Solid-state bonding steel, solid-state bonding steel material, solid-state bonding joints and solid-state bonding structures

Abstract

The present invention provides steel for solid-state welding and a steel material for solid-state welding that are steel for which the tensile properties of the parent material and a joint obtained by solid-state welding are equivalent or superior to those of high tensile strength steel, and to which minimal amounts of only relatively inexpensive alloy elements have been added, wherein softening of a heat-affected part is suppressed by the addition of the minimal amounts of the alloy elements. The present invention also provides: a solid-state welded joint that is formed from steel for solid-state welding to which minimal amounts of only relatively inexpensive alloy elements have been added, wherein the tensile properties of the parent material and the joint are equivalent or superior to those of high tensile strength steel; and a solid-state welded structure including said joint. Steel for solid-state welding according to the present invention is characterized in that, in terms of mass%, the steel composition thereof contains C: 0.20-2.14%, Si and Al combined: 1.00-3.00%, Mn: 2.00-5.00%, and at least one substance from among V: more than 0.1% but not more than 3.0%, W: more than 1.0% but not more than 6.0%, and Mo: more than 1.0% but not more than 6.0%, with the remainder being only Fe and unavoidable impurities.

Description

[Technical Field]

[0001] The present invention relates to a steel for solid-state bonding suitable for use in solid-state bonding, a steel material for solid-state bonding, and a solid-state bonded joint and a solid-state bonded structure made of the steel for solid-state bonding. [Background technology]

[0002] Solid-state joining methods, which can reduce the decrease in strength of the joint compared to conventional fusion welding, have attracted attention, and in particular, solid-state joining methods that utilize frictional heating and plastic deformation of metallic materials are being actively studied. Examples of such solid-state joining methods include "friction stir welding (FSW)," in which a cylindrical tool rotating at high speed is pressed into the workpieces to join them, "friction welding," in which a rotating cylindrical workpiece is brought into contact with a fixed workpiece to join them, and "linear friction welding," in which the workpieces are joined by a reciprocating motion while in contact.

[0003] Conventional steel materials are often designed with alloys that are intended for use in fusion welding, but in recent years, research has been progressing on steel materials that are suitable for friction welding. For example, Patent Document 1 (Japanese Patent Laid-Open Publication No. 2008-31494) discloses a low-alloy structural steel for friction stir welding, which is characterized by the fact that the sum of the temperature range in which the steel becomes a single ferrite phase and the temperature range in which the steel becomes two phases, austenite and ferrite, in an equilibrium state of 600°C or higher is 200°C or higher.

[0004] In the low-alloy structural steel described in Patent Document 1, the deformation resistance of the steel during friction stir welding is significantly reduced by expanding the ferrite single-phase region and the austenite-ferrite two-phase region near the temperature reached by the weld, which results in improved durability of the rotary tool and mitigated restrictions on welding conditions such as welding speed.In addition, the frequency of tool replacement due to wear and breakage is reduced, and welding time is shortened, improving construction efficiency.

[0005] Furthermore, research is also underway into steels suitable for the friction stir process, which is a surface modification technology that utilizes the principles of friction stir welding. For example, Patent Document 2 (JP 2014-162971 A) discloses a steel for the friction stir process that is composed, in mass%, of 0.40 to 1.50% C, 0.15 to 2.00% Si, 0.30 to 2.00% Mn, 0.50 to 3.00% Cr, with the balance being Fe and unavoidable impurities.

[0006] The steel for friction stir processing described in Patent Document 2 above is said to be able to achieve excellent surface hardening by applying the friction stir process.

[0007] Furthermore, the present inventors have also disclosed in Patent Document 3 (JP 2018-16866 A) a steel for friction stir welding characterized in that the steel composition contains, in mass%, 0.20 to 0.45% C and 1.00 to 3.50% Cr, and the carbon equivalent CE defined by formula A is 0.40 to 1.00 mass%: CE = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5... The element symbols in formula (A) represent the content of each component in the steel for friction stir welding in mass%.

[0008] The steel for friction stir welding described in Patent Document 3 above is a steel that can obtain joint properties (such as tensile strength and fracture toughness of the stir zone) equivalent to or better than those of conventional high-tensile steel through friction stir welding, and it is said that it is possible to provide steel with a minimum addition of only relatively inexpensive alloy elements, as well as a friction stir welding method that uses this steel as the welded material. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-31494 [Patent Document 2] Japanese Patent Application Laid-Open No. 2014-162971 [Patent Document 3] Japanese Patent Application Publication No. 2018-16866 Summary of the Invention [Problem to be solved by the invention]

[0010] However, the low-alloy structural steel disclosed in Patent Document 1 facilitates the application of friction stir welding to steel by reducing the deformation resistance of the steel during the process, and little consideration is given to the mechanical properties of the weld (stirred part) or the cost and availability of the elements added to the steel.

[0011] Furthermore, the steel for friction stir processing disclosed in Patent Document 2 has a composition optimized for surface hardening using frictional heat, and its design guidelines are completely different from those of steel materials intended to ensure the mechanical properties of the joint.

