JP7192483B2 - Duplex stainless welded channel steel and its manufacturing method - Google Patents

Description

The present invention relates to a duplex stainless welded channel steel and a method for manufacturing the same.

Duplex stainless steel is used as a building or structural material because of its excellent corrosion resistance and particularly high strength. Among hot-rolled stainless steel plates and strips, steel grades of duplex stainless steel include SUS329J1 and SUS329J4L described in JIS G 4304.

Since these conventional duplex stainless steels contain a large amount of additive elements and are relatively expensive, lean-type duplex stainless steels in which the amount of additive elements is suppressed have been developed. Patent Documents 1 and 2 disclose inexpensive duplex stainless steels with low Ni content and utilizing austenitic elements such as Mn and N.

On the other hand, channel steel having a U-shaped cross section is used as a building material or structural material. Stainless channel steel is manufactured by hot forming (described in JIS G 4317) or cold forming (described in JIS G 4320). In addition to these methods, there is also a method of bending a steel plate to form a channel steel.

In addition, when it is difficult to manufacture by rolling or forming, channel steel is manufactured by welding two angle steels each having an L-shaped cross section. Unlike the above-described channel steel, this welded channel steel has a problem that the corrosion resistance of the weld is deteriorated and it is difficult to ensure dimensional accuracy. In particular, in the case of long lengths, bending poses a problem. The cause of the deterioration of corrosion resistance associated with welding is sensitization due to precipitation of Cr carbides and Cr nitrides.

In particular, in duplex stainless steel, N is often added for the purpose of ensuring corrosion resistance and strength, and there is concern about sensitization due to Cr nitrides. Furthermore, in the cooling process during welding and subsequent solution heat treatment, shape change occurs due to uneven thermal expansion and contraction. Duplex stainless steel is deformed compared to single-phase stainless steel because it consists of a mixed structure of ferrite and austenite phases with different coefficients of thermal expansion, and part of the austenite phase transforms into ferrite during the cooling process. is large, and bending is likely to occur in the case of a long length.

JP-A-61-56267 JP 2010-229459 A

"New Edition Special Welding and Joining Techniques" Sanpo Shuppan, edited by Welding Society (published first edition in 2005), page 178

Due to the recent expansion of the application of duplex stainless steel, there is a demand for welded duplex stainless channel steel with excellent corrosion resistance and dimensional accuracy. At present, however, there is no duplex stainless welded channel steel that satisfies these requirements.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a duplex stainless welded channel steel excellent in corrosion resistance and dimensional accuracy, and a method for manufacturing the same.

In order to solve the above-mentioned problems, the present inventors investigated the effects of manufacturing conditions on the corrosion resistance and dimensional accuracy of channel steel welded with duplex stainless steel. It is known that heat treatment after welding improves corrosion resistance (see, for example, Non-Patent Document 1). In particular, as a result of detailed studies on the relationship between heat treatment conditions after welding, corrosion resistance, and shape, the following findings were obtained.

The following (a) and (b) explain the knowledge about the corrosion resistance of the weld zone obtained by the present invention.

(a) When welding duplex stainless steel to which 0.050% or more of N is added, the corrosion resistance of the weld metal zone and the weld heat affected zone (hereinafter collectively referred to as the "weld zone") It deteriorates compared to the part (base material part) where it is not. When the critical pitting initiation temperatures (CPT) of the weld and base metal are measured and compared, the CPT of the weld is 10-15°C lower than that of the base metal.

(b) When duplex stainless steel is welded and then subjected to solution heat treatment, the corrosion resistance of the weld is improved, and the decrease is within 5°C compared to the base metal. The effect of improving corrosion resistance is large when the heat treatment temperature is high and the subsequent cooling rate is fast. Specifically, the heat treatment temperature must be 950° C. or higher, and the cooling rate must be 0.3° C./s or higher.

Furthermore, the following (c) and (d) explain knowledge about the dimensional accuracy in welding obtained by the present invention.

(c) If the temperature of the heat treatment is too high in the channel steel produced by welding duplex stainless steel, deformation will occur. This is probably because the fraction of the soft ferrite phase increased at high temperatures, the high-temperature strength decreased, and creep deformation occurred. Specifically, the heat treatment temperature must be 1050° C. or lower.

