JP2837733B2 - Manufacturing method of high heat input welding steel with excellent low temperature toughness - Google Patents

Description

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

(Prior Art) In recent years, development of oil, natural gas, and the like in the ocean has been vigorously performed due to an increase in energy demand. Particularly in recent years, huge marine structures have been constructed in cold regions such as the North Sea and the Arctic Ocean in search of more abundant oil resources.

Such offshore structures are exposed to low temperatures of -30 ° C or less and operate under complicated load stress conditions due to the effects of waves and the like. Excellent brittle fracture properties are required.

In particular, the toughness of the weld heat affected zone, where the toughness is lower than that of the base metal, directly affects the safety of the structure, so it is evaluated in impact tests and the like.For example, at -60 ° C, 3.5 kgf ・ m or more Impact value may be required.

In addition, the enlargement of the structure leads to an increase in construction costs.
By using a steel material of / mm ² or more, the weight of the upper structure is reduced and the welding cost is reduced by employing a large heat input welding method.

As a method for producing this steel material, for example,
As disclosed in Japanese Patent No. 103021, there are controlled rolling and production by an accelerated cooling method in which component elements are limited. Such a conventional technology is not suitable for ordinary welding heat input (50 kJ / cm or less).
Although it does provide a steel material having excellent toughness in the heat affected zone of the welding, its effect cannot be expected in large heat input welding.

As a technique for improving the toughness of the heat affected zone, for example, as described in JP-A-60-245768 and JP-A-60-152626, an oxide is used as a ferrite transformation nucleus and intragranular ferrite is used. Has been proposed to improve the toughness of the weld heat affected zone by generating.

However, these steels have the disadvantage that it is difficult to uniformly disperse the oxides in the casting process, and it is not possible to secure stable toughness of the heat affected zone.

(Problems to be Solved by the Invention) An object of the present invention is to provide a method for producing a steel plate for marine structures 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 the gist of the present invention is that C: 0.02 to 0.15
%, Si: 0.3% or less, Mn: 0.5-2.0%, Ni: 0.2-1.5%, C
u: 0.2 to 1.5%, provided that Mn, Ni and Cu are Mn / 6 + (Cu + Ni) / 15
= 0.28 to 0.40%, N: 0.0020 to 0.010%, B × 10000 +
B or B so that the value of the formula of Nb × 1000 becomes 4 to 10
Nb, Ti, Al with a ratio of Ti to N (Ti / N) of 2.0 to 4.0: 0.005
~ 0.1%, S: 0.003 ~ 0.008%, the remainder is continuously cast steel consisting of Fe and unavoidable impurities, prior to hot rolling, 11
After heating to 50 ° C. to 1250 ° C. and performing a pretreatment of holding for 3 hours or more and 10 hours or less, cool to 500 ° C. or less,
The present invention relates to a method for producing a large heat input welding steel excellent in low-temperature toughness, characterized in that the steel is heated to 0 ° C. or lower and hot-rolled.

(Action) Based on many experimental facts related to the improvement of the toughness of the weld heat affected zone (hereinafter referred to as HAZ), the present inventors found that the intragranular ferrite generated during the cooling process during welding is a composite precipitate of TiN and MnS. (Hereinafter referred to as TiN-MnS precipitates) to improve the toughness of HAZ. TiN-Mn that contributes to the improvement of HAZ toughness
It was found that the size of the S precipitate was 0.4 μm or more. And to achieve it, MnS at high temperature
Was found out.

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

FIG. 1 shows the number of TiN-MnS precipitates on the toughness after applying a welding thermal cycle equivalent to 200 kJ / cm.

The chemical components of the samples used in the experiment at this time are as shown in Table 1.

From this figure, as the number of TiN-MnS precipitates increases, HAZ
It is understood that the TiN-MnS precipitate has a remarkable effect on improving the HAZ toughness.

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

As can be seen from the figure, it is effective in improving HAZ toughness.
In order to generate a TiN-MnS precipitate having a size of 0.4 μm or more shown in the above, in the pretreatment conditions, 1150 ~ 1250
It is necessary to hold for 3 to 10 hours in the temperature range of ° C.

