JPH03264617A - Production of thick-walled 9%ni steel excellent in yield strength - Google Patents

Abstract

PURPOSE:To obtain the steel excellent in toughness at low temp. by specifying slab heating temp. before hot rolling, cumulative reduction of area at the time of hot rolling, and hardening treatment temp. after rolling, respectively, in a hardened, intermediate hardening treated, and tempered material. CONSTITUTION:A 9%Ni steel is produced by applying hot rolling to a slab of a steel containing 7.5-10wt.% Ni and then carrying out, in succession, primary hardening consisting of heating up to a temp. not lower than the Ac3 transformation point and cooling, secondary hardening consisting of heating up to a temp. between the Ac1 and the Ac3 transformation point and cooling, and tempering consisting of heating up to a temp. not higher than the Ac1 transformation point and cooling. In the above method, slab heating temp. before hot rolling is 800-900 deg.C, and subsequently, hot rolling in which cumulative reduction of area at 700-800 deg.C is regulated to 50-70% is performed, and hardening after rolling is carried out by exerting heating up to a temp. between Ac3 and 850 deg.C. By this method, the steel of >=40mm plate thickness having high yield strength and superior toughness at low temp. hitherto impossible by conventional methods can be produced.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention is a method for manufacturing a steel plate having a thickness of 40+nm or more with excellent yield strength.
The present invention relates to a method for manufacturing Ni steel.

(Prior Art) Due to increasing energy demand and concerns about the safety of nuclear power, the demand for natural gas as a clean energy source is rapidly increasing. Therefore, in recent years, the construction of LNG storage tanks has been actively promoted at home and abroad, and the demand for 9% Ni steel used in these tanks is also increasing. Furthermore, there is a trend toward increasing tank capacity due to an increase in storage efficiency, and even with 9% Ni steel as LNG tank material, the plate thickness conventionally manufactured is 30 m1Il.
It has become necessary to manufacture steel materials with a strength of 40+am or more.

Conventionally, from the viewpoint of tank safety, 9% has excellent low temperature toughness.
Many inventions have been made regarding methods for producing Ni steel.

Among these methods, there are many methods that include heating and quenching between Ac1 and Ac3 transformation points and then tempering because they can significantly improve low-temperature toughness.

For example, as in Japanese Patent Application Laid-Open No. 47-23317,
A method for improving the toughness of thick-walled 9% Ni steel, which is characterized by heating between c1 and Ac3 transformation points, quenching and air cooling, and then tempering at a temperature equal to or higher than the Acl transformation point, or JP-A-58-
As in JP-A No. 73717, JP-A No. 62-205227, etc., after heating to above the Ac3 transformation point and cooling, Ac
l ~ Ac, heat treatment that heats and cools between transformation points j! l! There is a heat treatment method that is characterized by tempering (hereinafter referred to as "intermediate quenching treatment") at a temperature below the Ac transformation point.

Also, JP-A-49-135813, JP-A-61-
As in JP-A No. 238911, JP-A No. 60-131916, JP-A No. 56-156715, etc., after hot rolling, the material is cooled at a speed higher than air cooling, and then heated between the Ac and Ac3 transformation points. There is a method for producing 9% Ni steel which is characterized by cooling and then tempering in Ac at a temperature t" below the transformation point.

(Problems to be Solved by the Invention) Among the above, Japanese Patent Application Laid-Open No. 47-23317 aims to improve the toughness in the thickness direction of a thick steel plate used for the equatorial support band of a spherical tank. be. According to this invention, improvement in plate thickness and toughness in the Z direction can certainly be expected, but as the thickness increases, there is naturally a concern that the strength will decrease. It is not suitable as a manufacturing method.

Also, JP-A-58-73717 and JP-A-62
JP-205227 is widely used as a heat treatment method for 9% Ni steel. However, these inventions focused only on improving low-temperature toughness, and did not pay equal attention to strength, which is the most important aspect of structural performance.As described later, these methods L with an unprecedentedly thick plate thickness of 40 [IIm]
It is difficult to manufacture 9% Ni steel for NG tanks.

