JP7390865B2 - Duplex stainless hot rolled steel and welded structures - Google Patents

Description

The present invention relates to duplex stainless steel and welded structures using the same.

In structures such as rivers and dam facilities, if ordinary steel is used in parts that come into contact with rainwater and river water, corrosion will occur in those parts, so paint or plating is applied, or stainless steel is used. . In particular, in sliding parts included in gates and the like, the deterioration of coating films and plating is significantly accelerated due to friction caused by equipment operations such as opening and closing of gates. For this reason, stainless steel, which is less affected by friction, is widely used.

For sliding parts, from the viewpoint of wear resistance, SUS304N2, which is SUS304 containing N (nitrogen) to increase surface hardness, is widely used. The surface hardness of the solution heat treated SUS304N2 material is approximately HB (Brinell hardness) 200. When further improvement in wear resistance is required, the strength is increased by cold working or hot TMCP production (mechanical heat treatment) before application.

Patent Document 1 discloses, as a stainless steel with increased surface hardness, an austenitic stainless steel with increased hardness due to V inclusion and TMCP production.

In addition, for large structures, it is necessary to manufacture members by welding together long strips. This is clearly different from a roll manufactured by forging or the like. Conventionally, TIG welding and FCAW welding have been applied to weld joints of these structures.

Patent Document 2 discloses a duplex stainless steel that exhibits good corrosion resistance in brackish water areas.

Japanese Patent Application Publication No. 8-209310 International Publication No. 2018/181990

Haruhiko Kajimura: 215th and 216th Nishiyama Memorial Lectures, edited by Japan Iron and Steel Institute, (2013), 17.

River facilities with gates, such as sluice gates at river mouths, are contaminated with seawater, which significantly increases the concentration of chloride ions in the river water, creating a harsh corrosive environment. In such places, SUS304N2 with a small Cr content may not be able to secure the desired corrosion resistance. Further, the same applies to the steel material disclosed in Patent Document 1, and no improvement in corrosion resistance is observed.

Furthermore, for large structures, it is necessary to manufacture members by welding together long strips. At this time, in order to ensure the flatness of the sliding surface against deformation due to welding, it is necessary to restrict construction conditions such as the upper limit of welding heat input, and to correct or grind deformed members. Since the steel material disclosed in Patent Document 1 has an austenitic structure, the amount of deformation during welding is large, and this welding has a problem.

On the other hand, among duplex stainless steels, general-purpose duplex stainless steel SUS329J4L contains large amounts of expensive Ni and Mo. Although it has sufficient corrosion resistance in environments where structures such as dam facilities are placed, it is expensive and its applications have been limited. Although SUS329J1 has good base metal corrosion resistance, the ferrite phase ratio in the welded part is excessive, making it impossible to ensure the desired corrosion resistance and toughness in the welded part, and its application has been limited to rollers and the like that do not involve welding. This tendency is particularly strong in laser welding, which has a faster cooling rate than commonly used welding methods such as FCAW, making it difficult to apply laser welding. The duplex stainless steel disclosed in Patent Document 2 has a steel composition with lower Ni and Mo content than SUS329J4L, and it is described that it exhibits excellent corrosion resistance in an environment with a high chloride ion concentration. It is not suitable for use on harsh sliding parts.

The present invention has sufficient corrosion resistance in both the base metal part and the welded part in structures in rivers and dam facilities where the concentration of chloride ions increases significantly due to seawater contamination at the mouth of the river. The objective is to obtain a stainless steel that has wear resistance (surface hardness) that can be used in moving parts and is highly economical.

As an evaluation index of the wear resistance of a sliding member, it is desirable that its surface hardness has a Brinell hardness equivalent to or higher than that of the roller that comes into contact with the sliding member. Since the Brinell hardness of commonly used SUS304N2 rollers is about HB190 to 230, it is desirable to use stainless steel with a surface hardness of HB230 or higher.

A duplex stainless steel containing a ferrite structure having a smaller coefficient of thermal expansion than an austenite structure has a smaller coefficient of thermal expansion than an austenitic stainless steel consisting only of an austenite structure. That is, thermal deformation due to welding is small.