In addition, this heat treatment cannot be performed also as a slab heat treatment at the time of normal hot rolling. This is because if the heating temperature before rolling exceeds 1150 ° C., the toughness of the base material is reduced, so that the heat treatment in the present invention should be performed prior to the heating during hot rolling.

From the above experimental facts, 3150-1250 ° C before hot rolling
Pre-treatment for up to 10 hours allows TiN to become an intragranular transformation nucleus
-MnS precipitates are precipitated, and as a result, HAZ toughness can be significantly improved.

The cooling rate after the heat treatment is preferably a cooling rate equal to or higher than air cooling from the viewpoint of preventing the formation of precipitates that inhibit toughness such as coarse AlN, and the cooling is performed at 500 ° C. where the form of MnS does not change.
Up to the following.

Further, the slab heating temperature for hot rolling needs to be 1150 ° C. or lower in order not to change the form of the TiN-MnS precipitate once formed and to ensure 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 for securing the strength, and it is necessary to add 0.02% or more for securing the strength. However, since the addition of a large amount causes a decrease in the toughness of HAZ, the upper limit is 0.1%.
5%.

Si is an element that lowers HAZ toughness when added in a large amount, and the upper limit is 0.3%.

Mn is required to be added in an amount of 0.5% or more from the viewpoint of securing the strength and controlling the microstructure of the HAZ portion described later. However, if added in a large amount, the HAZ toughness is reduced.
2.0%.

Ni is effective in improving the strength and toughness of the base material,
Like Mn, it is an element that improves HAZ toughness by controlling the microstructure of HAZ.Addition of 0.2% or more is necessary, but adding more than 1.5% causes a decrease in HAZ toughness. Limit the range to 0.2-1.5%.

Cu is an element that controls the microstructure of HAZ together with Mn and Ni and improves the toughness, while Cu is effective in improving the base metal strength. However, if it exceeds 1.5%, the range is limited to 0.2 to 1.5% in order to lower the HAZ toughness.

N is an important element that combines with Ti to form a nitride, but it needs to be added in an amount of 0.0020% or more. However, if it exists free in steel, it lowers the HAZ toughness.
10%.

In the present invention, B and Nb are added as elements that suppress the formation of coarse ferrite from the prior austenite grain boundaries that lower the HAZ toughness with a small amount of addition and bring about an improvement in toughness. FIG. 3 shows the effect of the amounts of B and Nb on the impact value when an impact test was performed at -60 ° C. after applying a reproducible heat cycle equivalent to a heat input of 200 kJ / cm.

 Table 2 shows the component systems used in the experiment.

As can be seen from FIG. 3, the impact value indicated by a circle is 6 kgf
-To obtain a high value of m or more, B or B and Nb,
It must be added so that the value of the formula of B × 10000 + Nb × 1000 in weight% is in the range of 4 to 10.

Ti is an essential element for the present invention, and is combined with N
Precipitates TiN and acts as MnS precipitation nuclei. Therefore, in order to obtain optimal TiN, it is necessary to control the amounts of Ti and N. That is, when the weight ratio of Ti and N is less than 2.0, N
If it exceeds 4.0, the HAZ toughness is reduced. If it exceeds 4.0, on the contrary, Ti becomes excessive and TiC precipitates, and the toughness of the base material is significantly reduced.

S is an element necessary for Mn precipitation. Fig. 4 shows a steel sheet with a thickness of 32 mm within the composition range shown in Table 2 and a heat input of 200k.
The effect of the amount of S on the impact value at −60 ° C. when performing J / cm three-electrode latent arc welding is shown.

As can be seen from this chart, if the addition is less than 0.003%, the amount of precipitation becomes insufficient, and the expected improvement in toughness cannot be obtained. Should be added in the range of 0.003% to 0.008%, but should preferably be added in the range of 0.003% to 0.005%.

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 is impaired, so the upper limit is 0.1%.

