A production method for eliminating full decarburization of Si-Cr spring steel wire rod and improving fatigue life

Abstract

The present application belongs to the technical field of rolling steel, and relates to a production method for eliminating full decarburization of Si-Cr spring steel wire rod and improving fatigue life. A heat state continuous casting blank is sprayed with a decarburization prevention coating, and is heated to 700-750 DEG C in a preheating section of a heating furnace, 750-850 DEG C in a first heating section, 900-1050 DEG C in a second heating section, and 1050-1200 DEG C in a soaking section. After the blank is discharged and descaled by high-pressure water, the opening rolling temperature is 950-1080 DEG C. The blank passes through a rough and intermediate rolling section, a pre-precision rolling section, a precision rolling section, and a water tank for cooling after precision rolling, so that the wire rod temperature is controlled at 800-850 DEG C. A Stelmor fan is used to rapidly cool the wire rod to 650-700 DEG C, and then the wire rod is slowly cooled in a holding cover, and the temperature of the wire rod after leaving the holding cover is not higher than 620 DEG C. The present application can eliminate full decarburization of the surface of Si-Cr spring steel wire rod and improve the fatigue life of the spring.

Description

Technical Field

[0001] This invention belongs to the field of steel rolling technology and relates to a production method for eliminating complete decarburization of Si-Cr spring steel wire rod. Specifically, it is a production method for eliminating complete decarburization of Si-Cr spring steel wire rod by spraying an anti-decarburization coating on the surface of a hot continuous casting billet and controlling the heating temperature, heating time, rolling temperature and post-rolling cooling process. Background Technology

[0002] Adding silicon (Si) to steel can improve its elastic limit and tempering stability, preventing elastic decay and extending the service life of springs. However, Si is a non-carbide-forming element, increasing the activity of carbon (C) in steel. During the oxidation and burning process on the steel surface, Si can easily cause decarburization, resulting in partial or even complete decarburization. Complete decarburization, in particular, severely reduces the surface hardness and strength of the steel, causing surface cracks and reducing the fatigue life of the spring.

[0003] Currently, the spring steel manufacturing industry generally mitigates decarburization by designing the steel's alloy composition, removing decarburized portions from the billet surface, applying anti-decarburization coatings, controlling heating temperature and time, controlling deformation temperature, and controlling heating and cooling rates. For Si-containing spring steel, adding medium-strong carbide-forming elements Cr and V can obtain Si-Cr-V series spring steel, increasing the activation energy of carbon and slowing down decarburization. Removing the decarburized portions from the surface of the rolled billet after the initial rolling process, then applying anti-decarburization coatings to the billet surface, controlling heating temperature and time, controlling deformation temperature, and controlling heating and cooling rates prevents prolonged residence in the two-phase region, which could lead to complete decarburization.

[0004] The aforementioned methods for controlling decarburization all have certain limitations. For example, strong carbide-forming elements such as Cr and V are expensive and have limited ability to inhibit decarburization; removing the decarburized layer from the billet surface after billet preparation is more costly, and the billet surface often has pits that prevent complete removal of the decarburized layer. Moreover, the peeling grinding wheel often creates steps and burrs on the billet surface, causing more severe decarburization in localized areas; after peeling the billet after billet preparation, applying anti-decarburization coating at room temperature requires using clamps to flip, lift, and stack the billet, causing the coating to be scraped off at contact points. Furthermore, due to the fast pace of continuous production, the coating is often applied but not yet dry before flipping, lifting, and stacking operations are required, resulting in severe coating loss in some areas, failing to prevent decarburization. Currently, decarburization is mitigated by controlling heating temperature and time, deformation temperature, heating rate, and cooling rate. For example, CN201210039991.5 describes a method for controlling surface decarburization of spring steel wire. By controlling the heating temperature and time, excellent decarburization control can be achieved. However, this method cannot solve the fatigue life problem. Although reducing the heating temperature and time can effectively reduce decarburization, for medium carbon spring steel with high alloy content, low-temperature short-time heating is not conducive to the diffusion of carbon and alloying elements, thus affecting the uniformity of subsequent microstructure transformation. Moreover, the low-temperature deformation determined by low-temperature heating is not conducive to the breakage of inclusions, all of which seriously affect the fatigue life of the spring.