Also, JP-A-49-135813, JP-A-61'
JP-A-238911, JP-A-60-131916, and JP-A-56-156715 achieve excellent base metal toughness or excellent weld zone toughness. However, as can be seen from the examples, these inventions were made with the purpose of improving the low-temperature toughness of steel plates having a thickness of 32m+n or less, that is, they were made with the existing steel plates for LNG tanks in mind. and the plate thickness is 40LIIIl
These methods cannot be applied to steel plates with a thickness of 11 or more.In general, the fracture toughness of steel materials decreases due to mechanical factors as the plate thickness increases, but at the same time, the fracture toughness of steel decreases due to metallurgical factors. , the toughness is also lowered.

4- Table 1 shows a comparison of conventionally known manufacturing methods for 9% Ni steel in terms of strength and fracture toughness values regarding brittle crack propagation arrest when the plate thickness is 42+I1 m.

As mentioned earlier, "quenching, intermediate quenching and tempering" is known as a heat treatment method that significantly improves the low-temperature toughness of 9% Ni steel. Although the fracture toughness is superior to that of the tempered material, the yield strength is significantly lower. therefore,
With conventional techniques, it is not possible to simultaneously obtain strength and low-temperature toughness comparable to conventional steel sheets using intermediate quenching.

(Means for Solving the Problems) The present invention has been made to solve the above problems, and the present invention includes Ni: 7.5 to 10% by weight.
After hot rolling using a steel slab containing A, a first quenching treatment of heating above the ACl transformation point and cooling, a second quenching treatment of heating to an Ac1 to Ac3 transformation point and cooling, and Ac, In the manufacturing method of 9% Ni steel, which involves heating to below the transformation point and tempering treatment, the slab heating temperature before hot rolling is 800 to 900°C, and then the cumulative rolling reduction rate of 700 to 800°C is 50 to 800°C. 80% hot rolling, and after rolling, the #S first quenching treatment was performed as Ac
Plate thickness 40 characterized by heating to 3 to 850°C
A method for producing a thick 9% Ni steel with an excellent yield strength of mm or more, and a steel slab containing 7.5 to 10% Ni by weight, and cooling at a rate of 10°C/s or more immediately after hot rolling. In the method for producing 9% Ni steel, the steel is cooled at a high speed, then quenched at a temperature between Ac and AC1 transformation point, and tempered at a temperature of 1 or higher than Ac This is a method for producing thick 9% Ni steel with a thickness of 40+am or more and excellent yield strength, which is characterized by applying a cumulative reduction of 50 to 80% at a temperature of 850°C. When cooling at a cooling rate of 10° C./s or more immediately after hot rolling, the slab heating temperature before hot rolling is preferably 800 to 1000° C.

(Function) The decrease in yield strength of the intermediately hardened material is due to the presence of a large amount of stable precipitated austenite generated during the tempering process. Therefore, in order to increase the yield strength of steel materials that do not contain precipitation-strengthening elements, such as 9% Ni steel, without impairing toughness, it is necessary to raise the yield point by thoroughly reducing the ferrite grains, or increase the dislocation density. Measures such as strengthening by increasing the amount of water are necessary. The present invention provides specific means for achieving this goal.

Figure 1 shows a plate thickness of 40 mmf) C: 0.05%, Mn:
After heating a 9% Ni steel slab containing 0.57% Ni and 9.42% Ni at the slab heating temperature shown on the horizontal axis, it was heated to 800°C.
Those that have been rolled with a rolling reduction of 50% (・) and those that have been rolled with a finishing temperature of 900°C (○) are air cooled, then quenched at 800°C, and then quenched at 670°C. FIG. 3 is a diagram showing the yield strength at 1/4 of the plate thickness after intermediate quenching and tempering at 570°C. The transformation point of this steel is Ac: 620°C, Ac3 near 20°C.

