JP3829189B2 - Grain boundary measurement method for steel with prior austenite grain boundaries. - Google Patents
Grain boundary measurement method for steel with prior austenite grain boundaries. Download PDFInfo
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- JP3829189B2 JP3829189B2 JP2002200600A JP2002200600A JP3829189B2 JP 3829189 B2 JP3829189 B2 JP 3829189B2 JP 2002200600 A JP2002200600 A JP 2002200600A JP 2002200600 A JP2002200600 A JP 2002200600A JP 3829189 B2 JP3829189 B2 JP 3829189B2
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Description
【0001】
【発明の属する技術分野】
この出願の発明はマルテンサイト鋼やベントナイト鋼等の旧オーステナイト粒界を組織に持つ鋼の旧オーステナイト(γ)結晶粒界を測定する方法に関するものであり、さらに詳しくはリンやイオウの含有量が極端に少ない旧オーステナイト粒界を組織に持つ清浄度鋼であっても旧オーステナイト(γ)結晶粒界を高精度に、簡単に測定することのできる新しい結晶粒界の測定法に関するものである。
【0002】
【従来の技術】
旧オーステナイト粒界を組織に持つ鋼に含まれるリンやイオウ等の不純物元素は旧オーステナイト(γ)結晶粒界に偏析しており、これが種々の脆化の原因とされている。したがって、旧オーステナイト粒界を組織に持つ鋼において、この不純物元素を低減することは強靭化のために欠かすことができないものとされている。 特に近年では製鋼技術や精錬技術の進歩によって純度が高く靭性が大きい旧オーステナイト粒界を組織に持つ清浄度鋼を製造することが可能になってきているが、このように純度が高く靭性が大きい旧オーステナイト粒界を組織に持つ清浄度鋼への指向が強くなれば強くなるほど、粒界破壊の要因となる旧オーステナイト(γ)結晶粒界を正確に測定する技術が重要になる。
【0003】
従来から旧オーステナイト粒界を組織に持つ鋼の旧オーステナイト(γ)粒界の測定法としては、ピクリン酸水溶液中に旧オーステナイト粒界を組織に持つ鋼を浸して旧オーステナイト粒界を組織に持つ鋼に含まれるリンやイオウを化学腐食する方法が知られている。
【0004】
これは不純物元素であるリンやイオウが旧オーステナイト粒界を組織に持つ鋼の旧オーステナイト(γ)結晶粒子の表面である粒界に偏析することを利用したものである。すなわち、この測定法はピクリン酸水溶液に旧オーステナイト粒界を組織に持つ鋼を浸して、旧オーステナイト粒界を組織に持つ鋼の結晶粒界に偏析しているリンやイオウを選択的に腐食させて正常な部分との差を測定するものである。この時に腐食剤として使用するピクリン酸水溶液としては、(イ)ピクリン酸飽和水溶液を使用する方法と、(ロ)ピクリン酸飽和水溶液に界面活性剤を添加した第I溶液とこの第I溶液に、さらに塩化第二鉄を添加した第II溶液を使用する方法があるが、現在では(ロ)の方法が主に採用されている。この(ロ)の方法を具体的に説明すると図1のようになる。
【0005】
【発明が解決しようとする課題】
しかしながら、図1のプロセス図で示されている測定法はSCM440鋼等のようにリンやイオウが比較的多い旧オーステナイト粒界を組織に持つ鋼の不純物を測定するには有効であるが、近年開発されているような純度の高い旧オーステナイト粒界を組織に持つ清浄度鋼の場合には、ピクリン酸水溶液によって腐食するリンやイオウの量が極めて少ないため結晶粒界での選択的腐食が難しいという問題がある。
【0006】
そこで、この出願の発明は純度の高い旧オーステナイト粒界を組織に持つ清浄度鋼であっても、旧オーステナイト(γ)結晶粒界を明確に測定できる新しい方法を提供することを課題としている。
【0007】
【課題を解決するための手段】
この発明は、上記の課題を解決するものとして、第1には、旧オーステナイト粒界を組織
