CN107075689A - 热轧用钛铸坯及其制造方法 - Google Patents

热轧用钛铸坯及其制造方法 Download PDF

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Publication number
CN107075689A
CN107075689A CN201480082251.7A CN201480082251A CN107075689A CN 107075689 A CN107075689 A CN 107075689A CN 201480082251 A CN201480082251 A CN 201480082251A CN 107075689 A CN107075689 A CN 107075689A
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Prior art keywords
titanium
melting
stabilizing elements
phase stabilizing
hot rolling
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CN201480082251.7A
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CN107075689B (zh
Inventor
立泽吉绍
国枝知德
森健
森健一
藤井秀树
高桥浩
高桥一浩
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Nippon Steel Corp
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Nippon Steel Corp
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
    • B21B1/02—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling heavy work, e.g. ingots, slabs, blooms, or billets, in which the cross-sectional form is unimportant ; Rolling combined with forging or pressing
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
    • B21B1/22—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B3/00—Rolling materials of special alloys so far as the composition of the alloy requires or permits special rolling methods or sequences ; Rolling of aluminium, copper, zinc or other non-ferrous metals
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D15/00—Casting using a mould or core of which a part significant to the process is of high thermal conductivity, e.g. chill casting; Moulds or accessories specially adapted therefor
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D19/00—Casting in, on, or around objects which form part of the product
    • B22D19/08—Casting in, on, or around objects which form part of the product for building-up linings or coverings, e.g. of anti-frictional metal
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D19/00—Casting in, on, or around objects which form part of the product
    • B22D19/16—Casting in, on, or around objects which form part of the product for making compound objects cast of two or more different metals, e.g. for making rolls for rolling mills
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D21/00—Casting non-ferrous metals or metallic compounds so far as their metallurgical properties are of importance for the casting procedure; Selection of compositions therefor
    • B22D21/002—Castings of light metals
