JP5692622B1 - Martensite steel - Google Patents

Abstract

Provided is a martensitic steel that imparts ductility while maintaining the excellent strength characteristics of the martensitic steel, that is, to achieve both strength and ductility. C: 0.18-0.30% by mass, Al: 1.0-2%, V: 0.3% or less, Cr: 2.0-5.0%, Ni: 10.5-15. 0%, however, Ni ≧ 7.0 + 3.5Al, Co: 5.0 to 7.0%, Mo alone or Mo + W / 2: 1.0 to 4.0%, selective elements B, Si, Mn, Ca, Rare earth element, Cu, Nb, B: 0.0050% or less, Si: 0.4% or less, Mn: 0.4% or less, Ca: 0.05% or less, rare earth element: 0.05% or less, Cu : Martensitic steel containing 1.0% or less, Nb: 0.1% or less, with the balance being composed of Fe and impurities, the martensitic steel is Mg: 40 ppm or less (excluding 0%) Further martensitic steel contained.

Description

  The present invention relates to martensitic steel.

Martensitic steel has excellent mechanical properties, high quenching hardness, and excellent wear resistance and fatigue strength. Therefore, steel for mechanical structures such as bearings, steel for knives such as knives and scissors, and for automobiles. It is used in a wide range of fields as sliding parts and aircraft parts.
Conventionally, these materials have a plurality of manufacturing processes depending on the intended use and required steel ingot cleanliness. For example, when used as an aircraft member, a molten metal having a predetermined composition is vacuum-melted into steel. A lump is manufactured, and after it is remelted at least once, such as vacuum arc remelting (VAR) or electroslag remelting (ESR), it is manufactured by hot working or hot working and cold working. Has been.

By the way, in the method for manufacturing an aircraft member described above, for example, Patent Document 1 discloses a heat treatment method that is performed during a manufacturing process or after adjusting to a final product shape in order to obtain required mechanical characteristics.
Specifically, the heat treatment method disclosed in Patent Document 1 includes a softening and tempering treatment for the purpose of softening the material, a solution treatment and a quenching treatment for dissolving alloy elements in the material, and a cooling treatment. Sub-zero treatment to change untransformed austenite remaining in martensite, softening treatment to remove stress in the material caused by martensite transformation, and aging treatment or tempering to precipitate fine carbides and intermetallic compounds It is a method consisting of processing.

Special table 2008-539331

In the aircraft member manufactured by the manufacturing method disclosed in Patent Document 1 described above, in the aging treatment stage, β-NiAl type intermetallic compound and M ₂ C type carbide precipitate, and several% between the lath martensite. The reverse transformed austenite is formed. Thereby, it is possible to obtain martensitic steel excellent in strength and ductility.
However, the aging treatment is originally a treatment for precipitating the above-described carbides and intermetallic compounds in the material, and it is necessary to change the aging treatment temperature in order to adjust the amount of retained austenite by the aging treatment. When the aging treatment temperature changes, the shape and distribution of aging precipitates change while the distribution of reverse transformed austenite also changes, so it is very difficult to balance strength and ductility in production. was there. Further, since reverse-transformed austenite undergoes austenite transformation in the state of inheriting high-density dislocations existing in martensite, there is a problem that ductility like the austenite phase obtained by recrystallization cannot be expected.
An object of the present invention is to provide martensitic steel for imparting ductility while maintaining the excellent strength characteristics of martensitic steel, that is, to achieve both strength and ductility.

The present inventor, in the martensitic steel described in the above-mentioned Patent Document 1, whether or not the ductility can be improved by the addition of alloy elements, not only optimization of heat treatment conditions conventionally performed. investigated. As a result, it was found that when Mg was added, the strength level did not change, but the Charpy impact value, the tensile test elongation, and the drawing value improved. Further, the addition of Mg has an appropriate range in the addition amount, and it has been found that the effect of developing ductility is further enhanced by controlling the addition amount within the range and adding it, thereby reaching the present invention.
That is, the present invention, in mass%, C: 0.18-0.30%, Al: 1.0-2.0%, V: 0.3% or less, Cr: 2.0-5.0%, Ni : 10.5 to 15.0%, but Ni ≧ 7.0 + 3.5Al, Co: 5.0 to 7.0%, Mo alone or Mo + W / 2: 1.0 to 4.0%, the balance Is a martensitic steel having a composition comprising Fe and impurities and further containing Mg: 0.0010 to 0.0040 % .
In the present invention, B: 0.0050% or less, Si: 0.4% or less, Mn: 0.4% or less, Ca: 0.05% or less, rare earth element: 0.05% or less, Cu: 1 It may further contain one or more elements selected from 0.0% or less and Nb: 0.1% or less.
Preferably, it is a martensitic steel mainly containing martensite at room temperature and containing 3.0 to 20.0% austenite by volume%.

