40cr hot-rolled steel coil and production method and heat treatment method thereof

Abstract

The application discloses a 40Cr hot-rolled steel coil and a production method and a heat treatment method thereof, and the production method comprises the following steps: hot metal pretreatment, converter smelting, LF furnace refining, RH furnace refining, continuous casting, heating, rolling, laminar cooling and coiling; through reasonable control of parameters of each process, the 40Cr hot-rolled steel coil with the following characteristics is produced: the microstructure is pearlite and ferrite, the banded structure is 1.5 grade, the upper and lower surfaces are free of decarburization, and the comprehensive performance is good.

Description

Technical Field

[0001] This invention belongs to the field of steel technology, specifically relating to a 40Cr hot-rolled steel coil and its production and heat treatment methods. Background Technology

[0002] 40Cr alloy structural steel is one of the most widely used steel grades in the machinery manufacturing industry. With the rapid development of the machinery manufacturing industry, its demand is increasing daily. After appropriate cold rolling, annealing, or heat treatment, the product can obtain good strength, plasticity, and hardness, meeting the processing requirements of complex mechanical parts. It is used to manufacture parts that withstand high loads, impacts, and wear, and is widely used in high-end chains, sprockets, automotive parts, and mechanical structural components. Therefore, special quality requirements are placed on the steel, requiring uniform and stable composition, low levels of harmful elements, high steel purity, and good internal and surface quality as well as processing performance.

[0003] Currently, most common 40Cr alloy structural steel products are round bars, which are commonly used to manufacture reinforcing bars, bolts, and various mechanical parts. For example, Chinese patent CN 108193136A discloses a 40Cr hot-rolled round bar and its production method. To broaden its applications, its product types should be expanded, such as coil products. However, existing technologies rarely disclose production methods for 40Cr hot-rolled steel coils. Compared to hot-rolled round bars, 40Cr hot-rolled steel coils present many challenges, such as decarburization issues due to high carbon content, edge cracking problems, thickness accuracy issues, and oxygen compression issues caused by high Cr content. Producing 40Cr hot-rolled steel coils with good overall performance is a very challenging task. Summary of the Invention

[0004] To solve the above-mentioned technical problems, the present invention provides a 40Cr hot-rolled steel coil and its production method and heat treatment method. The method can produce a 40Cr hot-rolled steel coil with a microstructure of pearlite and ferrite, a banded structure of grade 1.5, no decarburization on both the upper and lower surfaces, and good comprehensive performance.

[0005] The technical solution adopted in this invention is as follows:

[0006] A method for producing 40Cr hot-rolled steel coil, the method comprising the following steps:

[0007] 1) Hot metal pretreatment;

[0008] 2) Converter smelting;

[0009] 3) Refining: Reduce nitrogen addition in LF refining and RH refining processes, and control N to ≤50ppm. If the N content is too high, high carbon steel billets are prone to cracking.

[0010] 4) Continuous casting: The tundish temperature is controlled at 1520-1530℃, and high-carbon steel protective slag is used;

[0011] 5) Heating: The temperature of the billet entering the heating furnace is controlled above 600℃. The first heating temperature in the furnace is controlled at 1180~1220℃, the second heating temperature at 1210~1250℃, the third heating temperature at 1200~1240℃, and the soaking zone temperature at 1180~220℃. The total time for the second, third, and soaking zones is controlled at 60~80min, and the total time in the furnace is 130~150min. The billet exiting the furnace is controlled at 1180~1200℃. Through this heating process, the surface of the 40Cr hot-rolled steel coil is kept free of decarburization layer. If a decarburization layer exists on the surface of the steel coil or the decarburization layer is too deep, it will affect the surface hardness of subsequent products.

[0012] 6) Rolling: The descaling water at the inlet and outlet of the roughing mill is fully opened. The large iron oxide scale on the slab is removed by high-pressure descaling water to ensure the surface quality of the product during subsequent rolling processes. The starting rolling temperature of the finishing mill is controlled at 1000-1050℃, and the finishing rolling temperature is controlled at 860-890℃. This avoids rolling in the low-temperature two-phase region, which would cause excessive mill load and mixed crystal formation. It also avoids excessively high finishing rolling temperature, which would cause coarsening of austenite grains.

[0013] 7) Laminar flow cooling;

[0014] 8) Coiling: The coiling temperature is 700-740℃. If the coiling temperature is too high, too much pearlite will be generated. If the coiling temperature is too low, it will not be conducive to subsequent cold rolling. After coiling, the steel coil should be slowly cooled for at least 72 hours to avoid the outer ring of the steel coil from cooling too quickly and to ensure that the coiling performance fluctuates less.

