Rolling method of free-cutting flat steel

Abstract

The rolling method comprises the following steps that a steel billet enters a heating furnace to be heated, the temperature of a preheating section is smaller than or equal to 700 DEG C, the passing time ranges from 30 min to 45 min, the temperature of a heating section I ranges from 900 DEG C to 1100 DEG C, the passing time ranges from 30 min to 45 min, the temperature of a heating section II ranges from 1100 DEG C to 1300 DEG C, the passing time ranges from 15 min to 30 min, the temperature of a soaking section ranges from 1150 DEG C to 1250 DEG C, and the passing time ranges from 15 min to 30 min; and the heated steel billet is rolled, heat preservation and cooling are conducted after rolling, and the free-cutting flat steel is obtained. The initial rolling temperature is 1100-1180 DEG C, the final rolling temperature is 980-1030 DEG C, and the rolling is divided into three stages of rough rolling, intermediate rolling and finish rolling. MnS in the free-cutting flat steel produced by adopting the rolling method is uniformly distributed, and the anisotropy of the cutting performance is obviously reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of steel rolling, in particular to a rolling method for free-cutting flat steel. Background Art

[0002] Free-cutting flat steel, as a near-net-shape material, can be directly processed into various rectangular solenoid valve body parts for applications in automotive, engineering machinery, and other fields after simple processing. Solenoid valves, as core components of hydraulic servo control systems, have complex internal structures, often with intricately shaped holes and grooves machined in all directions around the core to facilitate hydraulic control. To improve efficiency and reduce costs, free-cutting steel is often used as the valve body material. Compared to structural steels with the same alloy content, free-cutting steel can withstand higher cutting speeds and achieve a higher surface finish when machining internal holes and grooves, ensuring the high-precision operation of electro-hydraulic servo valves.

[0003] The application requirements for free-cutting flat steel indicate that, due to the drilling and slotting processes in all directions, consistent cutting performance is essential to achieve high consistency in machining accuracy across all directions and meet the service requirements of the workpiece. The cutting performance of free-cutting steel relies on the uniform and dispersed distribution of free-cutting particles such as MnS and Pb within the steel. This requires high consistency in the size and morphology of the MnS within the free-cutting flat steel. However, free-cutting flat steel is mostly rolled from square or round billets. The morphology and size of the as-cast MnS at different locations across the cross-section of the billet itself vary significantly. Furthermore, unlike conventional round steel, the temperature and deformation during the rolling process are more uneven, making it difficult to control the deformation of the MnS during the rolling process. Currently, many free-cutting flat steels exhibit extremely uneven MnS distribution across their cross-section, significant anisotropy in cutting performance, and significant variations in surface accuracy after drilling and slotting. Therefore, it is crucial to develop a rolling method for free-cutting flat steel that achieves high internal MnS uniformity and low anisotropy in cutting performance. Summary of the Invention

[0004] The object of the present invention is to provide a method for rolling free-cutting flat steel to solve the problems existing in the above-mentioned prior art.

[0005] To achieve the above object, the present invention provides the following solutions:

[0006] One of the technical solutions of the present invention is a method for rolling free-cutting flat steel, comprising the following steps:

[0007] The billet enters the heating furnace for heating. The temperature of the preheating section is ≤700℃, and the passing time is 30-45min (i.e., the time for the billet to pass through the preheating section is 30-45min). The temperature of the heating section I is 900-1100℃, and the passing time is 30-45min. The temperature of the heating section II is 1100-1300℃, and the passing time is 15-30min. The temperature of the soaking section is 1150-1250℃, and the passing time is 15-30min.

[0008] Rolling the heated steel billet and then cooling it to obtain the free-cutting flat steel;

[0009] The starting rolling temperature is 1100-1180°C, and the final rolling temperature is 980-1030°C; the rolling is divided into three stages: rough rolling, intermediate rolling and finishing rolling; the rolling deformation rate of the rough rolling is 0.45-0.80s -1 , the deformation per pass is 15% to 25%;

[0010] The chemical composition of the steel slab includes Mn and S.

[0011] MnS inclusions are generated as the ingot solidifies, especially in the core of the ingot. S, as an element that is very easy to segregate, gathers between dendrites, resulting in the as-cast MnS in the core of the ingot being very large in size, mostly coral-shaped. The present invention has found that during the rolling deformation process, MnS has good plasticity, and its relative plasticity with the matrix is closely related to the deformation temperature, deformation amount and deformation rate. In the high-temperature section, the plasticity of MnS is affected by temperature, deformation amount and deformation rate, and there is a relatively low plasticity interval. Deformation in this interval will reduce the elongation of MnS inclusions, making it present a smaller aspect ratio after rolling, which is more beneficial to the isotropic uniformity of cutting performance. Based on the above-mentioned as-cast MnS morphological distribution characteristics and rolling deformation characteristics, the present invention effectively promotes the melting of coral-shaped MnS in the core of the ingot by increasing the furnace temperature and time of the heating furnace, and reduces the size difference of MnS between the core and edge of the ingot. During the rolling process, by controlling the rolling temperature, the rolling rate and the pass deformation in the rough rolling stage with the largest deformation, the deformation of MnS inclusions along the rolling direction can be reduced, thereby further reducing the differences in MnS size and shape in various directions after rolling, and obtaining more uniform cutting performance in all directions.

