JPS6112820A - Manufacture of low-alloy nickel steel having high strength and toughness - Google Patents

Abstract

PURPOSE:To manufacture a low-alloy Ni steel having 80-110kg/mm.<2> tensile strength and high toughness by subjecting a low-alloy Ni steel slab to soln. heat treatment, hot rolling under specified conditions, rapid cooling and tempering. CONSTITUTION:A low-alloy steel slab contg. 1.5-8wt% Ni is subjected to soln. heat treatment. The slab is hot rolled at 900-1,150 deg.C heating temp. and 700- 1,000 deg.C finishing temp., and it is rapidly cooled at once and then tempered at the A1 point or below. The desired low-alloy Ni steel is obtd. by combining soln. heat treatment with thermo-mechanical treatment as mentioned above.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for producing Ni-containing low alloy steel having excellent strength and toughness.

(Conventional technology) Liquefied natural gas (LNG) and liquefied petroleum gas (LPG) are
As it is a high-calorie, non-polluting energy source, and from the perspective of energy diversification, its demand is increasing year by year, and new LNG or LPG storage tanks are being built or expanded one after another. In addition, interest in the deep sea is rapidly increasing, such as undersea resource development and undersea crustal geological surveys, leading to the construction of offshore structures that will lead to this undersea development, and the construction of high-head penstocks for pumped storage power generation to adjust nighttime surplus power from nuclear and thermal power generation. The construction of energy-related steel structures is active, and this trend is expected to continue. Furthermore, in order to ensure safety, there is a demand for the development of steel with excellent strength and toughness for the materials used in these materials.

In order to meet the strong demands from the demand side for safer and more reliable materials, the steel industry has been working to improve the quality of Ni-containing low alloy steel, which has long been known as a low-temperature steel and has a proven track record. ing. For example, Ni-containing steel material after hot rolling is heated to a temperature higher than the A3 transformation point and quenched, and then A
□Special Publication No. 55-1208 to solve the unstable toughness of steel obtained by the conventional manufacturing method of tempering at a temperature below the transformation point.
3, a method for producing Ni-containing low-temperature steel that specifies the austenite grain size and upper bainite formation ratio of a hot-rolled steel sheet before quenching, and JP-A-55-104427, a method for manufacturing low-temperature steel, 1100-A3 slab of Ni-containing low alloy steel is a manufacturing method that provides steel with excellent strength.
A method for producing Ni-containing low-temperature steel, which is rolled in the temperature range of the transformation point, then quenched by heating and cooling to the temperature range between A3 and A□ transformation points, and then tempered at a temperature lower than A1 transformation. Furthermore, many types of manufacturing methods have been developed, including methods for manufacturing highly tough quenched and tempered Ni-containing steel treated with extremely low phosphorus and extremely low sulfur. Any manufacturing method is effective in improving toughness.

LNG storage tanks and offshore structures must not be deformed or destroyed by pressure or stress.Therefore, with safety as the top priority, Ni-containing low alloy steel with further improved strength and toughness has been developed. is desired to be used.

(Purpose, Structure, and Effects of the Invention) The present inventors have investigated manufacturing methods for steel with various compositions with the aim of developing high-strength Ni-containing low-alloy steel with further improved toughness. We discovered that the desired copper can be produced by combining processing and heat treatment. The present invention was constructed based on the knowledge of such facts, and the gist thereof is to perform intermediate rolling in which Ni-containing low alloy steel slabs are subjected to solution treatment or 1) heating to solution temperature, rolling, and immediately quenching. After applying, the heating temperature was increased to 900-11
A method for producing high-strength, high-toughness Ni-containing low-alloy steel that is hot-rolled at 50°C with a finishing temperature of 700-1000°C, rapidly cooled immediately after the completion of rolling, and then tempered at a temperature below the A□ point. be.゛The present invention will be explained in detail below.

N melted using a normal melting furnace such as a converter or electric furnace
A slab is manufactured from molten steel having an i-containing low-alloy steel composition by a continuous casting method or an ingot blooming method. Ni in steel reduces stacking fault energy, increases cross-over, facilitates stress relaxation, increases impact absorption energy, and improves steel toughness. This effect becomes remarkable when the Ni content is 1.5% or more. In addition, Ni is a component that improves hardenability and increases strength, and is included depending on the required strength and toughness of the steel. Based on the knowledge that reheated austenite grains become finer by making the structure after rapid cooling after rolling, that is, before reheating for the next step of hot rolling, into a structure mainly composed of bainite, we determined the Ni content. In particular, it is specified to be 1.5% or more. Moreover, the upper limit was set to less than 8%.゛The reason is 8%
The above range is defined in consideration of economical efficiency, since this is the region where the effect of Ni in the process of the present invention is saturated. This ultimately results in high strength and high toughness. Furthermore, in the present invention, low alloy steel does not particularly limit its steel composition; for example, as described in the above-mentioned patent publication, steel containing 0.2% or less carbon may contain Mn, Cr, Mo, and Cu.

Strength and toughness improving components such as V, Nb, Ta, Ti, Al, B, and Ca are selected according to the required properties and are contained in the necessary amounts.

The Ni-containing low alloy steel slab produced in this way is solution treated before hot rolling. This solution treatment can also be replaced by soaking heating during the blooming process. In that case, it is rapidly cooled after blooming.

