JPS604886B2 - Manufacturing method of heat-treated 60Kg/mm↑2 grade high tensile strength steel with excellent COD value - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a high tensile strength steel having a steel quality of 60k9/side 2 and having an excellent COD value.

In addition to weldability, the quality characteristics of conventional heat-treated 60k9/skin grade 2 high-strength steel were mainly evaluated by tensile tests and Charpy impact tests, and the conditions of use were often determined based on these values.

However, in recent years, a method has been devised for evaluating the characteristics of cell fracture occurrence using the COD value of a three-point notch bending test, and it is becoming widely popular due to the simplicity of the test method. This COD test method is standardized in detail in British Standard 5762, and generally COD is based on this standard.
The D value is being measured. On the other hand, recently, 60 kg/yanagi grade 2 high tensile strength steel has been used for reactor containment vessels in nuclear power plants.

The usage conditions for this steel type, which belongs to JIS G 3115 SPV 5, for nuclear reactors include the conventional Charpy impact test and gravity test, as well as the excellent CO
A D value is required, and the Ministry of International Trade and Industry requires that the limit COD value be on the 0.2 side or higher at the lowest operating temperature as a special condition. In order to increase the COD value of heat-treated 60k9/skin grade 2 high-strength steel, it is generally necessary to increase the ballability of the material, but it is not always the case that the COD value of a material with good Charpy impact value or Fushimi weight test is high. It was not always good, and it was extremely difficult to obtain an excellent COD value, especially in thick plates with a thickness of 50 or more.

An object of the present invention is to provide a method for producing high-strength steel having a tensile strength of 60 [9/2nd class] and exhibiting an excellent COD value even in thick plates of 50 to 10 m.

This object of the present invention is achieved by the invention as summarized below. That is, in terms of weight ratio, C: 0.15% or less, Si: 0
．． 10-0.40%, Mh: 1.0-1.5%, P: 0
．． 025% or less, S: 0.015% or less, Cu: 0.2
0% or less, Ni: 0.15-1.0%, Cr: 0.30
% or less, Mo: 0.05 to 0.25%, V: 0.02 to
0.05%, T. A〆: 0.010-0.030%, N
: 0.0060 to 0.0120%, and the balance is substantially Fe.
a step of heating and hot rolling, and a step of 5 after the hot rolling
A step of cooling at an average cooling rate of ℃/min or more, a step of heating and quenching to a temperature range of 880 to 93,030 degrees Celsius after the cooling step, and a step of quenching to a temperature of 580 to 700 degrees Celsius after the filling step.
This is a method for manufacturing grade 2 high tensile strength steel with an excellent COD value of 60 kg/side, which comprises the steps of firing at a temperature range of 0.05 m.

In other words, the present inventors found that the COD value of heat-treated 60k9/side grade 2 high-strength steel has a high correlation with the grain size, and regardless of whether the steel plate is manufactured by quenching or tempering, A 60k9/grade 2 high tensile strength steel with a good COD value was obtained by making the material into fine polygonal ferrite, and for this purpose, the chemical composition range and manufacturing conditions of the steel were limited as follows. It is something. First, the reason for limiting the chemical composition of the steel of the present invention is as follows.

C: A lower C content is preferable for the purpose of the present invention, but the C content is required to ensure minimum strength depending on the plate thickness.

However, if it exceeds 0.15%, the structure after quenching becomes a bainite structure in the surface layer, making it difficult to obtain fine polygonal ferrite and impairing weldability, so it was limited to 0.15% or less. Si: Si requires at least 0.10% for deoxidation, but 0.
．． If it exceeds 40%, it will have a negative effect on the quality, so 0.40
% range.

Mn: Mn is a useful element for improving strength without impairing rutting properties, and therefore requires at least 1.0%, but 1.5%
If it exceeds 1.0%, weldability deteriorates, so it is set in the range of 1.0 to 1.5%.

P, S: P and S are harmful elements, especially harmful to the uniformity and weldability of sardines, so 0.025% or less and 0.015%, respectively.
% or less.

