JPS5852007B2 - Suguri Takouzo Youkouno Seizouhou - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to structural steel with excellent high heat input weldability, and in particular proposes a method for significantly improving the low temperature toughness of high heat input weld bond parts.

According to this invention, the low-temperature toughness of the high heat input welded bond portion is approximately equal to that of the base material.

It is possible to provide 40 kg to 60 kg class non-tempered and tempered structural steels.

Recently, automatic welding methods that use high heat input, such as single-sided submerged arc welding, electrogas welding, and electroslag welding, are being adopted in the production of large welded structures in order to reduce welding man-hours and reduce welding costs.

However, when conventionally used 40 kg to 60 kg class welding steels are subjected to high heat input welding, the austenite crystal grains become coarse in the weld heat affected zone, especially the bond area, and the large mesh-like pro-eutectoid ferrite becomes coarse. This results in an upper bainitic structure, resulting in a significant deterioration of toughness, which prevents the steel material from increasing the heat input during welding.

Recently, there has been a strong demand for the development of steel materials suitable for high heat input welding, and steel engineers have been conducting intensive research.
"7O-1" in Welding (May 1959)
Investigation of mechanical properties and weldability of Type M steel with 60ixaa thickness
he properties and weldables
ity of 70～160*ttt thick
TypeMsteel)
rovskaya's paper states that the toughness of the heat-affected zone of high heat input welding is significantly improved by including steel plate i.

Here Type M steel is C10,11, Mn/1.53
.

5i10.50, S/0.028, Plo,
018, Cr10.11, Ni10.08. Cu
10.26. Ti10.011゜0210.0014
, has a composition of N210.0038.

Regarding recent research results, the applicant has specifically discovered that by coexisting rare earth elements (REM) and B in welding steel, the heat-affected zone of high heat input welding becomes fine ferrite + pearlite, which significantly improves toughness. Kosho 55-31819
This was disclosed in the publication.

However, although the toughness of the high heat input weld bond part of steel containing Ti or REM+B is significantly higher than that of conventional steel, it is far from the toughness of the base metal.

By the way, in many industries including the shipbuilding industry, it is often desired that not only excellent low-temperature toughness but also the toughness of any weld heat-affected zone satisfy the base metal standards when performing high heat input welding. That is, Ti or RE
In order to improve the toughness of the high heat input weld bond due to the inclusion of M+B, it is necessary to make the structure of the high heat input weld bond similar to the base metal, that is, during high heat input welding.
This corresponds to requiring a steel that shows no deterioration in toughness in the heat-affected zone, but from the perspective of a steel engineer, this is a difficult task that transcends modern common sense and can be called an impossible task.

The inventors have conducted intensive research on a method to further improve the effect of REMB+H used in "non-temperature steel for high heat input welding with heat input ioo, ooo Joule/cm or more" disclosed in the above-mentioned Japanese Patent Publication No. 55-31819. As a result of proceeding with REM+B
By adding iron oxide powder to the molten steel during casting after welding the steel containing the molten steel, we succeeded in making the toughness of the high heat input weld bond almost the same as that of the base metal.

That is, this invention has C; 0.03 to 0.18%, Si
: 0.10-0.60%+ Mn: 0.40-
1.70%, A/; Contains 0.1% or less, REM (rare earth element); 0.005-0.05% and B; 0.0003
Contains ~0.0050% in coexistence, and if necessary,
Roughly Nb: 0.03% or less + V: 0.05% or less + Cu: 0.3% or less, Nt: 0.3% or less.

Cr: 0.5% or less + Se: 0.1% or less,
After melting steel containing one or more of Ti: 0.04% or less and Zr: 0.04% or less, with the remainder being essentially iron, iron oxide is added to the molten steel during casting. This is a method for producing structural steel with excellent high heat input weldability, which is characterized by adding powder in an amount of 0.0!1 to 1 g per 1 kg of molten steel.

As will be described in detail below, in order to achieve the objects and effects of the present invention, the above-mentioned specific steel composition is made, and after the steel is melted, an appropriate amount of iron oxide powder is added to the molten steel during casting. This is essential.

The structural steel obtained in this invention may be as rolled or may be subjected to any heat treatment such as normalizing or quenching and tempering.

That is, the addition of iron oxide powder during casting after melting in REM-B composite copper helps to disperse and distribute a large number of ultrafine inclusions of 1μ or less in the steel.

Furthermore, these inclusions are generated during the cooling process during the welding heat cycle.
It functions as a precipitation site for compounds (mainly B nitride), and finely precipitates B nitride within the γ grains.

This B nitride has the function of directly and actively forming fine block-like ferrite within the γ grains independently of the γ grain boundaries.