    • B22D21/005—Castings of light metals with high melting point, e.g. Be 1280 degrees C, Ti 1725 degrees C
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D21/00—Casting non-ferrous metals or metallic compounds so far as their metallurgical properties are of importance for the casting procedure; Selection of compositions therefor
    • B22D21/06—Casting non-ferrous metals with a high melting point, e.g. metallic carbides
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
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    • B22D23/06—Melting-down metal, e.g. metal particles, in the mould
    • B—PERFORMING OPERATIONS; TRANSPORTING
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    • B—PERFORMING OPERATIONS; TRANSPORTING
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    • B32B1/00—Layered products having a non-planar shape
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    • B—PERFORMING OPERATIONS; TRANSPORTING
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    • B32B15/00—Layered products comprising a layer of metal
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    • B—PERFORMING OPERATIONS; TRANSPORTING
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    • B32B15/04—Layered products comprising a layer of metal comprising metal as the main or only constituent of a layer, which is next to another layer of the same or of a different material
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    • B32B5/14—Layered products characterised by the non- homogeneity or physical structure, i.e. comprising a fibrous, filamentary, particulate or foam layer; Layered products characterised by having a layer differing constitutionally or physically in different parts characterised by a layer differing constitutionally or physically in different parts, e.g. denser near its faces
    • C—CHEMISTRY; METALLURGY
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    • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
    • C22B34/00—Obtaining refractory metals
    • C22B34/10—Obtaining titanium, zirconium or hafnium
    • C22B34/12—Obtaining titanium or titanium compounds from ores or scrap by metallurgical processing; preparation of titanium compounds from other titanium compounds see C01G23/00 - C01G23/08
    • C22B34/1295—Refining, melting, remelting, working up of titanium
    • C—CHEMISTRY; METALLURGY
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    • C22B9/00—General processes of refining or remelting of metals; Apparatus for electroslag or arc remelting of metals