[0012] Furthermore, the chemical composition of the steel billet is as follows, by mass percentage: C 0.01%-0.20%, Si 0-0.60%, Mn 0.40%-1.70%, S 0.02%-0.55%, P 0-0.17%, Pb 0-0.50%, the remainder being Fe and unavoidable impurities, and Mn / S>4.5.

[0013] Limiting Mn / S to > 4.5 can optimize the morphology of as-cast MnS inclusions and ensure that the aspect ratio of the finished MnS inclusions after rolling reaches a control level of less than 3, thereby ensuring the chip breaking efficiency of the cutting process.

[0014] Furthermore, the steel billet is a rectangular billet with a cross-sectional size ranging from (120 to 400) mm×(120 to 400) mm or a round billet with a diameter of 120 to 400 mm.

[0015] Furthermore, during the heating process, the temperature deviation between the head and tail of the steel billet is ≤20°C, and the residual oxygen ratio of the heating furnace atmosphere is <2.0 vol%.

[0016] Furthermore, after the steel billet is heated and before being kept cool, a high-pressure water descaling step is also included, and the pressure of the high-pressure descaling water is ≥18 MPa.

[0017] Furthermore, the rough rolling is performed in 7 to 11 passes, the intermediate rolling is performed in 5 to 7 passes, and the finish rolling is performed in 5 to 7 passes.

[0018] Furthermore, the rough rolling adopts grooved rolling; the intermediate rolling and finishing rolling adopt flat roll grooveless rolling.

[0019] The use of groove rolling in the rough rolling stage, combined with the rolling temperature, rolling deformation and rolling deformation rate, can significantly reduce the wrinkle defects distributed along the rolling direction and ensure the yield rate of easy-to-cut flat steel.

[0020] Furthermore, the insulation cooling includes: placing the flat steel obtained by rolling on a cooling bed, entering an insulation cover, controlling the entry temperature to be greater than 800°C, the exit temperature to be less than 650°C, the time for passing through the insulation cover to be 3 to 5 minutes, and air cooling to room temperature after exiting the cover.

[0021] Furthermore, the free-cutting flat steel finished product is a rectangular material with a cross-sectional size ranging from (3 to 100) mm to (3 to 100) mm.

[0022] The second technical solution of the present invention: the free-cutting flat steel is obtained by rolling the free-cutting flat steel using the above-mentioned rolling method.

[0023] The third technical solution of the present invention: Application of the above-mentioned free-cutting flat steel in the preparation of electro-hydraulic servo valve block parts.

[0024] The present invention discloses the following technical effects:

[0025] The rolling method of the present invention causes the cast coral-like MnS to melt back and spheroidize during the heating process, thereby obtaining a rolled billet with a good MnS distribution. The rolling temperature, the rolling deformation amount and the rolling deformation rate in the rough rolling stage with the largest deformation are coordinated to significantly reduce the relative plasticity of MnS, so that the finished flat steel product obtains a MnS inclusion control effect with significantly improved anisotropy. The morphology of MnS is more uniform at the edge and core of the entire flat steel, and the cutting performance is significantly improved, ensuring that the holes and grooves processed at various positions of the flat steel have good and consistent surface finish, thereby meeting the working requirements of the hydraulic valve. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0027] Figure 1 This is a photo of the free-cutting flat steel product obtained in Example 1;

[0028] Figure 2 This is a photo of cracking of the intermediate billet during the rough rolling process of Comparative Example 1;

[0029] Figure 3 This is a photo of cracking of the intermediate billet during the rough rolling process of Comparative Example 2;

[0030] Figure 4 The following are photos of the microstructure and MnS morphology of the free-cutting flat steel product prepared in Example 1;

[0031] Figure 5 The following are photos of the microstructure and MnS morphology of the free-cutting flat steel product prepared in Example 2;

[0032] Figure 6 The following are photos of the structure and MnS morphology of the flat steel product prepared in Comparative Example 3;

[0033] Figure 7 This is a photo of cracking of the intermediate billet during the rough rolling process of Comparative Example 4;

[0034] Figure 8 This is a photograph of the surface defect morphology of the flat steel product prepared in Comparative Example 5;

[0035] Figure 9 These are photos of the chip morphology of the flat steel products prepared in Example 1, Example 2, and Comparative Example 3. DETAILED DESCRIPTION

[0036] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0037] It should be understood that the terms described herein are intended only to describe particular embodiments and are not intended to limit the present invention. In addition, for numerical ranges herein, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. The intermediate value within any stated value or stated range, and each smaller range between any other stated value or intermediate value within the stated range, is also encompassed within the present invention. The upper and lower limits of these smaller ranges may be independently included or excluded within the scope.

[0038] Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art. Although only preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein may also be used in the practice or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of any conflict with any incorporated document, the contents of this specification shall prevail.

[0039] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments described herein without departing from the scope or spirit of the invention. Other embodiments will be apparent to those skilled in the art from the description of the invention. The description and examples are intended to be exemplary only.

