A g20crni2moa steel and a manufacturing method thereof

Abstract

The application provides a G20CrNi2MoA steel and a manufacturing method thereof, and the manufacturing method comprises the following steps in sequence: steel ingot heating, steel ingot blooming, cooling, steel billet heating, rolling, cold bed collection and slow cooling; wherein the steel billet heating process comprises preheating zone, low temperature zone and high temperature zone heating; the high temperature zone heating temperature is 1140-1190 DEG C, and the high temperature zone heating time is 70-150 min; in the rolling process, the final rolling temperature is 960-990 DEG C; in the slow cooling process, 690 DEG C <= slow cooling pit temperature <= 850 DEG C, the slow cooling pit temperature is <= 150 DEG C, and the slow cooling time is >= 40 h. The G20CrNi2MoA steel material prepared by the method is free of surface iron oxide skin indentation, has no pit and spot, and the grain size and hardness meet the use requirements.

Description

Technical Field

[0001] This invention relates to the field of carburizing steel technology, and in particular to a G20CrNi2MoA steel and its manufacturing method. Background Technology

[0002] G20CrNi2MoA is a carburized bearing steel widely used in bearing rings for railways and heavy locomotives. The most common specifications are ∮80~120 mm. The steel requires an austenitic grain size ≥5 and a hardness ≤229 HBW. The chemical composition (mass percentage) is: C: 0.19~0.23%, Si: 0.15~0.40%, Mn: 0.40~0.70%, Cr: 0.40~0.65%, Ni: 1.60~2.00%, Mo: 0.20~0.30%, Cu≤0.20%, P:≤0.020%, S:≤0.015%, Al:≤0.05%, with the remainder being matrix Fe and unavoidable impurities. The steel production process is: ingot casting (electroslag remelting) – heating – ingot blanking – cooling – billet heating – rolling – cooling – annealing. To control grain size, the conventional process for heating steel billets involves high-temperature heating and holding. The high-temperature heating temperature is 1200-1260℃, and the heating time in the high-temperature zone is 150-300 minutes. Due to the high heating temperature and long holding time of the steel billets, coupled with their high Ni content and the presence of certain Cr and Mo alloying elements, the iron oxide scale on the surface of the billets is thick and firmly adheres to the substrate surface when they exit the furnace. This scale is difficult to remove during descaling and is thus pressed into the surface of the rolled product during the rolling process, forming pits and blemishes on the steel surface. Figure 3 As shown. Due to surface pits and blemishes, the steel is difficult to peel off using an abrasive wheel, such as... Figure 4 As shown. Therefore, the only solution is to roll the steel to a larger size and then machine it for delivery. Furthermore, after slow cooling, the steel's hardness is around 250 HBW, requiring annealing to meet the standard requirement of ≤229 HBW. The numerous production steps result in a long production cycle and high costs.

[0003] In view of this, the present invention is hereby proposed. Summary of the Invention

[0004] The purpose of this invention is to provide a method for manufacturing G20CrNi2MoA steel. The G20CrNi2MoA steel produced by this method has no iron oxide scale pressed into its surface, no pits or blemishes, and its grain size and hardness meet the requirements for use.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] This invention provides a method for manufacturing G20CrNi2MoA steel, which includes the following steps in sequence: ingot heating, ingot billet preparation, cooling, billet heating, rolling, cooling bed collection, and slow cooling;

[0007] The billet heating process includes preheating, low-temperature heating, and high-temperature heating; the high-temperature heating temperature is 1140–1190℃, and the high-temperature heating time is 70–150 min; during the rolling process, the final rolling temperature is 960–990℃; during the slow cooling process, the temperature entering the slow cooling pit is 690℃ ≤ 850℃, the temperature exiting the slow cooling pit is ≤ 150℃, and the slow cooling time is ≥ 40 h.

[0008] Furthermore, the temperature of the preheating zone is ≤750℃; and / or, the heating temperature of the low-temperature zone is 1000~1140℃, and the heating time is 30~60min; and / or, the total heating time of the billet is 150~270min.

[0009] Furthermore, the residual oxygen content in the furnace used for heating the steel billet is ≤3%.