[0005] Therefore, for high-speed wire rod production lines, there is a need to find a production method that can stably and completely eliminate the full decarburization of the surface of Si-Cr spring steel wire rods while being low in cost, so as to improve the fatigue life of springs. This invention attempts to provide a production method that eliminates the full decarburization of Si-Cr spring steel wire rods while also improving fatigue life. Summary of the Invention

[0006] The purpose of this invention is to address the above-mentioned problems by providing a production method that eliminates complete decarburization of Si-Cr spring steel wire rods while improving fatigue life. This method involves spraying an anti-decarburization coating on the surface of the hot continuous casting billet, controlling the heating temperature and heating time, controlling the deformation temperature, and controlling the heating rate and cooling rate to eliminate complete decarburization on the surface of the Si-Cr spring steel wire rods and improve the fatigue life of the springs.

[0007] To achieve the above-mentioned technical objectives, this invention provides a production method for eliminating complete decarburization of Si-Cr series spring steel wire rods. The production process is as follows: hot feeding of continuously cast billets → removal of oxide scale → spraying of anti-decarburization coating → billet heating → billet rolling → wire forming → controlled cooling of wire rods. The specific process steps are as follows:

[0008] (1) The hot continuous casting billet in the continuous casting workshop is collected and transferred to the conveyor roller table by the walking cooling bed, and then conveyed to the walking cooling bed in the steel rolling workshop by the roller table. The chain elevator then conveys the hot continuous casting billet on the walking cooling bed to the furnace feed roller table.

[0009] The temperature of the hot continuous casting billets in the continuous casting workshop is 500-600℃. Before collection by the walking beam cooling bed, each billet is sprayed with a identification number to provide a basis for the identification and tracking of subsequent production materials.

[0010] (2) The hot continuous casting billet moves forward on the furnace roller conveyor, and the four surfaces of the billet come into contact with and scrape against the steel brushes fixed on the top, bottom, left and right sides to remove the oxide scale from the surface of the hot continuous casting billet.

[0011] Furthermore, the temperature of the hot continuous casting billet on the furnace roller conveyor is 300-400℃, and the steel brush is covered with dense elastic steel wires.

[0012] (3) When the continuous casting billet reaches the photoelectric signal in front of the spraying device, the spraying device will automatically start after receiving the instruction. The nozzles on the four sides of the spraying device will start to spray anti-decarburization coating on the four surfaces of the slowly advancing hot continuous casting billet.

[0013] Furthermore, the temperature of the hot continuous casting billet reaching the spraying device is 300-400℃. The anti-decarburization coating is a mixture of powder and slurry in a 1:1 mass ratio and stirred evenly. The paste-like anti-decarburization coating is sprayed onto the hot continuous casting billet at 300-400℃. The liquid solvent evaporates rapidly, and the effective anti-decarburization powder particles are sintered on the surface of the hot continuous casting billet. It does not fall off during the friction process on the furnace roller conveyor. The intact anti-decarburization powder particles on the billet surface have an unexpected effect on protecting the high-temperature steel billet from oxidation and decarburization in the subsequent heating furnace.

[0014] This invention involves spraying a coating onto a hot steel billet at 300-400°C. The liquid solvent in the coating evaporates rapidly, and the effective component, the anti-decarburization powder particles, sintersects onto the surface of the hot continuously cast billet, bonding tightly with the billet matrix. Even when it comes into contact with and rubs against the conveyor rollers, it will not fall off, thus providing excellent anti-decarburization effect. Furthermore, spraying the anti-decarburization coating onto the surface of the hot continuously cast billet overcomes the limitation that high-Si spring steel must be heated at low temperatures to prevent decarburization. By using higher heating temperatures, the production difficulty is reduced, significantly decreasing the total decarburization layer depth of the wire rod while also exhibiting excellent tensile and fatigue properties.

[0015] (4) The hot continuous casting billet with anti-decarburization coating on its surface enters the regenerative walking beam furnace. The billet is slowly heated to 700-750°C in the preheating section. The billet passes through the temperature range of 750-850°C in the first heating section. The temperature in the second heating section is controlled at 900-1050°C. The temperature in the soaking section is controlled at 1120-1180°C.

