JPS58171526A - Manufacture of steel for extra-low temperature use - Google Patents

Abstract

PURPOSE:To obtain a steel having superior brittle cracking resistance and superior properties of stopping the propagation of brittle cracks, by treating a steel billet contg. specified percentages of C, Si, Mn, P, S, Al, Ni, Nb and N under specified rolling and cooling conditions. CONSTITUTION:A steel billet consisting of 0.005-0.08% C, <=0.6% Si, 0.2-2.0% Mn, <=0.020% P, <=0.006% S, 0.005-0.08% Al, 1.0-0.6% Ni, 0.005-0.03% Nb, <=0.005% N and the balance Fe with inevitable impurities is heated to 900- 1,000 deg.C in the final stage of hot rolling and rolled in a temp. region where austenite grains are recrystallized. It is further rolled sufficiently so as to adjust the draft at <=900 deg.C at which no recrystallization is caused to >=60% and the finishing temp. to 650-850 deg.C. Thus, the austenite grains are made thoroughly fine. The rolled steel is cooled to <=550 deg.C by accelerated cooling at 3-40 deg.C/sec cooling rate, and it is allowed to cool.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides special conditions for the composition of steel, and by controlling hot rolling conditions and cooling conditions immediately after rolling. The present invention relates to a method for producing steel having excellent brittle crack initiation and brittle crack arrest properties.

With the increase in energy demand in recent years, LNG,
Demand for cryogenic steel used in low-temperature storage tanks such as LEG and transportation has been steadily increasing. Steel for pressure vessels used in low-temperature storage tanks and transportation has low-temperature contents, so it has excellent weldability in terms of excellent toughness (base metal and welded parts) and workability in the extremely low temperature range. required. However, it is extremely difficult to fully satisfy these required characteristics using current manufacturing methods.

Current manufacturing methods for steel that satisfy these properties, although insufficient, are the controlled rolling method (CR method), which is widely used for line pipe materials, and the method of heat treatment after rolling. All of these have the disadvantage that there is a limit to improving toughness at extremely low temperatures (l), and increasing the Ni content deteriorates weldability and increases costs.

In order to develop a new cryogenic steel, the present inventors conducted intensive research on the composition system, heating, rolling, and cooling processes suitable for the manufacturing method of cryogenic steel. This led to the invention of a completely new method for manufacturing strong steel with excellent weldability and HAZ toughness.

This point will be explained in detail below.

The feature of the present invention is to extremely reduce the contents of impurities such as P, S, and N, and to heat a steel slab containing a trace amount of Nb and 1% or more of NI at a cryogenic temperature (900' to 1000°C) to form austenite grains. In addition to rolling in the recrystallized region, sufficient rolling (60% or more) is applied in the non-recrystallized region below 900°C, and 65
After finishing the rolling at 0 to 850° C. and sufficiently refining the austenite grains, the cooling rate (PCI) is set at a cooling rate of 3 to 850° C. or less to an arbitrary temperature.

Note that the above conditions are for the final step of the heating and rolling process 1. The continuous casting slab may be directly rolled under the above conditions, or it may be heated, rolled, cooled (
(including accelerated cooling) may be rolled under the above conditions.

According to the present invention, the structure after cooling becomes a mixed structure in which fine ferrite, fine pearlite, or fine bainite are mixed, so that it has excellent strength and toughness.

The improvement in strength in the steel of the present invention is due to: ■ Appropriate C9Si+M
Addition of rzNi and effective use of precipitation hardening element Nb, ■Nb
It is obtained as a result of microstructure refinement and bainite formation due to accelerated cooling after addition and rolling.

In addition, the toughness can be improved by adding 1.0% or more of Ni and by refining the microstructure.
00 to 1000℃), and refinement of heated austenite grains by suppressing austenite grain growth with fine Nb CN, ■ suppressing the growth of austenite grains recrystallized during rolling by Nb(C,N), and ■ suppressing the growth of austenite grains precipitated during rolling. Fine Nb (
C, N) suppresses recrystallization of austenite and applies sufficient low-temperature cumulative reduction (reduction amount of 60% or more at 900°C or less), so austenite grains are sufficiently stretched, resulting in an increase in ferrite transformation nuclei, This is obtained as a comprehensive effect of the grain refining process, such as an increase in the γ/α conversion ratio due to accelerated cooling after rolling.

