JP2013086134A - Welded joint and welding material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-strength and high-toughness welded joint which is obtained by welding a high-strength base material without preheating.SOLUTION: The welded joint consists of a base steel material containing the chemical composition of, C:0.03-0.19%, Si:0.03-0.90%, Mn:0.30-1.80%, P ≤0.030%, S≤0.010%, Cr:0.05-1.20%, Mo:0.05-1.00%, sol.Al:0.01-0.10%, N≤0.0050%, and the balance Fe with impurities, consisting of the structure that the constitution ratio of the martensitic phase is ≥95% in terms of area ratio, with the tensile strength of ≥950 MPa, and a welded metal having the chemical composition of C:0.03-0.08%, Si:0.2-1.0%, Mn:0.3-3.0%, Ni:4.0-7.0%, Cr:11.5-15.0%, and the balance Fe with impurities while satisfying the inequalities [Creq+0.5Nieq>16.5], [Creq+5.7Nieq<58.8], and [Creq-0.63Nieq<10.6].

Description

  The present invention relates to a welded joint and a welding material. Specifically, a high strength steel base material having a tensile strength of 950 MPa or more is obtained by performing gas shield arc welding with higher efficiency than pre-heating and TIG welding (hereinafter referred to as “TIG welding”). Includes a welded joint having high strength and toughness obtained by MIG welding (hereinafter referred to as “MIG” welding) or plasma welding, and welding for manufacturing the welded joint by MIG welding or plasma welding. Regarding materials.

High-strength steel, especially high-strength steel with a martensite phase composition ratio of 95% or more in area ratio, can reduce the thickness of the structure because it can reduce the thickness of the structure. is also effective in reducing ₂ emissions, it is widely used as a welded structure.

  In the following description, “a structure in which the composition ratio of the martensite phase is 95% or more in area ratio” may be referred to as “a structure mainly composed of the martensite phase”.

  However, high-strength steel composed mainly of a martensite phase has a great effect of thinning, but particularly in the case of high-strength thick plates and steel pipes having a tensile strength of 950 MPa or more, the welding process The following special considerations are necessary, which is disadvantageous in terms of economic efficiency, and this is a barrier to diffusion.

  That is, high-strength steel of the above strength level is highly susceptible to weld cracking, and therefore requires preheating when performing welding. Furthermore, in order to ensure toughness, it is necessary to apply a welding method with low efficiency, such as TIG welding disclosed in Patent Document 1.

  For this reason, Patent Documents 2 to 4 disclose techniques related to welding of high-strength steel having a tensile strength of 950 MPa or more.

  Patent Literature 2 discloses a technique for preventing welding cold cracking by using a low C—Mn—Ni—Cr—Mo—high O-based weld metal for a high-strength steel of 950 MPa or more. However, with the technique disclosed in Patent Document 2, it is difficult to ensure the yield strength of the welded joint, and further, it is not always possible to prevent weld cracking by welding in a wet environment.

  Patent Document 3 discloses a technique for preventing weld cracking by containing a retained austenite phase in a weld metal. However, the implementation No. shown as “Example of the Invention” in Patent Document 3 is shown. The retained austenite phase of the weld metal of the joints 1 to 14 is as low as 3.2% at most. For this reason, with the technique disclosed in Patent Document 3, welding cracks cannot always be prevented by welding in a wet environment.

  Patent Document 4 discloses a welding material having a high content of Cr and Ni, specifically, a weld containing Cr: 6.0 to 16.0% and Ni: 6.0 to 16.0% by mass%. A technique for preventing cold cracking by welding high-tensile steel up to 1180 MPa class without preheating using a material is disclosed. However, when the technique disclosed in Patent Document 4 is applied to high-strength steel having a tensile strength of 950 MPa or higher, the strength, particularly the yield strength, is insufficient.

JP 2009-248175 A JP-A-10-306348 JP 2002-115032 A JP 2001-246495 A

  When a steel base material is welded, the weld material is diluted by the base material composition to form a weld metal.

  Since the characteristics of the welded joint greatly depend on the structure of the weld metal, appropriate structure control is required in producing the welded joint.

  In general, in structure control, the use of austenite phase can prevent weld cold cracking and improve toughness, the use of ferrite phase during solidification can prevent weld hot cracking, and the use of martensite phase. High strength can be achieved. However, since these characteristics are in a trade-off relationship, a material design that clears these characteristics is required.

  In particular, in high-strength steels with a tensile strength of 950 MPa or more, it is not clear how the structure should be controlled. Based on the new component design philosophy, that is, based on the elucidation of the quantitative behavior of each element, Eliminating off-relationships is an urgent issue.