[0010] Furthermore, the rolling process includes roughing and finishing rolling;

[0011] And / or, the rolling process adopts an accelerated rolling mode, wherein the final rolling rate is increased from 1.0 m / s to 1.20 m / s.

[0012] Furthermore, in the rolling process, a heat insulation cover is installed on the roller table between the roughing and finishing rolling to reduce the temperature drop of the rolled piece.

[0013] Furthermore, during the heating process of the steel ingot, the holding temperature is 1220–1270℃, and the holding time is 3–6 hours;

[0014] And / or, the steel ingot is a die-cast or electroslag cast ingot.

[0015] Furthermore, in the rolling process, the initial rolling temperature is 1050–1160°C.

[0016] Furthermore, the height of the steel packed in the slow cooling pit is ≥ 1 / 2 of the pit height.

[0017] In addition, the present invention also provides a G20CrNi2MoA steel prepared by the manufacturing method described above, wherein the G20CrNi2MoA steel comprises the following chemical composition by mass percentage: C: 0.19-0.23%, Si: 0.15-0.40%, Mn: 0.40-0.70%, Cr: 0.40-0.65%, Ni: 1.60-2.00%, Mo: 0.20-0.30%, Cu≤0.20%, P:≤0.020%, S:≤0.015%, Al:≤0.05%, with the remainder being the matrix Fe and unavoidable impurities.

[0018] Furthermore, the G20CrNi2MoA steel has a grain size ≥ grade 5 and a hardness ≤ 229HBW.

[0019] Compared with the prior art, the technical solution of the present invention has at least the following technical effects:

[0020] The G20CrNi2MoA steel produced by the manufacturing method of this invention has no iron oxide scale indentation on its surface, no pits or blemishes, and its grain size and hardness meet the requirements for use. Specifically, by controlling the billet heating temperature, the holding time in the heat preservation zone, and the furnace atmosphere (residual oxygen value), the oxidation of the billet surface is reduced; by controlling the final rolling temperature and the post-rolling cooling rate, the grain size and surface quality of the steel are controlled; by using a heat preservation cover on the roller table between the roughing and finishing rolling mills to reduce the temperature drop of the rolled piece, the final rolling temperature is increased, and the rolling speed is increased within the equipment capacity; by controlling the temperature of the steel entering the slow cooling pit and the cooling effect of the steel in the slow cooling pit, the quality requirements of controlling the hardness of the steel and slow cooling instead of annealing are achieved, saving processes and reducing costs. Attached Figure Description

[0021] The accompanying drawings, which form part of this specification, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. Wherein:

[0022] Figure 1 Metallographic image of G20CrNi2MoA steel prepared in Example 1;

[0023] Figure 2 The surface quality of the G20CrNi2MoA steel prepared in Example 1;

[0024] Figure 3 The surface quality of rolled steel in existing technologies;

[0025] Figure 4 The surface quality of steel after peeling in existing technologies. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Those skilled in the art should understand that the embodiments described are merely illustrative of the invention and should not be considered as specific limitations thereof. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention. Process parameters not specifically specified in the following embodiments are generally performed under conventional conditions.

[0027] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this invention.

[0028] According to a first aspect of the present invention, a method for manufacturing G20CrNi2MoA steel is provided, comprising the following steps in sequence: ingot heating, ingot billet preparation, cooling, billet heating, rolling, cooling bed collection, and slow cooling;

[0029] The billet heating process includes a preheating zone, a low-temperature zone, and a high-temperature zone. The high-temperature zone heating temperature is 1140–1190℃ (e.g., 1140℃, 1150℃, 1160℃, 1170℃, 1180℃, 1190℃), and the high-temperature zone heating time is 70–150 min (e.g., 70 min, 80 min, 90 min, 100 min, 110 min, 120 min, 130 min, 140 min, 1...). (50 min); during the rolling process, the final rolling temperature is 960~990℃; during the slow cooling process, 690℃≤entry temperature into the slow cooling pit≤850℃ (e.g., 690℃, 700℃, 710℃, 720℃, 730℃, 740℃, 750℃, 760℃, 770℃, 780℃, 790℃, 800℃, 810℃, 820℃, 830℃, 840℃, 850℃), the exit temperature from the slow cooling pit≤150℃, and the slow cooling time≥40h.