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    • C22B9/22—Remelting metals with heating by wave energy or particle radiation
    • C22B9/221—Remelting metals with heating by wave energy or particle radiation by electromagnetic waves, e.g. by gas discharge lamps
    • C22B9/223—Remelting metals with heating by wave energy or particle radiation by electromagnetic waves, e.g. by gas discharge lamps by laser beams
    • C—CHEMISTRY; METALLURGY
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    • C22B9/225—Remelting metals with heating by wave energy or particle radiation by electromagnetic waves, e.g. by gas discharge lamps by microwaves
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    • C22B9/22—Remelting metals with heating by wave energy or particle radiation
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    • C—CHEMISTRY; METALLURGY
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Abstract

在利用电子束熔炼法、等离子体电弧熔炼法制造的包含工业用纯钛的钛铸坯的成为轧制面的表面处，在深度1mm以上的范围内具有熔融再凝固层，所述熔融再凝固层是添加一种或二种以上的β相稳定化元素并使其熔融和再凝固而得到的，至深度1mm为止的范围内的β相稳定化元素的浓度的平均值以质量％计比母材中的β相稳定化元素的浓度高出0.08质量％以上且1.50质量％以下。作为含有β相稳定化元素的原材料，使用粉末、碎片、线、箔。另外，作为使表层熔融的手段，利用电子束加热和等离子体电弧加热。

Description

热轧用钛铸坯及其制造方法
技术领域
本发明涉及包含工业用纯钛的热轧用钛铸坯的制造方法，特别是涉及即使省略初轧、锻造等开坯工序也能够良好地保持热轧后的表面性状的钛铸坯及其制造方法。
背景技术
工业用纯钛通常以海绵钛、钛废料作为原料，利用非消耗电极式电弧熔炼法、电子束熔炼法、等离子体电弧熔炼法等进行熔炼，从而形成钛铸锭(钛铸坯)。非消耗式电弧熔炼法中，以将海绵钛加压成形而得到的坯块作为电极，使电极和铸模进行电弧放电，使电极本身熔解，并在铸模内进行铸造，从而得到铸锭。因此，必须均匀地进行铸模与电极的放电，从而使铸模形状被限定为圆筒型，铸造后的铸锭形状成为圆柱状。另一方面，电子束熔炼法、等离子体电弧熔炼法中分别使用电子束和等离子体电弧，虽然熔炼法不同，但在熔炼时，将在炉床上熔炼的钛熔液注入至铸模，因此，铸模形状的选择是自由的，不限定于圆柱状，可以制造矩形、钢坯状等各种形状的铸锭。
目前的钛材料制造工序中，在之后经过铸锭的被称为开坯工序的初轧、锻造等热加工工序后，实施热轧，开坯工序是必需的。然而认为，从其形状出发，利用矩形铸锭(板坯状铸锭)制造板材时可以省略开坯工序，利用圆柱状和钢坯状铸锭制造棒材、线材时可以省略开坯工序，研究了省略开坯工序地进行热轧的技术。如果确立该技术，则可以期待通过省略工序和提高成品率来改善成本。
然而，利用电子束熔炼法、等离子体电弧熔炼法制造的钛铸坯由于保持了铸造时的状态，因此存在达到数十mm的粗颗粒。对于这样的钛铸坯，省略开坯工序地进行热轧时，由于由粗颗粒导致的晶粒内和各晶粒间的变形各向异性的影响，在表面产生凹凸，其成为表面瑕疵。为了去除热轧中产生的表面瑕疵，必须利用作为后续工序的酸洗工序来增加热轧材料表面的熔削量，成品率相应地恶化，担心成本增加。
因此，对于利用电子束熔炼法、等离子体电弧熔炼法制造的钛铸锭，一方面期待通过省略初轧、锻造等开坯工序来改善成本，另一方面担心因表面瑕疵的增加而导致成本增加，妨碍了省略开坯工序的钛铸坯的实用化。
专利文献1中公开了如下方法：在电子束熔炼炉中进行熔炼，从铸模内直接取出的钛板坯的截面组织中，从表层朝向内部的凝固方向与板坯的铸造方向所成的角度θ为45°～90°的情况下，或者在表层的晶体取向分布中，hcp的c轴与板坯表层的法线所成的角度为35°～90°的情况下，铸件表面良好，且即使省略铸锭的开坯工序也能够改善热轧后的表面瑕疵。即，通过控制表面的晶粒形状、晶体取向，能够抑制因这种粗大晶粒而产生的瑕疵。
专利文献2中，作为省略钛材料的铸锭的开坯工序并直接进行热轧的方法，通过利用高频感应加热、电弧加热、等离子体加热、电子束加热和激光加热等，使成为轧制面的表面的表层进行熔融并再凝固，从而进行自表层起1mm以上深的细粒化。通过利用该板坯表层的骤冷凝固而形成微细且不规则的晶体取向分布，从而防止表面瑕疵的产生。
现有技术文献
专利文献
专利文献1：国际公开2010/090353号公报
专利文献2：日本特开2007-332420号公报
发明内容
发明要解决的问题
本发明提供：尽管省略开坯工序且不需要对保持了铸造时的状态的钛铸坯表层进行切削精整的工序，仍然可抑制后续的热轧后的钛材产生表面瑕疵的钛铸坯及其制造方法。