[0015] In step 4), the high-carbon steel protective slag has a melting point of 1134±60℃, a melting rate of 25±10s at 1350℃, and a viscosity of 0.18±0.06Pa·s at 1300℃. The weight percentages of the components in the high-carbon steel protective slag are: CaO 29.0±4.0%, SiO2 32.2±4.0%, MgO 3.5±1.5%, Na2O 6.8±1.5%, and F 10.3±2.0%. The 40Cr hot-rolled steel in this invention requires not only good lubrication of the billet in the crystallizer but also protection from rapid cooling of the lower billet shell within the crystallizer. Therefore, a protective slag with high lubricity and good crystallization properties is used. The high-carbon steel protective slag used in this invention has both good lubricity and crystallization properties.

[0016] In step 4), the billet is hot-rolled, that is, when the billet temperature is ≥600℃, it is sent into the heating furnace.

[0017] In step 6), the descaling water pressure in the rough rolling is 18-22 MPa. Descaling is carried out in the first, third, fifth and sixth passes of the rough rolling, with descaling water pressures of 20 MPa, 22 MPa, 20 MPa and 20 MPa for each pass, respectively.

[0018] In step 6), the rough rolling adopts a 3+3 pass rolling mode instead of a 3+5 pass rolling mode, so that each pass can be rolled down to ensure grain refinement.

[0019] In step 6), the cumulative reduction rate of finishing rolling is ≥74%, and the grain size is refined by the cumulative large deformation; the finishing rolling speed is controlled at 10-15m / s. Too small or too large a finishing rolling speed is not conducive to the precise control of subsequent temperature.

[0020] In step 7), the laminar flow cooling is front-end laminar flow cooling, and the cooling rate is controlled at 25-30℃ / s. Too fast or too slow a cooling rate is not conducive to the formation of fine ferrite + pearlite structure.

[0021] In steps 6)-7), the total time t from the start of finishing rolling to coiling satisfies: 7.3s≤t≤10.2s, t=34 / V1+(T1-T2) / V2, where V1 is the finishing rolling speed, T1 is the finishing rolling temperature, T2 is the coiling temperature, and V2 is the laminar cooling rate. When calculating the formula, the values ​​of each parameter are substituted into the calculation.

[0022] The chemical composition and weight percentage of the 40Cr hot-rolled steel coil are as follows: C 0.38-0.42%, Si 0.17-0.30%, Mn 0.50-0.80%, P≤0.012%, S≤0.002%, Alt 0.015-0.035%, Cr 0.90-1.00%, N≤0.005%, with the remainder being Fe and unavoidable inclusions.

[0023] The metallographic structure of the 40Cr hot-rolled steel coil is pearlite + ferrite, with a banded structure of grade 1.5, and no decarburization on either the upper or lower surface.

[0024] The 40Cr hot-rolled steel coil has a yield strength of 359–402 MPa, a tensile strength of 666–724 MPa, an elongation of 19.5%–24.5%, a Brinell hardness of 186–199 HBW, and a tensile strength fluctuation of 68–85 MPa throughout the coil. After heat treatment, the 40Cr hot-rolled steel coil has a yield strength of 960–1010 MPa, a tensile strength of 1084–1140 MPa, and an elongation of 13.7%–17.9%.

[0025] The present invention also provides a heat treatment method for the 40Cr hot-rolled steel coil, the heat treatment method comprising: firstly, heating to 845-855℃ and holding for 20-30 minutes for quenching, and then reheating to 515-525℃ and holding for 55-65 minutes for tempering. After this heat treatment process, the steel coil exhibits excellent strength properties and elongation.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] The production method of 40Cr hot-rolled steel coil provided by this invention, through reasonable control of the parameters of smelting, continuous casting, hot rolling, cooling, and coiling processes, produces hot-rolled steel coils with excellent performance, and after heat treatment, obtains steel plates with excellent heat-treatable properties. The 40Cr hot-rolled steel coil provided by this invention has a microstructure of pearlite + ferrite, with a banded structure grade 1.5, and no decarburization on either the upper or lower surface. Its yield strength is 359–402 MPa, tensile strength is 666–724 MPa, elongation is 19.5%–24.5%, Brinell hardness is 186–199 HBW, and the tensile strength fluctuates between 68 and 85 MPa throughout the coil. After heat treatment, the yield strength of the steel coil is 960–1010 MPa, tensile strength is 1084–1140 MPa, and elongation is 13.7%–17.9%.