[0030] The inventors discovered that when steel billets are heated to temperatures above 1200℃, Ni causes selective oxidation at grain boundaries, resulting in Ni-rich metal particles and mesh-like structures in the iron oxide scale transition layer (the transition layer significantly increases when Ni and Cr coexist; the adhesion strength of the scale depends primarily on the thickness and structure of the transition layer). These Ni-rich metal mesh-like structures connect the iron oxide scale to the metal matrix. Due to their similar plasticity and coefficient of thermal expansion to the metal matrix, the scale exhibits strong adhesion and is difficult to peel off even under high-pressure water impact. Therefore, this invention controls the heating temperature of the rough steel billet between 1140 and 1190℃ and the heating time between 70 and 150 minutes. If the heating temperature and time are below this range, the grain size is unqualified; if they are above this range, the iron oxide scale on the billet surface is severe and cannot be removed by a descaling machine. This scale, pressed into the surface of the rolled product during rolling, will form pits. Simultaneously, controlling the final rolling temperature and the slow cooling effect in the slow cooling pit achieves the quality requirements for controlling the grain size and hardness of the steel.

[0031] Based on the above scheme, as a preferred embodiment, the temperature of the preheating zone is ≤750℃; and / or, the heating temperature of the low-temperature zone is 1000~1140℃ (e.g., 1000℃, 1020℃, 1040℃, 1060℃, 1080℃, 1100℃, 1120℃, 1130℃), and the heating time is 30~60min (e.g., 30min, 40min, 50min, 60min); and / or, the total heating time of the billet is 150~270min (e.g., 150min, 170min, 190min, 210min, 230min, 250min, 270min).

[0032] In this invention, the heating temperature in the low-temperature zone is lower than the heating temperature in the high-temperature zone.

[0033] Based on the above scheme, as a preferred embodiment, the residual oxygen content in the furnace for heating the steel billet is ≤3%. If the residual oxygen content is higher than 3%, there is excess air in the furnace, resulting in severe iron oxide scale on the surface of the steel billet.

[0034] Based on the above scheme, as a preferred embodiment, the rolling process includes roughing and finishing rolling; the rolling process adopts an increasing speed rolling mode, wherein the final rolling speed is increased from 1.0 m / s to 1.20 m / s.

[0035] Based on the above scheme, as a preferred embodiment, in the rolling process, a heat insulation cover is installed on the roller table between the roughing and finishing mills to reduce the temperature drop of the rolled piece. To increase the final rolling temperature, a heat insulation cover is used on the roller table between the roughing and finishing mills to reduce the temperature drop of the rolled piece.

[0036] Based on the above scheme, as a preferred embodiment, during the heating process of the steel ingot, the holding temperature is 1220-1270℃ (e.g., 1220℃, 1230℃, 1240℃, 1250℃, 1260℃, 1270℃), and the holding time is 3-6 hours (e.g., 3 hours, 4 hours, 5 hours, 6 hours). Optionally, the steel ingot is a die-cast or electroslag cast ingot.

[0037] Based on the above scheme, as a preferred embodiment, the initial rolling temperature in the rolling process is 1050-1160℃ (e.g., 1050℃, 1070℃, 1090℃, 1110℃, 1130℃, 1150℃, 1160℃).

[0038] Based on the above scheme, as a preferred embodiment, the steel loading height in the slow cooling pit is ≥ 1 / 2 of the pit height. To ensure the slow cooling effect, the steel loading height is preferably controlled within this range. If the amount of steel to be cooled in the pit is insufficient, additional steel is added to the pit to ensure the slow cooling effect. For example, the steel loading capacity of the slow cooling pit is 150t.

[0039] According to a second aspect of the present invention, a G20CrNi2MoA steel prepared by the above-described manufacturing method is provided, the G20CrNi2MoA steel comprising the following chemical composition by mass percentage: C: 0.19–0.23%, Si: 0.15–0.40%, Mn: 0.40–0.70%, Cr: 0.40–0.65%, Ni: 1.60–2.00%, Mo: 0.20–0.30%, Cu ≤0.20%, P: ≤0.020%, S: ≤0.015%, Al: ≤0.05%, the remainder being matrix Fe and unavoidable impurities.

