JP6614785B2 - Alloy-saving duplex stainless steel laser welded member with good laser weld characteristics and manufacturing method of alloy-saving duplex stainless steel laser welded member - Google Patents

Description

  The present invention relates to an alloy-saving duplex stainless steel laser welding member formed by laser welding a duplex stainless steel material and a method for manufacturing the same.

  Duplex stainless steel has both an austenite phase and a ferrite phase in the steel structure. Duplex stainless steel has long been used for petrochemical equipment materials, pump materials, chemical tank materials, and the like as a high-strength, high-corrosion-resistant material. Further, duplex stainless steel is generally a low Ni component system. For this reason, duplex stainless steel is attracting attention as a material with lower alloy costs and less fluctuations than austenitic stainless steel, which is the mainstream of stainless steel, due to the recent surge in metal raw materials.

By the way, the latest topic of duplex stainless steel is the development of an alloy-saving type and an increase in its usage.
The alloy-saving type is a steel type in which the content of expensive alloy components such as Ni and Mo is suppressed as compared with conventional duplex stainless steel, and the merit of low alloy cost is further increased. Among the alloy-saving duplex stainless steels, the steel types described in Patent Documents 1 and 2 are standardized by ASTM-A240. The steel type described in Patent Document 1 corresponds to S32304 (representative component 23Cr-4Ni-0.17N). The steel type described in Patent Document 2 corresponds to S32101 (representative component 22Cr-1.5Ni-5Mn-0.22N).

Conventionally, there are JIS SUS329J3L and SUS329J4L as main steel types of duplex stainless steel. These duplex stainless steels have higher corrosion resistance than the austenitic high corrosion resistance steel SUS316L, about 6 to 7% of expensive Ni (hereinafter,% for the component means mass%), about Mo. Contains 3-4%.
In contrast, the alloy-saving duplex stainless steel has a corrosion resistance of SUS316L or a level close to that of SUS304, which is a general-purpose steel. Reduced.

When welding a duplex stainless steel material (base material) formed of the above-mentioned alloy-saving duplex stainless steel to form a welded member, in the weld metal affected zone and the weld heat affected zone so-called HAZ (Heat Affected Zone) in the vicinity thereof, Extreme corrosion resistance and toughness degradation occur. This characteristic deterioration is caused by the following mechanism.
In the duplex stainless steel, the phase ratio between the ferrite phase and the austenite phase varies depending on the heating temperature. When the duplex stainless steel material is welded, the ratio of the ferrite phase in the portion that becomes the weld metal part and the HAZ part increases and the ratio of the austenite phase decreases due to heating for melting the base material. And at the time of cooling in which a weld metal part is formed, an austenite phase increases conversely. However, since the cooling rate is generally high when the weld metal part is formed, the amount of austenite phase in the weld metal part and the HAZ part is smaller than that of the base material.

  Moreover, most of N in the duplex stainless steel material which is a base material is dissolved in the austenite phase. However, in the weld metal part and the HAZ part, since the austenite phase is less than that of the base material, the N content in the ferrite phase is higher than that of the base material. The solid solubility limit of N in the ferrite phase is small. For this reason, N dissolved in the ferrite phase that becomes the weld metal part and the HAZ part by heating accompanying welding is precipitated as Cr nitride during cooling when the weld metal part is formed. When Cr is consumed by precipitation of Cr nitride, a so-called Cr-depleted layer is formed in the weld metal part and the HAZ part, and the corrosion resistance is lowered.

  In addition, the cooling rate at the time of forming a weld metal part is faster than a HAZ part, and the weld metal part which also produces a component segregation has a characteristic lower than a HAZ part by welding originally. However, in the weld metal part, this problem can be avoided by using a filler material having a component with higher corrosion resistance. Such a filler metal is usually used when a stainless steel thick plate is welded by FCAW (gas shield arc welding method using a flux-cored wire) or the like.

  As a countermeasure against the deterioration of the characteristics of the HAZ part due to welding, although seemingly contradictory, the addition of N into the duplex stainless steel material which is the base material is generally used. N is an austenite generating element having a high diffusion rate. Therefore, when the N content in the base material is increased, reprecipitation of the austenite phase is promoted during cooling when the weld metal part is formed, and N is reabsorbed by the austenite phase. Generally, an alloy-saving duplex stainless steel contains about 0.1 to 0.22% N.

  However, when a high heat input welding is performed on an alloy-saving duplex stainless steel material having a high N content, if the cooling rate during cooling is relatively slow, the following inconvenience occurs. That is, in the cooling process in which the weld metal portion is formed, the austenite phase is sufficiently precipitated by N in the base material. However, since the cooling rate is slow, the region that becomes the HAZ part in the cooling process is exposed to a Cr nitride precipitation temperature range of about 1000 ° C. to 600 ° C. for a long time. Eventually, Cr nitride precipitates in the HAZ portion of the welded member, and the corrosion resistance and toughness of the HAZ portion are reduced as compared with the base material.

  In order to overcome the problem of characteristic deterioration in the HAZ part of the welded member, we have disclosed an alloy-saving duplex stainless steel in Patent Document 3. This alloy-saving duplex stainless steel has C: 0.06% or less, Si: 0.1-1.5%, Mn: 2.0-4.0%, P: 0.05% or less, S: 0 0.005% or less, Cr: 19.0 to 23.0%, Ni: 1.0 to 4.0%, Mo: 1.0% or less, Cu: 0.1 to 3.0%, V: 0.00. Contains 0.5 to 0.5%, Al: 0.003 to 0.050%, O: 0.007% or less, N: 0.10 to 0.25%, Ti: 0.05% or less, the balance being It consists of Fe and inevitable impurities. Furthermore, Md30 value is 80 or less, Ni-bal. Is -8 or more and -4 or less, and the upper limit of the N content is Ni-bal. The austenite phase area ratio is 40 to 70% and 2 × Ni + Cu is 3.5 or more, and the corrosion resistance and toughness of the weld heat affected zone is good alloy-saving duplex stainless steel. The point of this invention is that, in addition to the addition of a very small amount of V at a solid solution level, Ni-bal. Therefore, by defining the upper limit of the N content according to the above, it is to suppress the precipitation of nitride in the HAZ part.

  By the way, laser welding has recently begun to be used as a new welding method. Laser welding is a method in which a laser beam is used as a heat source and the metal is locally melted, solidified and joined by converging and irradiating this light to a minimum diameter. Compared to conventional welding methods, the heat source has a higher energy density. For this reason, high-speed welding is possible, and the weld bead has a narrow width and deep penetration. By using laser welding, productivity can be significantly improved.

  Regarding laser welding of duplex stainless steel materials, there are techniques described in Patent Document 4 and Patent Document 5. Patent Document 4 discloses a method for producing a duplex stainless steel welded tube in which the ratio of the ferrite phase and the austenite phase in the weld metal can be made equal to that of the base material by mixing N in the shield gas. . Patent Document 5 discloses a duplex stainless steel welded pipe that controls the welding conditions so that the cooling rate at 700 ° C. of the weld metal is less than 10 ° C./sec, thereby reducing the ferrite ratio in the weld metal to 80% or less. A manufacturing method is disclosed. All of the target steel types are general-purpose duplex stainless steels such as SUS329J3L and SUS329J4L.

JP-A 61-56267 International Publication No. 2002/27056 International Publication No. 2009/119895 JP-A-8-132262 JP-A-8-155562 JP 2013-204044 A

Welding Journal vol.37 (1958), 210s-215s

  The heating history of laser welding is remarkably faster heating and faster cooling than conventional welding methods. Furthermore, in laser welding, it is not usual to use a filler metal as in other welding methods. Therefore, in a welded member obtained by welding a base material formed of duplex stainless steel, the characteristics of the weld metal part, which has a faster heating / cooling rate during welding than the HAZ part in the conventional welding method, Compared to a large drop.

