JP2007118025A - B-containing stainless steel and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method where, even under welding conditions of high efficiency, a B-containing stainless steel slab is hot-worked without generating welding cracks upon the operation and edge cracks after the rolling, and to provide a steel after the working. <P>SOLUTION: Regarding the B-containing stainless steel slab, a protect material composed of a stainless steel comprising, by mass, ≤0.3% B is joined and integrated into at least confronted two faces other than the working face of a stainless steel slab comprising 0.3 to 2.5% B by a stainless steel weld metal, the chemical composition of the stainless steel weld metal satisfies the relation expressed by fundamental inequalities of (1) to (3), and also, the shape coefficient Q (Wcm/Wn) in the cross-section of a weld bead shown by a ratio between the bead width Wcm at the central part of the cross-section in the weld bead and the bead width Wn at the inlet part is 0.8 to 1.4; wherein, 15≤Creq≤30 (1), 4≤Creq-Nieq≤17 (2), and Px≥0 (3). <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention, as a neutron shielding material for nuclear-related equipment such as a nuclear fuel transport container, a spent nuclear fuel storage rack, and the like, an application that exhibits an excellent function by containing 0.3% or more B in austenitic stainless steel, For example, the present invention relates to a B-containing stainless steel piece used as a fuel cell separator material and a method for producing a B-containing stainless steel material.

  Utilizing the excellent thermal neutron absorption effect of boron (B), austenitic stainless steel added with B is used in nuclear fuel transport containers, spent nuclear fuel storage racks, etc. as thermal neutron control and blocking materials Yes. In general, spent nuclear fuel used at a nuclear power plant is stored in a pool within the power plant until it is processed at a reprocessing plant. Due to the need to store as much spent nuclear fuel as possible within a limited site, the B content added to the B-containing austenitic stainless steel tends to increase, and the thickness of the steel material tends to be reduced.

  Austenitic stainless steel is excellent in corrosion resistance because a passive body film is formed on its surface, and by adding B, the electrical resistance characteristics are improved, and as a current-carrying electrical component that requires corrosion resistance. It becomes possible to use. As an example of application of a current-carrying electrical component that requires electrical resistance characteristics as well as excellent corrosion resistance, there is a fuel cell separator that generates DC power using hydrogen and oxygen.

  Hot working of B-containing stainless steel is performed while ensuring hot workability by preventing the temperature of the work piece from decreasing by repeatedly heating the slab with a heating furnace and processing such as forging and rolling. ing. When the B content increases, the hot workability is inferior, and therefore, it is necessary to perform processing while preventing the temperature of the workpiece from being lowered, and as a result, the number of repetitions of heating and processing must be increased. Therefore, an increase in the B content and a thin processing of steel cause an increase in manufacturing cost.

  Conventionally, various studies have been made in order to deal with the above-described problems. For example, Patent Document 1 discloses a material in which a side portion of an austenitic stainless steel material containing 0.3 to 2.0 wt% of B is coated with a steel material having a deformation resistance smaller than that of a stainless steel material by welding (53B + 700). A hot-rolling method of a steel material that prevents the occurrence of edge cracking by finish rolling at a temperature equal to or higher than ° C. (here, B: B content (wt%)) is disclosed.

  In the method disclosed in Patent Document 1, it is necessary to prepare a frame material having a groove shape with high accuracy, and to perform welding so that it does not peel off during hot working. Therefore, in order to apply this method to hot working of an ingot (cast steel ingot) or a forged slab having a thickness of 80 mm or more, a large number of welding steps are usually required.

  In the method of Patent Document 1, it is often difficult to secure a finishing temperature equal to or higher than the above temperature when rolling a wide material having a width exceeding 1000 mm. Have difficulty.

  In Patent Document 2, when hot-rolling an austenitic stainless steel piece containing 0.3 to 2.5% by mass of B, Ni: 4% or less, B: 0.1 to 0.4 on the side surface thereof. A method of hot working by providing a build-up weld coating layer having a thickness of 3 mm or more made of stainless steel containing 1% is disclosed.

  In the build-up welding method of Patent Document 2, the number of welding passes increases and the number of welding steps increases in order to ensure a sufficient welding thickness to prevent cracking. In addition, when a weld crack occurs, it may become a starting point and lead to the occurrence of an ear crack, and it is difficult to completely prevent the occurrence of the ear crack.

  As a means for solving these problems, the present inventors have developed a technique capable of rolling a stainless steel piece having a high B content to a predetermined plate thickness without generating an ear crack with a small number of welding steps. 3. B-containing stainless steel pieces welded to the side surfaces by electron beam welding of the protective material, B-containing stainless steel materials, and B-containing stainless steel materials that can prevent the occurrence of ear cracks in the material to be rolled during hot rolling A manufacturing method was proposed.

