JPS643941B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial field of application] This invention has an α grain size of 100 μm as hot-rolled.
The present invention relates to a method for rolling a large-diameter pure Ti billet having a diameter of 100 mm or more and having the following fine-grain recrystallized structure. [Prior Art] Pure Ti has a lower thermal diffusivity than steel, and this tendency becomes particularly large in the low temperature range, with a value 1/2 that of steel at room temperature. Figure 5 shows a heat conduction model that takes into account heat generation during machining based on thermal constants and deformation resistance values.
The average cooling rate from the end of rolling to 600°C for each billet diameter when rolling was performed at a rolling reduction rate of 10% per pass and a time between passes of 10 seconds and left to cool is shown.
As the billet diameter increases, the cooling rate of the shaft center decreases, especially when the billet diameter is 100 mm or more, the cooling rate decreases to 0.7℃/s.
It cools down at an extremely low rate. Therefore, when rolling such a large-diameter billet of pure Ti, there is a problem in that significant grain growth occurs during the cooling process after rolling, making it impossible to obtain fine grains. On the other hand, pure Ti billets are often processed and formed by final hot forging or hot extrusion at the user's end, but in this case, the α grain size of the billet should be 100 μm or less from the viewpoint of processing and formability. Very fine grains are often required. Therefore, when such a fine α grain size is specified, hot forging, which involves repeated heating and processing, has been used. [Problems to be Solved by the Invention] However, although fine grains can be obtained by the hot forging described above, the yield and productivity are inferior to those produced by hot rolling. On the other hand, in the conventional hot rolling method, no consideration is given to grain growth during the cooling process after rolling, and the billet size is uniformly reduced regardless of the billet diameter.
Since the billets are hot-rolled in the high temperature range of 850°C or higher or in the β range, coarse grains of 100 μm or more are currently formed in the axial center of large-diameter billets where the cooling rate after rolling is slow. Therefore, there has been a desire for a manufacturing method that can produce fine grains of 100 μm or less even in large-diameter billets with a diameter of 100 mm or more, even after hot rolling. [Means for Solving the Problems] In order to solve these problems, the present invention investigated the grain growth behavior of pure Tiα grains during the cooling process after multi-pass processing, and in pure Ti billets with a diameter of 100 mm or more, We have discovered a rolling method that allows a fine grain structure of 100 μm or less to be obtained. The present invention will be explained in detail below. Generally, in billet rolling of pure Ti, processing is repeated at a strain rate of 1 to 20/s and a rate of 5 to 15% per pass at intervals of 2 to 15 seconds between passes. As shown in FIG. 5, the cooling rate after processing varies depending on the billet diameter and also varies depending on the position in the axial direction of the billet. The present inventors simulated these conditions by interstitial compression tests, added the pattern shown in Figure 1, and tested the purity under various conditions.
The grain growth behavior of Tiα grains was investigated. Here, the pattern shown in Figure 1 means that 5 passes are performed at a heating temperature with a reduction rate of 10%, a strain rate of 2.5/s, and an interpass time of 10 seconds, and after processing, various cooling rates are applied, that is, cooling In addition, assuming accelerated cooling after processing, the speed is 0.1℃/s to 1℃/s.
s and then cooled. or,
In this experiment, in order to simultaneously examine the influence of the pre-rolling conditions on the α grain size during heating (before deformation), the α grain size before deformation was significantly varied from 80 μm to 1600 μm. Furthermore, as shown in Fig. 1, the machining is always carried out at an isothermal T.
The reason for this is that in the axial center of such a large-diameter billet, the heat generated by machining and the temperature drop due to heat conduction to the surface are balanced, resulting in almost no temperature drop. Figure 2 shows the changes in the recrystallized α grain size in the above experiment, using pure Ti containing 0.12% O, changing the initial α grain size and heating temperature, and cooling at 0.15°C/s after processing. shows. As is clear from the figure, the same deformation conditions (in this case, the above processing conditions)
In this case, the finer the initial α grain size (before deformation), the finer the recrystallized α grains can be obtained after processing, but the influence of the processing temperature (indicated by the heating temperature in the figure) is more important than the influence of the initial α grain size. dominant. At a cooling rate of 0.15°C/s, heating to a temperature lower than 825°C is required to reduce the α grain size after processing (indicated by the average α grain size in the figure) to 100 μm or less regardless of the initial α grain size. be. Also, considering the recrystallization rate shown in the same figure,
It can be seen that a complete recrystallized structure cannot be obtained by heating at 675°C. Furthermore, Figure 3 shows an investigation of the recrystallized α grain size when the heating temperature (equal to the processing temperature) and cooling rate after processing were changed when the initial α grain size was 1600 μm under the processing conditions and procedures described above. This shows the results. Same heating deformation conditions (same heating temperature)
In this case, as the cooling rate decreases, grain growth occurs during cooling, and the recrystallized α grain size increases. In addition, at any cooling rate, the change in the recrystallized α grain size is more pronounced as the heating temperature reaches a higher temperature range, and the slower the cooling rate is to obtain the same recrystallized α grain size, in other words, the larger the billet is, the more the hot rolling temperature is required. needs to be lowered. From the results shown in FIG. 3, in order to make the recrystallized α grain size 100 μm or less, the heating temperature Tr ₁ (°C) must satisfy the following equation in the α region as a function of the cooling rate R (°C/s). Tr ₁ ≦853+42.5logR... On the other hand, when heating to 700°C, if the cooling rate is fast, recrystallization will not be completed during cooling, resulting in an unrecrystallized structure. Therefore, in order to obtain a complete recrystallized structure, the finishing temperature Tr ₂
(°C) must satisfy the following formula in the α region as a function of the cooling rate R (°C/s). Tr ₂ ≧710+15logR ... In actual rolling, the temperature of the surface layer portion significantly drops during rolling, so the hot rolling temperature Tr ₂ described above corresponds to the surface finishing temperature. Therefore, in order to obtain a recrystallized grain size of 100 μm or less from the surface layer to the axial center, it is necessary to first set Tr ₁ ≦853+ to suppress grain growth in the axial center.
In order to heat to 42.3 logR and further recrystallize up to the surface layer, it is necessary to finish rolling at a surface temperature Tr ₂ ≧710+15 logR. Based on the fact that the billet diameter and the cooling rate are inversely proportional as mentioned above, the following experiment was further conducted in order to make the cooling rate in the above equation correspond to the billet diameter during cooling. That is, when pure Ti with a billet diameter ranging from 50 mm to 200 mm was heated to 850°C and then allowed to cool, the cooling rate at the axial center and 2 mm below the surface was measured, and the data is summarized in Figure 4. Ta. The temperature range of 800°C to 600°C, which has a large effect on grain growth, was measured in the axial center, and the temperature range of 700°C to 600°C, which is the critical temperature for recrystallization, was measured in the surface layer. As a result, the cooling rates of the axial center portion and surface layer portion of the billet are expressed by the following equations. Center part logR＝3.0−1.6logd ... Surface layer logR＝1.12−0.93logd ... d: Billet diameter (mm) Therefore, if the above formulas are substituted into the above formulas, Tr ₁ ≦853 + 42.5 (3.0− Tr ₂ ≧710+15 (1.12−0.93logd) …… By heating to a temperature below Tr ₁ shown in the above formula and then finishing rolling at a surface temperature above Tr ₂ in the above equation, It is possible to produce a pure Ti billet with a fine α grain size of 100 μm or less from the surface layer to the center. The above shows the critical heating temperature Tr ₁ and critical finishing temperature Tr ₂ as a function of the billet diameter when the billet is left to cool after rolling. However, when accelerated cooling is performed after rolling, the above-mentioned The critical heating temperature Tr ₁ and the critical finishing temperature Tr ₂ are applied as a function of the cooling rate given by the formula: In addition, what is called pure Ti here is H≦0.015.
%, O≦0.40%, N≦0.05%, Fe≦0.50% and other unavoidable impurities.
(Commercial Pure Ti). [Example] Next, an example of the present invention will be described. Table 1 below shows the α grain size of the billet axial center and surface layer when billet rolling is performed using a test material having the components shown in Table 2, and the α grain size at that time calculated using the above formula. The critical heating temperature Tr ₁ and the critical finishing temperature Tr ₂ calculated using the same formula are shown together with each rolling condition.

[Table] *…Continuous mill rolling

〔Effect of the invention〕

As explained above, according to the method of the present invention, it is possible to refine the alpha grains of pure Ti large-diameter billets by hot rolling, which has been difficult in the past, and as a result, yield and productivity can be improved. It has this excellent effect.

[Brief explanation of drawings]

Figure 1 is an explanatory diagram of processing patterns used in various experiments, Figure 2 is a graph showing changes in average α grain size and recrystallization rate according to heating temperature, depending on initial grain size, and Figure 3 is a graph showing the change in α grain size due to changes in heating temperature and cooling rate, Fig. 4 is a graph showing the average cooling rate for each billet diameter when the billet is allowed to cool, and Fig. 5 is a graph showing the cooling rate for each billet diameter. It is a graph diagram showing changes in speed.

Claims (1)

[Claims] 1. When rolling a pure Ti large-diameter billet with a diameter of 100 mm or more, after heating it to a temperature below the critical heating temperature Tr ₁ shown in the following formula (1), Finish rolling at a surface temperature above critical finishing temperature Tr ₂ ,
A method for rolling a pure Ti large-diameter billet having a fine-grain recrystallized structure as hot-rolled, characterized by allowing it to cool. Tr ₁ = 853 + 42.5 (3.0 - 1.6 logd) ... (1) Tr ₂ = 710 + 15 (1.12 - 0.93 logd) ... (2) Tr ₁ , Tr ₂ : Unit: °C d: Diameter of billet, unit: mm 2 Diameter When rolling a large-diameter pure Ti billet of 100 mm or more, after heating it to a temperature below the critical heating temperature Tr ₁ shown in equation (3) below, the surface reaches a critical finishing temperature Tr ₂ or above, also shown in equation (4) below. Finish rolling at temperature,
A method for rolling a large-diameter pure Ti billet having a fine-grain recrystallized structure as hot-rolled, which further comprises performing accelerated cooling at a cooling rate R shown in both equations. Tr ₁ = 853 + 42.5logR ... (3) Tr ₂ = 710 + 15logR ... (4) Tr ₁ , Tr ₂ : Unit: °C R: Unit: °C/s