に持つ鋼の表面を機械研磨および電解研磨を行ない、ピクリン酸水溶液中に浸漬した後にチオ硫酸ナトリウム水溶液中に浸漬して旧オーステナイト粒界を組織に持つ清浄度鋼の旧オーステナイト(γ)結晶粒界を選択腐食させて結晶粒界の形状および粒径を測定することを特徴とする測定法を提供するものであり、また、第2には、上記測定法において、ピクリン酸水溶液中に30秒間以上浸漬することを特徴とする方法を提供するものであり、そして、第3には、上記測定法において、チオ硫酸ナトリウム水溶液中に30秒間以上浸漬することを特徴とする方法を提供するものである。
【0008】
【発明の実施の形態】
この出願の発明は上記のとおりの特徴を持つものであるが、以下にその実施の形態について説明する。
【0009】
この出願の発明においては、なによりも、旧オーステナイト粒界を組織に持つ鋼をピクリン酸水溶液中に浸漬した後にチオ硫酸ナトリウム水溶液中に浸漬して旧オーステナイト(γ)粒界を選択腐食させることを特徴としている。
【0010】
この方法においては、たとえば図2に例示した手順をより好適な実施の形態として示すことができる。たとえば、まず、試料の表面をバフ研磨等の機械研磨により鏡面仕上げをおこない、次に電解研磨を施す。この電解研磨をおこなうことによってバフ研磨によって生じた残留応力層を除去すると共に清浄鋼の表面を極めて平滑にすることができる。引き続いて、ピクリン酸飽和水溶液中に10分間程度浸漬し、その後直ちにチオ硫酸ナトリウム飽和水溶液中に10分間程度浸すと、ピクリン酸飽和水溶液だけでは確認できないような旧オーステナイト粒界を組織に持つ清浄度鋼の結晶粒界のリンやイオウが選択的に腐食されて旧オーステナイト(γ)結晶の粒界形状および粒径の測定が可能になるものである。
【0011】
この出願の発明の方法によって、従来の不純物含有水準の旧オーステナイト粒界を組織に持つ鋼の場合はもとより、たとえば、リン含有量0.01masswt%以下、イオウ含有量0.01masswt%以下という高純度な、旧オーステナイト粒界を組織に持つ清浄度鋼の場合であっても、粒界腐食を促し、正確に高精度に旧オーステナイト(γ)結晶粒界の測定が可能になる。
機械研磨、そして電解研磨を行なう場合には、その表面粗度(Ra)は10nm以下程度までにするのが好ましい。
【0012】
以下に実施例を示す。もちろん、この例によって発明が限定されることはない。
【0013】
【実施例】
試料として、次の表1に示した化学組成を有する、不純物の少ないPC鋼棒と不純物含有量の多いSCM440鋼を使用した。
【0014】
【表1】
【0015】
この表1に示した組成のPC鋼棒は、リン含有量が0.006mass%、イオウ含有量が0.004mass%の低リン、低イオウの鋼材であり、一方、高純度化処理を施していないSCM440鋼は、リン含有量が0.02mass%、イオウ含有量が0.007mass%であり、比較的多量のリン及びイオウを含有している鋼材である。
【0016】
この二つの対照的な鋼材を使用して、この出願の発明の方法と従来法とにより、結晶粒界の腐食、すなわちリンやイオウの存在をどの程度明確に確認できるかを比較した。
【0017】
この出願の発明の方法は、図2のプロセスに沿うものとして、以下の条件によて行った。
【0018】
また、従来法は、図1のプロセスに沿うものとして行なった。
【0019】
図3は従来の測定法によって処理したSCM440鋼の結晶粒界の表面を光学顕微鏡で観察した組織図である。図面の黒い部分はリンやイオウが偏析している粒界をピクリン酸によって腐食した部分である。このように、リンやイオウの含有量が多いSCM440鋼の場合には、従来法でもリンやイオウの結晶臨界が腐食によって生じている粒径の輪郭を明瞭に判別することができる。
【0020】
しかしながら、従来法でリンやイオウ含量が少ないPC鋼棒を処理し、その後の結晶粒界の表面を光学顕微鏡で観察した場合には、図4(a)および(b)のようにしか観察できなかった。図4(a)は低倍率の光学顕微鏡で観察した時の組織図であり、(b)は高倍率の光学顕微鏡で観察した時の組織図であるが、いずれの場合もリンやイオウの腐食部分である結晶粒界の輪郭を明確に同定することはできない。このように、ピクリン酸の飽和水溶液だけを使用する従来の方法は、表1のSCM440鋼のようにリンやイオウが比較的多量に含有されている場合のリンやイオウの測定方法としては有効であるが、リンやイオウの含量が少ないPC鋼棒のような場合のリンやイオウの測定法としては充分機能しないことがわかる。
【0021】