  In this protection material welding method, under normal welding conditions (for example, a welding current of 300 mA or more and a welding speed of 200 mm / min or less), a predetermined effect is exhibited without causing solidification cracks in the welded portion. be able to. However, even with the proposed welding method of the protection material, there is a concern that solidification cracks may occur in the welded part under high-efficiency welding conditions, that is, under conditions where the welding current is large and the welding speed is large.

JP-A-4-253506 JP 2001-239364 A JP 2004-156132 A

  The problem of the present invention is that even if the number of welding man-hours is high under high-efficiency welding conditions, no cracking occurs at the time of construction. An object of the present invention is to provide a hot rolling method, a cold rolling method, and a steel material having a high B content. More specifically, for example, a crack is generated in the B-containing stainless steel piece welded to the side surface by high-efficiency electron beam welding, for example, a welding current of 500 mA or more and a welding speed of 200 mm / min or more. Furthermore, it is providing the manufacturing method of the B containing stainless steel material which can prevent generation | occurrence | production of an ear crack even if it is a B containing stainless steel material and the to-be-rolled material in hot rolling.

  In order to solve the above-mentioned problems, the present inventors can perform a stable hot rolling method and cold rolling method even when highly efficient welding is performed, and a high-quality steel material having a high B content can be obtained. As described above, various studies have been conducted on techniques capable of avoiding cracks during welding (weld solidification cracks).

  The weld solidification cracking is due to a superposition effect of the formation of a low melting point phase by B and the specific thermal stress in the welded part due to electron beam welding or the like. As disclosed in Patent Document 3 previously proposed, by setting the value of (Creq-Nieq) to 4 or more, the ferrite phase can remain until the late solidification stage and the low melting point phase can be dispersed. Solidification cracking can be avoided even under high thermal stress conditions such as welding.

  However, if the welding conditions become severe and the welding speed and heat input further increase, it becomes difficult to completely avoid weld solidification cracking, and it is certain to apply high-efficiency electron beam welding to B-containing stainless steel pieces. There will be restrictions.

  As these solutions, it has been found that it is effective to eliminate the low melting point phase due to B, which is a metallurgical factor of weld solidification cracking, at an early stage of the solidification process. Specifically, by adjusting the chemical composition of the weld metal and combining the control of the bead cross-sectional shape under appropriate welding conditions, it is possible to control the formation of boride and the residual melt composition without generating a ferrite phase, It was found that the low melting point phase due to B can be eliminated early.

The present invention has been completed based on the above findings, and the gist of the present invention is the following (1), (2) B-containing stainless steel pieces, (3), (4) B-containing stainless steel material manufacturing method And (5) the neutron shielding container and the fuel cell separator.
(1) A protective material made of stainless steel containing 0.3% by mass or less of B on at least two opposing surfaces excluding the processed surface of a stainless steel piece containing 0.3 to 2.5% by mass of B is stainless steel. It is joined and integrated by weld metal, the chemical composition of the stainless steel weld metal satisfies the relationship expressed by the following formulas (1) to (6), and between the stainless steel piece and the protect material: When the bead width Wcm at the center in the depth direction in the weld bead cross section and the bead width Wn at the entrance in the depth direction are the shape factors Q (Wcm / Wn) in the weld bead cross section indicated by these ratios ) Is 0.8 to 1.4, a B-containing stainless steel piece.

15 ≦ Creq ≦ 30 (1)
4 ≦ Creq−Nieq ≦ 17 (2)
Px ≧ 0 (3)
However,
Creq = Cr + 1.5 × Si + Mo−5 × B (4)
Nieq = Ni + 30 × (C + N) + 0.5 × Mn (5)
Px = Cr + 0.47 × Ni + 0.22 × Mo + 25 × B−33 (6)
Here, the element symbol in the formula represents the content (mass%) of each element contained in the weld metal. (2) The B-containing stainless steel piece described in (1) is a stainless steel piece and a protective material. In the meantime, it is desirable to interpose an insert material containing 0.4 to 2.5% by mass of B. Furthermore, it is desirable that the thickness of the protection material is 10 mm or more.
(3) A protective material made of stainless steel containing 0.3% by mass or less of B is bonded to at least two opposing surfaces excluding the processed surface of the stainless steel piece containing 0.3 to 2.5% by mass of B. And a chemical composition satisfying the relationship expressed by the above formulas (1) to (6) when joining the protect material using electron beam welding. When the welded stainless steel weld metal is melted and the bead width Wcm at the center in the depth direction and the bead width Wn at the entrance in the depth direction in the weld bead cross section between the stainless steel piece and the protect material In addition, the shape factor Q (Wcm / Wn) in the cross section of the weld bead indicated by these ratios is joined as 0.8 to 1.4, and after heating, it is processed. Law.
(4) In the method for producing a B-containing stainless steel material according to (3), an insert material containing 0.4 to 2.5% by mass of B is interposed and joined between the stainless steel piece and the protect material. It is desirable. Further, it is desirable that the thickness of the protect material is 10 mm or more.
(5) It is desirable to use the B-containing stainless steel material produced by the method described in (3) and (4) above as a neutron shielding container or a fuel cell separator.