  The present invention has been made in view of the above situation, and is a welded joint having high strength and high toughness obtained by MIG welding or plasma welding of a high strength steel base material having a tensile strength of 950 MPa or more without preheating. The purpose is to provide. It is another object of the present invention to provide a welding material for manufacturing the welded joint by MIG welding or plasma welding.

  In order to solve the above-described problems, the present inventors paid attention to Cr and Ni in a welded portion (welded metal) used for high-tensile steel of 950 MPa or more. This is because these elements greatly affect the contents of the austenite phase and martensite phase of the weld metal.

  On the other hand, the elements contained in the weld metal include elements that have the same effect as Cr and Ni on the structure control.

Therefore, the Cr equivalent represented by the following formula (i) (hereinafter referred to as “Creq”) and the Ni equivalent represented by the formula (ii) (hereinafter referred to as “Nieq”) are arranged. A detailed survey was conducted.
Creq = Cr + 1.5Si + Mo (i),
Nieq = Ni + 0.5Mn + 0.5Cu + 20C + 15N (ii).

  As a result, the following items (a) to (c) became clear.

(A) Both Creq and Nieq contribute to securing the austenite phase. In particular, the contribution of Creq is large, and it is necessary to satisfy the following formula [1] in order to secure a sufficient austenite phase and prevent welding cold cracking.
Creq + 0.5Nieq> 16.5 ... [1].

(B) Even if the formula [1] is satisfied, if the austenite phase is excessive in the structure, the strength is insufficient. The strength can be ensured by utilizing the martensite phase, and by satisfying the following equation [2], the yield strength that has a great influence on the strength, particularly the design stress of the welded portion, can be secured.
Creq + 5.7 Nieq <58.8 ... [2].

(C) On the other hand, in securing the austenite phase, by adjusting not only Nieq but also Creq, the structure at the time of solidification becomes a ferrite phase and it becomes possible to prevent hot welding cracking, but the toughness decreases. For this reason, it is necessary to satisfy the following equation [3].
Creq-0.63Nieq <10.6 ... [3].

  The present invention has been completed on the basis of the above matters, and the gist thereof is a welded joint shown in the following (1) to (4) and a welding material shown in (5).

(1) By mass%, C: 0.03 to 0.19%, Si: 0.03 to 0.90%, Mn: 0.30 to 1.80%, P: 0.030% or less, S: 0.010% or less, Cr: 0.05 to 1.20%, Mo: 0.05 to 1.00%, sol. Al: 0.01 to 0.10% and N: 0.0050% or less, the balance is Fe and impurities, and the composition ratio of the martensite phase is 95% or more in area ratio A steel base material composed of a certain structure and having a tensile strength of 950 MPa or more;
In mass%, C: 0.03-0.08%, Si: 0.2-1.0%, Mn: 0.3-3.0%, Ni: 4.0-7.0% and Cr: 11.5 to 15.0% is contained, the balance is made of Fe and impurities, and P, S, Al, V, Nb, Ti, B, Ca and N in the impurities are each P: 0.040% or less S: 0.008% or less, Al: 0.1% or less, V: 0.1% or less, Nb: 0.1% or less, Ti: 0.1% or less, B: 0.01% or less, Ca : 0.01% or less and N: 0.05% or less, and consisting of a weld metal having a chemical composition satisfying the following formulas [1] to [3]:
A welded joint characterized by that.
Creq + 0.5Nieq> 16.5 [1]
Creq + 5.7 Nieq <58.8 ... [2]
Creq−0.63Nieq <10.6 [3]
However,
Creq = Cr + 1.5Si + Mo (i)
Nieq = Ni + 0.5Mn + 0.5Cu + 20C + 15N (ii)
In the formulas (i) and (ii), the element symbol represents the content (% by mass) of the element.

  (2) The steel base material is in mass% instead of part of Fe, Cu: 2.0% or less, Ni: 3.0% or less, V: 0.1% or less, Nb: 0.1% The weld joint according to (1) above, having a chemical composition containing at least one of Ti: 0.1% or less and B: 0.01% or less.

  (3) The steel base material has a chemical composition containing, in mass%, Ca: 0.005% or less instead of a part of Fe, as described in (1) or (2) above Welded joints.

  (4) The weld metal has a chemical composition containing at least one of Mo: 2.0% or less and Cu: 2.0% or less in mass%, instead of a part of Fe. The weld joint according to any one of (1) to (3) above.

  (5) The composition according to any one of (1) to (4) above, characterized by containing, in mass%, Ni: 4.0 to 8.0% and Cr: 13.0 to 17.0%. Welding materials used for manufacturing welded joints of MIG or plasma welding.

