JPH069733B2 - Judgment method of steel slab excellent in hydrogen-induced cracking resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) Whether the present invention is suitable for a steel slab obtained by continuous casting as a steel slab excellent in hydrogen-induced cracking resistance used for oil country tubular goods and the like. It relates to a method of determining.

(Prior art) Quality evaluation methods for steel cast pieces (hereinafter referred to as continuous cast pieces) obtained by conventional continuous casting include the sulfur print method and the macro etching method. Generally, the former method is used. Has been done.

According to the sulfur print method, the segregation state of the cast piece, particularly in the vicinity of the central portion, can be easily evaluated, so that it has been widely used in continuous cast pieces as a criterion for judging by product type. This method was extremely excellent especially when manufacturing steel plates for general structures and shipbuilding materials.

However, the present situation is that the above-mentioned quality evaluation method is not an effective means for evaluating hydrogen-induced cracking resistance. This is because hydrogen-induced cracking has a strong relation with the segregation of elements other than S, such as P and Mn, that occur near the final solidified portion, and is inaccurate by the conventional sulfaprint method and macroetching method. This is because.

Therefore, at present, it is difficult to evaluate the hydrogen-induced cracking property after rolling at the slab stage for steel types in which cracking as described above poses a problem, so soaking diffusion treatment or breakdown soaking diffusion treatment, etc. Has been carried out, diffusion treatment of all the elements has been carried out, and then rolling has been taken as an excessive measure.

(Problems to be Solved by the Invention) The relationship between the hydrogen-induced cracking and the segregation amount of P, Mn, etc. as described above has not been clarified yet, and therefore, the composition adjustment,
Although the roll pitch has been shortened and the solidification structure has been equiaxed, the desired steel slab has not necessarily been obtained.

The present inventors investigated the relationship between the segregation state of P and Mn in a slab and the crack characteristics. As a result, it was found that there is a close relationship between the area ratio of the concentration portion where P ≧ 0.04 wt% (hereinafter simply referred to as “%”) and Mn ≧ 1.5% and the cracking property.

That is, the present invention provides a method for determining a soaking and diffusion treatment type continuous cast slab excellent in hydrogen-induced cracking resistance at the slab stage without performing a hydrogen-induced cracking test according to the NACE standard after rolling. is there.

(Means for Solving the Problem) The present inventors have made continuous cast slabs of molten steel components as described below,
Evaluation was made based on a new segregation evaluation method, and the hydrogen-induced cracking characteristics of the rolled product was investigated. As a result, as shown in FIG. 1, a certain relationship was found between the segregation of P and Mn and the cracking property of the product.

In the figure, ○ indicates no cracking and × indicates cracking. That is, the condition for preventing cracks is P in FIG.
It has been found that hydrogen-induced cracking does not occur by controlling the occupied area ratio AP and AMn so as to satisfy the equation (1), which is determined by the segregation state of Mn and the segregation state of Mn. .

log (AP) ≦ −0.4 × log (AMn) −1.2 (1) The occupied area ratios AP and AMn referred to here are the final solidifications in the thickness direction in the cross section of the continuous cast slab in the casting direction. When a sample with 40 mm in the casting direction and 20 mm in the thickness direction including the part is sampled, the ratio of the area of P ≧ 0.04% (occupied area ratio) and the ratio of the area of Mn ≧ 1.5% in the sample are shown. .

In the present invention, the method of calculating the occupied area ratio by the segregation of P and Mn is not particularly limited, but it is convenient to perform it by the microanalyzer process, that is, the mapping analyzer process according to JP-A-57-142551. is there.

This mapping analyzer is based on EPMA (Electro
This is a combination of a probe micro analyzer) and a data storage device, and the characteristic X-ray intensity of an element at each coordinate can be simultaneously measured for several elements. The measurement principle is
This is in that the specularly polished sample is irradiated with electrons and characteristic X-rays peculiar to the element generated from the sample surface are detected by the dispersive crystal, and simultaneous detection of multiple elements is possible depending on the number of dispersive crystals installed. The characteristic X-ray intensity value captured in the data storage device is converted into a concentration (% by weight) by a calibration curve. First
The ordinate and abscissa of the figure are obtained by image analysis of this density data.