As an evaluation index of corrosion resistance, pitting corrosion onset temperature (CPT) defined in JIS G0590:2013 was used as an evaluation index of pitting corrosion resistance. This is based on the assumption that the potential on the surface of the steel material is noble in a river environment, and a voltage of 745 mV vs. A method to determine the temperature at which pitting corrosion occurs by increasing the temperature of the solution while applying an SSE (SSE indicates that it is a silver-silver chloride reference electrode standard with a saturated KCl solution as the electrolyte solution) potential. be. As a result of intensive study of the corrosion behavior of stainless steel, the present inventors revealed that good corrosion resistance can be maintained in a river environment if the CPT is 25° C. or higher. It was also found that since the corrosion resistance of stainless steel welded parts is inferior to that of the base metal, there is no practical problem as long as the CPT of the bottleneck welded part is 25°C or higher.

When the corrosion resistance of the base metal part is improved, the corrosion resistance of the welded part also tends to be improved. Since the CPT of the welded part only needs to be 25°C, it is necessary to consider cost performance and efficiently improve the pitting corrosion resistance of the welded part. The inventors conducted extensive research and found that sufficient cost performance is achieved if the difference in CPT between the base metal and the welded part satisfies 18°C or less ((CPT of the base metal) - (CPT of the welded part) ≦18 (°C)). It was revealed that it is possible to obtain

Generally, the pitting corrosion resistance of stainless steel is ranked by pitting corrosion index, and various calculation formulas have been proposed. The pitting corrosion index (PRE) is often expressed as Cr+3.3Mo+16N for duplex stainless steel, and taking into account the negative effects of Mn and the effect of W, PRE _{W, Mn} = Cr+3.3(Mo+0.5W) )+16N−Mn has been proposed (Non-Patent Document 1). The present inventors calculated the pitting corrosion index of the sample material using this formula, and found that if PRE _W,Mn is 24.0 to 34.0, it satisfies the required corrosion resistance and is also economically viable. It was revealed that stainless steel can be obtained. Based on these findings, the present invention has been made, and its gist is as follows.

(1)
In mass%,
C: 0.001-0.030%,
Si: 0.01-1.50%,
Mn: 0.1 to less than 2.0%,
Cr: 20.0-26.0%,
Ni: 2.0 to 7.0%,
Mo: 0.5-3.0%,
Contains N: 0.10 to 0.25% and Al: 0.003 to 0.050%,
moreover,
W: 0.01-1.00%,
Co: 0.01 to 1.00%,
Cu: 0.01-2.00%,
V: 0.01-0.30%,
Nb: 0.005-0.100%,
Ta: 0.005-0.200%,
Zr: 0.001-0.050%
Hf:0.001~0.080%
Sn: 0.005-0.100%,
B: 0.0001 to 0.0050%,
Ca: 0.0005-0.0050%,
Contains one or more of Mg: 0.0001 to 0.0030% and REM: 0.005 to 0.100%,
The remainder is Fe and unavoidable impurities,
O as an impurity: 0.006% or less,
P: 0.050% or less,
S: steel limited to 0.003% or less,
The surface Brinell hardness is HB230 or higher,
PRE _W,Mn determined by formula 1 is 24.0 or more and 34.0 or less,
When the pitting corrosion onset temperature (CPT) specified in JIS G0590:2013 of the welded joint part and the base metal part other than the welded part of the steel is measured in a test piece in which the above-mentioned steels are welded together, it is determined that the joint by welding A duplex stainless hot rolled steel characterized in that the CPT of the part (welded part CPT) is 25°C or more, and the difference between the CPT of the base metal part (base metal part CPT) and the welded part CPT is 18°C or less Material .
PRE _W,Mn =Cr+3.3(Mo+0.5W)+16N-Mn...(Formula 1)
However, the element symbol in Formula 1 indicates the content (mass%) of each element, and if it is not contained, 0 is substituted.
(2)
The duplex stainless steel hot rolled steel material according to (1), wherein the duplex stainless steel hot rolled steel material is a strip-shaped steel material .
(3)
The duplex stainless hot rolled steel material according to (1) or (2), wherein the welding is laser welding.
(4)
The duplex stainless steel hot-rolled steel material according to (1) to (3), which is used for a structure having a sliding part.
(5)
A welded structure constructed by welding the duplex stainless steel hot rolled steel material according to (1) above.
(6)
The welded structure according to (5), wherein the welding is laser welding.
(7)
The welded structure according to (5) or (6), wherein the welded structure is a sliding component.
(8)
The welded structure according to any one of (5) to (7), wherein the welded structure is used in a brackish water environment or a river.