[0040] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0041] As a first aspect of the present invention, the present invention provides a method for rolling free-cutting flat steel, comprising the following steps:

[0042] The billet enters the heating furnace for heating. The temperature of the preheating section is ≤700°C, and the passing time is 30-45 minutes (i.e., the time for the billet to pass through the preheating section is 30-45 minutes, that is, the same position of the billet is heated in the preheating section for 30-45 minutes). The temperature of the heating section I is 900-1100°C, and the passing time is 30-45 minutes. The temperature of the heating section II is 1100-1300°C, and the passing time is 15-30 minutes. The temperature of the soaking section is 1150-1250°C, and the passing time is 15-30 minutes.

[0043] Rolling the heated steel billet and then cooling it to obtain the free-cutting flat steel;

[0044] The starting rolling temperature is 1100-1180°C, and the final rolling temperature is 980-1030°C; the rolling is divided into three stages: rough rolling, intermediate rolling and finishing rolling; the rolling deformation rate of the rough rolling is 0.45-0.80s -1 , the deformation per pass is 15% to 25%;

[0045] The chemical composition of the steel slab includes Mn and S.

[0046] As a preferred embodiment of the present invention, the chemical composition of the steel billet is as follows, by mass percentage: C 0.01% to 0.20%, Si 0 to 0.60%, Mn 0.40% to 1.70%, S 0.02% to 0.55%, P 0 to 0.17%, Pb 0 to 0.50%, the remainder being Fe and unavoidable impurities, and Mn / S>4.5; the steel billet is obtained by conventional smelting and casting methods in the art, and can also be referred to as a cast billet.

[0047] As a preferred embodiment of the present invention, the rough rolling is performed in 7 to 11 passes, the intermediate rolling is performed in 5 to 7 passes, and the finishing rolling is performed in 5 to 7 passes; the deformation amount of the intermediate rolling pass is 5% to 20%; the deformation amount of the finishing rolling pass is 5% to 10%.

[0048] As a preferred embodiment of the present invention, the rolling method of the free-cutting flat steel more specifically comprises the following steps:

[0049] (1) Heating: The billet (rectangular billet (including square billet) with a cross-sectional size range of (120-400) mm × (120-400) mm or round billet with a diameter of 120-400 mm) enters the heating furnace for heating. The preheating section temperature is ≤700°C and the transit time is 30-45 min. The heating section I temperature is 900-1100°C and the transit time is 30-45 min. The heating section II temperature is 1100-1300°C and the transit time is 15-30 min. The soaking section temperature is 1150-1250°C and the transit time is 15-30 min. The billet head and tail temperature deviation is ≤20°C. The residual oxygen ratio of the heating furnace atmosphere is <2.0 vol%.

[0050] This step is designed to use a higher heating temperature and longer heating time than conventional free-cutting steel production. Furthermore, the residual oxygen ratio (i.e., oxygen content) in the heating furnace atmosphere is controlled at a low level of <2.0 vol%, thereby ensuring the surface quality and metal yield of the heated ingot. The coral-like MnS in the ingot undergoes significant remelting and spheroidization during the heating process of step (1). In particular, the coral-like MnS in the core of the ingot is significantly broken down by the heating conditions of step (1), resulting in a reduction in the size and increase in the number of as-cast MnS inclusions, thereby improving the uniformity of MnS inclusions in the ingot before rolling.

[0051] (2) Descaling: The steel billet is descaled by high-pressure water after leaving the heating furnace. The pressure of high-pressure descaling water should be ≥18MPa.

[0052] The purpose of this step is to remove a large amount of iron oxide scale produced on the surface of the ingot in the heating furnace. Higher water pressure can ensure the removal effect.

[0053] (3) Rolling: The steel billet after descaling is rolled. The rolling process is divided into three stages: rough rolling, intermediate rolling and finishing rolling. The three stages are carried out continuously. The starting rolling temperature (i.e., the starting rolling temperature of the first rough rolling) is 1100-1180°C, and the finishing rolling temperature (i.e., the finishing rolling temperature of the last finishing rolling) is 980-1030°C. The rough rolling adopts the groove rolling method, and the horizontal and vertical directions are alternately rolled (i.e., one horizontal rolling pass and one vertical rolling pass are alternately carried out). The total number of rolling passes is 7-11. The rolling deformation rate of the rough rolling is 0.45-0.80s. -1 , the deformation per pass is 15% to 25%; the intermediate rolling adopts flat roll holeless rolling, the total rolling passes are 5 to 7 passes, the deformation per pass is 5% to 20%, in order to increase the flatness ratio, the vertical rolling passes in the intermediate rolling stage are less than the horizontal rolling passes; the finishing rolling adopts flat roll holeless rolling, and the horizontal and vertical directions are rolled alternately, the total rolling passes are 5 to 7 passes, and the deformation per pass is 5% to 10%, so as to ensure the dimensional accuracy of the flat steel.

[0054] Under the coordination of the starting rolling temperature, the final rolling temperature, the deformation amount of the rough rolling pass and the rolling rate in this step, the relative plasticity of the MnS inclusions is the lowest and the elongation after rolling is the smallest. The setting of the three stages of rough rolling, intermediate rolling and finishing rolling is because the present invention is aimed at the rectangular billet / round billet to be rolled and deformed into flat steel, so it is necessary to gradually control the flatness ratio of the rolled piece during the rolling process so that its shape becomes flat steel. In addition, the present invention only limits the rolling deformation rate in the rough rolling stage, but does not limit the rolling deformation rate in the intermediate rolling and finishing stages. This is because the deformation amount in the rough rolling stage is large, and the rolling deformation rate is too fast, which is prone to cracking, and the rolling deformation rate is likely to affect the rolling effect. In the intermediate rolling and finishing stages, the deformation amount is small, and the rolling deformation rate has little effect on the rolling effect. Therefore, there is no special limit on the rolling deformation rate in the intermediate rolling and finishing stages, as long as the corresponding intermediate rolling and finishing passes and deformation amount can be completed within the specified starting rolling temperature and final rolling temperature range.