用于解决问题的方案
本发明人等为了实现前述课题而进行了深入研究。其结果发现，作为纯钛制钛铸坯的熔炼方法，对于利用电子束熔炼法、等离子体电弧熔炼法制造的保持了铸造时的状态的钛铸坯而言，省略以往需要的开坯工序地进行热轧时，作为热轧的前工序，在保持了铸造时的状态的钛铸坯的轧制面表层设置或散布含有β相稳定化元素的原材料(粉末、碎片、线、箔)，连同原材料一起使钛材的表层熔融。如此，在钛材表层形成β相稳定化元素的浓度比母材高的层、即β相稳定化元素富集层。由此，会良好地保持热轧后的表面性状。
即，本发明如以下所述。
(1)一种热轧用钛铸坯，其特征在于，其为包含工业用纯钛的钛铸坯，
在成为轧制面的表面处，在深度1mm以上的范围内具有熔融再凝固层，所述熔融再凝固层是添加一种或二种以上的β相稳定化元素并使其熔融和再凝固而得到的，
至深度1mm为止的范围内的β相稳定化元素的浓度的平均值以质量％计比母材中的β相稳定化元素的浓度高出0.08质量％以上且1.50质量％以下。
(2)根据(1)所述的热轧用钛铸坯，其中，前述β相稳定化元素为Fe、Ni、Cr中的一种或二种以上。
(3)根据(1)所述的热轧用钛铸坯，其中，在含有前述β相稳定化元素的同时，还含有一种或二种以上的α相稳定化元素或者中性元素。
(4)一种热轧用钛铸坯的制造方法，其中，使包含工业用纯钛的钛铸坯的成为轧制面的表面与含有β相稳定化元素的原材料一起熔融后，使其凝固。
(5)根据(4)所述的热轧用钛铸坯的制造方法，其中，前述含有β相稳定化元素的原材料为粉末、碎片、线、箔中的任意形态。
(6)根据(4)所述的热轧用钛铸坯的制造方法，其中，利用电子束加热或等离子体加热，使前述包含工业用纯钛的钛铸坯的成为轧制面的表面熔融。
发明的效果
对于本发明的钛铸坯，即使省略以往需要的初轧(slabbing step)、锻造等开坯工序(breakdown step)地实施热轧，也能够制造具有与现有材料等同的表面性状的钛材。可以通过省略开坯工序来降低加热时间、通过与表层熔融带来的钛铸坯的表层平滑化相伴地降低切削修整、通过热轧后的钛材表面性状的提高来降低酸洗时的溶削量等，由此带来成品率的提高，因此对制造成本的削减是有效的，不可估量其产业上的效果。
附图说明
图1示出熔融再凝固层的浓度变化的示意图。
具体实施方式
以下，对本发明进行详细说明。
本发明中，工业用纯钛包括：JIS标准的1种～4种、以及与其对应的ASTM标准的Grade1～4、DIN标准的3·7025中规定的工业用纯钛。即，本发明中作为对象的工业用纯钛可以是指，C：0.1质量％以下、H：0.015质量％以下、O：0.4质量％以下、N：0.07质量％以下、Fe：0.5质量％以下、余量由Ti构成。需要说明的是，工业用纯钛中，与其它β相稳定化元素相比大量含有的Fe实质上在JIS1、2种中约为0.020～0.05质量％、在JIS3种中约为0.08质量％。
本发明中作为对象的钛铸坯为矩形铸锭(板坯状铸锭)、圆柱状铸锭、钢坯状铸锭。是使这些形状的钛铸坯的表层与含有β相稳定化元素的原材料一起熔融从而对热轧后的钛材抑制表面瑕疵的技术。
本发明中，仅将保持了铸造时的状态的钛铸坯的表层部加热，将深度1mm以上熔融。对于如此经过熔融的钛铸坯的表层部，在熔融后进行骤冷和再凝固并冷却至室温时的熔融再凝固层(将如此通过仅将保持了铸造时的状态的钛铸坯的表层部加热而使其熔融，之后进行骤冷并再次凝固的凝固层称为“熔融再凝固层”)的截面组织成为微细的针状组织。而且，表层熔融时通过将母材的表层与β相稳定化元素同时熔融，从而与母材相比，熔融再凝固层内的β相稳定化元素浓度变高，其结果，由于添加β相稳定化元素而提高淬透性，从而可以使熔融再凝固层形成更微细的组织。此处所谓“提高淬透性”是指，通过使钛铸坯的表层中含有β相稳定化元素，从而使连续冷却时的相变点(nose)向长时间侧位移，由此在低温下使其进行相变。目的在于，通过在低温下使其进行相变，从而增加核生成位点，使晶粒微细化。
如后述那样，本发明中，形成的熔融再凝固层产生深的部分和浅的部分。本发明中，使熔融再凝固层的深度为1mm以上，该深度是指，在与熔融焊珠的扫描方向垂直的方向的截面观察时的最浅部的深度。
通常，工业用纯钛在β相变点以下的温度域的α单相域中实施热轧。因此，钛铸坯被加热至作为热轧加热温度的α相高温域。一般来说，工业用纯钛微量地含有Fe等β相稳定化元素作为合金元素，虽然少量但是存在α+β二相温度域。然而，对于通常的工业用纯钛，α+β二相温度域为仅数十度的极窄的温度域。另一方面，对于向上述钛铸坯的表层添加β稳定化元素，使β稳定化元素与钛原材料的表层同时熔融并进行再凝固的钛铸坯，表层中的β稳定化元素的浓度变得高于母材。即，熔融再凝固层内的β相稳定化度高，因此，可以扩展α+β二相域的温度域，在热轧加热时，可以使熔融再凝固层内为α+β二相域的状态。由于在α相的晶界中产生β相，因此，α相的晶粒生长被抑制，可以将熔融再凝固后的微细晶粒维持到热轧加热后的热轧时为止。因此可知，可以抑制由粗大晶粒所导致的钛材表面的凹凸，可以制造不产生表面瑕疵的钛热轧材。
另外，通过进行上述那样的熔炼，在经过熔融的钛铸坯的表面，β相稳定化元素不会均匀分散，其结果，在局部产生前述元素的浓度高的区域，该部分中，可以进一步扩展α+β二相域的温度域，有可以进一步抑制热轧加热时的α相的晶粒生长的效果。