これに対して、この出願の発明の方法によって処理したものを示したのが図5である。この図5は、表1の(a)PC鋼棒および(b)SCM440鋼を電解研磨後ピクリン酸水溶液とチオ硫酸ナトリウム水溶液によって処理した結晶粒界の表面を光学顕微鏡の最大倍率で観察した場合のものである。(a)のPC鋼棒では、結晶粒界のリンやイオウを腐食させるために化学腐食液に長時間浸漬しているため、結晶粒内にも多少腐食している部分が見られるが結晶粒界の輪郭は従来法に比較してかなり明確に示されている。
【0022】
一方、(b)のSCM440鋼についてはリンやイオウの量が多いため、浸漬時間が短くても粒界の腐食が簡単におきるため、結晶粒界は明確に判別することができる。この結果得られた粒界の公称粒径は通常の方法で測定したのと同じ19μmであった。
【0023】
さらに、この出願の発明の方法で処理したものを、更に高度に解析できる走査型電子顕微鏡で結晶粒界を観察した場合を示したものが図6である。
【0024】
この図6はPC鋼棒を電解研磨後ピクリン酸とチオ硫酸ナトリウム水溶液で処理したものを走査型電子顕微鏡(SEM)で観察した組織写真である。
(a)は走査型電子顕微鏡の低倍率での写真あり、(b)は走査型電子顕微鏡の高倍率の写真である。図6の(a)および(b)からも明らかなように低リンおよび低イオウであるにもかかわらずPC鋼棒の結晶粒界は鮮明に識別できる。そして、図面からも明らかなように(a)の低倍率の写真より(b)の高倍率の写真の方が結晶粒界の輪郭をより明瞭に判別できる。この理由は、結晶粒界を腐食させるためには長時間腐食液に浸漬させる必要があるが、この際に粒内組織も多少腐食されている。このような場合には、高倍率写真の方が粒界と粒内の腐食程度の測定が容易になるためである。この方法で得られた結晶粒界の公称粒径は13μmであることが確かめられた。
【0025】
このようにこの出願の発明の方法によれば、リンやイオウが少ない高純度のマルテンサイト清浄鋼においてもリンやイオウを充分に測定できることが判明した。また、旧オーステナイト粒界を組織に持つ清浄度鋼のみならずマルテンサイト通常鋼においてもこの出願の発明の方法が有効であることは図5の(b)の結果からも確かめられている。
【0026】
【発明の効果】
以上詳しく説明したとおり、この出願の発明によって、リンやイオウが微量しか存在しない旧オーステナイト粒界を組織に持つ清浄度鋼においても、リンやイオウ等の不純物の存在を測定することができ、さらに高純度の旧オーステナイト粒界を組織に持つ清浄度鋼の製鋼および精錬が可能になる。
【図面の簡単な説明】
【図1】 従来の方法を例示したプロセス図である。
【図2】 この出願の発明の方法を例示したプロセス図である。
【図3】 従来法のピクリン酸水溶液でSCM440鋼の結晶粒界を選択腐食した面の光学顕微鏡の組織写真である。
【図4】 従来法のピクリン酸水溶液でPC鋼棒の結晶粒界を選択腐食した面の光学顕微鏡写真である。
(a) 低倍率の組織写真。
(b) 高倍率の組織写真。
【図5】 この出願の発明の電解研磨後のピクリン酸とチオ硫酸ナトリウム水溶液による結晶粒界の選択腐食した面の光学顕微鏡の最大倍率の組織写真である。
(a) PC鋼棒。
(b) SCM440鋼。
【図6】 この出願の発明の電解研磨後のピクリン酸水溶液とチオ硫酸ナトリウム水溶液によるPC鋼棒の結晶粒界を選択腐食した面の走査型電子顕微鏡写真である。
(a) 低倍率。
(b) 高倍率。[0001]
BACKGROUND OF THE INVENTION
The invention of this application relates to a method for measuring a prior austenite (γ) grain boundary of steel having a prior austenite grain boundary structure such as martensitic steel and bentonite steel, and more specifically, the content of phosphorus and sulfur. The present invention relates to a new grain boundary measurement method capable of easily measuring a prior austenite (γ) grain boundary with high accuracy even in a clean steel having a structure having extremely few former austenite grain boundaries.