  FIG. 1 is a diagram schematically showing a B-containing stainless steel piece of the present invention. FIG. 2 is a diagram schematically showing a cross section of a weld bead formed between a stainless steel piece and a protect material.

  In the present invention, the “stainless steel piece” refers to a continuously cast slab, a forged slab, a ingot rolled slab, and a cast ingot (steel ingot), and the base material 1 shown in FIG. 1 corresponds to this. These steel pieces are generally rectangular parallelepipeds, and are subjected to hot working such as hot rolling or forging so as to be stretched in the longitudinal direction (indicated by white arrows 6).

  “At least two opposing surfaces excluding processed surfaces” refers to at least two opposing surfaces among the surfaces other than the processed surface 3 subjected to processing such as rolling and forging. For example, in the case of rolling, two side surfaces in the longitudinal direction that do not come into contact with the rolling rolls, or end surfaces of the head and tail including these may be included. In the case of forging, two opposing side surfaces that do not contact the ram, or three to four side surfaces including these may be included. In addition, when chamfering the corner part of a steel piece, you may include the chamfered surface.

  The “thickness of the protect material” means the thickness 5 of the protect material from the side surface of the steel slab in the plane parallel to the processed surface before joining the protect material to the base material as shown in FIG. . In the steel piece after joining, it means the total thickness of the thickness of the protective material alone and the thickness of the weld metal in the protective material.

  The “welded metal” refers to a weld metal portion 7 which is a part of the joint portion and in which the base material and the protect material before joining are melted and solidified by joining. As shown in FIG. 2, the cross-sectional shape of the weld metal portion 7 can be schematically shown by a bead width Wn at the inlet portion in the depth direction and a bead width Wcm at the center portion in the depth direction. Therefore, the “shape factor Q in the weld bead cross section” defined in the present invention is indicated by the ratio of the bead width Wcm at the center and the bead width Wn at the inlet (Q = Wcm / Wn).

  The “insert material” is a material that is inserted or sandwiched between a stainless steel piece (base material) and a protect material, and specifically includes materials such as plates, foils, and powders.

  According to the method of the present invention, high-efficiency welding conditions (for example, a welding current of 500 mA or more and a welding speed of 200 mm / min or more) are used for electron beam welding with a high B content to reduce costs. Even when the protective material is joined to the side surface of the piece and rolled, it is possible to prevent the occurrence of weld cracking during construction and ear cracking after rolling, and to provide a B-containing stainless steel material having high productivity and excellent quality. Furthermore, if insert material is used at the time of welding joining, the cracking sensitivity of a weld metal can be reduced further.

  In order to produce a high-quality stainless steel with a high B content at low cost, the present inventors use a high-efficiency welding condition electron beam welding method to protect the steel piece with a certain thickness on the side surface. The method of joining the materials and making them steel by processing such as rolling was investigated. Table 1 summarizes the chemical composition of the base steel piece and the protective material used in the test.

1. Steel component composition of base material stainless steel piece B: 0.3 to 2.5%
If the B content in the B-containing stainless steel piece, which is a material to be hot-worked, is less than 0.3%, the thermal neutron absorption capacity is not sufficient, and the electrical resistance characteristics of the fuel cell separator material are not sufficiently improved. The B content is 0.3% or more. The thermal neutron absorption ability and electrical resistance characteristics improve with an increase in the B content. However, when the B content exceeds 2.5%, ductility and toughness at room temperature deteriorate significantly, so the content is 2.5. % Or less.

  Further, the base material may be austenitic stainless steel or ferritic stainless steel, but is limited to austenitic stainless steel when functioning as a fuel cell separator material.

  The B-containing stainless steel targeted by the present invention defines the above-described B content, but desirable ranges of other steel component compositions are as follows.

C: 0.08% or less C is an element having an effect of ensuring strength. However, if it exceeds 0.08%, it causes corrosion resistance deterioration and hot workability deterioration. Therefore, the content is preferably 0.08% or less. If it is 0.01% or more, it is more desirable.

Si: 1% or less Si is added as a deoxidizing agent, but is also an element having an action of improving oxidation resistance. However, if the content exceeds 1%, the weld cracking sensitivity becomes high. Therefore, the content is desirably 1% or less.

P: 0.04% or less P is an impurity element in steel, and if its content exceeds 0.04%, the weld cracking sensitivity becomes high, so 0.04% or less is desirable. .

