JP2016138320A - NiCrMo STEEL AND MANUFACTURING METHOD OF NiCrMo STEEL - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable efficient fine structuring of a NiCrMo steel.SOLUTION: It has a composition containing, by mass percentage, C:0.10 to less than 0.30%, Si:0.05 to less than 0.30%, Mn:0.20 to 1.00%, P:0.015% or less, S:0.015% or less, Cr:1.50 to 2.00%, Mo:0.10 to 0.50%, Ni:2.50 to 4.00%, Al:0.01 to 0.03%, N:0.005 to 0.015% and the balance Fe with inevitable impurities, consists of a martensite structure or a bainite structure or a combination structure thereof and makes refining for fine structuring efficient.SELECTED DRAWING: None

Description

  The present invention relates to grain refinement of NiCrMo steel having high hardenability.

NiCrMoV steel has long been applied to rotor shafts of large turbines, and even today, it is positioned as a highly practical low-alloy steel excellent in uniform strength and low-temperature toughness up to the shaft center. For this reason, the steel is often considered as a candidate material for other large members that require strength and toughness.
It is well known that strengthening by grain refinement is a method that can improve the toughness while improving the strength among the general strengthening methods, and the effect is also expected in NiCrMoV steel. The However, NiCrMoV steel is a steel type that is difficult to refine by gamma treatment (α / γ reverse transformation), and when NiCrMoV steel is applied to a thick member, crystal grain refinement due to rapid temperature rise or processing recrystallization is not possible. Since it becomes even more difficult, grain refinement of NiCrMoV steel thick members is often performed by repeating γ-heating and cooling (cooling generally to room temperature), and inevitably man-hours for heat treatment There is a problem that increases.

  As a method for refining crystal grains, it is generally known to precipitate a carbon nitride such as Nb or Ti, or a compound having a crystal grain boundary pinning effect such as AlN. In Patent Document 1, by adding appropriate amounts of Nb, Al and N and precipitating AlN and Nb (C, N) in the matrix, crystal grains are not only refined but also subjected to high-temperature carburizing treatment at 990 ° C. or higher. A technique that enables suppression of coarsening is shown. The technique disclosed in Patent Document 1 is considered to be brought about because Nb (C, N) and the like exist stably without being dissolved in the matrix even at a high temperature.

JP 2000-54069 A

  The high-temperature stability of Nb (C, N) is due to the strong tendency of Nb to form carbonitrides, but this stability is high when the member temperature is slowly lowered in a hot forging process or the like. Therefore, Nb (C, N) precipitates and becomes coarse during cooling. If the grain boundary pinning particles are coarsened, the grain boundary pinning effect in the subsequent heat treatment step may be reduced or eliminated, and the ductility may be reduced. When a large steel ingot is melted, elements having a strong tendency to form carbides such as Nb and Ti promote solidification segregation and can cause deterioration of internal properties. Therefore, the method of adding an element that forms a carbide or nitride with high temperature stability such as Nb or Ti is difficult to apply as a method for refining crystal grains in a thick member.

  The present invention has been made in the context of the above circumstances, focusing on AlN, which has low high-temperature stability compared to Nb, Ti, etc., and has determined the chemical composition for maximizing its pinning action. In the thick member made of NiCrMoV steel, which is difficult to refine crystal grains by this method, a method for producing NiCrMo steel and NiCrMo steel material that can refine crystal grains without repeating the γ-treatment is proposed.

  The present inventors have difficulty in physically increasing the heating and cooling rates, and in the thick member that requires a long time until the center reaches the target temperature, the γ-ization is less than that of the conventional NiCrMoV steel. The chemical composition necessary for obtaining fine crystal grains was examined by the number of times, and the following points were clarified. The present invention is not limited to the thick member.

  When a specific amount of Al and N is contained and V is limited to a content not contained or less than a specific amount, crystal grains are refined in a heat treatment at a predetermined temperature. This is because when the V content is reduced to 0 or less than that of a conventional general NiCrMoV steel, the precipitation amount of V (C, N) also decreases. By increasing the amount.

