JPS5814848B2 - Manufacturing method of non-tempered high-strength, high-toughness steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is directed to a method for producing a non-tempered, high-strength, high-toughness steel with excellent base metal and weld joint toughness.

Controlled rolling has been widely adopted as a manufacturing method for line pipe materials used at low or extremely low temperatures.

However, recently, the heating temperature has been increased to AC3 in this rolling method.
Low-temperature heating controlled rolling method lowered to directly above ~1000℃ and Ar3
A new controlled rolling method that combines so-called (γ-α) two-phase region rolling at a point of ~Ar is attracting attention.

The advantage of these methods is that the fracture surface transition temperature (a measure of brittle fracture arrest characteristics) of Charpy and DWTT can be dramatically improved by significantly reducing the grain size (including subgrains) and increasing the separation density. Therefore, the balance of strength and toughness (fracture surface transition temperature) can be maintained even if two-phase area rolling is performed.

Therefore, it may be possible to use it not only for line pipe materials but also for pressure vessels and the like in its rolled state.

However, the disadvantages of steel produced by this method are (1) the anisotropy of the steel material increases, the properties in the thickness direction deteriorate, and as a side effect of increased separation density, Charpy DW
(2) Even if the base material has excellent low-temperature toughness (brittle fracture arresting properties), the weld heat-affected zone (hereinafter referred to as H
The toughness of AZ) cannot keep up with the base metal.

Because of this drawback, steel products made by this manufacturing method have limited applications and are not widely used.

The present inventors aimed to solve the above-mentioned drawbacks by reducing the anisotropy and
As a result of intensive research into the manufacturing method of a well-balanced high-strength, high-toughness steel with high absorbed energy and extremely high weld toughness, Charpy has invented a completely new manufacturing method for welding steel. It's arrived.

This point will be explained below.

The feature of the present invention is that the S content is extremely reduced and the trace amount of Ca,
Steel with composite addition of Ti is continuously cast to a thickness of 300rn.
The objective is to prepare a slab with a diameter of less than m and then heat it at a low temperature of 9000 to 1000'C to perform controlled rolling.

According to this method, the anisotropy of the steel material is significantly reduced, the absorption energy of Charpy etc. is improved, and the HAZ toughness is dramatically improved.

The cause of the decrease in the absorbed energy of Charpy etc. is the occurrence of separation at the impact fracture surface, which is mainly due to the separation in the unrecrystallized region of stretched MnS and austenite or the (γ-α) two-phase region rolling. (10
0) It is due to the development of texture.

As a measure to improve absorbed energy, we reduced the separation based on MnS by drastically reducing the S content and controlling the shape of sulfide using Oa.

It was also found at the beginning that the addition of Oa was extremely effective in suppressing separation due to the development of the (100) texture.

On the other hand, we solved the problem of improving the toughness of welds by making steel containing small amounts of Ti and N into slabs using a continuous casting method with a high cooling rate, and then rolling the slabs at a low temperature of 900°C to 1000°C. .

The continuous casting method is used to produce slabs because the cooling rate is faster than in the ingot-forming method, and a large amount of fine TiN (0.05μ or less) can be obtained in the slabs.
The reason why the thickness is set to be less than L is because if the thickness is more than 3001n7IL, the cooling rate becomes slow and it is not possible to obtain sufficient fine TiN.

(As for the cooling rate, it is most desirable that the average cooling rate from just below the liquidus line of the molten steel to 1100°C in the center of the slab be 60°C/min or more.) However, the fine T
Even if a large amount of iN is obtained, if it becomes coarse in the next hot rolling process, a sufficient amount of fine TiN will not be obtained in the steel sheet, and the HAZ
It is not possible to refine the structure.

For this reason, the zero force temperature of the slab was limited to 900 to 1000°C.

It was first discovered that by limiting the heating temperature in this manner, the HAZ toughness was dramatically improved compared to conventional high-temperature heating.

The upper limit of the Calocermal temperature is a condition to prevent the fine TiN in the slab from becoming coarse, and the reason why the lower limit was set at 900°C is that at temperatures below this, the slab will not be sufficiently solutionized in the austenitic region, and the steel material will deteriorate. This is because the internal quality of the product has deteriorated and the product is unsatisfactory.

However, when the caloric temperature is 900°C or higher, the internal quality becomes sufficiently excellent due to the addition of Oa and extremely low S content.

Note that fine TiN, which does not coarsen during heating, refines the austenite grains during heating and the recrystallized grains during rolling, refines the rolling structure, and improves the toughness of the base material.

Next, regarding the rolling method in the present invention, controlled rolling is essential from the viewpoint of strength and toughness.

For this reason, in the present invention, the rolling conditions are limited to a rolling reduction of 60% or more below 850°C and a finishing temperature of 650 to 750°C.

If these conditions are followed, the strength and toughness will be greatly improved.

The reason for limiting the rolling conditions will be described below.

First, if the reduction amount at 850°C or less is 60% or more, the crystal grains become significantly smaller and the strength and toughness are greatly improved.

However, if the reduction amount is less than 60%, high strength and excellent toughness cannot be obtained.

