JPH02282417A - Production of 40kg/mm2 high toughness steel excellent in ssc resistance - Google Patents

Abstract

PURPOSE:To produce the high-toughness steel with the SSC resistance improved even if a nonmetallic inclusion is present and having high joint strength by controlling the steel composition, working condition after hot rolling, heat- treating condition, etc. CONSTITUTION:A billet contg., by weight, 0.05-0.35% C, 0.01-0.5% Si, 0.15-1.5% Mn, <=0.01% S, <=0.02% P, 0.005-0.1% Al 0.0020-0.0250% N and the balance Fe is hot-rolled. The hot-rolled steel is normalized, as required, and then worked below the Ac1 point at 0.5-10.0% draft, or the steel is further aged at 100-400 deg.C. Consequently, microscopic plastic deformation is controlled by the strengthening of the ferrite structure even if the nonmetallic inclusion, deposit, pearlite, etc., are present, the SSC resistance is improved, and a 40kg/mm<2> high-toughness steel is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for producing high-toughness 40 kg class steel materials such as steel plates, steel pipes, and shaped steels having excellent SSC resistance.

(Prior art) Steel materials with sulfide stress cracking resistance (hereinafter referred to as SSC resistance) are made of nonmetallic materials such as MnS, fiJz (h, etc.), which cause SSC in iron and M'80-3503 Reduction of inclusions, “Japan Steel Pipe Technical Report No. 87 (1)
9B? ) J is reported to be manufactured by homogenizing the structure by QT treatment. However, strength 4
Since QT-treated steel below 0 kgf/- has a relatively low C equivalent, the heat-affected zone of the joint weld tends to soften (which may cause problems in the design of structures.On the other hand, the occurrence of SSC can be prevented by the present invention). According to their speculations, this is thought to be caused by hydrogen embrittlement at the interface between ferrite and non-metallic inclusions under stress.In other words, hydrogen generated by the corrosion reaction of the steel penetrates into the steel. Hydrogen molecules are formed at the interface with nonmetallic inclusions, and the resulting pressure generates a stress concentration region in the matrix. In such a region, the hydrogen concentration increases and the matrix becomes brittle. In this case, the presence of eccentric force It is determined below that if the strength of ferrite is extremely lower than that of pearlite, the microscopic plastic deformation of ferrite will increase and hydrogen embrittlement will be accelerated.

However, high toughness steel with excellent SSC resistance is required for 40kg steel materials used in sulfide corrosive environments, but the manufacturing method requires the reduction of non-metallic inclusions, which are troublesome. The problem was in the steel manufacturing operation.

(Problems to be Solved by the Invention) The object of the present invention is to produce a 40 kg class steel material with excellent SSC resistance, without taking into account the above-mentioned problems in producing SSC-resistant high-toughness steel materials. The object of the present invention is to provide a method for purposefully manufacturing tough steel materials.

(Means for Solving the Problems) As a result of many experiments and studies aimed at producing high-toughness 40 kg-class steel materials with excellent SSC resistance, the present invention has been developed based on the results of many experiments and studies on the composition of steel, after hot rolling, or after hot rolling. Processing conditions after normalizing,
By controlling the heat treatment conditions, etc., even in the presence of nonmetallic inclusions, precipitates, pearlite, etc., the above-mentioned microscopic plastic deformation is controlled by strengthening the ferrite structure, and SSC resistance is significantly improved. It was discovered that a high-toughness steel material with high joint strength can be produced.

The present invention was constructed based on this knowledge, and the gist thereof is: C: 0.05 to 0.35%, Si: 0.
01-0.5%, Mn: 0.15-1.5%, s
: 0.01% or less, P: 0.02% or less, A7: 0
．． 005~0.1%, N: 0.0020~0.0
250% or further Nb: 0.00
5-0.1%, Mo: 0.05-0.4%, C
r: 0.1-1.5%.

