CN111771006A - 热冲压用镀锌钢板及其制造方法 - Google Patents

热冲压用镀锌钢板及其制造方法 Download PDF

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Publication number
CN111771006A
CN111771006A CN201980014431.4A CN201980014431A CN111771006A CN 111771006 A CN111771006 A CN 111771006A CN 201980014431 A CN201980014431 A CN 201980014431A CN 111771006 A CN111771006 A CN 111771006A
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Prior art keywords
steel sheet
hot
base steel
annealing
layer
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CN201980014431.4A
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Inventor
吉田贵敏
鹈川洋辅
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Kobe Steel Ltd
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Kobe Steel Ltd
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
    • B32B15/013—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • 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
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • 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
    • B32B15/043—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 of metal
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/18—Layered products comprising a layer of metal comprising iron or steel
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
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    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/26—Methods of annealing
    • C—CHEMISTRY; METALLURGY
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    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
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    • C—CHEMISTRY; METALLURGY
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    • C21—METALLURGY OF IRON
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    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • C—CHEMISTRY; METALLURGY
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    • C—CHEMISTRY; METALLURGY
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
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    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0257—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment with diffusion of elements, e.g. decarburising, nitriding
    • C—CHEMISTRY; METALLURGY
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
    • C21D8/0436—Cold rolling
    • C—CHEMISTRY; METALLURGY
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
    • C21D8/0457—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment with diffusion of elements, e.g. decarburising, nitriding
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
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Abstract

本发明一个方面涉及热冲压用镀锌钢板，其包括：基材钢板；以及镀层，设置于所述基材钢板的表面，其中，所述基材钢板以质量％计含有C：0.10～0.5％、Si：0.7～2.5％、Mn：1.0～3％、以及Al：0.01～0.5％，余部为铁和不可避免的杂质，所述基材钢板具有：内部氧化层，处于所述基材钢板的内部，厚度为1μm以上，并且包含氧化物，所述氧化物含有Si和Mn的至少其中之一；以及脱碳层，从与所述镀层的界面起朝向所述基材钢板的内部方向厚度为20μm以下。

Description

热冲压用镀锌钢板及其制造方法
技术领域
本发明涉及热冲压用镀锌钢板及其制造方法。
背景技术
汽车用部件一般通过将钢板压制成形来制造。作为汽车部件用的钢板，可以使用在热轧后经酸洗的钢板(以下称为“热轧酸洗钢板”)及冷轧钢板。此外，从提高耐腐蚀性的观点考虑，还可以使用对上述的钢板实施了镀覆的镀层钢板。镀层钢板主要被大致分为镀锌(Zn)系钢板及镀铝(Al)系钢板，但考虑到耐腐蚀性等，镀锌系钢板被广泛使用。
近年来，在汽车用部件的制造中，作为能够同时实现高强度化及复杂的形状的技术，提出了在高温下对钢板(热轧酸洗钢板、冷轧钢板、或者以这些钢板为基材钢板的镀层钢板)进行压制来进行制造的热冲压技术。热冲压也被称为热成形、热压等，是将上述钢板加热至奥氏体和铁素体的温度区间以上(Ac1相变点以上)的高温，并进行压制加工(冲压)的方法。通过热冲压，可以得到高强度且形状复杂的汽车用部件。以下，有时将通过钢板的热冲压而得到的部件称为“热冲压成形品”。
作为热冲压中所用的钢板，已知含有Si、Mn等元素，淬透性良好，压制时的生产率较高的钢板。
但是，在将该钢板作为热浸镀锌钢板的基材钢板使用的情况下，存在产生不镀部分或将镀层合金化时的合金化斑等外观不良的问题。
其原因在于：在热浸镀覆线中对基材钢板实施镀覆之前进行了还原退火的时候，Si及Mn扩散，在钢板表面富集(浓化)。通常被添加至钢中的元素中，Si及Mn是比Fe更容易氧化的元素(易氧化性元素)，因此富集的Si及Mn被选择性地氧化，在钢板的表面形成包含Si及Mn的氧化物的皮膜。对于Si、Mn及它们的氧化物而言，与熔融锌的润湿性差，因此，在对表面富集有Si及Mn的基材钢板实施镀覆的情况下，产生上述外观不良。
作为抑制这样外观不良的产生的技术，专利文献1公开了以下技术：将含有Si、Mn等易氧化性元素的热冲压用镀锌钢板的基材钢板，在Si、Mn等不会以氧化物的形式析出于表面的温度区间、或者即使析出而在表面的富集也极少的温度区间进行还原退火。
此外，专利文献2公开了以下技术：通过对含有C：0.1～0.5质量％、Si：0.05～0.5质量％、Mn：0.5～3％的钢板实施锌系镀覆，从而获得压制成形性及涂装后的耐腐蚀性优异的高强度热冲压钢板。关于专利文献2中记载的钢板，通过使成为不镀或合金化斑原因的钢中的Si含量设为0.5质量％以下，从而确保钢板表面中的镀覆性，抑制合金化斑的产生。
专利文献3公开了以下技术：通过在钢板表面形成厚度为10～200μm的脱碳层，从而抑制钢板表面的裂纹的产生，并且抑制使用该钢板制作的镀锌钢板中因液态金属脆化(Liquid Metal Embrittlement，LME)而导致的晶间裂纹的产生。
然而，本申请发明人们进行了进一步的研究，其结果：认为专利文献1记载的钢板在外观性状方面仍有一些改善的余地。
专利文献2记载的钢板存在以下问题：在热冲压的缓慢冷却步骤中进行退火，热冲压成形品的强度下降。此外，存在以下问题：由于Si含量为0.5％以下这样的较低值，因此难以生成残余奥氏体，热冲压成形品中难以获得良好的延性。
