JPS60152626A - Method for stabilizing toughness of high tension steel for welded structure - Google Patents

Abstract

PURPOSE:To inhibit the formation of abnormal grains by heating by hot rolling a steel with high tensile strength for high heat input welding contg. specified amounts of C, Si, Mn, Al and a component for increasing the strength and toughness, and a restricted amount of N after adjusting the composition with REM and Ti. CONSTITUTION:A slab of a steel with high tensile strength for high heat input welding having a composition contg., by weight, 0.01-0.15% C, 0.05-0.6% Si, 0.5-2% Mn and 0.01-0.8% A as essential components, one or more among 0.005-0.1% Nb, 0.005-0.15% V, 0.1-2% Ni, 0.1-1% Cu, 0.1-1% Cr, 0.05- 0.5% Mo and 0.0005-0.002% B, and 0.001-0.007% N is hot rolled at (the Ac3 point -30 deg.C)-1,150 deg.C after adjusting the composition by adding 0.005-0.025% Ti and 0.002-0.01% REM. One or more among said Nb, V, Ni, Cu, Cr, Mo and B are contained in accordance with the required strength and toughness.

Description

Detailed Description of the Invention (Technical Field) This specification relates to the reduction of variations in toughness in rolled base materials and welded parts of high-strength steel for welded structures to which so-called large heat input welding of 5000 GJ / C111 or more is applied. The technical content described in 2 proposes the development results regarding component adjustment in response to recent trends in the production of high-strength steel for high heat input welding.

(Background technology) In recent years, in the production of welded structures, single-sided single-layer submerged arc welding (SAW) and electrogas welding (EGW) have been used to reduce welding man-hours and reduce welding costs.
There is a growing momentum to adopt automatic welding that uses large heat input, such as welding or electroslag welding (ESW).

However, 4 has traditionally been used for welded structures.
When steel of 0 kgf/- grade or higher is subjected to large heat input welding, the structure of the weld heat affected zone (1-(AZ)), especially at the weld bond, becomes a structure consisting mainly of coarse upper bainite, resulting in significantly inferior toughness. Therefore, it was difficult to perform high heat input welding.

Since then, various steels suitable for high heat input welding have been developed.
Some of them are currently being put into practical use.

On the other hand, recently, technologies related to so-called processing heat treatments such as controlled rolling, controlled cooling after rolling, and direct quenching have been developed, and high heat input welded steel can also be manufactured using these methods. This is because by adopting such processing heat treatment technology, it is possible to reduce the amount of C, Mn, and other alloying elements that are considered to be bad for the toughness of high heat input welds. This is why it is expected to be a manufacturing method for steel materials with excellent welding properties under large heat input.

(Problem) In addition to further improving the toughness of the base material, these new manufacturing methods aim to reduce the fuel cost of the heating furnace.
During rolling, so-called low-temperature heating of 1,150°C or less is often performed, for example, as described in Japanese Patent Publication No. 55-30047
An example can be seen in the publication.

However, the base metal and welded part toughness of high-strength steel that is subjected to high heat input welding by combining low-temperature heating and processing heat treatment techniques is
Although certainly excellent, we have occasionally experienced the phenomenon of abnormally low toughness values.

In other words, it is a variation in toughness between the base metal and the high heat input welded part, and such unstable toughness is a serious problem from the perspective of structural stability, and is a serious problem for users of this type of steel. In addition to requests for improvement, this is a new issue that must be resolved as soon as possible for material manufacturers, in light of their mission to reliably manufacture and reliably supply steel products of stable quality.

(Motivation for the Invention) When low C equivalent steel is heated at a low temperature, coarse grains as large as 200 μm are generated in fine-grained austenite with a grain size of 20 to 50 μm, and such abnormal coarse grains cannot be refined by rolling. This is inherited by the structure after metamorphosis, resulting in variations in base material toughness. Furthermore, it is transferred to the structure of the affected zone of high heat input welding, resulting in variations in its toughness, and Ti is used as a component to suppress the generation of abnormal grains.
It has been found through the following experiments that the combined addition of REM and REM is particularly effective and can effectively reduce variations in toughness in the base metal and in high heat input welds.

