JPS60177128A - Production of 50-kg/cm2 class steel having excellent resistance to corrosion fatigue for oceanic structure - Google Patents

Abstract

PURPOSE:To obtain a titled 50-kg/cm<2> class steel having good toughness and weldability by hot-rolling a billet contg. C, Si, Mn, P and S respectively at specified contents at a prescribed temp. or below and subjecting the steel to finish rolling respectively at specified cumulative drafts in a recrystallization region and unrecrystallization temp. region then cooling the steel at a specific average cooling rate. CONSTITUTION:A billet contg., by weight, <=0.12% C, <=0.5% Si, 0.5-1.6% Mn, <=0.02% P, <=0.004% S, contg., if necessary, >=1 kind among 0.1-0.5% Cu, <=0.5% Ni and <=0.5% Cr, and consisting of the balance Fe is prepd. Such billet is heated to <=1,200 deg.C in order to form the finer austenite grains prior to rolling and is then subjected to rolling in which the cumulative drafts in the austenite recrystallization region and unrecrystallization temp. region are respectively made to >=20% and >=60% and the finish temp. is made 700-800 deg.C. The steel plate is immediately thereafter cooled down to 600-500 deg.C at an average cooling rate of 5-25 deg.C/sec and is then allowed to cool. The 50-kg/cm<2> class steel having excellent resistance to corrosion fatigue for an oceanic structure is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing 50 kg class steel for marine structures with excellent corrosion and fatigue resistance, and is an alternative to conventional normalized 50 kg steel with excellent corrosion and fatigue resistance. The present invention aims to provide a method that can economically produce 50 kg class steel materials for offshore structures.

In recent years, the construction of marine structures such as various rigs, platforms, and noise lines for the purpose of developing marine energy resources has become active, and the construction sites have expanded to polar regions and deep seas, and they are being used under increasingly harsh conditions. There is.

However, since these marine structures are subject to repeated external forces such as tides, waves, and wind in a corrosive seawater environment, it is essential to consider corrosion fatigue when selecting the steel materials used.
In the case of fatigue in the atmosphere, there is a fatigue limit below which no matter how much stress is applied repeatedly, it will not break, but the fatigue strength in seawater is significantly lower than in the atmosphere, and there is no clear fatigue limit. do not. However, today there is a growing tendency to use 50 kg class high tensile strength steel for offshore structures, and it is common to use low carbon steel that has been mainly normalized. It exhibits a ferrite-pearlite structure, and in seawater, cementite in pearlite is electrochemically nobler than ferrite, so a local battery is formed between the ferrite in pearlite or the surrounding ferrite matrix. As the ferrite in or around the pearlite dissolves, /4'-lite becomes the core and corrosion pits are formed, and the S in the steel forms inclusions MnS, and this -8 also dissolves in seawater. Since it is more electrochemically sensitive than the ground structure (ferrite), it also becomes a cathode and forms a local battery, which becomes the core of corrosion pit formation. Under repeated stress, when this corrosion pit grows to a certain critical dimension, fatigue cracks occur from the bottom of the pit, and the minute distributed cracks generated from the countless corrosion pits grow and coalesce repeatedly until they reach rupture. The Rukoto. Distributed cracks generated from corrosion pits are the cause of a significant decrease in fatigue strength in a corrosive environment, but in conventional carbon steels as described above, Cr.

Unless large amounts of alloying elements that improve corrosion resistance, such as N1 and Cu, are added, corrosion pits cannot be suppressed and the corrosion fatigue properties will not improve. However, the method of imparting corrosion resistance to this alloy element is uneconomical, and the production of such copper is currently not commercially available.On the other hand, the need for offshore structures continues to increase. It is difficult to respond to such demand immediately.