  The “impurities” in the “Fe and impurities” as the balance are components mixed in due to various factors of raw materials such as ores and scraps and manufacturing processes when industrially manufacturing steel. It means what is allowed as long as it does not adversely affect.

  According to the present invention, it is possible to provide a welded joint having high strength and high toughness by performing MIG welding or plasma welding on a high strength steel base material having a tensile strength of 950 MPa or more without preheating. This weld joint can be manufactured by MIG welding or plasma welding using the welding material according to the present invention.

  Hereinafter, each requirement of the present invention will be described in detail. In the following description, “%” display of the content of each element means “mass%”.

(A) Steel base material:
(A-1) Chemical composition:
C: 0.03-0.19%
C is the most effective and inexpensive element for improving the strength. However, if the C content is less than 0.03%, it is necessary to guarantee the strength with other elements, resulting in a loss of economic efficiency. On the other hand, when C is contained exceeding 0.16%, weldability will be inhibited and toughness will be deteriorated. Therefore, the C content is 0.03 to 0.16%. A desirable lower limit of the C content is 0.05%, and a desirable upper limit is 0.14%.

Si: 0.03-0.90%
Si is an element that contributes to strength improvement. However, when the Si content is less than 0.03%, the effect is small. On the other hand, when the Si content exceeds 0.60%, the base metal toughness and weld zone toughness are deteriorated. Therefore, the Si content is 0.03 to 0.60%. A desirable lower limit of the Si content is 0.05%, and a desirable upper limit is 0.35%.

Mn: 0.30 to 1.80%
Mn is an element necessary for ensuring strength, and it is necessary to contain 0.30% or more. However, when Mn is contained exceeding 1.80%, the base material toughness is deteriorated. Therefore, the Mn content is 0.30 to 1.80%. A desirable lower limit of the Mn content is 0.60%, and a desirable upper limit is 1.60%. A more desirable lower limit of the Mn content is 0.80%.

P: 0.030% or less P is present as an impurity in steel and not only impairs the low-temperature toughness of the base metal and the weld heat-affected zone, but also reduces weldability, and further reduces strain aging resistance. End up. For this reason, although it is preferable to make content of P as low as possible, excessive reduction causes a cost rise. Therefore, the P content is set to 0.030% or less as a range that does not cause actual harm. The P content is more preferably 0.020% or less.

S: 0.010% or less S is present in the steel as an impurity and not only impairs the low temperature toughness of the base metal and the weld heat affected zone, but also lowers the weldability. For this reason, although it is preferable to make S content as low as possible, excessive reduction causes a cost increase. Therefore, the content of S is set to 0.010% or less as a range that does not cause actual harm.

Cr: 0.05-1.20%
Cr has a function of improving hardenability, and is an element effective for improving strength and toughness, and therefore needs to be contained in an amount of 0.05% or more. However, if the Cr content exceeds 1.20%, the toughness is deteriorated. Therefore, the Cr content is 0.05 to 1.20%. A desirable lower limit of the Cr content is 0.4%, and a desirable upper limit is 0.8%.

Mo: 0.05-1.00%
Since Mo is effective for improving the strength, it is necessary to contain 0.05% or more. However, when Mo is contained exceeding 1.00%, toughness is impaired. Therefore, the Mo content is 0.05 to 1.00%. A desirable lower limit of the Mo content is 0.3%, and a desirable upper limit is 0.6%.

sol. Al: 0.01-0.10%
In addition to having a deoxidizing action, Al has an action of forming AlN to immobilize N and promoting tissue refinement. Further, when B is contained in the steel, N is immobilized as AlN, so that generation of BN is avoided, so effective B that contributes to improving hardenability (B in a solid solution state in the matrix) The amount of increases. In order to obtain the above-described N fixing effect of Al, Al is sol. It is necessary to contain 0.01% or more of Al. However, Al is sol. If the Al content exceeds 0.10%, not only the properties of the welded portion will deteriorate, but also the strain aging resistance and weldability will deteriorate. Therefore, the content of Al is sol. Al content was set to 0.01 to 0.10%. sol. A desirable lower limit of the amount of Al is 0.02%, and a desirable upper limit is 0.08%. In addition, “sol.Al” refers to “Al in a solid solution state that is not an inclusion in steel”.

N: 0.0050% or less N is present in steel as an impurity, and when it exceeds 0.0050%, the amount of solid solution N increases and the toughness deteriorates. In addition, when B is contained in the steel, if the N content is large, it becomes difficult to ensure the effective B amount, resulting in insufficient strength and increased toughness deterioration. For this reason, an upper limit is set for the N content to 0.0050% or less. The N content is desirably 0.0040% or less.