(Background of the experiment) Generally, steel is A. When heated above 8 degrees, the austenite grain size becomes coarser as the heating temperature increases, but
Over a certain temperature range, very coarse austenite grows within the fine austenite grains.

Figure 1 shows 0.09% (same below wt%) C-0,21
%Si -1,36%Mn -0,014% The relationship between heating temperature and austenite grain size of AJ2 steel is calculated from Ac8 points
Measured by quenching after holding at each temperature of 1200℃ for 30 minutes.
Give an example.

When heated at 920° C. or lower, it becomes regular austenite with an average grain size of 20 μm. On the other hand, even at temperatures above 1,000°C, the average grain size increases, but it is still austenite with a regular grain size of about 65 to 130 μm.

However, within the range of 920 to 1000℃, the temperature is 20 to 1000℃.
Coarse grains up to 200 μm are mixed into the 65 μm regular austenite, and the area ratio of these coarse grains is up to 30%.
It has also reached However, the upper limit temperature at which mixed grains do not occur depends on the steel composition, and in the composition range of the present invention, the upper limit is 1150°C.

This phenomenon can be seen to a greater or lesser degree in steel of any composition, but in particular, the smaller the alloying elements and impurity elements contained in iron, the larger the grain size and number of coarse grains, that is, the area ratio. This has been newly discovered, and this means that steel with a low C equivalent is likely to produce mixed grains containing abnormal grains when heated at low temperatures.

If a steel plate is produced by rolling starting from such an austenitic state, not only will the base metal have an abnormally low toughness value, but also the joint will suffer from stiffness when high heat input welding is performed. Some river edges were also found to have abnormally low values for sex.

Such variations in toughness tend to occur when low C-equivalent steel is manufactured by low-temperature heating, and detailed microscopic examination reveals that a particularly coarse structure exists at the bottom of the notch in Charpy and COD tests. I understand.

Here, the base metal has a coarse bainite structure, and the particularly coarse structure after welding is due to the coarse bainite structure that existed in the steel sheet before welding, and that bainite structure is formed at a low temperature before rolling. Abnormal growth caused during heating 2
Coarse austenite grains reaching 00 μm were transformed without being sufficiently refined even by subsequent rolling.

(Purpose of the Invention) Therefore, the inventors conducted research on a component system that can suppress the above-mentioned unstable brittleness that tends to occur in the base metal and high heat input welded parts of low carbon equivalent steel obtained by low-temperature heating method. After numerous experiments and studies, we found that the combined addition of REM and Ti helps to prevent abnormal grain growth of austenite during low-temperature heating, makes the austenite grain regular during low-temperature heating, and ultimately improves the base material and large heat input. The present invention was completed based on the discovery that this method effectively contributes to reducing variations in toughness in welded parts. In other words, the purpose of the present invention is to appropriately realize such a contribution, and by the combined addition of T1 and REM, the oxysulphida of TiN and REV effectively act in a composite manner, thereby forming austenite during low-temperature heating. It is based on the basic understanding that it prevents the abnormal growth of

(Structure of the Invention) This invention includes: C: 0.01 to 0.15 wt%, Si: 0.05
~o, ewt%, Mn: 0.5-2. Nb +
0.005-0.10wt%, V: 0.005-0
．． 15wt%, N1 0.1~2.0wt%, Cu
: 0.1 to 1.0 wt%, Cr: o, i to 1.
0wt%, MO: 0.05-0.5wt% and B
: Contains 0.0005 to 0.002wt% of one or more reinforcing components and N in steel: 0.001 to 0.007w
In manufacturing high tensile strength steel for high heat input welding with a composition suppressed to t% by hot rolling under slab heating in a temperature range from the temperature (A2B point - 30 ° C) to not exceeding 1100 ° C. , the slab has Ti: 0.005 to 0.
025wt% REM: 0.002~0.01wt
This is a method for stabilizing the toughness of high-strength steel for welded structures, which is characterized by controlling the formation of abnormal grains accompanying the heating by adjusting the composition containing a composite of % and %.

