JPS60174831A - Production of thick steel plate - Google Patents

Abstract

PURPOSE:To obtain inexpensively a thick steel plate having no work contraction without performing special refining and steel making and without decreasing considerably [P] by subjecting the thick steel plate which is hot rolled, hardened and tempered to weak water cooling at a specific cooling rate to suppress residual stress. CONSTITUTION:A thick steel plate consisting of, for example, 9% Ni steel is subjected to weak water cooling at 15 deg.C/sec cooling rate after tempering in the stage of assuring low-temp. toughness thereof by performing hardening and tempering after hot rolling. The steel plate is otherwise weakly cooled at a cooling rate of 5-15 deg.C/sec after tempering and is then subjected to leveling at ordinary curvature. The residual stress is suppressed by the above-mentioned methods. The residual stress occurring in water cooling after tempering is suppressed without deteriorating the low-temp. toughness simply by controlling the cooling rate after hardening and tempering according to the method of this invention. The thick steel plate having high toughness without having residual stress is thus inexpensively obtd. even with the kind of steel required to assure the low- temp. toughness such as 9% Ni steel, 3% Ni steel or the like.

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field of the Invention The present invention relates to a method for producing a thick steel plate of a steel type that undergoes quenching and tempering to ensure low-temperature toughness.

Technical background of the invention and its problems In general, residual stress is generated in steel plates, regardless of whether they are A4 Roll 1 products or heat-treated products, due to rolling distortion, uneven temperature of the steel plate, etc., which causes warpage, flatness defects, etc. It becomes. In order to suppress the generation of this residual stress, leveling is usually performed after rolling or heat aging. FIG. 1 is an explanatory diagram of after-sound, stress reduction and dispersion due to Roller Level Ichiban ζ. The function of a roller roller is to tilt the roll and apply strong repeated bending to reduce and disperse the maximum residual stress. Normally, residual stress rarely becomes a problem when a leveler is applied. there were.

However, in steel types such as 8.51Ni steel and 996Ni steel, which are subjected to water cooling after quenching and tempering in order to ensure low-temperature toughness, large residual stress occurs due to rapid water cooling. The reason for performing rapid water cooling after this tempering is as follows. That is, the 9th Nth 1ζ obtained by the normal manufacturing method contains o, oosqb or more CP), and this (P
) is densely segregated at austenite grain boundaries, increasing tempering susceptibility and deteriorating low-temperature toughness. For this reason, cooling after tempering is, for example, furnace cooling (40°C/Hr) or air cooling (35°C/Hr).
With cooling at a slow cooling rate such as 30°C/sec, the toughness falls within the embrittlement range, and the low-temperature toughness decreases with the content of (P); however, with water cooling, where the cooling rate exceeds 30°C/sec, the toughness falls outside the embrittlement range. The influence of P) is relatively small, making it possible to ensure low-temperature toughness.

For these reasons, residual stress occurs within the steel plate, and this −
If such a steel plate is used in a place where a large bending process is applied, such as a side plate of a tank, residual stress acts in the compressive direction due to the bending process, and a large process shrinkage occurs. for example,
In the case of an LNG tank made of 9% Ni steel, the outer circumference of the side plate 1
A welding gap of approximately 40 m/m occurs at 80 m. This welding gap of approximately 40m1m has a processing shrinkage rate of 0.02
Corresponds to %. In order to adjust this welding gap, welding is performed by adjusting the welding groove gap, but because it is not possible to have a large welding gap due to the safety of the tank, we have to reduce the welding gap in addition to suppressing the processing shrinkage rate mentioned above. It is not possible.

Therefore, the inventors investigated the influence of residual stress on processing shrinkage.

That is, a 9% Ni steel plate having a thickness of 31.3 ml and having the components shown in Table 1 was quenched and then tempered at 575°C, and the residual stress of the steel plate was measured after being water-cooled after tempering. The residual stress was measured by a sequential removal method using a resistance wire strain gauge. In this method, δX is
, δy, and assuming that these are uniform within the cross section and are a function of only the depth 2 from the surface of the plate, a strain gauge is attached to one side, and the outer layer is sequentially removed from the other side using a grinder. Strain during removal process ε9. This is a method of measuring C.

Table 1 Composition of 9% Ni steel plate (96) The results measured by the above method are shown in Figure 2, and it can be seen that due to thermal stress during water cooling, there is a compressive residual stress distribution on the surface and a tensile stress distribution on the inner surface.

When a plate with such a residual stress distribution is bent,
Since the starting point of plastic deformation and the yield region are different between the tension side and the compression side of bending stress, it is thought that the neutral line moves and the length of the plate decreases.

This also applies to the 8.51 Ni steel plate.

In order to avoid the shrinkage phenomenon of the steel plate due to the above-mentioned residual stress, it is desirable to ensure toughness without performing cooling after quenching and tempering. For example, if C is set to 0.04 or less, toughness can be ensured even if CP is 0.005 or more, but mechanical strength cannot be satisfied. In addition, another method is to reduce the (P) content to 0.003% or less (
S) A method of controlling the content to 0.001 or less is known. This method reduces the concentrated segregation of CP), ensures toughness, and does not reduce mechanical strength. ), and in addition to double slag treatment during converter blowing, ladle refining processes such as low-temperature tapping, RH, LF, and AOD are required, resulting in high costs and short refining times. The process takes a long time, which greatly impedes productivity, making it unsuitable for mass production.

