JPS6235452B2 - - Google Patents

Description

[Detailed description of the invention]

This invention relates to a steel material with excellent hydrogen-induced cracking resistance, and in particular, Mn is used to improve hydrogen-induced cracking resistance, which is required for applications such as line pipe steel and oil country tubular goods used in humid environments containing hydrogen sulfide. The purpose of this paper is to propose a method for manufacturing steel materials with excellent resistance to hydrogen-induced cracking, which improves the microstructure related to the steel composition, which is particularly improved even when the hydrogen content is 1.2% by weight or more. In recent years, line pipes and oil country tubular goods used to transport crude oil and natural gas containing hydrogen sulfide,
Cases of leakage or breakdown accidents caused by so-called hydrogen-induced cracking have been reported, and the resistance to hydrogen-induced cracking of this type of steel has become an important issue. Many studies have been conducted in recent years on the mechanism of hydrogen-induced cracking, and the following mechanism has been clarified. In other words, hydrogen-induced cracking occurs when hydrogen generated by the corrosion reaction of steel invades the steel, and the hydrogen that has penetrated into the steel accumulates and gasifies at the interface between nonmetallic inclusions in the steel and the base steel. This gas pressure causes cracks. Among non-metallic inclusions, A, such as MnS, tends to cause stress concentration due to the notch effect at the tip of the inclusion.
System inclusions are the most harmful to hydrogen-induced cracking, and low-temperature transformed abnormal structures (hereinafter referred to as abnormal structures) such as band-shaped martensite and bainite that occur in segregation areas are the structures that are most likely to cause cracks to propagate. It is known. Based on the above, the methods conventionally used as countermeasures against hydrogen-induced cracking are methods of reducing the amount of hydrogen that enters the steel, methods of reducing abnormal structures that are prone to propagation of cracks, and methods of reducing A-based inclusions that are the starting point of cracks. It can be roughly divided into three methods: dispersion and spheroidization. The most effective of these is Ca or REM.
The dispersion and spheroidization of inclusions due to the addition of Mn
In high Mn steel materials with Mn of 1.20% by weight or more, an abnormal structure develops, so the above measures alone are insufficient.
It is necessary to simultaneously take measures to reduce the abnormal structure, such as lowering P or reducing segregation, and heat treatment, so an increase in manufacturing costs is unavoidable. The inventors conducted research to overcome the drawbacks of the prior art and obtain a low-cost, high-Mn steel material with excellent hydrogen-induced cracking resistance, and as a result, they obtained the following new knowledge. In other words, in conventional ferrite/pearlite steel, the higher the Mn content, the more likely abnormal structures develop in the segregated areas, and the more the steel has a Vickers hardness of 350 or higher, resulting in higher cracking susceptibility. When a high Mn steel material is made of a particularly uniform fine ferrite/bainite steel, even the central segregation part has a structure with a Vickers hardness of 350 or less, and the hydrogen-induced cracking resistance is significantly improved. This is because the hardness of the bainite itself that is generated decreases due to lowering the carbon content, and by creating a composition system that generates bainite uniformly, the carbon that diffuses into the segregated area during low-temperature transformation is captured by the surrounding bainite. This is because the C content of bainite or island-like martensite that occurs in the segregated area is significantly lower than that that occurs in the segregated area of ordinary ferrite-pearlite steel. This invention is based on the above new knowledge,