(d) If a duplex stainless steel is welded and then heat-treated at a high cooling rate, the steel deforms during cooling. This is attributed to the mixed structure of the ferrite phase and the austenite phase, which have different thermal expansion coefficients, and is considered to be due to the fact that the thermal strain introduced during cooling varies depending on the site. Specifically, the cooling rate must be 10° C./s or less.

Based on the above, we clarified a duplex stainless welded channel steel that satisfies corrosion resistance and dimensional accuracy, and a manufacturing method thereof.

The present invention has been made based on the above findings, and its gist is the following duplex stainless steel welded channel steel and a method for producing the same.

(1) A channel steel having a U-shaped cross section extending in one direction and perpendicular to the one direction, and comprising a pair of side walls and a bottom connecting the pair of side walls,
The channel steel includes a pair of base metal parts having an L-shaped cross section perpendicular to the one direction, and a weld metal part sandwiched between the pair of base metal parts and extending in the one direction at the bottom part. has
A weld heat affected zone is formed at a boundary between the base metal portion and the weld metal portion,
The base material portion has a metal structure in which the area ratio of the austenite phase is 30 to 70%, and the balance is the ferrite phase and precipitates,
The critical pitting initiation temperature T1 (°C) in the weld heat affected zone and the critical pitting initiation temperature T2 (°C) in the base metal portion satisfy the following formula (i),
The bending tolerance in the one direction is 3 mm or less per unit length (m).
Duplex stainless welded channel steel.
T1≧T2−5 (i)

(2) In a cross section perpendicular to the one direction, when the side where the pair of side walls extends from the bottom is defined as the inner side and the opposite side is defined as the outer side,
The area ratio of austenite in the inner part of the weld metal part is higher than the area ratio of austenite in the outer part of the weld metal part,
The duplex stainless welded channel steel according to (1) above.

(3) The chemical composition of the base material portion is, in mass%,
C: 0.001 to 0.060%,
Si: 0.01 to 1.50%,
Mn: 0.1 to 6.0%,
P: 0.050% or less,
S: 0.0050% or less,
Cr: 19.0 to 26.0%,
Ni: 1.0 to 8.0%,
N: 0.050 to 0.25%,
Al: 0.003 to 0.050%,
Ti: 0 to 0.050%,
Nb: 0 to 0.15%,
Mo: 0-4.0%,
Cu: 0 to 4.0%,
W: 0 to 4.0%,
Mg: 0-0.0050%,
Ca: 0 to 0.0050%,
REM: 0-0.30%,
B: 0 to 0.0040%,
balance: Fe and impurities,
The duplex stainless welded channel steel according to (1) or (2) above.

(4) The chemical composition of the base material portion is, in mass %,
Ti: 0.01 to 0.050%,
Nb: 0.02 to 0.15%,
Mo: 0.05 to 4.0%,
Cu: 0.05 to 4.0%,
W: 0.05 to 4.0%,
Mg: 0.0002-0.0050%,
Ca: 0.0002 to 0.0050%,
REM: 0.005 to 0.30%, and
B: 0.0003 to 0.0040%,
containing one or more selected from
The duplex stainless welded channel steel according to any one of (1) to (3) above.

(5) (a) Two angle steels extending in one direction and having an L-shaped cross section perpendicular to the one direction are arranged symmetrically and joined by welding to form a channel steel having a U-shaped cross section. and
(b) After heating the channel steel to a temperature range of 950 to 1050 ° C., it is held in the temperature range for 3 to 15 minutes, and then the average cooling rate from the temperature range to 400 ° C. is 0.3 to 10 ° C. A step of cooling under conditions of / s,
A method for producing a duplex stainless welded channel steel.

(6) The channel steel includes a pair of sidewalls and a bottom connecting the pair of sidewalls, and in a cross section perpendicular to the one direction, the side extending from the bottom to the inside of the pair of sidewalls and
In the step (a), welding is performed from the inside in a state of being sealed with nitrogen gas.
A method for producing a duplex stainless welded channel steel according to (5) above.

(7) The angle iron has the chemical composition described in (3) or (4) above,
A method for producing a duplex stainless welded channel steel according to (5) or (6) above.

According to the present invention, it is possible to industrially stably obtain a welded duplex stainless channel steel excellent in corrosion resistance and dimensional accuracy.