Purpose of the Invention The purpose of the present invention is to propose a method for manufacturing thick steel plates without processing shrinkage at low cost, without performing special refining or steelmaking as described above, and without reducing the P value to an extremely low value. It is.

DISCLOSURE OF THE INVENTION As a result of various studies on methods that can maintain toughness and reduce residual stress during normal operations without special treatment, the inventors have found that even if the cooling rate is slower than before, the toughness is not as good as before. It has been found that residual stress can be reduced without causing deterioration.

This invention will be explained in detail below.

The method for producing a thick steel plate according to the present invention is a method for producing a thick steel plate in which quenching and tempering is performed after hot rolling to ensure low-temperature toughness. It is characterized by suppressing stress, and is characterized by suppressing residual stress by mild water cooling at a cooling rate of 5 to 15°C/sec after tempering, and then V belling at a normal curvature. be.

Here, the reasons for limitations in this invention will be explained based on examples.

FIG. 3 shows the cooling conditions of the present invention and the conventional cooling conditions of the normal cooling range for each plate thickness. Figure 3 shows a 9% Ni steel plate that was quenched at 800 to 830°C and tempered at 565 to 585°C using 9% N1tll with the composition shown in Tables 2 and 3 under these cooling conditions. Figure 4 shows the results of measuring the compressive residual stress in the surface layer for each plate thickness after cooling under the cooling conditions shown in Figure 4.

From FIG. 4, it can be seen that the residual stress is large in the normal cooling range, but under the cooling conditions of the present invention (cooling rate of 5 to 15°C/sec).
), it can be seen that the residual stress significantly decreases from 8 to 22 mm·f/-.

Table 2 Table 3 On the other hand, Table 3 shows the results of investigating the relationship between compressive residual stress and reduction rate using each plate thickness of 2t as a parameter when 9% Ni steel plates of various thicknesses were processed to R = 28.8m. It is shown in Figure 5.

Normally, when used in LNG tanks, etc., the reduction rate is 1Xl.
0' is the permissible limit, and a reduction rate greater than this will cause problems in construction.

As described above, from the results shown in FIGS. 4 and 5, it is possible to suppress the shrinkage rate to within 1X10' by simply increasing the cooling rate from 5 to 100 lbs for scraps up to a plate thickness of 20 g. However, when the plate thickness exceeds 20 fl, the compression residual stress is small, but the shrinkage ratio cannot be suppressed to 1 x 10''.

Moreover, FIG. 6 is a result of investigating the compressive residual stress in the surface layer of the steel plate in which residual stress was generated after cooling, in which a leveler was applied and in which a leveler was not applied.

As a leveling condition, the work roll diameter is 28081
11. Roll center distance 300mm, 3 rolls on top side,
The test was carried out using four rolls on the lower surface side, a work roll tilting amount of 2 degrees, and an initial curvature of 10 to 15 mR. In addition, the initial curvature is 10 to 15
mR is a curvature that is normally used when flatness correction is performed after heat treatment.

From FIG. 6, it can be seen that by leveling, the compressive residual stress in the surface layer is reduced compared to controlling only by cooling. In particular, the cooling condition of this invention has a residual stress of 10
It was possible to suppress the reduction rate to within 1X10'B even for a plate with a thickness exceeding 20W11.

Therefore, in this invention, the cooling rate is set at 5 to 15°C/sec.
And so.

Incidentally, FIG. 7 shows the examination of low-temperature toughness at -1.96 DEG C. Even under the cooling conditions of the present invention, no embrittlement of impact properties is observed compared to the conventional one.

As explained above, according to the method of the present invention, by simply controlling the cooling rate after quenching and tempering, it is possible to suppress residual stress caused by water cooling after tempering without deteriorating low-temperature toughness. Even if there are steel types that require low-temperature toughness, such as Ji steel or 9*Ni steel, high-toughness thick steel plates with no residual strain can be produced at low cost.

[Brief explanation of the drawing]

Fig. 1 is an explanatory diagram of the reduction and dispersion of residual stress by the roller repeller, Fig. 2 is a chart showing the residual stress distribution in the pressure direction of the steel plate, Fig. 3 is a chart showing the cooling rate after tempering by plate thickness, Fig. 4 Figure 5 is a diagram showing the relationship between cooling conditions and residual stress in the surface layer. FIG. 6 is a chart showing changes in compressive residual stress depending on cooling conditions under normal leveler conditions, and FIG. 7 is a chart showing impact characteristics between cooling in the cooling rate range of the present invention and conventional cooling. Fig. 1 Fig. 2 Residual g eaves (k<l・f, 41 cities -) Fig. 3 Coarse layer (mm) Fig. 4 Branch thickness (mm) 3% 5 Fig. 6 Tanita i71 (mm) ) Thickness // Armpit thickness (m/m)

Claims (1)

[Claims] 1. In a method for manufacturing a thick steel plate in which quenching and tempering is performed after hot rolling to ensure low-temperature toughness, the cooling rate after tempering is 5.
A method for manufacturing a thick steel plate, characterized in that residual stress is suppressed by mild water cooling at ~15° C./ac. 2. In a method for manufacturing a thick steel plate in which quenching and tempering is performed after hot rolling to ensure low-temperature toughness, the cooling rate after tempering is 5.
A method for producing a thick steel plate, characterized in that residual stress is suppressed by mild water cooling at ~15°C/sec and then leveling at a normal curvature.