On the other hand, even if the rolling reduction is 60% or more at 850°C or lower, a steel material with significantly superior strength and toughness cannot be produced if the finishing temperature is 750°C or higher.

By setting the finishing temperature to 750°C or less, grain refinement is promoted, and both strength and toughness can be improved, or strength can be improved without deteriorating toughness.

Furthermore, with the composition range and hot rolling conditions of the present invention, low-temperature toughness is good even if a considerable amount of rolling is performed in the ferrite-austenite region or ferrite region, which is effective for increasing strength. If the lower limit is 650°C or less, work hardening will occur and the toughness will deteriorate significantly.

For this reason, the finishing temperature was limited to 650 to 750°C.

Cooling after rolling is not particularly limited, but is 0.2 to 1
A range of 0°C/Sec is preferred.

Moreover, heating the steel material to a temperature below the Ac1 transformation point for the purpose of dehydrogenation or the like after rolling does not impair the characteristics of the present invention.

The steel produced by the method of the present invention has extremely superior base metal weld properties compared to conventional steel materials, and also has the characteristics of normalized or quenched and tempered steel, making it suitable for all kinds of uses (sour casing, cold region use). Applicable to line pipes, pressure vessels, offshore structures, shipbuilding, etc.).

The reason for limiting the component range will be explained below.

Among the steels of the present invention having the above characteristics, the composition range of the steel of the first invention shown in claim 1 is 00.01 to 0.15.
%, Si0.6 or less, Mn10. 5-2.0%,
Al0.0 1-0.0 8%, 80.004% or less,
Oa0.0005~0.005%, Ti0.008~0
．． 025% and N0.001 to 0.007.

The lower limit of 0.01% of C is to ensure the strength of the base metal and welded part, and Nb
, V and other carbide-forming elements are the minimum amount in order to fully exhibit their effects.

However, if the C content is too high, a large amount of coarse bainite or island martensite will be generated in the base metal and the HAZ, which will not only have a negative effect on toughness but also reduce weldability, so the upper limit is set at 0.15%. And so.

Si is an element that is naturally contained in steel for deoxidation,
Since Si is an unfavorable element in terms of weldability and HAZ toughness, its upper limit was set at 0.6%.

(Steel can be deoxidized using only Al, preferably 0.
2% or less is preferable) Mn is an extremely important element that lowers the transformation point of the steel of the present invention, enhances the effect of improving material quality by controlled rolling, and simultaneously improves strength and toughness.

If Mn is less than 0.5%, the strength and toughness of the steel material will deteriorate, so the lower limit was set at 0.5%.

However, if there is too much Mn, the hardenability will increase, a large amount of coarse paynite or island martensite will be generated, and the base metal toughness and HAZ toughness will deteriorate, so the upper limit is set at 2.0%.
And so.

Al is an element that is inevitably contained in this type of killed steel for deoxidation, but if Al is less than 0.01%, deoxidation will be insufficient and the toughness of the base material will deteriorate, so the lower limit is set at 0.01%. did.

On the other hand, if Al exceeds 0.08%, the cleanliness and HAZ toughness of the steel will deteriorate, so the upper limit was set at 0.08%.

The impurity S is limited to 0.004'% or less, and the O
The main reason for adding 0.0005 to 0.005% of a is to reduce the anisotropy of the base material and increase absorbed energy.

In the method of the present invention, rolling is performed near the Ar3 point or below the Ar3 point, but the low-temperature heated material has greater anisotropy than a normal OR material, and the Charpy absorbed energy is extremely reduced.

As mentioned above, this is due to MnS in the steel and the development of texture due to rolling.

The reason why the amount of S is limited is to reduce the absolute amount of MnS, and by setting the amount of S to 0.004% or less, a remarkable effect on toughness is recognized.

In this case, the lower S is, the better the ductility is, but 0.0015
% or less, it can be greatly improved.

However, no matter how small the amount of S in steel, it is impossible to completely remove MnS, and the development of texture cannot be controlled.

For this reason, Ca was added to control the shape of the sulfide and the texture.

Oa content from 0.0 0 0 5 to 0.0 0 5%
The reason why it is limited to Oa-O-
This is because a large amount of Ca inclusions such as S are generated, which impairs not only the toughness but also the cleanliness of the steel sheet, and also causes problems mainly in welding workability in carbon dioxide arc welding.

As mentioned earlier, the main purpose of adding TiN is to improve HAZ toughness with fine TiN dispersed in the steel material, and for this purpose, it is necessary to disperse as much fine TiN as possible in the slab. It is effective to keep the

However, if the Ti and N contents are too large, TiN will become coarse during solidification and cooling of molten steel even in continuous casting, so the upper limits were limited to 0.025% to 0.007%, respectively.

On the other hand, if the Ti and N contents are too small, no significant effect on improving HAZ toughness can be obtained, so the lower limits were set to 0.008% and 0.001%, respectively.

The steel of the present invention contains P as an impurity, but usually 0.03
It is 0% or less, and the lower the content, the better the base metal, weld zone toughness, and weldability.

