JPS647127B2 - - Google Patents

Description

[Detailed description of the invention]

<Industrial Application Field> This invention is applicable to Cr used at temperatures around 450°C.
- This relates to Mo steel, and aims to provide a method for manufacturing Cr-Mo steel that has both excellent creep strength and toughness even in extremely thick materials. <Conventional technology> When conventional Cr-Mo steel becomes extremely thick, the cooling rate during normalization (or quenching) inevitably decreases, resulting in a decrease in hardenability and deterioration in both strength and toughness. There is a tendency. To deal with this, there are methods to increase the content of components contained in this steel, especially carbon, or to add other alloying elements, but this is not necessarily a solution because weldability deteriorates. . In addition, a method of improving hardenability with B can be considered, but if a large amount of Al or Ti is added to ensure the amount of solid solution B that is effective for hardenability as in the past,
Coarse AlN and TiN precipitate and cause toughness deterioration. For this reason, the present applicant has developed a Ti-
A method has been proposed in Japanese Patent Application No. 121578/1987, in which N is reduced based on B-type steels, and Ti is added only in the minimum required amount. However, although this method provides sufficiently satisfactory values for strength and toughness at room temperature, there is a problem in that high temperature strength and creep strength are not necessarily satisfactory. <Summary of the Invention> The present invention was made to solve the above-mentioned conventional problems, and it is a Cr-Mo steel that simultaneously improves both creep strength and toughness without deteriorating weldability even in thick plates. The purpose of this invention is to provide a method for manufacturing. That is, the present invention improves both creep strength and toughness at the same time by rolling a steel having the specific components described below, normalizing or quenching it at the specific temperature range described below, and then tempering it. Ingredient range: Contains the following additional elements, with the balance consisting of iron and unavoidable impurities. Si: 0.01~0.60%, Mn: 0.20~1.20%, Cr:
1.80~3.50%~Mo: 0.80~2.00%, SolAl:
0.005-0.05%, P: 0.015% or less, S: 0.010%
Below, V: 0.25% ~ 0.35%, C: V/5.67 ~
0.15%, Ti amount: 0.010% or less and the amount that satisfies 3.43 [N] - 0.00583 < [Ti] < 3.43 [N] + 0.0050 B amount: 0.0003 to 0.0010% and [B] 0.77 [] - 3.16 An amount satisfying ×10 ⁻⁴ [However: N not fixed by Ti] In addition to the above, one or more of the following elements may be contained. Cu: 1.0% or less, Ni: 1.0% or less, Nb: 0.10%
Below, Ca: 0.07% or less, Mg: 0.07% or less. Normalizing or quenching temperature: (-10800/log(0.25) ^4/3・([C]-0.18([V
]-0.25))-7.06-273) Normalize or quench at a temperature range of ℃ or higher and 1020℃ or lower. The present invention will be explained in detail below. First, the concept of the N--Ti--B balance, CV balance, and normalizing (or quenching) temperature, which are the characteristics of the present invention, will be described. (N-Ti-B balance) The amount of solid solution B (abbreviated as below) required to improve the hardenability of Cr-Mo steel is 3 to 3, as shown in Figure 1.
10 ppm, and the effect becomes particularly noticeable at 5 to 6 ppm. When securing an amount effective for improving hardenability in this way, if the N content is relatively high, the amount of Ti required to fix N will inevitably increase, and a large number of coarse TiN will precipitate. Such coarse TiN is generated during solidification and hardly disappears into solid solution during the thermal cycle in the subsequent manufacturing process, which ultimately causes deterioration of the toughness of the steel sheet. Therefore, low Ti-low N is required to improve hardenability without impairing toughness. In addition, assuming the three elements Ti-N-B,
Since the stability of nitrides is TiN≫BN at the content and temperature within the range of the present invention, we first consider the formation of TiN, and then consider the formation of solid solution N, which cannot be fixed by Ti (hereinafter abbreviated as It is sufficient to consider the process of reacting with () to form BN. FIG. 2 shows the range of the amount of B in the present invention in relation to the amount (the amount of N that is not fixed by Ti). In the figure, each curve shown using temperature as a parameter is an equilibrium curve of B and N expressed by log[B][N]=13970/T(k)+5.24. In the present invention, the normalizing (or quenching) temperature is 900℃ or higher, and if an amount of 3 to 10 ppm is secured by normalizing (or quenching) from 900℃, the hardenability will be the same or higher if the temperature is higher than that. It can be effective. In Figure 2, if N is below the BN equilibrium curve at 900℃,
Since B in the steel remains as it is, the B content may be 0.0003 to 0.0010%. However, if is higher than this curve (i.e. to the right of the curve), to ensure that at 900 °C the precipitation of BN proceeds parallel to the BN stoichiometric line 1: 0.0003-0.0010% is B0.77−3.16×10 ^-4 ……
