A non-quenched and tempered steel 38MnVS6 bar and rolling method thereof

Abstract

The present invention belongs to the technical field of deformation heat treatment of metal materials, and relates to a non-quenched and tempered steel 38MnVS6 bar and a rolling method thereof. Specifically, the steel billet is heated, rough rolled, intermediate rolled, water-cooled, sizing-reducing finishing rolled, and heat-insulated cooling in sequence to obtain a hot-rolled bar; wherein, a water tank is arranged along the rolling line, and the water tank is used for water-cooling after intermediate rolling and before sizing-reducing finishing rolling, and the water volume in the water tank is controlled to be 150-350L / min, and the water pressure is 6-8MPa; the heat-insulated cooling is carried out in the heat-insulating cover of the cooling bed, and the temperature of the steel billet entering the heat-insulating cover is 760-800°C, and the temperature of the steel billet exiting the heat-insulating cover is 570-630°C. The obtained bar is exempt from annealing treatment, and its hot-rolled hardness is 227-238HBW, and the ferrite ratio is 51.5%-54.4%.

Description

Technical Field

[0001] The invention belongs to the technical field of deformation heat treatment of metal materials and relates to a non-quenched and tempered steel 38MnVS6 bar and a rolling method thereof. Background Art

[0002] Non-quenched and tempered steel is a type of steel that has microalloying elements added to traditional medium carbon steel. Through controlled cooling, it can achieve mechanical properties equivalent to those of quenched and tempered steel without quenching and tempering after processing. Compared with quenched and tempered steel, non-quenched and tempered steel has the advantages of energy saving and emission reduction, short production cycle, and low manufacturing cost. With the intensification of the world energy crisis, non-quenched and tempered steel has developed rapidly in recent years and has been widely used in the automotive industry.

[0003] 38MnVS6 is a commonly used ferrite-pearlite non-quenched and tempered steel, the main chemical composition (mass percentage) is: C: 0.38 ~ 0.42%, Si: 0.50 ~ 0.70%, Mn: 1.14 ~ 1.24%, P: ≤ 0.02%, S: 0.028 ~ 0.042%, Cr: ≤ 0.25%, Ni: ≤ 0.15%, Cu ≤ 0.25%, V: 0.15 ~ 0.20%, the rest is matrix Fe and inevitable impurities. It has the advantages of excellent mechanical properties and easy cutting, and is widely used in the production of crankshafts, engine high-pressure common rails and other automotive parts. However, the content of alloy elements in 38MnVS6 is relatively high. The small-size 38MnVS6 bars (Φ30 ~ Φ60) usually produced by the industry have a fast cooling rate after rolling. The ferrite content of the hot-rolled material is low and the hardness is high, which affects the subsequent processing of the material such as cutting. In order to reduce tool damage and ensure production efficiency, parts processing plants often need to anneal the raw materials before cutting them for use. This not only increases processing costs, but also weakens the advantages of non-quenched and tempered steel in energy conservation, emission reduction and green environmental protection.

[0004] The metallographic structure of 38MnVS6 small-size bars (Φ30~Φ60) commonly produced in the industry is as follows Figure 1 a. Figure 1 As shown in b, the metallographic structure is ferrite + pearlite, of which the proportion of ferrite is 40% to 45%, and the hardness is 260 to 275 HBW. When an automobile parts factory uses this material to produce engine high-pressure common rails, due to the high hardness of the hot-rolled material, the material is difficult to cut, and the tool damage rate is high, the material needs to be softened and annealed to between 200 and 248 HBW before use.

[0005] In order to reduce production costs and carbon emissions, it is planned to optimize the rolling process of 38MnVS6 small-size bars (Φ30~Φ60) so that the ferrite ratio of the material after rolling is ≥50%, the hardness meets the requirements of 200~248HBW, and annealing-free delivery is achieved. Summary of the invention

[0006] The present invention aims to provide a non-quenched and tempered steel 38MnVS6 bar and a rolling method thereof, and specifically relates to rolling 38MnVS6 bars with specifications of Φ30 to Φ60. By controlling the rolling temperature and post-rolling cooling, the microstructure and hardness of the bars after rolling are controlled to facilitate subsequent processing.