When the slab heating temperature is in the range of 1050 to 1150℃, the cumulative reduction rate from 800℃ is 50% at the same slab heating temperature.
In contrast, when the slab heating temperature is 900°C or less, the cumulative rolling reduction rate below 800°C is 5%.
Steel with 0% hot rolling has a yield strength of 2 kgf.
/lllIn2 or more” is rising.

Slab heating temperature should be 900℃ or less and 800℃
By rolling 50% or more in the non-recrystallization temperature range below ℃, it is possible to prevent coarsening of heated austenite grains and to refine the austenite during rolling, and the microstructure in the as-rolled state is improved by the above treatment. The particles have become finer than those without.

The thus refined rolled structure is subjected to a quenching treatment in order to obtain a uniform martensitic structure. 9
%Ni steel has a low Ac3 transformation point and is usually so
Hardening treatment is carried out at a temperature of about 1000 m, but at this temperature the diffusion of alloying elements and the recrystallization of the austenite structure are slower than in ordinary low carbon steel. For this reason, the slab heating temperature is low,
By controlling the rolling temperature, a uniform, fine-grained martensitic structure can be obtained after quenching.

The uniform martensitic structure thus obtained is subjected to intermediate quenching and tempering.

As with many of the examples mentioned above, the intermediate quenching process promotes the concentration of alloying elements and impurity elements in partially formed austenite by heating in the two-phase region, and then quickly cools it to achieve high purity. The purpose of this process is to generate a mixed structure of ferrite and martensite with a high concentration of component elements, and during the subsequent tempering process, stable precipitated austenite is generated and the low-temperature toughness is significantly improved. In this case, the Ae of 9% Ni steel, the transformation point is 7
Since the temperature is about 20° C., the size of the crystal grains after the heat treatment also depends on the microstructure before the heat treatment for the reasons mentioned above even in the process of the intermediate quenching treatment.

For the above reasons, as shown in FIG. 1, the effects of slab heating and rolling greatly influence the quality of the material after the heat treatment of quenching, intermediate quenching, and tempering.

On the other hand, as a method to further enhance the effect of this rolling, it is possible to use a direct quenching treatment after rolling.

Figure 2 shows a 9% Ni steel slab with the same chemical composition heated at the slab heating temperature shown on the horizontal axis, then heated to 50°C from 850°C.
% cumulative reduction rate to a plate thickness of 4011, and immediately
Steel that was cooled to room temperature at a cooling rate of °C/s, then subjected to intermediate quenching at 670 °C and tempering at 570 °C (○)
, and the yield strength at 1/4 of the plate thickness of steel (・) of the same plate thickness, which was rolled at a finishing temperature of 950°C, cooled and heat treated under the above conditions. This is a diagram.

The yield strength of a steel plate with a finishing rolling temperature of 950℃ is 60kg.
Compared to this, the yield strength of a steel plate manufactured by controlling the cumulative reduction rate to 50% at 850'C or higher after heating at the same slab heating temperature is 2 kgf 7 mm2.
It shows a higher value than above. Furthermore, the same slab is 1000'
After heating to the CC temperature, a steel plate rolled at a cumulative reduction rate of 50% below 850°C has a rolling reduction rate of 4 kgf/mm.
Shows an excellent increase in yield strength of 2 or more.

This is because the refinement of austenite crystal grains during rolling by regulating the hot rolling temperature and rolling reduction has a large effect on the refinement of martensite obtained by cooling after rolling. This is because the subsequent cooling also has the effect of introducing into the martensite the working dislocations introduced during low-temperature rolling at 850°C or higher.