(0.015% or less is desirable from the viewpoint of welding properties) Furthermore, the O content in the steel of the present invention is 0.008% or less, but it is desirable that it be as low as possible from the viewpoints of cleanliness, toughness, etc. of the steel.

In the second invention shown in claim 2,
In addition to the ingredients and manufacturing process of the steel of the first invention shown in Section 1, Nb 0. 0.8% or less, V0.10%
Hereinafter, Ni 20% or less, Cul. 0% or less, Orl. O
% or less, Mo 0. 4% or less of one or more of the following.

The main purpose of adding these elements is to improve the strength and toughness of the steel of the present invention and to increase the thickness of the manufactured plate without impairing the characteristics of the steel, and the amount of these elements should be limited by their own nature. It is.

Nb is included for grain formation and precipitation hardening of the rolled structure, and is an important element that improves both strength and toughness.
If it exceeds 0.08%, it is harmful to weldability and HAZ toughness, so the upper limit was limited to 0.08%.

■ has almost the same effect as Nb, but the upper limit is 0.10
% is acceptable.

Ni improves the strength and toughness of the base material without adversely affecting the hardenability and toughness of the HAZ, but if it exceeds 2.0 fb, it is unfavorable for the hardenability and toughness of the HAZ, so the upper limit was set at 2.0%. .

Cu has almost the same effect as Ni, and also has effects on corrosion resistance and the like.

However, if it exceeds 1.0%, Cu-cracks will occur during rolling even in low-temperature hot rolling as in the present invention, making manufacturing difficult.

Therefore, the upper limit is set to 1. It was set as o%.

Cr increases the strength of the base metal and the welded part and has an effect on hydrogen-induced cracking, etc., but if too much Cr is lost, it increases the hardenability of the HAZ, resulting in a decrease in toughness and weldability, which is undesirable.

Its upper limit is 1.0%. Mo is an element that improves both the strength and toughness of the base metal, but like Cr, if too much is lost, it increases hardenability, which is undesirable as it leads to a decrease in HAZ toughness and weldability.

The upper limit is 0. 4%. Note that it is desirable that the lower limit of the amount of these elements added be the minimum necessary amount to obtain significant effects on the material.
%, Ni, Ou, Or is 0. 1%, Mo is 0.05
%.

Next, examples of the present invention will be described.

Steel plates with a thickness of 18 to 35 mm were manufactured using slabs of various chemical compositions manufactured in a series of converter casting processes and by changing the 7IL hot rolling conditions.

The mechanical properties of the base metal and welded part are shown in Tables 1 and 2.

The steel roots produced by the method of the present invention all have excellent base metal and weld zone toughness, whereas the comparative steels not according to the present invention have unsatisfactory either base metal toughness or weld zone toughness. ,
Six lances are missing as steel materials for welding.

Steels 1 and 2 have the same composition, but steel 2 has poor base metal and welded part toughness because the heating temperature is high during manufacturing conditions.

Steels 3 and 4 have almost the same composition, but steel 4 does not have Ti added, so even though the manufacturing conditions are the same,
Base metal and weld zone toughness is poor.

Further, although Steel 5 has Ti added, since the amount of added caloric content is large, sufficient fine TiN cannot be obtained, and although the weld zone toughness is improved compared to no Ti added, it is not sufficient.

Among steels 6 and 7, steel 7 has no Ca added,
Even though the manufacturing conditions are the same, the Charpy absorbed energy of the base material is particularly inferior.

Steels 8 and 9 have the same composition, but steel 9 has a high S content and is inferior in Charpy absorbed energy in the base metal and weld.

Steel 10 has the same composition as the steel of the present invention, or the slab manufacturing method is an ingot-forming and one-piece crushing method, so that the effect of fine TiN cannot be sufficiently obtained and the base metal and weld zone toughness are inferior.

Claims (1)

[Claims] 1 00.01 to 0.15%, Si 0.6% or less, M
n0.5~2.0%, A70.01~0.08%, 80
．． 0.004% or less, Ca 0.0 0 0 5 to 0.0
0.05%, Ti0.008-0.025%, NO. 00
A slab with a thickness of 300 mm or less produced by a continuous casting method containing 1% to 0.007% of Fe and unavoidable impurities is heated to 900 to 1000°C, and then rolled at a temperature of 850°C or less. A method for producing non-thermal high strength, high toughness steel, characterized in that the downstream amount is 60% or more and the finishing temperature is 650 to 750°C. 2 00.01~0.15%, Si0.6% or less, M
B0.5-2.0%, A/0.01-0.08%, S0
．． 0.004% or less, Oa 0.0 0 0 5 to O. O
O 5%, Ti 0.008-0.025, N 0.001
~0007% NbO. 0.08% or less, V0.10% or less, Ni 2. O% or less, Cul. 0% or less, Cr1
．． 0% or less, Mo0.4% or less, and the balance is Fe and unavoidable impurities.
Heated to 1000°C and then rolled at 85°C.
The reduction amount is 60% or more below 0°C and the finishing temperature is 65%.
A method for producing non-thermal high-strength, high-toughness steel, characterized in that the temperature is 0 to 750°C.