CuCr: Both are effective in increasing the strength of plates with a thickness of 50 to 10 mm, but if Cu exceeds 0.20% and Cr exceeds 0.30%, it will have a negative effect on weldability. 0 each
．． The upper limits were set at 20% and 0.30%.

Ni: At least 0.15% of Ni is essential in order to increase the toughness of the matrix and ensure strength in this type of steel, but since it is an expensive element, it should be kept as the minimum amount to ensure a sufficient COD value. The upper limit is 1.0%, and 0.1
It was limited to a range of 5 to 1.0%.

It is necessary to add the optimum amount depending on the plate thickness. Mo, V: At least 0.05% Mo to increase burning resistance
, V is required to be 0.02%, and in particular, this type of steel has a polygonal ferrite structure in Sabairi, which is lower than the conventional 60 [9/side grade 2 steel]. It is characterized by its strength, with a yield strength of 50k9 even after resetting or stress-relieving annealing.
/ fence 2 or more, tensile strength 62k9 / rib 2 or more, it is necessary as an element to provide sufficient reversal resistance, but it is an expensive element, and 0.25% and 0, respectively.
．． Addition of more than 0.05% has little effect, so if Mo is added more than 0.05%, the effect will be small.
05 to 0.25%, V is 0.02 to o. The range was 0.05%.

A: N: T. A content: 0.010-0.030%, N: 0.006%
The steel of the present invention is characterized by having a content of 0 to 0.0120%.

A and N have the effect of forming A and N, refining austenite crystal grains, and refining the ferrite structure after competition. In other words, the generation of bainite is suppressed at dawn,
Addition of an appropriate amount is absolutely necessary to produce polygonal ferrite, which is the object of the present invention. Usually this kind of tempering6
0k9/Yanagi grade 2 high-strength steel is melted in a converter, but the absolute amount of AsoN produced is determined by the amount of N, and in order to obtain austenite pongee grains from AsoN, at least NO is required.
．． 0.0060% is required, but if it exceeds 0.0120%, weldability will be impaired, so the amount of N should be 0.0060 to 0.012%.
The range was 0%. In addition, in order to maximize the grain refining effect of AsoN, it is best to dissolve AsoN into solid solution by heating the slab and precipitate it as fine AsoN during heating with ants. be. When the total A content is less than 0.010%, this grain refining effect disappears, and when it exceeds 0.030%, the solidification temperature of AsoN rises, and A〆N is completely solidified during slab heating. The upper limit is 0.030% because it becomes difficult to remove the excess A that is not fixed by N, which tends to cause coarsening of austenite grains, and increases the bainite formation temperature, which causes damage to toughness. and 0.
The range was 0.010% to 0.030%. Next, the reasons for limitations in the manufacturing process of the steel of the present invention will be described.

First, the slab is heated to 1200°C to 1250qo, then hot rolled and cooled at an average cooling rate of 5°C/min or more.For thick plates of low cape or higher, the cooling rate is 5°C.
/min, resulting in fine A/N during heating.
The cooling rate was set to an average of 5° C./min or more because precipitation of

However, if the thickness is 100 mm or less, this cooling rate can be achieved even with air cooling. Next, the heat treatment is usually done by heating the steel plate in a hearth roller type non-oxidizing continuous heat treatment furnace, quenching it in a roller quench type competitive heating device, and then subjecting it to a heat treatment at the optimum temperature. What is important here is the temperature at dawn and the cooling rate at which the ferrite grains are injected.The faster the cooling rate is, the better, in order to make the ferrite grains finer.For this reason, it is difficult to obtain the desired performance with devices other than those described above. The slab heating temperature was limited to 1200qo to 1250oo because
In order to ensure solid solution of AsoN within the content range of AsoN and N specified in the present invention, it is necessary to heat the slab to 1200qC or more, and also to ensure that AZN is solidified.
From the perspective of improving the COD value, there is no particular need to regulate the upper limit of the heating temperature of the slab, but heating above 1250° will have an adverse effect on the surface quality due to scale eaves, so the slab heating temperature is usually set at 1250°. Never exceed the temperature.