1 is a schematic perspective view showing a duplex stainless welded channel steel according to an embodiment of the present invention; FIG. It is a figure which shows the schematic diagram of a channel steel test piece.

Each requirement of the present invention will be described in detail below.

1. Overall Configuration FIG. 1 is a schematic perspective view showing a duplex stainless welded channel steel according to an embodiment of the present invention. As shown in FIG. 1, the duplex stainless welded channel steel 10 according to the present embodiment has a U-shaped cross section extending in one direction (X direction in FIG. 1) and perpendicular to one direction, and a pair of side walls It includes portions 11a and 11b and a bottom portion 12 connecting the pair of side wall portions 11a and 11b.

In addition, the duplex stainless steel welded channel steel is sandwiched between a pair of base metal portions 13a and 13b having an L-shaped cross section perpendicular to one direction and the pair of base metal portions 13a and 13b, and is sandwiched between the pair of base metal portions 13a and 13b. and a weld metal portion 14 extending to the Furthermore, weld heat affected zones 15a and 15b are formed at the boundaries between the base metal parts 13a and 13b and the weld metal part .

2. Metallographic Structure The base material portions 13a and 13b have a metallographic structure in which the area ratio of the austenite phase is 30 to 70% at room temperature, and the balance is the ferrite phase and precipitates. Sufficient strength cannot be obtained when the area ratio of the austenite phase is less than 30%. On the other hand, if the area ratio of the austenite phase exceeds 70%, sufficient strength cannot be obtained, and surface cracks are likely to occur due to slight strain. The area ratio of the austenite phase is preferably 40-60%. Phases other than the austenite phase are the ferrite phase and precipitates. Precipitates may be carbides, nitrides, sulfides, intermetallic compounds, or the like.

As shown in FIG. 1, in a cross section perpendicular to one direction, when the side where the pair of side wall portions 11a and 11b extend from the bottom portion 12 is the inside and the opposite side is the outside, the inside portion of the weld metal portion 14 is Preferably, the austenite area fraction is higher than the austenite area fraction in the outer portion of the weld metal portion 14 . The austenite phase has a large coefficient of thermal expansion and shrinks in volume by cooling after welding. On the other hand, when the austenite phase transforms into the ferrite phase, the volume expands.

That is, in the cooling process after welding, by increasing the area ratio of the ferrite phase in the outer part of the weld metal part than in the inner part, the expansion of the outer part becomes relatively larger than the inner part, and the compressive stress on the inner side remains. It will be. As a result, deformation such as warpage or twisting of the channel steel is suppressed, and the dimensional accuracy is improved.

The inner portion of the weld metal portion 14 refers to a region 1.0 mm from the inner surface of the weld metal portion 14 in the thickness direction of the bottom portion 12, and the outer portion of the weld metal portion 14 refers to the thickness of the bottom portion 12. 1.0 mm from the outer surface of the welded metal portion 14 in the vertical direction. In addition, depending on the welding conditions, the weld metal may bulge on the inside and outside of the welded portion and protrude from the surface of the bottom portion. At that time, the protruding portion is ground to flatten the inside and outside. The aforementioned distance from the surface indicates the distance from the weld metal surface after grinding.

The area fraction of austenite is measured by an electron beam backscatter diffractometer (EBSD). Specifically, for the weld metal part, the width of the center of the welded part is 100 μm×the total plate thickness, and the measurement is performed at a measurement interval (step) of 1 μm. Then, the FCC phase is specified from the analysis result, the area ratio is obtained, and the area ratio is defined as the austenite area ratio. For a relatively homogenous base material, it is considered sufficient to cover 100 μm corners, and measurements shall be made in the same steps.

3. Corrosion Resistance In the duplex stainless steel welded channel steel 10 according to the present invention, the critical pitting corrosion initiation temperature T1 (°C) in the weld heat affected zones 15a and 15b and the critical pitting initiation temperature T2 (°C) in the base metal portions 13a and 13b ) satisfies the following formula (i).
T1≧T2−5 (i)

The critical pitting initiation temperature is measured by E method described in ASTM G48. The test temperature is changed by 5° C., and the lowest temperature at which pitting corrosion having a depth of 25 μm or more is recognized after the test is defined as the critical pitting initiation temperature. FIG. 2 is a diagram showing a schematic diagram of a channel steel test piece. Fig. 2 shows the sampling positions of the test pieces for the measurement of the critical pitting initiation temperature in the weld heat affected zone and the base metal.

