JPH0151526B2 - - Google Patents

Description

[Detailed description of the invention]

(Field of Industrial Application) The present invention relates to a method for producing ultra-high tensile strength steel that has excellent strength and toughness, and particularly has excellent resistance to stress corrosion cracking in stress corrosion environments such as seawater. (Conventional technology) In recent years, interest in the deep sea has increased, such as undersea resource development and undersea crustal geological surveys, and the construction of offshore structures such as drilling platforms for oil drilling and undersea oil production bases that lead to undersea development, Construction of energy-related steel structures, such as the construction of high-head penstocks for pumped storage power generation to adjust nighttime surplus power from nuclear and thermal power generation, is gaining momentum.
The materials used in these products are required to have high strength and excellent toughness to ensure safety, and furthermore, these high-strength materials are used in atmospheres and environments different from the atmosphere. In this case, sufficient consideration must be given to stress corrosion cracking in particular. In order to meet these demands for safer and more reliable materials, Ni-containing low alloy steels have been developed as high tensile strength steels and quality improvements have been made.
For example, as in Japanese Patent Application Laid-Open No. 56-9358, C+1/8
Ni−Cr− where Mo+V>0.26 and Cr≦0.8Mo
Mo-V series high strength and high toughness steel, JP-A-57-188655
As mentioned in the publication, Ni-Cr-Mo-V ultra-high tensile strength steel can achieve high strength and toughness at a wide range of cooling rates during quenching treatment, and also has high toughness quenched with ultra-low phosphorus and ultra-low sulfur treatment. Many types of manufacturing methods have been developed, including methods for manufacturing tempered Ni-containing steel. (Problems to be Solved by the Invention) All of these manufacturing methods are effective in improving the toughness of steel. However, when considering the working environment, for example, no study has been conducted that takes into account stress corrosion in seawater. In other words, regarding stress corrosion cracking in high-strength steel, the theory of linear fracture mechanics is adopted, and the K value of the crack tip is used to quantify the fracture behavior of the material in which the defect exists in a corrosive environment. A method is used to That is, in a working environment, a pre-cracked test piece is used to create extremely harsh conditions at the notch tip to facilitate delayed fracture, and constant load tests are conducted at various K value levels. Therefore, the stress corrosion cracking resistance is evaluated by determining the limit K _ISCC value at which destruction does not occur below a certain K value. However, no particular consideration has been taken to perform such evaluation on the steel obtained by the above-mentioned manufacturing method, and therefore it is difficult to say that it is sufficiently safe for use. Therefore, the present inventors have developed a method that has high strength and high toughness.
In order to manufacture steel with excellent resistance to stress corrosion cracking in seawater etc., we have studied methods for manufacturing steel of various compositions, assuming that it is mainly manufactured through a heating-rolling-direct quenching process. It has been found that the desired steel can be produced by adding Nb to Ni-containing steel and employing appropriate conditions for direct quenching and tempering after rolling for steel with reduced impurity elements P, N, and O. (Means for solving the problem) The present invention was made based on such knowledge, and the gist thereof is that C0.05 to 0.10% by weight,
Si0.20% or less, Mn0.4~1.5%, Ni5.0~8.0%,
Contains Mo0.3~1.5%, Nb0.005~0.05%, P0.010
% or less, O 0.0025% or less, N 0.0040% or less, and if necessary, Cu 0.2 to 1.5%, Cr 0.60% or less, V 0.02 to 0.12%, Total Al 0.08% or less,
Ti0.005~0.01%, Ca0.003% or less type 1 or 2
A steel billet containing at least 100% Fe, with the remainder consisting of unavoidable impurities and Fe, is heated to 1050 to 1250°C, hot rolling is completed at a temperature of Ar ₃ or higher, and water cooling is performed as it is.
This is a method for producing ultra-high tensile strength steel with excellent stress corrosion cracking resistance, which is characterized by stopping water cooling after the temperature reaches 100°C or lower, and then tempering at a temperature of 600°C or lower. (Function) First, the reason for limiting the chemical composition range of the target steel in the present invention will be explained. First, C is an element necessary to ensure hardenability and strength, but if the content is low, less than 0.05%, the required strength cannot be obtained, and if it exceeds 0.10%, the weld heat affected zone will be hardened significantly. toughness and stress corrosion cracking resistance. Therefore, it is set at 0.05 to 0.10%. Si is effective in increasing strength, but in the case of high Ni steel
A high Si content increases the susceptibility to temper embrittlement, which particularly impairs the notch toughness and K _ISCC of the weld heat affected zone. Therefore, a certain degree of strength is secured, and notch toughness and K _ISCC
In order not to reduce the value, the upper limit is set at 0.20%. Mn ensures hardenability and prevents cracking during hot working.
It is necessary to prevent hot cracking during welding, but in the case of Ni-containing steel, a high Mn content increases the susceptibility to temper embrittlement, so it must be kept at 1.5% or less. Also,