[0055] (4) Cooling: The flat steel obtained after rolling is placed on a cooling bed and then into an insulation cover. The temperature in and out of the cover is controlled. The temperature in the cover is greater than 800°C and the temperature out of the cover is less than 650°C. The time it takes to pass through the insulation cover is 3 to 5 minutes (i.e., the same position of the flat steel stays in the insulation cover for 3 to 5 minutes). After leaving the cover, it is air-cooled to room temperature and then packaged. The cooling bed adopts a one-tooth multi-branch method.

[0056] Compared to conventional free-cutting flat steel production processes, this step adds post-rolling insulation measures, specifically significantly increasing the dwell time in the high-temperature ferrite phase region (650-800°C). Under conventional production conditions, the corners of flat steel cool more quickly, resulting in a time lag between the core phase transition and the core phase transition. This can easily lead to internal and external stress differences and, later in the process, dimensional distortion in the drilled hole. This invention, by increasing the dwell time of the flat steel in the high-temperature ferrite phase region through insulation measures, ensures synchronous core-surface phase transitions.

[0057] The present invention mainly reduces the anisotropy of MnS in steel and improves the surface finish of flat steel after processing by designing the heating furnace and rolling process parameters during the steel rolling process, thereby meeting the processing and service requirements of materials such as electro-hydraulic servo valve blocks.

[0058] As a preferred embodiment of the present invention, the free-cutting flat steel product is a rectangular material with a cross-sectional size ranging from (3 to 100) mm×(3 to 100) mm.

[0059] As a second aspect of the present invention, the present invention provides a free-cutting flat steel obtained by rolling the free-cutting flat steel rolling method as described above.

[0060] As a third aspect of the present invention, the present invention provides the use of the above-mentioned free-cutting flat steel in the preparation of electro-hydraulic servo valve block parts.

[0061] The technical solution of the present invention will be further described below with reference to specific embodiments.

[0062] The room temperature involved in the specific implementation manner of the present invention specifically refers to 20-30°C.

[0063] Example 1

[0064] A method for rolling free-cutting flat steel, comprising the following steps:

[0065] (1) Heating: The billet (square billet with a cross-sectional size of 200 mm × 200 mm, the chemical composition of the billet is: C 0.06 wt%, Si 0.12 wt%, Mn 1.63 wt%, S 0.32 wt%, P 0.0068 wt%, Pb 0.23 wt%, and the balance is Fe and unavoidable impurities) enters the heating furnace for heating. The preheating section temperature is 680 ° C, the transit time is 40 min, the heating section I temperature is 1026 ° C, the transit time is 40 min, the heating section II temperature is 1238 ° C, the transit time is 20 min, and the soaking section temperature is 1220 ° C, the transit time is 20 min. The billet head and tail temperature deviation is ≤ 20 ° C, and the residual oxygen ratio of the heating furnace atmosphere is controlled below 2.0 vol%.

[0066] (2) Descaling: After the billet leaves the heating furnace, it is descaled with high-pressure water. The pressure of the high-pressure descaling water is 18 MPa.

[0067] (3) Rolling: Rolling is divided into three stages: rough rolling, intermediate rolling and finishing rolling. The three stages are carried out continuously. The starting rolling temperature is 1150℃ and the finishing rolling temperature is 1013℃. The rough rolling adopts the groove rolling method, and a total of 11 rolling passes are carried out. The rolling deformation rate of the rough rolling is 0.75s -1 , horizontal and vertical rolling are carried out alternately (horizontal rolling is carried out on a horizontal rolling mill, vertical rolling is carried out on a vertical rolling mill, and horizontal rolling is carried out first). The deformation of each rough rolling pass (1-11 passes) is 23%, 18%, 15%, 16%, 15%, 16%, 15%, 16%, 15%, 15%, 15%, and an intermediate billet with a cross-sectional size of 75mm×75mm is obtained; intermediate rolling adopts flat roll holeless rolling, and a total of 7 rolling passes are carried out, of which 5 passes (intermediate rolling passes 1, 2, 4, 5, and 6) are ) is rolling in the horizontal direction, and 2 passes (the 3rd and 7th passes of intermediate rolling) are rolling in the vertical direction. The deformation of each pass of intermediate rolling (1-7 passes) is 20%, 20%, 15%, 15%, 10%, 10%, and 8% respectively; the finishing rolling adopts flat roll holeless rolling, with a total of 7 rolling passes, alternating rolling in the horizontal and vertical directions (horizontal rolling is performed first), and the deformation of each pass of finishing rolling (1-7 passes) is 10%, 8%, 6%, 5%, 6%, 5%, and 5% respectively, to obtain flat steel with a cross-sectional size of 80mm×20mm.