S: 0.01% or less S is an impurity element in steel, and if its content exceeds 0.01%, the weld cracking sensitivity becomes high, so 0.01% or less is desirable. .

Cr: 5% or more Cr is an element having a function of improving the corrosion resistance, and its content is 5% or more, and a desirable effect is obtained. Therefore, the content is desirably 5% or more. On the other hand, if the content exceeds 30%, hot working may be difficult, so the content is more preferably 30% or less.

N: 0.05% or less N combines with B to deteriorate toughness. In order to ensure sufficient toughness, the content is desirably 0.05% or less.

Mo: 5% or less, Cu: 0.5% or less, and Al: 0.3% or less If these elements are contained as necessary within the above range of content, the corrosion resistance is further improved. It is effective. Therefore, when these effects are required, it is desirable to contain them alone or in combination within the above content range.
2. Component composition of weld metal The component composition of the weld metal that forms the joint between the protect material and the base material will be described. When the protective material is joined to the base material with high efficiency by electron beam welding or the like, it is essential to avoid cracking that occurs during welding and to prevent cracking during hot rolling of the protective material itself. Cracks generated during welding work include solidification cracks and ductility deficient cracks. In order to prevent all of these cracks, it is necessary that the chemical composition of the weld metal that constitutes the joint between the protect material and the base material satisfy the relationship represented by the following equations (1) to (6). The element symbol in the following formula indicates the content of each element contained in the weld metal.

15 ≦ Creq ≦ 30 (1)
4 ≦ Creq−Nieq ≦ 17 (2)
Px ≧ 0 (3)
However,
Creq = Cr + 1.5 × Si + Mo−5 × B (4)
Nieq = Ni + 30 × (C + N) + 0.5 × Mn (5)
Px = Cr + 0.47 × Ni + 0.22 × Mo + 25 × B−33 (6)
The reason why each of the above equations (1) to (3) must be satisfied when Creq, Nieq and Px are defined by the above equations (4) to (6) will be described in detail below.

15 ≦ Creq ≦ 30:
When the value of Creq is less than 15, due to dilution with B-containing stainless steel, a weld metal containing B is generated and the ductility is insufficient, and when thermal stress becomes large, such as welding by electron beam welding, Insufficient ductility cracking occurs. The lack of ductility is caused by the austenite phase becoming unstable with the formation of borides and partly martensite having poor ductility.

  However, by setting the value of Creq to 15 or more, the austenite phase becomes stable even when borides are generated, and the formation of martensite can be suppressed and ductility deficiency can be avoided.

  On the other hand, if the value of Creq exceeds 30, the hot workability of the weld metal deteriorates and cracks occur during hot rolling. Therefore, the value of Creq needs to satisfy the relationship represented by the above equation (1).

4 ≦ Creq−Nieq ≦ 17:
In order to avoid weld solidification cracks that occur during welding and to prevent ear cracks during hot rolling, it is not sufficient for the value of Creq to satisfy the relationship (1) above, (Creq-Nieq) The value of must also be adjusted to an appropriate range. If the value of (Creq-Nieq) is less than 4, weld solidification cracks occur, whereas if the value of (Creq-Nieq) exceeds 17, ear cracks after hot rolling occur.

  The weld solidification cracking is due to the superposition effect of the formation of a low melting point phase by B and the thermal stress peculiar to electron beam welding or the like. By setting the value of (Creq-Nieq) to 4 or more, it is possible to leave the ferrite phase up to the late stage of solidification and disperse the low melting point phase, so that solidification cracking can occur even under high thermal stress such as electron beam welding. Can be avoided. Therefore, the value of (Creq−Nieq) needs to satisfy the relationship of the above expression (2).

Px ≧ 0:
In addition, even in the case of high heat input welding with high efficiency and high industrial value, in order to prevent weld solidification cracking, the low melting point phase due to B, which is a factor on the metallurgical factor side, is introduced at an early stage of solidification. It is important to disappear. When the relationship of the formula (3) is satisfied, the activity of B in the remaining melt increases, and the boride crystallizes out of the melt rapidly. As a result, the B concentration in the residual melt rapidly decreases, and the melting point of the residual melt increases. An abrupt increase in the melting point of the residual melt means that solidification is rapidly completed, so that no weld solidification cracking occurs even in high heat input welding.

  On the contrary, when the relationship of the formula (3) is not satisfied, the activity of B in the residual melt does not increase, so that borides are hardly generated, and a low melting point melt remains until the latter stage of solidification. It will be. For this reason, in high heat input welding with a large thermal stress, it cannot resist the shrinkage strain due to solidification, and solidification cracks occur.