If it is less than 0.40%, there is a great risk of losing the balance between strength and toughness. Therefore, it is set at 0.40 to 1.5%. Ni obtains a mixed structure of lower bainite and martensite through quenching, is diffused and absorbed into austenite relatively quickly during tempering, and has the effect of stabilizing residual austenite against impact stress. Therefore, a minimum content of 5% is required depending on the content of other elements. On the other hand, if it exceeds 8%, the retained austenite transformed during tempering becomes unstable, the toughness deteriorates, and the hardness of the weld heat affected zone increases, causing the toughness or stress corrosion cracking resistance to deteriorate. Therefore, it is set at 5 to 8%. Mo is necessary to ensure strength and to prevent Ni-containing steel from tempering embrittlement. If it is less than 0.3%, the target strength cannot be obtained, and if it exceeds 1.5%, the number of carbides such as Mo ₂ C increases, reducing toughness and stress corrosion resistance. Therefore, it is set at 0.3 to 1.5%. Nb addition is the heating-rolling-direct quenching process of the present invention.
This is one of the most important points in the tempering process. In other words, Nb is added by sufficiently dissolving Nb carbon and nitride in the steel base when heating the steel billet.
Direct quenching after rolling - once melted during tempering
This is to precipitate Nb as Nb carbon/nitride and to expect hardening due to this, and the minimum content is required to be 0.005% or more. However, if it is contained in an excessive amount, there is a risk of deteriorating the toughness of the weld heat affected zone and welded steel part. The limit is 0.05%
Therefore, this is the upper limit. On the other hand, in the present invention, it is expected that the toughness of the base metal and the weld heat-affected zone will be improved by particularly limiting P, O, and N to a low level.
It is also effective in compensating for the decrease in toughness due to the precipitation of Nb carbon/nitride. First, P is an unavoidable impurity that cannot be avoided to some extent, but it greatly affects the toughness of the weld heat-affected zone, especially in high-strength steel with a yield strength of 100 Kgf/mm ² or more, which is the subject of the present invention. Particularly when stress corrosion cracking resistance is considered, P has a large influence on the K _ISCC value in the temper embrittlement region heated to a relatively low temperature in the weld heat affected zone, especially on the base metal side.
If it exceeds 0.010%, there is a risk that the K _ISCC value in this area will decrease significantly. Therefore, P is limited to 0.010% or less. O has a large influence on the toughness of ultra-high-strength steels, and in particular controls the absorbed energy on the upper shelf of the Shallpy transition curve, that is, the value of the Shelf energy.
Most of the O in steel forms oxides, which lower the absorbed energy at fracture, so the higher the tensile strength of the steel, the greater its influence. Therefore, it is desirable to reduce O as much as possible, and by reducing it to 25 ppm or less, the toughness of the steel, which is the object of the present invention, can be obtained. So O
Limit to 0.0025% or less. N has a large effect on the stress corrosion cracking resistance K _ISCC of the weld heat affected zone, and also has a large effect on ensuring the toughness of the base metal, so it must be reduced as much as possible.
In particular, even if the weld heat-affected zone contains sufficient Al, the N that combines with Al to form AlN in the base metal dissociates in the high temperature state during welding and cools as it is, becoming solid solution N. The toughness of coarse grain structures heated to high temperatures is significantly deteriorated. For high-strength steel,
This phenomenon is remarkable when N exceeds 0.0040%. Therefore, N is limited to 0.0040% or less. The above are the basic elements, but the present invention further includes Cu, Cr, V, Al, Ti,
By adding one or more types of Ca,
Ensures hardenability and strength according to the cross-sectional thickness of the steel material.
Toughness can be further improved. In other words, Cu is effective in increasing strength without deteriorating toughness, but if the content is less than 0.2%, no effective effect will be obtained, and if it exceeds 1.5%, the welded part will be damaged during welding. make hot cracks more likely to occur. Cr is effective in ensuring hardenability and strength, but if it exceeds 0.6%, it increases the hardness of the weld heat affected zone and risks lowering the K _ISCC value, so this is the upper limit. , the preferred range is 0.2-0.5
%. V is necessary to generate carbon/nitride during tempering and ensure strength. When used in combination with Nb, the strength increases due to precipitation hardening, so it is sometimes added to ultra-high tensile strength steels with especially high tensile strength. In this case, the addition amount is required to be 0.02% to ensure strength, while if it exceeds 0.12%, toughness will deteriorate. Therefore, it is set at 0.02 to 0.12%. Al combines with N in the steel to form AlN, which contributes to the refinement of the structure, but when added in an excessive amount, it actually inhibits toughness due to an increase in the amount of inclusions such as Al ₂ O ₃ . Therefore, the upper limit is set at 0.08%. Note that Al is normally used as a deoxidizing agent, but in the case of the present invention, the limit amount of O is separately specified, so if another means to reduce O, such as vacuum degassing, is used, it is not necessary to add Al. In some cases, this is not the case. Ti is effective in preventing deterioration of the toughness of the weld heat-affected zone by preventing coarse grains in the weld heat-affected zone, but if it is added in a large amount, it will deteriorate not only the toughness of the weld heat-affected zone but also the toughness of the base metal. The upper limit shall be 0.01%.