The duplex stainless steel strip including the welded portion obtained by the present invention has a surface hardness equal to or higher than that of SUS304N2, and has corrosion resistance superior to SUS304N2 even in the welded portion in addition to the general welding method. On the other hand, the thermal deformation is small, the number of man-hours can be reduced, and the alloy cost can also be suppressed, so it is economical. As a result, for example, by using the welded structure made of duplex stainless steel according to the present invention in the sliding parts of structures such as river facilities and facilities in brackish water environments, improvements can be made in terms of both performance and cost. The contribution made to the industrial and environmental aspects is extremely large.

Below, first, the reason for limitation of claim 1 of the present invention will be explained. In this specification, unless otherwise specified, % with respect to components represents mass %.

In order to ensure the corrosion resistance of stainless steel, the content of C is limited to 0.030% or less. If the content exceeds 0.030%, Cr carbide will be generated during hot rolling, resulting in deterioration of corrosion resistance and toughness. The content is preferably 0.025% or less, more preferably 0.023% or less. On the other hand, from the cost perspective of reducing the C content of stainless steel, the lower limit is set to 0.001%.

Si is contained in an amount of 0.01% or more for deoxidation. Preferably, the content is 0.10% or more, more preferably 0.15% or more. On the other hand, if the content exceeds 1.5%, the toughness will deteriorate. Therefore, it should be 1.5% or less. The content is preferably 1.2% or less, more preferably 1.0% or less.

Mn has the effect of increasing the austenite phase and improving toughness. Further, since it has the effect of lowering the nitride precipitation temperature, it is contained in an amount of 0.1% or more to ensure corrosion resistance of the base metal and welded parts. The content is preferably 0.3% or more, more preferably 0.5% or more. On the other hand, since Mn is an element that reduces the corrosion resistance of stainless steel, it is preferable to keep Mn to less than 2.0%. The content is preferably 1.8% or less, more preferably 1.5% or less.

Cr is contained in an amount of 20.0% or more in order to ensure basic corrosion resistance of the steel of the present invention. It is preferably 21.0% or more, more preferably 21.5% or more. On the other hand, when Cr is contained in an amount exceeding 26.0%, the ferrite phase fraction increases, which impairs the toughness and corrosion resistance of the weld zone. Therefore, the Cr content was set to 26.0% or less. It is preferably 25.0% or less, more preferably 24.5% or less.

Ni is contained in an amount of 2.0% or more in order to stabilize the austenite structure and improve toughness. By increasing the Ni content, it becomes possible to lower the nitride precipitation temperature. The content is preferably 3.0% or more, more preferably 4.0% or more. On the other hand, Ni is an expensive alloy, and from the viewpoint of cost, the content is limited to 7.0% or less. It is preferably 6.5% or less, more preferably 6.0% or less.

Mo is a very effective element that improves the corrosion resistance of both the base metal and welded parts of stainless steel, and is contained in an amount of 0.5% or more. The content is preferably 0.8% or more, more preferably 1.0% or more. On the other hand, Mo is an element that promotes precipitation of intermetallic compounds, and in the steel of the present invention, Mo is set at 3.0% or less from the viewpoint of suppressing precipitation during hot rolling. The content is preferably less than 2.5%, more preferably 2.0% or less, and even more preferably 1.5% or less.

Since N is an effective element that dissolves in the austenite phase to improve the hardness and corrosion resistance of duplex stainless steel and increases the amount of austenite phase precipitation in the weld zone, it is contained in an amount of 0.10% or more. The content is preferably 0.15%, more preferably 0.18% or more. The solid solubility limit increases with the Cr content, but in the steel of the present invention, if the content exceeds 0.25%, Cr nitrides will precipitate and impair toughness and corrosion resistance, so the content is was set to 0.25% or less. Preferably, it is 0.20% or less.