As a result of repeated experiments for the purpose of improving HAZ toughness during large heat input welding within the above component ranges, the present inventors found that
If TiN-MnS precipitates are dispersed in steel and elements with high hardenability such as Mn, Cu, and Ni are effectively used, the former austenite, which is the main cause of toughness reduction in the microstructure of HAZ, It has been found that the formation of coarse ferrite and upper bainite generated from grain boundaries can be suppressed and HAZ toughness can be significantly increased.

Fig. 5 shows a steel sheet having the composition range shown in Table 2 as a test piece with a thickness of 32 mm, which was welded by single-sided latent arc welding at a heat input of 200 kJ / cm, and then subjected to impact at -60 ° C at the HAZ. The average value of the impact values at the time of the test is shown on the vertical axis, and the horizontal axis is Mn / 6 in weight%.
9 is a table showing values of an equation of + (Cu + Ni) / 15.

In addition, the result shown by ○ is the case where the S content is 0.003% and 0.004% by weight and the TiN-MnS precipitates are finely dispersed. , T
The result when iN-MnS precipitate is hardly formed is shown.

As can be seen from this chart, in steel (○) in which TiN-MnS precipitates are dispersed, as the value of the above equation increases from 0.25 to 0.33, the impact value increases significantly. Toughness tends to decrease.

The improvement of this toughness was confirmed by observation of microstructure, Mn, Cu, Ni
As the hardenability of steel increases due to the addition, coarse ferrite generated from the grain boundaries is suppressed even at a slow cooling rate during large heat input welding, and at the same time fine intragranular TiN-MnS precipitates are formed in the grains. It is considered that ferrite is formed and the microstructure of HAZ is remarkably refined.
However, when the value of the above equation exceeds 0.33, hardenability rather increases excessively, and upper bainite and island-like martensite structure which inhibits toughness in the prior austenite grains during cooling are generated, so that toughness is reduced. descend.

On the other hand, steel (●) in which TiN-MnS precipitates are not dispersed shows low toughness regardless of the value of the above equation, and the difference from the steel of the present invention is clear.

From the above findings, it is possible to improve the HAZ toughness during large heat input welding by finely dispersing the TiN-MnS precipitates according to the present invention and limiting the value of the formula of Mn / 6 + (Cu + Ni) / 15. In addition, the range is the impact value that is usually required in industry.
Considering that it is about 3.5kg f · m, it should be 0.28 to 0.40.

After the steel having the components described above is melted in an electric furnace and a converter, it is cast by a continuous casting machine. This is unfavorable for the HAZ toughness, because the cooling rate during solidification is slow and the TiN does not precipitate finely in the case of the conventional ingot lump method.

Prior to heating this slab in hot rolling, 1150-1250
At 3 ℃ for 3-10 hours, then at a cooling rate higher than air cooling
Cool down to below 500 ° C.

This pretreatment, as described above, is optimal as a precipitation nucleus of intragranular ferrite generated by transformation during cooling during welding.
This is a process necessary for depositing TiN-MnS precipitates.
As shown in FIG. 2, at a temperature exceeding 1250 ° C., an appropriate precipitate cannot be obtained because MnS is dissolved,
At a temperature lower than 150 ° C., the aggregation of MnS is insufficient. Also,
Even in the temperature range of 1150 to 1250 ° C., if the holding time is less than 3 hours, the time for coagulating MnS 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 number density of composite precipitates required as transformation nuclei of intragranular ferrite is reduced, and the heat treatment cost is also increased, so the upper limit is set to 10 hours. .

Also, after this pretreatment, there may be no problem even if slab rolling is added.

Thereafter, reheating is performed for hot rolling, but the temperature at that time is to ensure the strength and toughness of the base material, and to not change the form of the TiN-MnS precipitate generated by the heat treatment. The temperature must be below 1150 ° C.

In addition, regarding the rolling after heating, in order to improve the strength and toughness of the base material, controlled rolling is performed, or after controlled rolling,
Since the TiN-MnS precipitate does not change at all even when cooled with water, a currently known production method can be appropriately selected and adopted.

(Examples) Table 3 shows the chemical components of the test materials.