  According to the present invention, ductility can be imparted while maintaining the excellent strength characteristics of martensitic steel, that is, both strength and ductility can be achieved.

In the present invention, the most characteristic addition of Mg will be described. The unit of content of each element shown below is mass%.
Mg: 0.0010 to 0.0040 %
Mg is an element having specific properties that can improve ductility and toughness while maintaining the strength level of martensitic steel in the present invention. In general, ductility, toughness, and strength are contradictory properties, so that when one property is increased, the other property is lowered. Therefore, it has been very difficult to achieve both strength, ductility and toughness. The mechanism by which ductility and toughness are improved by adding Mg has not been clarified at this stage, but is considered as follows.
The martensitic steel of the present invention is an alloy in which β-NiAl or M ₂ C (M: Cr, Mo, V, etc.) carbides of intermetallic compounds are formed in the steel and the strength is increased by precipitation strengthening. On the other hand, Al contained in the alloy is easily combined with N in the steel to form nonmetallic inclusion AlN. If AlN is in a fine state, the degree of influence on the material properties is small, but if it is a coarse inclusion exceeding 10 μm, it becomes a starting point of fatigue failure and not only significantly lowers the strength but also causes a reduction in ductility. Therefore, Mg is considered to have an effect of suppressing ductility deterioration due to AlN by becoming MgO as an oxide during melting and becoming AlN crystallization or precipitation nuclei and finely dispersing AlN.
Further, inevitably contained impurity elements tend to diffuse and segregate to the grain boundaries in the hot working and heat treatment steps. When such segregation occurs, the grain boundary strength is impaired, and ductility cannot be obtained. However, when Mg is added, the impurity element forms precipitates with Mg, and the effect of suppressing segregation to the grain boundary, that is, It is considered that there is an effect of fixing the impurity element as a precipitate. It is considered that the grain boundary ductility is improved by such an effect.
If Mg is not added, the above-mentioned effects cannot be obtained. On the other hand, if Mg exceeds 0.0040%, a large amount of MgO is generated or coarsened, and functions as crystallization or precipitation nuclei as described above. Not only can be obtained, but it is combined with Al in the steel to form an Al-Mg oxide, and both strength and ductility are lowered. Therefore, Mg is 0.0010 to 0.0040 % . A preferable upper limit of Mg is 0.0020% .

In the present invention, components other than Mg are limited to the following ranges.
C: 0.18 to 0.30%
C is an element necessary for forming M ₂ C carbides by combining with Cr, Mo, W, and V, and contributes to improvement of strength. If C is less than 0.18%, the amount of carbide that contributes to strength improvement cannot be obtained. On the other hand, when C exceeds 0.30%, austenite is stabilized, so that a large amount of retained austenite remains during quenching. Therefore, C is set to 0.18 to 0.30%.
Al: 1.0-2.0%
Al is an element that precipitates as an intermetallic compound β-NiAl during aging treatment and contributes to strength improvement. If the Al content is less than 1.0%, a sufficient amount of intermetallic compound for contributing to strength improvement cannot be obtained. On the other hand, when Al exceeds 2.0%, an intermetallic compound will increase and toughness and ductility will be reduced significantly. Therefore, Al is set to 1.0 to 2.0%.
V: 0.3% or less V is an element that stabilizes metastable M ₂ C carbides and contributes to strength improvement. When V is 0% (no addition), M ₂ C carbide cannot be present stably. Therefore, V is added in an essential amount exceeding 0% (no addition). On the other hand, if V exceeds 0.3%, the M ₂ C stabilizing effect becomes excessive. Therefore, the upper limit of V is set to 0.3%. (If possible, I would like to set the lower limit of V. Also, the upper limit is also preferable. If other elements can also describe the preferable range, it should be described.)