  Even in the case of performing laser welding, as a countermeasure for reducing the difference in characteristics between the weld metal part and the HAZ part and the base material, it is conceivable to increase the N content in the base material as described above. . However, in laser welding, the cooling rate when the weld metal part is formed is significantly faster than in the conventional welding method. For this reason, there is a case where the difference in characteristics between the weld metal part and the HAZ part of the weld member and the base material cannot be sufficiently reduced by simply increasing the N content in the base material. Furthermore, in the weld metal part of the weld member formed using laser welding, N added to the base material may be lost when the base material is melted. For this reason, even if the N content in the base material is increased, the austenite phase may not be sufficiently precipitated in the cooling process in which the weld metal part is formed.

The present invention relates to a welded member in which a base material having good toughness formed of an alloy-saving type duplex stainless steel material is welded by laser welding without using a filler material. It is an object of the present invention to provide an alloy-saving duplex stainless steel laser welded member in which a difference in characteristics from a base material is suppressed.
Moreover, this invention makes it a subject to provide the manufacturing method with which the difference in the characteristic of a weld metal part and a welding heat affected zone, and a preform | base_material is suppressed and which can obtain the alloy-saving duplex stainless steel laser welding member.

In order to solve the above-mentioned problems, the inventors have studied as follows.
First, an alloy-saving duplex stainless steel material having good toughness with Ni saving or Ni and Mo saving was welded by laser welding without using a filler material to form a plurality of welded members. And about the obtained welding member, the structure of the laser welding part of the welding member in which the corrosion resistance of the laser welding part (welded metal part and HAZ part) fell remarkably compared with the base material was investigated.

As a result, it was found that in such a laser weld, the grain boundary of the ferrite phase was not completely covered with the austenite phase due to the lack of the austenite phase. Moreover, in such a laser welded part, it was grasped that Cr nitride was concentrated and precipitated in the form of a film at the grain boundary between the ferrite phase and the ferrite phase.
Therefore, the inventors considered that it is important to reliably precipitate the austenite phase at the ferrite grain boundary of the laser weld, and evaluated the precipitation conditions in detail.

Specifically, for the alloy-saving duplex stainless steel materials with varying N content and other chemical compositions, laser welding with different welding conditions was performed to achieve various cooling rates, and austenite reprecipitation of the weld metal part The amount and the corrosion resistance of the laser weld were evaluated and arranged as follows.
First, the amount of austenite phase of the weld metal part necessary for ensuring the corrosion resistance of the laser weld part was evaluated. In this evaluation, it was determined that the average width of the austenite phase precipitated at the grain boundary of the ferrite phase in the cross section of the weld metal portion should be 2.5 μm or more.

  If the average width of the austenite phase is less than 2.5 μm, the grain boundary of the ferrite phase is not completely covered with the austenite phase, and Cr nitride precipitates in a film form at the grain boundary between the ferrite phase and the ferrite phase. I found out that The Cr nitride deposited in the film form is dissolved in the ferrite phase due to the decrease in the solid solution limit caused by cooling when the weld metal part is formed. It is no longer possible to deposit and form. By forming Cr nitride, it is estimated that a Cr-deficient layer is formed in the weld metal part and the corrosion resistance of the laser weld part is lowered.

  Moreover, in order to ensure the corrosion resistance of a laser welding part, it turned out that it is necessary to use what is an area ratio of an austenite phase 40% or more and 70% or less as an alloy-saving duplex stainless steel material.

As described above, if the width of the austenite phase is 2.5 μm or more on average, a sufficient amount of austenite phase can be secured in the weld metal part. However, if the N content in the weld metal part is excessively increased in order to ensure the amount of the austenite phase, the precipitation of Cr nitride from the ferrite phase increases, and the corrosion resistance deteriorates.
Therefore, the inventors have intensively studied and found that the calculated value _{N_preer} of the Cr nitride precipitation start temperature calculated by the following equation (1) can be used as an index of Cr nitride precipitation. And it confirmed that precipitation of Cr nitride can be suppressed by setting it as a welding member whose _Npler is 1250 degrees C or less, and the difference of the characteristic of a laser weld part and a base material can fully be suppressed.

_{N prer = 800N r -3Cr + 20Si} + 10Ni-4Mn + 1140 ··· (1)
“N _r ” in the formula (1) indicates the nitrogen concentration (mass%) in the weld metal part, and each element symbol indicates the content (mass%) of the element in the duplex stainless steel material.

We, when calculating the N _PRER, as shown in equation (1) using nitrogen concentration N _r in the weld metal portion. The reason is as follows.
N in the alloy-saving duplex stainless steel material that becomes the weld metal part is released in a certain amount into the atmosphere at the time of melting. Therefore, the N content in the weld metal part is less than the N content in the original steel material (base material). For this reason, the N amount contributing to the precipitation of Cr nitride is not the content in the steel material but the N content in the weld metal part. In addition, since the elements in the steel material other than N are difficult to be released at the time of melting, the contents in the steel material and the weld metal part can be regarded as the same.

  Next, the inventors welded an alloy-saving duplex stainless steel material by laser welding without using a filler material, and the average width of the austenite phase precipitated at the grain boundaries of the ferrite phase in the cross section was 2.5 μm or more. The method of forming a welded member having a weld metal part having a calculated value of the Cr nitride precipitation start temperature of 1250 ° C. or less was earnestly studied, and the following conclusions were obtained.

  In laser welding, the cooling rate when a weld metal part is formed is very fast. For this reason, only the light element can be diffused during cooling. Therefore, in the chemical composition of the alloy-saving duplex stainless steel, an austenite phase is formed only by diffusion of N in the cooling process in which the weld metal part is formed.

Inventors examined the manufacturing method of the welding member in which the austenite reprecipitation effect by N is exhibited in a welding process.
As a result, the nitrogen concentration estimated value (mass%) “N _re ” in the weld metal part, “DF _N ” indicating the ferrite phase stability in the weld metal part by elements other than _N, and 1000 ° C. of the weld metal part Of the weld metal part by the calculated value “γ _N ” (μm) of the austenite reprecipitation width of the weld metal part represented by the following formula (3) using the estimated cooling rate “V _CR ” (° C./s) in It has been found that the average value of the width of the austenite phase precipitated at the grain boundaries of the ferrite phase can be estimated. Then, by performing the welding step so that the calculated value N _PRER precipitation initiation temperature values Cr nitrides conditions become more 2.5μm of "gamma _N" becomes 1250 ° C. or less by experiments, above the austenite phase It has been found that it is possible to form a welded member having a weld metal part having an average width of 2.5 μm or more and in which the difference in properties between the weld metal part and the weld heat affected zone and the base material is suppressed.

γ _N = (1200N _re −1.2DF _N +100) / √ (V _CR ) (3)
“N _re ” in the equation (3) represents an estimated nitrogen concentration (mass%) in the weld metal part represented by the following equation (4), and “DF _N ” is other than N represented by the following equation (5). This is a numerical value indicating the stability of the ferrite phase in the weld metal part due to the element, and “V _CR ” is an estimated cooling rate (° C./s) at 1000 ° C. of the weld metal part represented by the following formula (6).

N _re = N−10 × N ² / √ (V _CR ) (4)
“V _CR ” in the formula (4) is a numerical value represented by the following formula (6), and “N” represents a nitrogen content (mass%) in the duplex stainless steel material.

DF _N = 7.2 (Cr + 0.88Mo + 0.78Si) −8.9 (Ni + 0.03Mn + 0.72Cu + 22C) −44.9 (5)
Each element symbol in the formula (5) indicates the content (mass%) of the element in the duplex stainless steel material, and is 0 when the element is not contained in the steel material.