The present invention was created based on the above-mentioned actual circumstances and after repeated studies, and is based on VT4 (hereinafter simply referred to as "excellent").
: 0.121 or less, St: 0.5% or less, Mn:
0.5 to 1.6, P: 0.021 or less, 8:0.
0041 or less, and further contains Cu: 0.1 as necessary.
~0.51i, Ni: 0.5% or less, cr: 0.
When hot rolling a steel billet containing one or more types of collapse of coefficient 5 or less and partly consisting of iron and unavoidable impurities, the billet heating temperature is set to 1200C or less and The cumulative reduction rate is t-204 or more, the cumulative reduction rate at the non-recrystallization temperature range (i.e. 950C-Ars transformation point is C0 coefficient or more, and the finishing temperature is 700-8o).

5 to 25 immediately after rolling.
C/see (7) 60o~so at average cooling rate.

It is proposed to cool at C1 and then leave to cool.

That is, to explain the present invention as described above, considering the mechanism of corrosion fatigue described above, the key to improving corrosion fatigue strength is first to limit the size of corrosion pits in order to suppress the occurrence of fatigue cracks. The second step is to reduce the number of pits in order to reduce the chances of distributed cracks coalescing, and to achieve this, the microstructure must be made finer, especially the pearlite finely dispersed, and the amount of S in the steel must be reduced. It is important to reduce the number of corrosion pit formation nuclei and to make them smaller.

Therefore, the present invention proposes the above-mentioned method, and the reason for the limitation on the preparation will be explained below.

C is an element that significantly increases strength, but as Cf# increases, the amount of pearlite that becomes the core of corrosion pit formation increases. Furthermore, since an increase in the amount of C significantly impairs toughness and weldability, the upper limit was set at 12.

& is added to increase strength and deoxidize, but o, s
If the content exceeds os, the toughness and weldability will deteriorate, so this is set as the upper limit.

Mn is necessary for increasing toughness and as a deoxidizing agent, and is necessary for increasing toughness and as a deoxidizing agent.
If it is less than 501, sufficient toughness cannot be obtained, so this is set as the lower limit, but if it exceeds 1.60%, a carbide band structure appears and the toughness is impaired, so this is set as the upper limit.

Since P impairs toughness, it is necessary to keep it at 0.021 or less.

Since S becomes non-metallic inclusions MMS and becomes the core of the formation of corrosion pits as described above, it is necessary to set it to 0.0041 or less. This S can be industrially reduced to o, oosst.

In addition to the above, the target steel according to the present invention may further contain Cu s Ni
One or more types of -Cr can be contained. That is, these will be explained as follows.

Cu is an effective element for increasing the strength of steel materials, but if it is less than 0.11%, the effect is poor, and if it is less than 0.5%
If it exceeds Cu, not only will weldability deteriorate, but the surface quality of the steel sheet will be damaged due to Cu embrittlement. Therefore, it is necessary to set it at 0.2-0.5%.

Although Ni improves the strength and toughness of the steel sheet, the upper limit was set at 0.5% in consideration of economic efficiency.

Cr imparts strength and corrosion resistance to the steel plate, but the upper limit was set at 0.51 in consideration of economic efficiency.

Next, in the present invention, in order to obtain the specified strength and toughness for 50 kg class steel with the above ingredients and at the same time improve corrosion fatigue resistance, the rolling conditions described below are combined to achieve thorough refinement of one structure. Regarding rolling conditions, first, slab heating must be performed at 12 ooc or less in order to refine the austenite grains before rolling.

Further, it is necessary to apply a certain amount of pressure in the austenite recrystallization region at 950 C'E from such a slab heating temperature to refine the recrystallized austenite. Therefore, in the present invention, the entire cumulative reduction of 20 inches or more is carried out in this austenite recrystallization region, and if the cumulative reduction is less than this, the microstructure of the final product cannot be refined.

The austenite unrecrystallized area following the above (950C-A
Rolling at the rB transformation point increases the grain boundary area and intragranular deformation zone density of austenite, which serve as ferrite nucleation sites, and results in significant refinement of ferrite, so rolling as much as possible should be performed in this region. is desirable, and at least 5
Make 0% reduction in this area. Further, since rolling near the Ar transformation point (730 to 780c for 50kg steel) has a significant effect on refinement, it is desirable that the rolling finish temperature be just above or just below the Arc transformation point.

