JPS6033340A - Extremely thick low carbon steel plate with excellent weldability - Google Patents

Abstract

PURPOSE:To provide an extremely thick low carbon steel plate enhanced in weldability while securing the marix strength thereof, obtained by especially using Cr and B or V as components to be contained in combination, and specifying a value of Pcm and an average cooling speed in the predetermined temp. range at the center part of a plate thickness during annealing. CONSTITUTION:An extremely thick low carbon steel plate contains, on a wt. basis, 0.12-0.22% C, 0.40% or less Si, 0.8-1.50% Mn, 0.01-0.10% Al, 0.4-0.8% Cr, 0.1-0.3% Mo and 0.03-0.1% B or/and 0.0003-0.0015% V and comprises the remainder Fe and inevitable impurities and has following chracteristics. That is, a value of Pcm of this steel plate prescribed by formula is 0.33% or less and the average cooling speed thereof between 800-400 deg.C at the center part of a plate thickness during annealing is 10 deg.C/sec. This extremely thick steel plate is excellent in weldability and the preheating temp. thereof for preventing welding crack can be made extremely low as compared with a conventional one and, as a result, energy cost or a time required in preheating work can be reduced to a large extent as compared with a conventional one.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to extra-thick steel plates used for manufacturing pressure vessels such as boilers, and more particularly to improving the weldability of extra-thick steel plates.

As is well known, pressure vessels such as boilers are made by welding extra-thick steel plates. These extra-thick steel plates have a slower cooling rate after normalizing than thin steel plates, especially at the center of the plate thickness, so it is difficult to achieve the required strength. is normal. Therefore, in order to ensure the required strength in such extra-thick steel plates, and to ensure the required strength even after tempering and SR (stress relief annealing) treatment, conventional extra-thick steel plates for boilers contain a large amount of carbon (carbon). ) was common.

By the way, when welding thick plates, preheating work is required before welding to prevent weld cracking, but especially when manufacturing boilers, etc., the steel used is extremely thick, so preheating work requires a lot of effort. and consumes time and energy. In particular, the above-mentioned extra-thick steel plate with a particularly high c content requires a high preheating temperature, which poses a problem in that the energy cost required for preheating work becomes extremely high. Furthermore, in the case of extremely thick copper plates with a high C content as mentioned above, even if preheating is performed to a sufficient degree, welding cracks often occur1, and repair work may be necessary. Ta. Furthermore, in the case of conventional mechanically thick steel plates with a high C content, it is difficult to obtain sufficient ductility and toughness of welded parts.

In order to eliminate the drawbacks of conventional extra-thick steel plates for boilers, which have a higher C content as described in 2.
Various methods have been proposed for improving strength through l-B treatment. However, in the case of this type of extra-thick steel plate, C0,0
Strength increase per weight% of 1.1.2 to 1.8 kgf/
Fni is reached, so the decrease in strength due to lower C is reduced to A7
In the method of supplementing by -B treatment, etc., there is a limit to the amount of C that can be reduced, and therefore there is a limit to the improvement of weldability such as reduction in preheating temperature.

This invention was made in view of the above circumstances, and aims to provide an extremely thick steel plate that secures the strength of the base material, lowers the preheating temperature of the welding pot, and improves weldability. It is something.

In order to eliminate the drawbacks of conventional extra-thick steel plates as described above, the present inventors have repeatedly conducted various experiments and found that the average cooling rate between 800 and 400°C at the center of the plate thickness during normalization is 10
For extra-thick steel plates where the temperature is less than °C/min,
It was discovered that by using Cr and B or V in combination, the contradictory problems of ensuring the strength of the base material and low preheating temperature can be solved, and this invention has been completed.

Specifically, the extra-thick steel plate of the first invention of the present application is CO,12
~0.22%, SiO, 40% or less, Mn0.8~
1.50%, Al0O1-0.10%, CrO, 4-0
, 8%, Mo 0.1-0.3%, and 0.0
3-0.1% V or 0.0003-0.0015
Containing at least one type of B of H, the balance being free of Fe and unavoidable impurities, and PC defined by the following formula.
The value of M is 0.33% or less, and the average cooling rate at the center of the plate thickness from 800°C to 400°C during normalizing is 10°C/Th in or less. It is characterized by having a plate thickness.