In equation (6), “λ” is the thermal conductivity (W / m / K) of the duplex stainless steel material, “c” is the specific heat (J / kg / K) of the duplex stainless steel material, and “ρ” is the duplex phase. The density of stainless steel (kg / m ³ ), “h” is the material thickness (m) of the welded portion of the duplex stainless steel, “V” is the welding speed (m / s), “P” is the laser output (W ), “T” indicates the temperature (1000 ° C.) for calculating the speed, and “T ₀ ” indicates the room temperature (20 ° C.). This estimation formula is based on Non-Patent Document 1.

As described above, in order to manufacture an alloy-saving duplex stainless steel laser welded member in which the difference in characteristics between the laser weld and the base material is sufficiently suppressed, “N _re ”, “DF _N ”, and “V _N ” It was found that “γ _N ” needs to be controlled using “ _CR ”.
From the above results, the inventors have invented an alloy-saving duplex stainless steel laser welded member and a method for producing the same that can solve the above-mentioned problems.

The gist of the present invention is as follows.
[1] Chemical composition in mass%,
C: 0.04% or less,
Si: 0.10 to 1.00%,
Mn: 0.50 to 6.00%,
P: 0.050% or less,
S: 0.0050% or less,
Cr: 20.0-25.0%,
Ni: 1.00 to 6.00%,
N: containing 0.120 to 0.250%, the balance being Fe and inevitable impurities, and the area ratio of the austenite phase being 40% or more and 70% or less, an alloy-saving duplex stainless steel material, a weld metal part, A welding member having
The chemical composition of the weld metal part is composed of the chemical composition of the alloy-saving duplex stainless steel material except N, and the N amount of the weld metal part is less than the N amount of the alloy-saving duplex stainless steel material,
The weld metal part has an average width of an austenite phase precipitated at a grain boundary of a ferrite phase in a cross section of 2.5 μm or more,
The welded member has a calculated value _{Nprer of} a Cr nitride precipitation start temperature calculated by the following formula (1) of 1250 ° C. or less, and is an alloy-saving duplex stainless steel with good laser weld characteristics. Laser welding member.

_{N prer = 800N r -3Cr + 20Si} + 10Ni-4Mn + 1140 ··· (1)
“N _r ” in the formula (1) indicates the nitrogen concentration (mass%) in the weld metal part, and each element symbol indicates the content (mass%) of the element in the duplex stainless steel material.

[2] The duplex stainless steel material is further in mass%,
Mo: 1.50% or less,
Cu: 1 type or 2 types selected from 2.00% or less,
N _welder calculated by the following formula (2) is 1250 ° C. or less, and the welded member has an excellent property of the laser welded portion according to [1], and is an alloy-saving duplex stainless steel laser welded member .

_{N prer = 800N r -3Cr + 20Si} + 6.5Mo + 10Ni-4Mn + 15Cu + 1140 ··· (2)
“N _r ” in the formula (2) indicates the nitrogen concentration (mass%) in the weld metal part, and each element symbol indicates the content (mass%) of the element in the duplex stainless steel material.

[3] The duplex stainless steel material is further in mass%,
V: 0.05 to 0.50%,
Nb: 0.010-0.200%
Ti: One or two or more types selected from 0.0030 to 0.050% are contained. Alloy-saving duplex stainless steel with good laser welded portion characteristics according to [1] or [2] Steel laser welding member.

[4] The duplex stainless steel material is further in mass%,
Al: 0.003-0.045%,
O 2: One or two elements selected from 0.007% or less are contained. Alloy-saving two-phase with good characteristics of laser welded portion according to any one of [1] to [3] Stainless steel laser welding member.

[5] The duplex stainless steel material is further in mass%,
Ca: 0.0050% or less,
Mg: 0.0050% or less,
REM: 0.050% or less,
B: One or two or more selected from 0.0040% or less are contained. The alloy-saving alloy 2 having good characteristics of the laser weld according to any one of [1] to [4] Phase stainless steel laser welded member.

[6] The duplex stainless steel material is further in mass%,
Zr: 0.020% or less,
Ta: 0.070% or less,
Co: 0.02 to 1.00%,
W: 1.00% or less,
Sn: One or two or more kinds selected from 0.100% or less are contained, and the alloy-saving alloy 2 having good laser welded part properties according to any one of [1] to [5] Phase stainless steel laser welded member.

[7] A welding step of welding the duplex stainless steel material having the chemical composition according to any one of [1] to [6] by laser welding without using a filler material,
The welding process is performed under the condition that the calculated value γ _N of the austenite reprecipitation width of the weld metal part calculated by the following formula (3) is 2.5 μm or more, and the Cr nitride calculated by the following formula (7) A method for producing an alloy-saving duplex stainless steel laser welded member, comprising forming a welded member having a calculated value of N precipitation of N _preer of 1250 ° C. or lower.

γ _N = (1200N _re −1.2DF _N +100) / √ (V _CR ) (3)
“N _re ” in the formula (3) is an estimated nitrogen concentration value (mass%) in the weld metal part represented by the following formula (4) and is 0.20 mass% or less, and “DF _N ” is (5) is a numerical value indicating the stability of the ferrite phase in the weld metal part by an element other than N represented by the formula (5), and “V _CR ” is a cooling rate estimation at 1000 ° C. of the weld metal part represented by the following formula (6). Value (° C./s).

N _re = N−10 × N ² / √ (V _CR ) (4)
“V _CR ” in the formula (4) is a numerical value represented by the following formula (6), and “N” represents a nitrogen content (mass%) in the duplex stainless steel material.

DF _N = 7.2 (Cr + 0.88Mo + 0.78Si) −8.9 (Ni + 0.03Mn + 0.72Cu + 22C) −44.9 (5)
Each element symbol in the formula (5) indicates the content (mass%) of the element in the duplex stainless steel material, and is 0 when the element is not contained in the steel material.

In equation (6), “λ” is the thermal conductivity (W / m / K) of the duplex stainless steel material, “c” is the specific heat (J / kg / K) of the duplex stainless steel material, and “ρ” is the duplex phase. The density of stainless steel (kg / m ³ ), “h” is the material thickness (m) of the welded portion of the duplex stainless steel, “V” is the welding speed (m / s), “P” is the laser output (W ), “T” indicates 1000 ° C. , and “T ₀ ” indicates room temperature (20 ° C.).

_{N prer = 800N r -3Cr + 20Si} + 6.5Mo + 10Ni-4Mn + 15Cu + 1140 ··· (7)
“N _r ” in the formula (7) indicates the nitrogen concentration (mass%) in the weld metal part, and each element symbol indicates the content (mass%) of the element in the duplex stainless steel material. 0 is not included in the steel material.

[8] The method for producing an alloy-saving duplex stainless steel laser welded member according to [7], wherein the _{VCR is set} to 500 ° C./s or more.

  According to the present invention, welding in which a base material formed of a duplex stainless steel material having good toughness of Ni-saving or Ni-Mo-saving alloy-saving type is welded by laser welding without using a filler metal. In the member, an alloy-saving duplex stainless steel laser welded member in which the difference in characteristics between the weld metal part and the weld heat affected part and the base material is suppressed is obtained. As a result, it is possible to provide an inexpensive Ni-saving duplex stainless steel laser welded member having corrosion resistance that is used in the air environment, water environment, etc., which greatly contributes to the industry.

It is a microscope picture of the cross section of the weld metal part in an example of the laser welding member of this invention.

The present invention is described in detail below.
First, the alloy-saving duplex stainless steel laser welding member of the present invention will be described. The alloy-saving duplex stainless steel laser welding member of the present invention is a welding member obtained by welding a duplex stainless steel material as a base material by a laser welding method without using a filler material.
Next, the chemical composition of the duplex stainless steel material will be described. In addition, in the following description,% which shows content means the mass%.