专利文献3记载的钢板存在以下问题：由于存在脱碳层，因此钢板表面的强度变得不稳定。进而存在以下问题：很难以厚度均匀的方式形成脱碳层，钢板的宽度方向及长度方向的材质缺乏稳定性。
本发明鉴于上述情况而作出，其目的在于提供：一种外观性状更好、并且能够得到质量稳定的热冲压成形品的镀锌钢板。
现有技术文献
专利文献
专利文献1：日本专利公开公报特开2014-159624号
专利文献2：日本专利公开公报特开2007-56307号
专利文献3：日本专利公开公报特表2013-513725号
发明内容
本发明人们进行了各种研究，其结果：发现通过以下的发明能够实现上述目的。
本发明一个方面涉及热冲压用镀锌钢板，其包括：基材钢板；以及镀层，设置于所述基材钢板的表面，其中，所述基材钢板以质量％计含有C：0.10～0.5％、Si：0.7～2.5％、Mn：1.0～3％、及Al：0.01～0.5％，余部为铁及不可避免的杂质，所述基材钢板具有：内部氧化层，处于所述基材钢板的内部，厚度为1μm以上，并且包含氧化物，所述氧化物含有Si和Mn的至少其中之一；以及脱碳层，从与所述镀层的界面起朝向所述基材钢板的内部方向厚度为20μm以下。
本发明的上述目的、特征以及其它的目的、特征及优点通过以下的详细记载将变得更为明了。
具体实施方式
在将专利文献1记载的热冲压用的钢板作为合金化热浸镀锌钢板的基材钢板使用的情况下，虽然比较少见但有时产生合金化斑。本发明人们对其原因进行了研究，其结果，发现其原因在于：该钢板中Si含量高，在表面及表面附近存在固溶Si，因此，在实施了镀覆后进行合金化时，Fe自钢板侧向镀层侧的扩散被抑制。
此外，针对抑制因固溶Si导致的合金化斑产生的方法进行了反复研究。其结果，发现：通过在钢板的还原退火时向炉内的氛围中添加水蒸气、即在高露点氛围下进行还原退火，从而在钢板的表面及表面附近会减少作为合金化斑原因的固溶Si。该固溶Si的减少是由于Si、Mn在钢板内部被氧化而形成内部氧化层所引起的，此时钢板的表面不会形成包含Si、Mn的氧化物的氧化皮膜。
但是，如果在高露点氛围下实施退火，则在钢板表面及表面附近碳含量少，会形成拉伸强度较差的层(以下称为“脱碳层”)。该脱碳层是通过炉内的水蒸气与钢板表面及表面附近的碳(C)原子反应生成一氧化碳(CO)而形成的。
本发明人们针对脱碳层也进行了研究，其结果，发现：通过使脱碳层设为指定的厚度以下，从而可以抑制脱碳层对镀锌钢板及使用其制得的热冲压成形品的强度所造成的影响。
本发明人们基于上述见解而完成了本发明。
根据本发明，可以提供外观性状更好的热冲压用镀锌钢板。通过对本发明的热冲压用镀锌钢板应用热冲压，从而可以得到质量稳定的热冲压成形品。
此外，根据本发明，可以稳定地得到热冲压用镀锌钢板。
以下，对本发明的实施方式进行详细说明。应予说明，本发明不受以下说明的实施方式的限定。
下文中，有时将热浸镀锌钢板(GI)和合金化热浸镀锌钢板(GA)统称为“镀锌钢板”，有时将热浸镀锌层及合金化热浸镀锌层统称为“镀层”。
(镀锌钢板的构成)
本发明的实施方式涉及的镀锌钢板，在基材钢板的表面具有镀锌层。基材钢板具有内部氧化层及脱碳层。
(内部氧化层)
内部氧化层是形成于基材钢板的内部且包含Si和Mn的至少其中之一的氧化物的层。内部氧化层，通过高露点退火而形成于基材钢板的表面附近，不会出现在基材钢板的表面。
本发明的实施方式涉及的镀锌钢板中，基材钢板的内部氧化层的厚度为1.0μm以上。通过形成该内部氧化层，可以充分地抑制镀锌钢板中不镀的产生，并且可以充分地提高所形成的镀层与基材钢板的密合性。即可以确保镀覆性。
其理由在于：如果形成该内部氧化层，则Si在基材钢板的表面附近形成氧化物，因此在基材钢板的表面及其附近减少固溶Si。固溶Si会降低基材钢板与锌的润湿性，并且具有延迟镀覆合金化的作用，会成为使镀覆性降低的原因。
本发明的实施方式涉及的镀锌钢板中，从基材钢板与镀锌层的界面起的内部氧化层的深度优选为1μm以上，更优选为1.5μm以上，进一步优选为2μm以上。内部氧化层的深度是指内部氧化层距该界面最近的部分的深度。内部氧化层的厚度优选为2μm以上，更优选为3μm以上。
(脱碳层)
脱碳层是通过高露点退火在基材钢板表面及表面附近形成的碳含量减少的区域。在镀锌钢板中，脱碳层位于从镀层与基材钢板的界面起朝向基材钢板内部方向的该界面的附近。
本实施方式中，脱碳层是通过退火在基材钢板表面及表面附近形成的层，并且是碳量为退火前的基材钢板的碳含量的80％以下的层。对于脱碳层而言，由于碳含量低于基材钢板的未脱碳的部分，故拉伸强度也低于未脱碳的部分。
如果脱碳层的厚度、即镀锌钢板中从镀层与基材钢板的界面起的脱碳层的深度不均匀，则将会引起镀锌钢板的特性的不均。因此，本实施方式涉及的镀锌钢板中，脱碳层的厚度为20μm以下。脱碳层的厚度优选为15μm以下，更优选为10μm以下。最优选不形成脱碳层，故脱碳层的厚度最优选为0μm。
