JPS63250417A - Manufacture of steel material excellent in resistance to stress corrosion cracking and having low yield ratio in sulfide-rich circumstances - Google Patents

Abstract

PURPOSE:To obtain a steel material excellent stress corrosion cracking resistance in sulfide-richcircumstances and having low yield ratio, by heating a steel slab having a specific composition containing Nb up to a temp. at which Nb is allowed to enter into solid solution in or after hot rolling and then applying natural cooling. CONSTITUTION:A steel slab consisting of 0.02-0.20% C, 0.01-0.50% Si, 0.2-2.5% Mn, <=0.025% P, <=0.02% S, <=0.1% Al, 0.01-0.10% Nb, and the balance essentially Fe is used as a stock, or, if necessary, trace amounts of one or more elements among Cu, Ce, Ca, and N or further one or more elements among Ti, B, Cr, Mo, and Ni are incorporated to the above steel. The above steels are subjected to heating up to a temp. where Nb is allowed to enter into solid solution in the course of hot rolling and then natural cooling after hot rolling, or, after rolling, the above steels are immediately heated up to the temp. where Nb enters into solid solution and then cooled naturally. By this method, the steel material having superior stress corrosion cracking resistance even in a sulfide-corrosion atmosphere and also having low yielding ratio can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for manufacturing a low yield ratio steel material that has good stress corrosion cracking resistance even in a sulfide corrosive environment.

(Prior art and problems to be solved by the invention) Conventionally,
Steel materials with sulfide stress corrosion cracking resistance (hereinafter referred to as SSC resistance) include, for example, MnS, AI!
Reduction of non-metallic inclusions such as 203, [Japan Steel Tube Technical Report Ugly 87 (
1987) QT! 2! ! It has been reported that it can be produced by refining and homogenizing crystal grains using the I-process. However, as grains become finer through QT treatment, the yield strength increases, resulting in a higher yield ratio (yield strength/tensile strength) of the steel, which poses a problem in the design of pipelines or other structures. .

According to the inventors' speculation, the occurrence of SSC is thought to be caused by hydrogen embrittlement phenomenon at the interface between ferrite and nonmetallic inclusions under stress. In other words, hydrogen generated by the corrosion reaction of steel penetrates into the steel, forms hydrogen molecules at the interface with nonmetallic inclusions, and the resulting pressure generates a stress concentration area in the matrix. In such a region, the hydrogen concentration becomes high and the matrix becomes brittle. In this case, it has been determined that if the strength of ferrite is extremely lower than that of pearlite in the presence of external stress, the microscopic plastic deformation of ferrite will increase and hydrogen embrittlement will be further accelerated.

However, steel materials used in sulfide corrosive environments have SS resistance.
Steels with excellent carbon properties and low yield ratios are required, but the steel manufacturing process has had the trouble of reducing non-metallic inclusions.

(Means for solving the problem) As a result of conducting many experiments and considering the purpose of manufacturing low yield ratio steel materials with excellent SSC resistance, the present inventors found that the composition of steel, hot rolling conditions, and By controlling the heat treatment conditions after rolling, even in the presence of nonmetallic inclusions, precipitates, pearlite, etc., the above-mentioned microscopic plastic deformation is suppressed by strengthening the ferrite, and the 5scQ resistance is significantly improved. It was found that steel materials with a low yield ratio can be manufactured. The present invention was constructed based on this knowledge, and the gist thereof is as follows: C: 0.02-0.20% Si: 0.04
~o, so %Mn: 0.2~2.54
P: 0.025% or less S: 0.02% or less
M: 0.1% or less Nb: o, o1 to 0.10%, and further added to the above components Cu: 0.05 to 0.5% Ce: 0.0
07~0.07%Ca: 0.001~0.07%
V: 0.01-0.12%N: 0.00
20-0.0250% of one or two selected from Group A
or Ti: 0.05~o, o 35% B: 0
．． 0003~0.003%Cr: 0.05~1.0
%MO: 0.05-0.5%Ni:
0.05-3.0% of one or two selected from group B
species or more, and one or two species selected from group A, and one selected from group B.
In the hot rolling process, a steel piece consisting entirely of Nb and the remaining Fe is heated to a temperature at which the Nb in the steel becomes a solid solution, followed by hot rolling and natural cooling. Or, after hot rolling, the steel is immediately heated to a temperature at which Nb and V are dissolved in solid solution, followed by natural cooling, and further tempered if necessary. Low yielding steel with excellent resistance to sulfide stress corrosion cracking. This is a manufacturing method for specific steel materials.