B0.77+8.46×10 ^-4 ... It is necessary to do so. However, if too much BN has already precipitated in this way during normalization or quenching, it will lead to deterioration of ductility and toughness, and to avoid this, it is necessary to set Bas BN to 0.0007% at 900℃. I understand. i.e. log([N]−0.0007)
([]-0.0009)-13970/900+273+5.24...
Although it would be sufficient to limit the amount of precipitated BN depending on the above, it was decided to control the upper limit of the amount of added B to 0.0010% in consideration of cost. In this way, the area surrounded by the shaded area can be improved in hardenability without deterioration in ductility.B
- within the range. At this time, it can be seen from the horizontal axis that it is 0.0017%. Regarding the fixation of N, as mentioned above, from the standpoint of stability and ductility of the steel, adding a small amount of Ti is most effective, and Ti is also added in the present invention. Figure 3 shows the Ti--N solubility product and the limited ranges of Ti and N amounts. Considering equilibrium at 1100℃, where TiN precipitation is almost complete (TiN is extremely thermally stable,
(It quickly precipitates as TiN during cooling even at a heating temperature of 1100℃ or higher), if [N] > 0.0017%,
Since TiN precipitation proceeds parallel to the stoichiometric straight line of TiN, in order to achieve 0.0017% at 1100°C, it is necessary to add Ti as follows: [Ti]3.43 [N]−0.00583... However, the precipitated TiN
Too much amount will lead to deterioration of ductility, so 1100
As the amount of TiN precipitated up to ℃, Tias TiN
It needs to be 0.010%. That is, it is necessary to limit the amount of precipitated TiN according to the line: log([Ti]-0.010)([N]-0.0029) =-15020/1100+273+3.82... On the other hand, if there is too much [Ti], even after the precipitation of TiN is completed, solid solution Ti (abbreviated as below) will exist in excess, and it will precipitate and harden as TiC during subsequent tempering, resulting in deterioration of the toughness of the base material. In order to invite
It needs to be 0.005%. That is, it is necessary to set [Ti]3.43[N]+0.005... However, as before, if the upper limit of added Ti is set to 0.010% from a cost perspective, the Ti-N balance that improves hardenability without deteriorating toughness will be in the area surrounded by diagonal lines. (CV balance) Figure 4 shows 0.002TN―0.2Si―0.55Mn―0.010P―
0.005S―0.010SolAl―0.006Ti―0.0006B, Cr:
1.8~3.5%, Mo: 0.8~2.0%, C: 0.03~0.17%,
For Cr-Mo steel with V: 0 to 0.5%, the amount of V that can precipitate as V ₄ C ₃ after subsequent tempering, SR, 450×10 ⁵ hr (normalizing The amount of V dissolved in solid solution at different temperatures was determined, and the creep rupture strength was determined by a creep test, and the relationship between the two was plotted. Further, FIG. 5 shows the VC range of the present invention. In the figure, each curve shown using temperature as a parameter is an equilibrium curve of V and C expressed by log [V] ^4/3 [C]=-10800/T(k)+7.06. At each normalizing temperature, V and C below the equilibrium curve form a solid solution, and this becomes V ₄ C ₃ in the next thermal cycle (tempering, SR, long-term high-temperature operation).
It precipitates finely in parallel to the stoichiometric straight line 8, increasing the creep strength. V and C above the equilibrium curve precipitate as V ₄ C ₃ during solidification in equilibrium with 8, and remain in the steel plate in a coarse form without forming a solid solution during normalization, which not only does not contribute to increasing creep strength but also deteriorates toughness. .
V ₄ C ₃ precipitates during the thermal cycle after normalization.
In order to ensure VasV ₄ C ₃ of 0.25% or more, it is necessary to make V at least 0.25% or more (straight line 9 or more in FIG. 5). As shown in Figure 4
V increases creep strength as V ₄ C ₃ The effect gradually increases as its amount increases, but 0.35%
In the above, the effect is almost saturated, and regardless of whether V exists as coarse or fine carbides, V dissolves during the welding thermal cycle and re-precipitates during SR, so welding SR It has a tendency to promote cracking, and if it exceeds 0.35%, this
The V content was reduced to 0.35% as SR cracking becomes noticeable.
It must be below (line 10 or less in Figure 5). In addition, when the mutual quantity relationship between CV and V is above the line 8 in Figure 5, that is, in the range of V > 5.67 × C, V becomes excessive and solid solution C
It is placed on the right side of the straight line 8 because the problem of insufficient quenching occurs and the toughness deteriorates in extremely thick materials. However, if the V and C contents exceed 0.15%, weldability deteriorates. In other words, it must be kept at 0.15% or less (to the left of straight line 11) since the weld cold cracking properties deteriorate. From what has been stated above, in the present invention, the C and V amounts are determined by the straight lines 8, 9, 10, 11 in Figure 5.
shall be kept within the range enclosed by. That is,
V: 0.25-0.35% C: V/5.67-0.15%. (Normalization or quenching temperature) By the way, just keeping the balance of the CV amount within the range of the above-mentioned invention will help improve the creep strength, as is clear from what has already been explained.
V as V ₄ C ₃ cannot be greater than 0.25%. Now within the range of the present invention, C = 0.10% V = 0.30%