  C limits the content to 0.04% or less in order to ensure the corrosion resistance of the duplex stainless steel. When C is contained exceeding 0.04%, Cr carbide is generated and the corrosion resistance is deteriorated. The C content is preferably 0.03% or less. On the other hand, since extremely reducing the C content significantly increases the cost, the lower limit of the C content is preferably set to 0.001%.

  Si is added in an amount of 0.10% or more for deoxidation. However, when Si is added exceeding 1.00%, toughness deteriorates. Therefore, the upper limit of Si content is limited to 1.00%. The range of preferable Si content is 0.20 to 0.60%.

  Mn increases the austenite phase in duplex stainless steel, suppresses the formation of work-induced martensite, improves toughness, increases the solid solubility of nitrogen, and suppresses the precipitation of nitride in the weld. To 0.50% or more. However, if Mn is added over 6.00%, the corrosion resistance deteriorates. Therefore, the upper limit of the Mn content is limited to 6.00%. A preferable range of the Mn content is 1.50 to 4.00%, and a more preferable range is more than 2.00 to less than 3.00%.

  P is an element inevitably contained in the duplex stainless steel, and is limited to 0.050% or less in order to deteriorate the hot workability. The P content is preferably 0.030% or less. On the other hand, reducing the P content extremely increases the cost significantly. For this reason, it is preferable that the lower limit of the P content is 0.005%.

  S is an element that is inevitably contained in steel like P, and is limited to 0.0050% or less in order to deteriorate hot workability, toughness, and corrosion resistance. The S content is preferably 0.0020% or less. On the other hand, reducing the S content extremely increases the cost significantly. For this reason, Preferably the minimum of S content shall be 0.0001%.

  Cr is an element that is basically necessary to ensure corrosion resistance. Cr is a relatively inexpensive alloy component, and in the present invention, it is contained in an amount of 20.0% or more. On the other hand, Cr is an element that increases the ferrite phase. If the Cr content exceeds 25.0%, the amount of ferrite in the component system of the present invention becomes excessive, which impairs corrosion resistance and toughness. For this reason, the Cr content is set to 20.0% or more and 25.0% or less. A preferable range of the Cr content is 20.5% to 22.0%.

  Ni is an element that increases the austenite phase in the duplex stainless steel. Ni is added in an amount of 1.00% or more in order to secure an austenite phase in the component system of the present invention, to suppress the formation of work-induced martensite and improve toughness, and to secure corrosion resistance against various acids. . On the other hand, since Ni is an expensive alloy, it is suppressed as much as possible in the present invention, and is made 6.00% or less. A preferable range of the Ni content is 1.50% to 3.00%.

  N is an effective element for increasing the austenite phase in the duplex stainless steel while at the same time increasing the strength and corrosion resistance by dissolving in the austenite phase. N is an important element particularly for enhancing the corrosion resistance of the austenite phase. In the present invention, particularly in laser welding, since it is necessary to ensure an austenite phase that reprecipitates during rapid cooling when a weld metal part is formed, N is contained in an amount of 0.120% or more. On the other hand, when N is contained exceeding 0.250%, N bubbles are generated during melt solidification. On the other hand, when the N content exceeds 0.250%, the amount of Cr nitride precipitated in the weld metal part is large, and the weld metal part has insufficient corrosion resistance. For this reason, the upper limit of N content was made 0.250%. A preferable N content is 0.150 to 0.200%.

Furthermore, the duplex stainless steel material may contain one or two selected from Mo and Cu.
Mo is a very effective element that greatly enhances the corrosion resistance of the duplex stainless steel, and is added as necessary in the present invention. On the other hand, Mo is a very expensive element. For this reason, in this invention, Mo content was suppressed as much as possible, and the upper limit of Mo content was prescribed | regulated as 1.50% or less. A preferable range of the Mo content is more than 0.10% and less than 0.50%.

  Cu is an element effective for increasing the austenite phase in the duplex stainless steel as well as Ni, suppressing the formation of work-induced martensite and improving toughness, and further improving the corrosion resistance against various acids. is there. Since Cu is an inexpensive alloy compared with Ni, it is added as necessary in the present invention. On the other hand, if Cu is contained in excess of 2.00%, precipitation of Cr nitride is promoted, so the upper limit of Cu content is set to 2.00%. The preferable range of the Cu content is more than 0.60% to 1.50%, the more preferable range is more than 0.80% to 1.35%, and the particularly preferable range is more than 1.00% to 1.20%. is there.

  Furthermore, the duplex stainless steel material may contain one or more selected from V, Nb, and Ti. V, Nb, and Ti are effective elements for lowering the activity of N and suppressing nitride precipitation, and are selectively added.

V is required to be added in an amount of 0.05% or more in order to obtain the above effect. On the other hand, if V is added over 0.50%, the HAZ toughness is reduced by precipitation of V nitride, so the upper limit was made 0.50%. A preferable range of the V content is 0.06% to 0.20%.
Nb needs to be added in an amount of 0.010% or more to obtain the above effect. The Nb content is preferably 0.020% or more. Nb has a relatively high affinity with N as compared with V, and Nb nitride is precipitated by addition of a small amount. For this reason, Nb content shall be 0.200% or less. The Nb content is preferably 0.100% or less.
To obtain the above effect, Ti needs to be added in an amount of 0.0030% or more. The Ti content is preferably 0.0050% or more. Ti has a higher affinity with N than Nb, and Ti nitride is precipitated with a small amount of addition. For this reason, Ti content shall be 0.050% or less. The Ti content is preferably 0.030% or less.

Furthermore, the duplex stainless steel material may contain one or two selected from Al and O (oxygen).
Al is an important element for deoxidation of steel and is selectively added to reduce oxygen in the steel. In order to acquire the said effect, it is necessary to contain Al 0.003% or more. The Al content is preferably 0.010% or more. On the other hand, Al is an element having a relatively large affinity with N, and if added excessively, AlN is generated and the toughness of the base material is inhibited. The degree depends on the N content, but when Al exceeds 0.045%, the toughness deteriorates remarkably, so the upper limit of the content is set to 0.045%. The Al content is preferably 0.030% or less.

  O is a harmful element constituting an oxide that is representative of nonmetallic inclusions, and excessive inclusion inhibits toughness. In addition, the formation of coarse clustered oxides causes surface defects. For this reason, the upper limit of the O content is set to 0.007%. The O content is preferably 0.005% or less. On the other hand, reducing the O content extremely increases the cost significantly. For this reason, it is preferable that the lower limit of the O content is 0.001%.

  Furthermore, the duplex stainless steel material may contain one or more selected from Ca, Mg, REM, and B. Ca, Mg, REM, and B are all elements that improve the hot workability of steel. On the other hand, excessive addition conversely reduces hot workability. For this reason, the range of the content of these elements was determined as follows.

Each of Ca and Mg is 0.0050% or less, REM is 0.050% or less, and B is 0.0040% or less. Here, REM is the total content of lanthanoid rare earth elements such as La and Ce.
In addition, about 0.005% or more about Ca and Mg, since the stable effect is acquired, a preferable range is 0.0005 to 0.0050%. If REM is made 0.005% or more, a stable effect can be obtained, so a preferred range is 0.005 to 0.050%. If B is 0.0003% or more, a stable effect can be obtained, so a preferable range is 0.0003 to 0.0040%.

Furthermore, the duplex stainless steel material may contain one or more selected from Zr, Ta, Co, W, and Sn.
Zr and Ta can suppress the adverse effect on the corrosion resistance of C and S by addition. The contents that stably exhibit this effect are 0.003% or more for Zr and 0.010% or more for Ta. When Zr and Ta are added excessively, adverse effects such as a decrease in toughness occur. For this reason, the content when Zr and / or Ta is selectively added is limited to 0.020% or less for Zr and 0.070% or less for Ta.