The relationship between the ultrafine inclusions generated by the addition of iron oxide powder and the precipitation of B nitride and the effect of B nitride on the raw skin of fine ferrite within the γ grains are completely new discoveries made by the inventors. The application of these to high heat input welding steels is new and appropriate.

The reason why the component range of the steel composition is limited to the above range in this invention is as follows.

First, the C content is limited to 0.03-018%.

In terms of strength as this type of structural steel, C is at least 0.03
% is necessary, and less than 0.03% is not preferable from the viewpoint of steel manufacturing.

On the other hand, the upper limit is 0.1 in terms of weld hardenability and weld crack sensitivity.
Limited to 8%.

Next, the Si content is required to be 0.10% or more from the viewpoint of steel manufacturing or the strength of this type of steel, but if it exceeds 0.60%, the toughness of the base metal will be significantly impaired, so the Si content should be o, io ~ 0. Limited to 60%.

Mu is required to be 0.40% or more in order to impart ductility and strength to the base metal, but if it exceeds 1.70%, the weld hardenability will increase significantly, so it is limited to 0.40 to 1.70%.

A/ is an element effective in improving strength and toughness through deoxidation and grain refinement, and is 0 for this type of structural steel.
．． An appropriate amount of 1% or less is required, but the effect is saturated if it exceeds 0.1%, so it is limited to 0.1% or less.

REM significantly improves the toughness of high heat input weld bond joints with B, but REMO, OO less than 5%, or 0.0
If it exceeds 5%, the effect will be almost gone, so 0.
It is limited to 0.005% to 0.05% or less.

B improves the toughness of the high heat input weld bond when coexisting with REM, but the effect is extremely large at 0.0003 to 0.0050%, so it is limited to 0.0003 to 0.0050%.

The addition of iron oxide powder, which can be called the gist of this invention, is R
Under the coexistence of EM and B, the above-mentioned mechanism has the effect of making the structure of the heat-affected zone of high heat input welding, especially the bond area, into ultra-fine ferrite + pearlite, which is useful for significantly improving notch toughness.

Iron oxide has almost no effect at concentrations up to 0.05 F per 1 kg of molten steel, but at concentrations above 1 g, giant inclusions may be mixed in, which is undesirable. Therefore, the amount is limited to 0.05 to 1 g per 1 kg of molten steel.

Next, the reason for limiting the content of the selected components will be explained.

Nb and ■ are useful for improving weld hardenability and cracking susceptibility in dwarf heat welding, and even a small amount of addition can significantly improve the strength of the base metal, but Nb is 0.03
%, ■ exceeds 0.05%, it reduces the toughness of the base metal and high heat input weld bond, so Nb is 0.03% or less.
(2) is limited to 0.05% or less.

Cu also contributes to an increase in strength, but if it exceeds 0.3%, the susceptibility to high temperature cracking during welding increases, so it is limited to 0.3% or less.

Note that at 0.3% or less, it also contributes to improving the corrosion resistance of steel.

Ni improves the strength and notch toughness of the base metal, but Ni
is an expensive element, and from the economical point of view of this type of steel, 0.
Limited to 3% or less.

Cr is also an effective element for increasing strength, but it increases weld hardenability and weld crack sensitivity, so it is limited to 0.5% or less.

Se is effective in improving strength and corrosion resistance as well as improving the toughness of the heat-affected zone of high heat input welding, but since it is an expensive element, it is limited to 0.1% or less.

Ti is not only effective in improving strength through deoxidation and grain refinement, but also in improving the ductility of the heat-affected zone of dwarf heat welding and eliminating anisotropy in the functional properties of the base metal.
If it exceeds 0.04%, the notch toughness of the base plate will be reduced, so it is limited to 0.04% or less.

In addition to being effective in increasing the strength of steel, Zr is useful in improving the shape of sulfides in steel and preventing coarsening of crystal grains.

If it exceeds 0.04%, the notch toughness of the base plate will be reduced, so it is limited to 0.04% or less.

In the present invention, unavoidable impurities contained in a normal steel manufacturing process are acceptable, but P and S are preferably 0.020% or less.

Now, in order to further clarify the constituent elements of the present invention as described above and specifically demonstrate the special effects of the present invention, Examples will be described below.

Example 1 Iron oxide powder was loaded into an iron pipe or wrapped with a thin iron plate and suspended inside the mold, and 1570 to 1600°C molten steel produced in the converter RH degassing process was poured using the bottom pouring method. did.

Next, a hot-rolled steel plate was prepared using a normal process.

Table 1 shows the composition of this steel plate and the amount of iron oxide powder added.