[0028] The 40Cr hot-rolled steel coil provided by this invention has a thickness range of 3.0 to 12.0 mm and is mainly used as a base material for cold rolling annealing or heat treatment. It is widely used in high-end chains, sprockets, automotive parts, mechanical structural parts and other fields. Attached Figure Description

[0029] Figure 1 The image shows the metallographic structure of the 40Cr hot-rolled steel coil in Example 1.

[0030] Figure 2 This is a schematic diagram of the decarburized layer on the surface of the 40Cr hot-rolled steel coil in Example 1. As can be seen from the figure, there is no decarburization on the surface.

[0031] Figure 3 This is a schematic diagram of the surface quality of the 40Cr hot-rolled steel coil in Example 1;

[0032] Figure 4 This is a schematic diagram of the surface quality of the 40Cr hot-rolled steel coil in Comparative Example 1. Detailed Implementation

[0033] This invention provides a method for producing 40Cr hot-rolled steel coils, comprising the following steps:

[0034] 1) Hot metal pretreatment;

[0035] 2) Converter smelting;

[0036] 3) Refining: The LF+RH refining process reduces nitrogen addition, with N controlled at ≤50ppm;

[0037] 4) Continuous casting: The tundish temperature is controlled at 1520-1530℃, and high-carbon steel protective slag is used;

[0038] 5) Heating: The temperature of the billet entering the heating furnace is controlled at greater than 600℃. The temperature of the first heating in the furnace is controlled at 1180~1220℃, the temperature of the second heating is controlled at 1210~1250℃, the temperature of the third heating is controlled at 1200~1240℃, the temperature of the soaking zone is controlled at 1180~220℃, the total time of the second heating, the third heating and the soaking zone is controlled at 60~80min, the total time in the furnace is 130~150min, and the temperature of the billet exiting the furnace is controlled at 1180~1200℃.

[0039] 6) Rolling: Descaling water is fully opened at the inlet and outlet of the roughing mill, the starting temperature of the finishing mill is controlled at 1000-1050℃, and the finishing temperature T1 is controlled at 860-890℃.

[0040] 7) Laminar flow cooling;

[0041] 8) Coiling: The coiling temperature T2 is 700-740℃, and the steel coil is slowly cooled for at least 72 hours after coiling.

[0042] In step 4), the billet is hot-rolled, that is, when the billet temperature is ≥600℃, it is sent into the heating furnace.

[0043] In step 4), the melting point of the high-carbon steel protective slag is 1134±60℃, the melting rate at 1350℃ is 25±10s, and the viscosity at 1300℃ is 0.18±0.06Pa·S. The weight percentages of each component in the high-carbon steel protective slag are: CaO 29.0±4.0%, SiO2 32.2±4.0%, MgO 3.5±1.5%, Na2O 6.8±1.5%, and F 10.3±2.0%.

[0044] In step 6), the descaling water pressure in the rough rolling is 18-22 MPa. Descaling is carried out in the first, third, fifth and sixth passes of the rough rolling, with descaling water pressures of 20 MPa, 22 MPa, 20 MPa and 20 MPa for each pass, respectively.

[0045] In step 6), the rough rolling adopts a 3+3 pass rolling mode.

[0046] In step 6), the cumulative reduction rate of finishing rolling is ≥74%; the finishing rolling speed is controlled at 10-15m / s.

[0047] In step 7), the laminar flow cooling is front-end laminar flow cooling, and the cooling rate is controlled at 25-30℃ / s.

[0048] In steps 6)-7), the total time t from the start of finishing rolling to coiling satisfies: 7.3s≤t≤10.2s, t=34 / V1+(T1-T2) / V2, where V1 is the finishing rolling speed, T1 is the finishing rolling temperature, T2 is the coiling temperature, and V2 is the laminar cooling rate. When calculating the formula, the values ​​of each parameter are substituted into the calculation.

[0049] The chemical composition and weight percentage of the 40Cr hot-rolled steel coil are as follows: C 0.38-0.42%, Si 0.17-0.30%, Mn 0.50-0.80%, P≤0.012%, S≤0.002%, Alt 0.015-0.035%, Cr 0.90-1.00%, N≤0.005%, with the remainder being Fe and unavoidable inclusions.

[0050] The present invention will now be described in detail with reference to the embodiments.

[0051] The chemical composition of the 40Cr hot-rolled steel coils in each embodiment and comparative example is shown in Table 1. Composition analysis was performed according to GB / T4336 "Spark Source Atomic Emission Spectroscopy Analysis Method (Conventional Method) for Carbon Steel and Medium-Low Alloy Steel".