The thus obtained fine martensitic structure having a high dislocation density is subjected to intermediate quenching and tempering treatments. As mentioned above, the intermediate quenching treatment is performed to improve low-temperature toughness, and during the heating, martensite having a high dislocation density changes to a mixed structure of hepteraite and austenite. At that time, the degree of concentration of alloying elements into partially transformed austenite is controlled by the diffusion behavior of alloying elements in ferrite, but if the dislocation density of martensite, which is the precursor structure of 7-erite, is In this case, the recovery of dislocations is delayed compared to when the dislocation density is low, and therefore the remaining dislocations are thought to become channels for commercial diffusion of alloying elements. As a result of this phenomenon, the concentration of alloying elements into austenite is also affected by the dislocation density of the previous structure, martensite, resulting in austenite containing high alloying elements, which becomes more concentrated after intermediate quenching. It metamorphoses into fine martensite.

Furthermore, when the thus obtained mixed structure of 7 erites and martensite is annealed, austenite containing more concentrated alloying elements is produced.

The strength of austenite in which the alloying elements are more concentrated is higher than that in other austenite, and the strength as a mixed structure of ferrite and austenite is also increased by 1.

For the following reasons, by introducing work dislocations during low-temperature rolling, it is possible to promote the enrichment of alloying elements in the austenite after tempering, and as a result, even after performing intermediate quenching, the strength is higher. It becomes possible to manufacture steel sheets with

Considering this point of view, although the numerous examples cited above all provide methods for producing steel plates with excellent low-temperature toughness, they are not effective for producing thicker 9% Ni steel plates. .

Next, regarding the reason for limiting the components in the present invention, Ni is very effective in improving toughness and stabilizing austenite, and in order to ensure toughness at LNG temperature, 7.5% Ni is required.
Although it is necessary to add more than 10%, the effect is saturated even if added in a large amount, so the upper limit is set at 10%. There is no need to specifically limit the components other than Ni. However, in order to ensure a predetermined strength and prevent a decrease in toughness, C: 0.04 to 0.
．． 10%, Si: 0.10-0.50%, Mn: 0.4
-1.0%, sol, AI: 0.005-0.10%
, P: 0.015% or less, and S: 0.010% or less.

After steel containing the above-mentioned elements is melted in an electric furnace or a converter, a slab is manufactured using a continuous caster or an agglomeration method. The slab heating temperature before hot rolling was 800-900'C, and then 700-8'C.
Hot rolling with a cumulative pressure F ratio of 50 to 80% at 00°C,
A quenching treatment in which the material is heated to an AC3 transformation point to 850°C and cooled in water, an intermediate quenching treatment in which it is heated to an Ac1 to Ac3 transformation point and then cooled, and a tempering treatment in which it is heated to a temperature below the ACl transformation point and then cooled. , or after applying a cumulative pressure F of 50 to 80% at a temperature of 700 to 850 °C in hot rolling, immediately cool to room temperature at a cooling rate of 10 °C / s or more, and then heated between Ac1 and Ac3 transformation points. An intermediate quenching process in which the material is cooled and a tempering process at a temperature of Ac3 vA7α or higher are performed.

The regulation of the slab heating temperature prior to rolling was established to obtain finer grain size of the heated austenite grains, and
As shown in Figure 1, a temperature of 900°C or lower is effective in increasing the yield point, but if the temperature is lower than 800°C, the deformation resistance during hot rolling increases considerably and the shape of the steel sheet deteriorates. . In addition, after rolling at 850°C or less with a cumulative pressure F ratio of 50% or more, the intermediate quenching treatment involves immediately cooling at a rate of 10°C/s or more, and then heating and cooling between the Ac1 and Ac3 transformation points, and the Ac2 transformation point. When performing tempering treatment at the following temperatures, as shown in Figure 2, the temperature is usually 1250℃.
Since it shows high yield strength even in the range of slab heating, there is no need to regulate the rolling temperature, but preferably 800 to 1000.
When a slab heating temperature of 'C is selected, the increase in yield point becomes more significant. The lower limit value of the slab heating temperature in this case is based on the same reason as mentioned above.