Al is an important element for deoxidizing steel, and is contained in an amount of 0.003% or more to reduce oxygen in steel. The content is preferably 0.005% or more. Furthermore, since the steel of the present invention is not subjected to solution heat treatment, distortion due to rolling remains in the steel. Since strain becomes a diffusion path for N (nitrogen), nitride precipitation tends to occur in the steel, and this tendency is particularly noticeable in welded parts that are affected by heat. Among nitrides, Cr nitride impairs toughness and corrosion resistance, so it is effective to contain Al, which nitrides preferentially over Cr. The reason for this is not clear, but the following mechanism can be considered. That is, Al is an element that has a relatively large affinity for N, and is thought to promote grain refinement by precipitating in the steel as fine nitrides. That is, it is thought that Al nitride has the effect of suppressing the coarsening of the ferrite grain size in the HAZ, and increases the number of ferrite/ferrite grain boundaries that serve as precipitation sites for the austenite phase. On the other hand, if it is contained in excess, nitrides of Al are generated and the toughness of stainless steel is impaired. The degree of this depends on the N content, but if Al exceeds 0.050%, the toughness will be significantly lowered, so the content is preferably set to 0.050% or less. The content is preferably 0.040% or less, more preferably 0.030% or less.

Furthermore, one or more of the following elements may be optionally contained.

Like Mo, W is an element that improves the corrosion resistance of stainless steel, and may be included. From the viewpoint of corrosion resistance, the content may be 1.00% or less. The content is preferably 0.70% or less, more preferably 0.50% or less. When it is contained, it is preferably contained in an amount of 0.01% or more, preferably 0.05% or more, and more preferably 0.10% or more in order to obtain the effect.

Co is an effective element for increasing the toughness and corrosion resistance of steel, and may be contained. Even if Co is contained in an amount exceeding 1.00%, since it is an expensive element, the effect commensurate with the cost will not be exhibited, so it is preferable to contain it in an amount of 1.00% or less. The content is preferably 0.70% or less, more preferably 0.5% or less. When it is contained, it is preferably contained in an amount of 0.01% or more, preferably 0.03% or more in order to obtain the effect. The content is preferably 0.03% or more, more preferably 0.10% or more.

Cu is an element that additionally increases the acid corrosion resistance of stainless steel and has the effect of improving toughness, so it may be contained. If Cu is contained in an amount exceeding 2.00%, εCu will precipitate beyond the solid solubility during cooling after hot rolling, resulting in embrittlement, so the content is preferably 2.00% or less. The content is preferably 1.70% or less, more preferably 1.50% or less. When Cu is contained, it is preferably contained in an amount of 0.01% or more, preferably 0.20% or more, more preferably 0.33% or more in order to obtain the effect.

V is an element that has an affinity for N and has the effect of reducing the precipitation rate of chromium nitride. For this reason, V may be included. However, if the content exceeds 0.30%, a large amount of V nitride will precipitate, impairing toughness, so the V content should be 0.30% or less, preferably 0.25% or less. , more preferably 0.20% or less. When V is contained, it is preferably contained in an amount of 0.01% or more, preferably 0.03% or more, and more preferably 0.08% or more in order to obtain the effect.

Nb is an element that has a stronger affinity with N than V and has the effect of further reducing the precipitation rate of chromium nitride. For this reason, Nb may be included. When contained, Nb is contained in an amount of 0.005% or more. The content is preferably 0.010% or more, more preferably 0.020% or more, and even more preferably 0.030% or more. On the other hand, if the Nb content exceeds 0.100%, a large amount of Nb nitride will precipitate and impair the toughness, so the content is set to 0.100% or less. The content is preferably 0.085% or less, more preferably 0.080% or less. Although Nb is an expensive element, the cost of raw materials for melting stainless steel can be reduced by actively utilizing Nb contained in scrap. It is preferable to reduce the cost of melting Nb-containing steel by such a method.

Ta is an element that has the same effect as Nb and has the effect of reducing the precipitation rate of chromium nitride. For this purpose, Ta may be included. When contained, Ta is contained in an amount of 0.005% or more. The content is preferably 0.010% or more. On the other hand, if Ta is contained in excess of 0.200%, a large amount of Ta nitride will precipitate, impairing toughness, so the content is set at 0.200% or less. Preferably it is 0.150% or less.

Zr is a strong nitride-forming element and has a crystal grain refining effect. For this purpose, Zr may be contained. When contained, Zr is contained in an amount of 0.001% or more. Preferably it is 0.005% or more. On the other hand, if Zr is contained in an amount exceeding 0.050%, coarse nitrides of Zr will precipitate and impair toughness, so the content is set at 0.050% or less. Preferably it is 0.030% or less.

Like Zr, Hf is a strong nitride-forming element and has a crystal grain refining effect. For this purpose, Hf may be contained. When contained, Hf is contained in an amount of 0.001% or more. Preferably it is 0.005% or more. On the other hand, if Zr is contained in an amount exceeding 0.080%, coarse nitrides of Hf will precipitate and impair toughness, so the content is set at 0.080% or less. Preferably it is 0.050% or less.