Considering V dissolved in solid solution during normalization or quenching of Cr-Mo steel, if 900℃ is adopted as the normalization (quenching) temperature, a straight line 8 will pass through point P (C=0.10, V=0.30). straight line PP′ parallel to , and VC at 900℃
This means that the amount of C and the amount of V at the intersection point _P1 of the equilibrium curves are respectively dissolved in solid solution. That is, even if the total C and total V are 0.10% and 0.30%, respectively, the C and V dissolved in solid solution at 900°C are approximately 0.075% and 0.17%, respectively, and 0.30−0.17=0.13
% of V is coarse V carbide precipitated during the casting or rolling stage, and has no function of improving creep strength. When 950° C. is adopted as the normalization (quenching) temperature, the solid solution V is approximately 0.27%, which satisfies the 0.25% or more required in the present invention. Moreover, if 970°C is adopted as the temperature, all of the 0.30% V contained will dissolve into solid solution and precipitate as fine V ₄ C ₃ during subsequent Temper, SR, and long-term high-temperature operation. After all, C: 0.10% V: 0.30% Cr-Mo
In order to dissolve 0.25% or more of V in solid solution during normalization in steel, it is necessary to use a temperature higher than the V--C equilibrium curve passing through the intersection _P2 of straight line 9 and straight line PP' where V=0.25%. Therefore, the normalizing (quenching) temperature of a Cr-Mo steel containing any C content V in the range surrounded by straight lines 8, 9, 10, and 11 is a point on straight line 9 [C-0.18
(V-0.25), 0.25], that is, (-10800/log[0.25] ^4/3・[C-0.18(V-0.25)
)] -7.06-273) Must be at least ℃. However, if treated at a temperature exceeding 1020°C, the austenite grains will become coarse and the toughness will deteriorate, so this temperature is set as the upper limit. (Other Additive Elements) Next, reasons for limiting other components will be explained. Mn: Mn must be at least 0.20% in terms of strength, but since it has a negative effect on weldability and temper embrittlement resistance, the upper limit was set at 1.20%. Si: From the viewpoint of deoxidizing effect and strength, Si should be 0.01% or more, but since it has a negative effect on toughness and tempering embrittlement resistance, the upper limit should be 0.60%. Cr: 1.80% or more is required to ensure oxidation resistance, hydrogen corrosion resistance, and strength at high temperatures, but the upper limit is set at 3.50% in consideration of weldability.
%. Mo: To ensure high temperature strength and hydrogen erosion resistance
Although it is necessary to add 0.80% or more, the upper limit is set at 2.00% from the viewpoint of cost increase and weldability deterioration. Acid-soluble Al: has the effect of improving the hardenability of B by refining crystal grains and identifying solid solution N, but
On the other hand, as with Ti, excessive addition produces coarse nitrides and impairs toughness, so 0.005 to 0.050%
The range shall be . P: It is extremely harmful to tempering embrittlement and SR cracking, so it should be kept at 0.015% or less. S: S causes toughness deterioration, anisotropy, and increased reheat cracking susceptibility, so it should be 0.010% or less. Next, the reasons for limiting the components of the elements of the second invention, which are added one or more depending on the required performance, will be described below. Cu: Increases strength, but too much Cu impairs hot workability, so the upper limit is set at 1.0%. Ni: It increases strength and improves toughness at the same time, but the increase in cost is large, so the upper limit is set at 1.0%. Nb: Forms thermodynamically stable carbides through tempering and improves high temperature strength and hydrogen attack resistance, but too much Nb impairs toughness and weldability, so the upper limit is set at 0.10%. Ca and Mg: each has the effect of controlling the shape of the sulfide, preventing the sulfide from elongating in the rolling direction, reducing anisotropy in various properties of the steel material. However, if the content is too high, a large amount of sulfides and oxides of these elements will be generated, impairing the cleanliness of the steel, so the upper limit is set at 0.07%. Incidentally, in the method of the present invention, there are no restrictions on rolling, and it may be carried out under normal conditions. In addition, tempering SR after normalization or quenching is normally performed.
Ac: Must be done with ₁ point or less. However, tempering SR at too low a temperature will result in high hardness and brittleness, making handling such as processing difficult, so it is desirable to carry out tempering SR in a temperature range of 650 to Ac ₁ . <Examples> Examples of the present invention will be described below. As shown in Table 1, the steels of each component were heat treated under various conditions, and their sharpy values and creep strengths were determined (the components of each steel are plotted in Figures 2, 3, and 5). . From this table, for example, for conventional steel,
It can be seen that the creep strength of the steel is low because the normalizing temperature is low or the amount of V added is low. In addition, the toughness of HO, RI, NU, RU, WO, and WA is low due to poor Ti-N-B balance, high normalizing temperature, etc. In contrast, it can be seen that the steel of the present invention has high creep strength and toughness.

