JPH06179909A - Production of steel material for very low temperature use - Google Patents

Abstract

PURPOSE:To produce a plate of Ni-containing steel having sufficient toughness at low a temp. even in the case of an extra thick material. CONSTITUTION:A steel having a composition which consists of, by weight, <=0.10% C, 8.0-10.0% Ni, <=1.0% Mn, and the balance Fe with inevitable impurities and where the contents of P and S among the impurities are regulated to <=0.001% and <=0.001%, respectively, is hot-rolled, hardened from a temp. not lower than the Ac3 transformation point, reheated up to 680-710 deg.C, rehardened, and tempered at 570-600 deg.C. By this method, the thick steel plate excellent in toughness at a low temp. and suitable for use in material, e.g. for vessels for transportation and storage of various liquefied gases, such as LNG, can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a container for transporting and storing various liquefied gases such as LNG, LPG, ethylene, ammonia, oxygen and nitrogen, and an extremely tough electrode used as a material for auxiliary devices thereof. The present invention relates to a method for manufacturing a low temperature steel material.

[0002]

2. Description of the Related Art In recent years, liquefied natural gas (LN) has been used as clean energy as global environmental problems have become more serious.
G) is drawing attention. Among fossil fuels, LNG emits relatively little carbon dioxide during combustion, and does not emit harmful substances such as sulfur dioxide as combustion waste gas because impurities such as sulfur are removed during liquefaction. It has the advantage of Generally, the production site of LNG is far from the consumption site, and therefore transportation and storage facilities are required. As suitable materials for such equipment, Ni-containing steel, aluminum alloys, austenitic stainless steel, etc. are used, but among them, Ni-containing steel is relatively inexpensive,
It is known that it has excellent low temperature toughness due to its effects such as improvement of matrix toughness by addition of Al, refinement of structure by heat treatment, improvement of toughness due to existence of stable retained austenite phase even under extremely low temperature conditions, etc. .

Among the Ni-containing steels, the steel containing 9% of Ni (9
% Ni steel. In the present specification, all% related to the content of alloy components are% by weight. ) Is L in 1963
Since it was used as a tank for NG storage, it has made many achievements as a material for tanks, and it is expected that its usage will increase in the future. In addition, as energy demand increases, L
The NG tank is also being studied for larger size, and the capacity of the NG tank was 80 to 140,000 kiloliters in the past, but the plan to construct a tank of 200,000 kiloliter class is in full swing. There is. Therefore, it is expected that the plate thickness of the 9% Ni steel used will increase from the previously manufactured 30-40 mm to more than 40 mm. For this reason, it has become necessary to develop a manufacturing method capable of ensuring stable performance at extremely low temperatures even in the case of thick-walled materials, as with conventional materials.

Conventionally, for 9% Ni steel, various methods have been proposed mainly for heat treatment in order to improve the low temperature toughness of the steel sheet. For example, there is a so-called direct quenching method in which a steel sheet is hot-rolled, immediately quenched, and then tempered at a temperature not higher than the Ac ₁ transformation point (JP-A-58-58).
217629, JP 60-181227, JP 63-12
8118 publication, Japanese Patent Publication No. 2-9649 publication, etc.). With this method, it is possible to manufacture steel sheets that have excellent low temperature toughness and strength with thick plate materials that have been conventionally used, such as 35-40 mm, but extremely thick plates with plate thickness exceeding 40 mm. In the case of the above material, there is a problem that the quenching rate is lowered due to the mass effect and the structure is changed, which causes deterioration of low temperature toughness and stable performance cannot be obtained.

Further, after the hot rolling of the steel sheet, the first quenching treatment is performed by heating the steel sheet to the Ac ₃ transformation point or higher and cooling, and the second quenching treatment is performed by heating between the Ac ₁ transformation point and the Ac ₃ transformation point. A series of heat treatment methods of quenching treatment and tempering treatment below the Ac ₁ transformation point, so-called reheating quenching method, have also been proposed (JP-A-62-205227, JP-A-3-264617, etc.). According to this method, it is possible to improve the low temperature toughness by increasing the amount of retained austenite in the steel sheet, but it is possible to improve the low temperature toughness,
When applied to steel production, the toughness at cryogenic temperatures is still insufficient.

