JPS581015A - Production of high-toughness ultralow carbon hot coil having high hydrogen-induced cracking resistance - Google Patents

Abstract

PURPOSE:To improve hydrogen-induced cracking resistance in the stage of producing a hot coil by reducing the contents of C and Mn of raw steel materials and performing finish rolling and coiling under specific temp. conditions. CONSTITUTION:The steel material contg. 0.001-0.01% C, <0.5% Si, 0.5-1.9% Mn, <0.015% P, <0.03% S, <0.07% Al, and 1 or >=2 kinds of 0.01-0.1% Ti, 0.01-0.1% Zr, 0.001-0.006% Ca, and further contg. 1 or >=2 kinds of 0.2-0.6% Cu, <0.5% Cr, <0.2% Ni, <0.5% Mo, <0.1% Nb, <0.1% V, 0.005-0.005% B and contg. ultralow carbon and <=1.9% Mn is used as a raw material. The steel plate which is subjected to hot rolling at 750-850 deg.C is coiled in a 500-700 deg.C temp. range. A hot coil having superior hydrogen induced cracking resistance is produced.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing an ultra-low carbon hot coil with excellent hydrogen-induced cracking resistance and high toughness, particularly for use in transportation pipes for natural gas in petroleum, etc. The present invention aims to provide a preferable method for producing ultra-low carbon steel hot coil material suitable for materials such as storage tanks.

In recent years, with the increase in oil consumption, there have been many plans for the development of oil fields and the installation of oil lines, but in such cases, storage tanks for the natural gas contained in the oil, etc., during transportation are often planned. Under the coexisting conditions of hydrogen sulfide and water, the steel surface is severely corroded. That is, hydrogen 9 generated by corrosion under the above conditions penetrates into the steel, is elongated, and aggregates on both sides of sulfide inclusions such as 9-8, increasing the internal pressure. Hydrogen-induced cracking occurs, and in some cases, it penetrates through the thickness direction and destroys the steel material. As a means to prevent hydrogen from entering steel in such a wet hydrogen sulfide corrosive environment, hydrogen induction has been conventionally achieved by adding CII of 25 or more to steel KO and forming a protective film with high corrosion resistance and bulk water permeability. This prevents the occurrence of cracks. However, the central segregation part of a practical steel ingot is S, C
It is difficult to completely prevent cracking by simply adding Cm to rounded grains, in which impurity elements such as , -, or elements that increase water volume-induced cracking susceptibility are concentrated;
Even with additive steel, if the environmental conditions in which it is used are severe, and the acidity of the coexisting moisture is -4.6 or less, there is a high possibility that the above-mentioned corrosion-resistant film will not be formed and hydrogen-induced cracking will occur frequently. Steel used in this harsh corrosive environment can be made by lowering the S content to reduce sulfide inclusions, which are the starting point for hydrogen-induced cracking, or by adding C1 or rare earth elements. It is necessary to control the shape of these nonmetallic inclusions to reduce the susceptibility to hydrogen-induced cracking. However, it is difficult to reduce the content of nonmetallic inclusions at a uniform level throughout the entire steel ingot, and it is also extremely difficult to effectively control the morphology of nonmetallic inclusions throughout the entire steel ingot. It is well known that Cm and rare earth element inclusions tend to accumulate particularly in one part of the steel ingot precipitate, resulting in increased cracking susceptibility.

On the other hand, the susceptibility of hot coil materials to hydrogen-induced cracking (compared to plate materials) is also greater, and the cracks are connected in a stepped manner, and the risk of penetrating through the plate thickness is extremely high.