[0016] Furthermore, the anti-decarburization coating adhering to the surface of the hot continuous casting billet is a dispersed powder particle. The heating time in the preheating section is 30-40 min, the heating time in the first heating section is 10-20 min, the heating time in the second heating section is 40-50 min, and the heating time in the soaking section is 40-50 min. The temperature of the second heating section is preferably 980-1050℃.

[0017] Si-Cr series spring steel has a high alloy content and poor heat transfer in the billet. The preheating section heats the billet slowly to ensure sufficient heat transfer and uniform temperature, preventing cracking caused by uneven heating due to rapid temperature rise. The first heating section has a temperature of 750–850℃. This temperature range is the ferrite + austenite two-phase region, where the two crystal structures have different carburizing abilities. Prolonged exposure to this temperature range can easily cause decarburization, so rapid heating is necessary to pass through it. The second heating section requires slow heating to ensure uniform temperature as much as possible. The soaking section has a high temperature and a long holding time. Its main function is to homogenize the billet temperature and alloy composition through high temperature, improve the steel's plasticity, and reduce deformation resistance.

[0018] (5) After the billet is descaled by high pressure water after exiting the furnace, the initial rolling temperature is 950-1080℃. Then the billet passes through the roughing and intermediate rolling mill and the pre-finishing rolling mill at 900-980℃, and then passes through the finishing rolling mill at 850-900℃.

[0019] Furthermore, the high-pressure water descaling pressure should not be less than 14 MPa, and the roughing, intermediate rolling and pre-finishing rolling combined need to complete more than 95% of the total deformation.

[0020] (6) After finishing rolling, the wire rod is cooled by a water tank to control the wire rod's extrusion temperature at 800-850℃;

[0021] Furthermore, only the section of the water tank closest to the finishing mill is opened, while the rest is closed.

[0022] (7) After the wire is spun into a coil, the wire rod is placed on the Stellmore roller conveyor. By setting the roller conveyor speed parameters, opening and closing the heat insulation cover, and turning on the Stellmore fan, the wire rod is quickly cooled to 650-700°C. Then it enters the heat insulation cover for slow cooling. The temperature of the wire rod exiting the heat insulation cover is not higher than 620°C.

[0023] Furthermore, the Steyrmo roller conveyor speed is set to 25–35 m / min, 2–8 insulation covers are activated, and 2–8 power units with a capacity of 2×10 are activated. 5 W's Stellmore fan has an air volume opening of 10-100%.

[0024] Setting a faster Steyrmo roller speed can reduce the stacking density between coils and make it easier to dissipate heat at the overlap between the coil and the roller. Several heat insulation covers near the spinning machine are opened and several fans are turned on, which helps the coil to pass quickly through the two-phase temperature range that is prone to complete decarburization, and quickly cool down to the pearlite transformation temperature range, so as to buy more time for the pearlite transformation in the heat insulation cover and complete the microstructure transformation on the Steyrmo roller.

[0025] The chemical composition of the Si-Cr spring steel wire rod obtained by this invention, by mass percentage, is as follows: C: 0.51-0.59%, Si: 1.20-1.60%, Mn: 0.50-0.80%, Cr: 0.50-0.80%, Ni≤0.35%, Cu≤0.25%, Al≤0.025%, P≤0.025%, S≤0.020%, with the remainder being iron and unavoidable impurities. The spring steel wire rod has a complete decarburization of 0 μm and a spring fatigue life ≥1.45 million cycles.

[0026] This invention prevents high-temperature oxidation and decarburization of steel billets by spraying an anti-decarburization coating on the surface of the hot-cast billets; controlling the heating temperature, heating time, and heating rate during billet heating; controlling the deformation temperature during rolling deformation; and controlling the cooling rate during wire rod cooling by setting the roller speed, switching the insulation cover on and off, and switching the Stellmore fan on and off. These measures can eliminate complete decarburization on the surface of Si-Cr spring steel wire rods and also improve the fatigue life of the springs.