如果基材钢板的脱碳层的厚度为20μm以下，则所得的镀锌钢板中可以充分地减少机械特性的不均。
此外，在镀锌钢板的热冲压时，基材钢板中的Fe经由基材钢板与镀层的界面而扩散至镀层侧。虽然也取决于热冲压的条件，但位于从基材钢板表面起深度为10～20μm左右之处的Fe被结合到镀层中。因此，如果热冲压前的镀锌钢板中脱碳层的厚度为20μm以下，则可以抑制热冲压成形品中脱碳层对强度的影响。
内部氧化层及脱碳层可以从钢板的表面朝向内部方向重复存在。如后述的实施例所记载那样，脱碳层的厚度可以通过以下方式来测定：利用辉光放电发射光谱法(GlowDischarge Optical Emission Spectrometry，GDOES)求出从钢板表面起的深度方向(厚度方向)中各元素的浓度曲线(浓度分布)。内部氧化层的深度及厚度可以通过使用扫描电子显微镜(SEM)拍摄的钢板截面的照片来测定。
(基材钢板的成分组成)
接着，对于构成本实施方式涉及的镀锌钢板的基材钢板的成分组成进行说明。下述成分组成中的“％”均表示“质量％”。
[C：0.10～0.5％]
C是作为固溶强化元素而有助于热冲压成形品的高强度化的元素。为了使热冲压成形品例如成为980MPa以上的高强度，将C含量的下限设为0.10％以上。C含量的下限优选为0.13％以上，更优选为0.15％以上，进一步优选为0.17％以上。
另一方面，如果C含量过剩，则热冲压成形品的焊接性降低。因此，C含量的上限为0.5％。C含量的上限优选为0.40％以下，更优选为0.35％以下，进一步优选为0.30％以下。
[Si：0.7～2.5％]
Si是有助于提高热冲压成形品的点焊部的接合强度的元素。此外，Si具有以下效果：防止热冲压的缓慢冷却步骤中的回火，保持热冲压成形品的强度。而且，Si又是有助于生成残余奥氏体并提高热冲压成形品的延性的元素。为了有效地发挥这些效果，Si含量的下限为0.7％以上。Si含量的下限优选为0.75％以上，更优选为0.80％以上，进一步优选为0.90％以上，更进一步优选为1.0％以上。
另一方面，如果Si含量过剩，则钢的强度变得过高，制造热轧酸洗钢板或冷轧钢板即基材钢板时的轧制负荷增大。此外，热轧时在基材钢板表面过度产生包含SiO2的氧化皮，镀覆后钢板的表面性状变差。因此，Si含量的上限为2.5％以下。Si含量的上限优选为2.3％以下，更优选为2.1％以下。
[Mn：1.0～3％]
Mn是为了提高淬透性、抑制热冲压成形品的高强度不均而有用的元素。此外，Mn又是有助于在后述的镀覆合金化处理中促进合金化、确保镀层中的Fe浓度的元素。为了发挥这些效果，Mn含量的下限为1.0％以上。Mn含量的下限优选为1.2％以上，更优选为1.5％以上，进一步优选为1.7％以上。
另一方面，如果Mn含量过剩，则钢的强度变得过高，制造基材钢板时的轧制负荷增大。因此，Mn含量的上限为3％以下。Mn含量的上限优选为2.8％以下，更优选为2.5％以下。
[Al：0.01～0.5％]
Al是对于钢的脱酸所必需的元素。因此，Al含量的下限为0.01％以上。Al含量的下限优选为0.03％以上。另一方面，如果Al含量过剩，则不仅上述效果饱和，而且氧化铝等夹杂物增加而加工性劣化。因此，Al含量的上限为0.5％以下。Al含量的上限优选为0.3％以下。
作为本实施方式涉及的镀锌钢板的基材钢板，可列举含有上述成分并且余部为铁(Fe)和不可避免的杂质的基材钢板。作为该不可避免的杂质，可列举例如P、S、N等。
P是对点焊部的接合强度产生不良影响的元素。如果P含量过剩，则在通过点焊形成的熔核的最终凝固面偏析，熔核脆化，接合强度降低。因此，P含量的上限优选为0.02％以下，更优选为0.015％以下。
S也是与P同样地，对点焊部的接合强度产生不良影响的元素。如果S含量过剩，则由于熔核内的晶界偏析而导致容易发生晶界断裂，接合强度降低。因此，S含量的上限优选为0.01％以下，更优选为0.008％以下。
N与B结合而减少固溶B量，对基材钢板的淬透性产生不良影响。如果N含量过剩，则氮化物的析出量增加，对基材钢板的韧性产生不良影响。因此，N含量的上限优选为0.01％以下，更优选为0.008％以下。应予说明，从制钢上的成本等方面考虑，通常的情况下，N含量为0.001％以上。
本发明中，除了上述成分之外，根据需要还可以含有B作为选择元素。
[B：0.005％以下(不包括0％)]
B是提高钢材的淬透性的元素。为了发挥该效果，优选含有0.0003％以上的B。B含量的下限更优选为0.0005％以上，进一步优选为0.0010％以上。另一方面，如果B超过0.005％，则在热冲压成形品中析出粗大的硼化物而使该成形品的韧性劣化，因此B含量的上限优选为0.005％以下，更优选为0.004％以下。
(镀锌钢板的制造方法)
本实施方式涉及的镀锌钢板例如可以通过依序进行以下各步骤来制造：铸造指定成分的钢→加热→热轧→酸洗→根据需要进行冷轧→热浸镀锌步骤→根据需要进行合金化步骤。
