JP7817738B2 - Solid-state bonding steel, solid-state bonding steel material, solid-state bonding joints and solid-state bonding structures - Google Patents

Description

The present invention relates to steel for solid-state welding and steel material for solid-state welding that can be suitably used for solid-state welding, as well as solid-state welded joints and solid-state welded structures made from the steel for solid-state welding.

Solid-state welding methods, which can reduce the loss of strength at the joint compared to conventional fusion welding, have attracted attention, with solid-state welding methods that utilize frictional heating and plastic deformation of metallic materials being actively investigated. Examples of solid-state welding methods include friction stir welding (FSW), in which a rapidly rotating cylindrical tool 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 moving them back and forth while in contact.

Conventional steel materials are often designed with alloys intended for use in fusion welding, but in recent years, research has also been conducted into steel materials suitable for friction welding. For example, Patent Document 1 (JP 2008-31494 A) discloses a low-alloy structural steel for friction stir welding, characterized in that the sum of the temperature range in which a ferrite single phase is formed and the temperature range in which two phases, austenite and ferrite, are formed at equilibrium temperatures of 600°C or above is 200°C or above.

The low-alloy structural steel described in Patent Document 1 above is said to significantly reduce the deformation resistance of steel during friction stir welding by expanding the ferrite single-phase region and the austenite-ferrite two-phase region near the temperature reached by the weld, thereby improving the durability of the rotary tool and easing restrictions on welding conditions such as welding speed. In addition, it is said to reduce the frequency of tool replacement due to wear and breakage, shortening welding time and improving construction efficiency.

In addition, research is also underway into steels suitable for the friction stir process, 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 consists, in mass percent, of C: 0.40-1.50%, Si: 0.15-2.00%, Mn: 0.30-2.00%, Cr: 0.50-3.00%, with the balance being Fe and unavoidable impurities.

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.

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

The steel for friction stir welding described in Patent Document 3 above is a steel that can be used to obtain joint characteristics (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 the minimum addition of only relatively inexpensive alloying elements, as well as a friction stir welding method that uses this steel as the welded material.

JP 2008-31494 A JP 2014-162971 A JP 2018-16866 A

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.

In addition, the steel for friction stir processing disclosed in Patent Document 2 above 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.

Furthermore, while the friction stir welding steel disclosed in Patent Document 3 can achieve joint properties equal to or better than those of general high-tensile steel, it uses relatively expensive chromium (Cr) as a primary additive element, and there is a demand for even higher mechanical strength in the base material and joint. While demand for ultra-high-tensile steel with a tensile strength of 1000 MPa or more has increased in recent years, the tensile properties of the friction stir welding steel disclosed in Patent Document 3 do not reach the level of ultra-high-tensile steel.

Furthermore, the characteristics of a joint are determined by the mechanical properties of its weakest area. Generally, the heat-affected zone formed at the outer edge of the joint softens, so there is a strong desire to prevent this softening. However, as steel becomes stronger, it has become extremely difficult to prevent the softening of the heat-affected zone.

In light of the above-mentioned problems with the prior art, the object of the present invention is to provide a steel for solid-state bonding and a steel material for solid-state bonding that has the tensile properties of both the base metal and the joint obtained by solid-state bonding equal to or greater than ultra-high tensile steel, and that contains a minimum of relatively inexpensive alloying elements, and that suppresses softening of the heat-affected zone due to the addition of 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 has the tensile properties of both the base metal and the joint equal to or greater than ultra-high tensile steel, as well as a solid-state bonded structure having such a joint.

In order to achieve the above objective, the inventors conducted extensive research into the relationship between the composition and mechanical properties of steel and the structure and mechanical properties of the joint obtained by solid-state welding. As a result, they discovered that adding appropriate amounts of Si and Mn to a carbon steel base, as well as including an appropriate amount of at least one of V, W, and Mo, is extremely effective, leading to the present invention.

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%;
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 being Fe and inevitable impurities;
The present invention provides a steel for solid-state bonding characterized by the above.

The steel for solid-state welding of the present invention contains a relatively large amount of carbon, thereby achieving high strength. However, by using solid-state welding, which has a lower joining temperature and allows for a slower cooling rate compared to fusion welding, it is possible to obtain a good joint free of cracks and defects. By setting the carbon content to 0.20% by mass or more, the strength of the steel and the solid-state weld can be sufficiently improved, and by setting it to 2.14% by mass or less, it is possible to suppress the deterioration of tensile properties due to the dispersion of graphite.