【table】

[Table] ○ indicates the steel of the present invention AC: Air cooling WQ: Water cooling * Cooling is a simulation ** Interpolated value by Larson-Miller method

[Brief explanation of drawings]

Figure 1 is a graph showing the relationship between the amount of solid solution B and the hardness as normalized, and Figure 2 is a graph showing the relationship between the amount of solid solution B and the hardness as normalized.
Figure 3 is a graph showing the range of the added Ti of the present invention.
Figure 4 is a graph showing the range of N added and the amount of N added.
A graph showing the amount of VasV _{4 C3} , and FIG. 5 is a graph showing the range of the amount of C and the amount of V in the present invention.

Claims (1)

[Claims] 1 Si: 0.01 to 0.60%, Mn: 0.20 to 1.20%, Cr:
1.80~3.50%, Mo: 0.80~2.00%, SolAl: 0.005
~0.05%, P: 0.015% or less, S: 0.010% or less,
V: 0.25% to 0.35%, C: V/5.67 to 0.15% and
After rolling a steel containing Ti, B, and N in the following limited amounts, with the remainder being ^iron and unavoidable impurities,
〕-0.25))-7.06-273) Production of Cr-Mo steel with excellent toughness and creep strength, characterized by normalizing or quenching in a temperature range of ℃ or higher and 1020℃ or lower, followed by tempering treatment. Method. Ti amount: 0.010% or less and the amount that satisfies 3.43 [N] - 0.00583 < [Ti] < 3.43 [N] + 0.0050 B amount: 0.0003 to 0.0010% and [B] 0.77 [] - 3.16 × 10 ^- Amount that satisfies ⁴ [However: N not fixed by Ti] 2 Si: 0.01 to 0.60%, Mn: 0.20 to 1.20%, Cr:
1.80~3.50%, Mo: 0.80~2.00%, SolAl: 0.005
~0.05%, P: 0.015% or less, S: 0.010% or less,
V: 0.25% to 0.35%, C: V/5.67 to 0.15% and
1 of Cu: 1.0% or less, Ni: 1.0% or less, Nb: 0.10% or less, Ca: 0.07% or less, Mg: 0.07% or less
After rolling a steel containing a species or two or more species, Ti, B, and N in the following limited amounts, with the remainder being iron and unavoidable impurities, (-10800/log(0.25) ^4/3・([C]- 0.18 ([V
〕-0.25))-7.06-273) Production of Cr-Mo steel with excellent toughness and creep strength, characterized by normalizing or quenching in a temperature range of ℃ or higher and 1020℃ or lower, followed by tempering treatment. Method. Ti amount: 0.010% or less and an amount that satisfies 3.43 [N] - 0.00583 < [Ti] < 3.43 [N] + 0.0050. Amount of B: 0.0003 to 0.0010% and an amount that satisfies [B] 0.77 [] -3.16×10 ^-4 [However: N not fixed by Ti]