JPS6334205B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing low temperature steel having excellent toughness and hydrogen-induced cracking resistance. Recently, in the shipbuilding industry, construction of carrier vessels and storage tanks for liquefied petroleum gases such as LNG and LPG has become active, and the low-temperature steel plates used for these tanks, secondary barriers, and internal structural members have excellent toughness at low temperatures. requested. Furthermore, as large-diameter line pipes for natural gas are used in colder regions and are operated at higher pressures, there is an increasing demand for strict low-temperature toughness, especially for thick-walled materials. In particular, in order to improve the propagation arresting characteristics of unstable ductile fractures, we have developed a method that has a high shear pie impact absorption energy and a fracture surface transition temperature of Drap Weight Tear Test (hereinafter abbreviated as BDWTT).
Below Appearance Transition Temperature
There is a demand for steel sheets for pipes with excellent properties (abbreviated as FATT). In particular, recently, the Shalpy impact absorption energy standard is not based on the design temperature, but is specified at the lowest temperature at which the ductile fracture surface shows 100%, and the impact absorption energy (abbreviated as Cv100) has become higher. Separation occurs in control-rolled materials used as steel plates, so even if they show a 100% ductile fracture surface, the absorbed energy is not saturated, resulting in a small Cv100 value, making it difficult to meet the required standards. On the other hand, embrittlement of materials due to corrosion has become a major problem in oil and natural gas pipelines. This is because crude oil and natural gas often contain hydrogen sulfide, which causes hydrogen-induced cracking to occur in a continuous manner in the thickness direction, leading to pipe failure. Today, it is common to use large steel ingots and continuously cast slabs to reduce costs when manufacturing steel plates for pipes.When forming these ingots, the solidification rate is slow in the center, resulting in segregation mainly consisting of Mn and P. As a result, a rough band-like structure consisting of martensite and bainite appears after rolling. This band-like structure not only adversely affects the hydrogen-induced cracking resistance but also deteriorates the toughness, so it has been difficult to take countermeasures against this problem. QT heat treatment can be considered as one solution to this problem, but in order to achieve BDWTT properties comparable to those of control-rolled materials that exhibit separation, it requires a large amount of expensive components such as Ni.
Even in the heat treatment process, simple quenching and tempering is not sufficient, and a complicated heat treatment of repeating quenching and tempering twice has to be performed, which is extremely disadvantageous from an economic standpoint. This invention has Cv100, hydrogen-induced cracking resistance, and
We have discovered a method to produce steel plates with excellent BDWTT characteristics at low cost using large steel ingots or continuously cast slabs. That is, C: 0.02 to 0.2 wt%, Si: 0.05 to 0.50 wt%, Mn: 0.5 to 2.0 wt%, Al: 0.005 to 0.1 wt% and Nb: 0.01 to 0.08 wt%, and S: 0.006 wt% or less. A steel slab containing iron with the balance Fe and unavoidable impurities is heated and rolled in a temperature range of 900 to 1200℃, and then rolled in an unrecrystallized austenite region or an austenite-ferrite two-phase region at a temperature of 900℃ or less. ,
Immediately after applying a cumulative reduction of 40% or more
Toughness characterized by being cooled to below the Ar transformation point, then reheated to a temperature range of A _c1 point to A _c3 point, then water-cooled, and then tempered at a temperature below A _c1 point. and a method for producing low-temperature steel with excellent hydrogen-induced cracking resistance. (First invention) C: 0.02-0.2wt%, Si: 0.05-0.50wt%, Mn: 0.5-2.0wt%, Al: 0.005-0.1wt% and Nb: 0.01-0.08wt%, S: 0.006wt% % or less, and further contains Ti: 0.003 to 0.04wt%, Ni: 1.0wt% or less, Mo: 0.3wt% or less, Cu: 0.5wt% or less, V: 0.1wt% or less, Cr: 0.4wt% or less. and B: any one or two selected from the group consisting of 0.003wt% or less