The competition temperature was limited to 880 to 930 pm.
When heated at less than 0.00000, the Soniri effect does not appear, and at 930oo
When heated at high temperatures exceeding
880-930q as it is difficult to obtain a fine ferrite structure.
It was limited to a temperature range of o.

In addition, the competitive return temperature was limited to 580 to 700 qo.
This is because the 80 qo heat flow does not produce the dawn effect, and the ferrite temperature exceeding 700 qo may cause the fine ferrite grains to become coarse. Examples Tables 1 and 2 show the chemical composition and manufacturing conditions of the steel of the present invention and the conventional Taishi 60k9/Yanagi grade 2 high tensile strength steel for comparison.

Their mechanical properties are shown in Table 3.

In addition, the sample collection position for measuring mechanical properties in Table 3 is No. 1.
Table 2 Table 3 The surface layer was measured perpendicular to the rolling direction at 1/4 part of the plate thickness.

As is clear from Table 3, the results of the Charpy impact test and the gravity test of the steel of the present invention show extremely excellent values, and in particular, the results of the Fuki gravity test show that the NDT is significantly lower than that of the comparative steel.
Shows temperature. The COD test was conducted using a notched three-point bending test piece according to the method of ASTM E399.

From the results, the limit COD was determined using DDI9 of the BS standard (British standard), and the temperature dependence of the limit COD is shown in the attached drawing. As is clear from this figure, there is a clear difference in the limit COD value between the steel of the present invention and the comparative steel, and it can be seen that the steel of the present invention has extremely excellent brittle fracture resistance. When using tempered 60k9/class 2 high tensile strength steel as a reactor containment vessel, excellent durability is required, specifically vE-25q○≧2.8k9 at -8℃, mTNoT
: It is necessary to satisfy the limit COD value 6c -8℃ ≧ 0.2 degrees below -25℃, but among these, the Charpy impact test value and the Fushimi weight test value can be satisfied relatively easily, but the board of 50 skin or more In the case of thick plates, it is difficult to satisfy the limit COD value with conventional comparison steels, but as is clear from the results shown in the attached diagram, it is possible to easily satisfy the limit COD value with the steel of the present invention. .

As is clear from the above examples, the steel of the present invention has a limited chemical composition, especially T. A 0.010-0.030%, N
By setting the austenite grains to 0.0060 to 0.0120% in A〆N and suppressing the formation of bainite during scale incorporation to form polygonal ferrite, other Ni, Mo, V, etc. Thermal treatment 60k9/ has an excellent COD value in combination with effectiveness and appropriate heat treatment conditions.
Although Yanagi grade 2 high tensile strength steel could be obtained, the effects of the steel of the present invention can be summarized as follows.

[A] The steel of the present invention has an excellent limit COO value, and can be used not only for reactor containment vessels, but also for use at lower temperatures and for low-temperature pressure vessels.

'Row The steel of the present invention has high rutting properties due to its fine polygonal ferrite structure, so the amount of expensive alloying elements such as Ni, Mo, and V used is small, making it not only economically efficient but also easy to weld. It is also advantageous in terms of

[Brief explanation of the drawing]

The attached drawing is a correlation diagram comparing the temperature dependence of the critical COD value of the steel of the present invention with that of a conventional comparative steel.

Claims (1)

[Claims] 1 C: 0.15% or less, Si: 0.10~ by weight ratio
0.40%, Mn: 1.0-1.5%, P: 0.025
% or less, S: 0.015% or less, Cu: 0.20% or less, Ni: 0.15 to 1.0%, Cr: 0.30% or less,
Mo: 0.05-0.25%, V: 0.02-0.05
%, T. Al: 0.010-0.030%, N: 0.0
060 to 0.0120%, with the remainder being substantially Fe.
A step of heating and hot rolling steel having a composition of 1200°C to 1250°C;
a step of cooling at an average cooling rate of n or more; a step of heating and quenching to a temperature range of 880 to 930°C after the end of the cooling step; and a step of quenching in a temperature range of 580 to 700°C after the quenching step. C, characterized by comprising the step of returning
A method for producing heat-treated 60kg/mm^2 class high tensile strength steel with an excellent OD value.