  Even if Nb is added in a state containing a specific amount of Al, N, and V showing the grain refinement effect, the additional grain refinement effect due to the addition of Nb under the same heat treatment conditions as described above is It does not appear.

  When the Cr content is increased within a specific composition range with a specific amount of Al, N showing a crystal grain refining effect, or further containing V if desired, the crystal grains are slightly increased. Refine.

  Ni, Si, Mn, and Mo in a specific composition range in accordance with target mechanical characteristics in a specific composition range containing a specific amount of Al, N that shows a grain refinement effect, or further containing V if desired. The content can be varied.

  The present invention has been made on the basis of the above findings, and the contents thereof are shown below.

  That is, among the NiCrMo steels of the present invention, the first present invention is in mass percentage, C: 0.10 to less than 0.30%, Si: 0.05 to less than 0.30%, Mn: 0.20 To 1.00%, P: 0.015% or less, S: 0.015% or less, Cr: 1.50 to 2.00%, Mo: 0.10 to 0.50%, Ni: 2.50 It is characterized by having 4.00%, Al: 0.01-0.03%, N: 0.005-0.015%, with the balance being composed of Fe and inevitable impurities.

  The NiCrMo steel according to the second aspect of the present invention is characterized in that, in the present invention, the composition further contains V: less than 0.10% by mass percentage.

  The NiCrMo steel of the third aspect of the present invention is characterized in that, in the present invention, the composition further contains Nb: less than 0.10% by mass percentage.

  The NiCrMo steel of the fourth aspect of the present invention is characterized in that, in the present invention, it comprises a martensitic structure, a bainite structure, or a mixed structure thereof.

  The manufacturing method of the NiCrMo steel material of the 5th this invention prepares the steel which has the composition in any one of the 1st-3rd this invention, and performs hot forging, normalizing, and tempering with respect to this steel The steel material whose crystal grain size based on JIS G0551 is 5.5 or more is obtained by performing the γ-treatment at 800-930 ° C. at least once.

  The contents defined in the present invention will be described below. In addition, all the components shown below are shown by mass percentage.

C: Less than 0.10 to 0.30% C dissolves in the matrix, gives solid solution strength, forms alloy carbide with other alloy elements, and precipitates in the matrix. In order to bring about an increase, 0.1% or more is added. However, if the amount is too large, workability and toughness are reduced. Therefore, the range is limited to 0.10 to less than 0.30%.

Si: 0.05 to less than 0.30% Si is a solid solution strengthening element of ferrite. However, since it is an element that promotes solidification segregation, if it is too much, the structure in the steel becomes uneven and the toughness is reduced. . Therefore, the range is limited to less than 0.05 to 0.30%. As a preferred embodiment, the upper limit is desirably 0.20%.

Mn: 0.20 to 1.00%
Since Mn is an austenite stabilizing element, it has the effects of improving hardenability and increasing strength. However, if it is less than 0.20%, the hardenability becomes insufficient. On the other hand, if it exceeds 1.00%, the material becomes hard and workability deteriorates. Therefore, the range is 0.20 to 1.00%. For the same reason, as a preferred embodiment, it is desirable that the lower limit is 0.30% and the upper limit is 0.90%.

P: 0.015% or less P is an element that segregates at the prior austenite grain boundaries and causes embrittlement of the grain boundaries, so the range is limited to 0.015% or less as impurities.

S: 0.015% or less S combines with Mn to form sulfide-based inclusions, but if it is too much, coarse sulfide-based inclusions increase, leading to a decrease in toughness. Therefore, the range of impurities is limited to 0.015% or less.

Cr: 1.50 to 2.00%
Cr, like Mn, is an element that provides improved hardenability and temper softening resistance. On the other hand, if the content is excessive, the material becomes hard and the workability decreases. Therefore, the range is limited to 1.50 to 2.00%.