Sn is an element that increases the corrosion resistance of stainless steel against acids. For this reason, Sn may be contained. When contained, Sn is contained in an amount of 0.005% or more. Preferably it is 0.010% or more. On the other hand, Sn is an element that inhibits the hot workability of steel, and therefore the upper limit of its content is limited to 0.100% or less. Preferably it is 0.050% or less.

B is an element that improves the hot workability of steel, and may be included if necessary. Further, B is an element that has a very strong affinity with N, and when it is contained in a large amount, B nitrides precipitate, which impairs toughness. Therefore, the content should be set to 0.0050% or less, preferably 0.0040% or less, and more preferably 0.0030% or less. When B is contained, it is preferably contained in an amount of 0.0001% or more, preferably 0.0005% or more, and more preferably 0.0014% or more in order to obtain the effect.

Ca and Mg are contained as necessary to improve the pitting corrosion resistance and hot workability of the invention steel. When containing Ca and Mg, in order to obtain the effect, the Ca content is preferably 0.0005% or more, and the Mg content is preferably 0.0001% or more. Preferably Ca is 0.0010% or more, Mg is 0.0003% or more, and more preferably Ca is 0.0015% or more and Mg is 0.0005% or more.
On the other hand, excessive contents of both Ca and Mg conversely deteriorate hot workability and toughness, so Ca and Mg are preferably kept at 0.0050% or less and Mg at 0.0030% or less, respectively. Preferably, Ca is 0.0040% or less, Mg is 0.0025% or less, more preferably Ca is 0.0035% or less, and Mg is 0.0020% or less.

REM is an element that improves the hot workability of steel, and may be contained in an amount of 0.005% or more for that purpose. When it is contained, it is preferably contained in an amount of 0.010% or more, more preferably 0.020% or more. On the other hand, excessive content will conversely reduce hot workability and toughness, so it is preferable to contain 0.100% or less. The content is preferably 0.080% or less, more preferably 0.070% or less. Here, the content of REM is the total content of lanthanoid rare earth elements such as La and Ce.

The remainder is Fe and unavoidable impurities. Unavoidable impurities are impurities that are unintentionally mixed into the steel during the manufacturing process and remain without being completely removed. In particular, O, P, and S in the impurities are limited for the following reasons.

O (oxygen) is an inevitable impurity and is an element that inhibits the hot workability, toughness, and corrosion resistance of stainless steel, so it is preferable to reduce it as much as possible. Therefore, the O content is preferably 0.006% or less. Moreover, since extremely high costs are required for refining to reduce the oxygen content extremely, the oxygen content may be 0.001% or more in consideration of economic efficiency.

P is an element that is unavoidably mixed in from raw materials, and since it deteriorates hot workability and toughness, it is better to have as little as possible, and it is limited to 0.050% or less. Preferably, it is 0.030% or less. In order to reduce the amount of P to an extremely low amount, the cost during refining becomes high, so in consideration of economic efficiency, the amount of P may be 0.001%.

S is also an element that is unavoidably mixed in from raw materials and also deteriorates hot workability, toughness, and corrosion resistance, so it is better to have as little S as possible, and the upper limit is limited to 0.003% or less. In order to reduce S to an extremely low amount, the cost at the time of refining increases, so in consideration of economic efficiency, the S amount may be 0.0001%.

[Surface Brinell hardness is HB230 or higher]
The surface of the strip-shaped sliding member is required to have a hardness comparable to that of the roller or the like that comes into contact with it. Since the surface hardness of commonly used rollers is Brinell hardness (HB) of 190 to 230, the surface of the sliding member preferably has a Brinell hardness of HB230 or higher. More preferably, HB250 or more is desirable.

