to the notch of the notched round bar tensile test and a load is applied at a high temperature.
FIG. 1 is a graph showing the relationship between the metal structure morphology and the molten zinc corrosion amount, and no change in the corrosion rate due to the difference in metal structure morphology was observed.
FIG. 2 is a graph showing the relationship between metallographic morphology and _SLM-400 value (%). In S _LM-400 value is an index of resistance to molten zinc cracking, the S _LM, in the case of galvanized the specimen, performed NBT test (notched round bar tensile test) at a test temperature 500 ° C., The value obtained by dividing the breaking strength by the breaking strength when plating is not applied. The _SLM-400 value represents the _SLM when the breaking time is 400 seconds, and the higher this value, the higher the resistance to zinc cracking. It shows that.

  As shown in the graph of FIG. 2, the present inventors have found that the zinc crack resistance of the steel sheet is higher when the metal structure includes a bainite structure. Further, the present inventors have found that the zinc cracking resistance is higher when the amount of granular ferrite is small, and the zinc cracking resistance is improved by including 70% or more of the bainite structure. Furthermore, it discovered that zinc cracking resistance improved greatly by setting it as the metal structure which contains 80% or more of a bainite structure. The steel sheet for hot dip zinc bath equipment of the present invention has been made based on the above-described knowledge, and can achieve extremely excellent hot galvanic corrosion resistance and zinc cracking resistance at the same time.

As described above, according to the steel sheet for hot dip zinc bath equipment excellent in hot galvanic corrosion resistance and zinc cracking resistance according to the present invention, C: 0.10 to 0.30% by mass, Si: 0 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, the balance being Fe and 70% in area ratio among the metallographic structure of the surface layer from the plate surface to t / 4 part in the plate thickness (t) direction in the plate thickness cross section made of unavoidable impurities, parallel to the rolling direction and perpendicular to the plate surface. % Or more is a bainite structure, so that it has excellent corrosion resistance against corrosion by molten zinc, and is less likely to crack due to molten zinc, and has the required strength, toughness and Economical molten zinc with weldability and capable of gas cutting and welding Equipment for the steel plate can be realized. Therefore, by constructing a hot-dip zinc bath kettle (tub) using the steel sheet for hot-dip zinc bath equipment excellent in hot-zinc corrosion resistance and zinc cracking resistance of the present invention, corrosion, cracking, etc. are unlikely to occur, and long life Therefore, the industrial effect is extremely large, and the contribution to society is immeasurable from the viewpoint of the safety of the structure.

[Method for producing steel sheet for molten zinc bath equipment]
The method for producing a steel sheet for hot dip zinc bath equipment excellent in hot galvanic corrosion resistance and zinc cracking resistance of the present invention is obtained by casting a steel having the above-described chemical composition into a slab, or the slab is used as it is, or After forming a cold piece, it is heated to 950 ° C. or higher, finish rolling in a temperature range where the plate surface temperature is Ar ₃ points + 150 ° C. or lower and Ar ₃ points or higher, and immediately water-cooled.
Ar ₃ (° C.) = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo-0.35 (t-8) (1)
However, in said Formula (1), t is the board thickness (mm) after finish rolling, and each component is the mass%.
Below, the reason for limitation in the manufacturing method of the steel sheet for molten zinc bath equipment of this invention is demonstrated.

“Heating temperature of slab before rolling”
In the manufacturing method of this invention, the heating temperature of the slab before rolling shall be 950 degreeC or more. In the present invention, after casting a steel having a prescribed chemical composition to form a slab (slab), the slab is used as it is or as a cold piece, and then heated to 950 ° C. or higher before rolling. It holds in a furnace so that the whole slab may become 950 degreeC or more.
When the heating temperature of such a slab before rolling becomes too low, the load applied to the rolling mill increases during rolling of the slab, so the lower limit temperature was set to 950 ° C. The heating temperature is preferably 1000 ° C. or higher, more preferably 1100 ° C. or higher. The upper limit of the heating temperature is not particularly limited, but is preferably 1250 ° C. in consideration of productivity. In order to suppress the coarsening of the metal structure, the upper limit of the heating temperature is more preferably set to 1150 ° C.

