US-12625125-B2 - Method of evaluating central segregation in steel

US12625125B2US 12625125 B2US12625125 B2US 12625125B2US-12625125-B2

Abstract

A method of evaluating central segregation in steel with excellent correlation with HIC susceptibility is provided. A method of evaluating central segregation in steel includes: taking a sample from a steel, the sample having a cross section including a central segregation area; measuring an area ratio of an inclusion containing a segregation metal element in a region to be measured including the central segregation area in the cross section; and evaluating central segregation in the steel based on the area ratio measured.

Inventors

Tomoharu Ishida
Seiya Sugawara
Kenji TSUZUMI

Assignees

JFE STEEL CORPORATION

Dates

Publication Date: 20260512
Application Date: 20210906
Priority Date: 20201218

Claims (6)

1 . A method of evaluating central segregation in steel, the method comprising: taking a sample from a steel, the sample having a cross section including a central segregation area; measuring an area ratio of an inclusion containing a segregation metal element in a region to be measured including the central segregation area in the cross section; and evaluating HIC susceptibility of the steel by evaluating central segregation in the steel based on the area ratio measured, where letting Mn x1 S y1 be the inclusion and Ag x2 S y2 be a compound with a smaller solubility product than Mn x1 S y1 , the method further comprising: treating the region to be measured in the sample with a solution containing Ag ions to replace Mn located on a surface of the inclusion exposed in the region to be measured with Ag; then, measuring an area ratio of Ag in the region to be measured; and using the area ratio of Ag measured as the area ratio of the inclusion, where x1 and y1 each denote a numerical value representing a composition ratio of the inclusion, and x2 and y2 each denote a numerical value representing a composition ratio of the compound.
2 . The method of evaluating central segregation in steel according to claim 1 , the method further comprising: performing mapping analysis of Ag in the region to be measured using EPMA; determining an area ratio of a region where Ag is detected at or above a predetermined concentration in the region to be measured; and using the area ratio as the area ratio of Ag.
3 . The method of evaluating central segregation in steel according to claim 1 , the method further comprising: acquiring an SEM image of the region to be measured; selecting a candidate region based on contrast in the SEM image, the candidate region being a candidate for the inclusion; performing SEM-EDS analysis on the candidate region to identify a region where Ag is detected at or above a predetermined concentration; determining an area ratio of the region in the region to be measured; and using the area ratio as the area ratio of Ag.
4 . The method of evaluating central segregation in steel according to claim 3 , wherein the SEM image is a reflected electron image.
5 . The method of evaluating central segregation in steel according to claim 1 , wherein the steel is a cast steel, a steel plate, or a steel sheet.
6 . The method of evaluating central segregation in steel according to claim 5 , wherein the cross section is a cross section perpendicular to a casting direction of the cast steel, a cross section perpendicular to a rolling direction of the steel plate, or a cross section perpendicular to a rolling direction of the steel sheet.

Description

TECHNICAL FIELD This disclosure relates to a method of evaluating central segregation in steel. BACKGROUND Central segregation, which occurs during the steelmaking process, is known to cause quality deterioration in various products. In particular, hydrogen induced cracking (HIC) has become a common problem in line pipes used to transport crude oil, natural gas, and other materials that contain large amounts of hydrogen sulfide, because hydrogen tends to penetrate into the steel from the surface. In a central segregation area in a steel material, there are inclusions such as stretched MnS, oxides, and carbides, which tend to accumulate hydrogen that has penetrated into the steel. Accordingly, in such central segregation area, hydrogen-induced cracking is likely to occur frequently. For this reason, a variety of technological developments have been made to measure and reduce central segregation. Typical methods of evaluating central segregation include the macro corrosion method, the slice method, and the H-print method, which are widely used. In the macro corrosion, a cut surface of a cast steel or a steel plate including a central segregation area is polished, and then macro-corroded with corrosion liquid such as picric acid to visually observe the occurrence of central segregation on the cut surface. In the slice method, the concentration distribution of the segregation metal is determined in the thickness direction by sequentially analyzing the components of the chips collected by slicing a cast steel or a steel plate in the thickness direction from the surface in a stepwise manner. In the H-print method, a cut surface of a cast steel or a steel plate including a central segregation area is macro corroded, and then measurements are made to determine the maximum grain size, etc. in the central segregation area from the print copied from the cut surface. However, JP H6-271974 A (PTL 1) describes that even after the macroscopic segregation that is the subject of the central segregation evaluation methods is eliminated, if there are spot segregation areas of Mn, MnS may form in groups in such spot segregation areas, and these groups of MnS may become initiation points of HIC. Several methods are known to evaluate microscopic central segregation as found in these Mn spot segregation areas. For example, PTL 1 describes a method of using an electron probe micro-analyzer (EPMA) to perform mapping analysis of Mn in a cross-section of a line pipe, including a central segregation area, to measure the size of Mn spot segregation areas with Mn concentrations at least 1.32 times higher than the average Mn concentration. JP H9-178733 A (PTL 2) describes a method of measuring the hardness of the axial center of a continuous cast steel and ascertaining the degree of central segregation of the continuous cast steel based on one or more of the average value, maximum value, and difference between the maximum and minimum values of the measured hardness values. In addition, JP 2009-236842 A (PTL 3) describes a method in which a C-section (a cross-section perpendicular to the casting direction or rolling direction) of a continuously cast steel or a steel plate is polished, mapping analysis of an indicator element (segregation metal element) is performed in the region to be measured including a central segregation area in the C-section using EPMA, etc. to determine the area of a region where the concentration of the indicator element is equal to or greater than a predetermined threshold concentration, and then the central segregation is evaluated based on that area or the area ratio relative to the region to be measured. Another conventional evaluation method is to determine the average concentration of an indicator element in a central segregation area and the base metal as the matrix, and to evaluate central segregation by dividing the average concentration in the central segregation area by the average concentration in the base metal. CITATION LIST Patent Literature PTL 1: JP H6-271974 APTL 2: JP H9-178733 APTL 3: JP 2009-236842 A SUMMARY Technical Problem In general, the degree of central segregation of a cast steel is not uniform in the C-section in the thickness and widthwise directions. Therefore, in order to investigate the central segregation of a cast steel or a steel plate, it is necessary to evaluate the central segregation over a wide area of the C-section. However, if an attempt is made to evaluate the full width of the cast steel by the method of PTL 2, it is necessary to measure the hardness of all the central segregation areas, which is very time-consuming to measure. Furthermore, when determining the maximum value of hardness, it is necessary to statistically measure hardness at many more positions to ensure that the value obtained at that position is the maximum value, which requires a long time for measurement. Although the methods described in PTLs 1 and 3 and the aforementioned conventional evaluation meth