[0002]
[Prior art]
Impurity elements such as phosphorus and sulfur contained in the steel having a prior austenite grain boundary as a structure segregate in the prior austenite (γ) crystal grain boundary, which is considered to cause various embrittlements. Therefore, in steels having a prior austenite grain boundary as a structure, reducing this impurity element is considered indispensable for toughening. Particularly in recent years, it has become possible to produce cleanliness steel with the structure of old austenite grain boundaries that have high purity and high toughness due to advances in steelmaking and refining technologies. As the orientation toward cleanliness steel having a prior austenite grain boundary as a structure increases, a technique for accurately measuring the prior austenite (γ) grain boundary that causes grain boundary fracture becomes more important.
[0003]
Conventionally, as a method for measuring the prior austenite (γ) grain boundary of steel having a prior austenite grain boundary as a structure, a steel having a prior austenite grain boundary as a structure is immersed in a picric acid solution to have the prior austenite grain boundary as a structure. A method of chemically corroding phosphorus and sulfur contained in steel is known.
[0004]
This utilizes the fact that impurity elements such as phosphorus and sulfur segregate at the grain boundaries which are the surfaces of the prior austenite (γ) crystal grains of the steel having the former austenite grain boundaries as a structure. That is, in this measurement method, steel having a prior austenite grain boundary is immersed in an aqueous picric acid solution to selectively corrode phosphorus and sulfur segregated at the crystal grain boundary of the steel having the prior austenite grain boundary. The difference from the normal part is measured. As the picric acid aqueous solution used as the corrosive agent at this time, (i) a method using a picric acid saturated aqueous solution, (b) a solution obtained by adding a surfactant to the picric acid saturated aqueous solution, and this solution I, Furthermore, there is a method using a solution II added with ferric chloride, but at present, the method (b) is mainly employed. The method (b) will be specifically described as shown in FIG.
[0005]
[Problems to be solved by the invention]
However, the measurement method shown in the process diagram of FIG. 1 is effective in measuring impurities in a steel having a structure having a prior austenite grain boundary with a relatively large amount of phosphorus and sulfur such as SCM440 steel. In the case of a clean steel with a structure of high-purity prior austenite grain boundaries as developed, the amount of phosphorus and sulfur corroded by the picric acid aqueous solution is extremely small, making selective corrosion at the grain boundaries difficult. There is a problem.
[0006]
Accordingly, an object of the invention of this application is to provide a new method capable of clearly measuring a prior austenite (γ) grain boundary even in a clean steel having a structure having a high purity prior austenite grain boundary.
[0007]
[Means for Solving the Problems]
In order to solve the above problems, the present invention firstly performs mechanical polishing and electrolytic polishing on the surface of steel having a prior austenite grain boundary as a structure, and is immersed in a picric acid aqueous solution and then a sodium thiosulfate aqueous solution. Providing a measurement method characterized by measuring the shape and grain size of grain boundaries by selectively corroding the former austenite (γ) grain boundaries of cleanliness steel immersed in the former austenite grain boundaries. And secondly, in the above measurement method, a method characterized by immersing in an aqueous picric acid solution for 30 seconds or more, and thirdly, the above measurement method is provided. in, Ru der provides a method characterized by immersion for more than 30 seconds in a aqueous solution of sodium thiosulfate.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
The invention of this application has the characteristics as described above, and an embodiment thereof will be described below.
[0009]
In the invention of this application, above all, a steel having a prior austenite grain boundary is immersed in an aqueous picric acid solution and then immersed in an aqueous sodium thiosulfate solution to selectively corrode prior austenite (γ) grain boundaries. It is characterized by.