[0012] Furthermore, although the steel for friction stir welding disclosed in Patent Document 3 can obtain joint properties equal to or better than those of general high-tensile steel, it uses chromium (Cr), which is relatively expensive, as a main additive element, and further improvements in the mechanical properties of the base material and the joint are required. In recent years, there has been an increasing demand for ultra-high-tensile steel with a tensile strength of 1000 MPa or more, but the tensile properties of the steel for friction stir welding disclosed in Patent Document 3 do not reach the level of ultra-high-tensile steel.

[0013] Furthermore, the properties of a joint are determined by the mechanical properties of the weakest region of the joint. Generally, the heat-affected zone (HAZ) formed at the outer edge of the joint softens, so there is a strong demand for suppressing the softening of the HAZ. However, as the strength of steel materials increases, it has become extremely difficult to suppress the softening of the HAZ.

[0014] In view of the above-mentioned problems in the prior art, an object of the present invention is to provide a steel for solid-state bonding and a steel material for solid-state bonding that is a steel in which the tensile properties of both the base metal and the joint obtained by solid-state bonding are equal to or greater than those of ultra-high tensile steel, and that contains a minimum of relatively inexpensive alloying elements, and that suppresses softening of the heat-affected zone by adding a minimum of alloying elements. Another object of the present invention is to provide a solid-state bonded joint made of a steel for solid-state bonding that contains a minimum of relatively inexpensive alloying elements, and that is equal to or greater than those of ultra-high tensile steel, and a solid-state bonded structure having such a joint. [Means for solving the problem]

[0015] In order to achieve the above object, the inventors have conducted extensive research into the relationship between the composition and mechanical properties of steel and the structure and mechanical properties of welds obtained by solid-state welding. As a result, they have discovered that it is extremely effective to use carbon steel as a base material and add appropriate amounts of Si and Mn, as well as to contain an appropriate amount of at least one of V, W, and Mo, and have arrived at the present invention.

[0016] That is, the present invention provides: The steel composition is, in mass%, C: 0.20~2.14%, Sum of Si and Al: 1.00 to 3.00% Mn: 2.00 to 5.00%, and Contains at least one of V: ​​more than 0.1% and not more than 3.0%, W: more than 1.0% and not more than 6.0%, and Mo: more than 1.0% and not more than 6.0%, The balance is composed of only Fe and unavoidable impurities. The present invention provides a steel for solid-state bonding characterized by the above.

[0017] The steel for solid-state welding of the present invention contains a relatively large amount of carbon to achieve high strength, while solid-state welding, which has a lower joining temperature and allows for a slower cooling rate than fusion welding, is used, thereby enabling the production of good welds free from cracks, defects, etc. Here, by setting the carbon content to 0.20 mass% or more, the strength of the steel and the solid-state weld can be sufficiently improved, and by setting the carbon content to 2.14 mass% or less, the deterioration of tensile properties due to the dispersion of graphite can be suppressed.

[0018] The carbon content is preferably 0.20 to 0.45 mass%, more preferably 0.20 to 0.30 mass%, and most preferably 0.20 to 0.25 mass%. In the steel for solid-state bonding of the present invention, the tensile strength of the base metal and the solid-state bonded joint is 1000 MPa or more at a carbon content of 0.20 mass%, which is the lower limit. Therefore, in order to ensure good ductility, it is preferable to set the upper limit to a small value as long as the desired tensile strength is obtained.

[0019] Furthermore, Si and Al have similar effects, and Si and / or Al are added primarily to ensure the ductility of the base material and the solid-phase welded joint. By making the total content of Si and Al 1.00 mass% or more, it is possible to suppress the formation of cementite, which reduces ductility, and stabilize austenite. On the other hand, adding more than 3.00 mass% does not improve these effects, so the upper limit of the addition amount is set to 3.00 mass%. The total content of Si and Al is preferably 1.50 to 2.50 mass%, more preferably 1.75 to 2.25 mass%. Si and Al are inexpensive alloying elements that are abundant. It is not necessary to add Si and Al together; either one may be added.

[0020] The addition of Mn also contributes to improving the ductility of the base metal and solid-state welded joints. In addition to the addition of Si, a Mn content of 2.00% by mass or more is expected to improve ductility through the TRIP (martensitic transformation-induced plasticity) effect caused by austenite stabilization. Furthermore, although depending on the solid-state welding conditions, adding 2.00% by mass or more of Mn maintains hardenability and refines the austenite grain size in the solid-state welded joint, resulting in the formation of a fine lath-shaped martensite structure from the austenite. As a result, a solid-state welded joint with high strength and sufficient ductility can be obtained. However, since this effect is hardly improved even if the Mn content is 5.00% by mass or more, the upper limit of the Mn content is set to 5.00% by mass. Here, when emphasis is placed on strength, the Mn content is preferably 3.00 to 5.00% by mass. When emphasis is placed on ductility, the Mn content is preferably 2.00% by mass or more but less than 3.00% by mass.