在钛铸坯中如上述那样使表层的深度1mm以上再熔融后进行凝固，从而自表层起的深度为1mm以上成为再熔融后凝固的微细针状组织，由于熔融再凝固层及其热影响部，钛材的板厚方向中央侧成为保持了铸造时的状态的组织。至少将与钛铸坯的成为轧制面的表层与含有β相稳定化元素的原材料一起再熔融后，进行凝固，使距离熔融再凝固层内的表层1mm深度为止的β相稳定化元素的浓度的平均值以质量％计比母材的β相稳定化元素的浓度高出0.08质量％以上且1.50质量％以下即可。β相稳定化元素可以将多种β相稳定化元素组合而添加，此时的β相稳定化元素的浓度是指含有的β相稳定化元素的各浓度的总和。母材与熔融再凝固层的β相稳定化元素的浓度之差低于0.08质量％时，无法充分得到β相稳定化元素的添加带来的淬透性提高效果和晶粒生长抑制效果，热轧后的钛材容易产生表面瑕疵。为了进一步发挥抑制表面瑕疵的效果，β相稳定化元素的浓度差优选超过0.2质量％，进而最优选超过0.5质量％。另外，母材与熔融再凝固层的β相稳定化元素的浓度差如果为前述范围内，则通过利用热轧及之后的工序、即喷丸和酸洗的工序的溶削，表层的β相稳定化元素的富集层被去除，熔融再凝固层中富集的β相稳定化元素被无害化。即，通过进行喷丸和酸洗的工序，从而使β相稳定化元素富集层消失，由此与由通常的制法制造的冷轧板呈现等同的成分和机械特性。然而，母材与熔融再凝固层的β相稳定化元素的浓度差高于1.50质量％时，热轧时钛铸坯表层的β相的比例变多，与母材相比，板坯表层部的氧化变剧烈。进而，钛铸坯的表层的β相稳定化元素的浓度变高，从而与母材相比，熔融再凝固层的硬度变高，热轧时有时产生表面裂纹等。出于这些因素，必须增加酸洗工序中的表面溶削量，成品率明显降低。此外，后续工序中，β相稳定化元素的富集层的无害化也变难，距离表层1mm深度为止的β相稳定化元素的浓度的平均值相对于母材的β相稳定化元素的浓度设为1.50质量％以下。此外，使熔融深度为1mm以上，如果熔融深度变得过深，则有喷丸和酸洗工序后β相稳定化元素的富集层仍残留的担心，因此期望熔融深度为5mm左右为止。
另外，通常，钛铸坯在铸造时从与铸模相接触的钛铸坯表层部起进行凝固，因此通过按元素的溶质分配，在钛铸坯的表层与内部，成分略有不同。Fe等β相稳定化元素是示出正偏析的元素，因此在凝固时、相变时，有钛铸坯的表层部的Fe浓度变低、越为钛铸坯的内部则Fe浓度越高的倾向。因此，通过将β相稳定化元素与母材同时熔融，对于使熔融再凝固层内的β相稳定化元素浓度与母材为等同以上是极其有效的。
此外，铸造钛材时，通过控制原料的投入量，从而调整成板坯整体的成分均匀的状态。然而，有时会局部产生成分的变动等。因此，即使是作为β相稳定化元素的Fe含量原本就高的工业用纯钛JIS3种、JIS4种，在熔融凝固层内，也与Fe的成分变动相应地存在微细晶粒化不充分的区域，在热轧后，经常局部产生表面瑕疵。因此，通过在熔融再凝固时添加β稳定化元素来提高β相稳定化元素的添加量是非常有效的，由此，也可以抑制局部产生的表面瑕疵。
从与熔融焊珠的扫描方向垂直的方向的截面观察时，有熔融再凝固层的形状在钛铸坯表层再熔融时的熔融焊珠中央变得最深的倾向，重叠熔融焊珠时，成为在相邻的熔融焊珠彼此中间变得最浅、且最深部和最浅部周期性地重复的形态。此时，最深部与最浅部的差异大时，热轧时由该差异而在变形阻力中产生差异，有时产生由其所导致的瑕瑕疵。因此，理想的是，上述差异低于2mm。需要说明的是，本发明中使熔融再凝固层的深度为1mm以上，该深度是指，从与熔融焊珠的扫描方向垂直的方向的截面观察时的最浅部的深度。
对熔融再凝固层的深度、熔融再凝固层内的不均匀性的测定方法进行说明。将从与熔融焊珠的扫描方向垂直的方向的截面的钛铸坯表层部切出的部分作为埋入研磨试样，将其供至SEM(Scaning Electron Microscopy)/EPMA(Electron Probe MicroAnalyser)，由此能够容易地判断。本发明中，将熔融再凝固层深度定义为最浅部的深度，因此利用元素映射分析求出熔融深度是简便的。图1中示出母材和熔融再凝固层的浓度变化的实测值的一例。这是从钛铸坯的轧制面表层附近的母材部分朝向轧制表面，沿厚度方向对β相稳定化元素浓度进行线分析的情况。可知，母材中的β相稳定化元素浓度低且大致均匀，但熔融再凝固层中的β相稳定化浓度高，进一步产生浓度的振动，存在不均匀性。
作为β相稳定化元素，可以举出：V、Mo、Fe、Cr、Mn、Ta、Nb、Ni、Co、Cu、W等。然而，钛中，熔点高的W、Ta等元素成为HDI(高密度夹杂物)的原因，保持未熔融、扩散不充分的状态地残留在钛材中时，成为疲劳的起点，因此使用时需要注意。另外，Mo、Nb等与W、Ta相比，虽然熔点低，但熔点仍然为2000℃以上，因此使用Mo、Nb时，理想的是，预先制成与Ti等元素的合金，作为降低了熔点的合金添加。β相稳定化元素可以分为V、Mo、Ta、Nb等完全固溶型和Fe、Cr、Mn、Co、Ni、Cu等共析型，共析型中，各β相稳定化元素的固溶度小，但β相稳定化能力大，因此共析型的β相稳定化元素以少量的添加也是有效的。共析型的Fe、Cr、Mn、Co、Ni、Cu中，相对于母材，如果熔融再凝固层的β相稳定化元素浓度高出0.08～0.60质量％左右，则可以抑制热轧后的表面瑕疵，因此优选上述范围。完全固溶型的V、Mo、Ta、Nb中，β相稳定化能力与共析型相比小，因此，相对于母材，以熔融再凝固层的β相稳定化元素浓度多0.60～1.50质量％左右地添加β相稳定化元素时，更容易发挥效果。另外，即使使用共析型的β相稳定化元素，再熔融后的凝固时也被骤冷，因此冷却速度快，不产生析出物，热轧加热时也变为α+β的二相域，因此不产生析出物。进而，含有β相稳定化元素的原材料中，也可以包含以Al为代表的α相稳定化元素、Sn、Zr等中性元素。也可以包含α相稳定化元素和中性元素中的任一者或者两者。另外，优选的是，相对于母材，将熔融再凝固层中的α相稳定化元素和中性元素的总量设为2.0质量％以下。与保持了铸造时的状态的铸坯的表层一起熔融的原材料优选使用属于β相稳定化元素且较廉价的Fe、Ni、Cr。使用Fe粉末等、不锈钢粉末等、或者运用粉碎普通钢、不锈钢的废料而得到的材料也是有效的。同样地，也可以使用粉碎钛合金的废料而得到的材料。