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

The reason for limiting the steel composition to the above in the present invention is 1.
Let me explain first. C and Nb are components that finely precipitate in the ferrite as N b C to strengthen the ferrite. Figure 1 shows the degree of hardening (ΔHv) of steel with a constant C content and varying Nb content. 0.02-0.2%, 0.01-0.10
%.

Sl 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 an adverse effect on weldability.Considering the effect and the influence on others, the content was set at 0.01 to 0.5%.

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 Mn
The content was 0.2 to 2.5%.

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 0.025% 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.
0.02% or less.

Increase in At content/Jll+ is caused by clustered Ag2O
, which significantly deteriorates SSC resistance, the upper limit was set to 0.1% or less.

Steel having the above-mentioned composition improves SSC resistance and produces a steel material with a low yield ratio. Further, in the present invention, in addition to the above-mentioned components, small amounts of SSC resistance improving components such as Cu, Ce, and Ca and reinforcing components such as Ti and B are selectively added.

Cu has the effect of suppressing hydrogen penetration into steel and works effectively on SSC resistance, but if it is too little, it is ineffective, and if it is too much, the effect is saturated and also has a negative effect on weldability. Therefore, the Cu content was set to 0.05 to 0.5 inches.

Ce and Ca prevent the stretching of sulfides and form clusters of A.
It is an effective component for preventing the formation of 2□03 and improving SSC resistance, but if it is too little, it will not be effective, and if it is too much, it will reduce the cleanliness of the steel and completely reduce the SSC resistance.

Therefore, the content of each component is between 0.007 and 0.007.
07%, o,oOl ~0.07%.

(2) and N are important components that finely precipitate in the ferrite as VN during natural cooling and strengthen the ferrite. Figure 2 shows the amount of hardening (
ΔHv), which is 0.01 to 0.0, respectively, because if it is too little, there is no effect, and if it is too much, the effect is saturated.
12% and 0.0020 to 0.0250%.

Ti is a component that generates nitrides, refines crystal grains, and improves toughness. However, if the content is less than 0.005%, there is no effect, and if the content exceeds 0.035%, giant nitrides are formed and the SSC resistance is deteriorated.

Therefore, the content of Ti component is 0.005 to 0.035.
%.

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%.

N1 works effectively to improve toughness by refining the crystal grains, but if it is too little, it will not have that effect, and if it is too much, the effect in item 7 will be saturated, and it is expensive. It was set as o%.

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

Steel with the above-mentioned composition is melted in a melting furnace such as a converter or electric furnace, and steel slabs are produced by continuous casting, ingot making, or blooming, and then reheated immediately or after cooling. The steel is then hot-rolled into the required shapes such as steel plates, steel pipes, and sections. Steel hot-rolled in this manner can achieve a low yield ratio, but has low SSC resistance and has problems in use in severe corrosive environments containing H2S gas and the like. Therefore, in order to solve this problem, the present invention heats the steel to a temperature at which Nb and V are dissolved in solid solution during hot rolling or immediately after hot rolling, and then naturally cools the steel, and if necessary, further tempers the steel. This treatment is performed on as-rolled or Nb.

This is done in order to prevent a significant decrease in the SSC resistance of steel materials that have been naturally cooled from a reheating temperature at which almost no V precipitates form a solid solution.

Therefore, it is possible to heat the precipitates such as Nb and V to a necessary and sufficient solid solution temperature, and in the cooling process, without affecting the strength and toughness, without increasing the yield ratio, and without creating excessive stress in the steel. It is necessary to perform natural cooling or even more gradual cooling to prevent this from happening. That is,
The heating during or after rolling causes fine precipitation during cooling to strengthen the ferrite, so it is necessary to ensure a sufficient solid solution temperature, but the heating temperature is 930 to 50
'C is sufficient. The holding time is not particularly limited, but may be short-time heating such as induction heating or long-time heating using furnace heating. However, if kept for too long (more than 100 minutes), depending on the heating temperature, crystal grains may grow significantly, making it resistant to S.
It tends to deteriorate SC properties.