  For the above-mentioned reasons, adjusting the component composition of the weld metal to the appropriate range represented by the formulas (1) to (6) can prevent the weld solidification cracks that occur during welding when the protective material is welded with high efficiency. This is an indispensable requirement to avoid and prevent cracking of the weld metal after hot rolling.

  The essential requirements to be included in the component composition of the weld metal of the present invention are as described above. The desirable ranges of the other component compositions are as follows.

C: 0.08% or less C is an element having an effect of ensuring effective strength for suppressing deformation during heating of a steel slab. However, if the content exceeds 0.08%, the hot workability deteriorates. Therefore, the content is preferably 0.08% or less. If it is 0.01% or more, it is more desirable.

Si: 1% or less Si is added as a deoxidizer, but is also an element having an effect of improving oxidation resistance. However, if the content exceeds 1%, the weld cracking sensitivity becomes high. Therefore, the content is desirably 1% or less.

P: 0.04% or less P is an impurity element in steel, and if its content exceeds 0.04%, the weld cracking sensitivity becomes high, so 0.04% or less is desirable. .

S: 0.01% or less S is an impurity element in steel, and if its content exceeds 0.01%, the weld cracking sensitivity becomes high, so 0.01% or less is desirable. .

Cr: 5% or more Cr is an element having a function of improving the corrosion resistance, and its content is 5% or more, and a desirable effect is obtained. Therefore, the content is desirably 5% or more. On the other hand, if the content exceeds 30%, hot working may be difficult, so the content is more preferably 30% or less.
3. Component composition of stainless steel for protective material B: 0.3% or less If the B content of steel for protective material exceeds 0.3%, the protective material itself will be applied to the ends where the tension during rolling is high. Cracks occur and the protective material is no longer effective. Therefore, the content is made 0.3% or less.

  The content of other elements such as Cr and Ni is substantially welded because the weld metal produced by melting and mixing with the base material needs to satisfy the expressions (1) to (6). Restricted by metal component composition.

  When the insert material is not used, the component composition of the weld metal generated when joining by electron beam welding is a value close to the arithmetic mean value of the respective component compositions of the base material and the protect material. Accordingly, when the component composition of the base material to be used is determined, the range of the component composition of Ni, Cr, etc. other than B in the protect material can be obtained using the above equations (1) to (6).

  In the case of using an insert material, the arithmetic average value of the base material and the protect material and the weighted average of the insert material are substantially the component composition of the weld metal. At this time, the weight ratio of the insert material in the weld metal is used as the weight of the insert material in the weighted average.

  The protective material targeted by the present invention defines the above-mentioned B content, but for the other component compositions, in addition to the component composition range obtained from the above conditions, the desirable component composition range of the protective material is: It is as follows.

C: 0.08% or less C is an element having an effect of ensuring effective strength for suppressing deformation during heating of a steel slab. However, if it exceeds 0.08%, it causes hot workability deterioration. Therefore, the content is preferably 0.08% or less. If it is 0.01% or more, it is more desirable.

Si: 1% or less Si is added as a deoxidizer, but is also an element having an effect of improving oxidation resistance. However, if the content exceeds 1%, the weld cracking sensitivity becomes high. Therefore, the content is desirably 1% or less.

P: 0.04% or less P is an impurity element in steel, and if its content exceeds 0.04%, the weld cracking sensitivity becomes high, so 0.04% or less is desirable. .

S: 0.01% or less S is an impurity element in steel, and if its content exceeds 0.01%, the weld cracking sensitivity becomes high, so 0.01% or less is desirable. .

Cr: 5% or more Cr is an element having a function of improving the corrosion resistance, and its content is 5% or more, and a desirable effect is obtained. Therefore, the content is desirably 5% or more. On the other hand, if the content exceeds 30%, hot working may be difficult, so the content is more preferably 30% or less.
4). Shape factor Q in weld bead cross section
The fact that it is necessary to control the bead cross-sectional shape in the present invention will be described. As described above, in order to prevent weld solidification cracking, it is important to eliminate the low melting point phase due to B at an early stage of the solidification process. However, in actual welding, it is not sufficient to control only “Px ≧ 0” according to the above equation (3), and a geometrical solid-liquid phase arrangement is also necessary to eliminate the low melting point phase at an early stage. Plays an important role.

  Specifically, in order to make the disappearance of the low melting point earlier, as shown in FIG. 2, the bead width at the center in the depth direction in the cross section of the weld bead between the stainless steel piece and the protect material. In the case of Wcm and the bead width Wn at the entrance in the depth direction, the shape factor Q (Wcm / Wn) in the weld bead cross section indicated by these ratios needs to be 0.8 to 1.4.