用于在铸坯的表层中添加前述β相稳定化元素而使用的原材料可以为粉末、碎片、线、箔中的任意形状，期望形成小片。有效的是，粉末使用粒径1μm～0.5mm的范围的原材料，碎片使用大小2mm见方～5mm见方的范围的原材料，线使用φ0.5mm～φ5mm的范围的原材料，箔使用膜厚1μm～0.1mm的范围的原材料。这些原材料放置或者散布于铸坯的表面时，通过均匀地配置于铸坯的表面，可以在钛铸坯的表层中减少与母材所含有的浓度为相同浓度的区域，可以得到表面性状更良好的钛铸坯。
另外，将表层与β相稳定化元素一起熔融的方法有电子束加热、电弧加热、激光加热和感应加热等方法，但钛是活性的金属，在大气中将表层熔融时，熔融部明显氧化，因此适合的是，能够在真空气氛或者非活性气体气氛中进行处理的电子束加热、电弧加热(特别是等离子体电弧加热、TIG(Tungsten Inert Gas)焊接等使用惰性气体的加热方法)、激光加热等，任意方法均可以进行前述处理。其中，能够一次性赋予高能量的电子束加热或者等离子体电弧加热在工业上是适合的，使用这些方法为宜。
实施例
以下，根据实施例对本发明进一步进行详细说明。
[表1]
[表2]
表1和表2所示的参考例、实施例和比较例中，钛铸坯是利用电子束熔炼、使用矩形铸模而制造得到的。需要说明的是，以后的实施例和比较例中，没有特别指定的情况下，记载的是利用由矩形铸模制造的保持了铸造时的状态的钛板坯制造热轧板的情况。由厚度200mm×宽度1000mm×长度4500mm的钛铸坯利用热轧制造厚度4mm的热轧板。钛的品种使用工业用纯钛JIS1种、JIS2种、JIS3种、JIS4种。另外，作为含有β相稳定化元素的原材料，使用粉末(粒径100μm以下)、碎片(2mm见方、1mm厚)、线(φ1mm)、箔(20μm)中的任意者。需要说明的是，含有β相稳定化元素时，在保持了铸造时的状态的钛铸坯(板坯)的保持铸件表面状态的面上直接放置或散布含有β相稳定化元素的原材料。从其上实施板坯表层的加热，利用电子束和等离子体电弧，使加热部进行扫描，从而对轧制面整个面进行处理，使得包含β相稳定化元素的原材料和轧制面的未熔融部没有残留。此外，保持了铸造时的状态的钛铸坯使用铸件表面较良好的材料，使得在表层熔融时不产生由铸件表面所导致的熔炼残留。另外，为了使β相稳定化元素均匀地添加至板坯整体，使含有β相稳定化元素的原材料均匀地分散于钛铸坯的轧制面整体。熔融再凝固层的深度的测定方法如下：将使表层再熔融后凝固的钛铸坯切出一部分，进行研磨和蚀刻，将所得样品用光学显微镜观察，测定成为微细针状组织(通过添加β相稳定化元素而成为微细的针状组织，因此可以判断)的层的深度(采用观察部位中最浅的部分的深度作为熔融再凝固层的深度)。另外，此时，从钛铸坯的轧制面的任意10处的表层1mm以内采集分析样品，进行ICP发射分光光度分析，取10处的平均值。另外，作为比较，在将钛铸坯的表层再熔融前，从钛铸坯的轧制面的任意3处的表层20mm以内采集分析样品，同样地进行ICP发射分光光度分析，取3处的平均值。对于该2种的分析结果，调查直至熔融再凝固层中的深度1mm为止的范围中的β相稳定化元素的浓度的平均值与母材中的β相稳定化元素的浓度的平均值的差异。另外，对于表面瑕疵的产生情况，热轧后，对热轧板进行喷丸和酸洗后，对钛材(热轧板)的表面进行目视观察，进行评价。需要说明的是，酸洗中，将每1次的轧制面的单面溶削约50μm左右(以两面计约100μm)，使热轧板进行1～2次酸洗后，评价热轧板的表面性状。需要说明的是，未实施表层的熔融处理的比较例中，从表层1mm以内采集分析样品，熔融再凝固层的厚度低于1mm的比较例中，从熔融再凝固层内采集分析样品。
首先，记载了对于使表层与包含廉价的Fe、Ni、Cr作为β相稳定化元素的原材料一起熔融而得到的钛铸坯的结果。
No.1、No.2的参考例和比较例中，不实施表层的熔融处理而实施热轧。由于不实施熔融处理，因此直至表层1mm为止的β相稳定化元素的浓度与母材的β相稳定化元素的浓度相同，其差低于0.08质量％。No.1的参考例是与通常的钛铸坯同样地实施初轧的情况。从厚度200mm进行初轧至100mm，之后进行再加热，实施热轧直至4mm。由于实施了初轧，因此，热轧后的表面性状中没有异常。No.2的比较例是不实施初轧的情况。由于也不实施初轧，因此，酸洗后的热轧板上产生粗大的表面瑕疵。
No.3的比较例是利用电子束加热使轧制面的表层熔融的情况，是熔融时不使用β相稳定化元素的情况。熔融再凝固层确保2mm以上，但是没有添加β相稳定化元素，因此，母材与熔融再凝固层的β相稳定化元素的浓度相等，其浓度差低于0.08质量％。热轧和酸洗后的热轧板表面上局部产生粗大的瑕疵。
No.4、No.5的比较例是使用Fe粉末作为包含β相稳定化元素的原材料、利用电子束加热使轧制面的表层熔融的情况。No.4的比较例是与母材相比熔融再凝固层的β相稳定化元素浓度高、其差低于0.08质量％、熔融再凝固层的深度为2mm的情况。由于Fe的浓度低于0.08质量％，因此，在酸洗后的热轧板的表面上局部产生粗大的瑕疵。No.5的比较例是与母材相比熔融再凝固层内的β相稳定化元素的浓度高0.22质量％、最浅部的熔融再凝固层的深度为0.5mm的情况。由于熔融再凝固层的深度小于1mm，因此，酸洗后的热轧板的表面上局部产生粗大的瑕疵。No.4、No.5的比较例与No.2所示的比较例相比，热轧板的表面性状得到改善，但也产生稍大的表面瑕疵，品质不充分。
No.6至No.9、No11至No.16的实施例、No.10的比较例中，作为板坯表层的熔融手法，使用电子束加热，改变包含β相稳定化元素的原材料的量和形状，进行热轧试验。
另外，No.6至No.9的实施例、No.10的比较例中，作为包含β相稳定化元素的原材料，使用粉末。
No.6的实施例是母材与熔融再凝固层的β相稳定化元素的浓度差为0.08质量％、熔融再凝固层的深度为3mm的情况。对于酸洗后的热轧板，局部产生稍粗大的表面瑕疵，但为能够允许的水准，与No.4、No.5的比较例相比，是非常良好的表面性状。