In order to measure the critical pitting corrosion initiation temperature in the weld heat affected zone, a test piece with a length of about 60 mm is sampled and tested so as to include the weld, weld heat affected zone and base metal. Since the weld heat-affected zone is within a range of about 3 mm from the weld metal zone, the corrosion resistance of the weld heat-affected zone can be sufficiently investigated with test pieces of the above size. As for the base metal portion, a test piece is taken from a position 10 mm or more away from the welded portion (weld heat affected zone), and the critical pitting initiation temperature is investigated as described above. The test is measured with an n number of 3, and the deepest pitting corrosion depth is taken as a representative value.

4. Dimensional Accuracy In the duplex stainless welded channel steel 10 according to the present invention, the bending tolerance in one direction is 3 mm or less per unit length (m). This is the same value as the bending tolerance in channel steel of hot-formed stainless steel shaped steel described in JIS G 4317.

5. Chemical composition The chemical composition of the base material is not particularly limited, but preferably has the chemical composition shown below. The reasons for limiting each element are as follows. In addition, "%" about content in the following description means "mass %."

C: 0.001 to 0.060%
Since C degrades corrosion resistance, the smaller the content, the better, and the C content is preferably 0.060% or less. However, excessive reduction leads to an increase in refining costs, so the C content is preferably 0.001% or more. From the viewpoint of manufacturability, the more preferable range of C content is 0.010 to 0.045%.

Si: 0.01-1.50%
Si is an element that increases strength and has a deoxidizing effect during refining, so its content is preferably 0.01% or more. On the other hand, excessive Si content causes cracking during manufacturing, so the Si content is preferably 1.50% or less. From the viewpoint of manufacturability, the Si content is more preferably 1.00% or less.

Mn: 0.1-6.0%
Since Mn is relatively inexpensive, it may be added instead of Ni. Since it is effective for increasing the strength and has a deoxidizing effect, its content is preferably 0.1% or more. On the other hand, excessive Mn content causes deterioration of corrosion resistance, so the Mn content is preferably 6.0% or less. In order to achieve both manufacturability and cost, the Mn content is more preferably 0.5 to 3.5%.

P: 0.050% or less P is an element that impairs manufacturability and weldability, and the smaller the content, the better. Therefore, it is preferable to set the P content to 0.050% or less. However, excessive reduction leads to an increase in refining cost, so it is preferable to set the P content to 0.003% or more. From the viewpoint of manufacturability and weldability, the P content is more preferably in the range of 0.005 to 0.040%, more preferably in the range of 0.010 to 0.030%.

S: 0.0050% or less S is an unavoidable impurity element contained in steel and lowers hot workability. Therefore, the lower the S content, the better, and preferably 0.0050% or less. From the viewpoint of hot workability, the lower the S content, the better. However, excessive reduction leads to an increase in raw material and refining costs. From the standpoint of manufacturability, the S content is more preferably in the range of 0.0001 to 0.0020%, more preferably in the range of 0.0002 to 0.0010%.

Cr: 19.0-26.0%
Cr is an element that improves oxidation resistance and corrosion resistance. In order to ensure sufficient corrosion resistance as a duplex stainless steel, the Cr content is preferably 19.0% or more. However, excessive Cr content promotes the formation of the σ phase, which is an embrittlement phase, when exposed to a high-temperature atmosphere, and causes an increase in alloy cost. preferably. From the viewpoint of manufacturability, a more preferable range of Cr content is 20.0 to 23.5%.

Ni: 1.0-8.0%
Ni improves corrosion resistance and stabilizes the austenite phase in duplex stainless steel. In order to improve corrosion resistance, it is preferable to set the Ni content to 1.0% or more. On the other hand, since the alloying cost of Ni is high, the content thereof is preferably 8.0% or less. From the viewpoint of manufacturability, the more preferable range of Ni content is 1.5 to 6.0%.

N: 0.050-0.25%
N is an element that improves corrosion resistance and, like Ni, stabilizes austenite, so it can be used as a substitute for Ni. Since sufficient corrosion resistance cannot be obtained when the N content is small, the N content is preferably 0.050% or more. The higher the N content, the more effective the corrosion resistance. From the viewpoint of manufacturability, the more preferable range of N content is 0.10 to 0.20%.