[0068] (4) Cooling: After rolling, the flat steel is placed on a cooling bed and then placed in a heat preservation cover. The temperature of the heat preservation cover is controlled. The temperature of the heat preservation cover is 840°C and the temperature of the heat preservation cover is 640°C. The time of passing through the heat preservation cover is 4 minutes. After leaving the cover, the steel is air-cooled to room temperature to obtain the final free-cutting flat steel product (actual photo as shown in the figure). Figure 1 As shown), for packing finished products, the cooling bed adopts a one-tooth-multiple-branch method.

[0069] Example 2

[0070] A method for rolling free-cutting flat steel, comprising the following steps:

[0071] (1) Heating: The billet (round billet with a diameter of 280 mm, the chemical composition of the billet is: C 0.08wt%, Si 0.32wt%, Mn 1.23wt%, S 0.23wt%, P 0.008wt%, the balance is Fe and inevitable impurities) enters the heating furnace for heating. The preheating section temperature is 680°C, the transit time is 40 min, the heating section I temperature is 1026°C, the transit time is 40 min, the heating section II temperature is 1238°C, the transit time is 20 min, the soaking section temperature is 1220°C, and the transit time is 20 min. The billet head and tail temperature deviation is ≤20°C, and the residual oxygen ratio of the heating furnace atmosphere is controlled below 2.0 vol%.

[0072] (2) Descaling: After the billet leaves the heating furnace, it is descaled with high-pressure water. The pressure of the high-pressure descaling water is 19 MPa.

[0073] (3) Rolling: Rolling is divided into three stages: rough rolling, intermediate rolling and finishing rolling. The three stages are carried out continuously. The starting rolling temperature is 1160℃ and the finishing rolling temperature is 1025℃. The rough rolling adopts the groove rolling method, and a total of 11 rolling passes are carried out. The rolling deformation rate of the rough rolling is 0.75s. -1 , horizontal and vertical rolling are performed alternately (horizontal rolling is performed on a horizontal rolling mill, vertical rolling is performed on a vertical rolling mill, and horizontal rolling is performed first), and the deformation of each pass (pass 1-11) is 25%, 22%, 18%, 20%, 18%, 20%, 18%, 20%, 18%, 20%, 16%, respectively, to obtain an intermediate billet with a cross-sectional size of 75mm×75mm; intermediate rolling adopts flat roll holeless rolling, and a total of 7 passes are performed, of which 4 passes (intermediate rolling passes 1, 2, 4, 6 passes) are horizontal rolling, 3 passes (the 3rd, 5th and 7th passes of intermediate rolling) are vertical rolling, and the deformation of each pass of intermediate rolling (1-7 passes) are 20%, 18%, 15%, 18%, 10%, 12% and 6% respectively; the finishing rolling adopts flat roll holeless rolling, with a total of 5 passes, alternating rolling in horizontal and vertical directions (horizontal rolling is performed first), and the deformation of each pass of finishing rolling is 6%, 5%, 6%, 5% and 5% respectively, to obtain flat steel with a cross-sectional size of 60mm×20mm.

[0074] (4) Cooling: After rolling, the flat steel is placed on a cooling bed and then enters an insulation cover. The temperature of the heat exchanger is controlled. The temperature of the heat exchanger is 840°C and the temperature of the heat exchanger is 630°C. The time of passing through the insulation cover is 4 minutes. After exiting the cover, the flat steel is air-cooled to room temperature to obtain the final free-cutting flat steel product. The finished product is packaged. The cooling bed adopts a one-tooth multiple-branch method.

[0075] Comparative Example 1

[0076] The rolling method with a shorter heating time is as follows:

[0077] (1) Heating: The steel billet (square billet with a cross-sectional size of 200 mm × 200 mm, the chemical composition of the steel billet is: C 0.06 wt%, Si 0.12 wt%, Mn 1.63 wt%, S 0.32 wt%, P 0.0068 wt%, Pb 0.23 wt%, the balance being Fe and unavoidable impurities) enters a heating furnace for heating. The preheating section temperature is 680°C, the transit time is 20 min, the heating section I temperature is 1026°C, the transit time is 20 min, the heating section II temperature is 1238°C, the transit time is 10 min, the soaking section temperature is 1220°C, and the transit time is 10 min. The residual oxygen content in the heating furnace atmosphere is controlled below 2.0 vol%.

[0078] (2) Descaling: After the billet leaves the heating furnace, it is descaled with high-pressure water. The pressure of the high-pressure descaling water is 18 MPa.

[0079] (3) Rolling: Rolling is divided into three stages: rough rolling, intermediate rolling and finishing rolling. The three stages are carried out continuously. The starting rolling temperature is 1150℃ and the finishing rolling temperature is 1013℃. The rough rolling adopts the groove rolling method, and a total of 11 rolling passes are carried out. The rolling deformation rate of the rough rolling is 0.75s -1 , horizontal and vertical rolling are performed alternately (horizontal rolling is performed on a horizontal rolling mill, vertical rolling is performed on a vertical rolling mill, and horizontal rolling is performed first). The deformation of each rough rolling pass (pass 1-11) is 23%, 18%, 15%, 16%, 15%, 16%, 15%, 16%, 15%, 15%, 15%. Due to the short heating time, the intermediate billet has serious rolling cracking problems during the rough rolling process, such as Figure 2 As shown, the intermediate billet cannot be used any more and subsequent intermediate rolling and finishing rolling are no longer performed.