Mo: 0.10 to 0.50%
Mo plays a role in reducing a reduction in the strength of tempering, but excessive addition causes a reduction in toughness. Therefore, the range is limited to 0.10 to 0.50%. As a preferred embodiment, the upper limit is desirably 0.30%.

Ni: 2.50 to 4.00%
Ni is an element that dissolves in the matrix and enhances hardenability and contributes to high strength and toughness. However, if it is less than 2.50%, the hardenability is insufficient and the strength is reduced. On the other hand, when the content is excessive, the material becomes too hard and the workability is lowered. Therefore, the range is limited to 2.50 to 4.00%. As a preferred embodiment, it is desirable that the lower limit is 3.00% and the upper limit is 3.80%.

V: Less than 0.10% V combines with C and N to form V (C, N), and thus greatly affects the formation of AlN. If it is contained in a large amount, a large amount of V (C, N) is formed, the amount of precipitation of AlN and the precipitation temperature are changed, and the pinning action of AlN is lowered. Therefore, when V is not added or contained, its content is limited to less than 0.10%. As a preferred embodiment, it is desirable that the upper limit is 0.07%. Moreover, in order to make V into a strengthening element, the lower limit of the content is preferably 0.03%. When V is not added, it may be contained as an inevitable impurity at less than 0.03%, and preferably 0.01% or less.

Al: 0.01-0.03%
Al precipitates as AlN and pins the grain boundaries. If the amount is too small, the number of AlN particles effective for pinning will be insufficient, and the crystal grains will not be refined. If the amount is too large, the AlN will become coarse and the ductility will be reduced. Therefore, the range is 0.01 to 0.03%. As a preferred embodiment, it is desirable that the upper limit is 0.025%.

N: 0.005 to 0.015%
N combines with Al and V and precipitates as AlN and V (C, N). If the amount is too small, the number of AlN precipitate particles effective for pinning will be insufficient, and the crystal grains will not be refined. If the amount is too large, the precipitates will become coarse, resulting in a reduction in ductility. Therefore, the range is made 0.005 to 0.015%. As a preferred embodiment, it is desirable that the upper limit is 0.010%.

Nb: less than 0.10% Nb can have a grain boundary pinning effect in a high temperature range such as during hot forging, for example, 1000 ° C. to 1150 ° C., and can be contained as desired. In that case, it is desirable that the lower limit amount be 0.01%. Furthermore, 0.03% or more is more desirable. Further, Nb may be contained in an amount of less than 0.01% as an inevitable impurity.

Structure The NiCrMo steel of the present invention has a martensite structure, a bainite structure, or a mixed structure thereof after tempering.
In the case of tempering (quenching), the structure can be formed even after the heating even if the average cooling rate from 800 to 200 ° C. is 50 ° C./min or less.

Temperature rising rate The present invention is suitably applied to a thick member made of NiCrMo steel, which has a thick member and the temperature rising rate at the center during heat treatment is 200 ° C./hour or less. For example, a member having a wall thickness of 100 mm or more can be given.

Final γ-ization temperature The final γ-ization treatment can be performed at 800 ° C. to 930 ° C. For example, even if it is maintained for more than 100 hours, the crystal grains are hardly coarsened, so that the tempering temperature can be freely selected within this temperature range according to the target mechanical characteristics and the thickness of the member.

  The NiCrMo (V) steel of the present invention has a crystal grain size in accordance with JIS G0551 in one γ-treatment after the general hot forging and heat treatment (normalizing and tempering) steps of forged steel members. Becomes 5.5 or more. The refined structure improves strength and provides excellent toughness and fatigue properties.

  As described above, according to the present invention, it is possible to refine crystal grains by utilizing the pinning effect of the grain boundary of AlN without repeated γ treatment even in a thick member that cannot be rapidly heated. Become. Crystal grain refinement is advantageous from the viewpoint of mechanical properties such as strength and toughness, and a NiCrMo steel member having improved material properties by crystal grain refinement can be provided.

