JP2002241888A - Steel for molten zinc vessel having high high- temperature strength and little erosion - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel for a molten zinc vessel which has high high- temperature strength and little erosion. SOLUTION: The steel for a molten zinc vessel having high high-temperature strength and little erosion has a composition containing, by mass, 0.01 to 0.2% C, <=0.05% Si, 0.01 to 2% Mn, <=0.02% P, <=0.02% S and 0.1 to 2% Mo, and further containing one or two kinds selected from 0.005 to 0.03% Nb and 0.005 to 0.05% V, and the balance Fe with inevitable impurities. Preferably, the steel further contains 0.005 to 0.05% Ti and 0.002 to 0.01% N and further contains 0.0002 to 0.06% Al and 0.0002 to 0.01% O, and one or more kinds selected from 0.0002 to 0.006% Mg, 0.0002 to 0.006% Ca and 0.0002 to 0.006% rare earth metals.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a hot-dip galvanizing tank for use in hot-dip galvanizing for ensuring the corrosion resistance of a steel structural member or a steel plate. It relates to a small amount of steel for hot-dip galvanizing.

[0002]

2. Description of the Related Art In order to increase the corrosion resistance of steel structural members or steel sheets, hot dip galvanizing is applied. For this purpose, 45
A so-called hot-dip galvanizing operation is performed in which zinc heated and melted at about 0 to 500 ° C. is stored in a pot or a container, and a structural member or the like is immersed therein. A problem in this case is that when a steel pot or container (hereinafter, referred to as a zinc pot) is used for a long time, the steel is melted and damaged by the molten zinc. Furthermore, the zinc pot is exposed to high temperature for a long time and is greatly deformed, which impairs safety. Regarding such damage to the hot-dip galvanizing pot, see, for example, “Lead and Zinc” No. 202 (issued in March 1998).
p. 11 to 21 have a detailed description.

[0003] When molten zinc and steel come into contact, an alloy layer of zinc and iron is formed. At this time, if a stable alloy layer is formed, erosion of the steel is small, but if the alloy layer is unstable, alloying proceeds and erosion of the steel proceeds. The effects of alloying elements that promote or suppress the progress of such erosion have already been studied in detail. For example, Nippon Steel Corporation's product catalog “Steel for hot-dip galvanized pots” (1980) April). According to this, at 500 ° C. where erosion is most remarkable, erosion is suppressed by reducing Si. This is because the lowering of Si reduces the unstable phase of the iron-zinc alloy layer and stabilizes the alloy layer.

[0004] On the other hand, regarding the deformation during use of the zinc pot, according to the above-mentioned document 1, for example, a width of 2.
There is a description that a 1 m pot may be deformed as much as 36 mm in the width direction. In order to avoid this, measures have been taken to reinforce the kettle structurally or to lower the temperature of the molten zinc as much as possible. However, these measures are not preferable because they may increase the production cost of the zinc pot and hinder the galvanizing workability. Furthermore, welding is applied when manufacturing a zinc pot, but it is difficult to secure the toughness of the heat affected zone (hereinafter, referred to as HAZ) with conventional zinc pot steel. I have to do it. In this case, the welding time becomes long and it takes a lot of time to manufacture the zinc pot.

[0005]

As described above, the steel used in the conventional zinc pot can be made to have low Si to reduce the melting loss of the zinc pot, but the strength at high temperatures is not high. It is sufficient, and the deformation of the zinc pot during use cannot be suppressed. Furthermore, it is necessary to secure the HAZ toughness by lowering the heat input during welding when manufacturing the zinc pot, which increases the manufacturing cost.

[0006] In order to improve the reliability of the zinc pot and enable long-term use, it is necessary to simultaneously reduce erosion due to molten zinc and reduce deformation due to long-term use. The object of the present invention is to solve the above-mentioned problems, to reduce the erosion by molten zinc, and to increase the high-temperature strength to suppress deformation during use of the zinc pot, and further, to apply a large heat input welding Another object of the present invention is to provide a steel for a hot-dip zinc pot capable of ensuring HAZ toughness.