When the shape factor Q is 1.4 or more, that is, when the bead width Wcm is relatively large at the center in the bead depth direction, the melt remains in the center of the bead in the process of solidification from the outside. And it becomes easy to produce a weld solidification crack in the center part. On the other hand, when the shape factor Q is 0.8 or less, a residual phenomenon of the melt occurs at the entrance portion in the bead depth direction similarly to the above-described central portion, and cracks easily occur at the entrance portion. In actual operation, the shape control of the weld bead cross section becomes possible by setting the welding conditions.
5). Necessity of insert material and its component composition As described above, when welding is performed in a combination in which a stainless steel piece is a low B-containing material of, for example, 0.3 to 0.8% and a protective material does not contain B, Crack susceptibility is increased and weld cracking may occur.

  This is because, in a weld metal diluted with a protect material, the content of B in the stainless steel piece is low, boride formation from the liquid phase is less likely to occur, and disappearance of the low melting point phase due to B tends to be delayed. As a result, the susceptibility of the weld metal to solidification cracking is increased, and weld cracking may occur when welding is performed under higher efficiency conditions.

  On the other hand, if the B content of the protect material is increased, the B amount of the weld metal will increase, but with this, the hot workability will deteriorate, and the original function of the protect material can be exhibited. Disappear. For this reason, it is desirable that the insert material containing B be used by being inserted or sandwiched between the stainless steel piece and the protective material. The B content and other component compositions at this time are preferably as follows.

B: 0.4 to 2.5%
If the B content of the insert material is less than 0.4%, the thermal neutron absorption capacity is not sufficient in consideration of dilution during welding, and the electric resistance characteristics of the fuel cell separator material are not sufficiently improved. The content is 0.4% or more. On the other hand, if the B content exceeds 2.5%, ductility and toughness at room temperature deteriorate significantly, so the content is set to 2.5% or less.

  When the insert material is used in the present invention, the B content of the insert material is as described above, but other desirable ranges of the component composition are as follows.

C: 0.08% or less C is an element having an effect of ensuring strength. However, if it exceeds 0.08%, it causes corrosion resistance deterioration and hot workability deterioration. Therefore, the content is preferably 0.08% or less. If it is 0.01% or more, it is more desirable.

Si: 1% or less Si is added as a deoxidizing agent, but is also an element having an action of improving oxidation resistance. However, if the content exceeds 1%, the weld cracking sensitivity becomes high. Therefore, the content is desirably 1% or less.

P: 0.04% or less P is an impurity element in steel, and if its content exceeds 0.04%, the weld cracking sensitivity becomes high, so 0.04% or less is desirable. .

S: 0.01% or less S is an impurity element in steel, and if its content exceeds 0.01%, the weld cracking sensitivity becomes high. .

Cr: 5% or more Cr is an element having a function of improving the corrosion resistance, and its content is 5% or more, and a desirable effect is obtained. Therefore, the content is desirably 5% or more. On the other hand, if the content exceeds 30%, hot working may be difficult, so the content is more preferably 30% or less.
6). Thickness of protect material In order to prevent cracking of the steel material, the thickness of the protect material is preferably 10 mm or more. As the thickness increases, the effect of preventing the ear cracks increases. However, excessively increasing the thickness is not preferable because the yield of the protective material deteriorates. For the above reasons, it is desirable that the thickness of the protect material be 50 mm or less.
7). Desirable Conditions for Electron Beam Welding Conventionally, stainless steel pieces with a high B content are rolled with a small number of welding steps, without causing weld cracks during construction, and at the same time, without causing edge cracks. As a condition for electron beam welding, a welding current of 300 mA or more and a welding speed of 200 mm / min or less have been proposed (for example, Patent Document 3).

  However, by adjusting the chemical composition of the weld metal and combining the control of the bead cross-sectional shape with appropriate welding conditions, it becomes possible to eliminate the low melting point phase due to B at an early stage of the solidification process. Even under the various welding conditions, it becomes clear that a stainless steel piece having a high B content can be rolled with a small number of welding steps without causing a weld crack and without causing an ear crack to a predetermined plate thickness.

Accordingly, it is required to produce a stainless steel having a high cost and a high B content at a low cost. In order to meet such demands, specifically, it is desirable to apply highly efficient electron beam welding in which the welding current is 500 mA or more and the welding speed is 200 mm / min or more.
8). Hot working and cold working Hot working refers to partial forging, thick plate rolling, hot rolling steel strip rolling and the like. It is desirable that the heating temperature of the steel slab is high as long as it does not cause melt brittleness. In the case of B-containing stainless steel, the temperature is preferably in the range of 1100 to 1200 ° C.

  A higher finishing temperature in hot forging or hot rolling is desirable from the viewpoint of preventing ear cracks. However, as long as the hot deformability of the protective material permits, it can be a low temperature finish of 600 to 700 ° C.