The thus heated slab is hot rolled, but if it is subjected to quenching, intermediate quenching, and tempering treatments after rolling,
Rolling at 00 to 800°C with a cumulative reduction rate of 50 to 80%,
When cooling immediately after hot rolling at a cooling rate of 10° C./s, rolling is performed at a cumulative reduction rate of 50 to 80% at 700 to 850° C.

This hot rolling restriction is carried out in order to obtain refinement of the structure after rolling and an increase in dislocation density due to working. Therefore, in order to achieve this, when performing quenching treatment after rolling, the reduction rate is 50% or more from 800°C or less, and when cooling at a cooling rate of 10°C/s or more after rolling, 850°C
It is necessary to roll at a rolling ratio i' of 50% or more from the following temperature.

However, if rolling is carried out at a temperature below 700°C, the steel plate will bend during rolling, deteriorating the shape of the product.
Rolling should be performed at a temperature of 700°C or higher. Further, for the above-mentioned reasons, the pressure gauge ratio is required to be 50% or more, but if it exceeds 80%, anisotropy in the toughness value will occur due to the development of rolled set wires, which is not preferable as steel for LNG tanks.

After cooling the steel plate rolled in this way, a quenching treatment is performed in which the steel plate is heated to A C3'& temperature ~850°C and then cooled, Ac1~A
Intermediate quenching treatment of heating to c3 transformation point and cooling, Ac,
The transformed fish gear 1 is subjected to a tempering treatment by heating and cooling, or immediately after hot rolling, it is cooled at a cooling rate of 10° C./s or more, and then an intermediate quenching treatment is performed at a temperature between ACl and Ac3 transformation points, ACl 'Perform tempering treatment below the temperature point below the Ji state point. In the present invention, the ACl transformation point refers to the point at which contraction starts abruptly in the thermal expansion curve.

The reason for performing the quenching treatment is to obtain fine martensite as a structure before performing the intermediate quenching treatment, as described above, and it is necessary to heat the material above the Ac3 transformation point.
If the temperature is too high, the grain refining effect achieved by hot rolling will disappear due to recrystallization behavior.
The test is carried out at a temperature of 0° C. or lower. This fine martensitic structure can also be obtained by rapid cooling immediately after hot rolling. In this case, a preferable martensitic structure cannot be obtained unless the average cooling rate at the center of the plate thickness is 10° C./s or more.

Quenching treatment after rolling or immediately at 10°C after rolling
A steel plate cooled at a cooling rate of /s or more has A c + ~ A
An intermediate quenching process is performed in which the material is heated to the e3 transformation point and then quenched, followed by a tempering process in which it is heated at a temperature higher than the Ac+transformation point and then cooled.

As described above, the intermediate quenching treatment is performed to obtain a mixed structure of 7-erite and fine martensite, and therefore, it is necessary to generate a mixed structure of ferrite and fine austenite during heating. Therefore, the heating temperature is limited to a temperature between the Act-Acz transformation points, and at the same time, it is necessary to water-cool the material after heating.

The tempering treatment is performed in order to lower the dislocation density of the martensitic structure obtained in the intermediate quenching treatment, and at the same time precipitate a large amount of stable austenite during heating, thereby significantly improving the low-temperature toughness. Therefore, the temperature is in the temperature range of the A c l * state fish gear as is normally carried out.

The present invention has excellent low-temperature toughness of 9% due to intermediate quenching treatment.
By improving the strength of Ni steel, we have created a plate thickness of 4, which was previously impossible to manufacture.
This makes it possible to manufacture steel materials with excellent low-temperature toughness and strength of 0II1m or more.

(Example) The chemical composition of the test steel is shown in Table 2.

Here, steel A satisfies the requirement for Ni content, but steel B satisfies the requirement for Ni content.
It is a steel with low i content.

