JPS6326339A - High tension steel having superior corrosion fatigue strength and manufacture thereof - Google Patents

Abstract

PURPOSE:To obtain an 80kg class high tension steel having superior corrosion fatigue strength and used for a marine structure by adding prescribed amounts of C, Si, Mn, P, S, Al, Ni and three or more among Cu, Mo, Cr, V, Ti and S. CONSTITUTION:This high tension steel having superior corrosion fatigue strength contains, by weight, <=0.12% C, <=0.5% Si, 0.5-1.4% Mn, <=0.01% P, <=0.003% S, <=0.07% Al, 4-6% Ni and three or more among <=0.5% Cu, <=0.7% Mo, <=0.7% Cr, <=0.1% V, <=0.1% Ti and <=0.005% S. The amount of S in the steel is reduced to reduce corrosion pit forming sources and >=4% Ni is added to form a protective film on the surface of the steel in seawater and to inhibit a pit forming reaction itself. When the steel having the two effects are used, an 80kg class steel sheet for a marine structure hardly causing corrosion cracking is obtd.

Description

[Detailed Description of the Invention] [Industrial Field of Application] This invention is intended to improve the corrosion fatigue strength of marine structures used in various rigs, pipelines, steel lifts, bamboo shoots, etc. for the purpose of marine energy resource development. This relates to 80 Giro class high tensile strength steel with excellent properties.

[Conventional technology]

Marine structures are exposed to tidal currents in the marine corrosive environment.

The steel materials used may suffer from corrosion fatigue as they are subjected to external forces such as waves and wind.

In other words, in fatigue in the atmosphere, is there a fatigue limit beyond which repeated stress will not cause fatigue failure?There is no clear fatigue limit in fatigue in seawater, and the fatigue strength is the same as that in the atmosphere. °C drops significantly.

It has been clarified that the origin of such corrosion fatigue cracks is corrosion bits that form around inclusions in a seawater environment due to local battery action between the inclusions and the steel base, and are caused by repeated stress. It is known that when this corroded bit grows to a certain critical dimension, fatigue cracks occur from the pit 1 to the bottom, and by repeating the growth and coalescence of these cracks, it leads to final breakage.

For conventional high-strength steel, effective prevention of corroded bits has not been achieved, although measures have been taken to reduce inclusions and segregation, or to add alloying elements to improve corrosion resistance. It has been generally thought that there is no countermeasure for steel materials. As prior art related to the present invention, there are the following documents (1) to (3).

(1) J, 1Iarrison; “Metal 5
election consideration for
0ffshore st, ruct, ure”Proc
, Intern, Conf, on 5teel inM
arine 5structures, 1981. P,
148-193.

(2) Haruo Shimada, “Materials that hold the key to ocean development”
, Chemical Industry, 32 (1981), No. 5 P, 181
~187 (3) Kenjibe Komai, Sadato Kita, Yoshibe Endo; “Corrosion behavior and spontaneous cracking progression behavior of 80kg class high-strength steel J Journal of Japan Society of Geometrical Mechanics vat 4!l, N.

445 (1982), 11.1029-1035 (Problems to be Solved by the Invention) Problems in the above conventional descriptions are as follows.

1) L-J composition design and structure control could not completely prevent the formation of corrosion pits, which are the starting point of corrosion fatigue cracks.

2) For this reason, it was not necessary to install a large number of pseudo electrodes on offshore structures and carry out thorough cathodic protection. The cost required for the design and management of cathodic protection is prohibitive, and achieving adequate corrosion protection poses considerable technical difficulties.

3) With conventional steels, high-strength steels have a large decrease in strength due to corrosion fatigue, so low-strength steels had to be used, and as a result, structures had to be thicker and heavier.

An object of the present invention is to provide a high tensile strength steel with excellent corrosion fatigue strength and a manufacturing method that solve the above-mentioned problems.

(Means for solving the problems) The present invention-1 Reducing the number of corroded bit formation nuclei by reducing sft in steel and reducing Ni fleas to 4* or more.
By adding 2, a protective film is formed on the steel surface in seawater and the bit generation reaction itself is suppressed.
Through the above two effects, corrosion cracking is less likely to occur80
The present invention provides a steel plate for kilo-class offshore structures and a method for manufacturing the same.

The high tensile strength steel with excellent corrosion fatigue strength and the manufacturing method of the present invention are as follows.