The temperature should be 700-800C. In other words, if it is more than 5oot, sufficient deformation band density cannot be obtained, and the desired strength and toughness cannot be obtained.Also, if rolling is performed in the ferrite/austenite two-phase region below the Arc transformation point, the development of the deformed structure becomes remarkable, resulting in poor corrosion resistance. The lower limit of the rolling finishing temperature is set at 700C in order to promote deterioration of the steel and anisotropy of mechanical properties.

Further, immediately after rolling is completed, online cooling is performed to suppress grain growth, thereby further improving grain refinement. The cooling rate and cooling stop temperature are important parameters; the faster the cooling rate and the lower the cooling stop temperature, the finer the structure becomes. That is, if the cooling rate is less than 5 C/see and the cooling stop temperature exceeds 600 C, sufficient microstructural refinement will not be achieved and the desired corrosion fatigue properties will not be exhibited. On the other hand, if the cooling rate is 25 C/sec or more or the cooling stop temperature is 50
Below 0C, martensite appears in the structure and toughness and weldability deteriorate. Therefore, the cooling rate should be increased from 5 to 25 tll'/
See, and it is necessary to set the cooling stop temperature to 500 to 600C.

As described above, in the rolling of the present invention, the main purpose is to refine the grains of ferrite from slab heating to water cooling stop 1, and to refine the ferrite, the second phase structure (carbide) t- is also refined at the same time. The hole becomes strong and the result is strength.
Improves toughness and corrosion fatigue resistance. In addition, it does not use expensive alloying elements and has a low carbon equivalent, so it has good weldability and is clearly an excellent product for welded structures used in seawater environments. be.

A specific example of the present invention will be described below.

That is, steels A-C as shown in Table 1 below were melted. A%
All B materials have steel components that satisfy the present invention,
For material B, Cu and Ni were added to obtain high toughness. Note that the C material has c, p, and s each larger than the specified range in the present invention, and Cu1
Nl.

Cr is also included in each.

Table 1 (qb) Each of the above-mentioned steel materials was subjected to heat treatment as shown in Table 2 below. That is, materials A and B were subjected to the heat treatment specified in the present invention, while material C was finished rolling in the austenite recrystallization region and normalized.

The microscopic structure of each village of A-C obtained as above is as shown in the micrograph in Figure 4 at a magnification of 400 times. It is clear that granulation appears.

Further, the mechanical properties of each of the above-mentioned rigid villages are as shown in Table 3 below.

In other words, the tensile test value in each village satisfies the standard of 50 kg, but the human resources are as simple as mild steel.The reason why the impact values of materials A and B are far superior to that of material C is because of this fine grain. This is the effect of

Hydraulic sensor for corrosion fatigue test? Testing was carried out using a testing machine in aerated artificial seawater at 20C at a repetition rate of 10 cpm and a stress ratio (minimum stress/maximum stress) of 0.1. That is, as shown in Fig. 1, a test piece with both ends 1 of 16 wφ and a reduced diameter part 2 of 10 mφ formed in the middle (total length 140 m, diameter reduced part 50 m) was collected from 3 At of each village. The test results were taken so that the longitudinal direction and the rolling direction coincided, and the relationship between the applied stress and the number of rupture cycles is as shown in the first table. That is, A
It has been observed that materials B and C generally tend to have a longer life, and the lower the force, the more remarkable the difference in fatigue life becomes.

Comparing the fatigue life at a load stress of 23h f /m'', the lifespan of human resources is 1.26 million times, material B is 1.11 million times, while material C is 760,000 times, and at this stress level, A , B material shows an increase in service life of 66 treble and 46 ts compared to C material.

As mentioned above, the fact that the material of the present invention has better corrosion fatigue properties in seawater than the comparative material is confirmed by the following facts.