[0007]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems and provides a steel for a hot-dip zinc pot having high strength at high temperature and little erosion. The gist of the invention is as follows. (1) In mass%, C: 0.01 to 0.2%, Si: 0.
Mn: 0.01 to 2%, P: 0.02% or less, S: 0.02% or less, Mo: 0.1 to 2%, and Nb: 0.005 to 0% 0.03%, V: 0.
005-0.05% of one or two kinds, with the balance being F
e. and high-temperature strength and little erosion, characterized by being composed of e and unavoidable impurities. (2) In mass%, Ti: 0.005 to 0.05
%, N: 0.002 to 0.01%, wherein the steel for a hot-dip zinc pot has a high high-temperature strength and a low melting loss according to the above (1). (3) In mass%, further, Al: 0.0002 to 0.0
6%, O: 0.0002-0.01%, and
Mg: 0.0002-0.006%, Ca: 0.000
2 to 0.006%, REM: 0.0002 to 0.006
% Or more of the steel for a hot-dip zinc pot as described in the above item (1) or (2), wherein the high-temperature strength is high and the erosion is small.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have conducted detailed experimental investigations on the erosion and high-temperature strength of a steel for a hot-dip zinc pot, and further on the HAZ toughness, and have obtained the following findings. That is, the steel for a hot-dip galvanizing furnace described in the Nippon Steel Corporation product catalog “Steel for hot-dip galvanizing kettle” (April 1980) stabilizes the iron-zinc alloy layer and suppresses erosion. Therefore, it is necessary to reduce the Si content as much as possible. Furthermore, it has been clarified that the incorporation of C further reduces erosion.

[0009] However, these steels do not take measures to ensure high-temperature strength, and cannot suppress deformation of the zinc pot during use. SUMMARY OF THE INVENTION The present inventors have added Mo to low Si-based steel for zinc pots, and further increased the high-temperature strength remarkably by adding a small amount of Nb or V, thereby reducing erosion and using zinc pots in use. It has been found that the deformation of can be suppressed as much as possible. Mo increases the high-temperature strength by solid solution strengthening, but when Nb or V is added in a small amount, carbonitrides of Nb or V are formed in the steel, and these carbonitrides are nucleated during use at high temperatures. As a result, Mo carbides are finely precipitated, so that the high-temperature strength further increases. The inventors, as shown in the examples, reduce Si, and contain Mo and then Nb or V, so that erosion by zinc is small and high-temperature strength is high. A steel that can exhibit excellent properties as a steel for zinc pots has been found.

The reasons for limiting the contained elements in the present invention are as follows. C is an essential element for securing the base material strength and the high-temperature strength. If it is less than 0.01%, the base material strength cannot be secured. If it exceeds 0.2%, the toughness of the base material and HAZ is greatly reduced. Therefore, C is 0.01-0.2%
Limited to.

Si is an element that promotes erosion by molten zinc, and the lower the better, the better the erosion if it exceeds 0.05%. Therefore, the upper limit is made 0.05%. But,
Less than 0.01% significantly reduces erosion.
% Is preferable.

Mn is an element effective for securing the base material strength. If it is less than 0.01%, the effect cannot be obtained. If it exceeds 2%, it becomes difficult to secure the toughness of the base material and HAZ. Therefore, M
The range of n was set to 0.01 to 2%.

[0013] P is an element that lowers the toughness of the base material and HAZ, and the lower the better. On the other hand, when P is high, the grain boundary strength is weakened, and zinc easily enters the grain boundary, so that erosion is increased and hot-dip galvanizing cracks are likely to occur. From such a viewpoint, the upper limit of P is set to 0.02%.

S is an element which lowers the toughness of the base material HAZ, and the lower the better. However, if it exceeds 0.02%, it becomes difficult to secure toughness, so 0.02% was made the upper limit.

Mo is an essential element in the present invention. 0.
If it is less than 1%, the effect of increasing the high-temperature strength is not significant.
When the content exceeds 2%, the high-temperature strength increases, but coarse carbides are formed and the toughness of the base material and the HAZ is significantly reduced. Therefore, the range of Mo is set to 0.1 to 2%.

Nb and V are elements that increase the high-temperature strength when Mo is contained together with Mo alone, and one or two of them can be contained as necessary. If Nb is less than 0.005%, the effect of increasing the high-temperature strength is not significant. If it exceeds 0.03%, the HAZ toughness significantly decreases. Therefore, the range of Nb is set to 0.005.
-0.03%. If V is less than 0.005%, the effect of increasing the high-temperature strength is not significant. On the other hand, if it exceeds 0.05%, H
The decrease in AZ toughness is significant. Therefore, the range of V is set to 0.
005 to 0.05%.