As the toughness of steel materials, there are crack generation inhibiting characteristics and crack propagation stopping characteristics (arrest characteristics), and when used as cryogenic steel materials such as LNG tanks, it is essential to secure both characteristics. The conventional method, as described in, for example, Japanese Patent Publication No. 2-9649, is mainly related to the improvement of the crack propagation stop characteristics of the steel material, it does not necessarily satisfy the characteristics of both crack generation prevention and crack propagation stop of the steel material. There is also the problem of not doing it.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing a steel plate of Ni-containing steel having sufficient low temperature toughness even with an extremely thick material having a thickness of more than 40 mm.

[0008]

DISCLOSURE OF THE INVENTION The gist of the present invention is the following method for producing a cryogenic steel material.

% By weight, C: 0.10% or less, Ni: 8.0-1
0.0% and Mn: 1.0% or less and the balance Fe and unavoidable impurities, and P in the impurities is 0.001% or less,
It is characterized in that steel having S of 0.001% or less is hot-rolled, quenched from a temperature of Ac ₃ transformation point or higher, reheated to 680 to 710 ° C, then rehardened, and tempered at 570 to 600 ° C. Cryogenic steel material manufacturing method.

[0010]

[Function] The present invention uses C, Ni and
Use Mn in the above range and keeping P and S in the impurities as low as 0.001% or less, and after hot rolling this steel under the specific conditions as described above. Applying twice hardening and tempering,
The above object is achieved by the combination of.

Below, the chemical composition of the raw material steel and the reasons for limiting various conditions in the manufacturing process will be explained together with the action and effect.

(1) Chemical composition of material steel C: 0.10% or less C is an element necessary for imparting a predetermined static strength to the steel, but on the other hand, it lowers toughness. It is necessary to pay particular attention to ensuring toughness in the cryogenic steel material having a plate thickness of more than 40 mm, which is the main object of the present invention, but if C exceeds 0.10%, the toughness sharply decreases, so its upper limit is set. 0.10%

Ni: 8.0 to 10.0% Ni is an element effective in improving the toughness of steel,
In particular, in order to improve the low temperature toughness, the retained austenite phase needs to exist stably even at a low temperature, and the Ni content effective for stabilizing the retained austenite phase is essential. In order to exert its effect, Ni must be contained in an amount of 8.0% or more, and when Ni is lower than this, the toughness at -165 ° C decreases, so use as a steel material for LNG tanks. Will be difficult. Further, even if the content exceeds 10.0%, the improvement effect is saturated and the economical efficiency is impaired, so 10.0% is made the upper limit.

Mn: 1.0% or less If the content of Mn exceeds 1.0%, it not only promotes the segregation of P, but also increases the temper embrittlement susceptibility of the steel sheet.
Therefore, Mn should be 1.0% or less.

In addition to the above alloy components, the balance consists of unavoidable impurities, and one of the features of the present invention is that the content of P and S among the impurities is kept low.

P: 0.001% or less P is an element that promotes temper embrittlement of steel, and in order to prevent this, it is desirable to suppress it as low as possible. This point must be taken into consideration especially for cryogenic steel materials with a plate thickness of more than 40 mm, which is the main subject of the present invention. 0.001%
Is the upper limit.

In FIG. 1, C: 0.06%, Si: 0.22%, Mn: 0.
57, Ni: 9.09%, S: 0.001% of the basic composition of the steel with only P content changed, heated at 1050 ℃, finish temperature
Using a test piece that was rolled under the conditions of 900 ° C, quenched from 810 ° C, reheated to 680 ° C and tempered, and further tempered at 570 ° C, the rolling direction (L direction) at -196 ° C was used. It is a figure which shows the result of having investigated the Charpy impact energy.

FIG. 2 shows a rolling direction (L direction) at -196 ° C. and a right angle to the rolling direction at 250 ° C. for 1 hour after applying 5% of tensile prestrain after the above heat treatment. It is a figure which shows the result of having investigated the Charpy impact energy of the direction (C direction). This test is a test for observing the transformation tendency of austenite to martensite (stability of retained austenite) due to application of work strain.

FIG. 3 shows a joint portion obtained by welding the steel material (after heat treatment) used in the test of FIG. 1 by the covered arc welding method (SMAW) under the conditions of X groove and heat input of 36 kJ / cm. CT at
It is a figure which shows OD value (the definition is described in the Example section).

As is clear from FIGS. 1 to 3, P is 0.0
By setting it to 01% or less, the low temperature toughness of the steel plate itself and the welded joint is dramatically improved. Based on the above test results, it was decided in the present invention that the P content of the material steel is suppressed to 0.001% or less.

In the steel melting process, preliminary dephosphorization of hot metal is combined with the technology of dephosphorization by converter blowing and subsequent ladle refining to obtain the above-mentioned ultra-low P steel. It is possible to manufacture.

S: 0.001% or less S forms an inclusion (MnS) and lowers the toughness of the steel in the direction perpendicular to the rolling direction, so it is desirable to suppress it to a low value as much as possible. In the present invention, which mainly targets extremely thick steel for cryogenic temperatures, the upper limit is 0.001%.

(2) Various Conditions of Manufacturing Process Hot Rolling Conditions Hot rolling is a slab manufactured by the ingot-agglomeration method or a continuously cast slab. The hot rolling conditions may be the same as the conventional 9% Ni steel. Desirable heating temperature is 1000 to 1150 ℃, and direct quenching is possible 800 to 900 ℃
Is to finish rolling.

Heat Treatment Conditions The decrease in fracture toughness of a steel material due to an increase in plate thickness is due to heat treatment, mainly due to a change in metallurgical structure due to a decrease in cooling rate during quenching. In the present invention, which is intended to secure the toughness at a low temperature, the heat treatment under the following conditions is important to complement and improve this metallurgical structure change.

First, in order to obtain a uniform martensite structure, it is reheated to the Ac ₃ transformation point or higher, water-cooled, and quenched. The heating temperature is preferably 800-900 ℃. If the temperature exceeds 900 ° C, the austenite grains become remarkably coarsened due to recrystallization, resulting in a decrease in the low-temperature toughness of the steel sheet.
If the amount is less than the above, quenching may be insufficient and strength may be insufficient.
If the hot rolling finish temperature is 800 to 900 ° C, quenching may be performed directly after the rolling is finished.

Next, reheating and re-quenching are performed. This is so-called two-phase region quenching, in which re-heating is performed to a two-phase region in which ferrite and austenite coexist and cooling is performed. In this reheating, austenite is partially formed, elements such as C, Ni, and Mn are concentrated in the austenite, and by subsequent quenching, a clean mixed structure of ferrite and martensite having a high solute element is formed.

By tempering the steel having a mixed structure of ferrite and martensite obtained as described above, a stable austenite (residual austenite) phase in which elements such as C, Ni and Mn are concentrated is partially formed. To generate. By appropriately setting the reheating temperature and the tempering temperature at this time, it is possible to control the generation amount and the degree of dispersion of the retained austenite phase in the steel material, and it is possible to obtain stable low temperature toughness.

The reasons for limiting the reheating temperature and the tempering temperature are as follows.

Reheating temperature: 680 to 710 ° C. When the reheating temperature is lower than 680 ° C., the crack initiation inhibiting property of the steel material can be obtained, but the sufficient crack propagation stopping property cannot be obtained. Further, when reheated at a temperature higher than 710 ° C, coarsening of the structure occurs, and both properties of preventing crack initiation and stopping crack propagation deteriorate. Therefore, the reheating temperature is 680-710 ℃.

Tempering temperature: 570 to 600 ° C. If this temperature is lower than 570 ° C., sufficient low temperature toughness cannot be obtained because the amount of retained austenite is small. On the other hand,
Tempering at a temperature above 600 ° C increases both the size and amount of retained austenite at room temperature, which leads to the transformation of austenite to martensite during use at low temperatures, and to the core where fracture occurs. And deteriorates the low temperature toughness. Also, from the viewpoint of ensuring strength, tempering temperature that is too high should be avoided. Therefore, the tempering temperature
570 to 600 ° C

The production method of the present invention is also effective when applied to the production of a conventional low temperature steel sheet having a sheet thickness of about 30 to 40 mm. Further, according to the method of the present invention, it is possible to relatively easily manufacture a thick plate having a plate thickness of more than 40 mm, and a thick material having a plate thickness of 50 mm or more, which has been extremely difficult to manufacture in the past. The cryogenic steel material produced by the method of the present invention has an excellent feature that both crack generation prevention and crack propagation arrest characteristics under cryogenic temperature are excellent. Therefore, in addition to LNG, LPG, ethylene, and ammonia. It is useful as a material for transporting and storing various liquefied gases such as oxygen, oxygen, and nitrogen, and their associated devices.

[0032]

[Example] 1080 continuous cast slabs having the chemical composition shown in Table 1 were used.
Heated to ℃ and hot rolled into a steel plate with a thickness of 50 mm.
The first quenching was performed by heating to 0 ° C. and cooling with water, and then the second reheating, requenching, and tempering treatment were sequentially performed. The reheating temperature and the tempering temperature were changed as shown in Table 2. In all cases, the heating time is the thickness of the steel plate 1
It was decided based on 1 hour per inch.

A test piece having a predetermined shape was cut out from the steel sheet that had been subjected to the heat treatment, and a three-sided slit Charpy impact test and C
A TOD test was conducted. The test temperature was -196 ° C. CT
The OD test was carried out in liquid nitrogen, and the three-sided slit Charpy impact test was carried out by immersing the test piece in liquid nitrogen, holding it for a predetermined time to adjust the temperature, and then taking out the test.

The three-sided slit Charpy impact test is 10
Width 0.14 mm, depth 2 on the central 3 faces of a 55 mm long, 55 mm long test piece
This is a test that can easily evaluate the crack propagation inhibition property in an impact test using a groove of mm (for details, refer to the Japan Society of Shipbuilding, No. 167 (1990), page 271).

The CTOD test is a test according to the BS-5762 standard, in which the relationship between load and crack opening displacement is measured by three-point bending of a test piece having a fatigue notch in order to evaluate crack initiation inhibition characteristics. is there. This test is often used to evaluate the fracture toughness of 9% Ni steel, and measures the crack growth length from the notch portion until the test piece is broken by three-point bending. The longer the length, the better the toughness.

The test results are shown in Table 3. As can be seen, the present invention examples (test Nos. 1 to 3) in which the chemical composition and heat treatment of the material steel are appropriate are excellent even at extremely low temperatures, despite the fact that the plate thickness is 50 mm. It shows the crack initiation inhibition property and the crack propagation stopping property.

On the other hand, with respect to the C steel in which the P content of the raw material steel deviates from the specified value of the present invention, the heat treatment was carried out under the conditions specified in the present invention, but the evaluation value of the low temperature toughness was higher than the other values. A steel which is bad and whose chemical composition is within the range defined in the present invention,
Even in the case of B steel, if the reheating and quenching temperature or the tempering temperature is inappropriate, either the crack initiation preventing property or the crack propagation stopping property is inferior. That is, a good cryogenic steel material can be obtained only when both the chemical composition of the material and the heat treatment conditions satisfy the conditions defined in the present invention.

FIG. 4 is a view showing the three-sided slit Charpy impact values of Nos. 1, 3 and 6 of the above test in relation to the P content of the raw steel. Here too, the effect of improving the toughness by suppressing P to 0.001% or less is conspicuous.

[0039]

EFFECT OF THE INVENTION The cryogenic steel material produced by the production method of the present invention has excellent low temperature toughness even if it is an extremely thick material having a plate thickness of more than 40 mm. In addition to LNG, LPG, ethylene,
It is suitable for use as a container for transportation and storage of various liquefied gases such as ammonia, oxygen, and nitrogen, and as a material for auxiliary devices thereof.

[0040]

[Table 1]

[0041]

[Table 2]

[0042]

[Table 3]

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a P content of a 9% Ni steel plate and a 2 mmV notch Charpy impact value.

FIG. 2 is a diagram showing the relationship between P content and strain-aged Charpy impact value.

FIG. 3 is a diagram showing a relationship between the P content and the limit CTOD value of the SMAW joint.

FIG. 4 is a diagram showing a relationship between P content and a three-sided slit Charpy impact value.

Claims (1)

[Claims] 1. By weight%, C: 0.10% or less, Ni: 8.0 to 1
0.0% and Mn: 1.0% or less and the balance Fe and unavoidable impurities, and P in the impurities is 0.001% or less,
A steel with S of 0.001% or less is hot-rolled, then quenched from a temperature of Ac ₃ transformation point or higher, then reheated to 680 to 710 ℃, then re-hardened, and tempered at 570 to 600 ℃. And a method for producing a cryogenic steel material.