[0080] Comparative Example 2

[0081] The rolling method using conventional rolling temperature is as follows:

[0082] (1) Heating: The billet (square billet with a cross-sectional size of 200 mm × 200 mm, the chemical composition of the billet is: C 0.06 wt%, Si 0.12 wt%, Mn 1.63 wt%, S 0.32 wt%, P 0.0068 wt%, Pb 0.23 wt%, and the balance is Fe and unavoidable impurities) enters the heating furnace for heating. The preheating section temperature is 680 ° C, the transit time is 40 min, the heating section I temperature is 1026 ° C, the transit time is 40 min, the heating section II temperature is 1238 ° C, the transit time is 20 min, and the soaking section temperature is 1220 ° C, the transit time is 20 min. The billet head and tail temperature deviation is ≤ 20 ° C, and the residual oxygen ratio of the heating furnace atmosphere is controlled below 2.0 vol%.

[0083] (2) Descaling: After the billet leaves the heating furnace, it is descaled with high-pressure water. The pressure of the high-pressure descaling water is 18 MPa.

[0084] (3) Rolling: Rolling is divided into three stages: rough rolling, intermediate rolling and finishing rolling. The three stages are carried out continuously. The starting rolling temperature is 1000℃ and the finishing rolling temperature is 950℃. The rough rolling adopts the groove rolling method, and a total of 11 rolling passes are carried out. The rolling deformation rate of the rough rolling is 0.75s -1 , horizontal and vertical rolling are performed alternately (horizontal rolling is performed on a horizontal rolling mill, vertical rolling is performed on a vertical rolling mill, and horizontal rolling is performed first). The deformation of each rough rolling pass (pass 1-11) is 23%, 18%, 15%, 16%, 15%, 16%, 15%, 16%, 15%, 15%, 15%. Due to the low rolling temperature, the intermediate billet has a serious rolling cracking problem during the rough rolling process (no cracking at the head, cracking at the middle and rear parts), such as Figure 3 As shown, the intermediate billet cannot be used any more and subsequent intermediate rolling and finishing rolling are no longer performed.

[0085] Comparative Example 3

[0086] The rolling method of the steel billet with a lower Mn / S ratio is as follows:

[0087] (1) Heating: The billet (square billet with a cross-sectional size of 200 mm × 200 mm, the chemical composition of the billet is: C 0.06 wt%, Si 0.12 wt%, Mn 0.85 wt%, S 0.32 wt%, P 0.0068 wt%, Pb 0.23 wt%, the balance being Fe and unavoidable impurities) enters the heating furnace for heating. The preheating section temperature is 680 ° C, the transit time is 40 min, the heating section I temperature is 1026 ° C, the transit time is 40 min, the heating section II temperature is 1238 ° C, the transit time is 20 min, the soaking section temperature is 1220 ° C, the transit time is 20 min; the billet head and tail temperature deviation is ≤ 20 ° C, and the residual oxygen ratio of the heating furnace atmosphere is controlled below 2.0 vol%.

[0088] (2) Descaling: After the billet leaves the heating furnace, it is descaled with high-pressure water. The pressure of the high-pressure descaling water is 18 MPa.

[0089] (3) Rolling: Rolling is divided into three stages: rough rolling, intermediate rolling and finishing rolling. The three stages are carried out continuously. The starting rolling temperature is 1150℃ and the finishing rolling temperature is 1013℃. The rough rolling adopts the groove rolling method, and a total of 11 rolling passes are carried out. The rolling deformation rate of the rough rolling is 0.75s -1 , horizontal and vertical rolling are carried out alternately (horizontal rolling is carried out on a horizontal rolling mill, vertical rolling is carried out on a vertical rolling mill, and horizontal rolling is carried out first). The deformation of each rough rolling pass (1-11 passes) is 23%, 18%, 15%, 16%, 15%, 16%, 15%, 16%, 15%, 15%, 15%, and an intermediate billet with a cross-sectional size of 75mm×75mm is obtained; intermediate rolling adopts flat roll holeless rolling, and a total of 7 rolling passes are carried out, of which 5 passes (intermediate rolling passes 1, 2, 4, 5, and 6) are ) is rolling in the horizontal direction, and 2 passes (the 3rd and 7th passes of intermediate rolling) are rolling in the vertical direction. The deformation of each pass of intermediate rolling (1-7 passes) is 20%, 20%, 15%, 15%, 10%, 10%, and 8% respectively; the finishing rolling adopts flat roll holeless rolling, with a total of 7 rolling passes, alternating rolling in the horizontal and vertical directions (horizontal rolling is performed first), and the deformation of each pass of finishing rolling (1-7 passes) is 10%, 8%, 6%, 5%, 6%, 5%, and 5% respectively, to obtain flat steel with a cross-sectional size of 80mm×20mm.

[0090] (4) Cooling: After rolling, the flat steel is placed on a cooling bed and then enters an insulation cover. The temperature entering and exiting the cover is controlled. The temperature entering the cover is 840°C and the temperature exiting the cover is 640°C. The time it takes to pass through the insulation cover is 4 minutes. After exiting the cover, it is air-cooled to room temperature to obtain the final flat steel product. The finished product is packaged. The cooling bed adopts a one-tooth multiple-branch method.