A steel slab containing C0.08% by weight or less and Ca addition with low S content to sufficiently disperse and spheroidize inclusions was heated to a temperature of 900°C or higher and 1350°C or lower. After that, it is rolled at a temperature of (Ar ₃ + 150℃) or higher so that the cumulative reduction ratio is 50% or more and 95% or less, and then it is rolled at a temperature of (Ar ₃ + 150℃) or lower and with ₃ or more Ar points. Rolling is carried out so that the cumulative reduction ratio is 50% or more and 95% or less within the temperature range of the austenite region, and then the temperature of the two-phase region of austenite and ferrite is lower than ₃ points of Ar and is at least (Ar ₃ −100℃). It is rolled so that the cumulative reduction ratio is 10% or more and 92% or less within the range, and then air-cooled or water-cooled to make the product mainly composed of ferrite, fine bainite, island martensite, and finely processed ferrite with a grain size of 5μ or less. It was discovered that the hydrogen-induced cracking resistance of steel can be significantly improved by creating a so-called fine ferrite-bainite structure. The following steel compositions are compatible with the common purpose according to the above basic knowledge through each component series, and each component series can further enrich the individual properties. 1 C: 0.08% by weight or less, Si: 0.01 to 0.50% by weight, Mn: 1.20 to 3.00% by weight, P: 0.020% by weight
Below, S: 0.003% by weight or less, Ca: 0.001~
0.010% by weight, Al: 0.01 to 0.10% by weight, and the remainder substantially consists of Fe. 2 Based on the component composition of 1, Cu: 0.15
~0.60 wt%, Ni: 0.10~0.60 wt% and
Cr: component composition containing at least one selected from 0.10 to 3.0% by weight. 3 In addition, Mo: 0.01 to 1.0% by weight, Nb:
A component composition containing at least one selected from 0.01 to 0.15% by weight, V: 0.01 to 0.15% by weight, and Zr: 0.01 to 0.15% by weight. 4 Similarly, B: 0.0005 to 0.005% by weight or B: 0.0005 to 0.005% by weight and Ti: 0.01 to
Ingredient composition containing 0.10% by weight. 5 Similarly, further Cu: 0.15 to 0.60% by weight, Ni:
At least one selected from 0.10 to 0.60% by weight, Cr: 0.10 to 3.0% by weight, and Mo:
0.01-1.0% by weight, Nb: 0.01-0.15% by weight,
A component composition containing at least one selected from V: 0.01 to 0.15% by weight and Zr: 0.01 to 0.15% by weight. 6 In addition, Cu: 0.15 to 0.60% by weight, Ni:
At least one selected from 0.10 to 0.60% by weight and Cr: 0.10 to 3.0% by weight, and B:
0.0005-0.005% by weight or B: 0.0005-0.005
% by weight and component composition containing Ti: 0.01-0.10% by weight. 7 In addition, Mo: 0.01 to 1.0% by weight, Nb:
At least one selected from 0.01 to 0.15% by weight, V: 0.01 to 0.15% by weight, and Zr: 0.01 to 0.15% by weight, and B: 0.0005 to 0.005% by weight, or B: 0.0005 to 0.005% by weight, and Ti. :
Ingredient composition containing 0.01-0.10% by weight. 8 Similarly, Cu: 0.15 to 0.60% by weight, Ni:
At least one selected from 0.10 to 0.60% by weight and Cr: 0.10 to 3.0% by weight, and Mo: 0.01
~1.0% by weight, Nb: 0.01-0.15% by weight, V:
0.01-0.15 wt% and Zr: 0.01-0.15 wt%
At least one type selected from among and B:
0.0005-0.005% by weight or B: 0.0005-0.005
% by weight and component composition containing Ti: 0.01-1.0% by weight. According to the inventors' research, it is very effective to reduce the hardness of the abnormal structure that occurs in the segregated area to a Vickers hardness of 350 or less in order to reduce the susceptibility to hydrogen-induced cracking. By reducing C to 0.08% or less and making the structure uniform and fine ferrite bainite, it was possible to reduce the hardness of bainite or island martensite occurring in the segregated areas to 350 or less through each of the above-mentioned component systems. Next, under low S conditions, inclusions are dispersed and spheroidized by adding Ca to reduce crack initiation points, and at the same time, by lowering the hardness of the abnormal structure that becomes the crack propagation structure, the steel becomes resistant to hydrogen-induced cracking. Significantly improves sex. After the above-mentioned three-step rolling process, it is possible to produce ferrite, fine bainite, island-like martensite, and micromachined ferrite with a crystal grain size of 5 μm or less without being affected by the cooling rate by simply subjecting it to cooling treatment by air or water cooling. A structure mainly composed of ferrite, that is, a fine ferrite-bainite structure is obtained. Next, the reason for limiting the ingredients of this invention will be described. C: 0.08% by weight or less (hereinafter simply expressed as % for simplicity) This is an element that improves the strength of steel, but if it exceeds 0.08%, the hardness of bainite or island martensite that occurs in the segregated area becomes Bitkers hardness. in
