CN113699337A - Heat treatment process for continuous casting large round billet of 9Cr heat-resistant steel - Google Patents

Abstract

The invention relates to a 9Cr heat-resistant steel continuous casting round billet heat treatment process, which belongs to the technical field of metal material heat treatment, and solves the technical problems of direct punching of large-caliber thick walls and high rejection rate of thin-walled tubes due to the fact that the microstructure, precipitated phases and structural abnormalities of a 9Cr heat-resistant steel round billet of a one-time long-time annealing process are large.

Description

Heat treatment process for continuous casting large round billet of 9Cr heat-resistant steel

Technical Field

The invention belongs to the technical field of metal material heat treatment, and particularly relates to a heat treatment process for a 9Cr heat-resistant steel continuous casting round billet.

Background

The 9Cr heat-resistant steel material has the characteristics of high temperature resistance, high pressure resistance and corrosion resistance, and is mainly used for manufacturing key components of a supercritical (super) critical thermal power generating unit. The high-quality 9Cr heat-resistant steel continuous casting round billet can be directly used for threading large-caliber thick-wall pipes and thin-wall pipes and applied to the fields of supercritical (super) critical thermal power generating units and petroleum and chemical industry. However, the 9Cr heat-resistant steel has more types of alloy elements, belongs to hypo-peritectic steel, has a complex solidification process, and easily generates large thermal stress and thick structure in the solidification process of large-specification continuous casting billets. The conventional one-time long-time annealing can release part of stress, but the problems of thick microstructure and incomplete structure transformation exist, so that the mechanical property is influenced, the pipe penetrating process is cracked, and the production stability and the product percent of pass are seriously influenced.

Disclosure of Invention

The invention aims to overcome the defects of the prior art and solve the technical problems of direct punching of large-caliber thick walls and high rejection rate of thin-walled tubes caused by large microstructure and precipitated phases and structural abnormality of a 9Cr heat-resistant steel large round billet by a one-time long-time annealing process.

The design concept of the invention is as follows:

the 9Cr heat-resistant steel has high alloy content and belongs to crack sensitive steel. The method is characterized in that the uneven microstructure and stress concentration are easily formed in the large-section continuous casting solidification process to cause cracks, the thermal stress of the large-section continuous casting blank is usually eliminated by adopting one-time long-time annealing, but the required heat preservation time is longer, the microstructure is easily caused, the precipitated phase is thick, and the residual martensite structure exists to cause the cracks easily in the subsequent pipe penetrating process.

The invention adopts the annealing and tempering process of the 9Cr heat-resistant steel round billet to replace the traditional one-time long-time annealing process. The method not only effectively releases the thermal stress of the large-section continuous casting billet, but also shortens the high-temperature annealing time of the casting billet, effectively reduces the cost, and simultaneously achieves the purposes of refining the microstructure, eliminating the residual martensite structure and reducing the subsequent rejection rate of direct pipe penetration.

In order to solve the problems, the technical scheme of the invention is as follows:

a heat treatment process for a 9Cr heat-resistant steel continuous casting round billet comprises the following steps:

s1, sequentially carrying out electric converter, LF ladle refining and VD vacuum degassing on the raw materials to obtain the product with the diameter of

The 9Cr heat-resistant steel continuous casting round billet;

s2, annealing process: carrying out red annealing on the 9Cr heat-resistant steel continuous casting round billet prepared in the step S1, wherein the temperature of the round billet in an annealing furnace is required to be more than or equal to 500 ℃; then, heating the steel plate to 765 +/-10 ℃ at a heating speed of less than 100 ℃/h by using an annealing furnace, wherein the heat preservation time is Dx 2.5-2.8 min/mm, and D is the diameter/mm of the continuous casting slab; finally, the large round billet is cooled to be less than or equal to 500 ℃ along with the furnace, taken out of the furnace and cooled to room temperature by air, and annealing is finished;

s3, tempering: and (4) heating the large round blank annealed in the step (S2) to 650 +/-10 ℃ at a heating speed of less than 100 ℃/h, preserving heat for Dx 1.2-1.5 min/mm, wherein D is the diameter/mm of the continuous casting blank, then cooling the large round blank to be less than or equal to 200 ℃ along with the furnace, taking out of the furnace, and air-cooling to room temperature to finish tempering.

Compared with the prior art, the invention has the beneficial effects that:

the invention breaks through the traditional one-time long-time high-temperature annealing (the annealing temperature is 765 +/-10 ℃, the annealing heat preservation time is Dx 3.7-4.3 min/mm, and D is the diameter/mm of a continuous casting billet and ranges from 300-800 mm) of the 9Cr heat-resistant steel continuous casting large round billet, invents the process system of annealing and tempering of the continuous casting large round billet, realizes the aim of low energy consumption in the heat treatment process of the 9Cr heat-resistant steel, can effectively refine the annealing structure and precipitated phase of the 9Cr heat-resistant steel continuous casting large round billet, improves the mechanical property of the 9Cr heat-resistant steel, and reduces the rejection rate of the 9Cr heat-resistant steel large round billet directly penetrating into a thick-wall pipe.

According to the statistics of field actual production data, the rejection rate of directly penetrating thick-wall pipes from the 9Cr heat-resistant steel large round blank subjected to conventional one-time long-time annealing is 6-8%, and the rejection rate of directly penetrating thick-wall pipes from the 9Cr heat-resistant steel large round blank subjected to the annealing and tempering process is 0.45%.