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 material and 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.

Furthermore, Si and Al have similar effects, and Si and/or Al are added primarily to ensure the ductility of the base material and solid-state welds. By ensuring that the total content of Si and Al is 1.00 mass% or more, it is possible to suppress the formation of cementite, which reduces ductility, and stabilize austenite. However, adding more than 3.00 mass% does not improve these effects, so the upper limit for the amount added is set at 3.00 mass%. The total content of Si and Al is preferably 1.50 to 2.50 mass%, and more preferably 1.75 to 2.25 mass%. Si and Al are inexpensive alloying elements that are abundant. Furthermore, it is not necessary to add Si and Al together; either one may be added.

The addition of Mn also contributes to improving the ductility of the base material 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. This results in a solid-state welded joint with high strength and sufficient ductility. However, this effect is hardly improved even if the Mn content is increased to 5.00% by mass or more, so the upper limit of the Mn content is set to 5.00% by mass. Here, when strength is emphasized, a Mn content of 3.00 to 5.00% by mass is preferred, while when ductility is emphasized, a Mn content of 2.00% by mass or more but less than 3.00% by mass is preferred.

Furthermore, the solid-state welding steel of the present invention 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%, which allows the formation of secondary carbides containing these elements and suppresses the hardness reduction in the heat-affected zone formed by solid-state welding. While other alloying elements also produce secondary carbides, V, W, and Mo have a moderate tendency to form carbides, which efficiently form secondary carbides due to the temperature history of the heat-affected zone at the outer edge of the solid-state weld, thereby extremely effectively suppressing the hardness reduction in that region. The commonly known carbide formation tendency of each alloying element is Ti > Ta > Nb > V > W > Mo > Cr > Mn > (Fe) > Ni, Co, Al, and Si. Ti, Ta, and Nb have too strong a 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, making it impossible to effectively harden the heat-affected zone.On the other hand, Cr and Mn have too weak a tendency to form carbides, and the hardness of the carbides is also relatively low, making it impossible to effectively harden the heat-affected zone.

In the solid-phase bonding steel of the present invention, it is preferable to add more than 1.0% but not more than 6.0% Mo, and it is even more preferable to add more than 2.0% but not more than 4.0% Mo. Adding Mo can produce more pronounced secondary hardening in the heat-affected zone than when adding V or W.

The solid-state welding steel of the present invention is composed of only Fe and unavoidable impurities, with the remainder being the above-mentioned elements. Based on carbon steel, the addition of rare metals is avoided, reducing manufacturing costs and ensuring the sustainability of production (reducing the risk of uneven distribution). While the solid-state welding steel of the present invention is based 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.

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 welding made from the above-mentioned steel for solid-state welding of the present invention, the solid-state welded portion of the steel material for solid-state welding has a fine lath-shaped martensite structure, and the average grain size of prior austenite in the martensite structure is 15 μm or less.

The solid-state welded joint of the present invention has at least one of the materials to be welded made of the solid-state welding steel of the present invention. The addition of expensive alloying elements is kept to a minimum, making it relatively inexpensive, while also providing high strength and ductility. Furthermore, the structure of the weld is lath-shaped martensite, and the carbon content in this region is 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 also be expected, resulting in good ductility.

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, imparting high strength and ductility to the solid-state welded joint. The average grain size of prior austenite is more preferably 10 μm or less, and most preferably 5 μm or less. The solid-state welded joint of the present invention contains at least one of V, W, and Mo, which 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.

Furthermore, it is preferable that the solid-state welded joint of the present invention has a Vickers hardness of 400 HV or more for the steel material for solid-state welding, a Vickers hardness of 400 HV or more for the solid-state welded portion, and a Vickers hardness of 400 HV or more for the heat-affected zone near the solid-state welded portion. A more preferable Vickers hardness is 450 HV or more for the steel material for solid-state welding, the solid-state welded portion, and the heat-affected zone. Here, if the Vickers hardness of the steel material is 400 HV, the tensile strength will be approximately 1200 MPa, and if the Vickers hardness is 450 HV, the tensile strength will be approximately 1350 MPa.