并且，本实施方式中，如下文中所详述的那样，为了形成本实施方式中所规定的内部氧化层及脱碳层，适当地控制热浸镀锌步骤中所含的退火步骤中的利用还原炉的退火、即还原性氛围下的热处理中的退火的条件。
以下，按照步骤顺序来说明本实施方式涉及的镀锌钢板的制造方法。
首先，铸造满足上述成分组成的钢，进行加热。加热条件没有特别限定，可以适当采用通常所用的条件，优选大致在1100～1300℃的温度下进行。
接着，对所铸造的钢进行热轧，得到热轧钢板。热轧条件没有特别限定，可以适当采用通常所用的条件。优选的条件大致如下。
精轧温度(Finisher Delivery Temperature，FDT)：800～950℃
卷取温度(Coiling Temperature，CT)：500～700℃
热轧钢板的板厚的上限优选为3.5mm以下。热轧钢板的板厚的上限更优选为3.0mm以下，进一步优选为2.5mm以下。热轧钢板的板厚的下限优选为2.0mm以上。
对于热轧钢板，在酸洗步骤中进行酸洗，制作热轧酸洗钢板。该酸洗步骤中，至少除去热轧氧化皮即可。
例如在热轧时的卷取温度高的卷材中，在热轧氧化皮与钢板的界面附近有时会形成基于Si、Mn的氧化物的晶界氧化层。但是，即使存在该晶界氧化层，在镀覆处理时也不会产生不镀等的不良影响。因此，在酸洗步骤中，不一定要除去晶界氧化层。
但是，从外观、粗糙度等使镀锌钢板的表面性状稳定化的观点考虑，优选尽量除去上述晶界氧化层。关于晶界氧化层的除去而言，可以适当采用通常所用的酸洗方法。作为热轧钢板的酸洗方法，可列举：例如，使用加热至80～90℃的盐酸等，进行20～300秒钟酸洗的方法。此时，优选在盐酸中加入适量的酸洗促进剂及抑制剂中的至少一者。关于酸洗促进剂而言，可以使用例如具有巯基的化合物。关于抑制剂而言，可以使用例如胺系有机化合物。
热轧酸洗钢板的优选的厚度亦与上述热轧钢板大致相同。
热轧酸洗钢板可以根据需要进行冷轧，制成冷轧钢板。本实施方式涉及的镀锌钢板可以尤其合适地用于以汽车的轻量化等为目的的汽车用部件。因此，从尺寸精度及平坦度的观点考虑，构成该镀锌钢板的基材钢板优选为冷轧钢板。
考虑到工厂中的生产率等，优选将冷轧率、即冷轧中的压下率大致控制在20～70％的范围内。冷轧钢板的板厚的上限优选为2.5mm以下。冷轧钢板的板厚的上限更优选为2.0mm以下，进一步优选为1.8mm以下。
接着，将如上所述获得的热轧酸洗钢板或冷轧钢板(以下，有时将它们统称为“基材钢板”)供至还原炉方式的连续镀覆步骤。
通常，在还原炉方式的热浸镀锌线中所进行的步骤分为前处理步骤、退火步骤、镀覆步骤。镀覆步骤中还根据需要进行合金化处理。
热浸镀锌线的退火步骤通常由还原炉和冷却区构成。本实施方式中，最大的特征在于：适当地控制还原炉中的退火条件、尤其是还原性氛围的露点。
当然，本实施方式涉及的方法并不限于上述方式，例如，上述热浸镀锌线也可以通过无氧化炉方式的连续退火线来进行。以下，基于上述方式进行说明。
首先，对上述基材钢板进行前处理。前处理是通常用以除去附着于基材钢板的表面的油(油脂)及污渍而进行的步骤，代表性的前处理是通过碱脱脂来进行。
碱脱脂中使用的脱脂液中所含的碱，优选使用例如苛性钠、硅酸盐或它们的混合物，只要可以将油脂等作为水溶性皂除去则没有特别限定。此外，为了提高脱脂性，还可以进行电解清洗、洗涤器处理、在脱脂液中添加表面活性剂及螯合剂的处理。
本实施方式中，只要可以将基材钢板的表面适当地脱脂，则前处理的方法没有限定，上述处理可以单独进行，也可以任意地组合。当进行碱脱脂作为前处理时，为了去除附着于基材钢板的脱脂液，进行热冲洗(热水洗涤)。热水洗涤后的基材钢板通过干燥器等进行干燥。
接着，将经过前处理的基材钢板投入还原炉，进行退火。即，对于基材钢板实施还原性氛围下的热处理。关于此时的退火条件，还原性氛围的露点为-20～0℃，退火温度为500～720℃。此外，该退火温度下的滞留时间、即退火时间为90～400秒。上述温度范围下的退火处理有时称为均热处理，这时退火温度称为均热温度，退火时间称为均热温度。
还原性氛围的露点的下限值优选为-15℃以上，更优选为-10℃以上。
还原性氛围的组成只要是还原性则没有特别限定，例如，在H2－N2混合气体中，优选H2浓度为1～30体积％。
退火温度的下限值优选为530℃以上，更优选为560℃以上，进一步优选为600℃以上。退火温度的上限值优选为700℃以下，更优选为680℃以下。
退火时间的下限值优选为120秒以上，更优选为150秒以上。退火时间的上限值优选为270秒以下，更优选为240秒以下。退火时间可以利用基材钢板通过还原炉中的速度(以下，有时称为“线速度”或者简称为“LS”)来控制。
应予说明，从节能的观点考虑，在投入还原炉之前，可以将经过前处理的基材钢板在使用了废气的还原性氛围的预热炉中进行预热。关于此时的预热条件而言，只要还原性氛围的露点为上述范围则没有特别限定。