The first steel of the present invention is a high tensile strength steel with excellent corrosion fatigue strength.

Based on weight.

c: o, 12* or less; Si: 0.5% C or less: Mn: 0.5 to 1.4! l; ;P
+0.01! If or less; S: 0.003 or less;
Al: 0.0i or less; Ni 4-6%; Cu: 0.5% or less: Mo: 0, 19
t, below.

Cr: 0.796 or less, V: 0.1%i or less.

Ti・0.1* or less, B: 0.005 or less:
Contains 3 or more types of carp.

The second steel is a high tensile strength steel with excellent corrosion fatigue strength, the balance being Fe and unavoidable impurities.

Based on weight, 'C: 0.1296 or less; Si+0.5%i or less; Mro: 05-14; P ・0.01 yen
Below; S: 0.003'l; Below, 711:
0.07! kjj bottom.

N1.4-6 zero.

Including Cu: 0 or less: Mo: 01 or less, Cr
: 0.7% or less, V: 0. ]% l;J lower 1T+・0.1'<Right: 11:0.0O
F! l: Below.

Including 3 inserts, Zr: 0.006%f or less; Ca: 0.0O1
Less than l.

It is a high-bow, long-strength steel that contains one or two of these, with the remainder being Fe and unavoidable impurities, and has poor corrosion fatigue strength.

Furthermore, the methods of the third and fourth inventions, which are the methods for producing the first and second invention steels, are the first and second invention steels having the respective compositions.
After austenitizing the invention and second invention steels at a temperature range of 800 to 1100°C, the invention and second invention steels were heated to 65
There is a method for producing high tensile strength steel with excellent corrosion fatigue strength, which is tempered in a temperature range of 0 to 700°C, and the fifth and sixth invention methods involve manufacturing the first and second invention steels having respective compositions. This is a method for producing high tensile strength steel with excellent corrosion fatigue strength, in which finish rolling is carried out at a temperature of 800°C, followed by direct water quenching and subsequent tempering at a temperature range of 650 to 700°C.

[Effect]

The reasons for limiting the steel components in the present invention are as follows.

C is an element that increases strength, but at the same time it significantly impairs toughness and weldability.

Considering these, the tensile strength of the steel of the present invention was set to 80 kg/mm2.
In order to keep it around, it is necessary to reduce the amount of C to [1.12%] or less.

Si is added to increase strength and deoxidize, but if added in excess of 0.50, toughness and weldability will deteriorate.
* is the upper limit.

Mn is an element that is effective in improving toughness and strength.

If it is less than 0.5*, sufficient toughness cannot be obtained, so this is set as the lower limit. On the other hand, if it exceeds 1.4*, the tensile strength of the steel of the present invention becomes 90 kg/mm2 or more, so this is set as the upper limit.

Since P impairs toughness, 001 was set as below.

Since S forms nonmetallic inclusions MnS and becomes the core of corrosion pit formation as described above, it is desirable to reduce S as much as possible.

Industrially, o, ooo4* may decrease, but 0.00
If the value exceeds 0.03*, the corrosion rate increases, pit formation becomes noticeable, and the corrosion fatigue strength deteriorates, so the upper limit is set to 0.003.
I did it.

At is necessary for deoxidation, but if it is added in an amount exceeding 0.07%, the toughness will deteriorate and non-metallic inclusions will increase.
The upper limit was set at 0.07%.

N1 has the effect of significantly improving toughness and suppressing the propagation of corrosion fatigue pits.As shown in the examples described later, when added in an amount of 4% or more, it forms a coating with excellent corrosion resistance in a seawater environment and prevents corrosion pits. It has the effect of preventing formation.

On the other hand, if it is added in an amount exceeding 6%, the tensile strength cannot be lowered to 90 kg/mm2 or less even if the tempering temperature is raised to just below the temperature point in the tempering treatment after quenching.

Therefore, the range of the Ni amount is limited to 4 to 6*.

Cu, Mo, Cr, V, Ti, and B are elements that contribute to improving strength and toughness, and it is necessary to add three or more of them. An explanation of these is as follows.

Cu is an element that is effective in increasing strength.If added in excess of 0.5%, weldability deteriorates and the so-called C
Since it causes scratches, the amount added should be less than o5*.