Further, the low carbon orange thick steel plate of the second invention of the present invention contains, in addition to the components of the first invention, 0.5% or less Cu or 0.5% Cu.
It contains at least one type of Ni listed below.

Below, the low carbon extra-thick steel plate of the present invention will be explained in more detail.

The steel sheet targeted in this invention Q is an extremely thick sheet that has a cooling rate of 10°C/+nin or less at the center of the sheet thickness between 800 and 400°C during normalizing. +: Ti plate. Specifically, the target is about 608 or more. In this way, 80
Average cooling rate between 0 and 400℃ is 10'Q/min
In the following cases, conventional steel has a structure of ferrite + pearlite or a structure with some paynite mixed in, whereas in the steel of this invention, which uses Cr and V or B in combination, carbides are finely dispersed. As a result, a base material strength comparable to that of conventional steel can be obtained even if the carbon content of the conventional steel is lowered. Furthermore, although the carbon equivalent value of the steel of this invention is almost the same as that of conventional steel, the hardness of the bond part and weld heat-affected zone during dwarf heat welding, where cold cracking is likely to occur, is lower than that of conventional steel. As a result, weld cracking can be prevented even if the preheating temperature during welding is lower than that of conventional steel.

Next, the reason for limiting the composition of the steel of this invention will be explained.

C: If the C content exceeds 0.22%, it is impossible to lower the preheating temperature during welding compared to conventional steel;
If the content is less than 0.124, it will be difficult to secure a predetermined strength. Therefore, C is 0.12 to 0.22
It was set within the range of %.

Sl: St is generally necessary to maintain strength, but if it exceeds 0.4 inch, the toughness will deteriorate, so 0.4
% or less.

Mn: Mn is effective in improving strength without impairing weldability, but if it is less than 0.8%, it is insufficient to ensure strength, and on the other hand, if it exceeds 1.50%, toughness and properties are reduced. is undesirable due to its low F, therefore, it is set in the range of 0°8 to 1,50°.

fi, e: Al is an effective element as a deoxidizing agent in the normal steelmaking process, and also functions to refine the structure and improve toughness. Such an effect is not noticeable when the content is less than 0.01%, and on the other hand, the toughness is adversely affected when the content exceeds 0.01%, so the content was limited to a range of 0.01 to 0.10%.

Cr: Cr is an important element in the steel of the present invention, and by adding it together with B or V, it is possible to compensate for the decrease in strength due to lower carbon content. In other words, it is difficult to compensate for the decrease in strength due to a reduction in C that is sufficient to sufficiently improve weldability by B addition treatment or V addition treatment alone, and it is difficult to compensate for the decrease in strength due to a reduction in C that is sufficient to sufficiently improve weldability. This makes it possible to compensate for the decrease in strength. This effect of Cr is insufficient if it is less than 0.4%,
On the other hand, if it exceeds 0.8 inches, weldability will be impaired, so it is limited to a range of 0.4 to 0.8 inches.

Mo: Mo is an element necessary to ensure strength, but 0.
If the amount is less than 1, the effect will not be noticeable, and if it is added in excess of 0.3, the toughness will be impaired, so 0.1 to 0.3%.
limited to the range of

(2) and B: These are important elements in the steel of this invention, and by adding either one or both together with Cr, the decrease in strength due to the reduction in C can be compensated for. The influence of ■ on the strength is not significant below 003φ, while V
If it exceeds 0.1 inch, it will have a negative effect on weldability, so ■
The amount added was limited to a range of 0.03 to 0.1 inch. Furthermore, if the amount of B added is less than 0.00034%, the effect of increasing strength will not be fully exhibited, while if the amount of B exceeds 0.0015%, it will have a negative effect on weldability, so the amount of B added is 0. 0
It was limited to a range of 0.003 to 0.0015 inches.