Hereinafter, an embodiment of the present invention will be described.
The NiCrMo steel of the present invention can be melted by a conventional method. The steel ingot obtained by melting is subjected to preliminary heat treatment such as forging and normalization as necessary, and further tempering. Such forging and heat treatment can be carried out by conventional methods, and the present invention is not limited to specific conditions.

However, it is desirable to appropriately set the γ-heating temperature for crystal grain refinement, and as a preferred mode, it is desirable that the temperature be in the temperature range of 800 ° C. to 930 ° C. Furthermore, the upper limit is more preferably 870 ° C. When the temperature is lower than 800 ° C., non-recrystallized grains remain and grain size cannot be obtained, and when the temperature is higher than 930 ° C., the crystal grains gradually become coarser as the temperature increases. Although heating time is not specifically limited, For example, it can carry out in 1 to 100 hours. When cooling by a conventional method from 800 to 930 ° C., a martensite structure, a bainite structure, or a mixed structure thereof is obtained.
For cooling during quenching, water cooling, oil cooling, air cooling, furnace cooling, or the like can be used depending on the required mechanical properties and the thickness of the member.

In addition, tempering in the tempering process has a great influence on toughness and cracking susceptibility, but it can be said that all heat treatments are irrelevant with respect to crystal grain size. Therefore, the tempering conditions suitable for the member to be applied may be performed by a conventional method. For example, the conditions of 150 to 200 ° C. can be shown when strength and hardness are required, and 550 to 600 ° C. can be shown when ductility is required. However, it is better to avoid carrying out at a temperature at which temper embrittlement occurs, for example, at 500 ° C. for a long time.
The grain size number and whether or not the grain size is sized can be judged by Japanese Industrial Standard G0551 “Austenite grain size test method for steel”, and can be judged using an apparatus such as an optical microscope. Tissues can also be determined using an optical microscope.
The steel material that has undergone the tempering of the present invention shows a grain size of 5.5 or more in terms of the crystal grain size according to the above criteria after general heat treatment conditions, that is, hot forging, normalizing, tempering, and quenching.

  Examples of the present invention will be described below. Steel types having the composition shown in Table 1 were melted in a 50 kg vacuum induction melting furnace, and the obtained steel ingot was forged into a prismatic shape having a 90 × 90 mm cross section. Then, normalization was performed at 900 ° C. × 6 hours, tempering was performed at 670 ° C. × 12 hours, and after holding at 900 ° C. × 20 hours, water quenching was performed. Then, in order to confirm the repetitive effect of the gamma treatment, water quenching was performed once or twice after holding at 840 ° C. for 5 hours. When the effect of grain refinement due to rapid temperature rise appears, the degree of grain refinement effect due to AlN cannot be confirmed. Therefore, the rate of temperature rise for all heat treatments was set to 40 ° C./hour assuming thick members. .

  Table 2 shows (1) Hot forging-Normalizing (N)-Tempering (T)-Quenching (Q), (2) Hot forging-NTQQ, (3) The crystal grain size measurement result after hot forging-NTQQQ is shown. The crystal grain size was measured according to JIS G0551.

  In this example, the presence or absence of a crystal grain refining effect is determined in comparison with the grain size number 4.5 after heat treatment of Sample 1 (1). From the comparison of Sample 1 and Sample 2, the crystal grain size number hardly changed when only Al and N were added, and the effect of refining crystal grains did not appear. However, when the amount of V was reduced while adding Al and N as in Samples 3, 4 and 5, the crystal grain size number was 6 or more, and the effect of crystal grain refinement was confirmed. Samples 6 and 7 were steels in which only the amount of V was varied without adding Al and N, but the grain size number was about 4, and the grain refinement effect was not recognized. From the above results, it was found that there is no crystal grain refining effect unless the V content is reduced to less than 0.1% while adding Al and N.