[0091] Comparative Example 4

[0092] The rough rolling stage adopts the rolling method with conventional rolling deformation rate, and the steps are as follows:

[0093] (1) Heating: The billet (square billet with a cross-sectional size of 200 mm × 200 mm, the chemical composition of the billet is: C 0.06 wt%, Si 0.12 wt%, Mn 1.63 wt%, S 0.32 wt%, P 0.0068 wt%, Pb 0.23 wt%, and the balance is Fe and unavoidable impurities) enters the heating furnace for heating. The preheating section temperature is 680 ° C, the transit time is 40 min, the heating section I temperature is 1026 ° C, the transit time is 40 min, the heating section II temperature is 1238 ° C, the transit time is 20 min, and the soaking section temperature is 1220 ° C, the transit time is 20 min. The billet head and tail temperature deviation is ≤ 20 ° C, and the residual oxygen ratio of the heating furnace atmosphere is controlled below 2.0 vol%.

[0094] (2) Descaling: After the billet leaves the heating furnace, it is descaled with high-pressure water. The pressure of the high-pressure descaling water is 18 MPa.

[0095] (3) Rolling: Rolling is divided into three stages: rough rolling, intermediate rolling and finishing rolling. The three stages are carried out continuously. The starting rolling temperature is 1150℃ and the finishing rolling temperature is 1013℃. The rough rolling adopts the groove rolling method, and a total of 11 rolling passes are carried out. The rolling deformation rate of the rough rolling is 1.5s -1 , horizontal and vertical rolling are performed alternately (horizontal rolling is performed on a horizontal rolling mill, vertical rolling is performed on a vertical rolling mill, and horizontal rolling is performed first). The deformation of each rough rolling pass (pass 1-11) is 23%, 18%, 15%, 16%, 15%, 16%, 15%, 16%, 15%, 15%, 15%. Due to the high deformation rate of rough rolling, the intermediate billet has serious rolling cracking problems during the rough rolling process, such as Figure 7 As shown, the intermediate billet cannot be used any more and subsequent intermediate rolling and finishing rolling are no longer performed.

[0096] Comparative Example 5

[0097] The rough rolling stage adopts the flat roll holeless rolling method, and the steps are as follows:

[0098] (1) Heating: The billet (square billet with a cross-sectional size of 200 mm × 200 mm, the chemical composition of the billet is: C 0.06 wt%, Si 0.12 wt%, Mn 1.63 wt%, S 0.32 wt%, P 0.0068 wt%, Pb 0.23 wt%, and the balance is Fe and unavoidable impurities) enters the heating furnace for heating. The preheating section temperature is 680 ° C, the transit time is 40 min, the heating section I temperature is 1026 ° C, the transit time is 40 min, the heating section II temperature is 1238 ° C, the transit time is 20 min, and the soaking section temperature is 1220 ° C, the transit time is 20 min. The billet head and tail temperature deviation is ≤ 20 ° C, and the residual oxygen ratio of the heating furnace atmosphere is controlled below 2.0 vol%.

[0099] (2) Descaling: After the billet leaves the heating furnace, it is descaled with high-pressure water. The pressure of the high-pressure descaling water is 18 MPa.

[0100] (3) Rolling: The rolling process is divided into three stages: rough rolling, intermediate rolling and finishing rolling. The three stages are carried out continuously. The starting rolling temperature is 1150℃ and the finishing rolling temperature is 1013℃. The rough rolling adopts flat roll non-hole rolling, and a total of 11 rolling passes are carried out. The rolling deformation rate of the rough rolling is 0.75s. -1 , horizontal and vertical rolling are carried out alternately (horizontal rolling is carried out on a horizontal rolling mill, vertical rolling is carried out on a vertical rolling mill, and horizontal rolling is carried out first). The deformation of each rough rolling pass (1-11 passes) is 23%, 18%, 15%, 16%, 15%, 16%, 15%, 16%, 15%, 15%, 15%, and an intermediate billet with a cross-sectional size of 75mm×75mm is obtained; intermediate rolling adopts flat roll holeless rolling, and a total of 7 rolling passes are carried out, of which 5 passes (intermediate rolling passes 1, 2, 4, 5, and 6) are ) is rolling in the horizontal direction, and 2 passes (the 3rd and 7th passes of intermediate rolling) are rolling in the vertical direction. The deformation of each pass of intermediate rolling (1-7 passes) is 20%, 20%, 15%, 15%, 10%, 10%, and 8% respectively; the finishing rolling adopts flat roll holeless rolling, with a total of 7 rolling passes, alternating rolling in the horizontal and vertical directions (horizontal rolling is performed first), and the deformation of each pass of finishing rolling (1-7 passes) is 10%, 8%, 6%, 5%, 6%, 5%, and 5% respectively, to obtain flat steel with a cross-sectional size of 80mm×20mm.

[0101] (4) Cooling: After rolling, the flat steel is placed on a cooling bed and then enters a heat preservation cover. The temperature of entering and exiting the cover is controlled. The temperature of entering the cover is 840°C and the temperature of exiting the cover is 640°C. The time of passing through the heat preservation cover is 4 minutes. After exiting the cover, it is air-cooled to room temperature to obtain the final free-cutting flat steel product. The cooling bed adopts a one-tooth multiple-branch method.