[0021] Furthermore, the steel for solid-state welding of the present invention contains at least one of V: ​​more than 0.1% and less than 3.0%, W: more than 1.0% and less than 6.0%, and Mo: more than 1.0% and less than 6.0%, which allows the formation of secondary carbides containing these elements and suppresses the decrease in hardness of the heat-affected zone formed by solid-state welding. While other alloying elements also form secondary carbides, V, W, and Mo have a moderate tendency to form carbides. These carbides are efficiently formed by the temperature history of the heat-affected zone at the outer edge of the solid-state weld, thereby effectively suppressing the decrease in hardness of the heat-affected zone. The generally known order of carbide formation tendency of each alloying element is Ti > Ta > Nb > V > W > Mo > Cr > Mn > (Fe) > Ni, Co, Al, and Si. Because Ti, Ta, and Nb have such a strong tendency to form carbides, for example, they easily form carbides even in austenite, reducing the amount of secondary carbides formed in the heat-affected zone and preventing effective secondary hardening of the heat-affected zone. On the other hand, Cr and Mn have too little tendency to form carbides, and the hardness of the carbides is relatively low, so they cannot effectively harden the heat-affected zone.

[0022] Here, in the steel for solid-state bonding of the present invention, it is preferable to add more than 1.0% to 6.0% of Mo, and more preferably more than 2.0% to 4.0% of Mo. Adding Mo can cause more significant secondary hardening in the heat-affected zone than when adding V or W.

[0023] The steel for solid-state welding of the present invention has a composition in which the remainder, other than the above elements, is Fe and inevitable impurities. Based on carbon steel, the addition of rare metals is avoided, thereby reducing manufacturing costs and ensuring the sustainability of production (reducing the risk of uneven distribution). While the steel for solid-state welding of the present invention is premised on solid-state welding, the method of solid-state welding is not particularly limited as long as it does not impair the effects of the present invention, and various conventionally known solid-state welding methods can be used. Representative solid-state welding methods include friction stir welding, friction welding, and linear friction welding.

[0024] The present invention also provides a solid-state welded joint, characterized in that at least one of the materials to be welded is a steel material for solid-state bonding made of the above-mentioned steel for solid-state bonding of the present invention, the solid-state welded portion of the steel material for solid-state bonding has a fine lath-shaped martensite structure, and the average grain size of prior austenite in the martensite structure is 15 μm or less.

[0025] The solid-state welded joint of the present invention has at least one of the workpieces made of the steel for solid-state welding of the present invention. The addition of expensive alloying elements is minimized, making it relatively inexpensive, while also providing high strength and ductility. Furthermore, the welded joint has a lath-shaped martensite structure with a carbon content of 0.20 to 2.14%, resulting in extremely high tensile strength. Furthermore, the addition of Mn refines the martensite, and depending on the welding conditions, the TRIP effect due to retained austenite can be expected, resulting in good ductility.

[0026] In the solid-state welded joint of the present invention, the average grain size of prior austenite in the martensite structure is 15 μm or less. By refining the prior austenite structure, the lath-shaped martensite structure is refined, thereby imparting high strength and ductility to the solid-state welded joint. The average grain size of the prior austenite is more preferably 10 μm or less, and most preferably 5 μm or less. In the solid-state welded joint of the present invention, at least one of V, W, and Mo is added, and this addition also has the effect of reducing the prior austenite grain size. The method for determining the average grain size of prior austenite is not particularly limited, and various conventionally known methods can be used. For example, a structure subjected to appropriate etching may be observed, or prior austenite may be reconstructed from martensite based on the orientation relationship between martensite and prior austenite, and the average grain size may be determined from the reconstructed prior austenite structure.

[0027] Furthermore, in the solid-state welded joint of the present invention, it is preferable that the Vickers hardness of the steel for solid-state welding is 400 HV or more, the Vickers hardness of the solid-state welded portion is 400 HV or more, and the Vickers hardness of the heat-affected zone near the solid-state welded portion is 400 HV or more. More preferably, the Vickers hardness of the steel for solid-state welding, the solid-state welded portion, and the heat-affected zone is 450 HV or more. Here, when the Vickers hardness of the steel is 400 HV, the tensile strength is about 1200 MPa, and when the Vickers hardness is 450 HV, the tensile strength is about 1350 MPa.

[0028] Furthermore, in the solid-state welded joint of the present invention, it is preferable that the Vickers hardness of the solid-state welded portion and the heat-affected zone is 80 to 120% of the Vickers hardness of the steel material for solid-state welding. Although the strengthening mechanisms of the steel material for solid-state welding, the solid-state welded portion, and the heat-affected zone are not the same, as a result, by reducing the variation in hardness, it is possible to obtain a good joint without any unique regions in mechanical properties.

[0029] Furthermore, it is preferable that the solid-state welded joint of the present invention has secondary carbides in the heat-affected zone that contain at least one of V, W, and Mo. To homogenize the hardness distribution throughout the joint, it is necessary to suppress a decrease in hardness in the heat-affected zone, and by dispersing high-hardness secondary carbides in that region, softening can be suppressed extremely effectively.

[0030] Furthermore, the solid-state welded joint of the present invention preferably has a tensile strength of 1000 MPa or more and an elongation of 20% or more at room temperature, more preferably 1200 MPa or more and an elongation of 20% or more, and most preferably 1500 MPa or more and an elongation of 20% or more. These values ​​can be achieved by imparting high tensile properties to the base material and the solid-state welded joint and suppressing softening in the heat-affected zone. Having these tensile properties, the solid-state welded joint can be fully used as a joint with mechanical properties equivalent to or better than those of ultra-high tensile steel in applications requiring high strength and reliability. While the results of tensile tests may be affected by the size and shape of the tensile test specimen, the values ​​obtained using a relatively small tensile test specimen with a parallel portion length of approximately 4 mm and a width of approximately 2 mm are used as the basis for this specification.