关于上述的退火条件而言，从“(1)通过在钢板形成内部氧化层，从而抑制固溶Si在基材钢板表面的富集、并抑制伴随Si富集的基材钢板表面上Si系氧化物的生成，进而抑制不镀、合金化斑的发生”以及“(2)减少钢板表面所形成的脱碳层，抑制镀锌钢板及热冲压后的部件的机械特性的不均”的观点考虑，通过多个基础实验来确定。
从上述(1)的观点考虑，在退火时的还原性氛围的露点的上限及下限、退火温度的上限及下限、退火时间的上限及下限分别处于上述范围外的情况下，会产生不镀。
尤其是，在还原性氛围的露点过低的情况下、退火温度过高的情况下、或者退火时间过长的情况下，容易在表面形成Si系氧化物，容易产生不镀。
另一方面，在退火温度过低的情况下、或者退火时间过短的情况下，容易残留Fe系氧化物，此时也容易发生不镀。此外，在还原性氛围的露点过高的情况下，可能会产生钢板及炉内设备的氧化等不良影响。
从上述(2)的观点考虑，在退火温度过高的情况下、或者退火时间过长的情况下，脱碳层容易成为20μm以上，且容易产生热冲压后的部件的机械特性的不均。
具体而言，优选通过还原性氛围的露点与退火时的温度及时间的平衡来适当控制上述退火条件，使得不会产生不镀、合金化斑及机械特性的不均。例如，在还原性氛围的露点低的情况下，可以降低退火温度或者缩短退火时间。另一方面，在还原性氛围的露点高的情况下，可以提高退火温度或者延长退火时间。
应予说明，除了热冲压用途之外，通常，如本实施方式那样，在对含有大量Si的钢进行镀锌的情况下，为了防止产生不镀，采用了例如在退火步骤之前进行预镀覆的方法、或者在还原炉中的还原退火之前实施进行氧化的氧化还原法的方法等。但是，在本实施方式中，如以下所详述的那样，在进行了适当的还原退火之后进行镀覆，因此不需要这些方法。进行预镀覆的方法中，引入特别的设备而导致成本提高。此外，在采用氧化还原法进行制造的情况下，形成于镀层与基材钢板的界面的氧化物层在热冲压加热时阻碍Fe向镀层的扩散，为了防止LME而使所需的加热时间变长，压制生产率降低。
接着，将离开还原炉的基材钢板在冷却区进行冷却。通常，冷却区由缓慢冷却区、急冷区、调节区构成。调节区有时被称为保持区。但是，对于冷却方法而言，以不产生不镀的方式在通常所用的条件下进行即可，可列举：例如，对钢板吹拂还原性氛围的气体来进行冷却等的方法。
在以如上方式进行了连续退火步骤之后，进行镀锌。详细而言，通过热浸镀锌步骤来制作热浸镀锌钢板(GI)。或者，可以将上述GI合金化，制作合金化热浸镀锌钢板(GA)。
上述热浸镀锌步骤没有特别限定，可以采用通常所用的方法。例如，热浸镀锌浴的温度控制为430～500℃左右即可。从确保耐腐蚀性的观点考虑，热浸镀锌层的附着量(与下述的合金化热浸镀锌层的附着量相同)优选为30g/m2以上，更优选为40g/m2以上，进一步优选为超过75g/m2。另一方面，从容易实现本发明所规定的镀层中Fe浓度的观点考虑，热浸镀锌层(尤其是合金化热浸镀锌层)的附着量越少越好。因此，热浸镀锌层的附着量优选为120g/m2以下，更优选为100g/m2以下。
上述合金化步骤也没有特别限定，可以采用通常所用的方法。合金化步骤中，镀层中的Fe浓度提高。从该观点考虑，合金化温度优选控制为例如500～700℃左右。合金化温度更优选为530℃以上，进一步优选为570℃以上，更进一步优选为600℃以上。另一方面，如果合金化温度过高，则镀层中的Fe浓度变得过高，因此，合金化温度优选为680℃以下，更优选为650℃以下。
镀覆步骤后的处理步骤也没有特别限定，可以采用通常所用的方法。通常进行了表皮光轧处理、拉伸矫直处理、涂油等，这些步骤根据需要在通常所用的条件下实施即可，若非必要也可以不实施。
前述再退火的推荐条件如下。即再退火时的加热温度(再退火温度)优选为400℃以上，更优选为450℃以上。另一方面，从抑制锌的蒸发的观点考虑，前述再退火温度优选为750℃以下，更优选为700℃以下。
此外，在上述再退火温度下保持的时间(再退火时间)可以根据加热方法等而适当设定。例如在炉加热的情况下，前述再退火时间优选为1小时以上，更优选为2小时以上。在感应加热的情况下，前述再退火时间优选为10秒以上。另一方面，从抑制锌的蒸发的观点考虑，在前述炉加热的情况下，前述再退火时间优选为15小时以下，更优选为10小时以下。在前述感应加热的情况下，前述再退火时间优选为3分钟以下，更优选为1分钟以下。
以如上方式得到的镀锌钢板(GI或GA)可以合适地用作热冲压用钢板。
本实施方式中，对于热冲压步骤没有特别限定，可以采用通常所用的方法。可列举例如下述方法：按照通常的热冲压方法，将上述钢板加热至Ac3相变点以上的温度而奥氏体化之后，例如将结束成形的时刻的温度、即模具到达下死点位置的时刻的温度设为约550℃以上的方法。作为前述加热的方法，可以采用炉加热、通电加热、感应加热等。