This steel plate was subjected to a thermal cycle equivalent to the bond part of a single-layer welded joint (shown in Figure 1) with a heat input of 230,000 joules/crrL, and a 2 mmV Charpy test was conducted on the steel plate.

Among the steels shown in Table 1, A to D are test steels according to the present invention, to which an appropriate amount of iron oxide powder was added during casting after melting.

Note that E is shown for comparison with an example in which the amount of iron oxide powder added is too small.

The iron oxide powder used here is hematite.
It is mainly composed of Fe2O3 crushed in a ball mill and has a particle size of 10 to 43μ, but it also contains magnetite (Fe3
04), wuistite (FeO), or a mixture thereof can also be used.

Further, F is an example showing one of the best high heat input weldability among conventional steels.

Table 2 shows the mechanical properties of the base material and the absorbed energy and Trs at -60°C of the simulated thermal cycle material.

In addition, the Charpy transition curves of the simulated thermal cycle materials of this invention steel, test steel B, and conventional steel E are shown in Figure 2, and the leading structures of the simulated thermal cycle materials of these steels are roughly shown in Figure 3'a.
, b.

As is clear from Table 2 and Figure 2, the toughness of the high heat input weld bond of the invented steel is extremely good compared to conventional steel, and as can be seen from Figure 3, the structure of the large heat weld bond of the invented steel is was an ultrafine ferrite + pearlite structure, and in particular, the ferrite grain size was almost the same as that of the base material.

Example 2 The mechanical properties of the base metal and the heat input of 230 KJ/
Table 4 shows the impact test results for the welded bond parts of cm.

In this case as well, when producing the test steel, the same iron oxide powder as described in Example 1 was added at 0.00% per molten steel during casting after melting.
Steel G-, in which iron oxide powder was added in the range of 0.05 to 0.05 l, and other ingredients were also within the range of the steel composition of the present invention, had a higher welding pound part than conventional steel M in which iron oxide powder was not added. It can be seen that the toughness has been significantly improved.

Example 3 Next, the reason why the structural steel according to the present invention is not limited to that as rolled but also includes those that have been subjected to any heat treatment will be demonstrated by an example.

Inventive steel B shown in Table 1 was subjected to a normalizing treatment of air cooling from 930°C, and a quenching-tempering treatment of water cooling from 950°C, reheating to 610°C, and then air cooling.

Table 5 shows the results of investigating the mechanical properties of the base metal and the toughness of the weld bond.

As rolled or normalized, or quenched.
It can be seen that the toughness of the bonded portion is good in all cases where tempering was performed.

In other words, the effect of adding iron oxide powder during pouring after melting in REM-B steel is not affected by the heat treatment of the base metal. Advantageous when used.

Pocket Example 4 The welding heat input and the toughness of the bond portion (-40°C 2 kg·m) were compared for the sample steel B and the conventional steel E shown in Table 1.

As shown in Table 6, in the present invention, the toughness improves as the heat input increases, similar to the conventional steel E for high heat input welding.

However, when comparing the amount of heat from the same person, the amount of heat input is 60,000J1
In terms of cmJd, test steel B of the present invention exhibits extremely high toughness compared to conventional steel F.

As shown above, the effect of RE on the formation of fine ferrite
In order to promote the combined action of M and B, it is extremely effective to add iron oxide powder to the molten steel during casting after melting in manufacturing the steel, and it is extremely effective to reduce the welding heat of steel for high heat input welding. For the first time, it is possible to solve the extremely difficult problem of making the toughness of the affected zone comparable to the base metal standard.

[Brief explanation of the drawing]

Figure 1 is a simulated thermal cycle curve corresponding to a single layer welded bond part with a heat input of 230 KJ/crrL, and Figure 2 is a diagram of a heat input of 230 KJ/crr.
A graph showing an example of a 2 mmV Charpy transition curve in an IrL weld bond, and FIG. 3 a+b are photographs comparing the microscopic structure of the same bond.

Claims (1)

[Claims] iC; 0.03-0.18%, Si; 0.10-
0.60%, Ml: 0.40-1.70%, A
/; Contains 0.1% or less, REM (rare earth element); 0.
005-0.05% and B; 0.0003-0.0050
% in coexistence, and if necessary, roughly Nb; 0
．． 03% or less, V: 0.05% or less, Cu: 0.3% or less, Ni: 0.3% or less, Cr: 0.5% or less, Se:
0.1% or less, Tt+0.04% or less, and Zr: 0.
After melting steel containing one or more of the following: 0.04% or less, the remainder being essentially iron, 0.059 to 1 liter of iron oxide powder is added per 1 kg of molten steel into the molten steel during casting. A method for manufacturing structural steel with excellent high heat input weldability.