JPS5861221A - Manufacture of high strength and high toughness steel - Google Patents

Abstract

PURPOSE:To obtain a steel capable of showing its strength and toughness to the utmost limits by specially combining ght Ceq of a steel with the draft by hot working and the cooling conditions producing a heat recuperating effect. CONSTITUTION:A steel with <=0.45% Ceq is hot worked at >=40% draft and <=900 deg.C. The worked steel is cooled at 5-40 deg.C/sec cooling rate to a temp. from which the surface temp. of the steel recuperates to 150-500 deg.C within 1min after stopping the cooling, and after carrying out heat recuperative treatment, it is cooled. The Ceq value is obtd. by the equation. A steel slab with <=0.45% Ceq is hot worked to form fine ferrite grains and to attain the purpose. Said temp. and draft are limit values at which the effect is noticeably recognized. When the cooling rate is below the lower limit, the strength is remarkably reduced, and when the cooling rate exceeds the upper limit, the strength is not increased. By stopping the cooling once during the cooling to ordinary temp. and by regulating the surface temp. of the steel to said temp. range after 1min, the strength and toughness can be considerably improved.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing steel (steel materials) such as steel plates, steel pipes, and shaped steels with high strength and toughness by processing and reheating treatment.

Steel with high strength and toughness is in high demand in all fields such as civil engineering, architecture, and shipbuilding because it ensures a high level of safety. In particular, accidents that threaten human life, such as damage to tankers and the collapse of low-temperature tanks, as well as changes in the energy situation, will lead to an increase in the number of these structures and demand for high-strength, high-toughness steel for large-sized buildings will continue to grow. It's getting bigger. Conventionally, high-strength, high-toughness steels have been developed by adjusting the alloying elements of the steel and adjusting the structure, shape, and distribution of precipitates through heat treatment. Recently, the JIJ heat treatment method (which combines the effects of processing and heat treatment) has been developed as an effective means of increasing the strength of steel while maintaining its toughness, and is now being used to manufacture steel. However, this steel is produced by reheating the steel material to a high temperature above the A3 transformation point after hot working, processing and quenching, and then subjecting it to tempering at a high temperature of 550°C or above. It was considered difficult to achieve high strength.

The present inventors aimed to explore a manufacturing method that maximizes the strength and toughness that steel is proud of.
q, - As a result of investigating factors related to steel manufacturing such as hot working conditions, cooling conditions, and heat treatment methods, it was found that C
It was found that by combining the cooling conditions within an appropriate range, steel for the desired purpose can be produced.

The present invention was constructed based on this knowledge, and its gist is that after hot working steel with a Ceq of 0.45% or less at a temperature of 90011 m or less and a rolling reduction of 40% or more, the cooling rate is 5 to 5%. b After stopping for 1 minute, the surface temperature of the steel is 150~5
This is a method for manufacturing high-strength, high-toughness steel in which the steel is cooled to a temperature that regenerates to 00°C, and then cooled after the reheating treatment.

The present invention will be explained in detail below.

Steel slabs with Ceq: 0.45% or less are produced by bath-making in a converter, air furnace, etc., and by a continuous casting method or an ingot-blowing method. Ceq is a numerical value determined by the following formula.

As shown in FIG. 1, this is a range in which a high degree of toughness can be obtained in the manufacturing method of the present invention.

FIG. 1 is a graph of rolling conditions at 900° C. or lower and a rolling reduction of 60 inches. ΔvTrs on the vertical axis was determined by the following formula.

△vTrs = (“v when cooled to room temperature after rolling
Trs) - (Cooling after rolling (30°C/sec); surface temperature 300°C after 1 minute of cooling stop vTrs) Also, although the composition of the steel is not specified in the present invention, the purpose of the present invention The most preferable steel is C: 0.02-0.1
5%, Si: 0.5% or less, Mn: 05-2.0%, P
:0.030% or less, S:0010% or less, Al:0.
Cr: 0.5% or less, MO
20. 5% or less, Ni: 5.0% or less, Cu: 0.5%
Below, Nb and V: 0.1% or less, B: Q, 00
50% or less, N: 200 ppm or less, Ca, Ce
, Mg: 0.015% or less of one or more types can be selected and included.

Steel slabs manufactured with Ceq: 0.45% or less are heated to high temperatures and hot-processed into steel plates, steel pipes, sections, etc. In order to maximize the toughness and strength that we are proud of, the temperature is 90℃.
Hot working is performed at a temperature of 0°C or lower with a rolling reduction of 40% or more. The temperature and reduction rate in this case are the limit values at which the effect is noticeable.

Thus, the hot worked steel is cooled at a cooling rate that does not reduce its strength. Regarding the cooling rate, in the manufacturing method of the present invention, the cooling rate after hot working is shown in Figure 2.
Various cooling rates (average cooling rates) between temperatures of 5°C and 5°C show an effect that does not reach the yield point, so slow speeds of less than 5°C/see significantly reduce the strength, and excessive cooling rates exceeding 40°C/see No increase in strength is observed at speeds of .

In addition, this cooling is not performed at room temperature 24°C, but during cooling, the cooling is stopped once, and after 1 minute has elapsed, the steel is cooled to a temperature at which the surface temperature of the steel reheats to 150 to 500°C, and then the steel is reheated. Then cool to room temperature at any rate. As shown in Figure 3, this recuperation effect can be achieved at a recuperation temperature of 150 to 500℃.
Significantly improves strength and toughness within the range of .

In Figure 3, when t = 25 mm, the rolling reduction of 60 inches at 900°C or less is represented as a, and the rolling reduction of 10 inches at 900°C or less is represented as b. The horizontal axis is water cooling (cooling rate 30℃/sec)
This is the plate surface temperature one minute after stopping. H indicates water cooling after rolling.

The reason for this is, according to the inventors' speculation, that the strength of the water-cooled material after hot rolling is lower than that of the reheated material is because martensite or a similar quenched structure forms during water cooling after rolling. This is because the ferrite expands during transformation, gives strain to the inside of the ferrite, and introduces mobile dislocations. On the other hand, the reason why the strength increases when cooling is stopped is because mobile dislocations are fixed, and the recovery of the hardened structure is considered to be a major reason why the toughness improves.

Next, an example will be described.

Table 1 shows the chemical components, and Table 2 shows the test results. As is clear from the above test results, the steel obtained by the method of the present invention exhibits superior properties compared to the conventional method.

Beg□1. CQ i 1 e') , , ---
−” −’, “One step JP-A-1987
8-61221 (3) A-1, B-1, C-1, D-1
(Steel of the present invention) 1000℃ heating - finishing temperature 820℃ (90℃
(50% pressure below 0°C) - Water cooling (30C/see) - The surface temperature was set to 350°C one minute after leaving the water cooling zone.

A-2, B-2 (comparative steel) The surface temperature K after water cooling was set to room temperature.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between carbon equivalent and toughness, Figure 2 is a diagram showing the relationship between 0.2% proof stress and average cooling speed, and Figure 3 is a diagram showing the relationship between toughness and 0.2% proof stress.
It is a relationship diagram between % proof stress and board surface temperature. 1st Vice Ce1 (o10)

Claims (1)

[Claims] Ceq: After hot working steel with a content of 0.45% or less at a temperature of 900°C or less and a reduction rate of 40% or more, the cooling rate is 5 to 40°C, and after 1 minute has passed after stopping cooling, A method for producing high-strength, high-toughness steel, which comprises cooling the steel to a temperature at which the surface temperature of the steel regenerates from 150 to 500°C, and then cooling after reheating treatment.