To explain the susceptibility to hydrogen-induced cracking in hot coil materials and plate materials for lids, hot coil materials do not have the process of rolling in the width direction like plate materials, and are rolled by strong rolling in one direction compared to plate materials. Since it has been elongated significantly, its susceptibility to cracking is increased accordingly. In addition, hot coil materials are generally thinner than plate materials, so even if the inclusion content is the same, hot coil materials will have considerably more inclusions in terms of cross-sectional area in the thickness direction. At the same time, there is a greater risk that water volume-induced cracks that occur as cracks parallel to the plate thickness direction will connect in a stepwise manner and develop into cracks penetrating the plate thickness. Furthermore, as mentioned above, hot coils are rolled into a thin wall under unidirectional strong reduction conditions, so the rolling length is considerable, and even within the same steel plate, the rolling conditions vary considerably between the initial stage of production and the vicinity of the final stage. In addition, in the production of hot coils, there is a unique process of rapid cooling with water on a funnel, followed by coiling. Although there is an advantage in that a wide range of materials can be obtained through control, if there are variations in rolling conditions within the same coil, it is important to note that such variations are unavoidable or deliberate. , high cracking susceptibility
There is a risk that the paynite structure will be formed in a band shape, and the hydrogen-induced cracking resistance will be significantly deteriorated.

The present invention was created after repeated studies in view of the above-mentioned actual circumstances. That is, the present invention has been studied from both the steel composition and the rolling method, and has developed a structure that can withstand use under the severe corrosive environment conditions mentioned above and has excellent hydrogen-induced cracking resistance against changes in rolling conditions. It also has excellent toughness and has a tensile strength of 45 to 60 #/Ganji grade! Successfully obtained hot coil material for 9.

To further explain the present invention, generally speaking, the controlling factors for water volume induced cracking in hot coil materials are alloying elements,
The microstructure is due to nonmetallic inclusions (especially elongated IthlB inclusions) and variations in hot coil rolling conditions. In particular, reducing the C content in steel improves the hydrogen-induced cracking resistance, and by making it an ultra-low carbon content of less than 0.0111, the hydrogen-induced cracking resistance improves significantly, and it also has high toughness and roundness. We confirmed that the steel can be obtained with extremely low carbon content, and even with variations in rolling conditions during hot fill manufacturing, the microstructure of the hot coil material exhibits a single ferrite phase. Coupled with the fact that it is kept below 1.901Z, the formation of a crack-sensitive high-i low-temperature deformation structure is completely eliminated as shown in the attached photograph substituted for the drawing. That is, the steel components in the present invention are: C: Q, GO 1~0.01%, St
: 0.50- or less, I! lI: 0.50-1.90%
, P: 0.020- or less, Ji: 0.003- or less, M: 0.070- or less, and n: 0.0.
1-0.10%, Zr: 0.005-6.1005
1% Ca: 0.0010 to 0.00601 is added singly or in combination, and the remainder is iron and unavoidable impurities. IfCCu: 0 .20-0.401. Cr: 0.5
0- or less, N1: 0.20S or less, 36: 0.50
- Below.

Nb: CL10S or less, V: Q, 1011 or less, B=0
．． There are nine types containing one or more of 0005-Q and 0050-. In addition, as a rolling method for such ultra-low carbon steel or ultra-low carbon low alloy steel, the final finish rolling is performed within the temperature range of 750 to 850 °C during hot rolling, and the rolling is performed within the temperature range Ii of 550 to 700 °C. By doing this in one liter (1 L) in this way, in combination with the above-mentioned component system, excellent improvements in toughness and hydrogen-induced cracking resistance can be achieved.

The reason for limiting the composition of the steel in the above-mentioned 9.1 is as follows.

If C is contained in an amount exceeding o or oi-, there is a possibility that bainite and grain boundary carbides, which are harmful to hydrogen-induced cracking, will be generated under the hot rolling conditions described below. Also this C
It is industrially difficult to make the value less than 0.001.
In addition, there is a risk that ferrite grain boundary embrittlement may occur due to an increase in oxygen in the steel.

& is added for rounding for deoxidation, but if it exceeds 0.5091%, the toughness will deteriorate, so this is the upper limit.

