JPH06104860B2 - Manufacturing method of high heat input welding steel with excellent low temperature toughness - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a steel material for large heat input having excellent low temperature toughness.

(Prior Art) In recent years, due to the increase in energy demand, development of oil, natural gas, etc. in the ocean has been vigorously carried out, and in particular, in search of abundant oil resources, recently, the North Sea, Arctic Ocean, etc. Huge marine structures are being built in cold regions.

Since such an offshore structure is exposed to a low temperature of −30 ° C. or lower and is operated under a complicated load stress condition due to the influence of waves, it is superior to the steel materials used for it. Brittle fracture characteristics are required.

In particular, the toughness of the heat-affected zone of the weld, which has a lower toughness than the base metal, directly affects the safety of the structure, so it is evaluated by impact tests, for example, at −60 ° C, 3.5 kg f ・ m or more. The impact value may be required.

In addition, the enormous size of the structure causes an increase in construction cost.Therefore, use of high-strength steel in the steel material used, for example, a yield point of 36 kg / m
By using steel materials of m ² or more, the weight of the upper structure is reduced and the welding cost is reduced by adopting the large heat input welding method.

As a method for producing such a steel material, for example, Japanese Patent Laid-Open No.
As described in Japanese Patent Laid-Open No. 63-103021, controlled rolling in which constituent elements are limited and production by an accelerated cooling method are known. This kind of conventional technology uses normal welding heat input (50 kJ / cm or less)
Does provide a steel material having excellent toughness in the heat-affected zone of welding, but its effect cannot be expected in high heat input welding.

As a technique for improving the toughness of the weld heat affected zone, for example,
As described in JP-A-60-245768 and JP-A-60-152626, a technique for improving the toughness of a heat-affected zone of a weld by forming an intragranular ferrite by using an oxide as a ferrite transformation nucleus Have been proposed.

However, these steels have a drawback that it is difficult to uniformly disperse the oxide in the casting process, and stable toughness of the weld heat affected zone cannot be ensured.

(Problems to be Solved by the Invention) An object of the present invention is to provide a method for producing a steel material for an offshore structure having high strength and excellent toughness of a weld heat affected zone used in cold regions and polar regions. .

(Means for Solving the Problems) The present invention has been made to solve the above problems, and its gist is (1)% by weight, as C: 0.02 to
0.3%, Si: 0.3% or less, Mn: 0.50 to 2.50%, Ni: 0.2 to 4.5
%, Nb: 0.003 to 0.010%, Cu: 0.2 to 2.0%, N: 0.01% or less and weight%, and the Ti to N ratio (Ti / N) is 2.0 to 4.0.
Contains Ti, S: 0.003 to 0.004%, Al: 0.001 to 0.1
%, (2) In addition to the above components, V: 0.2%
Hereinafter, one or more of the strength improving element group consisting of Mo: 1.0% or less and Cr: 1.0% or less is further contained, and the balance is Fe.
And steel containing unavoidable impurities are cast by a continuous casting machine and then heated to 1150 ° C to 1250 ° C for 3 hours or more 10
Manufacture of steel for high heat input welding with excellent low temperature toughness, characterized by cooling to 500 ° C or less, then heating to 1150 ° C or less, after pretreatment to retain for less than time, and then heating to 1150 ° C or less It is about law.

(Function) Based on a number of experiments, the present inventors found that the intragranular ferrite generated in the cooling process during welding was
TiN-MnS precipitates that contribute to the improvement of the toughness of the weld heat affected zone, which is generated even from the composite precipitate of N and MnS (hereinafter referred to as TiN-MnS precipitate) and improve the toughness of the weld heat affected zone It was found that the size needs to be a certain size or more, and in order to achieve it, a treatment for aggregating MnS at high temperature is necessary.

Hereinafter, the essence of the present invention will be described based on the above findings.

FIG. 1 shows the change in toughness after the addition of the welding heat cycle.

The chemical components of the sample at this time are as follows.

From this figure, it is understood that the toughness of the weld heat affected zone is improved as the TiN-MnS composite precipitate increases, and that the TiN-MnS composite precipitated zone is effective in improving the toughness of the weld heat affected zone.

Fig. 2 shows the precipitation state of TiN-MnS precipitates with respect to the pretreatment temperature before rolling and the holding time thereof.

Under the pretreatment conditions, there is an optimum region for promoting the precipitation of TiN-MnS composite precipitates, and the temperature range of 1150 to 1250 ° C is 3 to
It is necessary to hold for 10 hours. In addition, TiN at this time
The size of the —MnS composite precipitate is preferably 0.4 μm or more.