That is, the present invention, a cross-section is cut out in a region having a center segregation in the width direction or a center segregation and a segregation in the width direction of a steel slab obtained by continuous casting, and at least 40 mm in the casting direction in the cross section. After setting a visual field having a length and a width of 20 mm including the final solidified portion in the thickness direction, P and Mn in the visual field are processed by a mapping analyzer process.
And the occupied area ratio AP of P of 0.04% or more,
The occupied area ratio AMn of Mn of 1.5% or more is expressed by the formula log (A
P) ≦ −0.4 × log (AMn) −1.2 is satisfied, the slab is judged to be excellent in hydrogen-induced cracking resistance. This is an excellent method for judging steel slabs.

(Operation) It can be seen from FIG. 1 that the area of the P segregation portion may be slightly increased if the area of the Mn segregation portion is decreased.

According to the present invention, a sample (40 × 20 mm) is sampled from the central part (final solidified part) of the continuous cast slab, and this part is for example disclosed in
Mapping of 50μm beam diameter according to 57-142551
It is desirable to analyze the entire surface with an analyzer to evaluate the segregation state of P and Mn in the sample. In addition, the X-ray irradiation time is set to 1 in order to make the error within 10%, considering the measurement error of P.
It is desirable to set at least 100 msec per measurement point.

That is, as shown in FIG. 2, in a cross section of the continuous cast slab 10 in the casting longitudinal direction (hereinafter referred to as L direction), a sample having a desired area 12 including the final solidified portion in the normal thickness direction is taken. Specimen 12
Has an area of 40 × 20 mm or more in the L-direction cross section of the continuous cast slab, but it is preferable to make it a region having common center segregation and V segregation as shown in FIG.

Here, the size of the test piece was set to 40 × 20 mm for the following reason.

As the measurement conditions of the mapping analyzer, the beam diameter D is 50 μm, and the X-ray irradiation time t per measurement point is 100 mse.
c, X-ray irradiation current I was set to 5 μA. The measurement time T at this time is expressed by the equation (2) and becomes longer in proportion to the size of the measurement visual field.

T = (1000 x A / D) x (1000 x B / D) x (t / 1000) (2) However, T: measurement time (sec) A: thickness-direction measurement length (mm) B: Measured length in the width direction (mm) D: Beam diameter (μm) t: X-ray irradiation time per measurement point (msec) When a hydrogen-induced cracking test according to the NACE standard is performed, cracks are found at the center segregation part and V The field of view is generated in the segregation part and includes both the thickness direction and the width direction. There is a need. Considering this point, in order to minimize the measurement time T, the measurement visual field is specified.

First, in the thickness direction, the measurement field of view is located on the upper side of the center segregation so that center segregation, V segregation, reverse V segregation, etc.
mm, 5 mm on the lower surface side, totaling 20 mm.

In the casting direction, a visual field including at least three V segregations within the visual field macroscopically represents the actual condition of the slab, and the length was set to 40 mm. At this time, the difference due to changing the field of view
It was within 10% by weight.

By measuring a length of 20 mm in the thickness direction and 40 mm in the casting direction in this way, one sample can be measured within 10 hours in a short time, and the segregation of the slab is small and the macro It became possible to evaluate and quantify it.

FIG. 4 shows an example of measurement of a slab cast with a ladle P of 0.005%, which is a 40 × 20 mm test piece magnified 6 times.
In the figure, the black marking A is the P = 0.01% equi-segregated P concentration range, the shaded marking B is the P = 0.02% equi-segregated P concentration region, and the marking C is the P = 0.04% equi-segregated P concentration region. Area.

That is, in this case, the area ratio of the segregated P concentration of 0.04% or more is 0.002%, the area ratio of 0.02% or more is 0.27%, and the area ratio of 0.01% or more is 5.67% in the total area of 40 × 20 mm. Is shown. From this result, it is considered preferable to set the area ratio of the segregated P concentration of 0.01% or more as the occupied area ratio of P segregation in order to make a significant difference.