[0031] In addition, in the solid-state welded joint of the present invention, it is preferable that the solid-state welded portion is a friction stir welded portion. The solid-state welded portion may be formed by friction welding or linear friction welding, but by using a friction stir welded portion, it is possible to form a solid-state welded portion in any region of the steel plate, and it can also be used for manufacturing large structures.

[0032] The present invention also provides a solid-state welded structure characterized by having the solid-state welded joint of the present invention. The material, shape, and size of the structural parts other than the solid-state welded joint are not particularly limited, and various conventionally known structures can be used.

[0033] Furthermore, the present invention also provides a steel material for solid-state bonding, which is made of the steel for solid-state bonding of the present invention, has a lath-shaped martensite structure, and has an average grain size of prior austenite in the martensite structure of 15 μm or less. The steel material for solid-state bonding having this composition and structure has a tensile strength of 1000 MPa or more and an elongation of 20% or more at room temperature, and can be fully used as a steel material with mechanical properties equal to or better than ultra-high tensile steel in applications requiring high strength and reliability. [Effects of the Invention]

[0034] According to the present invention, it is possible to provide a steel for solid-state bonding and a steel material for solid-state bonding that is a steel in which the tensile properties of both the base metal and the joint obtained by solid-state bonding are equal to or greater than those of ultra-high tensile steel, and that contains a minimum amount of relatively inexpensive alloying elements, and that suppresses softening of the heat-affected zone by adding a minimum amount of alloying elements.Furthermore, it is also possible to provide a solid-state bonded joint that is made of a steel for solid-state bonding that contains a minimum amount of relatively inexpensive alloying elements, and that has tensile properties of both the base metal and the joint that are equal to or greater than those of ultra-high tensile steel, and a solid-state bonded structure having such a joint. [Brief explanation of the drawings]

[0035] [Figure 1] 1 is a schematic diagram of a solid-state welded joint having a solid-state weld formed by friction stir welding. [Figure 2] FIG. 1 is a schematic diagram of a solid-state welded joint having a solid-state weld formed by friction welding. [Figure 3] FIG. 1 is a schematic diagram of a solid state welded joint having a solid state weld formed by linear friction welding. [Figure 4] 1 shows the results of structural observation of the base material and stir zone of practical solid-state bonding steel 2, practical solid-state bonding steel 3, and comparative solid-state bonding steel 1. [Figure 5] 1 shows the results of structural observation of the base material, stir zone, and heat-affected zone of the example solid-state bonding steel 3. [Figure 6] This is the result of reconstructing prior austenite from martensite in the base material and stir zone. [Figure 7] 1 shows the Vickers hardness distribution in the cross section of the friction stir welded joints of Example Solid-State Welding Steel 1, Comparative Solid-State Welding Steel 1, Comparative Solid-State Welding Steel 2, and Comparative Solid-State Welding Steel 3. [Figure 8] 1 shows the Vickers hardness distribution in the cross section of the friction stir welded joints of Example Solid-State Welding Steel 2, Example Solid-State Welding Steel 3, Example Solid-State Welding Steel 4, and Comparative Solid-State Welding Steel 1. DETAILED DESCRIPTION OF THE INVENTION

[0036] Representative embodiments of the steel for solid-state bonding, the steel material for solid-state bonding, the solid-state bonded joint, and the solid-state bonded structure of the present invention will be described in detail below, but the present invention is not limited to these. Note that in the following description, duplicated explanations may be omitted.

[0037] (1)Steel for solid phase joining The steel for solid-phase bonding of the present invention is The steel composition is, in mass%, C: 0.20~2.14%, Sum of Si and Al: 1.00 to 3.00% Mn: 2.00 to 5.00%, and Contains at least one of V: ​​more than 0.1% and not more than 3.0%, W: more than 1.0% and not more than 6.0%, and Mo: more than 1.0% and not more than 6.0%, The balance is composed of only Fe and unavoidable impurities. It is a steel for solid-state bonding characterized by the above.

[0038] There is a strong demand for improvements in the specific strength of structural materials, and various high-tensile steels that incorporate rare metals have been proposed, but rare metals have a high risk of uneven distribution, which poses problems from the perspectives of cost and production stability. On the other hand, because the strength of steel generally improves with an increase in carbon content, the active use of carbon steel would enable a reduction in the amount of rare metals used.

[0039] Here, medium- and high-carbon steels with high carbon contents are considered to be extremely difficult to weld because they crack during fusion welding. However, the steel for solid-state welding of the present invention overcomes this problem by being designed for use in solid-state welding. Furthermore, the addition of appropriate amounts of Si, Al, and Mn imparts good ductility to the base material and welded joint. Furthermore, the addition of at least one of V, W, and Mo generates secondary carbides containing these elements, suppressing a decrease in hardness in the heat-affected zone. Each component will be described in detail below.