Here, steel B and steel C are component systems corresponding to the present invention,
Steels A and D to G are steels deviating from the present invention.

Table 4 shows the manufacturing conditions, base metal, and HAZ of the test materials.
Are also shown.

These steel sheets were melted in a converter and the thickness was
After being cast to a thickness of 0 to 250 mm and a width of 1300 to 1600 mm, it was subjected to pre-treatment and hot rolling for rolling, thereby producing a steel sheet having a thickness of 32 mm. Note that the HAZ toughness was evaluated by a Charpy impact test by taking an impact test from a 1 / 4t portion of the plate thickness after one-sided one-layer latent arc welding (heat input: 200 kJ / cm).

The steel sheet manufactured by the present invention: (sheet number: B1, C1) shows excellent toughness in both the base metal and the HAZ.

On the other hand, plate number A1 has a high pretreatment temperature, so HAZ
The toughness is reduced, and sheet No. A2 has a high slab heating temperature before rolling, and the toughness and HAZ toughness of the base material are reduced. Plate number A3
Although the pretreatment temperature is appropriate, the retention time is out of the range of the present invention, and the HAZ toughness is reduced.

In addition, the improvement in HAZ toughness was not observed in sheet number B2 because the cooling condition after the pretreatment was slow cooling, and the HAZ toughness was also lowered in sheet number C2 because the pretreatment temperature was low.

Further, D1, E1, F1, and G1 have component ranges that deviate from the present invention. That is, the plate number D1 is B × 10000 + Nb
The value given by the formula of × 1000 is out of the range of the present invention, the HAZ toughness is reduced, and the sheet number E1 is Mn / 6 + (Cu + N
i) The value given by the formula of / 15 exceeds the range of the present invention, and the HAZ toughness is also low. In addition, the sheet number F1 has a reduced HAZ toughness because Ti / N is out of the range of the present invention, and the sheet number G1 also has a reduced HAZ toughness because the S number is high.

(Effects of the Invention) As described above, according to the present invention, high-temperature steel can be obtained stably at a low temperature toughness of HAZ even by high heat input welding, and thus is extremely useful in industry.

[Brief description of the drawings]

Fig. 1 is a table showing the number of TiN-MnS precipitates contained in steel and the change in toughness after welding heat cycle, and Fig. 2 is TiN-MnS.
FIG. 3 is a graph showing the precipitation state of the precipitates with respect to the pretreatment temperature and its retention time, FIG. 3 is a graph showing the effect of the addition amount of B and Nb on the toughness after the welding heat cycle, and FIG. 200k
Fig. 5 is a chart showing the effect of S content on the impact value of HAZ after single-sided sub-arc welding at J / cm, and Fig. 5 shows the effect of Mn / 6 + (Cu + Ni) on the impact value after single-sided sub-arc welding at a heat input of 200kJ / cm. 4 is a chart showing the influence of Mn, Cu, and Ni represented by the formula of / 15.

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁶ , DB name) C21D 8 / 02,8 / 00

Claims (1)

(57) [Claims] (1) As% by weight, C: 0.02 to 0.15%, Si: 0.3% or less, Mn: 0.5 to 2.0%, Ni: 0.2 to 1.5%, Cu: 0.2 to 1.5%, provided that Mn, Ni, Cu Is Mn / 6 + (Cu + Ni) /15=0.28~0.40%, N: 0.0020 ~ 0.010%, weight%, B or B and Nb, so that the value of the formula of B × 10000 + Nb × 1000 becomes 4 to 10, weight %, The ratio of Ti to N (Ti / N) is 2.0 to 4.0, Ti: Al: 0.005 to 0.1%, S: 0.003 to 0.008%, and the balance is Fe and unavoidable impurities. Prior to hot rolling, heat treatment is performed at 1150 ° C to 1250 ° C, holding for 3 hours or more and 10 hours or less, then cooling to 500 ° C or less, and then heating to 1150 ° C or less and hot rolling. A method for producing a large heat input welding steel having excellent low-temperature toughness, characterized in that: