JP6897521B2 - Hot rolling method of titanium material - Google Patents

Description

The present invention relates to a method for hot rolling a titanium material.

Titanium material is generally produced by forming an ingot obtained from a melting step into a slab or billet shape by a slabbing step, cleaning the surface, hot rolling, and further annealing or cold working. In addition to the widely used vacuum arc remelting (VAR) method, the melting process includes an electron beam remelting (EBR) method in which melting is performed in a place other than the mold and poured into the mold, and plasma. There is a dissolution method and the like.
In the former case, since the mold is limited to a cylindrical type, a lump or forging process is indispensable for manufacturing the plate material. The latter has a high degree of freedom in mold shape, and a square mold as well as a cylindrical mold can be used. Therefore, if the electron beam melting method or the plasma melting method is used, a square ingot or a cylindrical ingot can be directly cast. Therefore, when the plate material is manufactured from the square ingot, or when the bar or wire is manufactured from the cylindrical ingot, the slabbing step can be omitted from the point of view of the ingot shape. In this case, since the cost and time required for the slabbing process can be omitted, it is expected that the production efficiency will be significantly improved.

However, in the as-cast structure of a large ingot used industrially, coarse particles having a crystal grain size of several tens of mm are formed. When such an ingot is directly hot-rolled without undergoing a slabbing step, the surface becomes uneven due to the influence of deformation anisotropy within and between crystal grains due to the coarse crystal grains. It becomes a surface defect. Therefore, if a square ingot or a cylindrical ingot is directly manufactured by the electron beam melting method or the plasma melting method and the slabbing step is omitted, surface defects will occur in the subsequent hot rolling. In order to remove surface defects generated by hot rolling, it is necessary to increase the amount of wrought on the surface of the hot-rolled sheet in the pickling process, which causes a problem of deteriorating cost and yield. That is, it is necessary to newly introduce a rectification process for removing surface defects. Therefore, there is a concern that the improvement in production efficiency expected by omitting the slabbing step will be offset by the new introduction of such a refining step. To solve such problems, a method for producing a material for hot rolling and a method for reducing surface defects by processing or heat-treating after the production have been proposed.

In Patent Document 1, when a titanium ingot is directly heat-rolled by omitting the slabbing step, the surface layer is strained in order to refine the crystal grains near the surface layer, and then the recrystallization temperature is obtained. A method of heating to a depth of 2 mm or more from the surface has been proposed. Examples of means for applying strain include forging, roll reduction, shot blasting, and the like.

In Patent Document 2, a titanium ingot is _{heated to T β} + 50 ° C. or higher _{, cooled to T β} -50 ° C. or lower, and then hot-rolled. A method of reducing waviness and wrinkles on the formed surface and reducing surface defects has been proposed.

In Patent Document 3, as a method for reducing surface defects of a rolled product when undergoing a slabbing process in a titanium material, the temperature at the end of the slabbing process is set to the α range, or the heating before hot rolling is set to the α range. A method has been proposed in which 60 μm or more from the surface is equiaxed. As a result, it is possible to prevent the oxygen-rich layer from becoming partially deep, the oxygen-rich layer can be removed in the descaling step, and the non-uniform hardness and ductility are eliminated, so that after cold working. It is said that the surface texture will improve.

In Patent Document 4, when a titanium ingot is directly hot-rolled by omitting the hot working step, the surface layer of the surface corresponding to the rolled surface of the ingot is subjected to high-frequency induction heating, arc heating, plasma heating, and electron beam heating. A method of improving the surface structure after hot rolling by granulating a depth of 1 mm or more from the surface layer by melting and resolidifying by laser heating or the like is mentioned. This prevents the occurrence of surface defects by forming a solidified structure having a fine and irregular orientation on the surface layer portion by quenching solidification. Examples of the method for melting the surface layer structure of the titanium slab include high frequency induction heating, arc heating, plasma heating, electron beam heating, and laser heating.

In Patent Document 5, a titanium material for hot rolling is provided with dimples having an average height of 0.2 to 1.5 mm and an average length of 3 to 15 mm of undulation contour curve elements by cold plastic deformation. Therefore, a method has been proposed in which surface defects caused by hot rolling are minimized even if the breakdown step of the ingot is omitted.

In Patent Document 6, in the cross-sectional structure of the slab in which the titanium slab melted in the electron beam melting furnace is directly drawn from the inside of the mold, the angle θ between the solidification direction from the surface layer to the inside and the casting direction of the slab is 45 ° to When the angle formed by the normal line between the c-axis of hcp and the surface layer of the slab is 35 ° to 90 ° in the crystal orientation distribution of 90 ° or the surface layer, the casting surface is cast even if the hot working step is omitted. A method is disclosed in which the surface defects after hot rolling can be improved. That is, by controlling the shape and crystal orientation of the crystal grains on the surface, it is possible to suppress the occurrence of defects caused by such coarse crystal grains.

Japanese Unexamined Patent Publication No. 01-156456 Japanese Unexamined Patent Publication No. 08-060317 Japanese Unexamined Patent Publication No. 07-102351 JP-A-2007-332420 International Publication No. 2010/090352 International Publication No. 2010/090353

The methods proposed in Patent Documents 1 to 5 require processing of the surface of the titanium material and heat treatment before hot rolling. Further, the method proposed in Patent Document 6 is difficult to control to a structure aiming at the entire surface of the ingot due to variations in operating conditions at the time of casting, and in some cases, surface defects due to a coarse casting structure occur. , There is a concern that the surface texture may deteriorate.

By the way, the main surface flaws generated by hot rolling of titanium material are heald flaws, edge healds, and diving flaws. Of these, the hege flaw is a flaw in which metallic titanium partially peeled off from the surface of the titanium material covers the surface of the titanium material, as shown in FIG. 1, which is a cross-sectional view of the surface after hot rolling. is there.

This bald defect also occurs on the surface due to the effects of deformation anisotropy within the crystal grains and deformation anisotropy between the crystal grains due to the coarse crystal grains. Of these, hesitation defects caused by the effects of deformation anisotropy within crystal grains (hereinafter referred to as "intra-grain origin hege defects") are hege defects caused by the effects of deformation anisotropy between crystal grains (hereinafter referred to as "intra-grain origin hege defects"). Hereinafter, the number is larger than that of "grain boundary-derived hesitation defects"), so it has a serious effect. However, on the other hand, intragranular-derived heald flaws tend to have shallower flaw depths and shorter flaw lengths than grain boundary-derived heald flaws. It has been empirically confirmed that when the number of intergranular-derived bald spots is small, almost no grain boundary-derived bald spots occur. Therefore, it is desired that the conditions of hot rolling are optimized without depending on the processing, heat treatment, and casting conditions before hot rolling to prevent the occurrence of intragranular blemishes.

Therefore, in the present invention, it is possible to suppress the occurrence of intragranular-derived bald spots that occur after hot rolling and maintain good surface properties without depending on processing before hot rolling, heat treatment, or casting conditions. , An object of the present invention is to provide a method for hot rolling a titanium material.

The gist of the present invention is as follows.
(1) A method for hot rolling a titanium material in which the titanium material is heated to a hot rolling temperature and hot rolled by a rolling roll.
The temperature of the titanium material during hot rolling is below the β transformation point at the surface temperature.
In the first 1 pass in hot rolling, the surface temperature of the titanium material one pass before the first pass is T ₀ [° C], the surface temperature of the rolling roll one pass before the first pass is T ₁ [° C], and the first 1 The thickness of the titanium material before the pass is h ₀ [mm], the thickness of the titanium material after the first pass is h ₁ [mm], the diameter of the rolling roll is D [mm], and the rotation speed of the rolling roll is r. A method for hot rolling a titanium material, characterized _{in that T 2} calculated by the following equation (1) is 670 or more when [rpm] is set.

(2) The method for hot rolling a titanium material according to (1), wherein the rolling roll is preheated and the surface temperature T ₁ [° C.] thereof is set to 200 ° C. or higher and 500 ° C. or lower.
(3) The method for hot rolling a titanium material according to (1) or (2), wherein the titanium material is a JIS standard industrial pure titanium or a titanium material of an α-type titanium alloy. ..
(4) The titanium according to any one of (1) to (3), wherein the titanium material before the first pass is characterized in that its crystal particle size is 500 μm or more in average particle size. Hot rolling method of material.