[Corrosion resistance: PRE _{W, Mn} ]
In a river environment, chloride ions contained in river water act as a corrosion factor, causing corrosion. In order to ensure sufficient corrosion resistance in this environment, the values of PRE _{W and Mn} determined by the following formula 1 should be 24.0 or more by containing Cr, Mo, N, and W and limiting Mn. Bye. It is preferably 25.0 or more, more preferably 27.0 or more. On the other hand, if the contents of Cr, Mo, and W are increased excessively in order to increase PRE _{W and Mn,} the alloy cost will increase, and if the content of N is excessively increased, adverse effects such as deterioration of toughness will occur. Moreover, if Mn is reduced too much, the amount of solid solution of N decreases, Cr nitrides precipitate, and the corrosion resistance decreases. Considering these cost performance, the upper limit of PRE _{W and Mn} is preferably set to 34.0.
PRE _W,Mn =Cr+3.3(Mo+0.5W)+16N-Mn...(Formula 1)
However, the element symbol in Formula 1 indicates the content (mass%) of each element, and if it is not contained, 0 is substituted.

[Corrosion resistance: CPT]
Near the river mouth, where seawater is mixed and the water is mainly brackish, the concentration of chloride ions is high, creating an extremely harsh corrosive environment. Furthermore, a phenomenon unique to river mouths is that the amount of freshwater flowing in from upstream and the amount of seawater flowing upstream from downstream fluctuate, causing the environment to change significantly in a short period of time. When considering the applicability of steel materials, it is necessary to consider both the chloride ion concentration, water temperature, and potential values that determine the severity of the corrosive environment, as well as the length of time such values are taken. . The inventors investigated in detail the corrosion of stainless steel in actual brackish water and the relationship between the corrosion factors, chloride ion concentration, water temperature, and potential transition. As a result, it was revealed that there are conditions to be most careful about, such as chloride ion concentration, water temperature, and electric potential. It was found that the generation temperature (CPT) was 25°C or higher.

On the other hand, the corrosion resistance of duplex stainless steel welds is lower than that of the base metal. Therefore, in order for a welded structure including a welded joint to withstand use under such an environment, it is preferable that the CPT of the welded portion of the structure is 25° C. or higher. Preferably the CPT is 28°C or higher, more preferably the CPT is 30°C or higher, and even more preferably the CPT is 35°C or higher.

As mentioned above, CPT is performed by applying a voltage of 745 mV vs. A method to determine the temperature at which pitting corrosion occurs by increasing the temperature of the solution while applying an SSE (SSE indicates that it is a silver-silver chloride reference electrode standard with a saturated KCl solution as the electrolyte solution) potential. be.

Generally, when an element that improves the corrosion resistance of the base metal is contained, the corrosion resistance of the welded part also tends to be improved. However, in consideration of economic efficiency, it is not preferable that the difference in corrosion resistance between the base metal and the welded portion be too large. Therefore, it has been found that the difference between the base metal CPT and the weld CPT can be used as an index for improving the corrosion resistance of the weld zone in consideration of cost performance. Here, the base metal portion CPT refers to the CPT of a steel portion (base metal) other than the welded portion in a welded structure obtained by welding steel materials. Further, the welded portion CPT refers to the CPT of a welded joint (welded joint) in the welded structure.

Specifically, if the difference between the base metal CPT and the weld CPT is 18° C. or less, it can be said that the cost performance is good. The difference in CPT is preferably 16°C or less, more preferably 14°C or less, and even more preferably 13°C or less.
By ensuring a welded part CPT of 25°C and making the difference in CPT between the base metal part and the welded part 18°C or less, it is possible to obtain duplex stainless steel with good cost performance while ensuring corrosion resistance.

Pitting corrosion specified in JIS G0590:2013 of the welded joint part and the base metal part other than the welded part of the steel in a test piece made by welding the steels together without creating a welded structure. The corrosion resistance of the steel can be evaluated based on the welded part CPT and the base metal part CPT obtained by measuring the occurrence temperature (CPT).

The welding method is not particularly limited for welded structures or test pieces made of steel welded together. Existing welding methods may be used. In particular, even in laser welding, which has a large cooling rate compared to welding methods such as FCAW and is difficult to apply to welds from the viewpoint of corrosion resistance, if the duplex stainless steel according to the present invention is applied, corrosion resistance in welds can be improved. can be ensured.
Therefore, when manufacturing a steel test piece, it may be manufactured by laser welding and evaluated.

The shape of the steel is not particularly limited. It may be a band-shaped steel material, a bar-shaped steel material, a wire-shaped steel material, or a tubular steel material. Of course, you may manufacture a welded structure by combining these.