Welding is inevitable in the production of zinc pots. In the conventional steel for a zinc pot, it was necessary to keep the welding heat input low in order to secure the toughness of the HAZ. As a result, the time required for welding is increased, and the production cost is increased. In order to solve this problem, HAZ toughness can be ensured even by applying large heat input welding by including the following elements.

By containing Ti together with N, fine TiN is formed in the steel, which can prevent the austenite crystal grains of the HAZ from becoming coarse. Further, MnS can be precipitated on TiN, and fine ferrite can be generated from the composite precipitate during the welding thermal cycle. Due to these effects, even in high heat input welding, HA
Z toughness can be ensured. If the Ti content is less than 0.005%, the above effects cannot be obtained. If it exceeds 0.05%, coarse TiN or T
Precipitation of iC lowers the toughness of the base metal and HAZ. Therefore, the range of Ti is set to 0.005 to 0.05%.

N is an essential element for producing TiN. If it is less than 0.002%, the TiN precipitation amount is small,
The effect of improving the HAZ toughness is not significant. If it exceeds 0.01%, the amount of N which forms a solid solution in the base iron increases, so that the toughness of the base metal and the HAZ decreases. Therefore, N was limited to 0.002 to 0.01%.

Mg, Ca, and REM alone generate oxides, sulfides, or composites thereof, or oxides composited with Al, and these fine particles are pinned to form austenite grains in the HAZ. Restrain growth,
Alternatively, in the cooling process of the welding heat cycle, these particles serve as nuclei to generate fine ferrite, thereby reducing the HAZ structure and improving the HAZ toughness. Such an effect is particularly remarkable in large heat input welding. In the present invention, large heat input welding in the production of a zinc pot can be made possible by including these elements in the steel for the zinc pot.

Al forms an oxide in combination with one or more of Mg, Ca, and REM. If the content exceeds 0.06%, a composite oxide is not generated and alumina is generated, so that the above effects cannot be obtained. 0.0002%
It is industrially unsuitable to make the amount less than the above because it increases the cost for steelmaking. Therefore, the range of Al is 0.0002-
0.06%. 0.00 for a noticeable effect
It is desirable to set it to 1 to 0.01%.

O is an element necessary for forming an oxide or a composite of a sulfide and an oxide. If it exceeds 0.01%, the above effect cannot be obtained due to the formation of a coarse oxide, and the strength ductility of the base material is impaired. Also, 0.0
If the content is less than 002%, the cost for steelmaking is increased, so that it is not industrially suitable. Therefore, the range of O is 0.000
2 to 0.01%. In order to obtain a remarkable effect, use 0.
001-0.005% is desirable.

Mg, Ca, and REM exhibit the above-mentioned effects by containing one or more of them to form fine oxides, sulfides, or mixtures thereof. When these elements are less than 0.0002%, a sufficient amount of oxides,
Inability to form sulfides or their composites. If it exceeds 0.006%, coarse particles are formed and the HAZ toughness is reduced. Therefore, the content range of these elements is set to 0.0002 to 0.006%. To maximize the effect of improving HAZ toughness, 0.001 to 0.00
It is desirable to set it to 3%.

In the present invention, the main object is a thick plate, but there is no particular limitation on the manufacturing method. For example, a continuous cast slab is heated to an austenite region, hot-rolled in a recrystallization region, and air-cooled. After rolling in the recrystallized and unrecrystallized regions, air cooling (control rolling), cooling to the ferrite region after hot rolling, heating again to the austenite region, air cooling (normalizing), rolling in the recrystallization region, water cooling or tempering ( Direct quenching, or
Direct quenching and tempering), recrystallization zone and non-recrystallization zone, controlled cooling after rolling or tempering afterwards (controlled rolling controlled cooling,
Or, control rolling control cooling tempering) or, after hot rolling, cooling to the ferrite region, heating again to the austenitic region, then water cooling or tempering (quenching, or
A manufacturing process such as quenching and tempering can be applied. These processes may be selected according to the required base material strength toughness level.

[0025]

EXAMPLES (Example 1) In order to confirm the above effects, the following experiment was conducted. A steel ingot having the alloy composition shown in Table 1 was produced in a laboratory vacuum melting furnace,
And then hot-rolled to a thickness of 20 mm to produce a steel sheet. A strip-shaped test piece was processed from this steel, and immersed in molten zinc at 500 ° C. to measure the amount of erosion. On the other hand, a creep test was performed to measure the high-temperature strength. With the test specimen kept at 500 ° C, a stress of 150 MPa was applied,
The break time was measured.