[0010]
In this method, for example, the procedure illustrated in FIG. 2 can be shown as a more preferred embodiment. For example, first, the surface of the sample is mirror-finished by mechanical polishing such as buffing, and then electrolytic polishing is performed. By performing this electropolishing, the residual stress layer generated by the buffing can be removed and the surface of the clean steel can be made extremely smooth. Subsequently, when immersed in a saturated aqueous solution of picric acid for about 10 minutes and then immediately immersed in a saturated aqueous solution of sodium thiosulfate for about 10 minutes, the cleanliness of the former austenite grain boundaries in the structure that cannot be confirmed only with the saturated aqueous solution of picric acid. The phosphorus and sulfur at the grain boundaries of steel are selectively corroded, and the grain boundary shape and grain size of the prior austenite (γ) crystal can be measured.
[0011]
According to the method of the invention of this application, not only in the case of steel having a conventional austenite grain boundary of the impurity content level in the structure, for example, high purity such as phosphorus content 0.01 masswt% or less, sulfur content 0.01 masswt% or less Even in the case of cleanliness steel having a prior austenite grain boundary as a structure, intergranular corrosion is promoted, and the prior austenite (γ) grain boundary can be measured with high accuracy.
When performing mechanical polishing and electrolytic polishing, the surface roughness (Ra) is preferably about 10 nm or less.
[0012]
Examples are shown below. Of course, the invention is not limited by this example.
[0013]
【Example】
As a sample, a PC steel bar having a low impurity content and an SCM440 steel having a high impurity content having the chemical composition shown in Table 1 below were used.
[0014]
[Table 1]
[0015]
The PC steel bar having the composition shown in Table 1 is a low phosphorus and low sulfur steel material having a phosphorus content of 0.006 mass% and a sulfur content of 0.004 mass%, and on the other hand, has been subjected to a high-purity treatment. No SCM440 steel has a phosphorus content of 0.02 mass% and a sulfur content of 0.007 mass%, and is a steel material containing a relatively large amount of phosphorus and sulfur.
[0016]
Using these two contrasting steels, the method of the invention of this application and the conventional method were compared to see how clearly the grain boundary corrosion, that is, the presence of phosphorus and sulfur can be confirmed.
[0017]
The method of the invention of this application was performed under the following conditions assuming that the process of FIG.
[0018]
Further, the conventional method was performed along the process of FIG.
[0019]
FIG. 3 is a structural diagram obtained by observing the surface of a grain boundary of SCM440 steel processed by a conventional measuring method with an optical microscope. The black part of the drawing is the part where the grain boundaries where phosphorus and sulfur are segregated are corroded by picric acid. As described above, in the case of SCM440 steel having a high phosphorus and sulfur content, it is possible to clearly discriminate the particle size contour in which the crystal criticality of phosphorus and sulfur is caused by corrosion even in the conventional method.
[0020]
However, if a conventional method is used to treat a PC steel rod with a low phosphorus or sulfur content and the subsequent grain boundary surface is observed with an optical microscope, it can be observed only as shown in FIGS. 4 (a) and 4 (b). There wasn't. FIG. 4A is a structure diagram when observed with a low magnification optical microscope, and FIG. 4B is a structure diagram when observed with a high magnification optical microscope. In either case, corrosion of phosphorus and sulfur is observed. The contour of the grain boundary that is a part cannot be clearly identified. Thus, the conventional method using only a saturated aqueous solution of picric acid is effective as a method for measuring phosphorus and sulfur in the case where a relatively large amount of phosphorus and sulfur is contained, such as SCM440 steel in Table 1. However, it can be seen that it does not function sufficiently as a method for measuring phosphorus and sulfur in the case of a PC steel rod having a low phosphorus and sulfur content.
[0021]
On the other hand, FIG. 5 shows what was processed by the method of the invention of this application. FIG. 5 shows a case where the surface of a grain boundary obtained by electropolishing the (a) PC steel rod and (b) SCM440 steel in Table 1 and treating with a picric acid aqueous solution and a sodium thiosulfate aqueous solution is observed at the maximum magnification of an optical microscope. belongs to. In the PC steel rod of (a), since it is immersed for a long time in a chemical corrosive solution in order to corrode phosphorus and sulfur at the grain boundaries, some corroded portions are also seen in the crystal grains. The contour of the field is shown quite clearly compared to the conventional method.