According to the present invention, by optimizing the hot rolling conditions, it is possible to suppress the occurrence of in-grain-derived scabs, so that processing and heat treatment before hot rolling are not required, and casting conditions that are difficult to control are managed. The surface texture of the titanium material can be kept good without the need for it.

It is a cross-sectional photograph which shows the example of the dent of the titanium material after hot rolling. It is a cross-sectional photograph showing the occurrence and extension process of a bald defect. It is a cross-sectional photograph showing a hot twin crystal which became a starting point of an intragranular scab.

The background of the development of the present invention will be described.
First, the findings obtained regarding the development and extension process of scabs will be described with reference to FIG. At the initial stage of rough rolling, which is after the first two passes of hot rolling (a rolling reduction of about two passes is 22%), a dent with a depth of about 50 to 15 μm is generated on the surface of the titanium material. This dent is found after rough rolling (7 passes including the initial pass in rough rolling, the reduction rate of rough rolling is 85%) and after finish rolling (from before rough rolling to after finish rolling, the reduction rate is 85 to 90%). ), The metallic titanium that has been stretched and largely peeled off from the surface becomes a scab that covers the surface of the titanium material.

Both the intragranular-derived baldness defect and the grain boundary-derived baldness defect are generally generated and extended by the extension of the dents generated at the initial stage of rough rolling. Therefore, if the occurrence of dents at the initial stage of rough rolling is suppressed, the occurrence of dents can also be suppressed.
Therefore, as shown in FIG. 3, the structure of the dented portion that causes the intragranular blemishes was analyzed. According to FIG. 3, it can be seen that a dented portion is generated on the surface of a specific structure generated in the crystal grain. Crystal orientation analysis of this structure by EBSD (Electron Back Scatter Diffraction Patterns) revealed that it was a coarse hot twin. Therefore, it is considered that the generation of dents can be suppressed by not generating the hot twin structure at the initial stage of rough rolling.

Specifically, as a result of examining various changes in the hot rolling conditions, the surface temperature of the titanium material in the first pass of rough rolling of the hot twins, which is the cause of the intragranular baldness, is lowered. It became clear that it tends to occur when rolling.

Therefore, under what conditions the first 1 pass of the rough rolling tends to lower the surface temperature of the titanium material after the first 1 pass of the rough rolling, and whether intragranular blemishes occur. It was investigated.
The effect on the surface temperature of the titanium material was considered to be the heat removal caused by the contact of the rolling roll at room temperature with the surface of the heated titanium material. The heat removal from the rolling roll increases as the contact time t [sec] between the rolling roll and the titanium material increases, and the surface temperature of the titanium material decreases. For t [sec], the thickness of the titanium material before the first pass is h ₀ [mm], the thickness of the titanium material after the first pass is h ₁ [mm], and the diameter of the rolling roll is D [mm]. ], If the rotation speed of the rolling roll is r [rpm], it is expressed by the following equation (2).

Experimentally, when this t [sec] is related to the ease of changing the surface temperature of the titanium material around the first pass (hereinafter referred to as the temperature reduction index ΔT (° C.)), ΔT is , Ln (t) was found to have a relationship as shown in the following equation (3).

_{On the other hand, it is the titanium material surface temperature T 0} [° C.] before rough rolling that most simply affects the surface temperature of the titanium material. The _{higher the T 0} [° C.], the higher the temperature after passing one pass, of course.
Further, as for heat removal from the rolling roll, the _{lower the surface temperature T 1} [° C.] of the rolling roll one pass before, the larger the cooling capacity of the rolling roll, and therefore the lower the surface temperature of the titanium material. Substituting equation (2) into equation (3), and further _{considering the effects of T 0} [° C] and T ₁ [° C], obtain the following equation (4) to obtain the temperature reduction index ΔT (° C). lead. This temperature decrease index ΔT cannot be calculated exactly as the temperature decrease, but is an index for calculating _{T 2 which can evaluate the degree of generation of hot twins, which will be described later.}

The surface temperature of the titanium material one pass before the first pass _{is subtracted from T 0} [° C.] by subtracting ΔT obtained by the equation (4) as in the following equation (5) to obtain T ₂ , and the titanium one pass after the first pass. The material surface temperature can be evaluated in a pseudo manner. Equation (5) is the same as Equation (1). From this T ₂ , the degree of decrease in the surface temperature of the titanium material, the degree of occurrence of hot twins that cause the dented portion that causes the intragranular blemishes, and the degree of the occurrence of the intragranular blemishes are predicted. And can be evaluated.