Solution treatment is a process in which coarse precipitates such as carbon, nitrides and intermetallic compounds generated during slow cooling during slab manufacturing are dissolved in the austenite matrix, which significantly improves the strength and toughness of the steel after tempering. It is something that can be improved. Further, in the present invention, in order to improve toughness, intermediate rolling is performed immediately after heating to the solution temperature and holding, or immediately after rolling, in which the material is rapidly cooled at an average cooling rate of 10° C./min or more. This step is also effective in forming many fine carbonitrides during reheating in the next step and refining heated austenite grains. Figure 1 shows a Ni-containing low alloy steel slab (C: 0.1%
, Si: 0.24%, Mn: 0.60%,
P: 0.008%.

The figure shows the impact value at low temperature of steel manufactured under various manufacturing conditions (S: 0.001%, Ni near, 94%). That is, the impact value of steel A, which has been subjected to intermediate rolling in which it is hot rolled after solution treatment and then rapidly cooled, is higher than that of steel B, which has only been subjected to solution treatment. The reason for this is thought to be that the grains become finer due to rolling.

However, when steel d is air-cooled after intermediate rolling, coarse carbonitrides precipitate during the air-cooling process, and the crystal grains are also coarser than in steel A, making it impossible to obtain a high impact value.

The solution-treated slab or the intermediate-rolled slab as described above is reheated and then hot rolled.

This refinement of the heated austenite grains during reheating is important for refining the structure in the final stage, which is why the above slab pretreatment is applied to slabs with a low transformation point containing 1.5% or more Ni. This can be achieved more effectively. Hot rolling is performed at a heating temperature of 900 to 1,000 ℃ in order to finish the shape and thickness of the product and give it high strength and toughness.
Rolling is carried out at 150° C. and a finishing temperature of 700 to 1000° C. After rolling is completed, it is necessary to rapidly cool the material to refine the grains and obtain a quenched structure (martensite, martensa, ito+bainite structure).

In addition, the heating temperature in this case must be a temperature at which fine heated austenite grains do not become coarse due to carbonitrides finely precipitated during reheating due to the solution treatment of the slab in the previous step, and for this reason, the heating temperature must exceed 1150°C. High heating temperatures are not preferred. Further, at a low temperature of less than 900° C., the deformation resistance during hot rolling is large and the rolled shape is poor, making it unsuitable as an industrial product. Therefore, the heating temperature range is 900
~1150°C.

In addition, the finishing temperature in hot rolling is specified in order to refine the austenite grains and at the same time obtain a quenched structure when rapidly cooled. Quench hardening is insufficient because pro-eutectoid ferrite begins to precipitate. On the other hand, 1000℃
If the temperature exceeds 100, the grains cannot be refined by rolling, resulting in coarse grains. The Ni-containing low alloy steel hot-rolled in this manner has extremely high strength due to fine martensite or martensite + bainite. However, the toughness is insufficient as it is, and in order to impart toughness, a tempering treatment is performed at a temperature below the A1 point.

Next, examples of the present invention will be described.

(Example) The chemical composition of the steel materials used is shown in Table 1. These steel materials were treated according to the present invention and the comparative method as shown in Table 2. Its mechanical properties are shown in Table 3. From these results, it is clear that according to the present invention, superior strength and toughness can be obtained compared to the comparative method. The austenite grain size of the steel produced by the method of the present invention is finer than that of the comparative method, which works effectively to improve toughness.

[Brief explanation of the drawing]

〃De1μ Figure 5-1 shows the absorbed energy (kg-m
). Kaya 1. Written amendment of national procedure 1. Indication of the case 1982 Patent Application No. 133527 2 Name of the invention Process for manufacturing high strength and high toughness Ni-containing low alloy steel 3 Person making the amendment Relationship to the case Patent applicant address 2-chome Otemachi, Chiyoda-ku, Tokyo 6 No. 3 name
(665) Nippon Steel Corporation Representative Takeshi 1) Yutaka 4, Agent 105 Tomb (503) 4877 Address 1-12-1 Nishi-Shinbashi, Minato-ku, Tokyo 8th Floor Voluntary 6, Mori Building 1, Subject to amendment Detailed explanation of the invention in the specification column 7, contents of the amendment Table 1 on page 10 of the specification, Table 2 on page 11, Table 3 on page 12
Correct the table as shown in the attached sheet.

Claims (2)

[Claims] (1) After solution-treating a low-alloy steel slab containing 1.5% to less than 8% Ni by weight, the heating temperature is 900 to 900℃.
80kg/mm^ characterized by hot rolling at 1150℃ with a finishing temperature of 700 to 1000℃, rapid cooling immediately after the completion of rolling, and then tempering at a temperature below A_1 point.
A method for producing high toughness Ni-containing low alloy steel having a tensile strength of 2 or more and 110 kg/mm^2 or less. (2) A low-alloy steel slab containing 1.5% or more and less than 8% Ni by weight is heated to a solution temperature, then rolled and immediately rapidly cooled. After performing intermediate rolling, the heating temperature is increased to 900 to
80kg/mm^ characterized by hot rolling at 1150℃ with a finishing temperature of 700 to 1000℃, rapid cooling immediately after the completion of rolling, and then tempering at a temperature below A_1 point.
A method for producing high toughness Ni-containing low alloy steel having a tensile strength of 2 or more and 110 kg/mm^2 or less.