[0040] Based on the above scheme, as a preferred embodiment, the grain size of the G20CrNi2MoA steel is ≥5 and the hardness is ≤229HBW.

[0041] The present invention will now be described in detail with reference to the accompanying drawings and embodiments thereof. Various examples are provided by way of explanation and not by way of limitation. Indeed, those skilled in the art will recognize that modifications and variations may be made to the invention without departing from its scope or spirit. For example, a feature shown or described as part of one embodiment may be used in another embodiment to produce yet another embodiment. Therefore, it is desirable that the present invention encompass such modifications and variations falling within the scope of the appended claims and their equivalents.

[0042] Examples 1-5

[0043] Examples 1-5 provide a method for manufacturing G20CrNi2MoA steel, using G20CrNi2MoA steel billet as raw material, and designing to roll and produce steel with specifications of φ80-95mm.

[0044] The main chemical composition and content (wt%) of the steel billets in Examples 1-5 are shown in Table 1:

[0045] Table 1

[0046] Example 1 0.21 0.36 0.57 0.61 1.68 0.24 0.08 0.014 0.007 0.021 Example 2 0.20 0.32 0.61 0.58 1.72 0.23 0.10 0.012 0.006 0.024 Example 3 0.22 0.30 0.52 0.62 1.65 0.26 0.06 0.009 0.009 0.025 Example 4 0.20 0.35 0.63 0.56 1.76 0.22 0.09 0.015 0.008 0.022 Example 5 0.22 0.33 0.54 0.60 1.73 0.25 0.07 0.013 0.007 0.020

[0047] Production process flow: billet heating → billet rolling (220~240 square billet) → cooling → transfer → billet heating → rough rolling → roller table heat preservation → finish rolling → cooling bed collection → slow cooling.

[0048] Specifically, the steel ingot is heated and held at a temperature of 1220–1270℃ for 3–6 hours.

[0049] In the billet heating process, the heating process includes a preheating zone, a low-temperature zone heating zone, and a high-temperature zone heating zone: the preheating zone requires a temperature ≤750℃ and the preheating time is unlimited; the temperature of the low-temperature zone is 1000~1140℃ and the heating time is 30~60min; the temperature of the high-temperature zone is 1140~1190℃ and the heating time of the high-temperature zone is 70~150min.

[0050] In the rolling process, the initial rolling temperature is 1050~1160℃, and the final rolling temperature is 960~990℃. Insulation covers along the roughing and finishing mill rollers are lowered to reduce the temperature drop of the rolled piece. During rolling, within the equipment's capacity, production is accelerated by increasing the rolling speed to control the final rolling temperature. To improve the grain size of the steel, after the steel is placed on the cooling bed, it is promptly sawed and collected. The sawing temperature is >780℃, the collection temperature is ≥700℃, the temperature entering the slow cooling pit is 690℃ ≤ 850℃, the slow cooling time is ≥40h, the temperature exiting the pit is ≤150℃, and the slow cooling time is ≥40h.

[0051] The specific process parameters in Examples 1-5 are shown in Table 2:

[0052]

[0053] Comparative Example 1

[0054] This comparative example provides a method for manufacturing G20CrNi2MoA steel. The steps and parameters of the manufacturing method are basically the same as those in Example 1, except that:

[0055] The high-temperature zone has a heating temperature of 1213-1246℃ and a heating time of 216 minutes.

[0056] Comparative Example 2

[0057] This comparative example provides a method for manufacturing G20CrNi2MoA steel. The steps and parameters of the manufacturing method are basically the same as those in Example 1, except that:

[0058] In the rolling process, the final rolling temperature is 953℃.

[0059] Comparative Example 3

[0060] This comparative example provides a method for manufacturing G20CrNi2MoA steel. The steps and parameters of the manufacturing method are basically the same as those in Example 1, except that:

[0061] During the slow cooling process, the temperature entering the slow cooling pit is 645℃, the temperature exiting the slow cooling pit is 127℃, and the slow cooling time is 47 hours.