Al: 0.003-0.050%
Al is used as a deoxidizing element. It is preferable to set the Al content to 0.003% or more because it is effective if it is contained in an amount of 0.003% or more as a deoxidizing element. On the other hand, since excessive content causes hardening, the Al content is preferably 0.050% or less. From the viewpoint of manufacturability, a more preferable range of Al content is 0.005 to 0.030%.

Ti: 0-0.050%
Ti is an element that combines with C and N and contributes to improving the corrosion resistance of the weld zone and increasing the strength, so it may be contained as necessary. On the other hand, an excessive Ti content causes a decrease in corrosion resistance and an increase in alloy cost, so the Ti content is preferably 0.050% or less. In order to obtain the above effects, it is preferable to set the Ti content to 0.01% or more.

Nb: 0-0.15%
Nb is an element that combines with C and N and contributes to improving the corrosion resistance of the weld zone and increasing the strength, so it may be contained as necessary. On the other hand, an excessive Nb content causes a decrease in corrosion resistance and an increase in alloy cost, so the Nb content is preferably 0.15% or less. In order to obtain the above effects, it is preferable to set the Nb content to 0.02% or more.

Mo: 0-4.0%
Cu: 0-4.0%
W: 0-4.0%
Mo, Cu and W are elements that contribute to the improvement of corrosion resistance, so they may be contained as necessary. On the other hand, an excessive content causes an increase in cost and a decrease in hot workability. Therefore, the content of any element is preferably 4.0% or less. In order to obtain the above effects, the content of one or more elements selected from these elements is preferably 0.05% or more.

Mg: 0-0.0050%
Ca: 0-0.0050%
REM: 0-0.30%
B: 0 to 0.0040%
Mg, Ca, REM, and B are elements that improve hot workability, and may be contained as necessary. On the other hand, excessive content leads to inhibition of manufacturability. Therefore, it is preferable to set the Mg content to 0.0050% or less, the Ca content to 0.0050% or less, the REM content to 0.30% or less, and the B content to 0.0040% or less. In order to obtain the above effect, Mg: 0.0002% or more, Ca: 0.0002% or more, REM: 0.005% or more, B: 0.0003% or more It is preferable to contain one or more selected from.

In the chemical composition of the base material portion of the present invention, the balance is Fe and impurities. Here, the term "impurities" refers to components mixed in by various factors in raw materials such as ores, scraps, etc., and in the manufacturing process when steel is manufactured industrially. means something

The chemical composition of the weld metal portion is not particularly limited, but in order to increase the corrosion resistance of the weld metal portion to the same level or higher than that of the base metal portion, the chemical composition of the weld metal is preferably within the following range. .

That is, the chemical composition of the weld metal part is, in mass %, C: 0.001 to 0.060%, Si: 0.01 to 1.50%, Mn: 0.1 to 4.0%, P: 0 .050% or less, S: 0.0050% or less, Cr: 19.0-27.0%, Ni: 1.0-10.0%, N: 0.050-0.25%, Al: 0.05% 003-0.050%, Ti: 0-0.050%, Nb: 0-0.15%, Mo: 0-6.0%, Cu: 0-4.0%, W: 0-4.0 %, Mg: 0-0.0050%, Ca: 0-0.0050%, REM: 0-0.30%, B: 0-0.0040%, balance: Fe and impurities.

The chemical composition of the weld metal part is determined by the inflow ratio of the base metal and the welding material during welding. A commercially available material may be used as the welding material, and for example, a duplex stainless steel welding rod DP8 can be used.

6. Manufacturing Method The manufacturing method of the duplex stainless welded channel steel of the present invention is not particularly limited, but it can be manufactured by the method shown below.

(a) Welding process First, two angle steels extending in one direction and having an L-shaped cross section perpendicular to the one direction are prepared. Although there is no particular limitation on the method of manufacturing the angle steel, it can be obtained, for example, by hot-rolling a billet having the chemical composition described above into an L-shape.

Then, two angle steels are arranged symmetrically and joined by welding to form a channel steel having a U-shaped cross section. A welding material is used for welding. The welding method is also not particularly limited, and although MIG welding is preferred, methods such as _CO2 welding or shielded arc welding (SMAW) may be employed.