If it exceeds 350, it deteriorates the hydrogen-induced cracking resistance, so it was limited to 0.08% or less. Si: 0.01 to 0.50% This is an element necessary for deoxidation, but if it is less than 0.01% it will not be effective and if it exceeds 0.50% it will damage the toughness of the steel, so it was limited to a range of 0.01 to 0.50%. Mn: 1.20-3.00% An element that improves strength, and is especially necessary to obtain a uniform fine ferrite bainite structure. If it is less than 1.20%, it is difficult to obtain the desired structure and strength. If it exceeds 1.20% to 3.00, it will have a negative effect on the toughness of the steel.
% range. P: 0.020% or less P is easily segregated, causing non-uniform structure and hardness, and also deteriorating toughness, so it is desirable to have less P, but since lowering P increases manufacturing costs, The content was limited to 0.020% or less, which is the upper limit that does not adversely affect the invention. S: 0.003% or less S is limited to 0.003% or less because if it exceeds 0.003%, even if Ca is added, the effect of improving hydrogen-induced cracking resistance due to dispersion and spheroidization of inclusions cannot be sufficiently obtained. Ca: 0.001-0.010% At least 0.001% is required for inclusion dispersion and spheroidization due to Ca addition, but 0.010%
If added in excess of
It was limited to the range of 0.001-0.010%. Al: 0.01~0.10% This element is necessary for deoxidation and improves the yield of Ca, but if it is less than 0.01%, it has no effect, and if it exceeds 0.10%, it causes coarsening of crystal grains and deteriorates the material. I don't want it to deteriorate, so
It was limited to the range of 0.01-0.10%. C, Si, Mn, P, S, Ca and
The basic composition to which this invention is advantageously applied has a limited range of components of Al, but the following components can also be further developed for the same purpose as expected in the basic composition within their limited ranges. . Cu: 0.15-0.60% Cu is contained at 0.60% or less with the main purpose of improving corrosion resistance, but it is also effective in improving hydrogen-induced cracking resistance, but less than 0.15% has little effect.
If it exceeds 0.60%, hot workability will be impaired, so 0.15
~0.60% range. Ni: 0.1-0.6% Ni is also contained at 0.6% or less with the main purpose of improving corrosion resistance, but it is also effective in improving toughness, and especially when Cu is contained at 0.2% or more, it is also used to prevent embrittlement due to Cu. Contributes to this by 0.10~0.60
%, it was limited to 0.10 to 0.60%. Cr: 0.10-3.0% Cr is also contained at 3.0% or less with the main purpose of improving corrosion resistance, but it is also effective in improving strength and toughness, but if it is less than 0.10%, it has no effect.
In addition, if it exceeds 3.0%, it will adversely affect workability, so it is limited to a range of 0.10 to 3.0%. Here, the above-mentioned Cu, Ni and Cr have the same effect within their respective component ranges. Mo: 0.01-1.00% Mo is effective in improving hardenability and strength at 1.00% or less, but less than 0.01% has little effect.
Addition of a large amount exceeding 1.00% may deteriorate the toughness, so the content was set in the range of 0.01 to 1.00%. Nb, V and Zr: 0.01-0.15% Nb, V and Zr have almost the same effect as Mo at 0.15% or less, and for the same reason as mentioned for Mo, Nb: 0.01-0.15% and V:
The range was limited to 0.01 to 0.15%, and Zr: 0.01 to 0.15%. B: 0.0005-0.005% B is an element that improves hardenability and is effective at 0.05% or less, but less effective at less than 0.0005%.
If it exceeds 0.005%, the toughness will be damaged, so 0.0005~