The _{higher the T 2} , the less likely it is that hot twins will occur, which will cause dents that cause intragranular baldness. For that purpose, T ₂ needs to be 670 or more. If it is less than 670, hot twins that cause dents that cause intragranular baldness are likely to occur in the first pass of hot rolling, resulting in dents in the grains. Therefore, it is not possible to prevent the occurrence and extension of intragranular twinning defects.
Further, by _{setting T 2} to 670 or more, grain boundary-derived hesitation defects are also reduced. This is due to the following reasons. By _{setting T 2} to 670 or more, the hot rolling temperature in the first pass becomes high. Therefore, even if there is a difference in deformation anisotropy between the crystal grains, the difference in deformation anisotropy is alleviated because the rolling in the first pass is performed at a high temperature so that the crystals are easily deformed.
As described above, in the rolling in the first pass of hot rolling, by _{setting T 2} to 670 or more, it is possible to produce a titanium material having no intragranular blemishes and good surface texture. A more preferable T ₂ is 700 or more. On the other hand, when T ₂ exceeds the same value as the temperature (β transformation point -30 ° C), the internal temperature exceeds the β transformation point due to processing heat generation, and a large difference in plastic deformability occurs between the surface layer and the inside. Since it may cause cracks and wrinkles, _{it is preferable that the upper limit of T 2} is the same as the temperature at (β transformation point −30 ° C.).

In the present invention, the _{reason why the pass having T 2} of 670 or more is defined as the first pass of hot rolling is that the crystal grain size is the coarsest when the first pass is performed. This is because hesitation defects are likely to occur regardless of whether they are intragrain-derived hesitation defects or grain boundary-derived hesitation defects. In the second and subsequent passes, since the crystal grain size is reduced by rolling in the first pass, baldness is less likely to occur, and T ₂ does not have to be 670 or more. From the second pass onward, both rough rolling and finish rolling may be rolled and molded as usual to obtain a predetermined shape. After the hot rolling, cold rolling may be performed if desired. The reduction rate in the first pass is preferably 3% or more.

Here, T ₀ [° C.] is preferably equal to or lower than the β transformation point. This is because when T ₀ [° C.] exceeds the β transformation point, the amount of oxide scale generated on the surface becomes significant.
T ₁ [° C.] is preferably 500 ° C. or lower. If T ₁ [° C.] exceeds 500 ° C., the strength of the roll may decrease and the roll may be severely damaged. It _{is not necessary to specify the lower limit of T 1} [° C.], but usually room temperature is the lower limit.
h ₀ [mm] and h ₁ [mm] are determined by the equipment specifications. h ₀ [mm] is usually 40 to 280 _{mm, and h 1} [mm] is usually 30 to 270 mm.
D [mm] is usually 300 to 1500 mm, and r [rpm] is usually 25 to 100 rpm.

Further, it is preferable to roll up to the second pass or up to the third pass before and after each pass (nth pass) _{so that T 3 of the following formula is 670 or more.}

Here, the titanium material surface temperature before the path T _{b [°} C.], the surface temperature of the reduction roll before the path T _{a [°} C.], the thickness of the titanium material before the path h _{b [mm],} the thickness of the titanium material after the path _h a [mm], the diameter of the rolling roll _D n [mm], the rotational speed of the rolling rolls and _r n [rpm].

In the present invention, the temperature of the titanium material during hot rolling is defined as the β transformation point or less in terms of surface temperature. This is because when the surface temperature exceeds the β transformation point, the amount of oxide scale generated on the surface becomes remarkable.