Next, the manufacturing method will be explained.
Common hot-rolled stainless steels are softened by solution heat treatment performed after hot rolling to remove strain introduced during rolling. In order to obtain a surface hardness suitable for a sliding member, the steel of the present invention controls the temperature of the steel plate during rolling, and utilizes the strain introduced by rolling to obtain the surface hardness, so solution heat treatment is omitted. Specifically, hot rolling involves heating a melted steel billet containing predetermined components to a temperature between 1050 and 1250°C before rolling, and then bringing the final rolling temperature (finish rolling temperature) to 800 to 1000°C. Apply rolling. There is a tendency that greater hardness can be obtained as the finish rolling temperature is lowered, but if it is lower than 800°C, the σ phase precipitates and the toughness deteriorates. Furthermore, in the cooling process after rolling, in order to suppress the precipitation of chromium nitrides, this temperature range is It is preferable to cool at a cooling rate of 1° C./second or more. The speed is preferably 3°C/second or more, more preferably 5°C/second or more. Thereby, it is possible to ensure surface hardness and obtain a steel material with good corrosion resistance. As mentioned above, in order to ensure surface hardness, solution heat treatment is not performed after cooling after hot rolling.

Examples are described below. Table 1 shows the chemical composition, PRE _{W, and Mn} of the test steel. These steels were obtained using a vacuum melting furnace. Regarding the components shown in Table 1, the portions whose contents are not listed indicate the unavoidable impurity level. REM means lanthanoid rare earth elements, and the content indicates the total of these elements. The slabs obtained by vacuum melting were heated at 1200° C. for 2 hours, and then hot forged to obtain steel ingots of a predetermined shape. Here, the shape of the steel ingot is 110mmw (width) x 150mmL (length) x 60mmt (thickness) if the final steel plate thickness is 20mm or less, and the final steel plate thickness is 20mm. For those exceeding 110 mmw x 150 mmL x 100 mmt.

Table 2 shows the conditions of hot rolling carried out to convert a steel ingot into a steel plate, the Brinell hardness of the obtained steel plate, the base metal CPT, the weld CPT, and the value obtained by subtracting the weld CPT from the base metal CPT ( difference). The manufacturing conditions of the steel plate are shown below. First, the steel ingot obtained by the forging described above was soaked at 1200° C. for 60 minutes. Thereafter, hot rolling was performed so that the finish rolling temperature was between 800 and 1000°C. Thereafter, it was cooled so that the cooling rate in the 800 to 600°C range (the post-rolling cooling rate in Table 2 indicates the cooling rate in this 800 to 600°C temperature range) was 1°C/sec or more.

To evaluate the welded area, a simulated thermal cycle test was conducted that simulated the thermal history during arc welding. That is, a rod-shaped test piece of 10 mmφ x 60 mm L was cut out from a steel plate, a thermocouple was welded to the center of the test piece, high-frequency induction heating was performed to 1360° C., and after soaking for 5 seconds, gas cooling was performed. The soaking area was 15 mmL. The cooling rate during this cooling was controlled to be 7.2°C/sec or 60°C/sec in the temperature range of 900 to 600°C. ). For the test piece that was cooled at 7.2°C/sec in the temperature range of 900 to 600°C, the cooling rate in the temperature range of 1360°C to 900°C was controlled to 22°C/sec. For the test piece cooled at 60°C/sec in the temperature range of 900 to 600°C, the cooling rate in the temperature range of 1360°C to 900°C was controlled to 60 to 100°C/sec.

In addition, regarding laser welding, which has an extremely fast cooling rate, joints were actually fabricated by laser welding and evaluated. The laser used for the evaluation was a fiber laser with an output of 5 kW, which was irradiated perpendicularly to the steel plate so that the spot diameter at the processing point was φ0.4 mm, and the welding speed was varied between 0.5 m and 4.0 m/min. The cooling rate was varied between 3300°C/sec and 750°C/sec. Ar was used as the shielding gas.

The method for measuring Brinell hardness is described below. A test piece of 30 mmw x 30 mmL was cut out from the steel material by sawing, and the oxidized scale on the rolled surface was removed by grinding to provide a measurement surface, and the test was conducted according to the method of JIS Z2243. A cemented carbide ball with a diameter of 10 mm was used as an indenter. The test force was 29.42 kN.

The method for measuring CPT will be described. A sample of 15 mmw x 30 mmL x 2 mmt was cut out from the surface layer of the steel material by machining, and a position ground 0.2 mm from the surface layer was polished as a test surface. Of this test surface, a 10 mm x 10 mm surface was left and the remaining portions were coated with resin to create an evaluation surface.