  As in Samples 8 to 11, even when the amount of Ni or Mn is increased while controlling the amounts of Al, N, and V to the amount showing the effect of refining crystal grains, the crystal grain size number shows 6 or more. In contrast, it has been found that an increase in Ni and Mn does not reduce the crystal grain refining effect of AlN.

  Sample 12 to which Nb was added with respect to the composition of sample 9 had a crystal grain size number of 6.9, which was almost the same as that of sample 9, so that the amount of Al, N, and V showed a grain refinement effect. It is presumed that there is almost no crystal grain refinement effect of Nb addition in a state where the amount is controlled.

Samples 13 and 14 are steels with increased Cr, but since the grain size number is slightly larger, it can be said that the increased Cr is effective for crystal grain refinement.
After the heat treatments (2) and (3) in which Q was added to the heat treatment (1), the crystal grains became fine, but the sample 1 had a crystal grain size of 6 or more. Under the condition of adding twice, it was found that Sample 1 had to add Q twice compared to the inventive steel.

Table 3 shows the tensile strength, elongation, and Charpy impact value at room temperature of Sample 1 and Sample 5 conditioned under the same conditions. Sample 5 with fine crystal grains showed an impact value 1.5 times higher than that of sample 1 of the comparative material, although the tensile properties were hardly changed.

  As described above, the present invention has been described based on the above embodiment, but appropriate modifications can be made without departing from the scope of the present invention.

Final γ conversion temperature The final γ conversion treatment can be performed at 800 ° C to 930 ° C. For example, even if it is maintained for more than 100 hours, the crystal grains are hardly coarsened, so that the tempering temperature can be freely selected within this temperature range according to the target mechanical characteristics and the thickness of the member.

In addition, tempering in the tempering process has a great influence on toughness and cracking susceptibility, but it can be said that all heat treatments are irrelevant with respect to crystal grain size. Therefore, the tempering conditions suitable for the member to be applied may be performed by a conventional method. For example, the conditions of 150 to 200 ° C. can be shown when strength and hardness are required, and 550 to 600 ° C. can be shown when ductility is required. However, it is better to avoid carrying out at a temperature at which temper embrittlement occurs, for example, at 500 ° C. for a long time.
The grain size number and whether or not the grain size is sized can be judged by Japanese Industrial Standard G0551 “Austenite grain size test method for steel”, and can be judged using an apparatus such as an optical microscope. Tissues can also be determined using an optical microscope.
The steel material that has undergone the tempering of the present invention shows a grain size of 5.5 or more in terms of the crystal grain size according to the above criteria after general heat treatment conditions, that is, hot forging, normalizing, tempering, and quenching.

Claims (5)

  By mass percentage, C: 0.10 to less than 0.30%, Si: 0.05 to less than 0.30%, Mn: 0.20 to 1.00%, P: 0.015% or less, S: 0 0.015% or less, Cr: 1.50 to 2.00%, Mo: 0.10 to 0.50%, Ni: 2.50 to 4.00%, Al: 0.01 to 0.03%, N : NiCrMo steel containing 0.005 to 0.015%, with the balance being composed of Fe and inevitable impurities.   The NiCrMo steel according to claim 1, wherein the composition further contains V: less than 0.10% by mass percentage.   The NiCrMo steel according to claim 1 or 2, wherein the composition further contains Nb: less than 0.10% by mass percentage.   The NiCrMo steel according to any one of claims 1 to 3, comprising a martensitic structure, a bainite structure, or a mixed structure thereof. A steel having the composition according to any one of claims 1 to 3 is prepared, and hot forging, normalizing and tempering the steel, followed by γ-heating at 800 to 930 ° C x 1 to 100 hours A method for producing a NiCrMo steel material, characterized by obtaining a steel material having a prior austenite grain size of 5.5 or more in accordance with JIS G0551 by performing the treatment at least once.