[0052] Table 1

[0053]

[0054]

[0055] The 40Cr hot-rolled steel coils in each embodiment and comparative example are produced by the following method: 1) Hot metal pretreatment; converter smelting, LF refining, RH refining, continuous casting, heating, rolling, laminar flow cooling, coiling.

[0056] In the continuous casting step, the tundish temperature in Examples 1-5 and Comparative Examples 1-4 is controlled at 1520℃, 1521℃, 1523℃, 1525℃, 1527℃, 1530℃, 1530℃, 1525℃, and 1524℃, respectively.

[0057] The same high-carbon steel protective slag was used for protective casting throughout the continuous casting process. The physicochemical properties of the high-carbon steel protective slag met the following requirements: melting point of 1134±60℃, melting rate of 25±10s at 1350℃, viscosity of 0.18±0.06Pa·S at 1300℃, and weight percentages of each component in the high-carbon steel protective slag of CaO 29.0±4.0%, SiO2 32.2±4.0%, MgO 3.5±1.5%, Na2O 6.8±1.5%, and F 10.3±2.0%.

[0058] The main rolling process parameters of the 40Cr hot-rolled steel coils in each embodiment and comparative example are shown in Table 3.

[0059] Table 3 Heating process parameters

[0060]

[0061] The descaling water pressure process parameters for the 40Cr hot-rolled steel coils in each embodiment and comparative example are shown in Table 4. The descaling is performed in the 1st, 3rd, 5th and 6th passes, with descaling water pressures of 20 MPa, 22 MPa, 20 MPa and 20 MPa, respectively.

[0062] Table 4 Descaling water pressure process parameters

[0063] Sample number Thickness / mm First pass of rough rolling Third pass of rough rolling 5th pass of rough rolling 6th pass of rough rolling Example 1 3.0 20 22 20 20 Example 2 5.0 20 22 20 20 Example 3 8.0 20 22 20 20 Example 4 10.0 20 22 20 20 Example 5 12.0 20 22 20 20 Comparative Example 1 5.0 <![CDATA[ 10 ]]> <![CDATA[ 12 ]]> <![CDATA[ 10 ]]> <![CDATA[ 10 ]]> Comparative Example 2 8.0 20 22 20 20 Comparative Example 3 10.0 20 22 20 20 Comparative Example 4 3.0 20 22 20 20

[0064] The main rolling process parameters of the 40Cr hot-rolled steel coils in each embodiment and comparative example are shown in Table 5.

[0065] Table 5 Main process parameters for rolling process

[0066]

[0067] The mechanical properties of the 40Cr hot-rolled steel coils in each embodiment and comparative example are shown in Table 5.

[0068] Table 5 Mechanical Properties

[0069]

[0070] The inclusion results of the 40Cr hot-rolled steel coils in each embodiment and comparative example are shown in Table 6.

[0071] Table 6 Non-metallic inclusions in various embodiments of the present invention

[0072]

[0073] The decarburization layer results of the 40Cr hot-rolled steel coils in each embodiment and comparative example are shown in Table 7.

[0074] Table 7 Decarburization layer of various embodiments of the present invention

[0075]

[0076]

[0077] The mechanical properties of the 40Cr hot-rolled steel coils in each embodiment and comparative example after heat treatment as shown in Table 8 are illustrated in Table 8. It is evident that the 40Cr hot-rolled steel coils in the embodiments exhibit good comprehensive mechanical properties after heat treatment. The hot-rolled steel coils in Comparative Examples 5 to 8 in Table 8 have the same composition and production process as in Example 1, except that they employ different heat treatment processes. It is clear that excessively low or high quenching temperatures, excessively long quenching times, and excessively long tempering times will result in lower strength properties, while excessively short tempering times will result in lower elongation.

[0078] Table 8 Heat treatment properties of the test steel of the present invention

[0079]

[0080]

[0081] In summary, the 40Cr hot-rolled steel coils produced according to the chemical composition, steelmaking, and hot rolling process provided by this invention have a yield strength of 359–402 MPa, a tensile strength of 666–724 MPa, an elongation of 19.5%–24.5%, a Brinell hardness of 186–199 HBW, a through-coil tensile strength fluctuation of 68–85 MPa, no decarburization on the upper and lower surfaces, and after heat treatment, a yield strength of 960–1010 MPa, a tensile strength of 1084–1140 MPa, and an elongation of 13.7%–17.9%, exhibiting excellent comprehensive mechanical properties after heat treatment.

[0082] The above detailed description of a 40Cr hot-rolled steel coil and its production and heat treatment methods, with reference to the embodiments, is illustrative rather than limiting. Several embodiments may be listed within the defined scope. Therefore, variations and modifications that do not depart from the overall concept of the present invention should be within the protection scope of the present invention.