No.7的实施例是母材与熔融再凝固层的β相稳定化元素的浓度差为0.87质量％、熔融再凝固层的深度为1mm的情况。No.8的实施例是母材与熔融再凝固层的β相稳定化元素的浓度差为1.50质量％、熔融再凝固层的深度为5mm的情况。No.7、No.8的实施例中，酸洗后的表面瑕疵是轻微的，可以得到非常良好的表面性状。
No.9的实施例是母材与熔融再凝固层的β相稳定化元素的浓度差为0.49质量％、熔融再凝固层的深度为7mm的情况。酸洗后的热轧板的表面性状是良好的，但熔融再凝固层的深度深至7mm，因此为了去除Fe富集层，与通常相比增加酸洗的次数，与No.8的实施例相比，成品率降低。
No.10的比较例是母材与熔融再凝固层的β相稳定化元素的浓度差为1.67质量％、熔融再凝固层的深度为4mm的情况。酸洗后的热轧板表面上局部产生粗大的瑕疵，而且还产生表面裂纹，品质不充分。
No.11至No.16的实施例中，以板坯表层的熔融再凝固层的深度为3mm的方式实施表层的加热。No.11、No.12的实施例中，作为包含β相稳定化元素的原材料使用碎片，No.13、No.14的实施例中，作为包含β相稳定化元素的原材料使用线，No.15、No.16的实施例中，作为包含β相稳定化元素的原材料使用箔。No.11至No.16的实施例中，母材与熔融再凝固层的β相稳定化元素的浓度差为0.08质量％以上，热轧板的表面瑕疵成为能够允许的水准，并且轻微。
由以上的结果，作为包含β相稳定化元素的原材料的形状，使用粉末、碎片、线、和箔中的任意者，热轧板的表面性状均可以得到良好的结果。
No.17、No.18的实施例中，作为包含β相稳定化元素的原材料使用粉末，改变板坯表层的熔融方法，进行热轧试验。No.17、No.18的实施例中，作为板坯表层的熔融方法，使用等离子体电弧加热，熔融再凝固层的深度为4mm。No.17、No.18的实施例中，酸洗后的热轧板的表面瑕疵是轻微的，非常良好。
由以上的结果，作为板坯表层的熔融方法，使用电子束加热和等离子体电弧加热的任一者，热轧板的表面性状均可以得到良好的结果。
接着，对于使用除了Fe之外包含Cr、Ni的不锈钢的情况的结果进行记载。
No.19至No.22的比较例和实施例是作为不锈钢使用SUS304的粉末，利用电子束加热使表层熔融，从而使熔融再凝固层的深度为2mm的情况。对于No.19的比较例，母材与熔融再凝固层的β相稳定化元素的浓度差低于0.08质量％，热轧板的表面瑕疵产生了粗大的瑕疵。No.20至No.22的实施例中，母材与熔融再凝固层的β相稳定化元素的浓度差为0.08质量％以上，热轧板的表面瑕疵成为能够允许的水准且轻微。
No.23至No.26的比较例和实施例是作为不锈钢使用SUS430的粉末，利用电子束加热使表层熔融，从而使熔融再凝固层的深度为2mm的情况。No.23的比较例中，母材与熔融再凝固层的β相稳定化元素的浓度差低于0.08质量％，热轧板的表面瑕疵产生了粗大的瑕疵。No.24至No.26的实施例中，母材与熔融再凝固层的β相稳定化元素的浓度差为0.08质量％以上，热轧板的表面瑕疵成为能够允许的水准且轻微。
接着，No.27至No.32的比较例和实施例是单独添加Cr或Ni时的结果。No.27至No.29的比较例和实施例是使用Cr碎片，利用电子束加热使表层熔融，从而使熔融再凝固层的深度为3mm的情况。No.27和No.28的实施例中，母材与熔融再凝固层的β相稳定化元素的浓度差为0.08质量％以上且1.5质量％以下，虽然也存在热轧板的表面的瑕疵局部稍粗大的情况，但基本上是良好的。No.29的比较例中，母材与熔融再凝固层的β相稳定化元素的浓度差为1.50质量％以上，酸洗后的热轧板表面上局部产生粗大的瑕疵，而且还随处可见表面裂纹，与No.27和No.28的实施例相比，表面性状恶化。
No.30至No.32的比较例和实施例是使用Ni碎片，利用电子束加热使表层熔融，从而使熔融再凝固层的深度为3mm的情况。No.30的比较例中，母材与熔融再凝固层的β相稳定化元素的浓度差低于0.08质量％，热轧板的表面瑕疵局部产生粗大的瑕疵。No.31至No.32的实施例中，母材与熔融再凝固层的β相稳定化元素的浓度差成为0.08～1.50质量％的范围，热轧板的表面瑕疵均抑制在能够允许的水准内。
No.33至No.37的实施例和比较例是由利用电子束熔炼法得到的直径170mm×长度12m的圆柱状铸锭利用热轧制造直径13mm的线材的情况。表面瑕疵的评价方法与热轧板同样地通过目视观察酸洗后的线材的表层来实施。
No.33的参考例中，不实施表层的熔融处理而实施热轧。由于不实施熔融处理，因此直至表层1mm为止的β相稳定化元素浓度与母材Fe浓度相等。是与通常的钛铸坯同样地实施初轧的情况，由于实施了初轧，因此热轧后的表面性状没有差异。
No.34和No.35的比较例和实施例是作为包含β相稳定化元素的原材料使用Fe粉末的情况。No.34的比较例中，母材与熔融再凝固层的β相稳定化元素的浓度差低于0.08质量％，热轧板的表面瑕疵局部产生粗大的瑕疵。No.35的实施例中，母材与熔融再凝固层的β相稳定化元素的浓度差为0.08～1.50质量％的范围，可以得到良好的表面性状。
No.36的实施例是作为包含β相稳定化元素的原材料使用Cr碎片的情况。No.36的实施例中，母材与熔融再凝固层的β相稳定化元素的浓度差为0.08～1.50质量％的范围，可以得到良好的表面性状。
No.37的实施例是作为包含β相稳定化元素的原材料使用Ni碎片的情况。No.37的实施例中，母材与熔融再凝固层的β相稳定化元素的浓度差也为0.08～1.50质量％的范围，与No.35和No.36的实施例同样地，可以得到良好的表面性状。
No.38至No.66的比较例和实施例是利用电子束在熔融深度成为4mm的条件下使钛材的表层熔融的情况。对添加各种β相稳定化元素的情况进行记载。