  Co is an element effective for enhancing the toughness and corrosion resistance of steel, and is selectively added. If the Co content is less than 0.02%, the effect is small. For this reason, content in the case of adding Co shall be 0.02% or more. The Co content is preferably 0.04% or more. However, if Co is contained in excess of 1.00%, since it is an expensive element, the effect corresponding to the cost cannot be exhibited. Therefore, the content when Co is added is determined to be 1.00% or less. The Co content is preferably less than 0.30% from the viewpoint of cost.

  W is an element that is selectively added to additionally enhance the corrosion resistance of the duplex stainless steel. The W content that stably exhibits this effect is 0.05% or more. The W content is preferably 0.10% or more. However, W is an expensive element, and excessive addition causes cost increase, so 1.00% or less is contained. The W content is preferably 0.50% or less.

  Sn is a selective element that additionally improves acid resistance. Sn content which exhibits this effect stably is 0.050% or more. Sn can be added up to 0.100% from the viewpoint of hot workability.

  In order to obtain good characteristics in the duplex stainless steel laser welded member of the present invention, it is necessary to make the area ratio of the austenite phase in the duplex stainless steel material as the base material in the range of 40 to 70%. When the area ratio of the austenite phase is less than 40%, the toughness is poor. If the area ratio of the austenite phase is more than 70%, the hot workability is insufficient and the problem of stress corrosion cracking occurs. Moreover, even if the area ratio of an austenite phase is less than 40% or exceeds 70%, corrosion resistance becomes poor.

  Next, a laser welding member obtained by laser welding such a duplex stainless steel material will be described. A laser welded portion having a weld metal portion and a HAZ portion is formed on the laser welded member by welding.

  With respect to the metal structure of the weld metal part, the average width of the austenite phase precipitated at the grain boundary of the ferrite phase in the cross section is defined as 2.5 μm or more. When the average width of the austenite phase is less than 2.5 μm, as described above, the grain boundary of the ferrite phase is not covered with the austenite phase, and a nitride containing Cr at the grain boundary between the ferrite phase and the ferrite phase is a film. The corrosion resistance is greatly reduced. For this reason, the average width of the austenite phase is set to 2.5 μm or more, preferably 3.0 μm or more.

  The average width of the austenite phase precipitated at the ferrite phase grain boundaries in the cross section of the weld metal part is measured by the method described later.

The welding member of this embodiment has a calculated value _N.sub.prer of the Cr nitride precipitation start temperature calculated by the above equation (1) of 1250.degree.
In order to suppress the precipitation of Cr nitride in the weld metal part, it is necessary not only to reduce the N content but also to control the content of elements involved in the precipitation of Cr nitride contained in the duplex stainless steel material. is important. The magnitude of the driving force of Cr nitride precipitation, that is, the difference in chemical potential (energy) (ΔG) corresponds to the degree of supercooling indicated by the difference between the Cr nitride precipitation start temperature and the actual temperature. It is thought that. The greater the degree of supercooling, the easier the nucleation of Cr nitride and the easier it is for Cr nitride to precipitate in the weld metal.

  Therefore, the present inventors have determined the precipitation start temperature of Cr nitride when forming the weld metal part by simulation calculation on the premise that the nitrogen concentration of the weld metal part is lower than that of the original steel material. Furthermore, the magnitude | size of the contribution to precipitation of Cr nitride of each component contained in a duplex stainless steel material was calculated and formulated by simulation calculation. Using these results, an attempt was made to define the nitrogen concentration of the weld metal part and the component range of the duplex stainless steel material in which Cr nitride hardly precipitates on the weld metal part.

Specifically, the influence of each additive element contained in the duplex stainless steel material on the Cr nitride precipitation start temperature was calculated by simulation calculation using thermodynamic data. Furthermore, it was confirmed by experiment that the simulation calculation results coincided with the experimental results, and the equation (1) for calculating the calculated value N _prer at the Cr nitride precipitation start temperature was created.

Usually, since the cooling rate when a weld metal part is formed in laser welding is very fast, most of the structure of the weld metal part is a ferrite phase. For this reason, the simulation calculation was performed under the following conditions.
That is, in the ferrite phase separated from the austenite phase in the weld metal part, the N content before the re-precipitation of austenite is maintained even though it is in the ferrite phase where the solid solution limit of N is small. It is thought that there is also. Therefore, in the simulation calculation, the austenite phase was excluded, and the equilibrium calculation of the Cr nitride precipitation start temperature was performed using only the ferrite phase and Cr nitride.

The present inventors have found that the calculated value N _PRER the deposition starting temperature of Cr nitrides calculated by the equation (1) created in this way, the difference in corrosion resistance between the laser weld and the base material, checked by experiment It was. From the experimental results, it was confirmed that the difference in characteristics between the laser welded portion and the duplex stainless steel material that is the base material can be sufficiently suppressed by _{setting N.sub.preer} to 1250.degree. C. or less.

That is, in the welding member of the present embodiment, the degree of supercooling is reduced by suppressing N _prer calculated by the equation (1). Thereby, the precipitation driving force of Cr nitride corresponding to the difference in chemical potential (ΔG) is reduced, and the weld member in which the deterioration of the corrosion resistance of the weld metal part due to the precipitation of Cr nitride is suppressed, and Become.

(1) exceeds 1250 ° C. Calculated N _PRER the precipitation start temperature of Cr nitrides calculated by the formula, sensitization is caused fiercely, corrosion resistance of the weld metal is reduced. Sensitization is caused by the precipitation of Cr nitride on the weld metal part, so that the Cr element around the Cr nitride is consumed for the formation of the Cr precipitate, and a locally low Cr concentration region is formed around the Cr nitride. Occur. When N _preer is 1250 ° C. or lower, the Cr nitride precipitation start temperature in the cooling process in which the weld metal part is formed is lowered. For this reason, the precipitation amount of Cr nitride does not increase too much, and sensitization is suppressed. In order to suppress sensitization more effectively, it is preferable that _N.sub.prer is _1240.degree .

As shown in the formula (1), Cr nitride is difficult to precipitate when the nitrogen concentration (mass%) “N _r ” in the weld metal part is small. If the nitrogen concentration in the weld metal part is too high, _Npler increases, the amount of Cr nitride deposited in the weld metal part becomes large, sensitization occurs, and the corrosion resistance decreases. Therefore, “N _r ” is preferably 0.20% or less, and more preferably 0.18% or less. Further, when “N _r ” is 0.14% or more, it is preferable that the austenite phase is sufficiently reprecipitated at the time of rapid cooling when a weld metal part is formed in laser welding. “N _r ” of 0.16% or more is more preferable because the amount of reprecipitation of the austenite phase is larger.
The nitrogen concentration _Nr in the weld metal part can be measured by chemically analyzing the weld metal part with a nitrogen analyzer or the like.

As shown in the equation (1), the Cr nitride precipitation start temperature is related to the contents of Si, Ni, and Mn. Mn lowers the precipitation start temperature of Cr nitride and suppresses a large amount of Cr nitride from being deposited on the weld metal part. Si and Ni increase the Cr nitride precipitation start temperature and promote the precipitation of Cr nitride. For this reason, Si and Ni are contained in a range where N _preer is 1250 ° C. or lower.

The calculated value _N.sub.prer of the Cr nitride precipitation start temperature is calculated by the above equation (2) when the duplex stainless steel material that is the base material of the laser welding member contains Mo and / or Cu.
As shown in the formula (2), the precipitation start temperature of Cr nitride is related to the contents of Mo and Cu. Cu and Mo increase the precipitation start temperature of Cr nitride and promote the precipitation of Cr nitride. For this reason, Cu and Mo are contained in a range where N _preer is 1250 ° C. or lower.

  The alloy-saving duplex stainless steel laser welding member of the present invention may be any shape as long as it is welded by laser welding without using a filler material. For example, a welded joint or a welded pipe may be used.