  One of the steel base materials constituting the welded joint of the present invention has a chemical composition containing the elements C to N described above, with the balance being Fe and impurities.

  Another one of the steel base materials constituting the welded joint of the present invention is made of Cu, Ni, V, Nb, Ti, B, and Ca instead of a part of Fe in the “Fe and impurities” as the remainder. It has a chemical composition containing one or more selected elements.

  Hereinafter, the effect of the above-mentioned optional elements Cu, Ni, V, Nb, Ti, B, and Ca and the reason for limiting the content will be described.

  First, any of the above elements from Cu to B has an effect of improving the strength. For this reason, you may contain these elements.

  Hereinafter, elements from Cu to B will be described.

Cu: 2.0% or less Since Cu has an effect of improving strength, it may be contained. However, if the Cu content increases and exceeds 2.0%, the surface properties of the steel material are deteriorated due to the generation of scale, and further, the toughness is deteriorated. Therefore, the amount of Cu when contained is set to 2.0% or less. When Cu is contained, the amount of Cu is preferably 0.5% or less.

  On the other hand, in order to stably obtain the effect of Cu described above, the amount of Cu is preferably 0.05% or more, and more preferably 0.1% or more.

Ni: 3.0% or less Ni has an effect of improving strength. Ni also has the effect of increasing toughness. Therefore, Ni may be included. However, when the Ni content exceeds 3.0%, the effect is saturated and the cost is increased. Therefore, the amount of Ni in the case of inclusion is set to 3.0% or less. When Ni is contained, the amount of Ni is preferably 2.0% or less.

  On the other hand, in order to stably obtain the effect of Ni described above, the amount of Ni is desirably 0.05% or more, and more desirably 0.3% or more.

V: 0.1% or less V has an effect of improving strength. That is, V precipitates as VC during tempering and has a function of improving strength. Therefore, V may be contained. However, the content of V exceeding 0.1% causes the strength improvement effect to be saturated, resulting in an increase in cost and a decrease in toughness. Therefore, the V content in the case of inclusion is 0.1% or less. When V is contained, the amount of V is preferably 0.08% or less.

  On the other hand, in order to stably obtain the effect of V described above, the amount of V is preferably 0.005% or more, and more preferably 0.01% or more.

Nb: 0.1% or less
Nb has a strength improving effect. That is, Nb precipitates as Nb (C, N) in the grains during tempering, and has a function of improving yield strength. Furthermore, Nb is effective in manufacturing fine steel materials in units of fracture surfaces because it suppresses crystal grain coarsening during slab heating and exhibits the same effect during quenching. Therefore, you may contain Nb. However, when the Nb content increases and exceeds 0.1%, the deterioration of the toughness of the heat affected zone becomes remarkable. Therefore, the amount of Nb in the case of inclusion is 0.1% or less. When Nb is included, the amount of Nb is preferably 0.03% or less.

  On the other hand, in order to stably obtain the effect of Nb described above, the amount of Nb is preferably 0.005% or more, and more preferably 0.01% or more.

Ti: 0.1% or less Ti has an effect of improving strength. In addition to acting as a deoxidizing element, Ti forms an oxide phase composed of Al, Ti, and Mn, and contributes to refinement of the structure. Therefore, Ti may be included. However, if the Ti content increases and exceeds 0.1%, the oxide formed will be Ti oxide or Ti-Al oxide, and the dispersion density will decrease. The effect of refining the structure in the heat affected zone is lost. Therefore, the amount of Ti when contained is 0.1% or less. When Ti is contained, the amount of Ti is preferably 0.08% or less.

  On the other hand, in order to stably obtain the effect of Ti described above, the amount of Ti is preferably 0.005% or more, and more preferably 0.01% or more.

B: 0.01% or less B has an effect of improving strength. That is, B segregates at the austenite grain boundary to remarkably improve the hardenability, thereby increasing the strength. Therefore, B may be contained. However, when the B content increases and exceeds 0.01%, the toughness deteriorates. Therefore, the amount of B when contained is 0.01% or less. When B is contained, the amount of B is preferably 0.009% or less.

  On the other hand, in order to stably obtain the effect of B described above, the amount of B is preferably 0.0005% or more, and more preferably 0.002% or more.

  In addition, B combines with N relatively easily to form BN. And if B exists as BN, the above-mentioned hardenability improvement effect of B cannot be acquired. Therefore, when B is contained, in order to prevent B from binding to N as much as possible, it is desirable to fix N with another element so that B can exist as effective B.

  Said Cu, Ni, V, Nb, Ti, and B can be contained only in any 1 type in them, or 2 or more types of composites. The total amount when these elements are contained in combination may be 5.31%, but is preferably 5.0% or less, and more preferably 3.0% or less.