Mo and Cr are elements that have the effect of increasing strength, but 07*
If it exceeds 0, the effect will be saturated and it will be economically disadvantageous.
The upper limit was 7°6.

V, Ti, and B may be added to achieve the required strength level, but V, Ti, and 0.1! Even if the amount exceeds 0.001, the result will be saturation, which is economically disadvantageous. Therefore, the upper limits were set to O1* and 0.005%, respectively.

Furthermore, Zr and Ca added in the second invention steel are added in order to effectively control the morphology and structure of sulfide-based nonmetallic inclusions in the steel, but even if they are added in excess of o and ooe96, respectively, The result is saturation and is economically disadvantageous. Therefore, o, ooe96 was set as two limits.

Next, in the method for manufacturing the steel of the present invention having the above components, in order to adjust the tensile strength to around 80 kg/mm2, the steel is hot rolled to a predetermined thickness, then austenitized in a temperature range of 800 to 1100°C, and water quenched. Continue from 650
Tempering treatment is performed in a temperature range of 700°C. Alternatively, in hot rolling, finish rolling is performed at a temperature of 3800°C or higher, water quenching is performed immediately after rolling is completed, and subsequently tempering treatment is performed at a temperature range of 650 to 700°C.

The reasons for limiting these heat treatment conditions and rolling conditions will be described.

First, regarding the austenitizing temperature, if the austenitizing temperature exceeds 1100°C, austenite crystal grains will undergo abnormal grain growth and become coarse.

For this reason, even if the material is subsequently tempered, good toughness cannot be obtained, and the susceptibility to temper brittleness increases, and at the same time, the resistance of the material to propagation of cracks due to corrosion fatigue in a seawater environment is also drastically reduced.

Therefore, the upper limit temperature for austenite formation must be 1100° C. or lower.

The lower the austenitizing temperature, the finer the grains after austenitizing, which improves the toughness after tempering and resistance to corrosion fatigue cracking, so it is desirable to keep it as low as possible, or the lower limit is limited by the Ar3 transformation point. .

The Ar3 transformation point is 800°C in the Ni content range of the steel of the present invention (4 to 6).For the above reasons, the austenitizing temperature range was limited to 110θ to 800°C.

Next, regarding the temperature range of the tempering treatment, if the tempering temperature is less than 650°C, the tensile strength cannot be lowered to the strength level of 80 kg steel even if the tempering treatment is performed for a long time.

Furthermore, since tempered cementite is not sufficiently precipitated from the martensite produced by quenching after austenitization, the toughness value is significantly inferior and the resistance to corrosion fatigue cracking is also low.

On the other hand, the two limiting values of the tempering temperature are determined by Ac and the transformation point.

The temperature range from these J1 mountains to the tempering plant Juri is 650℃.
The temperature was limited to ~700°C.

Water quenching after hot rolling of the fourth and sixth invention methods,
If the austenitizing temperature before rolling is too high, the initial austenite grain size will become coarse and it will be difficult to completely refine the grains by subsequent rolling, resulting in a mixed grain structure and deterioration of toughness. It is desirable that the austenitizing temperature be 1100°C or less.

Regarding rolling, if the finishing temperature of the rolling reaches the Ar3 vortex, ferrite will be generated during rolling, and even if water quenching is subsequently performed, uniform martensite will be produced! No. 11 weave cannot obtain a good toughness value even if it is tempered.

For this reason, the rolling temperature is Ar3? Must be at least M degree. Regarding the tempering treatment temperature, the above-mentioned reason for limitation also applies in this case.

Examples of the present invention will be described below.

〔Example〕

Example 1 Table 1 shows the components of test steels A-1.

There is a slight increase in the amount of Ni when moving from steel to steel I,
Steel E to Steel 11 are uninvented steels.

These eight steels were subjected to the heat treatments shown in Table 2.

In order to keep the tensile strength within the range of 80 to 90 kg/mm2, the higher the Nii, the higher the tempering temperature.

The mechanical properties after heat treatment are also shown in Table 2.

In steel (2) in which the Ni content exceeds the range of the present invention, even if the tempering temperature was increased to 700°C, the tensile strength could not be lowered to 907 mm2 or less.

Corrosion fatigue test pieces shown in FIG. 1 were taken from 1/2 t of the Konera sample so that the longitudinal direction and rolling direction coincided.