Furthermore, it is preferable that the Vickers hardness of the solid-state welded joint of the present invention be 80 to 120% of the Vickers hardness of the solid-state welded portion and the heat-affected zone. Although the strengthening mechanisms of the solid-state welded steel, the solid-state welded portion, and the heat-affected zone are not identical, reducing the variation in hardness results in a good joint with no areas of unique mechanical properties.

It is also 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.

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 while suppressing softening in the heat-affected zone. By possessing these tensile properties, the solid-state welded joint can be fully used as a joint with mechanical properties equivalent to or greater 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 present specification uses values obtained using a relatively small tensile test specimen with a parallel portion approximately 4 mm long and 2 mm wide.

Furthermore, 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, making it suitable for the manufacture of large structures.

The present invention also provides a solid-state welded structure characterized by having the solid-state welded joint of the present invention. The materials, shapes, and sizes of the structural components other than the solid-state welded joint are not particularly limited, and can be any of a variety of conventionally known structures.

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 is characterized in that the average grain size of prior austenite in the martensite structure is 15 μm or less. A steel material for solid-state bonding with 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 equivalent to or better than ultra-high tensile steel in applications requiring high strength and reliability.

The present invention provides a steel for solid-state bonding and a steel material for solid-state bonding that has the tensile properties of both the base material and the joint obtained by solid-state bonding equal to or greater than ultra-high tensile steel, with the addition of only relatively inexpensive alloying elements to a minimum extent, and in which softening of the heat-affected zone is suppressed by the addition of a minimum of alloying elements. The present invention also provides a solid-state bonded joint made of a steel for solid-state bonding that has the addition of only relatively inexpensive alloying elements to a minimum extent, with the tensile properties of both the base material and the joint equal to or greater than ultra-high tensile steel, and a solid-state bonded structure having such a joint.

1 is a schematic diagram of a solid-state welded joint having a solid-state weld formed by friction stir welding. FIG. 1 is a schematic diagram of a solid-state welded joint having a solid-state weld formed by friction welding. FIG. 1 is a schematic diagram of a solid state welded joint having a solid state weld formed by linear friction welding. 1 shows the results of observation of the structure of the base material and the stir zone of the practical solid-state bonding steel 2, practical solid-state bonding steel 3, and comparative solid-state bonding steel 1. 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. This is the result of reconstructing prior austenite from martensite in the base material and stir zone. 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. 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.

Representative embodiments of 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 described in detail below, but the present invention is not limited to these. Note that in the following description, redundant explanations may be omitted.

(1) Steel for solid-state bonding The steel for solid-state 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%;
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 being Fe and inevitable impurities;
It is a steel for solid-state bonding characterized by the above.

There is a strong demand for improvements in the specific strength of structural materials, and various high-tensile steels containing rare metals have been proposed. However, 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.

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 solid-state welding steel 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 produces secondary carbides containing these elements, suppressing a decrease in hardness in the heat-affected zone. Each component is explained in detail below.

1. Essential additional elements C: 0.20 to 2.14 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 the deterioration of 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 the lower limit of the carbon content of 0.20% by mass. 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.

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%.

Mn: 2.00 to 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 joining 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%.

At least one of V, W, and Mo V: more than 0.1% but not more than 3.0% Adding more than 0.1% V can generate secondary carbides to a degree that suppresses softening in the heat-affected zone of a solid-state bonded joint. On the other hand, adding more than 3.0% V saturates the effect, and there is a risk that the V that does not dissolve in the matrix may reduce the mechanical properties of the solid-state bonded joint. In addition, this is undesirable from an economic standpoint. A more preferable amount of V is 0.25 to 0.75%.

W: More than 1.0% and up to 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. On the other hand, 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%.

Mo: More than 1.0% and up to 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. On the other hand, adding Mo in excess of 6.0% saturates the effect, and the Mo that is not dissolved 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%.

Of the three elements V, W, and Mo, Mo is the most preferred additive element. These elements may be added singly or in combination. When adding multiple elements, it is advisable to take into account the carbon content required to form carbides and to avoid adding excessive amounts.

2. Optional added elements Cr: 1.00 to 3.50 mass%
The inclusion 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 weld and improving the ductility of martensite (or bainite). The Cr content is more preferably 1.50 to 3.00 mass%.