A steel slab containing Fe and unavoidable impurities is heated and rolled in a temperature range of 900 to 1200℃, and then the steel slab is rolled into an unrecrystallized austenite region or austenite-ferrite two phase below 900℃. In the area,
Immediately after applying a cumulative reduction of 40% or more
Toughness characterized by being cooled to below the Ar transformation point, then reheated to a temperature range of A _c1 point to A _c3 point, then water-cooled, and then tempered at a temperature below A _c1 point. and a method for producing low-temperature steel with excellent hydrogen-induced cracking resistance. (Second invention) C: 0.02 to 0.2 wt%, Si: 0.05 to 0.50 wt%, Mn: 0.5 to 2.0 wt%, Al: 0.005 to 0.1 wt% and Nb: 0.01 to 0.08 wt%, S: 0.006 wt% % or less, and further contains Ti: 0.003 to 0.04wt%, Ni: 1.0wt% or less, Mo: 0.3wt% or less, Cu: 0.5wt% or less, V: 0.1wt% or less, Cr: 0.4wt% or less. and B: any one or two selected from the group consisting of 0.003wt% or less
Contains one or two of the following groups: Ca: 0.001 to 0.1 wt%, REM: 0.001 to 0.1 wt%, and the remainder
A steel slab made of Fe and unavoidable impurities is
Heating and rolling in a temperature range of ~1200℃, followed by rolling in an unrecrystallized austenite region or austenite-ferrite two-phase region below 900℃,
Immediately after applying a cumulative reduction of 40% or more
Toughness characterized by being cooled to below the Ar transformation point, then reheated to a temperature range of A _c1 point to A _c3 point, then water-cooled, and then tempered at a temperature below A _c1 point. and a method for producing low-temperature steel with excellent hydrogen-induced cracking resistance. (Third invention) It is generally known that making the structure of a steel plate finer has the effect of simultaneously improving its impact properties, that is, fracture surface transition temperature and absorbed energy, and increasing its strength. If controlled rolling is used to increase the rolling amount below 900°C to make grains finer,
At the same time, it promotes the development of rolling texture, resulting in generation of separation and enhancement of anisotropy. Therefore, the impact absorption energy in the direction perpendicular to the rolling direction is extremely reduced. Therefore, we considered a method to improve the absorbed energy of Shalby impact, especially the absorbed energy in the direction perpendicular to the rolling direction, by making the fine grain structure obtained by controlled rolling even finer. After rolling, we found that the result was that after rolling, it was again between the Ac ₁ point and the Ac ₃ point. heated to a temperature, then put into water to quench, then
It was found that tempering below the Ac ₁ transformation point not only improves Cv100 absorbed energy but also improves BDWTT properties and hydrogen-induced cracking resistance. In order to obtain a low-temperature steel suitable for the purpose of this invention, the following limitations must first be made regarding its chemical composition. C; 0.02 to 0.2% If C is less than 0.02%, the strength of the steel material will decrease, and the weld heat affected zone (HAZ) will be greatly softened, so the lower limit of C content is set to 0.02%, while C
If it exceeds 0.2%, the toughness of the base metal deteriorates, hardening of the welded part, and deterioration of cracking resistance are significant, so the upper limit was set at 0.2%. Si: 0.05-0.50% Si is an element that is inevitably included for deoxidation during steel refining, but if it is less than 0.05%, the toughness of the base material will deteriorate, so the lower limit is set at 0.05%. Since toughness decreases due to solid solution hardening, the upper limit was set at 0.50%. Mn: 0.5-2.0% If Mn is less than 0.5%, the strength and toughness of the steel will decrease, and the softening of the HAZ will increase, so the lower limit must be set.