Claims

1. A method for producing 40Cr hot-rolled steel coils, characterized in that, The production method includes the following steps: 1) Hot metal pretreatment; 2) Converter smelting; 3) Refining: Reduce nitrogen addition in LF refining and RH refining processes, and control N to ≤50ppm; 4) Continuous casting: The tundish temperature is controlled at 1520-1530℃, and high-carbon steel protective slag is used; 5) Heating: The temperature of the billet entering the heating furnace is controlled at greater than 600℃. The temperature of the first heating in the furnace is controlled at 1180~1220℃, the temperature of the second heating is controlled at 1210~1250℃, the temperature of the third heating is controlled at 1200~1240℃, the temperature of the soaking zone is controlled at 1180~220℃, the total time of the second heating, the third heating and the soaking zone is controlled at 60~80min, the total time in the furnace is 130~150min, and the temperature of the billet exiting the furnace is controlled at 1180~1200℃. 6) Rolling: Descaling water is fully opened at the inlet and outlet of the roughing mill, the starting temperature of the finishing mill is controlled at 1000-1050℃, and the finishing temperature T1 is controlled at 860-890℃. 7) Laminar flow cooling; 8) Coiling: The coiling temperature T2 is 700-740℃, and the steel coil is slowly cooled for at least 72 hours after coiling; In step 4), the melting point of the high-carbon steel protective slag is 1134±60℃, the melting rate at 1350℃ is 25±10s, and the viscosity at 1300℃ is 0.18±0.06Pa▪S. The weight percentage of each component in the high-carbon steel protective slag is CaO 29.0±4.0%, SiO2 32.2±4.0%, MgO 3.5±1.5%, Na2O 6.8±1.5%, and F 10.3±2.0%.

2. The method for producing 40Cr hot-rolled steel coil according to claim 1, characterized in that, In step 6), the descaling water pressure in the rough rolling is 18~22MPa. Descaling is carried out in the first, third, fifth and sixth passes of the rough rolling, with descaling water pressures of 20MPa, 22MPa, 20MPa and 20MPa for each pass, respectively.

3. The method for producing 40Cr hot-rolled steel coil according to claim 1, characterized in that, In step 6), the cumulative reduction rate of finishing rolling is ≥74%; 10m / s≤finishing rolling speed V1≤15m / s.

4. The method for producing 40Cr hot-rolled steel coil according to claim 1, characterized in that, In step 7), the laminar flow cooling is front-end laminar flow cooling, and the cooling rate V2 of the laminar flow cooling is controlled at 25-30℃ / s.

5. The method for producing 40Cr hot-rolled steel coil according to claim 1, characterized in that, The total time t from the start of finishing rolling to coiling satisfies 7.3s≤t≤10.2s, t=34 / V1+(T1-T2) / V2, where V1 is the finishing rolling speed, T1 is the finishing rolling temperature, T2 is the coiling temperature, and V2 is the laminar cooling rate. When calculating the formula, the values ​​of each parameter are substituted into the formula.

6. The method for producing 40Cr hot-rolled steel coil according to claim 1, characterized in that, The chemical composition and weight percentage of the 40Cr hot-rolled steel coil are as follows: C 0.38~0.42%, Si 0.17~0.30%, Mn 0.50~0.80%, P ≤0.012%, S≤0.002%, Alt 0.015~0.035%, Cr 0.90~1.00%, N ≤0.005%, with the remainder being Fe and unavoidable inclusions.

7. The 40Cr hot-rolled steel coil produced by the production method according to any one of claims 1-6, characterized in that, The metallographic structure of the 40Cr hot-rolled steel coil is pearlite and ferrite, with a banded structure of grade 1.5, and no decarburization on either the upper or lower surface.

8. The 40Cr hot-rolled steel coil according to claim 7, characterized in that, The 40Cr hot-rolled steel coil has a yield strength of 359–402 MPa, a tensile strength of 666–724 MPa, an elongation of 19.5%–24.5%, a Brinell hardness of 186–199 HBW, and a tensile strength fluctuation of 68–85 MPa throughout the coil. After heat treatment, the 40Cr hot-rolled steel coil has a yield strength of 960–1010 MPa, a tensile strength of 1084–1140 MPa, and an elongation of 13.7%–17.9%.

9. The heat treatment method for 40Cr hot-rolled steel coil as described in claim 7 or 8, characterized in that, The heat treatment method includes: first, heating to 845~855℃ and holding for 20~30 minutes for quenching, and then heating again to 515~525℃ and holding for 55~65 minutes for tempering.

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