Furthermore, if the Ti content is less than 0.005%, the above effects cannot be effectively exhibited. Therefore, the range of Ti is set to 0.005 to 0.01%. Ca generally has the effect of changing the shape of inclusions such as MnS, improving toughness and reducing the directionality of toughness.
The upper limit is set at 0.003%, where the effect is saturated. Note that when low S steel is targeted, it is not necessary to particularly add Ca. Next, the present invention uses steel having the above chemical composition.
It is heated to 1050-1250℃, hot rolling is completed at a temperature of Ar ₃ points or higher, water-cooled as it is, water-cooled to a temperature of 100℃ or lower, and then heated and tempered again to a temperature of Ac ₁ or lower. One of the important points of the invention is to carry out this process, and the reasons for limiting the process conditions will be described below. First, the heating temperature of the steel slab was set at 1050°C to 1250°C, but the lower limit temperature was set at 1050°C as a strengthening mechanism to obtain high-strength steel with low stress corrosion cracking resistance, which is the objective of the present invention. When utilizing the strengthening caused by the precipitation of Nb carbon and nitrides, it is necessary to fully dissolve the Nb carbon and nitrides present in the steel slab into the steel base when heating the steel slab, but the temperature is lower than 1050℃. This is because at temperatures above 1250°C, this solid solution effect is not sufficient, and therefore sufficient precipitation hardening cannot be expected during tempering after water cooling.On the other hand, at temperatures above 1250°C, solid solution of Nb carbon and nitrides is sufficient. However, when the steel contains a large amount of Ni, such as the steel subject to the present invention, specific oxides tend to appear on the surface of the steel slab, and these tend to remain on the surface of the steel sheet after rolling and cause surface flaws. Therefore, the heating temperature is
The temperature shall be 1050-1250℃. After heating in this way, hot rolling is performed, and after rolling, the steel plate is water-cooled while still hot, but the water-cooling start temperature at this time needs to be at least ₃ points of Ar, which each steel has. This is due to the necessity of making the structure sufficiently martensitic. In this case, if the martensitic structure obtained after quenching is made finer, the toughness after tempering will be even better.To achieve this, sufficient rolling should be carried out within the range of Ar ₃ points to _{Ar 3} +100℃ during rolling before water cooling. It is also possible to adopt a reduction schedule such as the following, but this invention does not particularly specify it. In addition, when performing water cooling from a temperature of Ar ₃ or higher, it is preferable to adjust the water cooling conditions depending on the thickness of the rolled plate. Next, the reason why the water-cooling stop temperature is specified as 100℃ or less is because if water-cooling is stopped at a temperature exceeding 100℃, the martensitic morphology may not yet be completed in the case of the steel subject to the present invention, which significantly affects the strength and toughness. This is because it may have an impact. In addition, tempering is performed in a reheating tempering furnace, but since the Ac ₁ point of steel with the chemical composition specified in the present invention is 540 to 570°C, tempering at a temperature slightly higher than this is necessary in order to improve the strength-toughness balance. This is the best, and when the temperature exceeds 600℃, the toughness deteriorates due to the precipitation of unstable austenite.
Therefore, the tempering temperature was limited to 600°C or less. Note that, although not particularly stipulated, water cooling may be performed after tempering in order to prevent tempering embrittlement. Next, examples will be described in order to clarify the effects of the present invention. (Example) Chemical components A-1 to A-10 as shown in Table 1,
Steel having B-1 to B-5 is produced, hot worked and rolled to a thickness of 32 to 180 mm, and directly quenched under the conditions of No. 1 to No. 18 shown in Table 2. Tempering was performed. For these, the mechanical properties of the base metal and the K _ISCC values of the base metal and weld heat affected zone were investigated. Welding is TIG welding with a heat input of 25KJ/cm. Mechanical properties obtained by various combinations of steels having chemical compositions shown in Table 1 and heat treatment conditions shown in Table 2, and mechanical properties in 3.5% artificial seawater.
Table 3 shows the K _ISCC test results using the test pieces specified in ASME E399. Chemical components No. B-1 to B-5 in Table 1 are examples of components that deviate from the chemical component range specified in the present invention, and heat treatment conditions No. 16 to 18 in Table 2 are examples of chemical components that are outside the range of chemical components specified in the present invention. This is an example of deviating from the scope defined by the invention.