Further, this heat treatment may be performed in combination with the slab heat treatment during hot rolling that is usually performed, but if the heating temperature before rolling is set to a temperature higher than 1150 ° C, the strength of the base metal,
The heat treatment in the present invention should be performed prior to heating during hot rolling because it causes a decrease in toughness.

From the above experimental facts, 1150 ~ 1250 ℃ 3 ~ 10 before hot rolling
By performing the pretreatment for a time, TiN-MnS composite precipitates, which become the transformation nuclei of intragranular ferrite, are precipitated, and as a result, the toughness of the weld heat affected zone can be significantly improved.

Incidentally, the cooling rate after the heat treatment is preferably a cooling rate of air cooling or higher from the viewpoint of preventing the formation of precipitates that hinder the toughness such as coarsening of AlN, and the cooling does not change the morphology of MnS 500
Up to ℃ or less.

Further, the heating temperature of the slab for hot rolling needs to be 1150 ° C. or lower in order not to change the morphology of the TiN-MnS precipitate that has been generated once and for the reason of securing the strength and toughness of the base material. .

Next, the reasons for limiting the components in the present invention will be described.

C is an element necessary to secure the strength, and 0.02% or more is required to secure the strength. However, a large amount of addition causes a decrease in the toughness of the weld heat affected zone, so its upper limit is set. 0.3%

When Si is added in a large amount, it impairs the toughness of the weld heat affected zone, so its upper limit is made 0.3%.

Mn needs to be added in an amount of 0.5% or more to secure the strength, but if it is added in a large amount, the toughness decreases, so the upper limit is made 2.5%.

Ni is an element effective in toughness and hardenability, and at the same time, it is effective in reducing hot cracking, which is a problem when adding Cu.
If less than 2% is added, the effect is not recognized, and if a large amount is added, Ni is expensive, so the content is limited to 4.5% or less.

N is an important element that combines with Ti to form precipitates, but if it exists free in the steel, it causes a decrease in the toughness of the weld heat affected zone, so its upper limit is made 0.010%.

Ti is an essential element for the steel of the present invention, and when combined with N,
Precipitates TiN and acts as a precipitation nucleus for MnS. Therefore, it is necessary to control the amounts of Ti and N in order to obtain the optimum TIN. That is, when the weight ratio of Ti and N is less than 2.0, N becomes excessive, leading to a decrease in the toughness of the weld heat affected zone. At Ti / N exceeding 4.0, on the contrary, Ti becomes excessive and TiC precipitates. Toughness decreases.

Nb is an element necessary to secure the strength and toughness of the base metal, and when 0.003% or less is added, the effect of suppressing recrystallization is lost, and the toughness of the base metal is reduced. Since the toughness of the affected zone is reduced, it is limited to the above range.

Cu is an effective element for increasing the strength. If it is less than 0.2%, it has no effect. If it exceeds 2.0%, it causes cracking during hot working and impairs weldability. Limited to the range.

S is an important element for the precipitation of MnS. As shown in Fig. 3, when the addition amount is 0.002% or less, the precipitation becomes insufficient, and when it exceeds 0.008%, a large amount of MnS is precipitated. On the contrary, in order to inhibit the toughness, the content is limited to the range of 0.003 to 0.008%, but preferably it should be added to the range of 0.003 to 0.005%.

The base component in Fig. 3 is 0.05C-0.11Si-1.57Mn-0.005P.
It is -0.30Cu-0.30Ni-0.010Nb-0.008Ti-0.0030N.

Al is an element necessary for deoxidation, and it is necessary to add 0.005% or more. However, if added in a large amount, the toughness significantly decreases, so 0.1% is made the upper limit.

In the present invention, one, two or more kinds of Cr, V and Mo are added in addition to the above basic component system. These components have an equal effect of improving the strength of steel, and in order to secure the desired effect, the lower limit of the content is Cr: 0.1%, V:
It is necessary to set 0.01% and Mo: 0.1%. But each
If the content of Cr: 1.0%, V: 0.2%, and Mo: 1.0% is exceeded, the weldability and base material toughness will be reduced, so the above limits are set.

Steel having the components described above is melted in an electric furnace and a converter, and then cast by a continuous casting machine. This is not preferable for the toughness of the weld heat affected zone because the conventional ingot-agglomeration method has a slow cooling rate during solidification and does not cause fine precipitation of TiN.

This slab is held at 1150 to 1250 ° C for 3 to 10 hours before heating in hot rolling, and then 500 ° C at a cooling rate of air cooling or higher.
Cool to below.

As described above, this pretreatment is performed with the optimum Ti as the precipitation nuclei for the intragranular ferrite that is formed by transformation during cooling during welding.
This is a necessary treatment for precipitating N-MnS composite precipitates.

