JPH0331784B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a method for producing a β-type titanium alloy material having excellent strength and ductility. [Prior art] Ti-15%V-3%Cr-3%Sn-3%Al alloy,
Ti-3%Al-8%V-6%Cr-4%Mo-4%
β-type titanium alloys such as Zr alloys have good cold workability and are sometimes used as cold-rolled sheets, cold-drawn bars, and wire rods. The strength of these β-type titanium alloy materials can be increased by direct aging treatment after cold working, and the strength increases with increasing cold working rate.
%V-3%Cr-3%Sn-3%Al alloy cold-rolled sheet can achieve a maximum strength of 165Kgf/ ^mm2 . However, in this case, the elongation is approximately 1%, and as the strength increases, the ductility value decreases while maintaining an inverse correlation, so in practical use, heat treatment conditions are adopted that can suppress the strength and secure the ductility value. These cold-worked materials of β-type titanium alloys are subjected to solution treatment and aging treatment after cold working, or cold working and aging treatment before use. If the solution temperature is set to a relatively low temperature by cold working or by solution treatment, and a moderate amount of cold working strain remains, the precipitation and refinement of α crystals will be promoted during aging, and the strength will be reduced by cooling. This can be increased by increasing the machining rate. For example, recrystallization progresses in the solution treatment step after cold working, but by selecting the solution treatment conditions as described above, a structure in which a uniform fine substructure remains within the grains can be obtained. When the solution-treated material in which this lower dislocation structure remains is aged, the existence of these uniform and fine lower dislocations promotes and uniformizes the α-crystal aging precipitation. As a result, high strength can be obtained. However, dislocations are more likely to accumulate at grain boundaries and near grain boundaries than inside grains.
For this reason, α crystals tend to precipitate in layers at grain boundaries during aging. Therefore, in cold-worked materials manufactured using conventional methods, as the cold working rate increases, the precipitation of α-crystals at grain boundaries is significantly accelerated compared to inside the grains, making grain boundary fracture more likely to occur. Therefore, the strength limit of cold-worked β-type titanium alloy material is about 165 Kgf/mm ² , and when it is made into high-strength material, it has the drawback of low ductility. [Problems to be solved by the invention] With this, we have filed a patent application for a new cold rolling method as a method for manufacturing high-strength β-type titanium alloy cold-rolled sheets.
Proposed as No. 237140. This new cold rolling method is
(Solution treatment) - Cold rolling - Intermediate solution treatment - Cold rolling - Solution treatment - Aging steps are taken. That is, the structure into which strain has been introduced by cold rolling in the preceding stage of final cold rolling is transformed into a recrystallized structure with a uniform and fine lower dislocation structure remaining within the grains by intermediate solution treatment, and the intermediate solution treated material with this lower dislocation structure remaining is formed by intermediate solution treatment. By applying mild cold working to the material and further performing solution treatment, only the recovery phenomenon progresses, and a uniform fine substructure containing even more uniform and fine dislocations, including within the grains and at the grain boundaries, is obtained. Therefore, during aging, the aging precipitation of α-crystals is promoted and made uniform through the grain boundaries and within the grains, and as a result, grain boundary fracture is less likely to occur, resulting in higher strength and higher strength than conventionally produced materials. A ductile cold-rolled sheet (strength of 170 Kgf/mm ² or more, elongation of 5% or more) will be manufactured. However, this new cold rolling method had the disadvantage that it was only effective for relatively thin sheets with a thickness of about 2 mm or less. The present invention was devised in view of the above-mentioned problems, and further improves the new cold rolling method described above to combine both the cold working process and the solution treatment process in the heat treatment after the cold working. Through strict control, it is possible to apply the effects of the new cold rolling method regardless of the sheet thickness. [Means for Solving the Problems] Therefore, in the present invention, a β-type titanium alloy material is cold-worked at a processing rate of 30% or more before the final cold-working, and then subjected to two-stage cold working in the β-range temperature range. After completing recrystallization by increasing the temperature at a temperature increase rate of ℃/sec or more2
The intermediate solution treatment is completed by cooling to 300℃ or less at a cooling rate of ℃/sec or more, and final cold working is performed at a processing rate of 3% or more but less than 30%, and in the subsequent final solution treatment, β 2 in the temperature range
It is characterized in that the temperature is raised at a temperature increase rate of ℃/sec or more and then cooled to 300℃ or less at a cooling rate of 2℃/sec or more. That is, in the present invention, the heating and cooling rates during intermediate and final solutioning are controlled to such a rate that precipitation of α crystals is not observed during heating and cooling in the α+β region. If the temperature increase rate during solutionization is slow, α crystals will precipitate on the underlying structure in the α + β region during heating to the β single phase region, but the precipitated α crystals will be