Table 2 shows the amount of erosion of steel in molten zinc. Steel 2 in which Si is out of the range of the present invention shows remarkable erosion, but the other steels have a small amount of erosion. Mo, Nb,
V-containing steel also has a small amount of erosion if the amount of Si is within the range of the present invention. In particular, steel 4 and steel 6 with less than 0.01% Si
In particular, the amount of erosion is particularly small. Table 3 shows the results of the creep test. In steels 1 and 2, which do not contain any of Mo, Nb and V, the rupture time is short. On the other hand, the steel 3 containing Mo has a longer rupture time. In steels 4 to 6 containing Nb and V in addition to Mo, the rupture time is longer than that of steel 3.

[0027]

[Table 1]

[0028]

[Table 2]

[0029]

[Table 3]

Example 2 Steel was melted by a converter and a slab having a thickness of 240 mm was manufactured by continuous casting. Table 4
The chemical composition of steel is shown below. Table 5 shows the thickness of the steel sheet manufactured by air cooling after hot rolling in the recrystallization region and the mechanical properties of the base material. Table 6 shows that a strip test piece processed from a steel plate was 500
The result of measuring the amount of erosion by immersion in molten zinc at ℃ is shown. As is clear from the table, the steel of the present invention has a small amount of erosion. The table also shows the creep rupture time at 500 ° C. and a load stress of 150 MPa. The steel of the present invention exhibits high creep strength. Comparative steel 14 has a small amount of erosion but a short creep rupture time. Comparative steels 16 and 17 have a long creep rupture time, but have a large amount of erosion.

In order to confirm the effect of suppressing the deformation during use of the zinc pot by improving the creep strength, the invention steel 8 and the comparative steel 14 were 2.1 m in width, 7 m in length and 1.9 m in depth (sheet thickness of 50 m).
mm) was manufactured, and the amount of deformation in the width direction after three years of use was measured. Whereas the comparative steel was 30-36mm,
In the case of the invention steel, it was 5 to 13 mm. The pot using the invention steel has a small deformation amount and exhibits excellent characteristics.

Table 7 shows the results of preparing submerged arc welded joints and evaluating the HAZ toughness. Here, the welding heat input was set to 75 kJ / cm. It was confirmed that the steel of the present invention corresponding to claim 2 or 3 has high toughness of the high heat input welding HAZ as compared with the comparative steel, and enables high efficiency welding with high heat input.

[0033]

[Table 4]

[0034]

[Table 5]

[0035]

[Table 6]

[0036]

[Table 7]

[0037]

As described above, the steel of the present invention is excellent in erosion resistance, which is the most important property as a steel for a zinc pot, and has little deformation of the pot during long-term use, and has the characteristics as a zinc pot. Excellent. In order to produce a highly reliable zinc pot, it is necessary to simultaneously achieve low erosion and low deformation, and this can only be achieved by using the steel of the present invention. Furthermore, even if the large heat input welding is performed, the toughness of the HAZ is hardly reduced, and the zinc pot can be efficiently manufactured by applying the large heat input welding, thereby contributing to a reduction in manufacturing cost. Therefore, it can be said that the present invention is an invention having extremely high industrial value.

Claims (3)

[Claims] 1. Mass%, C: 0.01 to 0.2%, Si: 0.05% or less, Mn: 0.01 to 2%, P: 0.02% or less, S: 0.02 % Or less, Mo: 0.1 to 2%, Nb: 0.005 to 0.03%, V: 0.005 to 0.05%, and the balance Fe And a high-temperature strength and low melting loss steel for molten zinc pots, characterized by being composed of unavoidable impurities. 2. The high-temperature strength according to claim 1, wherein the steel further contains 0.005 to 0.05% of Ti and 0.002 to 0.01% of N in mass%. Steel for hot-dip zinc pot with low erosion. 3. The composition further contains, by mass%, Al: 0.0002 to 0.06%, O: 0.0002 to 0.01%, Mg: 0.0002 to 0.006%, and Ca 3. High temperature strength and melting damage according to claim 1 or 2, characterized by containing one or more of 0.0002 to 0.006% and REM: 0.0002 to 0.006%. Low-temperature steel for hot-dip zinc pots.