[Table] Rolled directly from steel ingot to test plate thickness.
○ Ar ₃ points (°C) were calculated using the following formula.
Ar ₃ (℃)=863−396C+24.6Si−68.1Mn−36.
1Ni−20.7Cu−24.8Cr＋29.6Mo

【table】

【table】

【table】

[Table] As can be seen from the results in Table 3, in order to obtain good K _ISCC for the strength and toughness of the base metal and the weld heat affected zone, it is necessary to comply with the chemical composition specified by the present invention and the manufacturing conditions specified by the present invention. Direct quenching based on the premise that
This can only be achieved by using a manufacturing method using tempering, and even if the chemical components are outside the range specified by the present invention, or even if the chemical components are within the specified range, the manufacturing conditions specified by the present invention cannot be met. If this is not the case, the high tensile strength steel with excellent stress corrosion cracking resistance, which is the object of the present invention, has not been obtained. (Effects of the Invention) As is clear from the above examples, according to the present invention, it is possible to produce high-strength steel that has excellent toughness and exhibits sufficient soundness even in corrosive environments such as seawater. The industrial effects are extremely significant.

Claims (1)

[Claims] 1. Contains C 0.05-0.10%, Si 0.20% or less, Mn 0.4-1.5%, Ni 5.0-8.0%, Mo 0.3-1.5%, Nb 0.005-0.05%, P 0.010% Below, O is limited to 0.0025% or less, N is limited to 0.0040% or less, and if necessary, Cu 0.2 to 1.5%, Cr 0.60% or less, V 0.02 to 0.12%, Total Al 0.08% or less, Ti 0.005 to 0.01%, Ca. A steel billet containing 0.003% or less of one or more kinds, with the remainder consisting of unavoidable impurities and Fe is heated to 1050-1250℃,
Stress corrosion resistance characterized by finishing hot rolling at a temperature of Ar ₃ or higher, directly water cooling, stopping water cooling when the temperature reaches 100°C or lower, and then tempering at a temperature of 600°C or lower. A manufacturing method for ultra-high tensile strength steel with excellent crackability.