The channel steel to be joined has a structure including a pair of side wall portions and a bottom portion connecting the pair of side wall portions. At this time, when the side where the pair of side walls extend from the bottom is defined as the inside in the cross section perpendicular to one direction of the channel steel, it is preferable to perform the welding in a state of being sealed with nitrogen gas from the inside.

By sealing the welded portion with nitrogen gas, it is possible to suppress the decrease in nitrogen contained in the steel that accompanies welding. By suppressing the decrease in the nitrogen concentration inside, the ratio of the austenite phase inside can be relatively increased with respect to the outside where nitrogen gas sealing is not performed.

When welding from the outside, the nitrogen in the steel is reduced by welding in the atmosphere, and the ferrite phase, which has a small coefficient of thermal expansion and does not undergo transformation, should be increased.

(b) Heat Treatment Process A heat treatment process is performed after the welding process. In the heat treatment step, the channel steel after joining is heated to a temperature range of 950 to 1050°C. Then, this temperature range is maintained for 3 to 15 minutes. After that, the channel steel is cooled under the condition that the average cooling rate from the above temperature range to 400°C is 0.3 to 10°C/s.

If the heat treatment temperature is less than 950° C. or the holding time is less than 3 minutes, the carbides and nitrides that cause deterioration in corrosion resistance are not sufficiently dissolved. On the other hand, if the heat treatment temperature exceeds 1050° C. or the holding time exceeds 15 minutes, the channel steel may be deformed during the heat treatment. This is because, in duplex stainless steel, the soft ferrite phase ratio increases at high temperatures, and creep deformation occurs at high temperatures.

In the cooling that follows the heat treatment, if the average cooling rate from the heat treatment temperature to 400° C. is less than 0.3° C./s, carbides and nitrides will precipitate during cooling and the corrosion resistance will deteriorate. In particular, the difference in the critical pitting corrosion initiation temperature between the base metal portion and the weld heat affected zone is 10° C. or more, and the weld heat affected zone corrodes preferentially. On the other hand, if the average cooling rate exceeds 10° C./s, twist deformation of the channel steel occurs during cooling, resulting in deterioration of the dimensions and shape.

In order to ensure the above cooling rate, it is better not to use cooling with water after the heat treatment. This is because if water is directly applied to the steel material, the cooling rate will partially increase, causing deformation.

EXAMPLES The present invention will be described in more detail with reference to examples below, but the present invention is not limited to these examples.

Duplex stainless steels having chemical compositions shown in Table 1 were melted, heated under various conditions, and then hot-rolled to obtain angle steels having an L-shaped cross section with a thickness of 6 mm. After pickling the surface, it was cut to a length of 200 mm. Two of the above angle steels were placed side by side symmetrically, and 200 mm length portions were MIG welded to each other to form a channel steel.

Welding was performed using 3.2 mmφ duplex stainless steel (Low C-22Cr-6Ni-3Mo-0.1N: equivalent to SUS329J3L) as a wire, with a voltage of 30 V from the inside or outside, a current of 520 A, and a speed of 55 cpm. . The gas blown during welding was mainly nitrogen gas, and partly used Ar-3% O ₂ gas.

After grinding the back surface of the welded portion, heat treatment was performed under various conditions shown in Table 2. After cooling, the channel steel was examined for warpage. The warp was evaluated by measuring the maximum height h above the ground when the groove steel was placed on a flat surface with the outside of the surface (bottom) including the welded portion as the bottom (installation surface).

Next, the corrosion resistance was investigated. First, a test piece of 15 mm×60 mm was taken from a position including the welded part and a position separated from the welded part by 10 mm or more, and the entire surface was polished with #600. Then, the above test piece was subjected to a critical pitting corrosion initiation temperature measurement test based on the ASTM G48 E method.

After the test, the depth of focus was adjusted to each of the maximum pitting depth and the surface with an optical microscope, and the difference was measured as the pitting depth. The test temperature was changed by 5° C., and the lowest temperature at which pitting corrosion having a depth of 25 μm or more was observed after the test was taken as the critical pitting initiation temperature. Since the test solution freezes below -5°C, the lowest temperature of the test was -5°C. When the corrosion resistance of the base material was ≤ -5°C, the corrosion resistance was poor (x).

The results are also shown in Table 2.

As is clear from the results shown in Table 2, the examples of the present invention satisfying the requirements of the present invention exhibited excellent corrosion resistance and dimensional accuracy.