[0062] Mechanical properties of the G20CrNi2MoA steels prepared in Examples 1-5 and Comparative Examples 1-3 after slow cooling were tested. Grain size was determined according to the standard test method for average grain size determination (ASTM E112); tensile strength, elongation after fracture, and reduction of area were determined according to the standard test method (ASTM E8); room temperature impact energy was determined according to the impact test method (ASTM E23); and hardness was determined according to the Brinell hardness test method (ASTM E10). The test results are shown in Table 3.

[0063] Table 3

[0064]

[0065] As can be seen from Examples 1-5 and Comparative Examples 1-3 of the present invention, the comprehensive performance of the G20CrNi2MoA steel obtained in Examples 1-5 is higher than that in Comparative Examples 1-3; after the heating temperature in the high-temperature zone of Comparative Example 1 is higher than the range defined by the present invention, the comprehensive performance of the G20CrNi2MoA steel obtained is lower than that in Example 1; after the final rolling temperature of Comparative Example 2 is lower than the range defined by the present invention, the comprehensive performance of the G20CrNi2MoA steel obtained is significantly lower than that in Example 1; the specific parameters in the slow cooling process of Comparative Example 3 are not within the range defined by the present invention, and the grain size and hardness of the G20CrNi2MoA steel obtained cannot meet the requirements for use.

[0066] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for manufacturing G20CrNi2MoA steel, characterized in that, The steps are as follows: Steel ingot heating, steel ingot roughing, cooling, steel billet heating, rolling, cooling bed collection, slow cooling; The billet heating process includes preheating, low-temperature heating, and high-temperature heating; the high-temperature heating temperature is 1140~1190℃, and the high-temperature heating time is 70~150min; during the rolling process, the final rolling temperature is 960~990℃; during the slow cooling process, 690℃≤entry temperature into the slow cooling pit≤850℃, exit temperature from the slow cooling pit≤150℃, and the slow cooling time≥40h; The heating temperature in the low-temperature zone is 1000~1137℃, and the heating time is 30~60min; The G20CrNi2MoA steel comprises the following chemical composition by mass percentage: C: 0.19-0.23%, Si: 0.15-0.40%, Mn: 0.40-0.70%, Cr: 0.40-0.65%, Ni: 1.60-2.00%, Mo: 0.20-0.30%, Cu≤0.20%, P:≤0.020%, S:≤0.015%, Al:≤0.05%, with the remainder being matrix Fe and unavoidable impurities; The G20CrNi2MoA steel obtained after slow cooling has a grain size ≥ 5 and a hardness ≤ 229HBW.

2. The method for manufacturing G20CrNi2MoA steel according to claim 1, characterized in that, The temperature of the preheating zone is ≤750℃; and / or the total heating time of the billet is 150~270min.

3. The method for manufacturing G20CrNi2MoA steel according to claim 1 or 2, characterized in that, The residual oxygen content in the furnace used for heating the steel billet is ≤3%.

4. The method for manufacturing G20CrNi2MoA steel according to claim 1, characterized in that, The rolling process includes roughing and finishing rolling; And / or, the rolling process adopts an accelerated rolling mode, wherein the final rolling rate is increased from 1.0 m / s to 1.20 m / s.

5. The method for manufacturing G20CrNi2MoA steel according to claim 4, characterized in that, In the rolling process, a heat insulation cover is installed on the roller table between the roughing and finishing rolling to reduce the temperature drop of the rolled piece.

6. The method for manufacturing G20CrNi2MoA steel according to claim 1, characterized in that, During the heating process of the steel ingot, the holding temperature is 1220~1270℃ and the holding time is 3~6h; And / or, the steel ingot is a die-cast or electroslag cast ingot.

7. The method for manufacturing G20CrNi2MoA steel according to claim 1, characterized in that, In the rolling process, the initial rolling temperature is 1050~1160℃.

8. The method for manufacturing G20CrNi2MoA steel according to claim 1, characterized in that, The height of the steel packed in the slow cooling pit is ≥ 1 / 2 of the pit height.

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