Ca: 0.005% or less Ca increases the ductility / brittle crack growth resistance by controlling the form of sulfide-based nonmetallic inclusions, and consequently contributes to the improvement of toughness. Therefore, Ca may be contained. However, if the Ca content increases and exceeds 0.005%, the amount of non-metallic inclusions increases, and on the contrary, the toughness is impaired. Therefore, the amount of Ca in the case of containing is 0.005% or less. When Ca is contained, the amount of Ca is preferably 0.003% or less.

  On the other hand, in order to stably obtain the effect of Ca described above, the amount of Ca is preferably 0.001% or more, and more preferably 0.002% or more.

(A-2) Organization:
In order for the steel base material having the chemical composition described in the section (A-1) to have a tensile strength of 950 MPa or more, the structure of the steel base material is a structure mainly composed of a martensite phase, that is, The composition ratio of the martensite phase must be composed of a structure having an area ratio of 95% or more.

  The “martensite” may be either “lass martensite” or “lens martensite”. The remaining phases are not particularly limited, and may be phases usually observed such as a grain boundary ferrite phase, a granular ferrite phase, and a pearlite phase. The composition ratio of the martensite phase may be 100%.

  In addition, manufacturing conditions such as rolling and heat treatment are appropriately adjusted according to the chemical composition of the slab obtained by melting and continuously casting steel having the chemical composition described in the section (A-1). Thus, it is possible to obtain a steel base material having a structure mainly composed of a martensite phase and having a tensile strength of 950 MPa or more.

(B) About weld metal:
C: 0.03-0.08%
C is an element effective for stabilizing the austenite phase in the same manner as N described later. However, if the C content is less than 0.03%, stabilization of the austenite phase becomes insufficient, and if it exceeds 0.08%, the sensitivity to welding cold cracking increases. For this reason, content of C shall be 0.03-0.08%. A desirable lower limit of the C content is 0.04%, and a desirable upper limit is 0.07%.

Si: 0.2 to 1.0%
Si is an element effective in preventing weld defects by ensuring the fluidity of the molten metal during welding. However, if the Si content is less than 0.2%, fluidity cannot be ensured and welding defects occur, and if it exceeds 1.0%, the toughness decreases. For this reason, content of Si shall be 0.2-1.0%. A desirable lower limit of the Si content is 0.25%, and a desirable upper limit is 0.85%.

Mn: 0.3 to 3.0%
Mn is an element effective for improving hot workability by desulfurization and deoxidation effects during welding. However, if the Mn content is less than 0.3%, these effects are insufficient, and if it exceeds 3.0%, the toughness decreases. For this reason, content of Mn shall be 0.3-3.0%. A desirable lower limit of the Mn content is 0.5%, and a desirable upper limit is 1.0%.

Ni: 4.0-7.0%
Ni acts as an austenite phase forming element and is an effective element for reducing toughness and susceptibility to welding cold cracking. However, if the Ni content is less than 4.0%, these effects are insufficient, and if it exceeds 7.0%, the strength is insufficient. For this reason, content of Ni shall be 4.0 to 7.0%. A desirable lower limit of the Ni content is 4.5%, and a desirable upper limit is 6.5%.

Cr: 11.5 to 15.0%
Cr is an element effective for preventing weld hot cracking by generating a ferrite phase during solidification. However, if the content of Cr is less than 11.5%, the above effects are insufficient, and if it exceeds 15.0%, the toughness decreases. For this reason, content of Cr shall be 11.5-15.0%. A desirable lower limit of the Cr content is 13.0%, and a desirable upper limit is 14.0%.

  One of the weld metals composing the welded joint of the present invention contains the above-mentioned elements from C to Ni, the balance is made of Fe and impurities, and P, S, Al, V, Nb, Ti, B in the impurities. , Ca and N are limited to the ranges described below, and have chemical compositions that satisfy the formulas [1] to [3] described below.

  The impurities in the weld metal include those mixed from the steel base material composition by dilution when the welded joint is manufactured.

P: 0.040% or less P is present as an impurity in the welding material, and is also mixed into the weld metal. When the P content exceeds 0.040%, the weld hot cracking susceptibility increases. Therefore, the P content is 0.040% or less. The P content is preferably 0.030% or less.

S: 0.008% or less S is present as an impurity in the welding material, and it is also mixed into the weld metal. When the content of S exceeds 0.008%, hot workability is insufficient when manufacturing a welding material, and processing into a wire becomes difficult. Therefore, the S content of the weld metal when the welded joint is manufactured is 0.008% or less.