[0007] To achieve the above object, the present invention adopts the following technical solutions:

[0008] In a first aspect, a rolling method of a non-quenched and tempered steel 38MnVS6 bar is provided, wherein the steel billet is sequentially heated, rough rolled, intermediate rolled, water-cooled, sizing-reducing finish rolled, and heat-insulated cooled to obtain a hot-rolled bar;

[0009] Among them, water tanks are arranged along the rolling line. After intermediate rolling and before sizing and finishing rolling, water tanks are used for water cooling, and the water volume in the water tank is controlled to be 150-350L / min and the water pressure is 6-8MPa.

[0010] The heat preservation cooling is carried out in the heat preservation cover of the cooling bed. The temperature of the steel billet entering the heat preservation cover is 760-800°C, and the temperature of the steel billet exiting the heat preservation cover is 570-630°C.

[0011] Furthermore, the heating soaking temperature is 1160-1220° C., and the heat preservation time is 50-120 min.

[0012] Furthermore, the starting rolling temperature of the rough rolling is 1070-1130°C.

[0013] Furthermore, the final rolling temperature of the finish rolling is 780-830°C.

[0014] Furthermore, the final rolling temperature of the finish rolling is 790-820°C.

[0015] Furthermore, the steel billet is surface treated after heating and before rough rolling.

[0016] Furthermore, the surface treatment includes high-pressure water dephosphorization treatment.

[0017] Furthermore, the steel billets are sheared, collected and bundled after being cooled and kept warm.

[0018] In a second aspect, a non-quenched and tempered steel 38MnVS6 bar is obtained by the aforementioned rolling method.

[0019] Furthermore, the bar material is exempt from annealing treatment, has a hot-rolled hardness of 227-238 HBW, and a ferrite ratio of 51.5%-54.4%.

[0020] The present invention uses the deformation-induced ferrite phase transformation mechanism and combines the characteristics of 38MnVS6 non-quenched and tempered steel to design a rolling process for controlling the ferrite structure ratio and hardness. Compared with the prior art, the present invention has the following beneficial effects:

[0021] (1) The 38MnVS6 non-quenched and tempered steel rolling process of the present invention controls the final rolling temperature to about 810°C by using water tanks along the rolling line, so that the material is deformed at a temperature slightly higher than the Ar3 phase transformation point, accumulating a large number of dislocations to promote ferrite nucleation, increasing the driving force for the transformation of austenite to ferrite, inducing ferrite precipitation, and thus increasing the proportion of ferrite tissue.

[0022] (2) In the present invention, water tanks are arranged along the rolling line. The final rolling temperature is controlled by controlling the water volume in the water tanks, and the steel after rolling is insulated and slowly cooled by using a cooling bed insulation cover. This technology can accurately control the temperature of each key point to obtain the desired round bar, and has the characteristic of high controllability.

[0023] (3) The 38MnVS6 non-quenched and tempered steel bar rolled by the process has a specification of 30-60 mm, a hot rolled hardness of 227-238 HBW, and a ferrite ratio of 51.5%-54.4%, which meets the user's direct material requirements without the need for heat treatment such as softening annealing. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 The metallographic structure of 38MnVS6 under ordinary rolling process (1a. Surface image; 1b. Core image);

[0025] Figure 2 This is the metallographic structure of 38MnVS6 of Example 2 of the present invention (2a. Surface image; 2b. Core image). DETAILED DESCRIPTION

[0026] In order to make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. The embodiments of the present invention are implemented on the premise of the technical solution of the present invention, and detailed implementation methods and processes are given, but the protection scope of the present invention is not limited to the following embodiments. Those skilled in the art should understand that the embodiments are only to help understand the present invention and should not be regarded as specific limitations of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0027] The endpoints and any values ​​of the ranges disclosed in the present 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 each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be regarded as specifically disclosed in the present invention.

[0028] In the present invention, unless otherwise specified and / or described, all numerical values ​​involving the amount of components are "weight or weight percentage" from beginning to end. The process parameters without specific conditions in the following examples are usually based on conventional conditions. The raw materials described in the following examples can all be obtained from public commercial channels.