Drawings

FIG. 1 is a microstructure of a prior art tissue after a single long-time anneal;

FIG. 2 is a microstructure diagram of the annealed and tempered structure of the present invention;

FIG. 3 is a microstructure diagram of a precipitated phase structure after a long-time annealing in the prior art;

FIG. 4 is a microstructure of a precipitated phase structure after annealing and tempering treatment according to the present invention.

Detailed Description

The invention is described in further detail below with reference to the figures and examples.

A heat treatment process for a 9Cr heat-resistant steel continuous casting round billet comprises the following steps:

s1, sequentially carrying out 80-ton electric converter, LF ladle refining and VD vacuum degassing on the raw materials to obtain the product with the diameter of

The material of the 9Cr heat-resistant steel continuous casting large round billet is typical steel types P91 and P92 of 9Cr steel in the implementation;

TABLE 1 control of chemical composition of P91 (unit:%)

Element(s)

C

Si

Mn

P

S

Cr

Mo

Content (wt.)

0.08-0.12

0.20-0.50

0.30-0.60

≤0.020

≤0.010

8.00-9.50

0.85-1.05

Element(s)

Nb

V

Al

Ni

Ti

Zr

N

Content (wt.)

0.06-0.10

0.18-0.25

≤0.020

≤0.40

≤0.010

≤0.010

0.030-0.070

TABLE 2P 92 chemical composition control (unit:%)

S2, annealing process: carrying out red annealing on the 9Cr heat-resistant steel continuous casting round billet prepared in the step S1, wherein the temperature of the round billet in an annealing furnace is 550 ℃; then, the annealing furnace is heated to 765 ℃ at the heating speed of 100 ℃/h, and the heat preservation time is 30 h; finally, the large round billet is cooled to 450 ℃ along with the furnace, taken out of the furnace and cooled to room temperature in air, and annealing is finished;

s3, tempering: and (4) heating the large round blank annealed in the step (S2) to 650 ℃ at a heating speed of 100 ℃/h, preserving heat for 15h, then cooling the large round blank to 200 ℃ along with the furnace, taking out of the furnace, and air cooling to room temperature to finish tempering.

Usually, the thermal stress of a large-section continuous casting billet is eliminated by one-time long-time annealing, but the required heat preservation time is long, the microstructure is thick and thick, high-temperature ferrite is remained easily, and cracks are easily caused in the subsequent pipe penetrating process. By adopting the annealing and tempering process of the 9Cr heat-resistant steel round billet, the thermal stress of the large-section continuous casting billet is eliminated, the high-temperature annealing time of the casting billet is shortened, the cost is effectively reduced, the microstructure is refined, and the subsequent pipe-penetrating crack generation rate is reduced.

Fig. 1 and 2 show the microstructure of a 9 Cr-based heat-resistant steel in a conventional one-time long-time annealing and the "annealing + tempering" process of the present invention, respectively. The microstructure under the primary long-time annealing process is ferrite + pearlite + partially coarse residual martensite, while the microstructure under the 'annealing + tempering' process is ferrite + pearlite, and the coarse martensite structure is completely eliminated.

As can be seen from comparison of fig. 3 and 4, the precipitated phase of the 9 Cr-based heat-resistant steel in the "annealing + tempering" process is significantly refined.

The strength and plasticity of two steels P91 and P92 were compared under different heat treatment regimes, and the results are shown in tables 3 and 4, respectively.

TABLE 39 Cr series heat-resistant steel P91 mechanical properties

Item	RP0.2,MPa	Rm,MPa	A50,％	Z,％	Room temperature AKV,J	HB
							The invention 'annealing + tempering'	526	681	28	68	215	210
Conventional primary annealing	515	678	28	67	185	210

TABLE 49 mechanical properties of heat-resistant Cr steel P92

Item	RP0.2,MPa	Rm,MPa	A50,％	Z,％	Room temperature AKV,J
						The invention 'annealing + tempering'	538	705	26	63	207
Conventional primary annealing	536	703	25	62	204

As can be seen from tables 3 and 4: the strength and plasticity indexes of the blank prepared by the method are superior to those of the conventional one-time long-time annealing.

The above description is only for the specific embodiments of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present invention are included in the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the appended claims.

Claims (1)

1. A heat treatment process for a 9Cr heat-resistant steel continuous casting round billet is characterized by comprising the following steps:

s1, sequentially carrying out electric converter, LF ladle refining and VD vacuum degassing on the raw materials to prepare a 9Cr heat-resistant steel continuous casting round billet with the diameter of phi 300-phi 800 mm;

s2, annealing process: carrying out red annealing on the 9Cr heat-resistant steel continuous casting round billet prepared in the step S1, wherein the temperature of the round billet in an annealing furnace is required to be more than or equal to 500 ℃; then, heating the steel plate to 765 +/-10 ℃ at a heating speed of less than 100 ℃/h by using an annealing furnace, wherein the heat preservation time is Dx 2.5-2.8 min/mm, and D is the diameter/mm of the continuous casting slab; finally, the large round billet is cooled to be less than or equal to 500 ℃ along with the furnace, taken out of the furnace and cooled to room temperature by air, and annealing is finished;

s3, tempering: and (4) heating the large round blank annealed in the step (S2) to 650 +/-10 ℃ at a heating speed of less than 100 ℃/h, preserving heat for Dx 1.2-1.5 min/mm, wherein D is the diameter/mm of the continuous casting blank, then cooling the large round blank to be less than or equal to 200 ℃ along with the furnace, taking out of the furnace, and air-cooling to room temperature to finish tempering.

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