Ni: 0.1-2.0%, V: 0.01-0.1%,
Ti: 0.01-0.1%, B: 0.000
3-0.003%, REM (rare earth element) 70.0
01-0.05%, Ca: 0.001-0.02
%.

After hot rolling a steel piece containing one or more of Cu: 0.1 to 0.5% and the remainder substantially consisting of Fe, or after further skewering treatment, a reduction rate of 0.5 to 10.0 % processing is carried out at a temperature below the Ac1 point of the steel, or further
This is a method for producing high-toughness 40 kg class steel with excellent SSC resistance, which is characterized by performing aging treatment at a temperature of 00 to 400°C.

(effect) Hereinafter, the manufacturing method of the present invention will be explained in detail.

The reason why the present invention is limited to the above-mentioned steel components will be explained first.

C is a component necessary for strengthening steel, and if it is too little, the strength of the welded joint will be reduced, and if it is too much, hardening due to working will increase and the toughness will drop significantly. Further, since it has an adverse effect on weldability, it is set at 0.05 to 0.35%.

S4 is a necessary component for increasing the strength due to the solid solution effect. However, if it is too small, it will not be effective, and if it is too large, it will cause deterioration of toughness and have a negative effect on weldability.

Mn is an effective component that refines crystal grains and improves toughness, but if it is too small, it has no effect, and if it is too large, it creates low-temperature transformation products and deteriorates toughness. Therefore, the Mn content was set to 0.15 to 1.5%.

P segregates into pearlite to promote SSC generation and improve S resistance.
Since it is a harmful component that significantly deteriorates SC properties, its content was set to 0.02% or less.

S produces stretched sulfides, promotes the generation of SSC, and significantly deteriorates SSC resistance, so the upper limit should be set at 0.0.
It was set to 1% or less.

An increase in the M content produces cluster-like AL (h), which significantly deteriorates SSC resistance, so its content is reduced to o, o.
os~0.1%.

N is an important element that diffuses and adheres to dislocations generated by processing and strengthens the ferrite. 0.0020 because if it is too small, the effect will not be effective, and if it is too large, the effect will be saturated.
~0.0250%.

According to the present invention, the steel having the above-mentioned composition becomes a starting material for manufacturing a high-toughness steel material having excellent SSC resistance and high joint strength.

In addition to the above-mentioned components, the starting materials include Cu and REM.

SSC resistance improving components such as Ca, Nb, Mo, Cr,
Ni, V.

Small amounts of reinforcing components such as Ti, B, etc. may optionally be included.

Cu has the effect of suppressing hydrogen penetration into steel and works effectively to improve SSC resistance, but if it is too little, it is ineffective, and if it is too much, the effect is saturated and it also has a negative effect on weldability.

Therefore, the Cu content was set to 0.1 to 0.5%.

REM and Ca are effective components for SSC resistance by preventing the stretching of sulfides and the formation of clustered AlzOx, but if there is too little, there is no effect, and if there is too much, the cleanliness of the steel decreases and the SSC resistance is reduced. decrease.

Therefore, the content of each component is 0.001 to 0.
05% and 0.001 to 0.02%.

Ti is a component that generates nitrides, refines crystal grains, and improves toughness. However, if the content is less than 0.01%, there is no effect, and if the content exceeds 0.1%, giant nitrides are formed and the SSC resistance is deteriorated. Therefore, the content of Ti component should be reduced to 0. O was set at 1% to 0.1%.

B is a component that has the effect of suppressing ferrite transformation during natural cooling and is effective in increasing strength, but if it is too small, it will not have this effect, and if it is too large, the effect will be saturated, so 0.0003 to 0.0
03%.

Ni works effectively to improve toughness by refining the crystal grains, but if it is too little, it has no effect, and if it is too much, the effect is saturated, and it is expensive, so it was set at 0.1 to 2.0%. .