Also, t! Table S3 shows the tensile test results and DT test results when slabs having the chemical components shown in Table 2 were manufactured under the heating, rolling, and heat treatment conditions shown in Table 3.

Steels A1 to A5 manufactured by the method of the present invention have a frequency of 61kHz
exhibiting a yield strength of f/mm m2 or more and at the same time D
It has a strong brittle crack arrest performance indicated by T energy.

On the other hand, steels A6, A7, and All are comparative examples in which the steels are subjected to quenching treatment, intermediate quenching treatment, and tempering treatment after rolling.

The slab heating temperature for Steel 6 is 1000℃, and the rolling bite temperature for Steel A7 is 850℃, so even if appropriate quenching, intermediate quenching, and tempering are performed afterwards, the heating temperature is 59 to 60 kgf/ It showed a low yield strength of about mm2.

Although the slab heating and rolling conditions for All steel were appropriate, the quenching temperature was as high as 860°C, and therefore the yield strength was lower than that of the examples.

Steels A8, A9, and AIO are comparative examples in which they are cooled at 10° C./s or more immediately after rolling.

Steel A8 has a rolling biting temperature of 900℃, and steel A9
The cumulative rolling reduction rate below 850°C is as low as 30%.

Moreover, the cooling rate of steel AIO after rolling is as slow as 5° C./s.

Therefore, the yield strength of steel 8, A9 is 60 kg g f
/ tn tn 2 or less, which is lower than the example, and the yield strength of steel A10 is 62 and 3 kgf/mm2.
However, the DT energy value is 124 compared to the example.
kgf-m and low I/).

Steels B1 and B2 have a low Ni content of 7.12 wt%. Therefore, even if the hot rolling and heat treatment conditions are appropriate, the DT energy is significantly lower than in the example.

(Effects of the Invention) As described above, the method of the present invention has a plate thickness of 4.5 mm, which has a high yield strength and excellent low-temperature toughness that could not be obtained with the conventional method.
It is clear that this makes it possible to manufacture 9% Ni steel with a thickness of 0 mm or more, and greatly contributes to improving the safety of LNG storage tanks and the like.

[Brief explanation of drawings]

Figure 1 shows the relationship between slab heating temperature and 0.2% PS organized by rolling conditions. Figure 2 shows the relationship between slab heating temperature and 0.2% PS for materials water-cooled after rolling. It is a figure organized and shown by rolling conditions.

Claims (3)

[Claims] (1) Using a steel slab containing Ni: 7.5 to 10% by weight, after hot rolling, the first quenching treatment involves heating to Ac_3 transformation point or higher and cooling, and heating to Ac_1 to Ac_3 transformation point. Second quenching treatment and Ac_1
9% subjected to tempering treatment by heating below the transformation point and cooling
In the Ni steel manufacturing method, the slab heating temperature before hot rolling is 800 to 900°C,
Next, hot rolling is performed at 700 to 800°C with a cumulative reduction rate of 50 to 80%, and after rolling, the first quenching treatment is performed as A.
c_A method for producing thick-walled 9% Ni steel having an excellent yield strength and having a thickness of 40 mm or more, which is carried out by heating to 3 to 850°C. (2) Using a steel slab containing Ni: 7.5 to 10% by weight, immediately after hot rolling, it is cooled at a cooling rate of 10°C/s or more, and then quenched at a temperature between Ac_1 and Ac_3 transformation points. A method for producing 9% Ni steel in which the steel is subjected to a tempering treatment at a temperature below the Ac_1 transformation point, characterized in that during the hot rolling, a cumulative reduction of 50 to 80% is applied at a temperature of 700 to 850 ° C. A method for manufacturing thick-walled 9% Ni steel with excellent yield strength when the plate thickness is 40 mm or more. (3) Slab heating temperature before hot rolling is 800-1000
The thickness of the plate according to claim 2 is 40 mm.
The above method for producing thick 9% Ni steel with excellent yield strength.