No.38至No.42的比较例和实施例是作为含有β相稳定化元素的原材料使用Ti-Mo合金的碎片的情况。No.38的比较例中，母材与熔融再凝固层的β相稳定化元素的浓度差低于0.08质量％，热轧板的表面上局部产生粗大的瑕疵。No.39至No.41的实施例中，母材与熔融再凝固层的β相稳定化元素的浓度差为0.08～1.50质量％的范围，任意热轧板的表面状态与No.38的比较例相比也良好。No.42的比较例中，母材与熔融再凝固层的β相稳定化元素的浓度差超过1.50质量％，酸洗后的热轧板表面上局部产生粗大的瑕疵。而且还随处可见表面裂纹，表面性状差。
No.43至No.45的比较例和实施例是作为含有β相稳定化元素的原材料使用V碎片的情况。No.43的比较例中，母材与熔融再凝固层的β相稳定化元素的浓度差低于0.08质量％，热轧板的表面性状差，局部产生粗大的瑕疵。No.44和No.45的实施例中，母材与熔融再凝固层的β相稳定化元素的浓度差为0.08～1.50质量％的范围，任意热轧板的表面状态也为能够允许的水准。
No.46至No.48的比较例和实施例是作为含有β相稳定化元素的原材料使用Mn碎片的情况。No.46的比较例是母材与熔融再凝固层的β相稳定化元素的浓度差低于0.08质量％的情况，在热轧板的表面上局部产生粗大的瑕疵。No.47和No.48的实施例中，母材与熔融再凝固层的β相稳定化元素的浓度差为0.08～1.50质量％的范围，与No.46的比较例相比，表面状态为能够允许的范围。
No.49至No.51的比较例和实施例是作为含有β相稳定化元素的原材料使用Fe-Nb合金的碎片的情况。No.49和No.50的实施例中，母材与熔融再凝固层的β相稳定化元素的浓度差为0.08～1.50质量％的范围，表面状态良好，瑕疵轻微。No.51的比较例中，母材与熔融再凝固层的β相稳定化元素的浓度差超过1.50质量％，酸洗后的热轧板表面上局部产生粗大的瑕疵。
No.52至No.54的比较例和实施例是作为含有β相稳定化元素的原材料使用Co碎片的情况。No.52的比较例中，母材与熔融再凝固层的β相稳定化元素的浓度差低于0.08质量％，热轧板的表面上局部产生粗大的瑕疵。No.53和No.54的实施例中，母材与熔融再凝固层的β相稳定化元素的浓度差为0.08～1.50质量％的范围，与No.52的比较例相比，表面状态良好。
No.55至No.57的比较例和实施例是作为含有β相稳定化元素的原材料使用Cu碎片的情况。No.55的比较例是母材与熔融再凝固层的β相稳定化元素的浓度差低于0.08质量％的情况，酸洗后的热轧板表面上局部产生粗大的瑕疵。No.56和No.57的实施例中，母材与熔融再凝固层的β相稳定化元素的浓度差为0.08～1.50质量％的范围，是非常良好的表面性状。
No.58至No.60的比较例和实施例是作为包含β相稳定化元素的原材料添加将作为α+β合金的Ti-6Al-4V(6-4V碎片)的废料粉碎而得到的钛合金碎片的情况。
No.58的比较例是母材与熔融再凝固层的β相稳定化元素的浓度差低于0.08质量％的情况，酸洗后的热轧板表面上局部产生粗大的瑕疵，No.59和No.60的实施例中，虽然添加了作为α相稳定化元素的Al，但是母材与熔融再凝固层的β相稳定化元素的浓度差为0.08～1.50质量％的范围，表面瑕疵是合格水准。
No.61至No.63的比较例和实施例是作为包含β相稳定化元素的原材料，添加将作为α+β合金的Ti-15V-3Cr-3Sn-3Al的废料(15-3-3-3碎片)粉碎而得到的钛合金碎片的情况。
No.61的比较例是母材与熔融再凝固层的β相稳定化元素的浓度差低于0.08质量％的情况，热轧板表面上局部产生粗大的瑕疵。No.62和No.63的实施例中，添加作为α相稳定化元素的Al、作为中性元素的Sn，母材与熔融再凝固层的β相稳定化元素的浓度差为0.08～1.50质量％的范围，各热轧板的表面瑕疵可局部观察到稍粗大的瑕疵，是轻微的，这些条件下也可以确认热轧板表面瑕疵抑制效果。
No.64至No.68的比较例和实施例是利用电子束在熔融深度成为3mm的条件下使纯钛和钛合金铸坯的表层与Fe碎片一起熔融的情况。
No.64至No.66的实施例是作为钛材使用纯钛JIS3种的钛板坯的情况。
No.64的比较例是母材与熔融再凝固层的β相稳定化元素的浓度差低于0.08质量％的情况，热轧板表面上局部产生粗大的瑕疵。No.65和No.66的实施例中，母材与熔融再凝固层的β相稳定化元素的浓度差为0.08～1.50质量％的范围，各热轧板的表面瑕疵是轻微的。
No.67至No.68的实施例是作为钛材使用纯钛JIS4种的钛板坯的情况。
No.67的比较例是母材与熔融再凝固层的β相稳定化元素的浓度差低于0.08质量％的情况，酸洗后的热轧板上局部产生粗大的表面瑕疵。No.68的实施例中，母材与熔融再凝固层的β相稳定化元素的浓度差为0.08～1.50质量％的范围，热轧板表面良好。

Claims (6)

1.一种热轧用钛铸坯，其特征在于，其为包含工业用纯钛的钛铸坯，
在成为轧制面的表面处，在深度1mm以上的范围内具有熔融再凝固层，所述熔融再凝固层是添加一种或二种以上的β相稳定化元素并使其熔融和再凝固而得到的，
至深度1mm为止的范围内的β相稳定化元素的浓度的平均值以质量％计比母材中的β相稳定化元素的浓度高出0.08质量％以上且1.50质量％以下。
2.根据权利要求1所述的热轧用钛铸坯，其中，所述β相稳定化元素为Fe、Ni、Cr中的一种或二种以上。
3.根据权利要求1所述的热轧用钛铸坯，其中，在含有所述β相稳定化元素的同时，还含有一种或二种以上的α相稳定化元素或中性元素。
4.一种热轧用钛铸坯的制造方法，其中，使包含工业用纯钛的钛铸坯的成为轧制面的表面与含有β相稳定化元素的原材料一起熔融后，使其凝固。
5.根据权利要求4所述的热轧用钛铸坯的制造方法，其中，所述含有β相稳定化元素的原材料为粉末、碎片、线、箔中的任意形态。
6.根据权利要求4所述的热轧用钛铸坯的制造方法，其中，利用电子束加热或等离子体加热，使所述包含工业用纯钛的钛铸坯的成为轧制面的表面熔融。
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