[0027] The beneficial effects of this invention are as follows: This invention eliminates the need for additional process flows; the hot-cast billet does not require a high-temperature, long-duration billet opening process; the surface of the hot-cast billet is free of decarburization layer and rolling defects; and it also eliminates the need for a peeling process to remove the decarburization layer and rolling defects from the billet surface, significantly reducing production costs. Only an anti-decarburization coating needs to be sprayed onto the surface of the hot-cast billet before it enters the furnace, overcoming the limitation that high-Si spring steel must be heated at low temperatures to prevent decarburization. Simultaneously controlling the heating temperature, heating time, heating rate, deformation temperature, and cooling rate allows for the elimination of complete decarburization of Si-Cr spring steel wire rods by using higher heating temperatures, significantly reducing the total decarburization layer depth while ensuring excellent tensile properties of the wire rods and excellent fatigue properties of the springs. The production method is highly efficient, has a large equipment load margin, and is suitable for mass production. Attached Figure Description

[0028] Figure 1 The image shows the decarburization morphology of the Φ14mm 55SiCr spring steel wire rod produced in Example 1.

[0029] Figure 2 The image shows the decarburization morphology of the Φ14mm 55SiCr spring steel wire rod produced in Example 2.

[0030] Figure 3 The decarburization morphology of Φ14mm 55SiCr spring steel wire rod produced for Comparative Example 1.

[0031] Figure 4 The decarburization morphology of Φ14mm 55SiCr spring steel wire rod produced for Comparative Example 2.

[0032] Figure 5 The decarburization morphology of the Φ14mm 55SiCr spring steel wire rod produced for Comparative Example 3.

[0033] Figure 6 The decarburization morphology of the Φ14mm 55SiCr spring steel wire rod produced for Comparative Example 4.

[0034] Figure 7 The decarburization morphology of the Φ14mm 55SiCr spring steel wire rod produced for Comparative Example 5.

[0035] Figure 8 The decarburization morphology of the Φ14mm 55SiCr spring steel wire rod produced for Comparative Example 6. Detailed Implementation

[0036] The present invention will now be described in conjunction with the production process of 55SiCr spring steel wire rod with a billet chemical composition of C: 0.55%, Si: 1.49%, Mn: 0.72%, Cr: 0.75%, Ni: 0.05%, Cu: 0.05%, Al: 0.009%, P: 0.006%, S: 0.005% and a finished product specification of Φ14mm.

[0037] Example 1

[0038] 1. Hot delivery of continuously cast billets

[0039] In the continuous casting workshop, each hot continuous casting billet is marked with a steel billet number. The hot continuous casting billets at 500-600℃ are collected and transferred to the conveyor roller conveyor via the walking beam cooling bed. The roller conveyor then transports the hot continuous casting billets from the walking beam cooling bed to the furnace feed roller conveyor.

[0040] 2. Remove oxide scale

[0041] The hot continuous casting billet at 300-400℃ advances on the furnace roller conveyor. The four surfaces of the billet come into contact with and scrape against a steel brush with densely distributed elastic steel wires on the top, bottom, left and right sides to remove the oxide scale from the surface of the hot continuous casting billet.

[0042] 3. Spray anti-decarburization coating

[0043] When the continuously cast billet reaches the signal point in front of the spraying device, the spraying device will automatically start after receiving the instruction. The nozzles on the four sides of the spraying device will start spraying a paste-like anti-decarburization coating with a mass ratio of 1:1 (coating mass ratio SiO2:MgO:Al2O3:CaO = 58:15:14:13) onto the four surfaces of the slowly advancing 300-400℃ hot continuously cast billet.

[0044] 4. Heating of steel billets

[0045] Hot continuously cast billets with anti-decarburization coating on their surface enter a regenerative walking beam furnace. The billets are slowly heated to 700-750°C in the preheating section for 35 minutes. In the first heating section, the billets are rapidly heated to 750-850°C for 15 minutes. In the second heating section, the temperature is controlled at 980-1050°C for 45 minutes. In the soaking section, the temperature is controlled at 1120-1180°C for 45 minutes.

[0046] 5. Billet rolling

[0047] After the billet is descaled by 18MPa high-pressure water, the initial rolling temperature is 950-1080℃. Subsequently, the billet passes through the roughing and intermediate rolling mill and the pre-finishing rolling mill at 900-980℃, and then passes through the finishing rolling mill at 850-900℃. The roughing and intermediate rolling and the pre-finishing rolling together need to complete 97% of the total deformation.

[0048] 6. Spinning silk into circles

[0049] After finishing rolling, the wire rod is cooled by a water tank. Only the section of the water tank closest to the finishing mill is opened, while the rest is closed to even out the core surface temperature of the wire rod and control the wire rod's extrusion temperature at 800-850℃.