It was limited to a range of 0.005%. Ti: 0.01 to 0.1% Ti coexists with B and not only makes the effect more effective, but also contributes to improving strength and corrosion resistance. If Ti is less than 0.01%, the coexistence with B is insufficient, while if it exceeds 0.10%, the toughness deteriorates. It is. Next, the reasons for limiting the processing steps applied to the steel material of this invention will be described. First, the slab heating temperature is set at 900°C or higher because rolling is difficult in practice unless it is higher than 900°C. However, if the slab heating temperature is increased unnecessarily, hot cracking occurs during rolling due to the appearance of δ phase or liquid phase, which is preferable. do not have. In order to prevent such hot cracking, in normal rolling, the slab heating temperature is kept below the _A4 point (1400℃), but considering that the _A4 point varies depending on the composition or segregation, the slab heating temperature is increased to 1350℃. was set as the upper limit. In this invention, fine ferrite and fine bainite structures are essential requirements, but in order to generate these fine particles, it is necessary to reduce the austenite crystal grain size to 20μ or less, and for this,
First of all, it is necessary that the cumulative reduction ratio in the high temperature recrystallized austenite region of (Ar ₃ + 150℃) or higher with respect to the steel slab heated to a temperature of 900℃ or higher is 50% or more, and if it is lower than this, the fine ferrite bainite structure will be formed. I can't get it. In addition, the cumulative reduction rate in this region exceeding 95% is appropriate for rolling in the unrecrystallized austenite region from (Ar ₃ +150℃) to Ar ₃ and in the region from Ar ₃ to (Ar ₃ -100℃). Since it would be difficult to reduce the amount by a certain amount, the upper limit was set at 95%. Next, by applying rolling of 50% or more in the unrecrystallized austenite region from (Ar ₃ +150℃) to Ar ₃ , the formation of bainite and island martensite is promoted, and from Ar ₃ to (Ar ₃ -100 A fine ferrite-bainite structure can be obtained by rolling at 10% or more in the (γ + α) two-phase region up to (°C). If the rolling of the non-recrystallized region from (Ar ₃ +150℃) to Ar ₃ is less than 50% and is insufficient, the formation of bainitic martensite outside the segregated areas will be delayed, and C will diffuse into the segregated areas. This produces a so-called abnormal structure in the segregated area, which deteriorates the hydrogen-induced cracking resistance. In addition, the fact that the cumulative rolling reduction rate in this region exceeds 95% means that the appropriate amount of rolling in the high-temperature recrystallized austenite region above (Ar ₃ +150℃) and in the region from Ar ₃ to (Ar ₃ -100℃) is required. Since this makes rolling down difficult, the upper limit was set at 95%. Furthermore, rolling in the region from Ar ₃ to (Ar ₃ −100℃)
Even if it is less than 10%, the transformation of retained austenite is delayed, resulting in the formation of a small amount of abnormal structure, and at the same time, the ferrite grain size cannot be made sufficiently fine, making it difficult to obtain the toughness of steel, and (Ar ₃ -100℃)
If the following low-temperature rolling is performed, the toughness of the steel may be impaired. In addition, the cumulative reduction rate in this region exceeds 92% when rolling in the high-temperature recrystallized austenite region above (Ar ₃ +150℃) and in the unrecrystallized austenite region from (Ar ₃ +150℃) to Ar ₃ . Since it is difficult to reduce the appropriate amount of steel, the upper limit was set at 92%. For the above reasons, in the present invention, the rolling conditions are limited as described above. Regarding cooling after the above-mentioned rolling step, as mentioned above, the desired fine ferrite-bainite structure can be obtained by performing air cooling or water cooling. Examples The inventors conducted tests to clarify the improvement in hydrogen-induced cracking resistance according to the present invention based on four types of low-C, high-Mn steels with different C and Mn levels. (Table 1) shows the chemical components of the samples used in the test. Steels 1 to 24 are all fine ferrite bainite steels that have been rolled to satisfy the rolling conditions specified in this invention. Steels 1 to 20 are air-cooled after rolling, and steels 21 to 24 are shower water-cooled to 500℃ after rolling. I did this. Steels 8A to 8D have the same composition as Steel 8, but only the rolling conditions were changed, and the effects of the rolling conditions were observed.