That is, Fig. 5 shows scanning electron micrographs of the surface of each test piece fractured at a load stress of 2 a Kyt/w'', and the surfaces of the human resources and B materials of the present invention shown as A and B in the figure are extremely While the surface of the comparative material is smooth, the macrostructure is exposed and the degree of unevenness is significant.Since the degree of surface unevenness due to corrosion depends on the grain size, the grain refinement by the present invention is less likely to result from corrosion. This can be said to reduce the surface unevenness due to

Second, pits are formed in the cross section near the surface of the comparison material in FIG. 6, and the material according to the present invention is A? - It can be said that the fine dispersion of light and the reduction of MnB due to the reduction of S suppressed the formation of corrosion pits due to dissolution and falling off of these second phase particles and their surroundings. In addition, 9-light amount, MnS
In order to clarify the effect of corrosion fatigue life on corrosion fatigue life,
As shown in the figure, the amount of C and Sli are taken as coordinates, and the load stress 2
3Kof/m", it was determined whether the life exceeds 106 cycles.In other words, the figure also plots the f-time of 50 kg class steel, which was announced in 1. It is clear that controlling the amount of C and the amount of S within the range specified by the invention is an essential requirement for improving corrosion fatigue properties.

Thirdly, as a result of these, as shown in Figure 5 above, the material of the present invention shows almost no fatigue cracks even though the test was conducted for a longer time, whereas the comparative material showed no fatigue cracks. It can be said that a large number of cracks were generated at an early stage, and coalescence between cracks occurred frequently, leading to a decrease in life.The fatigue crack suppressing effect of the present invention is clear.

According to the present invention as explained above, it is possible to obtain a steel material that has extremely excellent corrosion fatigue resistance, has good toughness and weldability, and is not made of special alloying elements, so it can be used for marine structures with excellent economic efficiency. This invention is capable of providing 50 kg class steel for commercial use, and is industrially highly effective.

[Brief explanation of drawings]

The drawings show the technical content of the present invention, and Fig. 1 is an explanatory diagram of a corrosion fatigue test piece, Fig. 2 is a chart showing the relationship between applied stress and the number of rupture cycles, and Fig. 3 is a Fig. 4 is a photograph showing the properties of one surface of the examples of the present invention cA material, B material) and the comparative material cC material), Fig. 6 is a micrograph showing the state of formation of corrosion pits at a magnification of 400 times of a cross section near the surface of a comparative material (cC material). Patent applicant: Nippon Kokan Co., Ltd. Inventor: Fuji 1) Taka Butsuma Yusuke Inagaki / Peng 2nd Okadai J Situation, 5 layers (/,) Procedural amendment (1 person) q8Iu-'98 Commissioner of the Japan Patent Office Person Kazuo Sugi 1. Indication of the case Relationship to Showa Tata Patent Application No. ♂2Δd2 q# Name of applicant (name) Nippon Steel Tube Co., Ltd. 4. Contents of amendment by agent 12 Description of the present application On page 17, lines 6 to 13, [Figure 4 shows...]. ].
Figure 4 is a 400x magnification micrograph showing the metal structure on the surface of the material according to the embodiment of the present invention (material 1. B) and the comparative material (material C), and Figure 5 is the corrosion fatigue thereof. A photograph showing the metal structure on the surface of each specimen after the test at a magnification of 75 times, and Figure 6 shows the state of corrosion pit formation in the metal structure of a cross section near the surface of a comparison material (material C) at a magnification of 400 times. This is a microscopic photograph. ” he corrected.

Claims (1)

[Claims] C: 0.1 Zwt or less, St: 0.5 wt or less,
MFI: 0.5 to 1.6 wt%, P: 0.02 wt1 or less. Contains S: 0.004wt% or less, and further contains Cu: 0.1''-0.5wt%, Nl: 0.
When hot rolling a steel billet containing one or more of 5wt% or less, Cr: 0.5wt% or less, and the remainder consisting of iron and inevitable impurities, the billet heating temperature is 1200t:' or less. The cumulative reduction rate in the austenite recrystallization area is 20 degrees or more, the cumulative reduction rate in the non-recrystallization temperature area is 60 degrees or more, and the finishing temperature is 70 degrees or more.
Rolling is carried out to 00 to 800C, and then immediately after the above rolling, the temperature is reduced to 600 to 50C at an average cooling rate of 5 to 25C/sec.
A method for manufacturing 50 kg class steel for marine structures with excellent corrosion resistance and fatigue properties, which is characterized by cooling to 0C and then allowing it to cool.