Cr: 2.0-5.0%
Cr is an element that has the effect of stabilizing metastable M ₂ C carbides. If Cr is less than 2.0%, the effect of stabilizing M ₂ C cannot be obtained. On the other hand, when Cr exceeds 5.0%, M ₂₃ C ₆ is formed, and the carbide structure changes. Since the change in the carbide structure changes the strength characteristics, Cr is set to 2.0 to 5.0%.
Ni: 10.5 to 15.0% (However, Ni ≧ 7.0 + 3.5Al)
Ni is an element that combines with Al to form β-NiAl, an intermetallic compound, contributes to improving the strength, and also affects the martensite transformation start temperature (Ms point). Therefore, it is necessary to adjust Ni appropriately with respect to the amount of Al added. If the amount of Ni is less than 10.5%, the amount of β-NiAl in the intermetallic compound decreases and the strength decreases. On the other hand, when Ni exceeds 15.0%, the Ms point is significantly lowered, and a large amount of retained austenite is present during quenching. Therefore, Ni is made 10.5 to 15.0%.
However, the Ni content needs to satisfy Ni ≧ 7.0 + 3.5Al. As described in Patent Document 1, Ni ≧ 7.0 + 3.5Al is an index for obtaining an appropriate amount of an intermetallic compound necessary for maintaining high strength. If Ni does not become 7.0 + 3.5Al or more, the intermetallic compound decreases and the strength decreases.
Co: 5.0-7.0%
Co does not significantly affect the stability of the martensitic structure of the matrix and increases the solid solubility of aging precipitate-forming elements such as Mo and Al during the solution treatment, so that the solid solution of Mo and Al in the aging precipitation temperature range is increased. It is an important element contributing to aging precipitation strengthening by promoting precipitation of fine intermetallic compounds by lowering solubility. If Co is less than 5.0%, sufficient strength cannot be obtained. On the other hand, when Co exceeds 7.0%, austenite is stabilized and it becomes difficult to obtain a martensite structure. Therefore, Co is set to 5.0 to 7.0%.

Mo alone or Mo + W / 2: 1.0 to 4.0%
Mo and W are elements that combine with C to form M ₂ C carbides that contribute to strength improvement. If Mo alone or the total amount of Mo and W / 2 is less than 1.0%, carbide formation is insufficient. On the other hand, when Mo alone or the total amount of Mo and W / 2 exceeds 4.0%, a μ phase (Fe ₇ Mo ₆ ) of an intermetallic compound is formed and forgeability is lowered. Therefore, the total amount of Mo alone or Mo + W / 2 is set to 1.0 to 4.0%.
The elements described above are essential elements of the present invention, and the balance is Fe and impurities. However, when it is going to acquire the effect of the further strength improvement, it can contain any 1 type, or 2 or more types of elements of B, Si, Mn, Ca, rare earth elements, Cu, and Nb. In addition, since intensity | strength will fall when it exceeds the upper limit of each element shown below, if it adds selectively, it will limit to the following component range.
B: ≦ 0.0050%, Si: ≦ 0.4%, Mn: ≦ 0.4%, Ca: ≦ 0.05%, rare earth element: ≦ 0.05%, Cu: ≦ 1.0%, Nb : ≦ 0.1%

The martensitic steel of the present invention has a metal structure mainly composed of martensite at room temperature. The metal structure preferably contains 3.0 to 20.0% austenite phase by volume. This is because it is preferable to introduce a metal structure rich in workability in order to more reliably increase ductility (particularly elongation in tensile testing) without lowering the strength level.
In the present invention, the ductility can be further enhanced by forming an austenite phase of 3.0 to 20.0% by volume in martensite. When the volume fraction of the austenite phase is less than 3.0%, it becomes difficult to obtain the effect of imparting ductility. On the other hand, if the austenite phase is more than 20.0%, the ductility increases, but the strength may decrease. A more preferred lower limit of the volume fraction of the austenite phase is 4.0%, and a more preferred upper limit of the volume fraction of the austenite phase is 15.0%.
In addition, formation of this austenite phase is made into 3.0 to 20.0% residual austenite by volume% by making subzero treatment into the temperature range of -50 degreeC or more and 0 degreeC or less for 0.5 to 5 hours. It is possible to form.

Example 1
10 kg of each ingot of martensite steel shown in Table 1 was produced by a vacuum induction melting furnace, and a bar having a cross-sectional shape of 30 mm × 30 mm was obtained through homogenization annealing and hot forging.
Thereafter, normalization was performed at 870 ° C. for 1 hour in the air to obtain a material for annealing. An annealing material was obtained by annealing at 650 ° C. × 8 h using the annealing material described above, and then a solid solution treatment was performed at 900 ° C. × 1 h using the annealing material to obtain a solid solution treatment material.
Using the solution treatment material, a sub-zero material was obtained by performing a sub-zero treatment for 2 hours in an ethyl alcohol solution adjusted to a sub-zero temperature of −78 ° C., and then a low temperature aging treatment of 200 ° C. × 8 h and 495 An age hardening treatment at 10 ° C. for 10 hours was performed.