According to the present invention, due to the Ni addition microstructure refinement and reduction of impurities P, S, and N, both fracture surface transition temperature and shock absorption energy are excellent, and separation is difficult to occur for cryogenic use. It is possible to manufacture steel.

The steel manufactured according to the present invention has superior strength and toughness with Ni + low Ceq compared to conventional steel materials, so it is not only inexpensive, but also has low hardenability and cracking susceptibility during welding. The toughness of the parts is extremely good.

The reasons for limiting the hot rolling cooling conditions in the present invention will be explained in detail below.

The reason why the heating temperature is limited to 900 to 1000°C is to keep the austenite grains as small as possible during heating and to refine the rolled structure.

1000℃ is the upper limit temperature at which austenite grains do not become mixed grains during heating, and if the heating temperature exceeds this temperature, austenite grains become coarse mixed grains, and the microstructure after cooling also becomes mixed grains, which deteriorates the toughness of the steel. do.

On the other hand, if the heating temperature is too low, the internal quality of the steel deteriorates and the temperature at the final stage of rolling is too low, making it impossible to expect a sufficient effect of improving material quality through controlled cooling. Therefore, the lower limit is 9
It is necessary to set the temperature to 00°C.

However, even if the heating temperature is limited to a low level as described above, if the rolling conditions are inappropriate, good material cannot be obtained. , the finishing temperature is in the range of 650 to 850°C. This is to thoroughly refine and stretch the austenite grains by applying sufficient rolling in the non-recrystallization temperature range, and to make the fine grains of the transformed structure formed after cooling uniform.

By sufficiently stretching the fine-grained austenite in this manner, the microstructure formed after rolling and cooling is sufficiently refined, and the toughness is greatly improved.

However, if the finishing temperature is inappropriate, good strength and toughness cannot be obtained. The lower limit of the finishing temperature is set to 650° C. in order to prevent deterioration of rolling toughness due to excessive rolling in the (γ+α) region below the transformation point. Furthermore, if the finishing temperature is less than 650°C, a sufficient strength increase effect cannot be expected due to controlled cooling.

On the other hand, if the finishing temperature is too high, the effect of refining austenite grains due to controlled rolling cannot be expected and the toughness decreases. Therefore, it is necessary to set the upper limit to 850°C.

Next is cooling, from immediately after rolling to 550℃ or less.
~b It is necessary to The reason for this is that if the temperature is less than 3°C/sec, the microstructure will not be refined sufficiently and toughness cannot be expected to be improved sufficiently, and if it exceeds 40°C/sec, a large amount of low-temperature transformation products will be produced, which will reduce the toughness. This is to cause deterioration.

The reason why the cooling stop temperature was specified as an arbitrary temperature of 550° C. or less is because dehydrogenation effect and sufficient precipitation hardening cannot be obtained if the material is further cooled to a low temperature. In this case 350
It is desirable to stop cooling at around ~550°C and air-cool. However, if the cooling stop temperature is 550° C. or higher, sufficient improvement in strength and toughness cannot be expected. Note that spray water or water is generally suitable as the cooling medium.

Furthermore, when the steel manufactured according to the present invention is reheated for the purpose of dehydrogenation or the like, it is not preferable to heat the steel at 600° C. or higher, as this leads to deterioration in strength. However, reheating to a temperature of about 600° C. or lower does not result in the loss of the characteristics of the steel of the present invention, although there is a slight decrease in strength.

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 is C
0.05-0.08%, St O, 6% or less, MnO1
2-2.0%, Po, 0.020% or less, 80.006% or less, A10.005-0.08%, Ni 1.0-6.
0%, Nb O, 005-0.03%, NO, 005%
It contains the following:

The lower limit of 0.005% of C is the minimum amount in order to ensure the beauty of the base metal and the welded part and to fully exhibit the effects of Nb and V. However, if the C content is too high, island-shaped martensite will be generated when controlled cooling is performed, which will not only adversely affect ductility but also deteriorate internal quality, weldability, and HAZ toughness, so the upper limit is set at 0.08%. And so.

St is an element that is naturally included in deoxidized steel, but Si
Also, in order to improve weldability and HAZA two-part manufacturing properties, the upper limit was set at 0.6% (steel can be deoxidized with Al alone, and it is preferably 0.2% or less).