"Finish rolling temperature"
In the manufacturing method of the present invention, as described above, the steel sheet surface temperature is set to a temperature range of Ar ₃ points + 150 ° C. or lower and Ar ₃ points or higher with respect to the finish rolling temperature. The temperature range at the time of finish rolling is within this range, and by immediately cooling with water, 70% or more (area ratio) of the metal structure in the surface layer of the steel sheet, that is, the layer area from the plate surface to t / 4 part, is a bainite structure. It becomes possible.
The steel sheet surface temperature can be measured with a radiation thermometer.

In the production method of the present invention, since the ratio of the bainite structure to the metal structure of the steel sheet is in the above range, it is necessary to finish the finish rolling at the temperature of the austenite region and perform water cooling. It was set as the temperature of Ar ₃ point prescribed | regulated by (1) Formula. In addition, if the finish rolling temperature is too high, insufficient cooling or uneven cooling occurs in the subsequent water cooling, resulting in a non-uniform structure. Therefore, the upper limit of the finish rolling temperature was set to Ar ₃ points + 150 ° C.
In addition, as said (1) type | formula for calculating | requiring ₃ points | pieces of Ar, reference literature {"Control rolling and control cooling" by Minoru Gunao, Jijinshokan (1997)} p. The formula (2-3) described in 26 was used.

  In addition, the plate thickness after finish rolling can usually be set to a plate thickness of, for example, about 6 mm to 70 mm when the steel plate is used for a hot dip zinc bath, and the total rolling ratio at this time is 80 to 98. If it is about%, it becomes possible to manufacture a steel sheet for hot dip galvanizing bath equipment with high productivity without impairing the characteristics of the steel sheet.

"Water cooling conditions"
In the production method of the present invention, it is preferable that the steel sheet is immediately cooled by water cooling means after finish rolling in the above temperature range. Thus, the purpose of starting water cooling immediately after finish rolling is to suppress the formation of ferrite structure and to increase the area ratio of bainite structure to 70% or more, and air cooling during conveyance from the rolling mill to the water cooling device. Is acceptable. The water cooling stop temperature is not particularly limited, but is preferably 450 to 700 ° C. when it is necessary to suppress excessive curing and ensure toughness. In addition, since a cooling rate changes with plate | board thickness, a preliminary test etc. should be performed and the water quantity density of water cooling should be controlled so that a desired metal structure may be obtained.

As described above, according to the method for manufacturing a steel sheet for hot dip zinc bath equipment excellent in hot galvanic corrosion resistance and zinc cracking resistance according to the present invention, a steel having a predetermined steel component is cast into a slab, Heating to 950 ° C or higher, steel sheet surface is finished at a temperature range of Ar ₃ points + 150 ° C or lower, and Ar ₃ points or higher, and immediately cooled with water to provide excellent resistance to corrosion by molten zinc. It is possible to efficiently produce steel sheets for hot dip bath equipment that have corrosion resistance and are less susceptible to cracking due to molten zinc, and that have the required strength, toughness and weldability as steel for welded structures. It becomes. Here, the Ar ₃ point is defined as follows.
Ar ₃ (° C.) = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo-0.35 (t-8) (1)
{However, in the above formula (1), t is a plate thickness (mm) after finish rolling, and each component is mass%. }
Therefore, when a steel sheet for a molten zinc bath facility is manufactured by the manufacturing method of the present invention and a hot-dip zinc bath pot (tub) is formed using this steel sheet, corrosion, cracking, etc. are unlikely to occur, and long- life molten zinc Since a bath can be obtained, the industrial effect is extremely large, and the contribution to society is immeasurable from the viewpoint of the safety of the structure.