In the present invention, the titanium material to be hot-rolled may have any shape such as a slab or a billet. As the material of the titanium material, α-type titanium alloy and JIS standard industrial pure titanium are preferable. In particular, since it is easy to process, pure titanium containing impurities below the normal industrial standard is preferable.

Further, the hot rolling method for titanium material of the present invention is preferably applied to a direct casting slab which has not been subjected to slabbing or forging after electron beam melting or plasma melting. According to the present invention, since the occurrence of dents can be suppressed without performing bulk rolling or forging, the steps of bulk rolling or forging can be omitted.

Further, the present invention is preferably applied to a titanium material having a crystal particle size of 500 μm or more on average. Where a titanium material having a coarse crystal grain size is prone to blemishes, the present invention can suppress the occurrence of blemishes even in a titanium material having a coarse crystal grain size.

Using JIS type I pure titanium with an average crystal grain size of 750 μm as a material, hot rolling in the first pass was performed at T ₀ [° C.], T ₁ [° C.], H ₀ [mm] shown in Table 1. , H ₁ [mm], D [mm], r [rpm], and then hot rolling was performed until the rolling reduction was 80% or more.

After hot rolling, hedging defects were evaluated on the hot rolled plates that had been pickled so that the plate thickness was reduced by 50 μm per side. The hot-rolled plate had a total length of 10 m or more (the plate width dimension was arbitrary), and scabs on the entire surface were observed. Since it has been empirically confirmed that the length of the grain-derived hesitation flaw is 5 to 15 mm and the length of the grain boundary-derived hessian flaw is more than 15 mm, it is 5 to 15 mm in length. The number of scabs ^{was divided by the observation area [m 2} ] to obtain the frequency of occurrence of scabs (derived from the grain). "A" when the frequency of occurrence of baldness is ^{less than 0.1 [pieces / m 2} ], "B" when it is 0.1 [pieces / m ² ] or more and less than 0.2 [pieces / m ^2]. , 0.2 [pieces / m ² ] or more and less than 0.3 [pieces / m ² ] is defined as "C", and 0.3 [pieces / m ² ] or more is designated as "D". C was accepted. The evaluation of this baldness is also shown in Table 1.

As is clear from Table 1, in _{Comparative Examples 1 to 6 in which the value of T 2} deviated from the present invention, many bald defects occurred. On the other hand, in _{Invention Examples 1 to 19 in which the value of T 2} is within the range of the present invention, the occurrence of scabs could be sufficiently suppressed and a good surface could be maintained.

According to the present invention, since the occurrence of burr defects can be suppressed, the burden on the surface treatment step after hot rolling can be reduced. Yield is improved by reducing the amount of pickling by improving the surface quality. In addition, since special treatment before hot rolling such as disassembly rolling and forging is not required, it has a great effect not only on reduction of manufacturing cost but also on improvement of energy efficiency, and has special industrial applicability. ..

Claims (4)

This is a hot rolling method for titanium material, in which the titanium material is heated to a hot rolling temperature and hot rolled by a rolling roll.
The temperature of the titanium material during hot rolling is below the β transformation point at the surface temperature.
In the first 1 pass in hot rolling, the surface temperature of the titanium material one pass before the first pass is T ₀ [° C], the surface temperature of the rolling roll one pass before the first pass is T ₁ [° C], and the first 1 The thickness of the titanium material before the pass is h ₀ [mm], the thickness of the titanium material after the first pass is h ₁ [mm], the diameter of the rolling roll is D [mm], and the rotation speed of the rolling roll is r. A method for hot rolling a titanium material, characterized _{in that T 2} calculated by the following equation (1) is 670 or more when [rpm] is set.

The method for hot rolling a titanium material according to claim 1, wherein the rolling roll is preheated and the surface temperature T ₁ [° C.] thereof is set to 200 ° C. or higher and 500 ° C. or lower. The method for hot rolling a titanium material according to claim 1 or 2, wherein the titanium material is a JIS standard industrial pure titanium or an α-type titanium alloy titanium material. The heat of the titanium material according to any one of claims 1 to 3, wherein the titanium material before the first 1 pass has an average crystal grain size of 500 μm or more. Inter-rolling method.

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