[0040] 1. Essential additive elements C: 0.20~2.14% by mass A carbon content of 0.20% by mass or more can sufficiently improve the strength of the steel and the solid-state weld, and a carbon content of 2.14% by mass or less can suppress a decrease in tensile properties due to the dispersion of graphite. The carbon content is preferably 0.20 to 0.45% by mass, more preferably 0.20 to 0.30% by mass, and most preferably 0.20 to 0.25% by mass. In the steel for solid-state welding of the present invention, the tensile strength of the base metal and the solid-state weld is 1000 MPa or more at a carbon content of 0.20% by mass, which is the lower limit. Therefore, in order to ensure good ductility, it is preferable to set the upper limit to a small value as long as the desired tensile strength is obtained.

[0041] Total of Si and Al: 1.00 to 3.00 mass% By setting the total content of Si and / or Al to 1.00 mass% or more, it is possible to suppress the formation of cementite, which reduces ductility. On the other hand, adding 3.00 mass% or more does not improve this effect, so the upper limit of the addition amount is set to 3.00 mass%. The total content of Si and / or Al is preferably 1.50 to 2.50 mass%, more preferably 1.75 to 2.25 mass%.

[0042] Mn:2.00~5.00% by mass In addition to the addition of Si, by increasing the Mn content to 2.00 mass% or more, improved ductility can be expected due to the TRIP (martensitic transformation induced plasticity) effect caused by austenite stabilization. Furthermore, although depending on the solid-state welding conditions, adding 2.00 mass% or more of Mn can form a fine martensite structure in the solid-state welded joint. As a result, a solid-state welded joint with high strength and sufficient ductility can be obtained. However, since this effect is hardly improved even if the Mn content is increased to 5.00 mass% or more, the upper limit of the Mn content is set to 5.00 mass%. Here, when emphasis is placed on strength, the Mn content is preferably set to 3.00 to 5.00 mass%, and when emphasis is placed on ductility, the Mn content is preferably set to 2.00 mass% or more but less than 3.00 mass%.

[0043] At least one of V, W, and Mo V: More than 0.1% and less than 3.0% Adding V in excess of 0.1% can produce secondary carbides sufficient to suppress softening in the heat-affected zone of a solid-state bonded joint. However, adding V in excess of 3.0% saturates the effect, and the V that does not dissolve in the matrix may reduce the mechanical properties of the solid-state bonded joint. This is also undesirable from an economical standpoint. A more preferable V content is 0.25 to 0.75%.

[0044] W: More than 1.0% and less than 6.0% Adding W in excess of 1.0% can generate secondary carbides sufficient to suppress softening in the heat-affected zone of a solid-state bonded joint. However, adding W in excess of 6.0% saturates the effect, and the W that does not dissolve in the matrix may reduce the mechanical properties of the solid-state bonded joint. This is also undesirable from an economical standpoint. A more preferable W content is 2.0 to 4.0%.

[0045] Mo: More than 1.0% and less than 6.0% Adding Mo in excess of 1.0% can generate secondary carbides sufficient to suppress softening in the heat-affected zone of a solid-state bonded joint. However, adding Mo in excess of 6.0% saturates the effect, and the Mo that does not dissolve in the matrix may reduce the mechanical properties of the solid-state bonded joint. This is also undesirable from an economical standpoint. A more preferable Mo content is 2.0 to 4.0%.

[0046] Of V, W, and Mo, the most preferred added element is Mo. These elements may be added singly or in combination. When adding multiple elements, it is preferable to take into account the carbon content required to form carbides and to ensure that the amount added is not excessive.

[0047] 2. Optional additive elements Cr:1.00~3.50% by mass The addition of an appropriate amount of Cr can improve the strength and toughness of solid-state welds. Since toughness is a product of strength and ductility, the addition of Cr increases both strength and ductility, resulting in improved toughness. The mechanism by which the addition of Cr improves the properties of solid-state welds is not entirely clear, but it is believed that adding Cr to carbon steel suppresses the formation of pro-eutectoid ferrite during solid-state welding (the cooling process from austenite), thereby increasing the strength of the resulting solid-state welds and improving the ductility of martensite (or bainite). The Cr content is more preferably 1.50 to 3.00 mass%.

[0048] Cu: 3.0% by mass or less Cu is an element useful for ensuring the strength of the base metal, but if it is contained in excess of 3.0 mass %, the base metal and HAZ will harden, so it is preferable to keep it at 3.0 mass % or less.

[0049] Ni: 5.0% by mass or less Ni is an element that improves the strength and toughness of the base material, but if it exceeds 5.0 mass%, the HAZ hardens, so it is preferable to keep it at 5.0 mass% or less. Also, since Ni is expensive, it is preferable to keep it at 5.0 mass% or less.

[0050] Nb: 0.1% by mass or less Nb is an element useful for ensuring the strength and toughness of the base material and HAZ, but if it exceeds 0.1 mass%, it has a negative effect on toughness, so it is preferable to keep it at 0.1 mass% or less. Also, since Nb is expensive, it is preferable to keep it at 0.1 mass% or less. Note that Nb is thought to have the effect of refining austenite, resulting in the formation of a fine lath-shaped martensite structure.

[0051] Ti: 0.1% by mass or less Ti is an element useful for ensuring the strength and toughness of the base material and HAZ, but if it exceeds 0.1%, it has a negative effect on toughness, so it is preferable to keep it at 0.1 mass% or less.Ti is thought to have the effect of refining austenite, resulting in the formation of a fine lath-shaped martensite structure.