  Hereinafter, examples of the steel sheet for hot-dip zinc bath equipment excellent in hot-zinc corrosion resistance and zinc cracking resistance and the manufacturing method thereof according to the present invention will be described, and the present invention will be described more specifically. Of course, the present invention is not limited to the following examples, and the present invention can be carried out with appropriate modifications within a range that can be adapted to the gist of the preceding and following descriptions, all of which are included in the technical scope of the present invention. Is.

[Production of sample]
In the steelmaking process, the deoxidation / desulfurization of the molten steel and the chemical composition were controlled, and slabs having the chemical composition shown in Table 1 below were produced by continuous casting. At this time, the heating temperature was set to the temperature shown in Table 2 below.
Next, the slab is reheated (finish rolling temperature) under the production conditions shown in Tables 2 and 3 below, finished to a sheet thickness of 32 to 70 mm by finish rolling, and immediately accelerated cooling by water cooling to provide a molten zinc bath facility Steel plates were manufactured. In addition, some steel plates were air-cooled after finish rolling for comparison (rolling condition B1 in Table 2 below, “water cooling after rolling: none”).

[Evaluation test]
The following evaluation tests were performed on the steel sheets for hot dip zinc bath equipment manufactured by the above method.
For the evaluation of the metal structure, the surface layer of t / 4 part from the plate surface of the steel sheet was observed with a microscope, and the ratio of the bainite structure in the metal structure of the surface layer was investigated by image analysis from a photograph taken at a magnification of 100 times. did. And the thing whose bainite structure in the metal structure of a steel plate surface layer is 70% or more was evaluated as "(circle)", and the thing less than 70% was evaluated as "x".

For evaluation of corrosion resistance to molten zinc, a test piece of 40 mm × 25 mm × 4 mm collected from the surface layer of the steel plate was immersed in zinc having a purity of 99.99% at a temperature of 500 ° C. for 24 hours. And the thing whose corrosion after 24 hours immersion is 200 mg / cm < ² > or less was set as "(circle)", and the thing exceeding 200 mg / cm < ² > was evaluated as "*".
Moreover, about the evaluation of zinc cracking resistance, it evaluated by the NBT test (round bar tensile test with a notch) (refer to Nippon Steel Technical Report 348, 1993, p.63-70). Then, a zinc wire is wound around the notch of the test piece and heated to adhere the molten zinc, the test temperature is 500 ° C., and the _SLM value ( _SLM-400 value) at a rupture time of 400 seconds is 80% or more. Was evaluated as “◯”, and less than 80% was evaluated as “x”.

  Further, as a weldability test, an evaluation was made based on the presence or absence of occurrence of a cross-sectional crack by a y-type weld crack test (JIS Z3158). That is, with each final plate thickness, with a root interval of 1.0 mm, a groove is prepared, and a solid wire material of YGW15 (JIS Z3312) standard is used as a welding material, with a heat input of 30 to 36 kJ / cm, Multi-layer welding by MAG welding was performed to produce a y-type weld specimen, and the presence or absence of a cross-sectional crack was visually confirmed on the mirror-polished surface of each of the five cross sections of the y-type weld. A case where no cross-sectional cracks were observed with the naked eye at each of the five cross sections was evaluated as “◯”, and a cross section crack confirmed with one or more cross-sections was evaluated as “×”.

  A list of chemical composition of the steel sheets (present invention steel and comparative steel) in this example is shown in Table 1 below, and a heating condition before finish rolling, finish rolling temperature, and a rolling condition list with or without water cooling are shown in Table 2 below. Show. In addition, Table 3 below shows a list of combinations of steels having chemical composition shown in Table 1 below and manufacturing conditions, and a list of evaluation results.