Mn is an extremely important element that enhances the effect of improving material quality by low-temperature heating rolling and controlled cooling in the steel of the present invention, and simultaneously improves strength and toughness.

If Mn is less than 0.2%, strength and toughness cannot be ensured, and the effect of improving toughness is also small, so the lower limit was set at 0.2%.

However, if the hardenability increases due to too much Mn, low-temperature transformation products such as martensite tend to be generated in large quantities, and the toughness of the base metal and HAZ deteriorates, so the upper limit is set at 2.0%.
And so.

The reason why P and s, which are impurities, are limited to 0.020% or less and 0.006% or less in the steel of the present invention is that the base metal, weld zone toughness, weldability, and internal quality improve as the content is lower. (p
, s are preferably 0.010 and 0.002% or less, respectively.) Al is an element that is inevitably contained in this type of killed steel for deoxidation, but if Al is less than 0.005%, deoxidation is insufficient. However, since the toughness of the base material deteriorates, the lower limit was set at 01005%. On the other hand, when Al exceeds 0.08%, the cleanliness of steel and H
Since AZ toughness deteriorates, the upper limit was set to 0.08%.

Ni is an essential element for the steel of the present invention and has the property of improving the strength and toughness of the base metal without adversely affecting the hardenability and toughness of the HAZ, but if it is less than 1.0%, it may cause the necessary brittleness in the cryogenic region It is difficult to obtain crack stopping properties. Moreover, if it exceeds 6.0%, it is unfavorable in terms of the hardenability and toughness of T(AZ), so the lower limit was set to 1.0% and the upper limit was set to 6.0%.

As mentioned above, Nb is an essential element in the present invention and has an extremely large effect on the quality of the material. Nb is included to refine the microstructure and harden the precipitation, and is an important element that improves both strength and toughness.
．． Adding more than 0.03% has no significant effect on the material quality, and is harmful to weldability and HAZ toughness, so the upper limit is set at 0.0%.
It was limited to 3%. Further, the lower limit of 0005% is the minimum amount that has an effect on the material.

N inevitably mixes into molten steel and deteriorates the toughness of the steel. In particular, a large amount of freeN (<7) tends to generate island-like martensite in the HAZ, which greatly deteriorates the HAZ toughness. For this reason, it was limited to 0.005% or less.

Next, in the second invention, Vo, 01 to 0.05%, TiO
,005~0.03%,Bo,0005~0.0030
%, CuO, 1-1.0%, CrO, 1-1.0%
, Mo 0.05-030%, Ca O, 0005-0
．． 005% and REM 0.003 to 003%.

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 manufactured plates without impairing the characteristics of the steel. There are things that naturally have to be restricted.

(2) has almost the same effect as Nb, but there is no significant effect below 0.01%, and an upper limit of 0.05% is permissible.

Ti is added in a small amount range (Ti 0.005 to 0.0
3%), fine TiN is formed and the rolled structure and HAZ
It is effective in making the grains finer and improving the toughness of the grain. In this case N
(!-Ti is preferably near equivalent stoichiometrically.T
The upper limit of the amount of i added is the minimum amount that produces the effect on the material, and the upper limit is 0.0 under the conditions that fine TiN can be obtained in the steel billet by the normal manufacturing method and that no deterioration of toughness due to TiC occurs.
It was set at 3%.

B segregates at austenite grain boundaries during rolling, improves hardenability and facilitates the formation of bainite structure, and also
Depending on the amount added, it combines with N, becomes fine N, and has an effect similar to that of Ti, in that it improves the toughness of the welded joint. However, if it is less than 0.0005%, there is no significant hardenability improvement effect or toughness improvement effect, and if it exceeds 0.003%, K will generate B constituent, which will actually deteriorate the toughness of the base material and HAZ. Therefore, the lower limit is 0.0005% and the upper limit is 0.003%. %.

It has almost the same effect as Cu FiNt, and is also effective in corrosion resistance, hydrogen-induced cracking resistance, etc.

However, if it is less than 0.1%, there is no noticeable effect like Ni,
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 lower limit was set to 0.1% and the upper limit was set to 1.0%.