As shown in FIG. 2, at a temperature higher than 1250 ° C., MnS is dissolved, so that an appropriate composite precipitate cannot be obtained, and at a temperature lower than 1150 ° C., the aggregation of MnS is insufficient. Also, 1150-1
Even in the temperature range of 250 ° C., if the holding time is less than 3 hours, the MnS aggregation time is still insufficient, so the lower limit of the holding time is 3 hours.

However, holding for more than 10 hours, due to the coarsening of MnS, the appropriate number density of the composite precipitates as transformation nuclei of intragranular ferrite decreases, and at the same time, the heat treatment cost also increases, so the upper limit is 10 hours. To do.

In addition, slump rolling may be added in some cases after this pretreatment.

After that, reheating is performed for hot rolling, but the temperature at that time is in order to secure the strength and toughness of the base metal and to prevent the morphology of TiN-MnS precipitates from changing by the above-mentioned heat treatment.
It must be below 1150 ℃.

Regarding the rolling after heating, in order to improve the strength and toughness of the base metal, there is no change in TiN-MnS composite precipitate even if controlled rolling is performed or water cooling is performed after controlled rolling. Therefore, currently known manufacturing methods can be appropriately selected and adopted.

(Example) Table 2 shows the chemical composition of the test material.

Here, Steel A to Steel G are component systems corresponding to the present invention,
Steels H to J are steels that deviate from the present invention.

Further, Table 3 also shows the manufacturing conditions of the test material, the base metal, and the toughness value of the weld heat affected zone.

These steel plates are melted in a converter and made to a thickness of 24 by a continuous casting machine.
After being cast to a width of 0 mm and a width of 1600 m, it was subjected to heat treatment for pretreatment and rolling to produce a steel plate of 32 mm. In addition,
The toughness of the heat-affected zone is determined by latent arc welding (heat input: 200
kJ / cm) was evaluated by an impact test.

Steel plate manufactured by the present invention (plate number: 1,4,6,7,9,10,11)
Shows excellent toughness in both the base metal and the weld heat affected zone.

On the other hand, the plate No. 2 has a high pretreatment temperature, which lowers the toughness of the heat-affected zone of welding, and the plate No. 3 has a high slab heating temperature before rolling, which reduces the toughness of the base metal. Plate No. 5 showed a decrease in toughness of the weld heat affected zone because the cooling after pretreatment was slow cooling, and plate No. 8 also showed a decrease in toughness of the weld heat affected zone due to the low pretreatment temperature. There is.

Further, the plate numbers 12, 13 and 14 are those in which the composition range deviates from the present invention. In other words, plate No. 12 has a high Nb content, so the base metal strength is high, but the toughness of the weld heat affected zone decreases,
Since the Ti / N of plate No. 13 deviates from the conditions of the present invention, the toughness of the welded portion also deteriorates. Further, since plate No. 14 has a high S content, the toughness of the weld heat affected zone is also reduced.

(Effects of the Invention) As described above, according to the present invention, since the low temperature toughness of the weld heat affected zone is stable and a high level steel material can be obtained even by high heat input welding, it is extremely useful industrially. is there.

[Brief description of drawings]

Fig. 1 is a chart showing the change in toughness after the TiN-MnS composite and welding heat cycle, and Fig. 2 is a chart showing the precipitation state of the TiN-MnS composite precipitate with respect to the pretreatment temperature and its holding time, Third
The figure is a chart showing the effect of S content on the toughness of the heat-affected zone of the weld after single-sided single-layer latent arc welding (heat input: 200 kJ / cm).

Claims (2)

[Claims] [Claim 1] C: 0.02 to 0.3% Si: 0.3% or less Mn: 0.50 to 2.50% Ni: 0.2 to 4.5% Nb: 0.003 to 0.015% Cu: 0.2 to 2.0% N: 0.01% or less %, Ti: N ratio (Ti / N) becomes 2.0 to 4.0, Al: 0.005 to 0.1% S: 0.003 to 0.008% Continuous casting of steel with the balance being Fe and unavoidable impurities Prior to rolling, it is heated to 1150 ° C to 1250 ° C and subjected to a pretreatment of holding for 3 hours to 10 hours, then 500 ° C.
A method for producing a steel for high heat input welding with excellent low temperature toughness, which comprises cooling to the following, then heating to 1150 ° C or less, and hot rolling. 2. The composition further comprises, as a weight%, one or more elements selected from the group of strength improving elements consisting of Cr: 0.1-1.0% V: 0.01-0.2% Mo: 0.1-1.0%, with the balance being Fe. The method for producing a steel for high heat input welding having excellent low temperature toughness according to claim 1, wherein the steel is composed of unavoidable impurities.