[0052] B: 0.0040% by mass or less B has the effect of segregating to the austenite grain boundaries during rolling to improve hardenability, but if it exceeds 0.0040 mass %, it will deteriorate the toughness of the HAZ, so it is preferably set to 0.0040 mass % or less.

[0053] Other impurities include N, which, if contained in large amounts, forms nitrides and reduces toughness, so the amount of N mixed in is preferably 0.010 mass % or less.

[0054] (2) Solid-state bonded joints With regard to the solid-state welded joint of the present invention, one embodiment having a solid-state weld formed by friction stir welding is shown in Fig. 1, one embodiment having a solid-state weld formed by friction welding is shown in Fig. 2, and one embodiment having a solid-state weld formed by linear friction welding is shown in Fig. 3. When friction welding or linear friction welding is used, a solid-state weld 6 is formed on the friction surface of the workpieces (2, 4), and when friction stir welding is used, the solid-state weld 6 is formed in the area where the friction stir welding tool has passed.

[0055] Friction stir welding includes the following four modes (1) to (4) and combinations thereof: (1) joining in which the ends of metal plates are butted together to form a joint, and a rotating tool is moved while rotating along the longitudinal direction of the processed area to join the metal plates; (2) spot joining in which the ends of metal plates are butted together to form a joint, and a rotating tool is rotated at the joint without moving, (3) spot joining in which metal plates are overlapped at the joint, a rotating tool is inserted into the joint, and the rotating tool is rotated at that point without moving to join the metal plates; and (4) joining in which metal plates are overlapped at the joint, a rotating tool is inserted into the joint, and the rotating tool is moved while rotating along the longitudinal direction of the joint to join the metal plates.

[0056] In the solid-state welded joint 1 of the present invention, at least one of the workpieces (2, 4) is a steel material for solid-state welding made of the steel for solid-state welding of the present invention. The structure of the solid-state welded joint 6 has a lath martensite structure. By making most of the region of the solid-state welded joint 6 a lath martensite structure, extremely high strength can be obtained. On the other hand, it is difficult to achieve sufficient ductility with a general martensite structure. However, the present inventors have conducted extensive research into the relationship between the martensite structure and tensile properties and have found that adding Mn to refine the lath martensite structure can impart good ductility.

[0057] In the solid-phase welded joint 6, the average grain size of prior austenite in the martensite structure is 15 μm or less. By refining the prior austenite structure, the lath-shaped martensite structure is refined, thereby imparting high strength and ductility to the solid-phase welded joint. Here, the average grain size of the prior austenite is more preferably 10 μm or less, and most preferably 5 μm or less. Note that the method for determining the average grain size of the prior austenite is not particularly limited and various conventionally known methods can be used. For example, a structure subjected to appropriate etching may be observed, or prior austenite may be reconstructed from martensite based on the orientation relationship between martensite and prior austenite, and the grain size may be determined from the reconstructed prior austenite structure.

[0058] It is also preferable that the Vickers hardness of the workpieces (2, 4) is 400 HV or more, the Vickers hardness of the solid-state welded portion 6 is 400 HV or more, and the Vickers hardness of the heat-affected zone near the solid-state welded portion 6 is 400 HV or more. A more preferable Vickers hardness is 450 HV or more for all of the workpieces (2, 4), the solid-state welded portion 6, and the heat-affected zone.

[0059] Furthermore, it is preferable that the Vickers hardness of the solid-state welded portion 6 and the heat-affected zone be 80 to 120% of the Vickers hardness of the welded materials 2, 4. Although the strengthening mechanisms of the welded materials 2, 4, the solid-state welded portion 6, and the heat-affected zone are not the same, ultimately reducing the variation in hardness among them makes it possible to obtain a good joint without any unique regions in mechanical properties.

[0060] It is also preferable that the heat-affected zone contains secondary carbides containing at least one of V, W, and Mo. To homogenize the hardness distribution of the entire solid-state welded joint 1, it is necessary to suppress a decrease in hardness in the heat-affected zone, and by dispersing high-hardness secondary carbides in this region, softening can be suppressed extremely effectively.

[0061] Furthermore, the solid-state welded joint 1 preferably has a tensile strength of 1000 MPa or more and an elongation of 20% or more at room temperature, more preferably 1200 MPa or more and an elongation of 20% or more, and most preferably 1500 MPa or more and an elongation of 20% or more. These values ​​can be achieved by imparting high tensile properties to the welded materials (2, 4) and the solid-state weld 6 and suppressing softening in the heat-affected zone. Because the solid-state welded joint 1 has these tensile properties, it can be fully used as a joint with mechanical properties equivalent to or better than ultra-high tensile steel in applications requiring high strength and reliability. Here, while the results of tensile tests may be affected by the size and shape of the tensile test specimen, the values ​​obtained using a relatively small tensile test specimen with a parallel portion approximately 4 mm long and 2 mm wide are used as the basis for this specification.

[0062] The solid-state welded portion 6 of the solid-state welded joint 1 may be formed by friction welding or linear friction welding, but by using a friction stir welded portion, the solid-state welded portion 6 can be formed in any region of the steel plate, making it possible to use it in the manufacture of large structures.