First, the reason for limiting the range of the steel composition in this invention will be explained.

C: 0.01 to 0.15% If the C content is less than 0.01%, the required strength will not be obtained, and the weld heat affected zone will soften;
If it exceeds 0.01 to 0.15, weldability will be impaired.
%.

3i: 0.05-0.60% 8i promotes deoxidation of steel and increases strength, so it must be added in an amount of at least 0.05%, but if it exceeds 0.6%, the toughness will increase. and weldability is significantly impaired.
05 to 0.60%.

Mn: 0.5 to 2.0% If MO is less than 0.5%, the strength and toughness of the steel sheet will decrease and the HAZ will soften, and if Mn is too high (more than 2.0%) (0.0 because the toughness of AZ deteriorates.
It was set at 5 to 2.0%.

AJ2: 0.01 to 0.08% Aβ requires a minimum content of o and oi% for deoxidizing steel, but if solid solution A℃ exceeds 0.08%, 1-IA
A: Not only Z but also the mechanical properties of the weld metal will deteriorate significantly.
j2 was set to 0.01 to 0.08%.

The above are the basic components necessary to advantageously apply the method of the present invention.
07% of the REM oxysu l already mentioned
It is essential for the production of TiN, which has a synergistic effect with f ide.

In this invention, by containing Ti and REM, T
Together with REM's oxysulphide, iN prevents abnormal grain growth of austenite. However, Ti
If the content of N and N is too large, TiN will become coarse and the effect of suppressing abnormal austenite grain growth will be lost, and on the contrary, the toughness of high heat input welded joints will be impaired, so the upper limit of T1 is 0.
025%, and the upper limit of N was 0.007%. On the other hand, Ti
, even if the N content is too small, there is no effect of suppressing abnormal austenite grain growth, so the lower limits of Ti and N are each 0.005.
%, 0.001%.

Regarding REM, in the coexistence of TiN as its oxysu if ide, REM exhibits the effect of suppressing abnormal austenite grain growth, but excess REM exceeding 0.01% impairs the cleanliness of the steel and causes internal defects. The upper limit is 0.
0.01%, while less than 0.002% has no effect.

In addition to the above, this invention further provides Nb: 0.1
0% or less, V: 0.15% or less, Ni: 2.0% or less, Cu: 1.0% or less, Cr: 1.0% or less, MO: 0.5% or less, and B: 0.002 %
1 of the following! Alternatively, two or more of these elements can be contained, and the main purpose of containing these elements is to enable an increase in plate thickness while improving strength and toughness without losing the characteristics of the present invention. The amount added is limited by the following reasons.

Nb is included to refine the grains of the rolled structure and harden it by precipitation, and is an important element that improves both strength and toughness, but if it exceeds 0.10%, it not only affects weldability but also the weld metal. The upper limit was set at 0.10% because it also deteriorates the properties.

V has almost the same effect as Nb, but the upper limit is 0.15
% is acceptable.

N1 is added because it improves the strength and toughness of the base material without adversely affecting the hardenability and toughness of HAZ, but since it is expensive, the upper limit was set at 3.0%.

C1 not only has almost the same effect as N1, but also improves corrosion resistance, but if it exceeds 1.0%, it tends to cause hot embrittlement and the surface quality of the steel sheet deteriorates, so the upper limit is set at 1.0%.

MO makes the austenite grains fine and regular during rolling,
Furthermore, since fine bainite and martensite 1- are produced, strength and toughness are improved, but since it is expensive, the upper limit was set at 0.50%.

Or produces fine bainite and martensite and improves strength and toughness, but addition of 1.0% or more impairs weldability, so the upper limit was set at 1.0%.

B improves strength and toughness because it generates fine bainite and martensite, but it has no effect if added in excess of 0.002%, and hardening of the HAZ is significant.
The upper limit was set at 0.002%.

Note that the minimum necessary conditions for the significant addition effect of these elements are 0.005% for Nb and V, and 0.005% for Ni and Cu.
and Cr 0.1%, MO 0.05% and B
is 0,0005%.