Since it remains for a while even after the temperature has been raised in the region, it locally inhibits the recovery phenomenon of the underlying structure, resulting in a non-uniform recovery structure and α crystal precipitation during aging resulting in non-uniformity and low strength. In addition, if the cooling rate during solution treatment is slow, α crystals will precipitate on the recovered substructure during the cooling process, and the precipitated α crystals will enlarge during aging, resulting in a non-uniform aged structure with low strength. Become. However, in the present invention, by controlling the temperature rise and cooling rate during solution treatment, a homogeneous recovered structure can be obtained regardless of the plate thickness, so the effects of the new cold rolling method can be fully utilized. Numerical limitations in the configuration of the present invention will be explained below. The reason why the cold working rate in the previous stage of the final cold working is specified as 30% or more is because if the cold working rate is less than 30%, recrystallization does not progress to intermediate solution formation, and the final product becomes coarse grained. However, the distribution of residual cold working strain after intermediate solution treatment is uneven and the density is sparse, and therefore, due to the non-uniformity of residual cold working strain, solution treatment after final cold working is difficult. This is because the distribution of residual cold working strain becomes uneven and the density becomes coarse, making it impossible to obtain a cold worked material with material properties of high strength and high ductility after aging. Additionally, there is no upper limit to the processing rate due to the material, but if the processing rate exceeds 90%, edge cracks will occur at the edges of the cold-rolled sheet, and in order to prevent deterioration of product properties, the processing rate should be 90%.
The following are desirable. In addition, the cold working rate in the final cold working was specified as 3% or more because at a cold working rate of less than 3%, strain is introduced unevenly due to cold working. This is because aging precipitation of α crystals becomes non-uniform and the material properties of high strength and high ductility are lost. In addition, the cold working rate of the final cold working was specified to be less than 30% because at a cold working rate of 30% or more, recrystallization progresses during final solutionization, and the effect of adding processing strain due to the final cold working. This is because it will be lost. Furthermore, the temperature rise and cooling rates during intermediate solution treatment and final solution treatment were specified to be 2℃/sec or more.
Temperature rise at less than sec, cooling rate α
This is because the aging precipitation of α crystals in the final aged material becomes uneven or enlarged, and the material properties of high strength and high ductility are lost. Note that the conditions for heating and cooling rates are not particularly specified, but if they exceed 100°C/sec, distortion will occur in the shape of the material, so the upper limit for these is 100°C/sec.
It is preferable to set it to sec. Furthermore, the heating temperature during intermediate solution formation and final solution formation is defined as the β range temperature range because α crystals will precipitate during modification if heating to a temperature below the β transformation point. The upper limit of the heating temperature is not particularly specified either, but if it exceeds the β-transformation point +200°C, grain growth will occur and the material will deteriorate, so it is preferable to set the upper limit to the β-transformation point +200°C. Furthermore, the reason why the cooling rate during intermediate solution formation and final solution formation is specified to be up to a temperature of 300℃ or less is that if the cooling rate is controlled to a temperature of 300℃ or higher, cooling from that temperature to a temperature of 300℃ is also specified. This is because α crystals precipitate inside. The reason why the precipitation of these α crystals deteriorates the quality of the final aged material is the same as the above-mentioned use. Further, although regulation of the lower limit of the cooling stop temperature is not particularly necessary from the viewpoint of the material, room temperature is adopted from an industrial standpoint for cost reasons. [Example] Typical β-type alloy Ti-15%V-3%Cr
- 550mm diameter ingot of 3%Sn-3%Al alloy at 1050℃
After heating, a slab was created by hot forging to a thickness of 200mm. Table 1 shows the chemical composition of the sample materials used (β transformation point Tβ = 729°C). The above slab was heated to 950°C, hot-rolled to a thickness of 80mm, and then subjected to solution treatment at 800°C for 20 minutes to obtain a cold-rolled material. In cold rolling, samples with a thickness of 55 mm to 7.0 mm are cut from the above hot rolled sheet (80 mm thick) and used as cold rolling materials, which are then subjected to primary cold rolling (cold rolling before final cold rolling: rolling). Rate 20~
80%) and secondary cold rolling (final cold rolling: rolling reduction 0 to 50
%) to produce a cold-rolled plate with a final thickness of 5 mm (10 mm in some cases). The intermediate and final solution treatment conditions for cold-rolled materials are
The temperature was 710°C to 900°C for 1 to 20 min, and the temperature raising and cooling rates during solutionization were varied from 1.0°C/sec to 10°C/sec. The aging conditions were 510°C x 14 hours → air cooling, and the mechanical properties of the heat-treated materials were 12.5 mm in width at the parallel part.
A plate-shaped tensile test piece of mmG.L.50mm was taken in the L direction and investigated. Table 2 shows the cold rolling, solution treatment conditions, and material properties of the cold rolled material obtained thereby. From the same table, it can be seen that excellent material properties such as strength of 170 Kgf/mm ² or more and elongation of 5% or more are obtained only when the cold rolling and heat treatment conditions of the present invention are satisfied.

【table】

【table】

〔Effect of the invention〕

As detailed above, according to the method for manufacturing β-type titanium alloy material of the present invention, both the cold working process and the solution treatment process in the heat treatment after cold working are strictly controlled (especially By controlling the heating rate and cooling rate during solution treatment, it has the excellent effect of making it possible to manufacture β-type titanium alloy materials with excellent strength and ductility even when the plate thickness increases. .

Claims (1)

[Claims] 1 Cold working is performed on the β-type titanium alloy material at a processing rate of 30% or more before the final cold working, and then,
After completing recrystallization by raising the temperature to the β region temperature range at a heating rate of 2°C/sec or more, the intermediate solution treatment is completed by cooling to 300°C or less at a cooling rate of 2°C/sec or more. , further perform final cold working at a processing rate of 3% or more and less than 30%, and in the subsequent final solution treatment, the temperature is raised to the β region temperature range at a heating rate of 2°C/sec or more, and then 2°C/sec or more. with a cooling rate of 300
A method for producing a β-type titanium alloy material with excellent strength and ductility, characterized by cooling to below ℃.