- is needed to increase toughness and as a deoxidizer. That is, 0.
If it is less than 50, sufficient toughness cannot be obtained, so this is set as the lower limit, but if it exceeds 1.9, it promotes the formation of a low-temperature transformed structure that is harmful to hydrogen-induced cracking, so this is set as the upper limit.

In the case of a hot coil material, P may cause temper brittleness due to slow cooling after winding, and in order to suppress the promotion of this temper brittleness, it is necessary to set the P content to 0.020 s or less.

The rounding upper limit at which the hydrogen-induced cracking susceptibility increases as the SFi content increases is set to 0.003-. In addition, it is possible to lower it industrially by o, ooos-t.

M is necessary for deoxidizing rounding and is also effective for refining the microstructure, but if it exceeds 0.07, it will cause deterioration of toughness and increase of non-metallic inclusions, which is undesirable. The upper limit was set.

n%Zr and Cm are added for the purpose of effectively controlling the morphology and composition of sulfide-based nonmetallic inclusions in steel, but 7Z and Zr are added at 0.10%, and Ca#i0.006
Even if the content exceeds 1, the effect will be saturated and it will be economically disadvantageous.

On the other hand, Cu, Cr%Nl, Mo, Nb%V%B
These elements are added as necessary for the purpose of further improving the properties of manufactured steel.These elements contribute to improving hydrogen-induced cracking resistance, strength, and toughness, and improve the properties required for steel materials. Type 1 or 2 within an appropriate range considering usage conditions, etc.
More than one species can be used in combination. That is, a description of these items is as follows.

Cm is an element that is effective in improving strength and preventing hydrogen-induced cracking, but if it is less than 0.20, this effect will be poor, and if it is more than 0.60, it will cause deterioration in weldability, and Adversely affects workability.

Cr is effective in improving strength, but even if it exceeds O, SO-, the effect is saturated and there are 4 disadvantages economically.

N1 is an element that improves the strength and toughness of steel, but if it increases, it promotes hydrogen-induced cracking, so it is
− is the upper limit.

Nb, Me, V, l may be added to obtain the required strength level, but Nb, V is 0, 1-1 irises is 0.
．． Even if 5-1l is added in excess of o, oos-, the effect will be saturated and it is economically disadvantageous.

Next, the rolling conditions of the present invention will be explained. First, the final finishing temperature must be 750 to 850°C. That is 8
If the temperature exceeds 50℃, the austenite grains after finishing will become polygonal and large, and the ferrite grains that will be transformed and generated from this will also be large, and corrosion will easily occur in the environment where HsS and water coexist. It becomes impossible to increase hydrogen-induced cracking resistance. Also, if the final finishing temperature is less than 750℃, rolling will occur in the ferrite-austenite two-phase region, and even in a hot coil state, the ferrite still has considerable distortion from rolling, making it difficult to induce hydrogen resistance. This will worsen the cracking characteristics. The winding temperature needs to be 580 to 700°C, and as mentioned above, this winding temperature is adjusted by water cooling on the 57 out table. When it is gradually cooled, it becomes ζro. When this winding temperature exceeds 700°C, the hot coil material is slowly cooled from a high temperature, and the strength aimed at by the present invention cannot be obtained. If the coil material is subjected to large processing distortions during winding, low-temperature transformation products may easily form in pad/do shapes, which may worsen the hydrogen-induced cracking resistance. becomes.

Specific embodiments of the present invention will be described below.

The chemical compositions of the steel according to the present invention and the comparative round steel that the present inventors have specifically used for KN are as shown in Table 1 below, where steel is based on the basic composition, and steel is based on the basic composition. n%
Steel C contains Nk, and steel C is a composite addition of each component. Steel steel is a steel that does not contain Cu and Ni. Comparative steel is a steel with components corresponding to these seven elements, and C is a steel that does not contain Cu or Ni. High, steel g, H is 2% 84hK, steel Ge1-
It's also expensive.