Al: 0.1% or less, V: 0.1% or less, Nb: 0.1% or less, Ti: 0.1% or less, B: 0.01% or less, Ca: 0.01% or less, and N: 0.05% or less In the weld metal constituting the welded joint of the present invention, the contents of P, S, Al, V, Nb, Ti, B, Ca and N in impurities are each Al: 0.1% or less V: 0.1% or less, Nb: 0.1% or less, Ti: 0.1% or less, B: 0.01% or less, Ca: 0.01% or less, and N: 0.05% or less As a result, there is no problem in the performance of the welded joint.

  For example, when N is mixed as an impurity, if N is contained in the weld metal in an amount exceeding 0.05%, the toughness is reduced. However, if the content is 0.05% or less, N effectively acts as an austenite phase forming element to reduce toughness and sensitivity to weld cold cracking, so that there is no problem in the performance of the welded joint.

  Another one of the weld metals constituting the weld joint of the present invention contains one or more elements of Mo and Cu instead of a part of Fe in the “Fe and impurities” as the remainder, And it has a chemical composition which satisfy | fills [1]-[3] formula mentioned later.

  Hereinafter, the effect of the said Mo and Cu which are arbitrary elements, and the reason for limitation of content are demonstrated.

Mo: 2.0% or less Mo is effective in preventing weld hot cracking by generating a ferrite phase during solidification. Therefore, you may contain Mo. However, when the content of Mo exceeds 2.0%, the toughness of the welded joint decreases. Therefore, the amount of Mo in the case of containing is set to 2.0% or less. When Mo is contained, the amount of Mo is preferably 1.5% or less.

  On the other hand, in order to stably obtain the effect of Mo described above, the amount of Mo is preferably 0.05% or more, and more preferably 0.1% or more.

Cu: 2.0% or less Cu acts as an austenite phase forming element, and has an effect of improving toughness and reducing weld cold cracking sensitivity. However, when the Cu content exceeds 2.0%, the hot workability decreases. Therefore, the amount of Cu when contained is set to 2.0% or less. When Cu is contained, the amount of Cu is preferably 1.5% or less.

  On the other hand, in order to stably obtain the effect of Cu described above, the amount of Cu is preferably 0.05% or more, and more preferably 0.1% or more.

  Said Mo and Cu can be contained only in any 1 type or 2 types of composites. The total amount when these elements are combined and contained may be 4.0%, but is preferably 3.0% or less.

In order to eliminate the mutual trade-off relationship between weld cracking, toughness, and strength, the weld metal needs to satisfy not only the above-described element content range but also the following formulas [1] to [3].
Creq + 0.5Nieq> 16.5 [1]
Creq + 5.7 Nieq <58.8 ... [2]
Creq−0.63Nieq <10.6 [3]
However,
Creq = Cr + 1.5Si + Mo (i)
Nieq = Ni + 0.5Mn + 0.5Cu + 20C + 15N (ii)
In the formulas (i) and (ii), the element symbol represents the content (% by mass) of the element.

  By satisfying the formula [1], an austenite phase is secured and welding cold cracking can be prevented.

  However, an excessive austenite phase causes strength reduction. Therefore, by satisfying the formula [2], the amount of austenite phase is suppressed, and the yield strength can be ensured.

  Furthermore, by satisfy | filling Formula [3], it becomes possible to suppress the embrittlement of a matrix, ensuring an austenite phase, and toughness can be ensured.

(C) About welding material:
In order to manufacture a welded joint composed of the above-mentioned steel base material and weld metal, it is necessary to prepare a welding material in consideration of the compatibility with the steel base material having the chemical composition described in the section (A). .

  In other words, when the steel base material is welded, the chemical composition of the weld metal is diluted by the elements in the steel base material, so the chemical composition of the welding material must be determined in consideration of the chemical composition of the steel base material. Don't be.

  Basically, a welded joint may be manufactured using a welding material in the chemical composition range of the weld metal described in the above section (B). However, when the difference in the content of the specific element between the steel base material and the welding material is large, there is a high possibility that the chemical composition of the weld metal deviates from the above range due to the dilution effect.

  Regarding the chemical composition of the weld metal and steel base material in the welded joint of the present invention, the content ranges of elements other than Ni and Cr at least partially overlap. On the other hand, for Ni and Cr, the ranges of the contents of the weld metal and the steel base material do not overlap and the difference is also large. In such a case, elements other than Ni and Cr are rarely affected by dilution during welding, but Ni and Cr are greatly affected by dilution. However, if a welding material containing Ni: 4.0 to 8.0% and Cr: 13.0 to 17.0% is used, the contents of Ni and Cr in the weld metal can be easily determined by the item (B). It can be adjusted to the stated range.