Cu, Ni: Cu and Ni are effective in improving strength without impairing toughness based on their hardenability increasing effect and solid solution strengthening effect, respectively, and therefore, in the second invention of the present application, either one is used. Or add both as essential ingredients. However, since Cu impairs hot workability and increases weld cracking susceptibility if it exceeds 0.5%, it is limited to 0.5% or less F. Further, since teeth are an expensive element, the number of teeth was limited to 0.5 inch or less in view of cost for this type of steel material.

In the steel of the present invention, the composition range of each element is limited as described above, and the preheating temperature during welding is lower than that of conventional steels, so that P is known as a composition susceptible to weld cracking. It is necessary to keep the M value at 0.33% or less.

The embodiments of this invention will be described below in comparison with conventional steel.

Steels A-N and conventional steels 0-Q within the composition range of the present invention shown in Table 1 were subjected to normalizing, tempering, and stress-relieving annealing treatments, and were subjected to a tensile test and a Charpy impact test to determine their tensile strength. TS ) and absorbed energy value (v
Eo) was investigated. Further, a diagonal Y-type weld cracking test was conducted on the same steels A to Neo to Q, which had been similarly normalized and tempered, using low hydrogen welding slag, and the crack inhibition temperature was investigated. The results are also shown in Table 1. The thickness of each plate is 20w1I, and as shown in Table 1, the cooling rate between 800 and 400°C at the center of the plate thickness after normalization is 10'C/1 nin or less. Also, among the steels shown in Table 1, A to E, H-N are the invention steels conforming to ASTM standard A299.
, P is the corresponding conventional steel, and steel F, G is JIS
The steel of the present invention conforms to the standard 5B49, and WIQ is the conventional steel corresponding to it.

From Table 1, inventive steels A-E and H-N are compared with conventional steels o and p, and inventive steels F and G are compared with conventional steel Q.
While both have the same strength, their toughness is much superior, and the cracking prevention temperature in the diagonal Y-type weld cracking test, in other words, the preheating temperature required to prevent weld cracking is 50 to 100°C. It is clear that it is low.

As is clear from the above explanation, the thick steel plate of the present invention has excellent weldability, and the preheating temperature for preventing weld cracking is much lower than that of the conventional one, resulting in lower energy costs and time required for preheating. can be significantly reduced compared to conventional methods,
In addition, it has the same strength as conventional extra-thick steel plates, and it also has excellent toughness and ductility, making it much superior to conventional steel.

Applicant: Kawasaki Steel Corporation Agent: Patent attorney Taketo Toyota (1 other person)

Claims (2)

[Claims] (1) C'0.12-0.22% (weight%, same below), Si0.40% or less, Mn 0.8-1.50%
, A10.01-0.10%, Cr 0.4-0.8
%, including Mo0.1~0.3%, Katsu0.03~
0.1% (DV or 0.0003-0.0015
% of B, the balance is Fe and unavoidable impurities, and P defined by the following formula:
A plate thickness such that the CM value is 0.33% or less and the average cooling rate from 800°C to 400°C at the center of the plate thickness during normalization is 10°C/in in in or less. Low carbon extra thick steel plate with excellent weldability. 10 - [Chi V) + 5 [Chi B] 0 (2) CO, 12~0°22%, SiO, 40 degrees, Mn 0.8~1.50%, A10.01~0.1
0%, CrO, 4-0.8%, MoO, 1-0.3
%, or -) 0.03 to 0.1% V or 0.
At least one type of B of 0003 to 0,0015%,
It contains at least one of 0.5% or more of Cu or 0.5% or less of Ni, and the balance consists of Fe and unavoidable impurities, and furthermore, the value of Po defined by the following formula is (1 , 3:;% or less, and the weldability of the plate thickness such that the average cooling rate from 800°C to 400'C at the center of the plate thickness during normalizing is 10°c/lm1n or less. Excellent low carbon extra-thick steel plate. PcM -C%C: ] + -C% Si ]+ -C
%1', 4n] + "C%Cub)020 + Engineering [%Ni] 10±C%Cr] 10±[%Mo]60 2
0 15 +±[%V':l+5[chi B] 0