Next, the manufacturing method of the alloy saving duplex stainless steel laser welding member of this invention is demonstrated.
The method for producing an alloy-saving duplex stainless steel laser welded member of the present invention includes a welding process in which a duplex stainless steel material having the chemical composition described above is welded by laser welding without using a filler material. .
The manufacturing method of the duplex stainless steel material used as a base material in the laser welding member of this embodiment is not particularly limited, and a conventionally known method can be used.

The welding process in the present embodiment is performed under the condition that the calculated value γ _N of the austenite reprecipitation width of the weld metal part calculated by the following expression (3) is 2.5 μm or more, and the expression (1) or (2) _This is carried out so that N _prer calculated by the _equation is 1250 ° C. or lower. By performing the welding process, a weld member having an average width of the austenite phase precipitated at the grain boundary of the ferrite phase in the cross section is 2.5 μm or more and _Npler is 1250 ° C. or less is formed. it can. As a result, a laser welded member is obtained in which the difference in characteristics between the laser welded portion and the base material is suppressed.

Formula (3) is an estimation formula for the austenite reprecipitation width of the weld metal part obtained by experiments. The calculated value γ _N of the austenite reprecipitation width of the weld metal part can be calculated from the equation (3).
γ _N = (1200N _re −1.2DF _N +100) / √ (V _CR ) (3)
“N _re ” in the equation (3) represents an estimated nitrogen concentration (mass%) in the weld metal part represented by the following equation (4), and “DF _N ” is other than N represented by the following equation (5). This is a numerical value indicating the stability of the ferrite phase in the weld metal part due to the element, and “V _CR ” is an estimated cooling rate (° C./s) at 1000 ° C. of the weld metal part represented by the following formula (6).

N _re = N−10 × N ² / √ (V _CR ) (4)
“V _CR ” in the formula (4) is a numerical value represented by the following formula (6), and “N” represents a nitrogen content (mass%) in the duplex stainless steel material.

DF _N = 7.2 (Cr + 0.88Mo + 0.78Si) −8.9 (Ni + 0.03Mn + 0.72Cu + 22C) −44.9 (5)
Each element symbol in the formula (5) indicates the content (mass%) of the element in the duplex stainless steel material, and is 0 when the element is not contained in the steel material.

In equation (6), “λ” is the thermal conductivity (W / m / K) of the duplex stainless steel material, “c” is the specific heat (J / kg / K) of the duplex stainless steel material, and “ρ” is the duplex phase. The density of stainless steel (kg / m ³ ), “h” is the material thickness (m) of the welded portion of the duplex stainless steel, “V” is the welding speed (m / s), “P” is the laser output (W ), “T” indicates the temperature (1000 ° C.) for calculating the speed, and “T ₀ ” indicates the room temperature (20 ° C.).

The reprecipitation of the austenite phase in the weld metal part is caused by the diffusion of N. From this, as shown in the equation (3), the reprecipitation width of the austenite phase is the square root of the estimated cooling rate (° C./s) “V _CR ” at 1000 ° C. of the weld metal part represented by the equation (6). It is considered to be inversely proportional to. As shown in the equation (3), the estimated nitrogen concentration “N _re ” in the weld metal part represented by the equation (4) contributes to the γ _N shown in the equation (3) as a positive coefficient. It is considered that the ferrite phase stability “DF _N ” in the weld metal part due to an element other than N represented by the formula (E) contributes as a negative coefficient. The equation (3) is obtained by arranging the coefficients of the three parameters “N _re ”, “DF _N ”, and “V _CR ” by experiment.

Γ _N calculated by the equation (3) can be used as an index of the austenite reprecipitation width of the weld metal part. By performing the welding process under the condition that γ _N calculated by the formula (3) is 2.5 μm or more, precipitation of film-like Cr nitride can be suppressed, and the austenite phase precipitated at the grain boundary of the ferrite phase in the cross section. A weld member having a weld metal part having an average width of 2.5 μm or more can be formed. The difference in the characteristics of the laser welding unit and the base material, in order to obtain a suppression laser welding member, under the condition that gamma _N is equal to or greater than 3.0 [mu] m, it is preferable to carry out the welding process.

The above formula (4) is an estimation formula for the nitrogen concentration (mass%) in the weld metal part. Equation (4) is found by changing the welding conditions so as to obtain various cooling rates, performing laser welding, analyzing the nitrogen concentration “N _r ” in the weld metal part, and using the results. is there. The estimated nitrogen concentration value (mass%) “N _re ” in the weld metal part can be calculated by the equation (4).
The estimated nitrogen concentration (mass%) “N _re ” in the weld metal part calculated by equation (4) is 0.14 so that γ _N calculated by equation (3) is 2.5 μm or more. % Or more is preferable, and 0.16% or more is more preferable. On the other hand, if the nitrogen concentration in the weld metal part is too high, the amount of Cr nitride deposited in the weld metal part becomes large, sensitization occurs, and the corrosion resistance decreases. Therefore, “N _re ” is preferably 0.20% or less, and more preferably 0.18% or less.

The estimated cooling rate (° C./s) “V _CR ” at 1000 ° C. of the weld metal part in the equations (3) and (4) can be calculated by the equation (6). The expression (6) calculates an estimated value of the cooling rate at 1000 ° C. of the weld metal part as described in Non-Patent Document 1, for example.

  As described above, when the duplex stainless steel material is welded by laser welding, the cooling rate when the weld metal portion is formed is very fast, and therefore the austenite phase is precipitated very little compared to normal welding. Therefore, in order to promote the reprecipitation of the austenite phase, it is preferable to control the cooling rate when the weld metal part is formed as slow as possible as shown in the equations (3) and (4). Conceivable. However, when the inventors actually conducted an experiment, the faster the cooling rate, the smaller the deterioration in corrosion resistance at the laser weld. This is because the faster the cooling rate, the smaller the release of N during melting, and the fact that the nitride precipitation is suppressed because it passes through the nitride precipitation temperature range in a very short time. Conceivable.

Therefore, the estimated cooling rate (° C./s) “V _CR ” at 1000 ° C. of the weld metal part represented by equation (6) is within the range where γ _N calculated by equation (3) is 2.5 μm or more. High is preferred. _{Specifically, V CR} is preferably 500 ° C. / s or more, more preferably 1500 ° C. / s or higher. By setting the _VCR to 500 ° C./s or more, the release of N and the precipitation of nitride in the weld metal part accompanying welding are suppressed. As a result, a laser welding member in which the difference in characteristics between the laser welded portion and the base material is further suppressed can be obtained. _{Also, V CR} is (3) as gamma _N calculated is greater than or equal to 2.5μm in expression, preferably not more than 10000 ° C. / s, more preferably at most 6000 ° C. / s.

“DF _N ” expressed by the formula (5) is a numerical value indicating the stability of the ferrite phase in the weld metal portion by an element other than N. Expression (5) is obtained by removing the N term from the DF value shown in Patent Document 6. As shown in the above formula (5), the more the Ni, Mn, Cu, and C elements that increase the austenite phase, the lower the ferrite phase stability “DF _N ” in the weld metal part due to elements other than _N. Thus, the chemical stability of the austenite phase is increased.

  As described above, in the chemical composition of the alloy-saving duplex stainless steel, the austenite phase is formed only by N diffusion. However, as a factor for determining the amount of reprecipitation of austenite, the austenite phase stability due to elements other than N affects. This is because when the austenite phase stability by elements other than N is high, the amount of N for generating austenite is small.

The ferrite phase stability “DF _N ” in the weld metal part by an element other than N represented by the formula (5) is 100 or less so that γ _N calculated by the formula (3) is 2.5 μm or more. It is preferable that it is 85 or less. Further, when the DF _N is less than 50, amount of austenite matrix also N content is a lower limit in the steel is likely to exceed 70%. Therefore, DF _N is more preferably preferably 50 or more, 70 or more.