  That is, in order to manufacture the welded joint of the present invention, a welding material containing Ni: 4.0 to 8.0% and Cr: 13.0 to 17.0% may be used. It is more preferable to use welding materials whose lower limits of Ni and Cr are 4.5% and 13.5%, respectively, and whose upper limits are 7.5% and 16.5%, respectively. Even more preferred.

  As for elements other than Ni and Cr, if the welding material satisfies the chemical composition conditions of the weld metal described in the above section (B), the content of the element in the weld metal can be easily determined in the section (B). It can be adjusted to the stated range.

  That is, in addition to the above-described amounts of Ni and Cr, C: 0.03-0.08%, Si: 0.2-1.0% and Mn: 0.3-3.0%, the balance Consists of Fe and impurities, and P, S, Al, V, Nb, Ti, B, Ca and N in the impurities are respectively P: 0.040% or less, S: 0.008% or less, Al: 0. 1% or less, V: 0.1% or less, Nb: 0.1% or less, Ti: 0.1% or less, B: 0.01% or less, Ca: 0.01% or less, and N: 0.05% If a welding material having a chemical composition satisfying the above-mentioned formulas [1] to [3] is used below, the chemical composition of the weld metal can be easily adjusted to the range described in the section (B). it can.

  Thus, if MIG welding or plasma welding is performed using a welding material whose chemical composition is adjusted, weld cracking can be produced without preheating and with high toughness and high strength of 950 MPa or more. .

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

  Steels P0 to P3 having chemical compositions shown in Table 1 were melted in a vacuum melting furnace to form ingots. Next, from each ingot, an X groove with a groove angle of 30 degrees was provided at the end of the long side by hot forging, hot rolling, heat treatment and machining with a thickness of 30 mm × width of 200 mm × length of 300 mm. A test steel plate (steel base material) was produced.

  In addition, about said test steel plate, it confirmed that the tensile strength was 950 Mpa or more by the separate tensile test. Furthermore, it was confirmed by a micro test performed separately that the structure of the test steel sheet was a structure mainly composed of a martensite phase having a martensite phase composition ratio of 96 to 100% in area ratio.

  Moreover, steel WW1-WW7 which has the chemical composition shown in Table 2 was melted in the vacuum melting furnace, and it was set as the ingot. Next, each ingot was heated to 1200 ° C. and finished into a round bar having a diameter of 50 mm by hot forging. The above round bar having a diameter of 50 mm cooled to room temperature was heated to 1250 ° C., and a strand having a diameter of 6 mm was obtained by hot rolling. The wire thus obtained was repeatedly annealed at 1050 ° C. and cold rolled to produce a welding material (welding wire) having a diameter of 1.2 mm.

  Next, welded joints A0 to A8 and B1 to B4 were produced by combinations of steel base materials and welding materials shown in Table 3.

  Specifically, the steel base materials described in the “Steel Base Material” column of Table 3 were butted together, and MIG welding was performed under conditions where the heat input amount was 10 to 28 kJ / cm to prepare a welded joint.

  As the shielding gas, a mixed gas of Ar + 50% He (that is, a mixed gas having a volume ratio of Ar to He of 1: 1) was used. The chemical composition of the weld metal was changed by controlling the dilution rate by adjusting the heat input.

  An analytical sample was collected from the weld metal of the welded joint thus produced, and the chemical composition of the weld metal was investigated by chemical analysis.

  Table 3 shows the component analysis results of the weld metal.

  Furthermore, a joint tensile test piece, a weld metal tensile test piece, and a Charpy impact test piece were sampled by machining from the central portion of the plate thickness of the obtained welded joint.

  The joint tensile test piece was sampled so that the parallel portion had a diameter of 6 mm and a length of 30 mm in the orthogonal direction with the weld line as the center, and a tensile test was performed at room temperature to measure the tensile strength. The target tensile strength of the welded joint was 980 MPa or more.

  Weld metal tensile test specimens were taken in the direction of the weld line so that all parallel parts with a diameter of 6 mm and a length of 30 m were weld metal, and subjected to a tensile test at room temperature to measure 0.2% yield strength and yield strength. I asked for it. The yield strength of the weld metal was targeted at 600 MPa or more.

  For the Charpy impact test piece, a test piece having a size of 10 mm × 10 mm × 55 mm was taken with a V notch having a depth of 2 mm in the center of the weld metal. A Charpy impact test at −40 ° C. was performed, and the toughness was evaluated by measuring the absorbed energy. The absorbed energy at −40 ° C. was set to 47 J or more.

  Table 4 summarizes the above test results. In Table 4, “◯” in each column of “Tensile strength of welded joint”, “Yield strength of weld metal” and “Toughness” indicates that each of the above characteristics has cleared the target. On the other hand, “x” indicates that each of the above characteristics does not reach the target.