[0029] A rolling method for non-quenched and tempered steel 38MnVS6 bars, wherein the steel billet is sequentially heated, rough rolled, intermediate rolled, water cooled, sizing finished rolled, and heat-insulated cooled to obtain hot-rolled bars;

[0030] Among them, water tanks are arranged along the rolling line, and water tanks are used for water cooling after intermediate rolling and before sizing and finishing rolling, and the water volume in the water tank is controlled to be 150-350L / min (such as 160L / min, 180L / min, 200L / min, 220L / min, 240L / min, 260L / min, 280L / min, 300L / min, 320L / min, 340L / min), and the water pressure is 6-8MPa (such as 6.1MPa, 6.3MPa, 6.5MPa, 6.7MPa, 6.9MPa, 7.1MPa, 7.3MPa, 7.5MPa, 7.7MPa, 7.9MPa);

[0031] The thermal insulation cooling is carried out in a thermal insulation cover of a cooling bed, and the temperature of the steel billet entering the thermal insulation cover is 760-800°C (such as 765°C, 770°C, 775°C, 780°C, 785°C, 790°C, and 795°C), and the temperature of the steel billet exiting the thermal insulation cover is 570-630°C (such as 575°C, 580°C, 585°C, 590°C, 595°C, 600°C, 605°C, 610°C, 615°C, 620°C, and 625°C).

[0032] The invention provides a rolling method for non-quenched and tempered steel 38MnVS6 bars. The 38MnVS6 non-quenched and tempered steel bars rolled by the method have a specification of 30-60 mm, a hot-rolled hardness of 227-238 HBW, and a ferrite ratio of 51.5%-54.4%, which meets the direct material cutting requirements of users without the need for heat treatment such as softening annealing.

[0033] Since 38MnVS6 is a hypoeutectoid steel, ferrite is the first precipitated phase. For hypoeutectoid steel, deformation-induced ferrite phase transformation can be used to increase the proportion of ferrite after rolling and reduce the hardness of the material. Deformation-induced ferrite phase transformation (DIFT) is a phenomenon caused by the increase of system free energy and the increase of nucleation positions due to deformation. It is an important theoretical basis for online microstructure control technology. It accumulates a large number of dislocations in austenite grains, especially at grain boundaries, through lower rolling temperatures. These dislocations induce the nucleation of ferrite phase transformation at grain boundaries, thereby inducing the ferrite phase transformation. Combined with controlling the cooling rate after rolling, the purpose of regulating metallographic structure and mechanical properties is achieved.

[0034] The Ar3 phase transformation point of 38MnVS6 non-quenched and tempered steel is 784℃, and the Ar1 phase transformation point is 733℃. In order to improve the deformation-induced phase transformation ability and increase the amount of ferrite precipitation, the final rolling temperature is selected to be about 810℃ (slightly higher than the Ar3 temperature); in order to ensure the precipitation environment of ferrite, a heat preservation cover is used for slow cooling after rolling, and the temperature entering the heat preservation cover is controlled at about 780℃ and the temperature exiting the heat preservation cover is controlled at about 600℃, so that the organization transformation is completed in the heat preservation cover. The present invention controls the final rolling temperature by controlling the water volume of the water tank, and uses the heat preservation cover of the cooling bed to keep the rolled steel warm and slowly cool. This technology can accurately control the temperature of each key point to obtain the required round bar, and has the characteristics of high controllability.

[0035] In addition, during the heat preservation cooling of the present invention, pit cooling can replace the heat preservation cover cooling to achieve the required tissue hardness, but pit cooling is complicated and labor-intensive, and it is simpler to use the heat preservation cover.

[0036] As an optional embodiment of the rolling method of the present invention, the heating soaking temperature is 1160-1220°C (such as 1165°C, 1170°C, 1175°C, 1180°C, 1185°C, 1190°C, 1195°C, 1200°C, 1205°C, 1210°C, 1215°C), and the holding time is 50-120min (such as 60min, 70min, 80min, 90min, 100min, 110min).