The upper limit was set at 0.5%, and since too much Mn deteriorates the toughness of the HAZ. Al; 0.005-0.1% Al is essential for deoxidizing steel and requires a minimum of 0.005%, but if it exceeds 0.1%, it will not only deteriorate the toughness of the HAZ and weld metal but also the surface shape, so 0.1%
% was set as the upper limit. Nb: 0.01-0.08% If Nb is less than 0.01%, there will be a lack of Nb (CN) precipitates and there will be no fine grain effect. However, if Nb is contained in excess of 0.08%, the toughness of the weld metal and HAZ will deteriorate significantly, and this Therefore, it was set in the range of 0.01 to 0.08%. S: 0.006% or less S is an unavoidable element as an impurity in steel,
In order to improve toughness and ensure hydrogen-induced cracking resistance according to the present invention, the content must be 0.006% or less. Therefore, the upper limit of S was set at 0.006%. Next, in this invention, as a group (a) element,
0.003~0.04% Ti, 1.0% or less Ni, 0.3% or less
One or more elements among Mo, 0.5% or less Cu, 0.1% or less V, 0.4% or less Cr, and 0.003% or less B all further improve the strength and toughness of the base material. It is useful as a selective component because it satisfies a common purpose by making it possible to expand the thickness of the plate that can be manufactured, and exhibits the same effect under the reason for restricting the content described below. Ti can be used to fix N in the form of TiN to refine the austenite grains and provide strength, but in this case, a minimum of 0.003% is required; on the other hand, if it exceeds 0.04%, not only TiN but also TiC is generated. The toughness of the material and welded parts deteriorates. Yotsute 0.003~
The range was set at 0.04%. Ni improves the strength and toughness of the base material without adversely affecting the hardenability and toughness of HAZ, but for the subject of this invention, it is not necessary to add 1.0%, and no more than 1.0% is required for manufacturing. The upper limit was set at 1.0% because it would lead to an increase in costs. Mo makes the γ grains regular during rolling, and also produces fine baitite during quenching, so it is useful for improving strength and toughness, but it is not necessary to add more than 0.3%. The upper limit was set at 0.3% because it would only lead to an increase in manufacturing costs. In addition, Cu not only has almost the same effect as Ni but also improves corrosion resistance, but if it exceeds 0.50%, cracks are likely to occur during hot rolling and the surface shape of the steel sheet deteriorates, so the upper limit was set at 0.50%. . If Cr and V exceed 0.40% and 0.1%, respectively, they will impair not only the toughness of the steel material but also the toughness of the welded part, so upper limits should be set for each value. It is advantageous to add B in an amount of 0.003% or less, which has the effect of increasing the toughness of the base metal in addition to increasing the hardenability. In addition to the above-mentioned group (a) selection component, in this invention, the group (b) element Ca or REM (rare earth element) is optionally added.
By containing at least one of these in the range of 0.001 to 0.1% (in the case of two types, the total amount), the toughness of the steel plate in the direction perpendicular to rolling can be improved. The reason for this range limitation is that it is less than 0.001%.
Ca and REM are disadvantageous because they do not sufficiently reduce MnS, which is elongated during rolling and has harmful effects, and if it exceeds 0.1%, oxide formation becomes excessive, damaging the internal properties of the steel. It's for a reason. As mentioned above, (i) Group (a) elements are used to improve the strength and toughness of the base material.