On the other hand, when the shape of the steel material is particularly thin, the cooling rate after rolling or during normalizing treatment becomes relatively fast, so that carbides and nitrides may not be sufficiently precipitated in the ferrite.

Ac for such steel materials.

It is necessary to perform a tempering treatment below the point and completely strengthen the ferrite through the fine precipitation of carbon atoms. The tempering method is not particularly limited either, but may be short-time heating such as induction heating or long-time heating such as furnace heating. However, if it is kept for too long, precipitates may grow depending on the heating temperature. Ft't38C! + tends to deteriorate.

Next, examples of the present invention will be described.

The table shows steel that has been melted in a converter and produced through continuous casting, hot rolled into seamless steel pipes, then normalized, or reheated during hot rolling and then final finish rolled. Both steel and tempered steel exhibit strength, yield ratio, ferrite hardness, and SSC resistance when naturally cooled. The rerolling heat and normalizing temperature during hot rolling is 93
It was conducted at 0 to 1150"C. In addition, for SSC Special 1st students, NAC
σth by constant load method according to ETM-01-77
(ThresholdStress) was calculated and evaluated.

It can be seen that the steel manufactured by the present invention has SSC resistance improved by about 0.20y in σth compared to the comparative method, and also exhibits a low yield ratio.

(Effect of the invention) Steel produced by the method of the present invention as described above has SS resistance.
It has excellent C properties and a low yield ratio, and is widely used in various applications.

[Brief explanation of drawings]

Figure 1 shows the influence of Nb on the increase in ferrite hardness.
FIG. 2 shows the influence of V and H on the increase in ferrite hardness. Figure 1 Nb (X)

Claims (8)