Each steel billet as described above is heated to 1150-1250℃, and then the final finishing temperature in a hot strip mill is 750-850℃ to make a hot coil with a final plate thickness of 13m, and the subsequent winding temperature is was set in the range of 550 to 700°C.

These hot coil materials or the electrically welded pipes made using the same were subjected to a tensile test, a 21EIIV notched mechanical impact test (all in the 4C direction), and a hydrogen-induced cracking test. The shape of the hydrogen-induced cracking test piece is 100mj
20wW x 11Wt, collected from the area where the central segregated area was exposed by gill king, and artificial seawater (B, pH condition,
pit di51) or Q, 5-acetic acid + 5-saline solution (N
ACl [fun ph! Each of 3.8) was saturated with hydrogen sulfide and immersed in a corrosive environment for 96 hours with no stress applied. After that, a specimen was taken out and subjected to a microscopic examination of 9 cross sections of 1 steel type to determine hydrogen cracking. I did it.

This table shows the results of each test at its specific pressure, that is, the steel in the composition range of the present invention has excellent toughness (-100℃) and corrosion resistance under any hot coil manufacturing rolling conditions. There was no occurrence of hydrogen-induced cracking even under the severe environmental conditions of −4 and 6.

On the other hand, substances outside the range of ingredients in the present invention are highly corrosive and cause slight cracking even in hydrogen sulfide-saturated artificial seawater, and even more corrosive hydrogen sulfide-saturated 0.51 acetic acid +
In the 5s saline solution, many hydrogen-induced cracks occur regardless of the hot rolling conditions, and it is clear that the 5s saline solution is highly susceptible to hydrogen-induced cracks.

In addition, according to the above-mentioned friend example, for the steel in the present invention,
1200°C slab heating → 850°C finishing compression → 580°C rolling The microscopic structure of the ERW steel pipe using the bottle material is clear as shown in Figure li1, and the same <1200°C slab heating → 750°C finishing pressing → 580°C rolling There is no formation of a low-temperature transformed structure that is highly susceptible to micro-''' cracking in the ERW steel pipe due to the bottle material.

As explained above, according to the present invention, the hydrogen-induced cracking resistance does not deteriorate even with changes in rolling conditions, and it exhibits excellent toughness and hydrogen-induced cracking resistance, making it suitable for lines, green ingots, and other applications. This invention is one of the most effective industrially, since it can be obtained accurately using nine hot coil materials.

[Brief explanation of the drawing]

The drawings show the technical content of the present invention, and Figure 1 shows the heating of a slab at 1200°C → 85G according to an embodiment of the present invention.
℃ finish compression → 580℃ rolling rolling, and Figure 2 is nine photographs showing each microscopic structure of 1200℃ slab heating → 750℃ finishing rolling → 580℃ rolling. Patent applicant Nippon Kokan Co., Ltd. Inventor Yutaka Inagaki Wama
Toshio Nakazawa! 1
11 Treasure 211I

Claims (1)

[Claims] 1, C: Q, 001-0.01%, st:o, s* or less, m-&5-191, P:Q, 015% or less, g:o
, oos* and below, #:0.0? -Contains the following, n: 0.01~0.11G, Zr: 0.01~
0.14% Ca: G, final finish rolling of steel containing one or more of 001 to &0061i, with the balance consisting of iron and unavoidable impurities at a temperature of 750 to sso°C. A method for producing an ultra-low carbon hot coil with excellent hydrogen-induced cracking resistance and high tip toughness, characterized by rolling the coil at 580 to 700°C. L Cm: 0.2-0. @-1Cr:O,S
11 or less, N1:Q111i or less, Mo:Q, 5- or less, Nb:Q, 1- or less, V:Q, 1- or less, l:0. o
A method for manufacturing an ultra-low carbon hot coil with excellent hydrogen-induced cracking resistance and high toughness according to claim 1, using steel containing one or more of o05 to o, and oos-. .