[0063] (3) Solid phase bonded structure The solid-state welded structure of the present invention is characterized by having a solid-state welded joint 1. The materials, shapes, and sizes of the structural parts other than the solid-state welded joint 1 are not particularly limited, and various conventionally known structures can be used.

[0064] As long as the effects of the present invention are not impaired, the solid-phase welded structure is not particularly limited, and examples thereof include structural parts of transportation equipment such as automobiles, ships, and railway vehicles, various architectural structures, bridges, iron pipes, etc.

[0065] (4) Steel materials for solid phase joining The steel material for solid-state bonding of the present invention is made of the steel material for solid-state bonding of the present invention, has a lath-shaped martensite structure, and is characterized in that the average grain size of prior austenite in the martensite structure is 15 μm or less. The steel material for solid-state bonding having the composition and structure has a tensile strength of 1000 MPa or more and an elongation of 20% or more at room temperature, and can be fully used as a steel material with mechanical properties equal to or better than ultra-high tensile steel in applications requiring high strength and reliability.

[0066] Representative embodiments of the present invention have been described above, but the present invention is not limited to these, and various design modifications are possible, all of which are included in the technical scope of the present invention.

[0067] The solid-state bonding steel, solid-state bonding steel material, solid-state bonded joint, and solid-state bonded structure of the present invention will be further described in the following examples, but the present invention is not limited to these examples in any way. [Example]

[0068] Example 1 Steel ingots (φ35 × 20-25h) having the composition shown in Table 1 were produced by high-frequency melting, and the plate thickness was adjusted to 3 mm by hot rolling at 1000°C. Then, in order to completely dissolve Mo in austenite and obtain a martensite structure, the steel was subjected to a soaking diffusion treatment at 1000°C for 10 minutes, followed by water cooling to obtain a steel plate (Steel 1 for solid-state bonding). The values ​​shown in Table 1 are in mass%.

[0069] [Table 1]

[0070] The obtained steel plates were friction stir welded using a cemented carbide tool (probe without a screw) with a shoulder diameter of 15 mm, a probe diameter of 6 mm, and a probe length of 2.9 mm under the following conditions: tool rotation speed: 400 rpm, welding speed: 150 mm / min, welding load: 2.5 ton, tool advance angle: 3°, and welding atmosphere: Ar.

[0071] Example 2 A steel plate (experimental solid-state bonding steel 2) was obtained in the same manner as in Example 1, except that the composition of Example 2 shown in Table 1 was used. Friction stir welding was also carried out in the same manner as in Example 1.

[0072] Example 3 A steel plate (experimental solid-state bonding steel 3) was obtained in the same manner as in Example 1, except that the composition of Example 3 shown in Table 1 was used. Friction stir welding was also carried out in the same manner as in Example 1.

[0073] Example 4 A steel plate (Steel 4 for solid-state bonding) was obtained in the same manner as in Example 1, except that the composition of Example 4 shown in Table 1 was used and the temperature of the soaking diffusion treatment was set to 1125° C. Further, friction stir welding was performed in the same manner as in Example 1.

[0074] Comparative Example 1 A steel plate (comparative solid-state welding steel 1) was obtained in the same manner as in Example 1, except that the composition of Comparative Example 1 shown in Table 1 was used. Friction stir welding was also carried out in the same manner as in Example 1.

[0075] Comparative Example 2 A steel plate (comparative steel 2 for solid-state bonding) was obtained in the same manner as in Example 1, except that the composition of Comparative Example 2 shown in Table 1 was used. Friction stir welding was also carried out in the same manner as in Example 1.

[0076] Comparative Example 3 A steel plate (comparative steel 3 for solid-state bonding) was obtained in the same manner as in Example 1, except that the composition of Comparative Example 3 shown in Table 1 was used. Friction stir welding was also carried out in the same manner as in Example 1.

[0077] [Evaluation test] (1) Microstructure observation The region including the stir zone was cut out perpendicular to the friction stir welding direction, and the cross section was polished and etched (4% nital), after which the structure was observed using a scanning electron microscope (FE-SEM, JEOL Ltd., JSM-7001FA). Emery paper (#600 to #3000) and diamond paste (grain sizes 3 μm and 1 μm) were used for polishing. Samples for base metal observation were also prepared in the same way.

[0078] (2) EBSD measurement EBSD measurements were performed to measure the prior austenite grain size in the base material and the stir zone of the friction stir welded joint. EBSD measurements were performed using a FE-SEM (JSM-7001FA manufactured by JEOL Ltd.) and TSL's OIM data collection ver. 5.31.

[0079] (3) Vickers hardness measurement Vickers hardness measurements were performed on the cross sections of the friction stir welded joints obtained in the above examples and comparative examples. The measurement was performed at the center of the plate thickness, and the hardness distribution in the horizontal direction was measured. The measurement device used was an ARS 10K manufactured by FUTURE-TECH, and the measurements were performed under conditions of 1 kg and 10 s.