[Table] (Table 2) shows the rolling conditions and mechanical properties of the test steel. BP is a method for evaluating hydrogen-induced cracking resistance.
Test conditions (96-hour immersion in _H2S -saturated artificial seawater,
pH approx. 5.2) and NACE conditions ( _H2S saturated 0.5% acetic acid + 5%
Two test conditions were used: immersion in saline solution for 96 hours and liquid pH of approximately 3.0).

【table】

[Table] Test pieces were taken in the rolling direction as shown in Figure 1 from a position corresponding to the center of the width of the continuously cast slab where segregation is considered to be the largest. Figure 2 shows the shape of the test piece. Cracking was determined by immersing the specimen in each test solution for 96 hours under no load, and then examining 3 cross sections of each test piece (3 cross sections of each type of steel, 9 cross sections in total, 10x magnification) as shown in Figure 3. Ta. Table 3 shows the test results and the maximum value of the Vickers hardness (50 g) of the central segregation part of the sample.

【table】

[Table] As can be seen from the following, each test steel of the present invention, which is a fine ferritic bainitic steel with a C content of 0.08% or less, and which has sufficient inclusion dispersion and spheroidization, exhibits excellent hydrogen-induced cracking resistance. Samples 1 to 4 in which Ca is less than 0.0010% or S content is more than 0.003%,
Comparative steels Nos. 9, 10, 14, 15, 21, and 22 have inferior hydrogen-induced cracking resistance. Further, in steel samples 16 and 17 in which C exceeds 0.08%, the Vickers hardness of the segregated portion is 350 or more, and therefore sufficient hydrogen-induced cracking resistance cannot be obtained. Further, when the rolling conditions of the present invention are not satisfied, such as steels 8-A to 8-D, an abnormal structure having a Vickers hardness of 350 or more is formed, so that sufficient hydrogen-induced cracking resistance cannot be obtained. From the above results, it is clear that the present invention provides a steel material with excellent resistance to hydrogen-induced cracking.

[Brief explanation of the drawing]

Figure 1 is an explanatory diagram showing the procedure for collecting test pieces from plate materials, Figure 2 is a perspective view of a hydrogen-induced cracking test piece, and Figure 3 is a perspective view of a hydrogen-induced cracking test piece.
The figure is an explanatory diagram of the hydrogen-induced crack microscopy procedure.

Claims (1)

[Claims] 1 C: 0.08% by weight or less, Si: 0.01 to 0.50% by weight, Mn: 1.20 to 3.00% by weight, P: 0.020% by weight or less, S: 0.003% by weight or less, Ca: 0.001 to 0.010% by weight. %, and Al: 0.01 to 0.10% by weight after heating at a temperature of 900°C or higher and 1350°C or lower.
Cumulative reduction rate is 50% at temperatures above (Ar ₃ +150℃)
Rolling where the cumulative reduction rate _is 50% or more and 95% or less at (Ar ₃ + 150℃) or less and within the temperature range of the unrecrystallized austenite region with Ar ₃ points or more Rolling with a cumulative reduction rate of 10% or more and 92% or less within the two-phase temperature range of austenite and ferrite of 100% or more (Ar ₃ -100℃), followed by cooling treatment by air cooling or water cooling. A method for producing a steel material having excellent hydrogen-induced cracking resistance, the method comprising: obtaining a structure mainly consisting of ferrite, fine bainite, island martensite, and microfabricated ferrite with a grain size of 5 μm.