  Also, when using B-containing stainless steel as a fuel cell separator material, after hot working, cold-rolled steel strip rolling is applied as a cold working to finish the cold-rolled steel sheet. Press-molded into a predetermined cross-sectional shape. The B-containing stainless steel material supported by the high reliability and productivity obtained as described above is suitable as a steel material for applications that exhibit functions such as a neutron shielding container and a fuel cell separator material.

Using a base material and a protective material whose chemical composition satisfies the range defined in the present invention, a laboratory test was performed to evaluate the occurrence of weld cracks during welding and ear cracks after rolling.
(Production of test materials and rolling conditions)
As a material to be rolled, a slab having a width of 200 mm, a thickness of 50 mm, and a length of 100 mm made of B-containing austenitic stainless steels having steel numbers M1 to M5 in Table 1 is used. The test materials were manufactured by joining the protective materials made of stainless steels Nos. P1 to P7 with electron beam welding.

  The conditions for electron beam welding were welding current: 560 mA, welding speed: 210 mm / min, beam amplitude: ± 2 mm, and welding work was performed under conditions with high construction efficiency. At this time, the focus position and the objective distance (conditions A to D) shown in Table 2 were changed to obtain weld metal parts having weld bead cross-sections having different shape factors Q.

  When electron beam welding was performed under the above conditions, the B-containing austenitic stainless steel base material and the stainless steel protect material were each melted over about 5 mm, and a weld metal part having a total thickness of about 10 mm was formed. . Table 3 shows the chemical composition of the weld metal part. For the test material T4 only, a metal plate (thickness: 3 mm, component composition: 35% by mass, Cr: 35%, Ni: 4%, the balance being Fe and inevitable impurities) was inserted for component adjustment.

  The protective material was polished after electron beam welding, and the thickness of the protective material alone (the thickness of the protective material excluding the weld metal) from the side surface end surface of the base material slab in the plane parallel to the processed surface was adjusted to 10 mm.

  The obtained slab was heated at 1180 ° C. for 1 hour or longer in a heating furnace and rolled at a finishing temperature of 600 to 700 ° C. Hot rolling of multiple passes is performed using a two-stage rolling mill with a work roll diameter of 350 mm so that the finished sheet thickness is 1 mm and the total reduction ratio (initial sheet thickness / finished sheet thickness) is 50.0. It was.

In this example, the total reduction ratio was set to 50.0, which was the same as or higher than the value of the total reduction ratio assumed in actual operation. That is, when the total rolling reduction ratio is small, it is difficult for the ear cracks to occur, and this is not a laboratory test that can accurately evaluate the ear cracks simulating actual operation.
(Evaluation results)
Based on the results of the above laboratory tests, the shape factor Q of the weld bead section, the presence or absence of weld cracks, and the evaluation of the ear cracks after rolling were evaluated.

  The shape factor Q of the weld bead cross section is measured by polishing the cut surface at a cross section perpendicular to the weld line from a so-called steady bead portion with a stable weld bead width to a roughness of # 600, and then etching to form a penetration shape. And the bead width Wn at the inlet portion in the depth direction and the bead width Wcm at the center portion (see FIG. 2) were measured. Based on the measured values, the shape factor Q (Wcm / Wn) of the weld bead cross section was obtained.

  Next, the presence or absence of weld cracks at the time of construction was confirmed by ultrasonic inspection, and indicated by no cracks (◯ mark) and cracks (x mark). The evaluation of the occurrence of ear cracks after rolling was performed by visually observing the crack occurrence situation with respect to the entire width end portion of the test material after rolling. ), The case where the length of the crack was 0.1 mm or more was regarded as having a crack (x mark).

  Furthermore, from the chemical composition of the weld metal part shown in Table 3, the value of Creq, the value of (Creq-Nieq), and the Px value represented by the formulas (1) to (3) were determined for each test material. . These results are shown in Table 4 together with the measurement of the shape factor Q of the weld bead section, the presence or absence of weld cracks, and the evaluation results of the ear cracks after rolling.

  As is clear from Table 4, the present invention example (test material T1) satisfying the relationship represented by the formulas (1), (2) and (3) defined in the present invention and satisfying the shape factor Q of the weld bead section. ~ T4), welding current: 560 mA, welding speed: 210 mm / min, even when a large construction efficiency is applied, there is no weld cracking at the time of construction and no occurrence of ear cracks after rolling. A high quality B-containing stainless steel material was obtained.

  In contrast, the test materials T5 and T6 that do not satisfy the formula (1) defined in the present invention, and the test materials T7 and T8 that do not satisfy the formula (2) cause weld cracks or ear cracks, and good results are obtained. I couldn't.

  On the other hand, all of the test materials T9 to T11 that did not satisfy the shape factor Q of the weld bead cross section even when the expressions (1), (2), and (3) defined in the present invention were satisfied, caused weld cracks. Moreover, all of the test materials T12 to T15 that did not satisfy the formula (3) defined in the present invention also caused weld cracks.