[0050] 7. Wire rod cooling

[0051] After being spun into coils, the wire rod is placed on the Steyrmo roller conveyor, with the roller speed set at 30 m / min. Four heat insulation covers are turned on, and four 2×1000 heat exchangers are activated. 5 The W-type Stellmore fan, with an airflow opening of 90%, rapidly cools the wire rod to 650-700°C, then it enters the insulation hood for slow cooling. The temperature of the wire rod exiting the insulation hood does not exceed 620°C.

[0052] Example 2

[0053] Compared with Example 1, Example 2 involves the following steps: the billet is heated for 40 minutes in the preheating section, 10 minutes in the first heating section, 40 minutes in the second heating section, and 40 minutes in the soaking section. The high-pressure water descaling pressure is 16 MPa. All other operations are the same as in Example 1.

[0054] Comparative Example 1

[0055] The main difference between Comparative Example 1 and Example 1 is that the device for spraying anti-decarburization coating in step 3 of Example 1 was powered off and shut down, while other conditions were the same as in Example 1.

[0056] Comparative Example 2

[0057] The main difference between Comparative Example 2 and Example 1 is that the heating time of the first heating section in step 4 of Example 1, which was 15 min, is replaced with 30 min, while other conditions are the same as in Example 1.

[0058] Comparative Example 3

[0059] The main difference between Comparative Example 3 and Example 1 is that the finishing rolling temperature of step 5 in Example 1 (850-900℃) is replaced with 780-830℃, while other conditions are the same as in Example 1.

[0060] Comparative Example 4

[0061] Compared with Example 1, the main difference of Comparative Example 4 is that step 7 of Example 1, which involves turning on 4 insulation covers and 4 fans with an air volume of 90%, is replaced by turning on 10 insulation covers and 10 fans with an air volume of 30%. Other conditions are the same as in Example 1.

[0062] Comparative Example 5

[0063] Compared with Example 1, the main difference of Comparative Example 5 is that the device for spraying anti-decarburization coating in step 3 of Example 1 is de-energized and shut down, the temperature of the second heating section in step 4 (980-1050℃) is replaced with 880-950℃, and the temperature of the heat spreader section (1120-1180℃) is replaced with 1020-1080℃. Other conditions are the same as in Example 1.

[0064] Comparative Example 6

[0065] The main difference between Comparative Example 6 and Example 1 is that the temperature of the second heating section in step 4 of Example 1, which is 980-1050°C, is replaced with 860-930°C, and the temperature of the heat spread section, which is 1120-1180°C, is replaced with 990-1050°C. Other conditions are the same as in Example 1.

[0066] The results of full decarburization and total decarburization layer depth of Φ14.0mm 55SiCr spring steel wire rods produced in Examples 1, 2, 1, 2, 3, 4, and 5 of this invention are compared in Table 1.

[0067] Table 1. Depth of total decarburization and decarburization layer for Φ14.0mm 55SiCr spring steel wire rod

[0068]

[0069] This invention, through the design of the steel's alloy composition, mitigates decarburization, resulting in a continuously cast billet with extremely shallow decarburization. Furthermore, it prevents high-temperature oxidation and decarburization of the billet by spraying an anti-decarburization coating on the hot-cast billet surface. During billet heating, the heating temperature, heating time, and heating rate are controlled to prevent complete decarburization in the two-phase region and severe decarburization under prolonged high temperatures. During rolling deformation, the deformation temperature is controlled to prevent deformation and dwell in the two-phase region from easily causing complete decarburization. During wire rod cooling, the cooling rate is controlled by setting the roller speed, switching the insulation cover, and switching the Stellmore fan, allowing the wire rod to quickly pass through the two-phase temperature range that easily causes complete decarburization. Therefore, the Φ14.0mm 55SiCr spring steel wire rod obtained in this embodiment has a complete decarburization layer depth of 0, while the total decarburization layer depth is very shallow, the wire rod has excellent tensile properties, and the spring has excellent fatigue properties. In contrast, the Φ14.0mm 55SiCr spring steel wire rod obtained in the comparative example all exhibit complete decarburization, and the total decarburization layer depth is deeper than that of the embodiment.

[0070] Unless otherwise specified, the raw materials and equipment used in this invention are all commonly used in the art; the methods used in this invention, unless otherwise specified, are all conventional methods in the art. The above descriptions are merely preferred embodiments of the present invention and are not intended to limit the invention. Any modifications made to the above embodiments based on the technical essence of the present invention are included within the protection scope of the present invention.