The molten steel components of the sample steel of FIG. 1 are mainly C 0.5% or less, Si.01 to 1.0%, Mn 0.7 to 1.5%, P0.0.
2% or less, S0.005 or less, O0.004% as a basic component, and if necessary, Ca70ppm or less, REM0.0
4 or less, Nb0.01-0.5%, V0.01-0.5%, Cu
1% or less, Cr 1% or less, Mo 1% or less, Zr0.01-
0.5%, 1% or less of Ni, 0.01 to 0.3% of Ti, etc. are appropriately contained.

On the other hand, Mn is necessary to secure the strength required for the material,
0.7% to 1.5% is added, but if 1.5% or more is added, cracking susceptibility is remarkably strengthened, so that hydrogen-induced cracking resistance is taken into consideration, so that it is limited by the correlation with P. Therefore, it is appropriate to set the area ratio of the segregated Mn concentration of 1.5% or more as the occupied area ratio of Mn segregation.

By the way, as described above, P ≧ 0.04%, Mn ≧ 1.5
The factors that affect the occupied area ratio are, of course, the ladle P% and the ladle Mn% in the molten steel component, and the solidification conditions (tandish heating degree ΔT, roll pitch lp, It is the molten steel static pressure P in the final solidification part, and the slab interface temperature [° C] (shell thickness).

Therefore, by considering the above-mentioned occupied area ratios of P and Mn and selecting the solidification conditions and molten steel components at the time of continuous casting, it is possible to manufacture a slab with induced hydrogen-induced cracking resistance. Especially for the roll pitch and the molten steel static pressure at the final solidification part, P, Mn
It is important to reduce the occupying area ratio of the sheet, and therefore, the optimal end-stage coagulation reduction by the dense roll shown in JP-A-62-33048 and the walking bar shown in JP-A-62-89555. Is effective.

(Example) C = 0.075%, Mn = 1.03%, Si = 0.256%, P
= 00029%, S = 0.015%, V = 0.31%, Nb = 0.041
%, Ca / S = 1.53 molten steel was cast by a 1.05R continuous casting machine while surface supporting near the final solidified portion with a walking bar.

Take out a 40 × 20 mm sample from the vicinity of the center of the obtained slab, and subject it to the mapping analyzer treatment,
When the occupied area ratio of P ≧ 0.04% and Mn ≧ 1.5% is obtained,
Since they were 0.05% and 0.3% respectively, they were sent to the rolling process as they were without being subjected to diffusion heat treatment, and NAC after rolling was performed.
A hydrogen induced cracking test according to the E standard was carried out. As a result, all were acceptable, and hydrogen-induced cracking of the sample after rolling could be judged by the NACE standard in the cast piece stage by the mapping analyzer.

(Effects of the Invention) The present invention is effective as a criterion for evaluating a casting apparatus and grasping optimum casting conditions. In addition, since hydrogen-induced cracking can be determined based on the NACE standard in the as-cast state, it is possible to determine the need for soaking and diffusion of the slab, and the product is sent to the plate-making process after being appropriately processed until rolling. Therefore, manufacturing conditions without hydrogen-induced cracking can be established.

[Brief description of drawings]

FIG. 1 is a graph of AP-AMn correlation, FIG. 2 is an explanatory view of sampling of the present invention, FIG. 3 is an explanatory view of a judgment visual field for evaluation, and FIG. 4 (a) is a P occupied area ratio. An analysis diagram by a microanalyzer for calculation, and FIG. 4 (b) is a partially enlarged view of FIG. 4 (a).

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-57-73162 (JP, A) JP-A-58-6961 (JP, A) JP-A-60-228655 (JP, A)

Claims (1)

[Claims] 1. A cross section in the casting direction is cut out at a central portion in the width direction of a steel slab obtained by continuous casting or in a region having a center segregation and V segregation in the width direction in common, and the casting direction is cut in the cross section. After setting a visual field having a length of at least 40 mm and a width of 20 mm including the final solidified portion in the thickness direction, the P / P in the visual field is processed by a mapping analyzer process combining an EPMA and a data storage device. And Mn are analyzed, and the occupied area ratio AP of P is 0.04% by weight or more,
The occupied area ratio AMn of Mn of 1.5% by weight or more is expressed by the formula lo
When the g (AP) ≦ −0.4 × log (AMn) −1.2 is satisfied, it is determined that the slab has excellent hydrogen-induced cracking resistance. A method for determining steel slabs with excellent properties.