Nb precipitates in the steel as carbonitrides, prevents austenite crystal grains from becoming coarser, and effectively acts to improve toughness by refining ferrite crystal grains after α transformation. However, too little will have no effect, and too much will saturate the effect, so 0
．． 0.005 to 0.1%.

(2) finely precipitates in the ferrite as VC and strengthens the ferrite to improve SSC resistance.

If it is too small, the effect will not be achieved, and if it is too large, the effect will be saturated, so it was set at 0.01 to 0.1%.

The purpose of Cr and Mo is to increase strength, but too little will have no effect, and too much will cause deterioration of weldability and toughness, so they were set at 0.1 to 1.5% and 0.05 to 0.4%, respectively. .

Next, the manufacturing method of the present invention will be explained.

Steel with the above composition is melted in a melting furnace such as a converter or electric furnace, made into steel slabs by continuous casting, ingot making, or blooming, and then reheated immediately or after being cooled once. , steel plates, steel pipes, and sections are hot-rolled into the required shape or further subjected to standard processing. Steel manufactured in this manner can achieve high joint strength and high toughness, but has low SSC resistance, and has problems in use in severe corrosive environments containing UZS gas and the like. Therefore, in order to solve this problem, the present invention proposes to carry out processing at a reduction rate of 0.5 to 10.0% at a temperature below the Acn point of the steel after hot rolling or further standard treatment, or to further perform processing at a temperature below the Acn point of the steel. Aging treatment is performed at ~400°C.

The cold working rate in the present invention is 0.5% as shown in Figure 1.
The above processing rate can already have the effect of improving ferrite hardness, which is considered to be a factor in improving SSC resistance.
If the processing rate exceeds 0%, the decrease in toughness will be significant.
The range was set to 0.5 to 10.0%. The processing method with the above-mentioned processing rate does not need to be particularly limited, and any of tension, twisting, rolling with rolls, tube expansion, etc. may be used. These treatments are performed to prevent a significant decrease in the SSC resistance of the as-rolled or as-normalized steel material.

That is, since the strength of ferrite directly affects SSC resistance, it is necessary to increase the stress at which ferrite plastic deformation starts. The working temperature was set to 1 point or less of the Ac of the steel because if it was too high, the amount of work hardening would be small. Aging treatment is performed to strengthen the ferrite structure by diffusing and fixing C, N, etc. to dislocations generated by processing. If the aging treatment is too low, the effect will be low; if the aging treatment is too high, the effect will decrease due to dislocation recovery phenomenon. . Therefore, the temperature was set at 100 to 400°C.

The retention time is not particularly limited, but the effect will be saturated even if the retention time is 100 minutes or more.

(Example) Next, examples of the present invention will be described.

Table 1 shows steels produced by melting in a converter and hot-rolling steel slabs into seamless steel pipes through the continuous casting method, or steels subjected to standard treatment after hot rolling, both of which are natural. After cooling, process 0.5-10.0% and heat to 100-400℃.
Shows the strength, joint strength, toughness, and ferrite hardness after aging treatment. The standard temperature was 880 to 980°C. In addition, SSC resistance was determined using a constant load method according to NACE TMOI-77.
5tress) was calculated and evaluated.

The joint strength was determined by a tensile test of a joint portion butt welded at a heat input of 10 to 30 kJ/cm.

It can be seen that the 40 kg class steel manufactured by the present invention has SSC resistance improved by 0.2σ compared to the comparative method, and also exhibits high joint strength. On the other hand, in terms of toughness value,
Although it is slightly lower than the conventional method, it is hardly a problem in practice, and considering other properties such as sour resistance, it can be said to be a useful manufacturing method.