[0080] Figure 4 shows the results of structural observations (SEM images) of the base material and stir zone of Experimental Solid-State Bonding Steel 2, Experimental Solid-State Bonding Steel 3, and Comparative Solid-State Bonding Steel 1. In all of Experimental Solid-State Bonding Steel 2, Experimental Solid-State Bonding Steel 3, and Comparative Solid-State Bonding Steel 1, a full martensite structure was formed in the base material due to water cooling from the austenite region. In the stir zone, a full martensite structure similar to that of the base material was also formed due to friction stir welding, in which the joining temperature was above the A3 point. Fine spherical particles were observed in both the base material and the stir zone, and these are thought to be V carbides that were not dissolved in the matrix.

[0081] Figure 5 shows the results of structural observations of the base material, stir zone, and heat-affected zone of the embodied solid-state bonding steel 3. Compared to the base material, it can be seen that the structure of the heat-affected zone is slightly coarsened. On the other hand, no significant changes were observed in the V carbides that were not dissolved in the matrix. Furthermore, the results of the Vickers hardness measurements described below indicate that the addition of V effectively suppresses the softening of the heat-affected zone, which is thought to be why fine secondary V carbides that cannot be clearly observed by SEM observation are formed.

[0082] Figure 6 shows the results of reconstructing prior austenite from martensite in the base material and stir zone based on the orientation relationship between martensite and prior austenite from the EBSD measurement results for Experimental Solid-State Bonding Steel 2, Experimental Solid-State Bonding Steel 3, and Comparative Solid-State Bonding Steel 1. It can be seen that the prior austenite grains are refined by the addition of V. In particular, when 1.0% V is added to the stir zone, the prior austenite grain size is extremely fine, at approximately 4 μm.

[0083] Figure 7 shows the Vickers hardness distribution in the cross section of the friction stir welded joints of the practical solid-state welding steel 1, the comparative solid-state welding steel 1, the comparative solid-state welding steel 2, and the comparative solid-state welding steel 3. Figure 7 also shows a schematic diagram of the cross section of the joint corresponding to the Vickers hardness measurement position. In the comparative solid-state welding steel 1, the comparative solid-state welding steel 2, and the comparative solid-state welding steel 3, significant softening was observed in the heat-affected zone. In contrast, in the practical solid-state welding steel 1, which contains 3.0% Mo, this softening was effectively suppressed, with a minimum hardness of 450 HV or more. Additionally, the Vickers hardness of the stir welded zone and the heat-affected zone was 80 to 120% of the Vickers hardness of the base material.

[0084] Figure 8 shows the Vickers hardness distribution in the cross-sections of friction stir welded joints for Experimental Solid-State Welding Steel 2, Experimental Solid-State Welding Steel 3, Experimental Solid-State Welding Steel 4, and Comparative Solid-State Welding Steel 1. Figure 8 also shows a schematic diagram of the cross-sections of the joints corresponding to the Vickers hardness measurement locations. For Comparative Solid-State Welding Steel 1, significant softening was observed in the heat-affected zone. In contrast, for Experimental Solid-State Welding Steels with 0.5% and 1.0% Mo addition, this softening was effectively suppressed, with minimum hardness exceeding 400 HV in both cases. Additionally, the Vickers hardness of the stir and heat-affected zones was 80–120% of that of the base metal. Here, there was little difference between the addition of 0.5% and 1.0% V, indicating that the addition of a relatively small amount suppresses softening in the heat-affected zone. Furthermore, the soaking temperature did not significantly affect the minimum hardness. On the other hand, compared to when 3.0% Mo is added, the minimum hardness is slightly lower. [Explanation of symbols]

[0085] 1. Solid-state bonded joints, 2,4...material to be joined, 6...Solid phase junction.

Claims

1. The material to be welded is a steel material for solid-state welding made of steel for solid-state welding, The steel composition of the steel for solid-state bonding is, in mass%, C: 0.20-2.14%, Sum of Si and Al: 1.00 to 3.00% Mn: 2.00 to 5.00%; Contains at least one of V: ​​more than 0.1% and not more than 3.0% and Mo: more than 1.0% and not more than 6.0%; The balance is composed of only Fe and inevitable impurities, The tensile strength of the steel material for solid-state bonding at room temperature is 1000 MPa or more, The solid-state welded portion of the steel material for solid-state bonding has a lath-shaped martensite structure, The average grain size of prior austenite in the martensite structure is 15 μm or less, The tensile strength at room temperature is 1000 MPa or more, The solid-state welded portion is any one of a friction stir welded portion, a friction welded portion, and a linear friction welded portion; A solid-state bonded joint characterized by:

2. The C content is 0.20 to 0.45 mass%; 2. The solid-state welded joint according to claim 1, wherein:

3. The Vickers hardness of the steel material for solid-state bonding is 400 HV or more, The Vickers hardness of the solid-phase bonded joint is 400 HV or more, The Vickers hardness of the heat-affected zone in the vicinity of the solid-state weld is 400 HV or more; 3. The solid-state welded joint according to claim 1 or 2, characterized in that:

4. the Vickers hardness of the solid-state welded portion and the heat-affected portion is 80 to 120% of the Vickers hardness of the steel material for solid-state welding; The solid-state welded joint according to any one of claims 1 to 3, characterized in that:

5. The heat-affected zone has secondary carbides containing at least one of V and Mo; The solid-state welded joint according to any one of claims 1 to 4, characterized in that:

6. Having the solid-state welded joint according to any one of claims 1 to 5, A solid-state bonded structure characterized by:

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