Furthermore, in order to increase the plate thickness, (ii) (b) group elements each have the same effect in improving the toughness of the steel plate in the direction perpendicular to the rolling direction. Next, the reason why the treatment according to the present invention is added under the specified chemical components as described above to improve various properties suitable for the purpose is derived from the following technical significance. The most important point of this invention is that the slab is heated to a temperature of 900 to 1200°C or higher, then rolled, and then the area reduction is achieved in the unrecrystallized austenite region or austenite-ferrite (hereinafter abbreviated as γ+α) two-pair region at a temperature of 900°C or lower. After applying a cumulative pressure of 40% or more, it is cooled to below the Ar transformation point, then reheated to a temperature range of Ac ₁ to Ac ₃ , and then water-cooled (γ + α). , when reheated to the two-phase region,
Some of the ferrite grain boundaries in the rolled structure become austenite grains, and when the material is poured into water after reheating, it becomes martensite, resulting in a fine mixed two-phase structure of ferrite and martensite. That is, since martensite is generated by dividing the original ferrite grains, the average grain size of the mixed structure is smaller than the ferrite grain size as rolled. Also, although the separation number decreases by reheating in this (γ+α) two-phase region, it does not become zero unlike reheating at _three or more Ar points. In other words, it is thought that reheating in the (γ+α) two-phase temperature range combines a moderate separation effect and a refinement effect of the mixed two-phase structure, thereby significantly improving the BDWTT characteristics. This water cooling treatment and tempering heat treatment improve the shear pie impact absorption energy due to the relaxation of separation and the tempering effect, and the strain relief effect of the martensite and bainite structures near the expanded inclusions such as MnS and the like improves the resistance. It is thought that the hydrogen-induced cracking properties will also be improved. In this invention, as described above, a controlled rolled steel plate with limited chemical composition is reheated to a specific temperature and then subjected to water cooling and tempering treatment to adjust the number of separations and improve impact absorption energy.
Furthermore, it can be said that this was actively used to improve the BDWTT characteristics. Next, the reasons for limiting the constituent elements of this invention will be described. First of all, the reason why the slab heating temperature is set in the range of 900 to 1200℃ is that if the slab heating temperature is lower than 900℃, heating will occur in the (γ + α) two-phase region, making it difficult to roll in the unrecrystallized γ region below 900℃. Therefore, the temperature should be 900℃ or higher. In addition, if the slab heating temperature exceeds 1200℃, the γ grain size during heating will become significantly coarsened, so even if rolling is performed in the non-recrystallized γ region at 900℃ or less, the grains will not be refined. It becomes difficult and the BDWTT characteristics deteriorate. 900 as the slab heating temperature
-1200℃ range. Next, the unrecrystallized γ region below 900℃ and (γ + α)
The cumulative reduction in the two-phase region is given, and the larger the reduction, the better; 85% of the BDWTT.
The lower limit was set at a cumulative reduction rate of 40% at which SATT was below -30°C (plate thickness 20mm). After rolling, the material is cooled once below the Ar ₁ transformation point.
This is because a sufficient fine grain effect cannot be obtained unless the temperature is cooled to Ar ₁ or less. The major feature of this invention is that after rolling cooling (γ + α)
The reason for this is that the material is heated to a two-phase region, water-cooled, and then tempered in a normal temperature region. Next, embodiments of the present invention will be described. Table 1-1 shows the chemical composition of steels A, B1, and B2 manufactured from large slabs and continuous cast slabs, and the chemical composition of these steels in the direction perpendicular to the rolling direction when manufactured under different rolling and heat treatment conditions. Table 1-2 shows the results of the investigation of various characteristics. Table 1 Example 1

【table】

[Table] ** Evaluation of cracks ○: No cracks, △: Small cracks, ×
: Large crack *** Presence or absence of separation ○: Yes, ×: No
Not only comparative steel A, which does not contain Nb, but also test steels B1 and B2, which contain Nb and satisfy the composition range of this invention, were quenched at a temperature of ₃ points or more of normal AC. for
It is not possible to reduce the 85% SATT of the BDWTT test to less than -30°C, and on the other hand, if rolled, the impact absorption energy is extremely low and the hydrogen-induced cracking resistance is poor. In Table 1-2, the invention steels B1 and B2 that simultaneously satisfy these characteristics are given a cumulative reduction rate of 40% or more at 900°C or less, then cooled once to below the Ar transformation point, and then subjected to Ac ₁ Only those that have been reheated to a temperature of _{3 to 3} Ac and then water-cooled. Next, Table 2-1 shows examples of steels with different compositions, and Table 2-2 lists various results obtained by subjecting these steels to heat treatment according to the present invention.