A reproduction heat cycle test piece simulating a welded part was obtained by cutting the heat cycle test piece into 10 mmφ x 30 mmL, which was then cut in half and polished as a test surface of 10 mmW x 30 mmL. Of this test surface, a 7 mm x 14.5 mm surface at the center of the width was left and the remaining portions were coated with resin to prepare an evaluation surface.

For laser welding, after cutting off the welding excess, the steel material was machined to a size of 15 mmw x 30 mm L x 2 mmt so that the weld line crossed the center in the longitudinal direction of the sample, and ground by 0.2 mm from the base metal surface layer. The position was used as a test surface and polished. Furthermore, an evaluation surface was prepared by leaving a 10 mm x 10 mm surface so that the weld line crossed the center of the evaluation surface, and applying resin to the remaining portions. In addition, regarding the test pieces subjected to laser welding, "laser welding" is written in the remarks column of Table 2.

Tests were conducted using these samples according to the method of JIS G0590. However, JIS G0590 sets the starting temperature at 25°C, but it was set at 5°C. The measurement was carried out until the anode current density reached 1 mA/cm ² , and the pitting corrosion onset temperature (CPT) was defined as the liquid temperature at the time when the current density exceeded 100 μmA/cm ² . These results are shown in Table 2.

As can be seen from the above examples, the present invention has produced a duplex stainless steel strip material that has surface hardness suitable for sliding members, excellent corrosion resistance even in welded joints, and has good economic efficiency. It has become clear what can be achieved.

Duplex stainless steel, which is the object of the present invention, can be used in structures that require corrosion resistance and wear resistance. In particular, it is more economical than existing stainless steels (SUS304N2, SUS329J1, and SUS329J4L), and has excellent long-term corrosion resistance and corrosion resistance of welded parts, making it suitable as a component that contributes to resource conservation and reduced maintenance costs for river and dam structures. It is.

Claims (8)

In mass%,
C: 0.001-0.030%,
Si: 0.01-1.50%,
Mn: 0.1 to less than 2.0%,
Cr: 20.0-26.0%,
Ni: 2.0 to 7.0%,
Mo: 0.5-3.0%,
Contains N: 0.10 to 0.25% and Al: 0.003 to 0.050%,
moreover,
W: 0.01-1.00%,
Co: 0.01 to 1.00%,
Cu: 0.01-2.00%,
V: 0.01-0.30%,
Nb: 0.005-0.100%,
Ta: 0.005-0.200%,
Zr: 0.001-0.050%
Hf:0.001~0.080%
Sn: 0.005-0.100%,
B: 0.0001 to 0.0050%,
Ca: 0.0005-0.0050%,
Contains one or more of Mg: 0.0001 to 0.0030% and REM: 0.005 to 0.100%,
The remainder is Fe and unavoidable impurities,
O as an impurity: 0.006% or less,
P: 0.050% or less,
S: steel limited to 0.003% or less,
The surface Brinell hardness is HB230 or higher,
PREW,Mn determined by formula 1 is 24.0 or more and 34.0 or less,
When the pitting corrosion onset temperature (CPT) specified in JIS G0590:2013 of the welded joint part and the base metal part other than the welded part of the steel is measured in a test piece in which the above-mentioned steels are welded together, it is determined that the joint by welding A duplex stainless hot rolled steel characterized in that the CPT of the part (welded part CPT) is 25°C or more, and the difference between the CPT of the base metal part (base metal part CPT) and the welded part CPT is 18°C or less Material .
PREW, Mn=Cr+3.3(Mo+0.5W)+16N-Mn...(Formula 1)
However, the element symbol in Formula 1 indicates the content (mass%) of each element, and if it is not contained, 0 is substituted. The duplex stainless steel hot rolled steel material according to claim 1, wherein the duplex stainless steel hot rolled steel material is a strip-shaped steel material . The duplex stainless hot rolled steel material according to claim 1 or 2, wherein the welding is laser welding. The duplex stainless steel hot rolled steel material according to any one of claims 1 to 3, which is used for a structure having a sliding part. A welded structure constructed by welding the duplex stainless hot rolled steel material according to claim 1. The welded structure according to claim 5, wherein the welding is laser welding. The welded structure according to claim 5 or 6, wherein the welded structure is a sliding component. The welded structure according to any one of claims 5 to 7, wherein the welded structure is used in a brackish water environment or a river.