According to the present invention, it is possible to industrially stably obtain a welded duplex stainless channel steel excellent in corrosion resistance and dimensional accuracy.

REFERENCE SIGNS LIST 10 Duplex stainless welded channel steel 11a, 11b Side wall 12 Bottom 13a, 13b Base material 14 Weld metal 15a, 15b Weld heat affected zone

Claims (7)

A channel steel having a U-shaped cross section extending in one direction and perpendicular to the one direction, and comprising a pair of side walls and a bottom connecting the pair of side walls,
The channel steel includes a pair of base metal parts having an L-shaped cross section perpendicular to the one direction, and a welded metal part sandwiched between the pair of base metal parts and extending in the one direction at the bottom part. has
A weld heat affected zone is formed at a boundary between the base metal portion and the weld metal portion,
The base material portion has a metal structure in which the area ratio of the austenite phase is 30 to 70%, and the balance is the ferrite phase and precipitates,
The critical pitting initiation temperature T1 (°C) in the weld heat affected zone and the critical pitting initiation temperature T2 (°C) in the base metal portion satisfy the following formula (i),
The critical pitting initiation temperature is measured by E method described in ASTM G48, and when the test temperature is changed by 5 ° C. and measured three times at a time, at least one measurement out of the three measurements is the lowest temperature at which pitting corrosion having a depth of 25 μm or more is observed after the test,
The bending tolerance in the one direction is 3 mm or less per unit length (m).
Duplex stainless welded channel steel.
T1≧T2−5 (i) In the cross section perpendicular to the one direction, when the side where the pair of side wall portions extend from the bottom portion is the inside and the opposite side is the outside,
The area ratio of austenite in the inner part of the weld metal part is higher than the area ratio of austenite in the outer part of the weld metal part,
The duplex stainless welded channel steel according to claim 1. The chemical composition of the base material portion is, in mass %,
C: 0.001 to 0.060%,
Si: 0.01 to 1.50%,
Mn: 0.1 to 6.0%,
P: 0.050% or less,
S: 0.0050% or less,
Cr: 19.0 to 26.0%,
Ni: 1.0 to 8.0%,
N: 0.050 to 0.25%,
Al: 0.003 to 0.050%,
Ti: 0 to 0.050%,
Nb: 0 to 0.15%,
Mo: 0-4.0%,
Cu: 0 to 4.0%,
W: 0 to 4.0%,
Mg: 0-0.0050%,
Ca: 0 to 0.0050%,
REM: 0-0.30%,
B: 0 to 0.0040%,
balance: Fe and impurities,
The duplex stainless weld channel steel according to claim 1 or claim 2. The chemical composition of the base material portion is, in mass %,
Ti: 0.01 to 0.050%,
Nb: 0.02 to 0.15%,
Mo: 0.05 to 4.0%,
Cu: 0.05 to 4.0%,
W: 0.05 to 4.0%,
Mg: 0.0002-0.0050%,
Ca: 0.0002 to 0.0050%,
REM: 0.005 to 0.30%, and
B: 0.0003 to 0.0040%,
containing one or more selected from
Duplex stainless welded channel steel according to any one of claims 1 to 3. A method for manufacturing a duplex stainless welded channel steel according to any one of claims 1 to 4,
(a) a step of arranging two angle steels symmetrically extending in one direction and having an L-shaped cross section perpendicular to the one direction and joining them by welding to form a channel steel having a U-shaped cross section; ,
(b) After heating the channel steel to a temperature range of 950 to 1050 ° C., it is held in the temperature range for 3 to 15 minutes, and then the average cooling rate from the temperature range to 400 ° C. is 0.3 to 10 ° C. A step of cooling under conditions of / s,
A method for producing a duplex stainless welded channel steel. When the channel steel includes a pair of side wall portions and a bottom portion connecting the pair of side wall portions, and in a cross section perpendicular to the one direction, the side where the pair of side wall portions extend from the bottom portion is the inside. to the
In the step (a), welding is performed from the inside in a state of being sealed with nitrogen gas.
The method for manufacturing the duplex stainless welded channel steel according to claim 5. wherein the angle iron has the chemical composition according to claim 3 or claim 4,
The method for manufacturing the duplex stainless welded channel steel according to claim 5 or 6.

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