【table】

【table】

[Table] ** Evaluation of cracking ○: No cracking *** Presence or absence of separation ○: Yes, ×: No In addition, the hydrogen-induced cracking test in Tables 1-2 and 2-2 was conducted using a 75 x 75 mm specimen. The size of the cracks was determined after immersion in artificial seawater saturated with H ₂ S for 96 hours. As described above, according to the present invention, the severe requirements in the use of low-temperature steel can be met by requiring 2.5% Ni in the conventional general QT treatment, and in addition, repeated quenching, or QQ'T treatment, is required. This requirement can be advantageously met with a low Ni content reduced to 1.0% or less by simple Q'T treatment, and the BDWTT properties are good in so-called controlled rolling. BDWTT is also dissatisfied with absorbed energy and hydrogen-induced cracking resistance, and adding QT treatment to improve absorbed energy can advantageously avoid the disadvantages associated with BDWTT deterioration due to grain coarsening, and Significant improvements in the toughness and hydrogen-induced cracking resistance of the steel can be achieved.

Claims (1)

[Claims] 1 C: 0.02-0.2wt%, Si: 0.05-0.50wt%, Mn: 0.5-2.0wt%, Al: 0.005-0.1wt% and Nb: 0.01-0.08wt%, S: A steel slab containing 0.006wt% or less with the remainder Fe and unavoidable impurities is heated and rolled in a temperature range of 900 to 1200℃, and then the unrecrystallized austenite region or austenite- In the ferrite two-phase region,
Immediately after applying a cumulative reduction of 40% or more
Toughness characterized by being cooled to below the Ar transformation point, then reheated to a temperature range of A _c1 point to A _c3 point, then water-cooled, and then tempered at a temperature below A _c1 point. and a method for producing low-temperature steel with excellent hydrogen-induced cracking resistance. 2 C: 0.02-0.2wt%, Si: 0.05-0.50wt%, Mn: 0.5-2.0wt%, Al: 0.005-0.1wt% and Nb: 0.01-0.08wt%, S: 0.006wt% or less Contains Ti: 0.003 to 0.04wt%, Ni: 1.0wt% or less, Mo: 0.3wt% or less, Cu: 0.5wt% or less, V: 0.1wt% or less, Cr: 0.4wt% or less, and B: 0.003. Any one or two selected from the group consisting of wt% or less
A steel slab containing Fe and unavoidable impurities is heated and rolled in a temperature range of 900 to 1200℃, and then the steel slab is rolled into an unrecrystallized austenite region or austenite-ferrite two phase below 900℃. In the area,
Immediately after applying a cumulative reduction of 40% or more
Toughness characterized by being cooled to below the Ar transformation point, then reheated to a temperature range of A _c1 point to A _c3 point, then water-cooled, and then tempered at a temperature below A _c1 point. and a method for producing low-temperature steel with excellent hydrogen-induced cracking resistance. 3 C: 0.02-0.2wt%, Si: 0.05-0.50wt%, Mn: 0.5-2.0wt%, Al: 0.005-0.1wt% and Nb: 0.01-0.08wt%, S: 0.006wt% or less Contains Ti: 0.003 to 0.04wt%, Ni: 1.0wt% or less, Mo: 0.3wt% or less, Cu: 0.5wt% or less, V: 0.1wt% or less, Cr: 0.4wt% or less, and B: 0.003. Any one or two selected from the group consisting of wt% or less
Contains one or two of the following groups: Ca: 0.001 to 0.1 wt%, REM: 0.001 to 0.1 wt%, and the remainder
A steel slab made of Fe and unavoidable impurities is
Heating and rolling in a temperature range of ~1200℃, followed by rolling in an unrecrystallized austenite region or austenite-ferrite two-phase region below 900℃,
Immediately after applying a cumulative reduction of 40% or more
Toughness characterized by being cooled to below the Ar transformation point, then reheated to a temperature range of A _c1 point to A _c3 point, then water-cooled, and then tempered at a temperature below A _c1 point. and a method for producing low-temperature steel with excellent hydrogen-induced cracking resistance.