These were subjected to a corrosion fatigue test using a hydraulic surf testing machine in aerated artificial seawater at 20° C. at a repetition rate of 10 cpm and a stress ratio of O1.

FIG. 2 shows a graph of the relationship between the number of repeated surfaces until fracture and the amount of Ni when the applied stress amplitude is 20 J/nun'. Compared to specific mild steel, the Ni amount range of the present invention is 10 to 6.0*
It can be seen that the fatigue life is improved by 2 to 3 times within this range.

Unexamined Japanese Patent Application, '763-26339 (6) Table 2 Heat treatment conditions and mechanical properties of test steel Ij1 The reason why the corrosion in seawater is greater than that of the inventive steel or comparative steel is as follows: The following points can be raised from metallographic micrographs.

FIG. 3 is a metallographic micrograph (x45) showing corrosion pits and corrosion products (stress repetition rate, 5X]05 cycles) formed on the surface of comparative steel.

Figure 4 shows the cracks observed on the bottom of the bit after removing corrosion products (comparison steel, number of stress cycles).

5X105 cycles) metallographic micrograph (X300
).

FIGS. 5(a) and 5(b) show the comparison steel and the invention i, respectively.
It is a metal structure photograph showing a comparison of the corrosion state of the parallel part of the test piece G.

First, in the comparative steel, many corrosion pits as shown in Figure 3 were formed during the corrosion fatigue test, and a fourth pit was formed from the bottom of the bit.
Bonds as shown in the figure are generated, and these interconnections lead to final breakage. On the other hand, in the steel of the present invention containing 4 or more Ni, an iJ silver-gray protective coating is formed in a seawater environment, as shown in FIG. 5(b).

These protective coatings are mechanically stable without breaking even under the external stress applied in fatigue tests.

For this reason, as shown in FIG. 5(a), in the comparison steel, significant surface roughness and numerous pits were observed due to corrosion, whereas in the steel of the present invention, only a few pits were formed.

Figure 6 shows the relationship between the number of bits formed on the surface of the corrosion fatigue test piece and the number of repetitions of the corrosion fatigue test for comparison steel and invention steel G.
Show about.

The same figure also clearly shows that bit formation is significantly suppressed in the steel of the present invention in which a corrosion-resistant protective film is formed.

In addition, good corrosion fatigue strength was also obtained for steel (2), but even after tempering at 700°C, the tensile strength remained at 100k.
It was excluded from the scope of the present invention because it could not be lowered to below g/mm2.

Example 2 Table 3 shows the components of test steels 1 to 20.

The components of Steels 1 to 11 belong to the components of the steel of the present invention.

On the other hand, in steels 12 to 20, the Ni amount was 1llFi according to the present invention.
However, other elements (beyond 11-fΦ) are beyond the scope of the claims.

Table 4 shows the heat treatment strips 1 of these steels.

'j! iIl to 3. Steel 6 and 8. steel 12 to steel 11
i 1r: J-1'1・i-pu1 (1) For quenching and tempering treatment. The others are products made from direct quenching and tempering IZ after rolling, and these are rolled at a temperature of 100 degrees or 100 degrees FJ.

Table 4 shows the yield stress, tensile strength and J of these steels, and the number of repetitions until failure when the applied stress amplitude is 20 kg/mm'.

For all steels, the sawtooth rupture life is significantly tilted when N1■ is in the range G of 40 to 60*, or the tempering temperature is set to B' [capacity Even if the temperature is raised to 700℃, the tensile strength is 10
It can be seen that GJ exceeds 0 kg/mm2 and far exceeds the standard range of 80 kg class high tensile strength steel.

JP-A ■rjG3-26339 (8) [Effects of the Invention] The high tensile strength steel with excellent corrosion fatigue strength and the manufacturing method of the present invention improve corrosion fatigue strength by forming a mechanically strong protective film based on the addition of Ni. It can significantly improve
It has great industrial value.