(Effects of the Invention) According to the present invention, a steel material having excellent SSC resistance, high joint strength, and high toughness can be provided.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the influence of processing rate on the ferrite hardness and toughness of steel materials. Figure 1 Processing rate (%)

Claims (7)

[Claims] (1) C: 0.05-0.35%, Si: 0.01-0
．． 5%, Mn: 0.15-1.5%, S: 0.01% or less, P: 0.02% or less, Al: 0.005-0.1%
, N: 0.0020 to 0.0250%, and the remainder substantially consists of Fe, after hot rolling, a rolling reduction ratio of 0.
A method for producing a high-toughness 40kg steel material with excellent SSC resistance, characterized in that processing of 5 to 10.0% is carried out at a temperature below the Ac_1 point of the steel. (2) C: 0.05-0.35%, Si: 0.01-0
．． 5%, Mn: 0.15-1.5%, S: 0.01% or less, P: 0.02% or less, Al: 0.005-0.1%
, N: 0.0020-0.0250%, further Nb: 0.005-0.1%, Mo: 0.05-0.4
%, Cr: 0.1-1.5%, Ni: 0.1-2.0%
. V: 0.01-0.1%, Ti: 0.01-0.1%,
B: After hot rolling a steel piece containing one or more of 0.0003 to 0.003% and the remainder substantially consisting of Fe, processing is performed at a reduction rate of 0.5 to 10.0%. A method for manufacturing a high toughness 40 kg class steel material with excellent SSC resistance, characterized in that the manufacturing method is carried out at a temperature below the Ac_1 point of steel. (3) C: 0.05-0.35%, Si: 0.01-0
．． 5%, Mn: 0.15-1.5%, S: 0.01% or less, P: 0.02% or less, N: 0.005-0.1%,
Contains N: 0.0020 to 0.0250%, and
REM: 0.001~0.05%, Ca: 0.001~
After hot rolling a steel piece containing Cu: 0.02%, Cu: 0.1 to 0.5%, and the remainder substantially consisting of Fe, a reduction rate of 0.5 to 10. 0% machining of steel Ac_
A method for producing high-toughness 40 kg class steel with excellent SSC resistance, characterized in that the process is carried out at a temperature of 1 point or less. (4) C: 0.05-0.35%, Si: 0.01-0
．． 5%, Mn: 0.15-1.5%, S: 0.01% or less, P: 0.02% or less, N: 0.005-0.1%,
N: 0.0020-0.0250% and Nb: 0.005
~0.1%, Mo: 0.05~0.4%, Cr: 0.1
~1.5%, Ni: 0.1~2.0%, V: 0.01~
0.1%, Ti: 0.01-0.1%, B: 0.000
Contains one or more of 3 to 0.003%, furthermore, REM: 0.001 to 0.05%, Ca: 0.00
After hot rolling a steel piece containing one or more of Cu: 1 to 0.02% and Cu: 0.1 to 0.5%, with the remainder substantially consisting of Fe, a rolling reduction of 0.5 to 10 .0% machining of steel Ac
A method for producing high-toughness 40 kg class steel with excellent SSC resistance, characterized by carrying out the process at a temperature of _1 point or less. (5) After hot rolling the steel billet according to claims 1 to 4, normalization treatment is performed, and then processing is performed at a reduction rate of 0.5 to 10.0% to obtain the Ac
A method for producing high-toughness 40 kg class steel with excellent SSC resistance, characterized by carrying out the process at a temperature of _1 point or less. (6) After hot rolling the steel billet according to claims 1 to 4, the reduction rate is 0.
．． Production of high-toughness 40 kg-class steel material with excellent SSC resistance, characterized by performing 5 to 10.0% processing at a temperature below the Ac_1 point of the steel, and then subjecting it to aging treatment at a temperature of 100 to 400°C. Law. (7) After hot rolling the steel billet according to claims 1 to 4, normalization treatment is performed, and then processing is performed at a reduction rate of 0.5 to 10.0% to obtain the Ac
A method for producing a high-toughness 40 kg class steel material with excellent SSC resistance, characterized by carrying out aging treatment at a temperature of _1 point or less, and then aging treatment at a temperature of 100 to 400°C.