[0102] Figure 8The morphology of the surface defects of the flat steel finished product obtained in comparative example 5 (due to the use of flat rolls without holes in the rough rolling stage, the width of the rolled piece is unconstrained during the rolling process, resulting in wrinkles at the corners of the ingot, which are inherited to the finished product Figure 8 defect shown).

[0103] Effect verification

[0104] (1) Characterization of tissue morphology

[0105] The morphology of the flat steel products obtained in Examples 1-2 and Comparative Example 3 was observed using an optical metallographic microscope. Figure 4 The following are photos of the microstructure and MnS morphology of the free-cutting flat steel product obtained in Example 1 (the average aspect ratio of MnS is 2.6). Figure 5 The following are photos of the microstructure and MnS morphology of the free-cutting flat steel product obtained in Example 2 (the average aspect ratio of MnS is 2.8). Figure 6 The following is a photo of the structure and MnS morphology of the flat steel product obtained in Comparative Example 3 (the average aspect ratio of MnS is 4.8). Figure 4-6 It can be seen that the free-cutting flat steel produced by the method of the present invention has a more uniform structure and a smaller MnS aspect ratio.

[0106] (2) Cutting performance evaluation

[0107] The same turning speed (50m / min) was used to conduct cutting tests on the flat steel products obtained in Examples 1-2 and Comparative Example 3. The morphology of the chips is as follows: Figure 9 As shown by Figure 9 It can be seen that the easy-to-cut flat steel chips produced in Examples 1 and 2 are both C-type chips with better chip breaking effect, while the flat steel chips produced in Comparative Example 3 are spiral chips with poor chip breaking effect (spiral chips are easy to wrap around the knife, affecting cutting efficiency).

[0108] A piezoelectric three-axis dynamometer was used to measure the cutting force applied to the tool tip during turning. The magnitude of this force is used to evaluate the material's cutting performance. Under the same turning parameters, the lower the cutting force, the better the material's cutting performance. Cutting force tests were conducted on the finished flat steel products from the above examples and comparative examples, and the results are shown in Table 1.

[0109] Table 1 Cutting force comparison

[0110] Turning speed, m / min Cutting force, N Example 1 50 126 Example 2 50 120 Comparative Example 3 50 168

[0111] As can be seen from Table 1, the cutting forces of Example 1 and Example 2 are both smaller than the cutting force value of Comparative Example 3.

[0112] The embodiments described above are merely descriptions of preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by persons skilled in the art should fall within the scope of protection defined by the claims of the present invention.

Claims

1. A method for rolling free-cutting flat steel, characterized in that: The following steps are involved: The billet enters the heating furnace for heating. The temperature of the preheating section is ≤700℃, and the passing time is 30-45min. The temperature of the heating section I is 900-1100℃, and the passing time is 30-45min. The temperature of the heating section II is 1100-1300℃, and the passing time is 15-30min. The temperature of the soaking section is 1150-1250℃, and the passing time is 15-30min. Rolling the heated steel billet and then cooling it to obtain the free-cutting flat steel; The starting rolling temperature is 1100-1180°C, and the final rolling temperature is 980-1030°C; the rolling is divided into three stages: rough rolling, intermediate rolling and finishing rolling; the rolling deformation rate of the rough rolling is 0.45-0.80s -1 , the deformation per pass is 15% to 25%; The chemical composition of the steel slab includes Mn and S.

2. The method for rolling free-cutting flat steel according to claim 1, wherein: The chemical composition of the steel billet is as follows by mass percentage: C 0.01%-0.20%, Si 0-0.60%, Mn 0.40%-1.70%, S 0.02%-0.55%, P0-0.17%, Pb 0-0.50%, and the balance is Fe and unavoidable impurities, and Mn / S>4.

5.

3. The method for rolling free-cutting flat steel according to claim 1, wherein: The steel billet is a rectangular billet with a cross-sectional size ranging from (120 to 400) mm×(120 to 400) mm or a round billet with a diameter of 120 to 400 mm.

4. The method for rolling free-cutting flat steel according to claim 1, wherein: During the heating process, the temperature deviation between the head and tail of the steel billet is ≤20° C., and the residual oxygen content of the heating furnace atmosphere is <2.0 vol%.

5. The method for rolling free-cutting flat steel according to claim 1, wherein: After the steel billet is heated and before being cooled, a high-pressure water descaling step is also included, and the pressure of the high-pressure descaling water is ≥18 MPa.

6. The method for rolling free-cutting flat steel according to claim 1, wherein: The rough rolling is performed in 7 to 11 passes, the intermediate rolling is performed in 5 to 7 passes, and the finish rolling is performed in 5 to 7 passes.

7. The method for rolling free-cutting flat steel according to claim 6, wherein: The rough rolling adopts groove rolling; the intermediate rolling and finishing rolling adopt flat roll grooveless rolling.

8. The method for rolling free-cutting flat steel according to claim 1, wherein: The heat preservation cooling comprises: placing the rolled flat steel on a cooling bed, entering a heat preservation cover, controlling the temperature entering the cover to be greater than 800°C, and the temperature exiting the cover to be less than 650°C, the time for passing through the heat preservation cover being 3 to 5 minutes, and air cooling to room temperature after exiting the cover.

9. A free-cutting flat steel obtained by rolling the free-cutting flat steel rolling method according to any one of claims 1 to 8.

10. Use of the free-cutting flat steel according to claim 9 in the preparation of electro-hydraulic servo valve block parts.

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