  Regarding the insert material, the test materials T4 and T15 had the same base material, protect material, and welding conditions, but the test material T4 inserted with metal foil as the insert material had no weld cracks and no rolling cracks. On the other hand, in the test material T15 in which no insert material was inserted, the expression (3) defined in the present invention could not be satisfied, and a weld crack occurred.

  According to the method of the present invention, high-efficiency welding conditions (for example, a welding current of 500 mA or more and a welding speed of 200 mm / min or more) are used for electron beam welding with a high B content to reduce costs. Even when the protective material is joined to the side of the piece and rolled, it is possible to prevent the occurrence of weld cracking during construction and ear cracking after rolling, and to provide a B-containing stainless steel material with high productivity and excellent quality. Furthermore, if insert material is used at the time of welding joining, the cracking sensitivity of a weld metal can be reduced further.

  Thereby, the B-containing stainless steel material of the present invention can be widely applied as, for example, a B-containing stainless steel material that exhibits functions such as a neutron shielding container and further a fuel cell separator material.

It is a figure which shows typically the B containing stainless steel piece of this invention. It is a figure which shows typically the weld bead cross section formed between a stainless steel piece and a protection material.

Explanation of symbols

1. 1. Base material Protective material 3. Processing surface, 4. Thickness 5. 5. thickness of protective material Rolling direction Weld metal part Wn: Bead width at the inlet in the depth direction in the weld bead section Wcm: Bead width at the center in the depth direction in the weld bead section

Claims (8)

A protective material made of stainless steel containing B in an amount of 0.3% by mass or less is formed of stainless steel weld metal on at least two opposing surfaces excluding the processed surface of a stainless steel piece containing B in an amount of 0.3 to 2.5% by mass Joined and integrated,
The chemical composition of the stainless steel weld metal satisfies the relationship represented by the following formulas (1) to (6), and the central portion in the depth direction in the weld bead cross section between the stainless steel piece and the protect material When the bead width is Wcm and the bead width is Wn at the entrance in the depth direction, the shape factor Q (Wcm / Wn) in the weld bead cross section indicated by these ratios is 0.8 to 1.4. A B-containing stainless steel piece characterized by the above.
15 ≦ Creq ≦ 30 (1)
4 ≦ Creq−Nieq ≦ 17 (2)
Px ≧ 0 (3)
However,
Creq = Cr + 1.5 × Si + Mo−5 × B (4)
Nieq = Ni + 30 × (C + N) + 0.5 × Mn (5)
Px = Cr + 0.47 × Ni + 0.22 × Mo + 25 × B−33 (6)
Here, the element symbol in a formula represents content (mass%) of each element contained in a weld metal.   The B-containing stainless steel piece according to claim 1, wherein an insert material containing 0.4 to 2.5 mass% of B is interposed between the stainless steel piece and the protect material.   The B-containing stainless steel piece according to claim 1 or 2, wherein the thickness of the protect material is 10 mm or more. A protective material made of stainless steel containing 0.3% by mass or less of B is joined to and integrated with at least two opposing surfaces excluding the processed surface of the stainless steel piece containing 0.3 to 2.5% by mass of B. A method for producing a B-containing stainless steel material,
When joining the protective material using electron beam welding, a stainless steel weld metal having a chemical composition satisfying the relationship represented by the following formulas (1) to (6) is melted, and the stainless steel piece and the When the bead width Wcm at the center in the depth direction and the bead width Wn at the entrance in the depth direction in the weld bead cross section between the protective materials, the shape factor Q in the weld bead cross section indicated by these ratios is used. A method for producing a B-containing stainless steel material characterized in that (Wcm / Wn) is joined as 0.8 to 1.4, and is processed after heating.
15 ≦ Creq ≦ 30 (1)
4 ≦ Creq−Nieq ≦ 17 (2)
Px ≧ 0 (3)
However,
Creq = Cr + 1.5 × Si + Mo−5 × B (4)
Nieq = Ni + 30 × (C + N) + 0.5 × Mn (5)
Px = Cr + 0.47 × Ni + 0.22 × Mo + 25 × B−33 (6)
Here, the element symbol in a formula represents content (mass%) of each element contained in a weld metal.   5. The production of a B-containing stainless steel material according to claim 4, wherein an insert material containing 0.4 to 2.5 mass% of B is interposed between the stainless steel piece and the protect material. Method.   The method for producing a B-containing stainless steel material according to claim 4 or 5, wherein the thickness of the protection material is 10 mm or more.   A neutron shielding container using the B-containing stainless steel material produced by the method according to claim 4. A separator for a fuel cell using the B-containing stainless steel material produced by the method according to any one of claims 4 to 6.

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