Cu: 3.0% by mass or less Cu is a useful element for ensuring the strength of the base material. However, if it is contained in excess of 3.0% by mass, the base material and the HAZ portion will harden, so it is preferable to keep it at 3.0% by mass or less.

Ni: 5.0% by mass or less Ni is an element that improves the strength and toughness of the base material, but if it is contained in excess of 5.0% by mass, the HAZ hardens, so it is preferable that the content be 5.0% by mass or less. In addition, since Ni is expensive, it is preferable that the content be 5.0% by mass or less.

Nb: 0.1% by mass or less Nb is a useful element for ensuring the strength and toughness of the base material and the HAZ, but if it exceeds 0.1% by mass, it has an adverse effect on toughness, so it is preferable to set it to 0.1% by mass or less. Also, since Nb is expensive, it is preferable to set it to 0.1% by mass or less. Note that Nb is thought to have the effect of refining austenite, and as a result, forming a fine lath-shaped martensite structure.

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

B: 0.0040% by mass or less B has the effect of segregating at austenite grain boundaries during rolling to improve hardenability, but if it exceeds 0.0040% by mass, it will deteriorate the toughness of the HAZ portion, so it is preferable to make it 0.0040% by mass or less.

Another impurity is N, which, if present in large quantities, forms nitrides and reduces toughness, so it is preferable to keep the amount of N contained to 0.010 mass% or less.

(2) Solid-state welded joint 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 surfaces 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.

Friction stir welding includes the following four modes and combinations: (1) joining in which the ends of metal plates are butted together to form a joint, and a rotating tool is moved along the length of the processed area while rotating 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 along the length of the joint while rotating to join the metal plates.

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

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, imparting high strength and ductility to the solid-phase welded joint. Here, the average grain size of prior austenite is more preferably 10 μm or less, and most preferably 5 μm or less. 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, the average grain size of prior austenite can be determined by observing a structure that has been appropriately etched, or by reconstructing prior austenite from martensite based on the orientation relationship between martensite and prior austenite, and then determining the average grain size from the reconstructed prior austenite structure.

It is also preferable that the Vickers hardness of the workpieces (2, 4) be 400 HV or more, the Vickers hardness of the solid-state welded portion 6 be 400 HV or more, and the Vickers hardness of the heat-affected zone near the solid-state welded portion 6 be 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.

It is also 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 materials to be welded (2, 4). Although the strengthening mechanisms of the materials to be welded (2, 4), the solid-state welded portion 6, and the heat-affected zone are not identical, ultimately reducing the variation in their hardness can result in a good joint with no areas of unique mechanical properties.

It is also preferable for the heat-affected zone to contain secondary carbides containing at least one of V, W, and Mo. In order to homogenize the hardness distribution throughout the solid-state welded joint 1, it is necessary to suppress a decrease in hardness in the heat-affected zone. However, by dispersing high-hardness secondary carbides in this region, softening can be suppressed extremely effectively.

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 while suppressing softening in the heat-affected zone. Because the solid-state welded joint 1 possesses 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. While the results of tensile tests may be affected by the size and shape of the tensile test specimen, the present specification uses values obtained using a relatively small tensile test specimen with a parallel portion approximately 4 mm long and 2 mm wide.

The solid-state welded joint 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 joint, the solid-state welded joint 6 can be formed in any area of the steel plate, making it possible to manufacture large structures.

(3) Solid-state welded 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.

Solid-phase welded structures are not particularly limited as long as they do not impair the effects of the present invention, but examples include structural parts of transportation equipment such as automobiles, ships, and railway vehicles, various architectural structures, bridges, iron pipes, etc.

(4) Steel for solid-state bonding The steel for solid-state bonding of the present invention is made of the steel for solid-state bonding of the present invention, and is characterized in that it has a lath-shaped martensite structure, and the average grain size of prior austenite in the martensite structure is 15 μm or less. The steel 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 having mechanical properties equal to or greater than those of ultra-high tensile steel in applications requiring high strength and reliability.

The above describes representative embodiments of the present invention, but the present invention is not limited to these and various design modifications are possible, all of which are included within the technical scope of the present invention.

The following examples further explain the solid-state bonding steel, solid-state bonding steel material, solid-state bonded joints, and solid-state bonded structures of the present invention, but the present invention is not limited to these examples in any way.