Various test pieces were cut out from the martensitic steel rods subjected to age hardening treatment, and subjected to a tensile test and a Charpy impact test.
In the tensile test, a block material of an arbitrary size was cut out from the bar material of martensite steel, and after processing into a tensile test piece shape having a parallel part length of 32 mm, a parallel part diameter of 6.35 mm, a test piece total length of 90 mm, and a grip part diameter of 10 mm, It carried out according to ASTM-E8. The Charpy impact test was carried out in accordance with JIS-Z2242, by cutting out a block material of an arbitrary size from a martensitic steel bar and applying a V-notch process of 10 mm × 10 mm × 5 mm and 2 mm in depth. The tensile test and Charpy impact test were performed at room temperature. Table 2 shows the measurement and test results.

  As shown in Table 2, the tensile strength hardly changes even when the Mg addition amount changes, and is constant, whereas the elongation and the Charpy impact value obtained in the tensile test are maximum when the Mg addition amount is 18 and 32 ppm. It can be seen that the value is shown. Thus, it can be seen that the addition of Mg in an appropriate range is a technique capable of improving the ductility while maintaining the strength level, and achieves both strength and ductility.

(Example 2)
No. 1 which is the martensitic steel of the present invention shown in Table 1. 1 and 2 were subjected to sub-zero treatment for 2 hours in an ethyl alcohol solution adjusted to -12 ° C., which is higher than the conventional conditions in order to leave austenite, and further low temperature aging treatment at 200 ° C. × 8 h. And an age hardening treatment of 495 ° C. × 10 h. The martensitic steel after the treatment is No. 1A and 2A.
Various test pieces were cut out from the martensitic steel bar subjected to age hardening treatment, and the amount of retained austenite was measured, a tensile test, and a Charpy impact test were performed.
The amount of austenite is measured by cutting a 10 mm x 10 mm x 5 mm size test piece from a martensitic steel bar, mechanically polishing the surface of the test piece, removing residual stress on the surface by electrolytic polishing, and measuring the peak of the X-ray diffraction method. Measured by intensity comparison. The amount of retained austenite was measured using a Rigaku X-ray diffractometer (RINT200) under the conditions of a Co source, a voltage of 40 kV, and a current of 200 mA (110) α, (200) α, (211) α, (200) γ , (220) γ, and (311) γ were calculated using the diffraction line intensity ratio obtained from each surface.
In the tensile test, a block material of an arbitrary size was cut out from the bar material of martensite steel, and after processing into a tensile test piece shape having a parallel part length of 32 mm, a parallel part diameter of 6.35 mm, a test piece total length of 90 mm, and a grip part diameter of 10 mm, It carried out according to ASTM-E8. The Charpy impact test was carried out in accordance with JIS-Z2242, by cutting out a block material of an arbitrary size from a martensitic steel bar and applying a V-notch process of 10 mm × 10 mm × 5 mm and 2 mm in depth. The tensile test and Charpy impact test were performed at room temperature. Table 3 shows the measurement and test results.

As shown in Table 3, the No. of the present invention having a high austenite amount. 1A and 2A have the same tensile strength as that of the conventional example, but the elongation, drawing and Charpy impact value of the tensile test are increased as compared with the conventional example, and it can be seen that the ductility is improved. In addition, No. of the present invention 1A and 2A are also No. 1 of the present invention. Compared with 1 and 2, the Charpy impact value and the drawing value of the tensile test are the same level, but it can be confirmed that the elongation is high and the ductility is improved. It can be seen that austenite has the effect of enhancing the effect of increasing ductility while maintaining the strength characteristics. The metal structure of the test piece was a martensite structure except for the retained austenite (γ _R ) in the amount shown in Table 3 except for precipitates such as carbides.

As described above, according to the present invention, it is possible to achieve both excellent ductility while maintaining the excellent strength characteristics of martensitic steel.

Claims (3)

C: 0.18 to 0.30% by mass, Al: 1.0 to 2.0%, V: 0.3% or less, Cr: 2.0 to 5.0%, Ni: 10.5 to 15.0%, but Ni ≧ 7.0 + 3.5Al, Co: 5.0-7.0%, Mo alone or Mo + W / 2: 1.0-4.0%, the balance being Fe and impurities In martensitic steel having the composition
Mg: A martensitic steel characterized by further containing 0.0010 to 0.0040 % .   B: 0.0050% or less, Si: 0.4% or less, Mn: 0.4% or less, Ca: 0.05% or less, rare earth element: 0.05% or less, Cu: 1.0% or less, and Nb The martensitic steel according to claim 1, further comprising one or more elements selected from 0.1% or less.   The martensitic steel according to claim 1 or 2, wherein the metal structure is mainly composed of martensite and contains 3.0 to 20.0% austenite by volume.