[0037] In the above technical solution, the heating can be carried out by a heating device such as a walking beam furnace. During the heating, the absorptive temperature is controlled to be 1160-1220°C, and the absorptive holding time is 50-120 minutes.

[0038] As an optional implementation mode of the rolling method of the present invention, the starting temperature of the rough rolling is 1070-1130°C (such as 1075°C, 1080°C, 1085°C, 1090°C, 1095°C, 1100°C, 1105°C, 1110°C, 1115°C, 1120°C, 1125°C).

[0039] As an optional embodiment of the rolling method of the present invention, the final rolling temperature of the finish rolling is 780-830°C (such as 785°C, 790°C, 795°C, 800°C, 805°C, 810°C, 815°C, 820°C, 825°C), preferably 790-820°C (such as 791°C, 793°C, 795°C, 797°C, 799°C, 801°C, 803°C, 805°C, 807°C, 809°C, 811°C, 813°C, 815°C, 817°C, 819°C).

[0040] As an optional embodiment of the rolling method of the present invention, the steel billet is subjected to surface treatment after heating and before rough rolling, such as high-pressure water dephosphorization treatment, in order to remove the iron oxide scale formed on the surface of the steel billet during the heating process and prevent the iron oxide scale from being pressed into the subsequent rolling process and affecting the surface quality of the finished product.

[0041] As an optional implementation of the rolling method of the present invention, the steel billet is further sheared, collected and bundled after being cooled and kept warm.

[0042] The present invention will be further described in detail below with reference to specific embodiments.

[0043] The mass percentages of the various element components in the 38MnVS6 continuous casting billet used in the following examples are shown in Table 1, and the remainder is Fe and unavoidable impurities.

[0044] Table 1

[0045] element C Si Mn P S Cr Ni Cu V Mass percentage (%) 0.39 0.57 1.2 0.015 0.035 0.2 0.1 0.05 0.16

[0046] Examples 1-4

[0047] Design of rolling process to control ferrite structure proportion and hardness:

[0048] 1. Process flow:

[0049] 38MnVS6 continuous casting billet - walking beam furnace heating - high pressure water descaling - rough rolling - medium rolling - water cooling - sizing and finishing rolling - heat preservation and cooling - shearing - collection - bundling

[0050] 2. Heating temperature: 1190±30℃, holding time 50~120min.

[0051] 3. Rolling temperature: 1100±30℃.

[0052] 4. Final rolling temperature: 780~830℃.

[0053] 5. Upper cooling bed temperature: 780±20℃.

[0054] 6. Temperature out of insulation cover: 600±30℃

[0055] 7. After the intermediate rolling, a water tank is used to control the final rolling temperature by controlling the water volume in the water tank. The water volume in the water tank is 150-350L / min, and the water pressure is 6-8MPa.

[0056] 8. After sizing and finishing rolling, the steel is quickly transported to the cooling bed through the conveyor roller to ensure that the temperature of the upper cooling bed is near the Ar3 temperature; the cooling bed uses an insulation cover for insulation cooling to reduce the cooling speed after rolling, control the temperature of the insulation cover, and ensure that the steel completes the organizational transformation in the insulation cover.

[0057] According to the designed rolling process, 4 batches of non-quenched and tempered steel bars with a grade of 38MnVS6 (respectively, Examples 1-4) were produced, with a specification of 30-60 mm. The ferrite structure ratio test was carried out according to GB / T 13298-2015, and the Brinell hardness test was carried out according to GB / T231.1-2018. The test results are shown in Table 2:

[0058] Table 2 Ferrite ratio and Brinell hardness in the bars after rolling of Examples 1-4

[0059]

[0060] By using the deformation-induced ferrite phase transformation mechanism, controlling the final rolling temperature and post-rolling cooling, the ferrite ratio and hardness of 38MnVS6 non-quenched and tempered steel bars are improved. The hardness of the hot-rolled material is 227-238 HBW, and the ferrite ratio is 51.5%-54.4%, which meets the delivery requirements without the need for heat treatment such as softening annealing.