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. Thereafter, 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 practical solid-state bonding). The values shown in Table 1 are in mass %.

Friction stir welding was performed on the obtained steel plates using a cemented carbide tool (probe without threads) with a shoulder diameter of 15 mm, probe diameter of 6 mm, and probe length of 2.9 mm under the following conditions: tool rotation speed: 400 rpm, joining speed: 150 mm/min, joining load: 2.5 tons, tool advance angle: 3°, and joining atmosphere: Ar.

Example 2
A steel plate (Steel 2 for practical solid-state welding) 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 performed in the same manner as in Example 1.

Example 3
A steel plate (Steel 3 for practical solid-state welding) 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.

Example 4
A steel plate (Steel 4 for practical 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. Furthermore, friction stir welding was performed in the same manner as in Example 1.

Comparative Example 1
A steel plate (comparative solid-state bonding 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 performed in the same manner as in Example 1.

Comparative Example 2
A steel plate (comparative solid-state welding steel 2) 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 performed in the same manner as in Example 1.

Comparative Example 3
A steel plate (comparative solid-state welding steel 3) 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 performed in the same manner as in Example 1.

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

(2) EBSD Measurement EBSD measurement was performed to measure the prior austenite grain size in the base material and the stir zone of the friction stir welded joint. FE-SEM (JSM-7001FA manufactured by JEOL Ltd.) and OIM Data Collection ver. 5.31 manufactured by TSL were used for the EBSD measurement.

(3) Vickers hardness measurement Vickers hardness measurement was performed on the cross section of the friction stir welded joint 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 ARS 10K manufactured by FUTURE-TECH, and the measurement was performed under the conditions of 1 kg and 10 s.

Figure 4 shows the results of structural observation (SEM images) of the base material and stir zone of the example solid-state bonding steel 2, the example solid-state bonding steel 3, and the comparative solid-state bonding steel 1. In all of the example solid-state bonding steel 2, the example solid-state bonding steel 3, and the comparative solid-state bonding steel 1, a full martensite structure was formed in the base material by water cooling from the austenite region. Also, in the stir zone, a full martensite structure similar to that of the base material was formed by friction stir welding in which the joining temperature was _A3 point or higher. Fine spherical particles were observed in both the base material and the stir zone, which are thought to be V carbides that were not solid-solved in the matrix.

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 suggest that the addition of V effectively suppresses softening of the heat-affected zone, resulting in the formation of fine secondary V carbides that cannot be clearly observed by SEM observation.

Figure 6 shows the results of EBSD measurements of practical solid-state bonding steel 2, practical solid-state bonding steel 3, and comparative solid-state bonding steel 1, which show the reconstruction of prior austenite from martensite in the base material and stir zone based on the orientation relationship between martensite and prior austenite. It can be seen that the addition of V refines the prior austenite grains. In particular, when 1.0% V was added to the stir zone, the prior austenite grain size was extremely fine, at approximately 4 μm.

Figure 7 shows the Vickers hardness distribution in the cross section of the friction stir welded joints of Experimental 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 7 also shows a schematic diagram of the cross section of the joint corresponding to the Vickers hardness measurement positions. Significant softening was observed in the heat-affected zone in Comparative Solid-State Welding Steel 1, Comparative Solid-State Welding Steel 2, and Comparative Solid-State Welding Steel 3. In contrast, in Experimental Solid-State Welding Steel 1, which contains 3.0% Mo, this softening was effectively suppressed, with a minimum hardness of 450 HV or higher. Additionally, the Vickers hardness of the stir and heat-affected zones was 80-120% of the Vickers hardness of the base material.

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 material. Here, there was little difference between the addition of 0.5% and 1.0% V, indicating that softening in the heat-affected zone was suppressed even with a relatively small amount of V. Furthermore, the soaking temperature did not significantly affect the minimum hardness. On the other hand, compared to the case where 3.0% Mo is added, the minimum hardness is a slightly lower value.

1...Solid-state bonded joint,
2, 4... material to be joined,
6...Solid phase junction.

Claims (6)

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: The C content is 0.20 to 0.45 mass%;
2. The solid-state welded joint according to claim 1, wherein: 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: 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: 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: The solid-state welded joint according to any one of claims 1 to 5 is included.
A solid-state bonded structure characterized by: