JP4710488B2 - Method for producing 9% Ni steel with excellent low temperature toughness - Google Patents

Description

  The present invention relates to a method for producing low-temperature steel: 9% Ni steel used in LNG storage tanks and the like.

  The demand for natural gas as a clean energy source is increasing rapidly due to the increase in energy demand and consideration for the global environment. Therefore, in recent years, the construction of LNG storage tanks has been actively promoted both at home and abroad, and the demand for 9% Ni steel used for tank construction is also increasing. At the same time, due to the effective use of the tank site, the tank to be constructed tends to be enlarged, and the production of a steel plate with high yield strength is desired.

  In such tanks, many researches and developments have been made to improve toughness from ensuring safety against brittle fracture. As a method for producing 9% Ni steel having excellent low-temperature toughness, JIS G 3127: a quenching and tempering method (hereinafter referred to as a QT process) is specified for a nickel steel steel sheet for a low-temperature pressure vessel (yield point or proof stress is 590 MPa or more). It is generally known to perform two-stage quenching and tempering (hereinafter referred to as the QQ'T process) as a production method that can obtain excellent low temperature toughness while ensuring a predetermined strength. This has been shown to be available if necessary.

According to p801 of Non-Patent Document 1, the concept of low temperature toughness improvement by QQ'T is as follows. In the first stage quenching (Q), martensite is obtained by quenching from the austenite region as in normal quenching. The second stage quenching (Q ′) is performed from the (γ + α) two-phase region below the Ac ₃ transformation point. The grain structure is refined by Q ′ and distribution of the alloy element occurs, so that tempered martensite, martensite enriched with the alloy element, and a small amount of retained austenite are formed. When this mixed structure is tempered in the vicinity of the Ac ₁ transformation point (T), stable austenite enriched with alloying elements is precipitated, and impurities harmful to toughness such as C and N in the tempered martensite are introduced into austenite. Transition. That is, since the final structure is a mixed structure of tempered martensite, which is fine and extremely high in toughness, and an austenite phase that is highly stable even at extremely low temperatures, the low temperature toughness is remarkably improved in the QQ'T process.

Utilizing the advantages of the QQ'T process, many conventional technologies based on this have been disclosed. For example, Patent Document 1 discloses that the mechanical properties are improved by setting the amount of austenite formed at the time of two-phase region heating within a specified range. Patent Document 2 discloses that, in the QQ'T process, excellent mechanical properties can be obtained by suppressing both the impurity elements P and S to 0.001% or less. Further, Patent Document 3 discloses that the mechanical properties are improved by performing the heating rate during Q heating and Q ′ heating at 50 ° C./min or more.
Japanese Unexamined Patent Publication No. 09-256039 Japanese Patent Laid-Open No. 06-179909 JP 08-27517 A The Japan Institute of Metals, “4th edition metal manual” Maruzen

  By using the QQ'T process, it is possible to obtain more stable and excellent low-temperature toughness while ensuring the prescribed strength. However, since the heat treatment process is multi-stage, it is costly and the delivery time after receiving an order is also high. There is a problem of becoming longer. The present invention achieves a 9% Ni steel sheet having excellent low temperature toughness equivalent to the case of using the QQ'T process by a two-stage heat treatment, excellent in productivity, and 9% Ni excellent in low temperature toughness. It aims at providing the manufacturing method of steel.

  An important point for stably obtaining low temperature toughness is to finely disperse a highly stable austenite phase in a large amount so that the strength does not decrease. In the QQ'T process, in order to precipitate stable austenite that contributes to low temperature toughness in T, it is important to prepare martensite enriched with C and Ni in the structure in advance in the process of Q '. is there.

  Conventionally, it has been difficult to finely disperse a highly stable austenite phase by two-stage heat treatment. In order to obtain stable austenite by heat treatment after Q, it is necessary to set the treatment temperature to a low temperature range of a two-phase region. In this case, the nucleation frequency of austenite is small, and in order to ensure a sufficient amount of precipitation, it is necessary to heat for a long time, but rather than austenite newly nucleated, austenite already nucleated is Since it grows, the austenite finally obtained becomes coarse and low temperature toughness deteriorates.

  On the other hand, when the processing temperature is set to a high temperature range of the two-phase region, a sufficient amount of austenite is precipitated, but alloy elements such as C and Ni are considerably diluted as compared with the case of heating to a low temperature range. Austenite becomes unstable and becomes martensite after cooling (same as Q 'in QQ'T process). Although the amount of precipitation can be suppressed by shortening the heating time, in this case, Ni cannot be sufficiently concentrated to austenite, and austenite becomes unstable.

  The idea of the present invention is that, as shown in “Process of the present invention” in FIG. 1, 9% Ni steel in a quenched state (direct quenching after rolling or reheating quenching) is rapidly heated to a high temperature range of two phases for a short time. First, a large amount of fine austenite is precipitated, and the temperature is immediately lowered from 500 ° C. to 600 ° C. at which austenite grain growth is unlikely to occur. In addition, C and Ni are concentrated to stabilize it.

The present invention is based on such an idea.
1st invention is the mass%, C: 0.03-0.10%, Si: 0.05-0.5%, Mn: 0.2-1.0%, Ni: 7.0-10 The steel plate containing 0.0% and the balance consisting of Fe and inevitable impurities is directly quenched after rolling, or reheated to a temperature range of Ac ₃ transformation point to 850 ° C. and subjected to quenching treatment. ° C. / in s or more heating rate, Ac ₃ heated rapidly briefly to a temperature range of transformation temperature -80 ° C. to Ac ₃ transformation point -20 ° C., and subsequently heated and maintained at a temperature range of 500 ° C. to 600 ° C. and baked This is a method for producing 9% Ni steel excellent in low temperature toughness characterized by being returned.

  The second invention is a method for producing 9% Ni steel excellent in low-temperature toughness according to the first invention, characterized by further containing, by mass%, Mo: 0.04 to 0.5%. is there.

  According to the present invention, a 9% Ni steel sheet having excellent low temperature toughness equivalent to the case of using the QQ'T process is achieved by two-stage heat treatment, excellent in productivity and excellent in low temperature toughness. It has become possible to provide a method for producing steel.

  Hereinafter, the present invention will be described in detail. As a steel plate of the present invention, C: 0.03-0.10%, Si: 0.05-0.5%, Mn: 0.2-1.0%, Ni: 7.0 in mass%. Although it contains 10.0%, it can further selectively contain Mo: 0.04 to 0.5%.

1. Next, chemical components of the 9% Ni steel of the present invention will be described. In the following description, all the units indicated by% are mass%.

C: 0.03-0.10%
C is an important element for imparting strength and needs to be added in an amount of 0.03% or more, but if added over 0.10%, the low temperature toughness is deteriorated, so the addition of C Is in the range of 0.03 to 0.10%.

Si: 0.05-0.5%
Si is added as a strength imparting or deoxidizing material, but a large amount is added in the range of 0.05 to 0.5% in order to increase the susceptibility to temper embrittlement.

Mn: 0.2 to 1.0%
If Mn is less than 0.2%, hot ductility deteriorates, so 0.2% or more is added. On the other hand, Mn is an element that contributes to an increase in strength, but even if added over 1.0%, the increase in strength is reduced, and conversely, low-temperature toughness is reduced and susceptibility to temper embrittlement is increased. Therefore, the addition is made 0.2 to 1.0% in range.

Ni: 7.0 to 10.0%
Ni is an element that imparts low temperature toughness and contributes to the stabilization of retained austenite, and needs to be added in an amount of 7.0% or more. However, if it exceeds 10.0%, the effect is saturated and effective. Therefore, the addition is made in the range of 7.0 to 10.0%.

Mo: 0.04 to 0.5%
Furthermore, Mo can be selectively added in a range of 0.04 to 0.5%.
The addition of Mo is performed mainly for the purpose of increasing the strength, but at the same time, the susceptibility to temper embrittlement can be reduced. In order to achieve this effect, 0.04% or more of addition is necessary. However, if the addition exceeds 0.5%, the toughness is reduced, so the addition is 0.04 to 0.5%. The range.

2. Production Conditions The steel sheet having the above component composition is directly quenched after rolling, or reheated between the Ac ₃ transformation point and 850 ° C. and subjected to a quenching treatment, and then subjected to Ac at a temperature rising rate of 3 ° C./s or more. between ₃ transformation point -80 ° C. to Ac ₃ transformation point -20 ° C., rapidly short heating and subsequently tempered by heating maintained at 600 ° C. from 500 ° C.. The reason for limiting the temperature will be described below. A schematic diagram of heat treatment is shown in the process of the present invention in FIG.

The first stage heating temperature range: Ac ₃ transformation point to 850 ° C.
This treatment is performed in order to obtain a uniform martensite structure as a structure before the subsequent second stage heat treatment, and needs to be heated at a temperature equal to or higher than the Ac ₃ transformation point. On the other hand, when the heating temperature exceeds 850 ° C., the austenite crystal grains at the time of heating become coarse, so the heating temperature range is set to Ac ₃ transformation point to 850 ° C., and then quenching is performed. This first stage heat treatment can be replaced by directly quenching the steel sheet after rolling.

Second-stage heating temperature range: Ac ₃ transformation point −80 ° C. to Ac ₃ transformation point −20 ° C.
Temperature rising rate: 3 ° C./s or higher The second stage heat treatment is the most important heat treatment in the present invention, and at a temperature rising rate of ₃ ° C./s or higher, the Ac ₃ transformation point −80 ° C. to Ac ₃ transformation point −20. It is rapidly heated to a temperature range of ° C. for a short time, and subsequently heated and maintained in a temperature range of 500 ° C. to 600 ° C. Here, the rate of temperature increase refers to the average heating rate from the temperature after quenching in the first stage until reaching the heat treatment temperature in the second stage. In this heat treatment, when heating to a two-phase region at a temperature increase rate of less than 3 ° C./s, the austenite nucleation frequency decreases and the grain growth of precipitated austenite becomes remarkable. , Low temperature toughness deteriorates. Therefore, the rate of temperature increase to the two-phase region is 3 ° C./s or more. For such rapid heating, for example, induction heating equipment can be used.

At this time, if the two-phase region heating temperature is lower than the Ac ₃ transformation point of −80 ° C., a sufficient amount of precipitated austenite cannot be obtained, so the Ac ₃ transformation point is set to −80 ° C. or more. When the temperature is higher than the Ac ₃ transformation point of −20 ° C., the precipitated austenite is not sufficiently stable even if the temperature is maintained in the temperature range of 500 ° C. to 600 ° C. Thereafter, the heating temperature range is set to the Ac ₃ transformation point of −80 ° C. ~Ac ₃ and transformation point -20 ℃. Holding at the highest heating temperature is as short as possible to provide rapid growth of precipitated austenite, and is preferably 5 seconds or less.

Thus, after heating rapidly to a two-phase area and precipitating fine austenite, it heat-holds at 500 to 600 degreeC continuously. In this process, C and Ni are concentrated into precipitated austenite, and the stabilization of precipitated austenite proceeds. This heating and holding can be realized, for example, by setting a heat treatment furnace behind the induction heating equipment at this temperature in advance and inserting the plate conveyed from the induction heating equipment into the heat treatment furnace as it is.
When this holding temperature is less than 500 ° C., the recovery of the matrix is delayed, and the effect of improving the toughness of the matrix due to the tempering effect is hindered. On the other hand, when the holding temperature exceeds 600 ° C., the precipitated austenite increases and the strength decreases, so the holding temperature is set in the range of 500 ° C. to 600 ° C. The holding time at this time is not particularly specified, but is preferably 5 minutes or more in order to obtain the effect of distributing the alloy elements to austenite. Cooling after this treatment may be left cooling or water cooling.

Table 1 shows the chemical composition of the test steel. Steel types A, B, and C are steels of the present invention, and steel type D is a comparative steel and the amount of Ni added is as low as 6.5%. These test steels were rolled and heat-treated under the production conditions shown in Table 2. A JIS No. 4 round bar tensile test piece and a V-notch Charpy test piece were collected in the rolling direction from the center of the thickness of the heat-treated steel sheet, and subjected to a room temperature tensile test and an impact test, respectively. In the impact test, the absorbed energy was measured 5 times at a test temperature of -196 ° C., and the average value was used. The results of the tensile test and Charpy impact test are also shown in Table 2 along with the manufacturing conditions. No. 4 to 10 are comparative examples, and the others are invention examples.

In Steel Nos. 1 to 3 and Steel Nos. 11 to 15 of the invention examples, both chemical components and production conditions are appropriate, and have excellent strength and low temperature toughness. On the other hand, steel No. 4 uses a steel type having a low Ni addition amount of 6.5%, so that the low temperature toughness is deteriorated even if a predetermined heat treatment is performed. In Steel No. 5, the quenching holding temperature is as high as 910 ° C., the heated austenite grain size is coarsened, and the low temperature toughness after heat treatment is deteriorated. In Steel No. 6, since the heating rate of the second stage heat treatment is slow, the coarse austenite is present after the heat treatment, so that the low temperature toughness is deteriorated. In steel numbers 7 and 8, since the maximum heating temperature of the second stage heat treatment is not appropriate, the low temperature toughness is deteriorated. Steel No. 9 has a high holding temperature, so that the tensile strength is remarkably reduced. Steel No. 10 has a low holding temperature, which causes a deterioration in low temperature toughness.

According to the present invention, 9% Ni steel having excellent strength and low temperature toughness can be manufactured at low cost, and it can contribute to the increase in size and safety of the LNG tank.

It is the schematic diagram which compared this invention process with the QT process and QQ'T process which are prior art.

Claims (2)

In mass%, C: 0.03-0.10%, Si: 0.05-0.5%, Mn: 0.2-1.0%, Ni: 7.0-10.0% Further, the steel plate comprising the balance Fe and inevitable impurities is directly quenched after rolling, or reheated to a temperature range of Ac ₃ transformation point to 850 ° C. and subjected to quenching treatment, and then the temperature is increased by ₃ ° C./s or more. at a rate, and rapid short heating to a temperature range of Ac ₃ transformation point -80 ° C. to Ac ₃ transformation point -20 ° C., the low temperature, characterized in that tempered subsequently heated maintained at a temperature range of 500 ° C. to 600 ° C. A method for producing 9% Ni steel with excellent toughness.   Furthermore, the manufacturing method of 9% Ni steel excellent in low temperature toughness according to claim 1, characterized by containing Mo: 0.04 to 0.5% in mass%.

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