[0061] In Example 2, the surface and core metallographic structures are respectively as follows: Figure 2 a. Figure 2 As shown in b, it can be seen that the surface and core structure of the material are composed of ferrite + pearlite structure, which is different from the ordinary rolling process. Figure 1 compared to, Figure 2 The proportion of ferrite in the surface and core tissues has increased to a certain extent, and the proportion of ferrite tissue has reached more than 50% ( Figure 2 a is 53.8%, Figure 2 b is 54.4%), and Figure 2 Grain size ratio of the metallographic structure Figure 1 The grain size of the medium metallographic structure is finer.

[0062] Comparative Example 1

[0063] A rolling method for non-quenched and tempered steel 38MnVS6 bars, which differs from Example 1 only in that the water volume of the water tank is 80 L / min, the water pressure is 5 MPa, and the final rolling temperature is controlled to be 840° C. The rest of the settings are the same as those of Example 1.

[0064] Comparative Example 2

[0065] A rolling method for non-quenched and tempered steel 38MnVS6 bars, which differs from Example 1 only in that the water volume of the water tank is 180 L / min, the water pressure is 6.5 MPa, and the final rolling temperature is controlled to be 780° C. The rest of the settings are the same as those of Example 1.

[0066] Comparative Example 3

[0067] A rolling method for non-quenched and tempered steel 38MnVS6 bars, which differs from Example 1 only in that the heat preservation cooling is replaced by air cooling. The rest of the settings are the same as those of Example 1.

[0068] Comparative Example 4

[0069] A rolling method for non-quenched and tempered steel 38MnVS6 bars, the difference from Example 1 is that the temperature of the upper cooling bed is 720° C. The rest of the settings are the same as those of Example 1.

[0070] Comparative Example 5

[0071] A rolling method for non-quenched and tempered steel 38MnVS6 bars, the difference from Example 1 is that the temperature out of the heat preservation cover is 660° C. The rest of the settings are the same as those of Example 1.

[0072] Table 3 Ferrite ratio and Brinell hardness of the bars after rolling in comparative examples 1-5

[0073]

[0074] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention are within the scope of protection of the claims of the present invention.

Claims

1. A rolling method for non-quenched and tempered steel 38MnVS6 bars, characterized in that: The steel billet is sequentially heated, rough rolled, intermediate rolled, water cooled, sizing finished rolled, and heat-insulated cooled to obtain a hot-rolled bar; the non-quenched and tempered steel 38MnVS6 has the following main chemical components by mass percentage: C 0.39%, Si 0.57%, Mn 1.2%, P 0.015%, S 0.035%, Cr 0.2%, Ni0.1%, Cu 0.05%, V 0.16%, and the rest is matrix Fe and unavoidable impurities; the bar is exempt from annealing treatment, and its hot-rolled hardness is 227~238HBW, and the ferrite ratio is 51.5%~54.4%; The starting rolling temperature of the rough rolling is 1070-1130°C; water tanks are arranged along the rolling line, and water tanks are used for water cooling after intermediate rolling and before sizing and finishing rolling, and the final rolling temperature is controlled by controlling the water volume in the water tank, and the final rolling temperature is 780-830°C; when the bar specification is 30mm, the water volume in the water tank is controlled to be 150L / min and the water pressure is 6MPa; when the bar specification is 40-60mm, the water volume in the water tank is controlled to be 200-350L / min and the water pressure is 6.5-8MPa; The heat preservation cooling is carried out in the heat preservation cover of the cooling bed. The temperature of the steel billet entering the heat preservation cover is 760-795°C, and the temperature of the steel billet exiting the heat preservation cover is 585-630°C.

2. The rolling method according to claim 1, characterized in that: The heating is carried out at a uniform temperature of 1160-1220° C. and a heat preservation time of 50-120 min.

3. The rolling method according to claim 1, characterized in that: The steel billet is further subjected to surface treatment after heating and before rough rolling.

4. The rolling method according to claim 3, characterized in that: The surface treatment includes high-pressure water dephosphorization treatment.

5. The rolling method according to claim 1, characterized in that: The steel billets are further sheared, collected and bundled after being cooled and kept warm.

6. A non-quenched and tempered steel 38MnVS6 bar, characterized in that: The bar is obtained by the rolling method according to any one of claims 1-5.

Images