The steel whose composition is limited as described above has almost no variation in toughness in the base material and the high heat input welded part even if it is heated and rolled at a low temperature, and can fully enjoy the effects aimed at by the present invention.

The upper limit of the heating temperature for this hot rolling is 1150° C., and if it exceeds it, the average austenite grain size becomes excessive, which is not preferable. Further, as mentioned above, the upper limit of the heating temperature at which mixed grains occur is also 1150°C. On the other hand, the lower limit is A for the purpose of turning the steel into austenite. 8 temperature, but in practical terms Ac
a Can be tolerated up to -30°C.

Note that the conditions for rolling and cooling after heating to (A3B-30°C) to 1150°C are not particularly specified, but
For the purpose of this invention, it is optimal to employ a controlled rolling method, an accelerated cooling method, or a direct quenching method.

Next, embodiments of the present invention will be described.

Using slabs with the components shown in Table 1 produced in a continuous converter casting process, by changing the heating-rolling-cooling conditions, the plate thickness was 25 mm to 75 mm.
11 steel plates were manufactured.

Table 2 shows the strength and toughness values of the base material, and the variations in toughness for each Charpy test and COD test. Note that the symbols in Table 2, for example, I
A indicates that the steel of Perfume 1 in Table 1 was used.

In addition, so-called large heat input welding was performed under the welding conditions shown in Table 3, and 10 Charpy test pieces were taken from the bonded portion and tested. The results are shown as the average value and the lowest value of the 10 pieces.

The steel sheet manufactured by the method of the present invention has excellent properties with no variation in toughness in the base metal and in the weld bond part with large heat input, whereas the comparative steel has significant variation in toughness.

Since 111A does not contain T1 and REM, the toughness of the base metal varies, and the toughness of joints made by high heat input welding is low.

Because Steel 2A has Ti added, its toughness in the base metal and large heat input welds is superior to Steel 1A, but since REV is added in combination and C is not present, variations in the toughness in the base metal and joints are still affected. The rest is unstable.

Although the components of Steel 3B are within the range of the present invention, the heating temperature is outside the range of the present invention, so the toughness of the base material itself is poor. Also, since REM is not added to Steel 5A, there are variations in toughness between the base metal and the welded part.

3A, 3C, and 4A according to the present invention.

48.6A, 7A, 8A, 9A, IOA, IIA, 12
A, 13A, 14A, 15A and 16A
The average values of the Charpy absorbed energy and COD value of the base material were also high, and the values were not abnormally low. Furthermore, both the average value and the minimum value of the high heat input welded joint are high.

(Effects of the Invention) According to the method of the present invention, a high-strength steel material with excellent toughness and uniformity in the base metal and the high heat input weld zone can be reliably obtained.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between austenite grain size, coarse grain size, coarse grain area ratio, and heating temperature when steel is heated to an austenite region.

Claims (1)

[Claims] 1.0: 0.01 to 0.15 wt%, Si:
0.05-0.6 wt%, Mn: 0.5-2
．． 0 wt%, and AJ2: 0.01-0.08 wt
% as a basic component, and further contains Nb: 0,005-0, io wt%, V: 0.0 according to requirements for strength and toughness.
05-0.15 wt%, Ni: 0.1-2. Owt%, Cu: 0.1 to 1. Owt%, Cr: 0.1 to 1.0 wt%, 1ylo:
0.05-0.5 wt% and 3: 0.0005
A high tensile strength steel for high heat input welding that contains at least 0.002 wt% of one or more reinforcing components and suppresses N in the steel to 0.001 to 0.007 wt%.
In manufacturing by hot rolling under slab heating in a temperature range from 8 points - 30 ° C) to 1150 ° C., the above slab contains Ti: 0.005 to 0,025 wt.
% and REM: 0.002 to -0.01 wt%, and the composition is adjusted to contain a composite of 0.002 to 0.01 wt% to suppress the generation of abnormal grains accompanying the heating. Sexual stabilization method.