JPS59180361A - Simple evaluating method of microsegregation in steel - Google Patents

Abstract

PURPOSE:To evaluate easily segregation with god reproducibility by placing the surface for evaluating segregation of a high carbon steel contg. a specific amt. or above of C, Si and Mn on the front, subjecting the steel to austenitization at a specific temp. or below then to constant temp. transformation in a lead bath then cooling the steel thereby converting the segregated part to micromartensite. CONSTITUTION:The section of a high carbon steel contg., by wt%, >=0.40 C, >=0.15 Si, >=0.30 Mn is cut out and the section is heated to <=1,200 deg.C to austenitize the steel. The section is then dipped in a lead bath kept at 400-600 deg.C and is quickly cooled and after the section is held for 120-180sec, the section is water or air cooled to convert the segregated part to micromartensite by making use of the delay in the end time of the pearlite transformation occurring in segregation. When the section is polished and is treated with an etching soln. such as nital or picral, the segregated part which are more resistant to etching than the pearlite part provides a gloss and is therefore distinctly and visually identified. The detail in the microsegregated part is detected by using an optical microscope in combination. The evaluation with more than half the previously required time is thus made possible.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a simple micro-segregation evaluation method for high carbon steel.

Regardless of whether the ordinary ingot method or continuous casting method is used, evaluation of segregation, which can lead to material defects, is an important issue. Various analytical instruments have been developed for segregation evaluation. For example, EPM analyzes the characteristics x hp generated using an electron beam.
Method A can detect micro-segregation with high accuracy, but there are restrictions on the method of preparing samples, the size, etc. It also requires a vacuum atmosphere.

Recent research has revealed that microsegregation has a major effect on the material properties of steel. Conventionally, S-print and macrostructure have been used as simple evaluation methods, but this only provides information about macro segregation and is invalid for micro segregation. Therefore, it has been desired to develop a simple evaluation method that can determine the state of microsegregation without using analytical equipment.

The ability to easily process large quantities of large samples has great industrial advantages. Conventionally, high carbon steels were removed using lead patenting treatment to ensure tensile strength and wire drawability. After being austenitized, it is rapidly cooled and held in a lead bath, and isostatically transformed into a micro-strengthened pearlite structure. In this case, the retention time required to complete the transformation was estimated from the TTT curve. The present inventors focused on this lead patenting treatment and invented a simple micro-segregation evaluation method from a different perspective.

The present invention will be explained in detail below.

The shape of the T T T Fm line is determined by the components and austenitization conditions. If the austenitizing conditions are kept constant, T
The time at which the transformation of the T T curve ends is temporarily determined by the components. Residual austenite can be converted to martensite by interrupting isothermal transformation and cooling with water or air. As a result of a series of studies, it was found that microsegregation can be easily evaluated by applying this research. It was also found that the method can be applied to large cross-section samples such as cast slabs and steel slabs. This method is expensive analysis! 7! Sample comrade's ratio 11 who changed Tsunemasa metamorphosis time without using a vessel, TTT
It is also possible to semi-quantitatively understand micro-segregation by comparison with curves, which is a great industrial advantage.

That is, the present invention is made of high carbon steel of 0.30% or more (02040%, sr=>o, is ratio, granite) (at C, the segregation evaluation surface is the surface, and austenite is formed at 1.200°C or less. After curing, immerse in a lead bath at 400-600°C,
This is a simple micro-segregation evaluation method for steel that involves water cooling or air cooling after isothermal transformation, and detecting the segregated portion as micro-martensite by utilizing the delay in the completion time of pearlite transformation caused by segregation.

Here, the reason for determining the scope of claims of the present invention will be explained.

Target C20, 40%, S1≧0.15%, Mn above 06
The reason for using high carbon steel with a coefficient of 0 is to stably detect the segregated portion as micro martensite with good reproducibility during 4-temperature transformation using a lead bath. Outside this 4u range, the judgment AX cannot be stable enough to be adopted as a simple evaluation method that can be used industrially. Setting the austenitizing temperature and lead bath holding time becomes extremely difficult.

One point is to ensure that the segregation evaluation surface is the front surface. This is to instantly bring the surface to be inspected to the temperature of the lead bath after being immersed in the lead bath. Especially when the sample crab gets larger, this effect becomes more pronounced on the dog. The delay in time for metamorphosis can be prevented and the influence of sample size 75 can be ignored. By adopting such a method, it is possible to evaluate not only small samples such as caterpillar insects, but also dog samples such as cast slabs and steel slabs, resulting in versatility. The purpose of austenitizing at temperatures below 200°C is to prevent the effects of re-energizing solar heat. 1,2
This is because when the value exceeds 00'O, the state of micro-segregation changes, with active diffusion of elements. Lead bath temperature 400~60
The reason for setting it to 0'O is to keep the transformation time in the base component within 6 minutes, which is industrially acceptable. 4 D
The transformation time is too long even if the temperature is below O ''O or above 600°C!It cannot be used as a simple evaluation method for Sl.In this case, lead is most suitable, but it is also possible to use a wet bath or an oil bath.