Examples will be described below.
Tables 1 and 2 show the chemical composition (mass%) of the duplex stainless steel material used for welding. In addition, it is Fe and an unavoidable impurity element other than the component described in Table 1 and Table 2, and it shows that the blank is not added. REM in the table means lanthanoid rare earth elements, and the content indicates the total of these elements.

The duplex stainless steel material was manufactured by the method shown below.
Steels having the components shown in Tables 1 and 2 were melted in a MgO crucible by a laboratory 50 kg vacuum induction furnace and cast into a flat steel ingot having a thickness of about 100 mm. The raw material for hot rolling is processed from the main body part of the steel ingot, the obtained material is heated to a temperature of 1180 ° C. for 1 to 2 hours, and then rolled at a finishing temperature of 950 to 850 ° C. A hot rolled steel sheet was obtained.
In addition, spray cooling was implemented with respect to the steel material immediately after rolling to 200 degrees C or less from the state with a steel material temperature of 800 degrees C or more. Further, the final solution heat treatment was performed on a hot-rolled steel plate under conditions of water cooling after soaking at 1050 ° C. for 20 minutes.

Two steel materials (base materials) obtained by cutting the steel sheet after the final solution heat treatment were prepared, and the joining surfaces of the two steel materials were processed into an I-shaped groove. Then, the joining surfaces of the two steels are opposed, by performing butt welding using a CO ₂ laser welding machine to obtain a weld member number Nanba1～nanba23 (weld joint).

Note that no filler metal (welding rod) was used during laser welding. Further, the welding conditions were the following three AC.
A: Thickness 10 mm of two steel materials, output 7 kV, welding speed 0.6 m / min.
B: Plate thickness of two steel materials 20 mm, output 7 kV, welding speed 0.6 m / min.
C: Plate thickness of two steel materials 10 mm, output 7 kV, welding speed 2 m / min.
The estimated cooling rate (° C./s) “V _CR ” at 1000 ° C. under each of the conditions A to C was calculated using the above equation (6). As a result, the condition of A was 419 ° C./s, the condition of B was 1676 ° C./s, and the condition of C was 4654 ° C./s.

No. 1-No. For the 23 welded members, a calculated value N _prer of the Cr nitride precipitation start temperature calculated by the above formula (1) or (2) was calculated. As the nitrogen concentration (mass%) “N _r ” in the weld metal part in the formula (1) or (2), about 1 g of the weld metal is cut out from the weld metal part of the weld member, and an oxygen / nitrogen analyzer is used. The numerical values measured by performing chemical analysis were used.

In addition, the number No. 1-No. For the 26 welded members, the estimated nitrogen concentration (mass%) “N _re ” in the weld metal part represented by the above formula (4), and the weld metal part by an element other than N represented by the above formula (5) A numerical value “DF _N ” indicating the stability of the ferrite phase is calculated, and the austenite reprecipitation width of the weld metal portion calculated by the above equation (3) using “N _re ”, “DF _N ”, and “V _CR ”. The calculated value γ _{N of} was calculated.

The measurement results of “N _r ” and the calculation results of “N _prer ”, “N _re ”, “DF _N ”, “V _CR ”, and “γ _N ” are shown in Table 3.

In addition, by the following method, the number No. 1-No. About each steel material used as a base material of 26 welding members, the area ratio of the austenite phase was investigated. The results are shown in Table 2.
A sample in which a cross section parallel to the rolling direction of the steel material was embedded in a resin was prepared, and the cross section of the sample was mirror-polished and then subjected to electrolytic etching in a KOH aqueous solution to reveal the cross-sectional structure. Thereafter, the exposed cross section of the sample was photographed at a magnification of 100 using an optical microscope. Then, the area ratio of the ferrite phase was measured by performing image analysis of an image having a field of view of about 30 mm ² taken with an optical microscope, and the remaining portion was defined as the area ratio of the austenite phase.

In addition, by the following method, the number No. 1-No. In 26 welded members, the average width of the austenite phase precipitated at the ferrite phase grain boundaries in the cross section of the weld metal part was examined.
The average value calculated from the results obtained by measuring the width of the austenite phase for any 30 locations on the image taken with an optical microscope of about 30 mm ² used for the measurement of the area ratio of the ferrite phase, The average width of the austenite phase was used.

  FIG. 1 shows an example of a micrograph of a cross section of a weld metal part in a weld member. As shown in FIG. 1, it was found that the grain boundary of the ferrite phase was completely covered with the austenite phase, and precipitation of the film-like Cr nitride was suppressed. Moreover, as shown in FIG. 1, in the weld metal part, the austenite phase precipitated in the ferrite phase grain boundary and the austenite phase precipitated in the ferrite phase grain can be easily distinguished.

In addition, by the following method, the number No. 1-No. The pitting corrosion potential difference of 26 welding members was calculated, and corrosion resistance was evaluated.
From the surface layer from which the excess was removed, a 25 mm wide test piece with the welding line by laser welding as the center was collected, and a test piece having a 12 mm width at the center as the measurement surface was used as a laser welded part (welded metal part and HAZ part) test piece. did. A sample taken from a position 30 mm away from the weld line was used as a base material test piece. Six laser welded specimens and base metal specimens were collected.

Then, the surface of each test piece was polished with an abrasive grain size of # 600, and a current density of 100 μA / cm using a saturated silver chloride electrode (SSE) by a method defined in JIS G0577 for a surface of 1 mm epidermal surface. ^The pitting potential (Vc′100) corresponding to ² was measured. Then, the average value of the result of six test pieces was calculated | required about each of a base material and a laser welding part. And the difference (laser welding part-base material) of the average value of the pitting corrosion potential of a laser welding part and the average value of the pitting corrosion potential of a base material was calculated | required, and it was set as the pitting corrosion potential difference.
When the pitting corrosion potential difference between the laser weld and the base material was 0.15 V or less, it was evaluated that the pitting corrosion potential difference was small and the corrosion resistance was sufficient.
Moreover, when the pitting potential of the base material was 0.25 V (vs SSE) or more, it was evaluated that the base material had sufficient corrosion resistance.

In addition, by the following method, the number No. 1-No. The toughness of the steel used as a base material for 26 welded members was evaluated.
Cut three JIS No. 4 V-notch Charpy specimens from the direction perpendicular to the rolling direction of the steel material, machine the V-notch so that the fracture propagates in the rolling direction, and perform an impact test with a testing machine with a maximum energy of 500 J. The impact value at −20 ° C. was measured. When the impact value of the base material was 100 J / cm ² or more, it was evaluated that the base material had sufficient toughness.

Table 3 shows the evaluation results of the average width of the austenite phase (gamma phase width average value), pitting potential difference, pitting corrosion potential of the base metal, and impact value of the base metal, which were precipitated at the ferrite phase grain boundaries in the weld metal section. Shown in
Numbers shown in Table 3 1-No. It is confirmed that the average value of γ phase width can be estimated with high accuracy by calculating “γ _N ” using the equation (3) from the calculated value of γ _N and the average value of γ phase width of 26 austenite reprecipitation width. did it.

As shown in Table 3, number no. 1-No. 15 weld members have an average value of γ phase width of 2.5 μm or more, N _pler of 1250 ° C. or less, a small pitting potential difference between the base metal and the welded portion, The impact value of was high.
Among these, numbers No. 2 to No. 8 and Nos. 10 to 15 in which the estimated cooling rate (° C./s) “V _CR ” at 1000 ° C. of the weld metal part is 500 ° C./s or more are Nos. Compared with No. 1 and No. 9, the pitting potential difference was small although the γ phase width average value was small.

No. 16 had a high N _preer due to a low Mn content. For this reason, Cr nitride was precipitated, the corrosion resistance of the weld metal part was lowered, and the pitting corrosion potential difference between the base metal and the weld part was increased. In No. 17, although the N _range was high although the component ranges of the respective component elements were satisfied, the difference in pitting potential between the base material and the welded portion was increased as in No. 16.