Cr increases the strength of the base material and has an effect on hydrogen-induced cracking resistance, etc., but if it is less than 0.1%, there is no significant effect, and 1.
If it exceeds 0%, it increases the hardenability of the HAZ and deteriorates toughness and weldability, which is undesirable. Therefore, the lower limit was set to 0.1% and the upper limit was set to 1.0%.

Mo1ti is an element that improves both the strength and toughness of the base material, but if it is less than 0.05%, it has no significant effect.

On the other hand, if the amount is too high, it increases the hardenability like Cr,
This is undesirable as it causes deterioration of weld toughness and weldability, and the upper limit is 0.30%. For this reason, the lower limit is set to 0.05%,
The upper limit was set at 0.30%.

Ca, REM makes MnS spheroidal and improves the Charpy absorbed energy, as well as Mn stretched by rolling.
Prevents internal defects caused by S and hydrogen. Regarding REM content, if it is less than 0.001%, it has no practical effect, and if it is added in excess of 0.03%, REM-8
Alternatively, a large amount of REM-0-8 is generated and becomes large inclusions, impairing not only the toughness but also the cleanliness of the steel, and also having an adverse effect on the weldability. For this reason, the upper limit was set at 0.03%. Ca
It also has the same effect as REM, and its effective range is 0.0005 to 0.005%.

Next, examples of the present invention will be described.

Steel plates with thicknesses of 15 to 351+1 were manufactured using slabs of various chemical compositions manufactured in a continuous converter casting process and by changing the manufacturing process. Table 1 shows examples of mechanical properties of base metals and welded parts. All of the steel plates manufactured by the method of the present invention have excellent base metal and weld zone properties, whereas comparative steels not according to the present invention are unsatisfactory in either the base metal strength or the weld zone properties. We are trying to balance it as a steel material for welding.

Table 1 (continued) Note 1) Charpy value in rolling direction is the value at % plate thickness position.

Note 2) Value based on a simulated thermal cycle test equivalent to a heat input of 35,000 J/cIIL.

Among the comparative steels, Steels 6 and 7 were subjected to appropriate hot rolling conditions, but since Steel 6 did not contain Nb, the low-temperature toughness of the base metal was poor, and Steel 7 had P.

The toughness of the weld is poor due to the high N content.

Although Steels 8 to 10 have the same composition as Steel 1 of the present invention, the low-temperature toughness of the base metal is inferior to Steel 1 because the conditions such as hot rolling are inappropriate.

In Steels 8 and 9, the heating temperature is high and the reduction amount is low at 900° C. or less, respectively, so that the microstructure is insufficiently refined.

In Steel 10, since the cooling stop temperature is high, the microstructure is insufficiently uniform and grain refinement is insufficient, resulting in poor toughness and low strength.

Claims (1)

[Claims] 1. CO, 005 to 0.08%, St O, 6% or less, MnO 2 to 2.0%, Po, 020% or less, 80.
006% or less, AIo, 005-0.08%, Ni 1
．． 0-6.0%, NbO, 005-0.03%, NO
,005% or less of the remaining Fe and unavoidable impurities.
Heating to a temperature range of 0℃, reduction of 60℃ below 900℃
% or more and a finishing temperature of 650° C. to 850° C., and after the rolling is completed, accelerated cooling is performed at 3 to 3 b, and then allowed to cool. 2. C0,005~0.08%, SiO, 6% or less, Mn0.2~2.0%, Po, 020% or less, 80
．． 006% or less, AlO, 005-0.08%, Ni
1.0-6.0%, NbO, 005-0.03%, N
In addition to 0,005% or less, vo, oi ~ 0.05%,
TiO, 005~0.03%, BO, 0005~0
．． 0030%, CuO, 1-1.0%, CrO
, 1-1.0%, Mo 0.05-0.30%,
Ca0.0005-0005%, REM 0.003
~0.03% of one or more types, and the remainder F
In the final step of the hot rolling process, the steel billet consisting of E and inevitable impurities is heated to a temperature range of 900 to 1000°C, and the rolling reduction is 60% or more of 900°C or less, and the finishing temperature is 650°C to 850°C. 1. A method for producing cryogenic steel, which comprises rolling the steel so as to achieve the desired temperature, and after the rolling is completed, accelerated cooling is performed at 3-b, and then allowed to cool.