  For welded joints in which the tensile strength of the welded joint, the yield strength of the weld metal, and the toughness all met the targets, the weld cold cracking was evaluated by the following test.

  That is, a test steel plate having a thickness of 30 mm, a width of 200 mm, and a length of 300 mm and having an X groove with a groove angle of 30 degrees at the end of the long side is placed on a commercially available SM400B steel plate with a thickness of 80 mm. Four rounds were restrained. Thereafter, without preheating at all, three-pass welding was performed in the groove under the same welding conditions as those for producing the welded joint, thereby producing a welded joint.

  After leaving for 48 hours, cross-sectional samples were taken from five locations of each welded joint and mirror polished. Next, the magnification was set to 100, and an optical microscope was used to examine the presence of cracks, that is, welding cold cracks. All five samples observed with an optical microscope were targeted to have no weld cold cracks.

  Table 4 also shows the above test results. In Table 4, “◯” in the “Welding cold crack” column indicates that no weld cold cracking was observed in all five samples and the target was cleared by observation with an optical microscope. On the other hand, “x” indicates that welding cold cracking was observed in at least one of the five samples observed with an optical microscope. Further, “-” indicates that the test is not performed.

  According to Table 4, the welded joints A1 to A8 of the “examples of the present invention” satisfying the conditions specified in the present invention are not preheated, but no weld cold cracks are observed even under strong restraint conditions. Moreover, it is clear that the tensile strength of the welded joint, the yield strength and the toughness of the weld metal have also cleared the targets and have high strength and high toughness.

  On the other hand, for the welded joints B1 to B4 of “Comparative Examples” that deviate from the conditions defined in the present invention, at least one of the tensile strength of the welded joint, the yield strength of the weld metal, and the toughness has reached the target. (Welded joints B1 to B3) or weld cold cracking occurred (welded joint B4).

  According to the present invention, it is possible to provide a welded joint having high strength and high toughness by performing MIG welding or plasma welding on a high strength steel base material having a tensile strength of 950 MPa or more without preheating. This weld joint can be manufactured by MIG welding or plasma welding using the welding material according to the present invention.

Claims (5)

In mass%, C: 0.03 to 0.19%, Si: 0.03 to 0.90%, Mn: 0.30 to 1.80%, P: 0.030% or less, S: 0.010 % Or less, Cr: 0.05 to 1.20%, Mo: 0.05 to 1.00%, sol. Al: 0.01 to 0.10% and N: 0.0050% or less, the balance is Fe and impurities, and the composition ratio of the martensite phase is 95% or more in area ratio A steel base material composed of a certain structure and having a tensile strength of 950 MPa or more;
In mass%, C: 0.03-0.08%, Si: 0.2-1.0%, Mn: 0.3-3.0%, Ni: 4.0-7.0% and Cr: 11.5 to 15.0% is contained, the balance is made of Fe and impurities, and P, S, Al, V, Nb, Ti, B, Ca and N in the impurities are each P: 0.040% or less S: 0.008% or less, Al: 0.1% or less, V: 0.1% or less, Nb: 0.1% or less, Ti: 0.1% or less, B: 0.01% or less, Ca : 0.01% or less and N: 0.05% or less, and consisting of a weld metal having a chemical composition satisfying the following formulas [1] to [3]:
A welded joint characterized by that.
Creq + 0.5Nieq> 16.5 [1]
Creq + 5.7 Nieq <58.8 ... [2]
Creq−0.63Nieq <10.6 [3]
However,
Creq = Cr + 1.5Si + Mo (i)
Nieq = Ni + 0.5Mn + 0.5Cu + 20C + 15N (ii)
In the formulas (i) and (ii), the element symbol represents the content (% by mass) of the element.   Instead of a part of Fe, the steel base material is in mass%, Cu: 2.0% or less, Ni: 3.0% or less, V: 0.1% or less, Nb: 0.1% or less, Ti The welded joint according to claim 1, having a chemical composition containing at least one of: 0.1% or less and B: 0.01% or less.   The welded joint according to claim 1 or 2, wherein the steel base material has a chemical composition containing Ca: 0.005% or less in mass% instead of part of Fe.   The weld metal has a chemical composition containing at least one of Mo: 2.0% or less and Cu: 2.0% or less in mass%, instead of a part of Fe, Item 4. The weld joint according to any one of Items 1 to 3.   The weld joint according to any one of claims 1 to 4, characterized by containing, in mass%, Ni: 4.0 to 8.0% and Cr: 13.0 to 17.0%. Or a welding material used for manufacturing by plasma welding.