对于上述加热的条件而言，通过将在Ac3相变点以上的温度下的保持时间(在炉加热的情况下，有时称为在炉时间。在通电加热、感应加热的情况下，是指从加热开始到结束为止的时间。)控制为优选30分钟以下、更优选15分钟以下、进一步优选7分钟以下，从而可以抑制奥氏体的晶粒生长，提高热状态下的可拉性(hot drawability)、热冲压成形品的韧性等特性。在Ac3相变点以上的温度下的保持时间的下限没有特别限定，只要在加热中达到Ac3相变点以上即可。
使用本实施方式涉及的镀锌钢板来制造热冲压成形品时，除了采用前述热冲压步骤之外，还可以采用基于部件形状进行的切削等通常所采用的步骤及条件。作为热冲压成形品，可列举：例如汽车用的底盘、所谓的悬架部件、补强部件等。
本说明书如上所述公开了各种实施方式的技术，其主要技术总结如下。
本发明一个方面涉及热冲压用镀锌钢板，其包括：基材钢板；以及镀层，设置于所述基材钢板的表面，其中，所述基材钢板以质量％计含有C：0.10～0.5％、Si：0.7～2.5％、Mn：1.0～3％、以及Al：0.01～0.5％，余部为铁和不可避免的杂质，所述基材钢板具有：内部氧化层，处于所述基材钢板的内部，厚度为1μm以上，并且包含氧化物，所述氧化物含有Si和Mn的至少其中之一；以及脱碳层，从与所述镀层的界面起朝向所述基材钢板的内部方向厚度为20μm以下。
根据该构成，可以得到例如不镀、合金化斑等外观不良的产生被进一步抑制的热冲压用镀锌钢板。
上述构成中，所述基材钢板可以还含有0.005％以下(不包括0％)的B。据此，可以提高基材钢板的淬透性并且提高热冲压用镀锌钢板的强度。
此外，本发明另一个方面涉及热冲压用镀锌钢板的制造方法，该方法是所述热冲压用镀锌钢板的制造方法，将满足所述热冲压用镀锌钢板的成分组成的冷轧钢板在露点为-20～0℃的还原性氛围中于500～720℃保持90～400秒；然后，实施镀覆。
根据该构成，可以得到如上所述的热冲压用镀锌钢板。
以下，列举实施例来更具体地说明本发明，但本发明不受下述实施例的限制，在可满足前述或后述的要旨的范围内亦可以进行变更而实施，这些均包含在本发明的技术范围内。
实施例
将由含有表1记载的成分组成的钢所形成的板坯加热至1200℃后，依序进行在表1记载的精轧温度(FDT)及卷取温度(CT)的条件下的热轧、基于酸洗步骤的除鳞处理、冷轧，得到了冷轧钢板。将该冷轧钢板作为镀层钢板的基材钢板。
表1
Figure BDA0002641776410000111
使用由此得到的各冷轧钢板，对以下的各项目进行了评价。
(内部氧化层的深度及厚度)
将上述冷轧钢板切断，得到了10mm×20mm的试片。将该试片埋入支承基材内，对截面进行研磨后以硝酸乙醇(Nital)进行了轻度腐蚀，然后，对于该截面中的镀层附近，使用FE-SEM(SUPURA35，ZEISS制)以倍率1500倍采用反射电子图像进行了观察。于是，在镀覆合金层与基材钢板的界面附近的基材钢板侧的基材钢板内部，将作为微小黑点被观察到的Si和Mn的至少其中之一的氧化物所分布的区域作为内部氧化层。将内部氧化层的平均厚度为1μm以上并且从镀覆合金层与基材钢板的界面起的深度为1μm以上的情况评价为○(合格)；将除此以外的情况评价为×(不合格)。该评价结果示于后述的表3。
(脱碳层的厚度)
针对由通过后述方法制作的模型方程(model equation)得到的数值纳入均热时间、均热温度及氛围的露点的影响而算出了脱碳层的厚度。在算出的脱碳层的厚度(从冷轧钢板表面起的深度)为20μm以下的情况下，视为可以确保机械特性，评价为○(合格)；将超过20μm的情况评价为×(不合格)。该评价结果示于后述的表3。
(模型方程的制作方法)
将上述冷轧钢板切断，得到了100mm×200mm的试片。对于该试片，采用镀覆模拟使退火温度、氛围的露点(以下，有时简称为“露点”)、退火时间变化，进行了退火处理。露点及退火温度如下述表2所示，对于各露点及退火温度，将退火时间设为0s、120s及240s。对于退火后的试片，通过GDOES求出从钢板表面起的深度方向(厚度方向)中的碳浓度曲线，测定了脱碳层的厚度。脱碳层的厚度是从试片的表面到碳浓度成为退火前基材钢板的碳浓度的80％的位置为止的深度。
表2
露点(℃) 退火温度(℃) 脱碳速率常数K(μm/s)
0 680 0.0864
0 700 0.1471
0 720 0.2411
0 740 0.3338
-10 680 0.0953
-10 700 0.1377
-10 720 0.2034
-10 740 0.2807
-15 680 0.0716
-15 700 0.1146
-15 720 0.1594
-15 740 0.2154
退火时间与脱碳层的厚度的关系以下述式(1)表示。
X＝Kt (1)
在此，X：脱碳层厚度(μm)，K：脱碳速率常数(μm/s)，t：退火时间(s)。
根据上述式(1)，作为横轴为退火时间t、纵轴为测得的脱碳层厚度X进行作图而得的曲线图的斜率，针对各退火温度及露点求出了脱碳速率常数K。求出的K示于表2。
此外，脱碳速率常数与退火温度的关系以下述阿伦尼乌斯(Arrhenius)公式表示。
K＝Aexp(-E/RT)