[Claims] (1) C: 0.02-0.20% Si: 0.01-0
．． 50% Mn: 0.2-2.5% P: 0.025% or less S: 0.02% or less Al: 0.1% or less Nb: 0.01-0.10% and the balance is Fe A steel billet containing unavoidable impurities is heated to a temperature at which Nb in the steel becomes a solid solution during the hot rolling process, followed by hot rolling and then allowed to cool naturally, or immediately after hot rolling, the steel billet is A method for producing a low yield ratio steel material having excellent resistance to sulfide stress corrosion cracking, characterized by heating the steel material to a temperature at which Nb becomes a solid solution, and then cooling it naturally. (2) C: 0.02-0.20% Si: 0.01-0
．． 50% Mn: 0.2 to 2.5% P: 0.025% or less S: 0.02% or less Al: 0.1% or less Nb: 0.01 to 0.10%, further Cu: 0 .05~0.5% Ce:0.007~0.0
7% Ca: 0.001-0.07% V: 0.01-0.1
2% Nb: In the process of hot rolling a steel billet containing one or more of 0.0020 to 0.0250%, with the balance consisting of Fe and unavoidable impurities, Nb in the steel solidifies. The steel is heated to a temperature at which it melts, followed by hot rolling, and then allowed to cool naturally, or after hot rolling, it is immediately heated to a temperature at which Nb and V in the steel become a solid solution, followed by natural cooling. A method for manufacturing low yield ratio steel materials with excellent sulfide stress corrosion cracking resistance. (3) C: 0.02-0.20% Si: 0.01-0
．． 50% Mn: 0.2-2.5% P: 0.025% or less S: 0.02% or less Al: 0.1% or less Nb: 0.01-0.10% and further Ti: 0 .005~0.035% B:0.0003~
Contains one or more of the following: 0.003% Cr: 0.05-1.0% Mo: 0.05-0.5% Ni: 0.05-3.0%, the balance being Fe and unavoidable During the hot rolling process, a steel billet containing impurities is heated to a temperature at which Nb in the steel becomes a solid solution, followed by hot rolling and natural cooling, or immediately after hot rolling, the Nb in the steel is removed. A method for producing a low yield ratio steel material with excellent resistance to sulfide stress corrosion cracking, which is characterized by heating to a temperature at which a solid solution occurs and then cooling naturally. (4) C: 0.02-0.20% Si: 0.01-0
．． 50% Mn: 0.2 to 2.5% P: 0.025% or less S: 0.02% or less Al: 0.1% or less Nb: 0.01 to 0.10% and further Cu: 0 .05~0.5% Ce:0.007~0.0
7% Ca: 0.001-0.07% V: 0.01-0.1
2% N: 0.0020~0.0250% of one or more types and Ti: 0.05~0.035% B: 0.0003~0
．． 003% Cr: 0.05-1.0% Mo: 0.05-0.5% Ni: 0.05-3.0% Contains one or more of the following, with the remainder being Fe and unavoidable impurities. During the hot rolling process, a steel billet is heated to a temperature at which Nb in the steel becomes a solid solution, and then the hot rolling is then naturally cooled, or immediately after hot rolling, Nb and V in the steel are dissolved in a solid solution. A method for producing a low yield ratio steel material with excellent sulfide stress corrosion cracking resistance, which is characterized by heating to a temperature of 100%, followed by natural cooling. (5) C: 0.02-0.20% Si: 0.01-0
．． 50% Mn: 0.2-2.5% P: 0.025% or less S: 0.02% or less Al: 0.1% or less Nb: 0.01-0.10% and the balance is Fe During the hot rolling process, a steel billet containing unavoidable impurities is heated to a temperature at which Nb in the steel becomes a solid solution, and then the hot rolling is allowed to cool naturally, or immediately after hot rolling, the Nb in the steel is A method for producing a low yield ratio steel material with excellent sulfide stress corrosion cracking resistance, which is characterized by heating to a temperature at which Nb becomes a solid solution, followed by natural cooling, and then tempering treatment. (6) C: 0.02-0.20% Si: 0.01-0
．． 50% Mn: 0.2 to 2.5% P: 0.025% or less S: 0.02% or less Al: 0.1% or less Nb: 0.01 to 0.10%, further Cu: 0 .05~0.5% Ce:0.007~0.0
7% Ca: 0.001-0.07% V: 0.01-0.1
2% Nb: In the process of hot rolling a steel billet containing one or more of 0.0020 to 0.0250%, with the balance consisting of Fe and unavoidable impurities, Nb in the steel solidifies. It is characterized by heating the steel to a temperature at which it dissolves, followed by hot rolling and then natural cooling, or immediately after hot rolling, heating to a temperature at which Nb and V in the steel become a solid solution, followed by natural cooling, followed by tempering treatment. A method for producing low yield ratio steel with excellent resistance to sulfide stress corrosion cracking. (7) C: 0.02-0.20% Si: 0.01-0
．． 50% Mn: 0.2-2.5 P: 0.025% or less S: 0.02% or less Al: 0.1% or less Nb: 0.01-0.10%, and Ti: 0. 005~0.035% B:0.0003~
Contains one or more of the following: 0.003% Cr: 0.05-1.0% Mo: 0.05-0.5% Ni: 0.05-3.0%, the balance being Fe and unavoidable During the process of hot rolling a steel billet containing impurities, it is heated to a temperature at which Nb in the steel becomes a solid solution, and then the hot rolling is subsequently allowed to cool naturally, or the Nb in the steel is removed immediately after hot rolling.
A method for producing a low yield ratio steel material with excellent resistance to sulfide stress corrosion cracking, which is characterized by heating to a temperature at which a solid solution occurs, followed by natural cooling, and then tempering. (8) C: 0.02-0.20% Si: 0.01-0
．． 50% Mn: 0.2 to 2.5% P: 0.025% or less S: 0.02% or less Al: 0.1% or less Nb: 0.01 to 0.10% and further Cu: 0 .05~0.5% Ce:0.007~0.0
7% Ca: 0.001-0.07% V: 0.01-0.1
2% N: 0.0020~0.0250% of one or more types and Ti: 0.03~0.033% B: 0.0003~0
．． 003% Cr: 0.05-1.0% Mo: 0.05-0.5% Ni: 0.05-3.0% Contains one or more of the following, with the remainder being Fe and inevitable impurities. In the process of hot rolling a steel billet, Nb in the steel is heated to a temperature at which it becomes a solid solution, and then the hot rolling is then allowed to cool naturally, or immediately after hot rolling, Nb and V in the steel are solidified. A method for producing a low yield ratio steel material with excellent sulfide stress corrosion cracking resistance, which is characterized by heating to a melting temperature, followed by natural cooling, and then tempering.