No. 18 had a low Ni content and therefore the toughness of the base material was low.
No. 19 had a high C content, and No. 20 had a low Cr content, so the pitting corrosion potential of the base material was low.
No. 21 has a large Cr content, a low area ratio of the austenite phase of the base material, and a small average value of the γ phase width of the welded portion, so that the impact value (toughness) of the base material becomes small and the base material The difference in pitting corrosion potential between the weld and the weld was increased. Further, No. 22 had a high N _preer because of the high N content. Therefore, in No. 22, Cr nitride was precipitated, and the pitting potential difference between the base material and the welded portion was increased.

No. 23 had a small N content and a small γ phase width average value, and therefore the pitting potential difference between the base metal and the welded portion was large.
No. No.24, because Cu content is _{high, N PRER} is increased. For this reason, Cr nitride precipitated and the pitting potential difference between the base metal and the welded portion increased.
No. 25 had a low area ratio of the austenite phase of the base material and a low toughness of the base material.
No. 26 is welded under the condition that the calculated austenite reprecipitation width γ _N is less than 2.5 μm because the cooling rate “V _CR ” corresponding to the components (“N _re ” and “DF _N ”) is too fast. It is an example of performing the process. In No. 26, since the average value of the γ phase width was small, the pitting corrosion potential difference between the base metal and the welded portion was large.

  As can be seen from the above examples, it has been clarified that the present invention can provide a duplex stainless steel laser welded member with good laser weld corrosion resistance.

  According to the present invention, a laser welding portion (welded metal), which is one of the major problems in welding Ni-saving or alloy-saving duplex stainless steel with Ni and Mo saved by laser welding without using a filler material. Part and HAZ part) can be prevented from decreasing in corrosion resistance. For this reason, the subject at the time of using an alloy-saving duplex stainless steel for structural materials etc. can be reduced. As a result, the application of the alloy-saving duplex stainless steel can be expanded to an application that replaces austenitic stainless steel at a low cost, and the place that contributes industrially is extremely large.

Claims (8)

The chemical composition is mass%,
C: 0.04% or less,
Si: 0.10 to 1.00%,
Mn: 0.50 to 6.00%,
P: 0.050% or less,
S: 0.0050% or less,
Cr: 20.0-25.0%,
Ni: 1.00 to 6.00%,
N: containing 0.120 to 0.250%, the balance being Fe and inevitable impurities, and the area ratio of the austenite phase being 40% or more and 70% or less, an alloy-saving duplex stainless steel material, a weld metal part, A welding member having
The chemical composition of the weld metal part is composed of the chemical composition of the alloy-saving duplex stainless steel material except N, and the N amount of the weld metal part is less than the N amount of the alloy-saving duplex stainless steel material,
The weld metal part has an average width of an austenite phase precipitated at a grain boundary of a ferrite phase in a cross section of 2.5 μm or more,
The welding member is represented by the following (1) saving alloy duplex stainless steel excellent characteristics of the laser weld, characterized in that calculated values N _PRER the deposition starting temperature of 1250 ° C. or less of Cr nitrides calculated by the formula Steel laser welding member.
_{N prer = 800N r -3Cr + 20Si} + 10Ni-4Mn + 1140 ··· (1)
“N _r ” in the formula (1) indicates the nitrogen concentration (mass%) in the weld metal part, and each element symbol indicates the content (mass%) of the element in the duplex stainless steel material. The duplex stainless steel material is further in mass%,
Mo: 1.50% or less,
Cu: 1 type or 2 types selected from 2.00% or less,
2. The alloy-saving duplex stainless steel laser welded member according to claim 1, wherein N _welder calculated by the following formula (2) is 1250 ° C. or less. .
_{N prer = 800N r -3Cr + 20Si} + 6.5Mo + 10Ni-4Mn + 15Cu + 1140 ··· (2)
“N _r ” in the formula (2) indicates the nitrogen concentration (mass%) in the weld metal part, and each element symbol indicates the content (mass%) of the element in the duplex stainless steel material. The duplex stainless steel material is further in mass%,
V: 0.05 to 0.50%,
Nb: 0.010-0.200%
The alloy-saving duplex stainless steel with good characteristics of the laser weld according to claim 1 or 2, characterized by containing one or more selected from Ti: 0.0030 to 0.050% Steel laser welding member. The duplex stainless steel material is further in mass%,
Al: 0.003-0.045%,
O 2: One or two kinds selected from 0.007% or less are contained. Alloy-saving two-phase with good characteristics of laser welded part according to any one of claims 1 to 3 Stainless steel laser welding member. The duplex stainless steel material is further in mass%,
Ca: 0.0050% or less,
Mg: 0.0050% or less,
REM: 0.050% or less,
B: 1 type or 2 types or more selected from 0.0040% or less are contained, Alloy-saving 2 with the characteristic of the laser-welded part of any one of Claims 1-4 characterized by the above-mentioned Phase stainless steel laser welded member. The duplex stainless steel material is further in mass%,
Zr: 0.020% or less,
Ta: 0.070% or less,
Co: 0.02 to 1.00%,
W: 1.00% or less,
Sn: One or two or more selected from 0.100% or less are contained, and the alloy-saving alloy 2 having good laser welded portion characteristics according to any one of claims 1 to 5 Phase stainless steel laser welded member. A welding step of welding the duplex stainless steel material having the chemical composition according to any one of claims 1 to 6 by laser welding without using a filler material,
The welding process is performed under the condition that the calculated value γ _N of the austenite reprecipitation width of the weld metal part calculated by the following formula (3) is 2.5 μm or more, and the Cr nitride calculated by the following formula (7) A method for producing an alloy-saving duplex stainless steel laser welded member, comprising forming a welded member having a calculated value of N precipitation of N _preer of 1250 ° C. or lower.
γ _N = (1200N _re −1.2DF _N +100) / √ (V _CR ) (3)
“N _re ” in the formula (3) is an estimated nitrogen concentration value (mass%) in the weld metal part represented by the following formula (4) and is 0.20 mass% or less, and “DF _N ” is (5) is a numerical value indicating the stability of the ferrite phase in the weld metal part by an element other than N represented by the formula (5), and “V _CR ” is a cooling rate estimation at 1000 ° C. of the weld metal part represented by the following formula (6). Value (° C./s).
N _re = N−10 × N ² / √ (V _CR ) (4)
“V _CR ” in the formula (4) is a numerical value represented by the following formula (6), and “N” represents a nitrogen content (mass%) in the duplex stainless steel material.
DF _N = 7.2 (Cr + 0.88Mo + 0.78Si) −8.9 (Ni + 0.03Mn + 0.72Cu + 22C) −44.9 (5)
Each element symbol in the formula (5) indicates the content (mass%) of the element in the duplex stainless steel material, and is 0 when the element is not contained in the steel material.
In equation (6), “λ” is the thermal conductivity (W / m / K) of the duplex stainless steel material, “c” is the specific heat (J / kg / K) of the duplex stainless steel material, and “ρ” is the duplex phase. The density of stainless steel (kg / m ³ ), “h” is the material thickness (m) of the welded portion of the duplex stainless steel, “V” is the welding speed (m / s), “P” is the laser output (W ), “T” indicates 1000 ° C. , and “T ₀ ” indicates room temperature (20 ° C.).
_{N prer = 800N r -3Cr + 20Si} + 6.5Mo + 10Ni-4Mn + 15Cu + 1140 ··· (7)
“N _r ” in the formula (7) indicates the nitrogen concentration (mass%) in the weld metal part, and each element symbol indicates the content (mass%) of the element in the duplex stainless steel material. 0 is not included in the steel material. The method for producing an alloy-saving duplex stainless steel laser welded member according to claim 7, wherein the _{VCR is set} to 500 ° C / s or more.