Claims

1. A production method for eliminating complete decarburization of Si-Cr series spring steel wire rod while improving fatigue life, characterized in that, The production method is as follows: hot delivery of continuously cast billet - removal of oxide scale - spraying of anti-decarburization coating - billet heating - billet rolling - wire forming - controlled cooling of wire rod. The specific process steps are as follows: (1) After hot delivery of the continuous casting billet and removal of the oxide scale on the surface of the hot continuous casting billet, anti-decarburization coating is sprayed on the four surfaces of the hot continuous casting billet; the temperature of the hot continuous casting billet sprayed with anti-decarburization coating is 300~400℃; the anti-decarburization coating is sprayed on the hot continuous casting billet at 300~400℃, the liquid solvent in the coating evaporates rapidly, and the effective anti-decarburization powder particles are sintered on the surface of the hot continuous casting billet and tightly bonded to the billet matrix; (2) The hot continuous casting billet with anti-decarburization coating on its surface enters the regenerative walking beam furnace. The billet is slowly heated to 700~750℃ through the preheating section. The billet passes through the temperature range of 750~850℃ in the first heating section. The temperature of the second heating section is controlled at 900~1050℃. The temperature of the soaking section is controlled at 1050~1200℃. The heating time of the preheating section is 30~40min, the heating time of the first heating section is 10~20min, the heating time of the second heating section is 40~50min, and the heating time of the soaking section is 40~50min. (3) After the billet is descaled by high pressure water after exiting the furnace, the initial rolling temperature is 950-1080℃. Then the billet passes through the roughing and intermediate rolling mill and the pre-finishing rolling mill at 900-980℃, and then passes through the finishing rolling mill at 850-900℃. (4) After finishing rolling, the wire rod is cooled by water tank to control the wire rod's extrusion temperature at 800-850℃; (5) After the wire is spun into a coil, the wire rod is placed on the Stellmore roller conveyor. By setting the roller conveyor speed parameters, opening and closing the heat insulation cover, and turning on the Stellmore fan, the wire rod is quickly cooled to 650~700℃. Then it enters the heat insulation cover for slow cooling. The temperature of the wire rod exiting the heat insulation cover is not higher than 620℃. The Steyrmo roller conveyor speed is set to 25~35m / min, with 2~8 insulation covers opened and 2~8 2×10 power units activated. 5 W's Stellmore fan has an air volume opening of 10~100%; the spring steel wire rod has a full decarburization depth of 0μm and a spring fatigue life of ≥1.45 million cycles; The chemical composition of the wire rod, by mass percentage, is as follows: C: 0.51-0.59%, Si: 1.20-1.60%, Mn: 0.50-0.80%, Cr: 0.50-0.80%, Ni≤0.35%, Cu≤0.25%, Al≤0.025%, P≤0.025%, S≤0.020%, with the remainder being iron and unavoidable impurities.

2. The production method for eliminating complete decarburization of Si-Cr series spring steel wire rod and improving fatigue life according to claim 1, characterized in that: Step (1) Hot delivery of continuous casting billet and removal of oxide scale from the surface of hot continuous casting billet refers to the process where hot continuous casting billets in the continuous casting workshop are collected and transferred to the conveyor roller conveyor after being cooled by the walking bed. The roller conveyor then transports the hot continuous casting billet on the walking bed to the furnace feed roller conveyor. As the hot continuous casting billet moves forward on the furnace feed roller conveyor, the four surfaces of the billet come into contact with and scrape against the steel brushes fixed on the top, bottom, left and right sides to remove the oxide scale from the surface of the hot continuous casting billet.

3. The production method for eliminating complete decarburization of Si-Cr series spring steel wire rod and improving fatigue life according to claim 1, characterized in that: The temperature of the second heating section in step (2) is 980-1050℃, and the temperature of the heat spreader section is 1120-1180℃.

4. The production method for eliminating complete decarburization of Si-Cr series spring steel wire rod and improving fatigue life according to claim 1, characterized in that: The high-pressure water descaling pressure mentioned in step (3) shall not be less than 14 MPa, and the total deformation amount of roughing, intermediate rolling and pre-finishing rolling shall be more than 95%.

Images