[Evaluation results]
As shown in Tables 1 to 3, a steel sheet for a hot dip zinc bath facility (invention steel) having a chemical component composition defined in the present invention and produced according to the production conditions defined in the present invention is a metal in the surface layer of the steel sheet. The bainite structure occupies 70% or more of the structure, and the evaluation of the molten zinc corrosion resistance, the zinc cracking resistance and the weld cracking resistance is all “good”, and the molten zinc corrosion resistance and the zinc cracking resistance It became clear that it was excellent in property.

On the other hand, the steel plate symbols C1 and C2, which are comparative steels, have a small amount of C, so that the bainite structure occupies the metal structure of the steel sheet surface layer, and both the molten zinc corrosion resistance and the zinc crack resistance are inferior. Further, since C3 and C10 have a large amount of Si, they are inferior in molten zinc corrosion resistance. Further, since C4 has a large amount of P, it is inferior in molten zinc corrosion resistance and weldability.
In addition, steel plate symbols C5 to C8, which are comparative steels, are examples in which the chemical composition is included in the range defined in the present invention, but none of them is subjected to water cooling after finish rolling, and the manufacturing conditions are the present invention. Therefore, although the bainite structure in the metal structure of the steel sheet surface layer is small and the molten zinc corrosion resistance is good, the zinc crack resistance is poor.

Further, the steel plate symbol C9, which is a comparative steel, has a large amount of C, and C11 has a large amount of Mn, so that the weld crack resistance is inferior. Moreover, since C12 has few amounts of Mn, there is little bainite structure in the metal structure of a steel plate surface layer, and its zinc cracking resistance is inferior.
Moreover, since steel plate symbol C15, which is a comparative steel, has a low finish rolling temperature, the bainite structure occupies the metal structure of the steel sheet surface layer, and the evaluation of zinc crack resistance is inferior.

  From the results of the examples described above, the steel sheet for hot-dip zinc bath equipment having excellent hot-zinc corrosion resistance and hot-zinc cracking resistance according to the present invention has excellent anti-corrosion resistance against hot zinc corrosion and melts. It is clear that cracks due to zinc are unlikely to occur and that the steel sheet has excellent steel sheet characteristics.

Claims (5)

C: 0% by mass. More than 12 to 0.30%,
Si: 0.05% or less,
Mn: 0.20 to 2.0%
Containing
P: 0.015% or less,
S: 0.030% or less,
Al: limited to 0.070% or less, the balance consists of Fe and inevitable impurities,
In the sheet thickness 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 t / 4 part in the sheet thickness direction is a bainite structure, and the remainder A steel sheet for hot dip zinc bath equipment excellent in hot galvanic corrosion resistance and zinc cracking resistance, characterized in that is a ferrite structure. Furthermore, in mass%,
Nb: 0.003 to 0.050%,
V: 0.01 to 0.10%,
Ti: 0.005 to 0.050%
The steel plate for hot-dip zinc bath equipment excellent in hot-zinc corrosion resistance and hot-zinc cracking resistance according to claim 1, comprising one or more of them. Furthermore, in mass%,
Cu: 0.1 to 0.5%,
Ni: 0.1 to 2.0%,
Cr: 0.1 to 2.0%,
Mo: 0.02-1.0%
The steel plate for hot-dip zinc bath equipment excellent in hot-zinc corrosion resistance and hot-zinc cracking resistance according to claim 1 or 2, characterized by containing one or more of them. Furthermore, in mass%,
Ca: 0.0002 to 0.0030%,
Mg: 0.0002 to 0.0030%,
REM: 0.0002 to 0.0030%
It contains 1 type or 2 types or more of these, The molten zinc bath equipment excellent in molten zinc corrosion resistance and zinc cracking resistance as described in any one of Claims 1-3 characterized by the above-mentioned steel sheet. Furthermore, in mass%,
B: 0.0002 to 0.0010%
Resistant molten zinc corrosion resistance and zinc cracking resistance excellent molten zinc bath equipment for steel sheet according to any one of claims 1 to 4, characterized in that it contains.

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