[0022]
On the other hand, since the SCM440 steel (b) has a large amount of phosphorus and sulfur, the grain boundaries are easily corroded even if the immersion time is short, so that the crystal grain boundaries can be clearly identified. As a result, the nominal grain size of the grain boundary was 19 μm, which was the same as measured by a usual method.
[0023]
Further, FIG. 6 shows a case where the crystal grain boundary is observed with a scanning electron microscope capable of analyzing the image processed by the method of the invention of this application with a higher degree.
[0024]
FIG. 6 is a photograph of a structure obtained by observing, with a scanning electron microscope (SEM), a PC steel bar treated with picric acid and an aqueous sodium thiosulfate solution after electrolytic polishing.
(A) is a photograph at a low magnification of a scanning electron microscope, and (b) is a photograph at a high magnification of a scanning electron microscope. As is clear from FIGS. 6A and 6B, the grain boundaries of the PC steel rod can be clearly distinguished despite the low phosphorus and low sulfur. As is clear from the drawing, the outline of the grain boundary can be discriminated more clearly in the high magnification photograph (b) than in the low magnification photograph (a). This is because, in order to corrode the crystal grain boundaries, it is necessary to immerse in a corrosive liquid for a long time, but the intragranular structure is also somewhat corroded. In such a case, the high-magnification photograph makes it easier to measure the degree of corrosion within the grain boundaries and grains. It was confirmed that the nominal grain size of the grain boundary obtained by this method was 13 μm.
[0025]
Thus, according to the method of the invention of this application, it has been found that phosphorus and sulfur can be sufficiently measured even in high-purity martensitic clean steel with little phosphorus and sulfur. Further, it is confirmed from the result of FIG. 5B that the method of the invention of this application is effective not only in cleanliness steel having a prior austenite grain boundary structure but also in martensite normal steel.
[0026]
【The invention's effect】
As described above in detail, according to the invention of this application, it is possible to measure the presence of impurities such as phosphorus and sulfur even in cleanliness steel having a former austenite grain boundary whose structure contains only a small amount of phosphorus and sulfur. Steelmaking and refining of cleanliness steel with a high-purity prior austenite grain boundary as a structure becomes possible.
[Brief description of the drawings]
FIG. 1 is a process diagram illustrating a conventional method.
FIG. 2 is a process diagram illustrating the inventive method of this application.
FIG. 3 is a structure photograph of an optical microscope of a surface where a crystal grain boundary of SCM440 steel is selectively corroded with a conventional picric acid aqueous solution.
FIG. 4 is an optical micrograph of a surface obtained by selectively corroding a crystal grain boundary of a PC steel rod with a conventional picric acid aqueous solution.
(A) Low magnification tissue photograph.
(B) High magnification tissue photograph.
FIG. 5 is a structure photograph of the maximum magnification of an optical microscope of a selectively corroded surface of a grain boundary by picric acid and an aqueous sodium thiosulfate solution after electropolishing according to the invention of this application.
(A) PC steel bar.
(B) SCM440 steel.
FIG. 6 is a scanning electron micrograph of a surface where a crystal grain boundary of a PC steel bar is selectively corroded by an aqueous picric acid solution and an aqueous sodium thiosulfate solution after electropolishing according to the invention of this application.
(A) Low magnification.
(B) High magnification.
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KR100843896B1 (en) * | 2006-12-22 | 2008-07-03 | 주식회사 포스코 | Observing Method Of Deformed Microstructure In Heavily Deformed Austenite Grain |
DE112010003086B4 (en) * | 2009-07-27 | 2021-11-11 | Hyundai Steel Co. | Method for evaluating the center segregation of a continuously cast slab |
CN110553892B (en) * | 2019-10-08 | 2023-01-24 | 华电邹县发电有限公司 | Erosion method suitable for T/P91 and T/P92 steel |
CN116699097B (en) * | 2023-07-28 | 2023-10-10 | 北京科技大学 | Nondestructive testing method for solidification structure of high-strength steel for automobile |
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