[Brief explanation of drawings]

Fig. 1 is an explanatory diagram of the shape of a corrosion fatigue test piece in the example, Fig. 2 is a graph of the relationship between the number of repetitions until fracture and the amount of Nl when the applied stress amplitude is 20 kg/mm2, and Figs. 5(a) and 5(b) are explanatory photographs of metallographic structures in Examples, and FIG. 6 is a graph showing the relationship between the number of corroded bits generated on the sample surface and the number of stress repetitions. Agent Patent Attorney Masatoshi Sato (,uuri/lr) *TJJIV(It', l *'
It procedural amendment 0. picture,

Claims (6)

[Claims] (1) Based on weight, C: 0.12% or less; Si: 0.5% or less; Mn: 0.
5-1.4%; P: 0.01% or less; S: 0.003%
Al: 0.07% or less; Ni: 4-6%; Cu: 0.5% or less; Mo: 0.7% or less; Cr: 0.
7% or less; V: 0.1% or less; Ti: 0.1% or less; B: 0.005% or less; 3 of these
A high tensile strength steel with excellent corrosion fatigue strength, characterized by containing at least 100% of Fe and the remainder consisting of Fe and unavoidable impurities. (2) On a weight basis, C: 0.12% or less; Si: 0.59% or less; Mn: 0
．． 5-1.4%; P: 0.01% or less; S: 0.003
% or less; Al: 0.07% or less; Ni: 4 to 6%; Cu: 0.5% or less; Mo: 0.7% or less; Cr: 0.
7% or less; V: 0.1% or less; Ti: 0.1% or less; B: 0.005% or less; 3 of these
and further contains one or two of the following: Zr: 0.006% or less; Ca: 0.006% or less; the remainder being Fe and unavoidable impurities. Excellent corrosion fatigue strength. High tensile steel. (3) Based on weight, C: 0.12% or less; Si: 0.5% or less; Mn: 0.
5 to 1.4%; P 0.01% or less; S: 0.003% or less; Al: 0.07% or less; Ni: 4 to 6%; further Cu: 0.5 or less; Mo: 0 .7% or less; Cr: 0.7% or less; V: 0.1% or less; Ti: 0.1% or less; B: 0.005% or less; 3 of these
A method for improving corrosion fatigue strength, which is characterized by water quenching after austenitizing a steel containing at least 100% of Fe and the remainder consisting of Fe and unavoidable impurities in a temperature range of 800 to 1100°C, and subsequently tempering in a temperature range of 650 to 700°C. A superior method of manufacturing high-strength steel. (4) Based on weight, C: 0.12% or less; Si: 0.5% or less; Mn: 0.
5-1.4%; P: 0.01% or less; S: 0.003%
Al: 0.07% or less; Ni: 4-6%; Cu: 0.5% or less; Mo: 0.7% or less; Cr: 0.
7% or less; V: 0.1% or less; Ti: 0.1% or less; B: 0.006% or less; 3 of these
A steel containing at least one of Zr: 0.006% or less; Ca: 0.006% or less, and the remainder consisting of Fe and unavoidable impurities is austenitized in a temperature range of 800 to 1100°C. A method for producing high-strength steel with excellent corrosion fatigue strength, which comprises water-scorching the steel and subsequently tempering it at a temperature range of 650 to 700°C. (5) Based on weight, C: 0.12% or less; Si: 0.5% or less; Mn: 0.
5-1.4%; P: 0.01% or less; S: 0.003%
Al: 0.07% or less; Ni: 4-6%; Cu: 0.5% or less; Mo: 0.7% or less; Cr: 0.
7% or less; V: 0.1% or less; Ti: 0.1% or less; B: 0.005% or less; 3 of these
Corrosion fatigue characterized by finish-rolling a steel containing at least 100% Fe and the remainder consisting of Fe and unavoidable impurities at a temperature of 800°C or higher, then immediately water quenching, and subsequently tempering at a temperature range of 650 to 700°C. A method of manufacturing high-strength steel with excellent strength. (6) Based on weight, C: 0.12% or less; Si: 0.5% or less; Mn: 0.
5-1.4%; P: 0.01% or less; S: 0.003%
Al: 0.07% or less; Ni: 4-6%; Cu: 0.5% or less; Mo: 0.7% or less; Cr: 0.
7% or less; V: 0.1% or less; Ti: 0.1% or less; B: 0.005% or less; 3 of these
A steel containing at least one of Zr: 0.006% or less; Ca: 0.006% or less; and the remainder consisting of Fe and unavoidable impurities is finish rolled at a temperature of 800°C or higher. A method for producing high tensile strength steel with excellent corrosion fatigue strength, which comprises immediately water quenching and tempering at a temperature range of 650 to 700°C.