在此，K：脱碳速率常数(μm/s)，A：常数(μm/s)，E：常数(J/mol)，R：气体常数(8.31J/mol·K)，T：退火温度(K)。
阿伦尼乌斯公式可以如下述式(2)那样变形。
lnK＝lnA+(-E/R)×1/T (2)
根据上述式(2)，在横轴取1/T(以绝对温度表示的退火温度的倒数)、在纵轴取lnK，由将表2所示的值作图而得到的曲线图的斜率求出了E，并且由切片求出了A。E与露点无关而恒定为146kJ/mol。A是以下述式(3)表示的值。
A＝2.49×105×DP+1.15×107 (3)
在此，DP：露点(℃)。
根据以上内容，上述式(1)中的K为以下述式表示的值。将该K代入式(1)而得的公式为模型方程。
K＝(2.49×105×DP+1.15×107)×exp((-146000/8.31)×1/T)
应予说明，在实验室的实验中，在退火温度为700℃以下的情况下，露点为-10℃以上则不会观察到露点的影响，因此，在退火温度为700℃以下且露点为-10℃以上的情况下，以DP＝-10℃的形式算出了K。
(镀覆状态)
将上述冷轧钢板切断，得到了100mm×150mm的试片。将试片在60℃的3％正硅酸钠中以20A、20秒的条件进行了电解脱脂后，在自来水中以流水进行了5秒水洗。由此进行了碱脱脂后，采用镀覆模拟，进行了退火(均热)。表2中，作为均热处理的条件，示出了均热温度、还原性氛围的露点、及线速度(LS)。线速度是通过退火炉的试片的速度。还原性氛围为含有5体积％的H2气体且余部为N2气体的混合气体。
具体而言，在上述还原性氛围中，从室温加热至均热温度，在表2所示的条件下进行了均热处理后，从该均热温度冷却至460℃。接着，采用Al含量为0.1质量％、温度为460℃的镀锌浴进行镀覆，并进行擦拭而得到了热浸镀锌钢板(GI)。并且，进行合金化温度为550℃、合金化时间为20秒的合金化处理，得到了合金化热浸镀锌钢板(GA)。
对于上述GA，以目视观察浸渍于镀锌浴中的范围(约100mm×120mm)内的钢板表面，求出了不镀的面积率，并且确认了合金化斑的有无。对于不镀而言，将不镀的面积率为5％以下的情况评价为○(合格)，将不镀的面积率超过5％的情况评价为×(不合格)。对于合金化斑而言，将Fe浓度为8％以上的情况评价为○(合格)，将Fe浓度低于8％的情况评价为×(不合格)。这些结果示于表3。应予说明，关于No.17而言，未进行针对内部氧化层的深度及厚度的测定。
表3
Figure BDA0002641776410000141
根据表3，可以进行如下的考察。
就No.1、2、4～6、8～13、15、16而言，在还原性氛围下的均热处理条件满足本发明的规定，针对脱碳层的厚度、不镀的面积率、及合金化斑的评价均为○。此外，虽然表3中未示出，但No.1～16中内部氧化层的深度为1～3μm，评价均为○。
就No.3、No.7及No.14而言，合金化斑的评价为×。认为其原因在于：还原性氛围的露点为-45℃这样的较低值，未形成内部氧化层、或者内部氧化层的厚度及深度不够。
其中，就No.7及No.14而言，不镀的面积率的评价也为×。认为其原因在于：在基材钢板的表面形成了Si系氧化物。
就No.17而言，脱碳层的厚度的评价为×。认为其原因在于：均热温度过高。
本申请以2018年3月2日申请的日本国专利申请特愿2018-037132为基础，其内容包含在本申请中。
为了表述本发明，上文中通过上述实施方式对本发明进行了适当并充分的说明，但应该认识到本领域技术人员容易对上述实施方式进行变更和/或改良。因此，本领域技术人员实施的变形实施方式或改良实施方式，只要是没有脱离权利要求书中记载的权利要求的保护范围的水平，该变形实施方式或改良实施方式可解释为被包含在该权利要求的保护范围内。
产业上的可利用性
本发明在有关热冲压用镀锌钢板的技术领域中，具有广泛的产业上的可利用性。

Claims (3)

1.一种热冲压用镀锌钢板，其特征在于包括：
基材钢板；以及
镀层，设置于所述基材钢板的表面，其中，
所述基材钢板以质量％计含有
C：0.10～0.5％、
Si：0.7～2.5％、
Mn：1.0～3％、以及
Al：0.01～0.5％，
余部为铁和不可避免的杂质，
所述基材钢板具有：
内部氧化层，处于所述基材钢板的内部，厚度为1μm以上，并且包含氧化物，所述氧化物含有Si和Mn的至少其中之一；以及
脱碳层，从与所述镀层的界面起朝向所述基材钢板的内部方向厚度为20μm以下。
2.根据权利要求1所述的热冲压用镀锌钢板，其特征在于：
所述基材钢板还含有0.005％以下且不包括0％的B。
3.一种权利要求1或2所述的热冲压用镀锌钢板的制造方法，其特征在于：
将满足权利要求1或2所述的成分组成的冷轧钢板在露点为-20～0℃的还原性氛围中于500～720℃保持90～400秒；
然后，实施镀覆。
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