The reason why water or air cooling is performed after immersion in a lead bath is to convert residual austenite caused by segregation into martensite. High Mn and alloy additive materials with good hardenability may be air cooled, but low Mn materials with poor hardenability require water cooling. Of course, any component system can be applied as long as it is water-cooled. As a result, the micro-segregation portion is detected as micro-martensite. To see micromartensite, a corrosive solution commonly used for steels such as nital and picral is sufficient.

Since the micromartensite part is less susceptible to corrosion than the pearlite part, the segregated part becomes glossy and can be visually determined. If an optical microscope is also used, it is possible to understand the detailed shape of the micro-segregation area. Furthermore, by using this sample, the segregation situation can be accurately determined by measuring the hardness distribution. When measuring using normal A S cast material or A S ROL I material, the segregation situation cannot be ascertained due to the influence of the difference in cooling rate between the surface and the center. The method of the present invention is capable of processing samples such as steel slabs and slabs, so it is perfect for S-print and macrostructure samples! One of the benefits is that it can be used for 2 hours.

As described above, the method of the present invention is a simple micro-segregation evaluation method with industrial merits. Next, Examples will demonstrate the usefulness of the method of the present invention.

Example 1 Table 1 shows the components and casting conditions of the continuously cast pieces used.

All samples in Table 1 are materials equivalent to 5WRH82B. The cross-sectional size of the continuous cast piece is 240 x 375 mra.

This continuous casting piece is 9 width 240 x length 50 x thickness 10Tn
An L cross section sample of m was cut out. This vJ piece is 950°
After being austenitized for 15 minutes, the sample was kept in a lead bath kept at 55 degrees Celsius for 20 seconds, 60 seconds, and 120 seconds, and then air-cooled. After this, the sample was polished and subjected to nital corrosion, and the occurrence of micromartensite was checked. The check results are shown in Table 2. Clear differences could be detected depending on casting conditions and locations.

Cut out a small sample containing micro martensite part B
PMA analysis was performed, and clear micro-segregation of Si, Mn, and P could be detected in the micro-martensite portion. The degree of segregation was greater in sample 1 than in sample 2.

When comparing the middle part and the center part of Sample 2, the degree of segregation in the center part was higher.

Therefore, it can be seen that the method of the present invention can accurately reflect the micro-individualization direction.

Example 2 Table 6 shows the components and casting conditions of the continuously cast pieces used.

All samples are materials equivalent to 5WRI-162A.

The cross-sectional size of the continuously cast piece is 240 x 375 mm.

This continuous casting piece has a width of 240 mm, a length of 100 mm, and a thickness of 1 mm.
A cross-sectional sample of 0 Dragon 0L was cut out. 1,000 pieces of this slab
After austenitizing at 0°C for 15 minutes, the sample was kept in a lead bath at 500°C for 2 seconds, 15 seconds, and 30 seconds, and then cooled in air. After this, the sample was polished and subjected to nital corrosion, and the occurrence of micromartensite was checked. The check results are shown in Table 4. Differences in microsegregation could be detected depending on casting conditions and locations.

The method of the present invention has the advantage that by making a table of the lead bath holding time and the presence or absence of micromartensite generation for samples with acceptable microsegregation limits, it is possible to easily judge pass/fail for each charge.

B, PMA analysis takes more than half a day from sample preparation to obtaining analytical results, but using the method of the present invention, it takes less than a minute. Therefore, it can be used in the same way as conventional 8-purine or macrostructure.

Procedural Amendment April 1980'' Kazuo Wakasugi, Commissioner of the Japan Patent Office 1 Display of the case 1988 Patent Application No. 52794 2 Name of the invention Simple micro-segregation evaluation method for steel Relationship with the case Patent applicant address: 2-6-6 Otemachi, Chiyoda-ku, Tokyo Name (
665) Nippon Steel Corporation Representative Takeshi 1) Yutaka 4th generation Director Address: (503) 4877, 105
Date of amendment order to 5th Floor, 8th Mori Building, Daiichi Mori Building, #1l-12-1 Nishijin, Minato-ku, Tokyo Subject of voluntary 6th amendment Contents of 07th amendment to the detailed explanation of the invention in the specification Attachment (1) Details as attached On page 3, line 7, insert the word ``detect'' between ``only'' and ``micro''.

(In the 8th line from the bottom of page 3 of the 21 Specification, [Ca2.
40% Insert the characters ``to be in charge of weight'' before J.

(3) On page 3, line 4 of the specification, “Ca2.
Insert the words ``by weight'' before ``40%''.

32

Claims (1)

[Claims] In terms of weight, C≧040%, Sj≧0.15%, Mn
≧0.60% or more high carbon steel with the segregation evaluation surface facing the surface, austenitized at 1,200°C or less, immersed in a lead bath at 400 to 600"C, and after isothermal transformation, water-cooled or A simple micro-segregation evaluation method for steel, which is characterized by detecting segregated portions as micro-martensite by air-cooling and utilizing the delay in pearlite transformation completion time caused by segregation.