A high-nitrogen cutting tool steel and its preparation method

Abstract

This invention relates to a high-nitrogen cutting tool steel and its preparation method. The chemical composition of the steel is: C 0.75%–0.85%, Si 0.10%–0.20%, Mn 1.50%–1.70%, P ≤0.015%, S 0.08%–0.10%, Cr 0.60%–0.80%, Ti 0.12%–0.15%, Alt 0.015%–0.055%, N 0.026%–0.030%, B 0.0040%–0.0060%, Nd 0.0020%–0.0030%, with the balance being Fe and impurities. This invention solves the problems of poor working stability, uneven performance, and easy breakage and tooth chipping in existing cutting tool steels. It reduces component segregation in cutting tool steel, improves surface quality, and possesses high strength, high toughness, and long fatigue life. It also exhibits good wear resistance, deformation resistance, impact resistance, and weldability.

Description

Technical Field

[0001] This invention relates to the field of cutting tool steel production technology, and in particular to a high-nitrogen cutting tool steel and its preparation method. Background Technology

[0002] With the rapid development of industries such as ore mining, ceramic production, building structures, and stone decoration in my country, the demand for cutting tool steel is increasing at a rate of 20% to 30% annually. To further improve yield and work efficiency, cutting tools (such as circular saw blades) are gradually developing towards higher speeds, thinner blades, and larger diameters. In harsh working environments, the increased speed of the cutting tool intensifies the impact, friction, and vibration it experiences, leading to increased stress, reduced working stability, and problems such as uneven performance, tooth breakage, severe segregation, deteriorated surface quality, and reduced wear and corrosion resistance, seriously affecting the service life of the cutting tool.

[0003] Harsh working environments place higher demands on the mechanical properties of cutting tool steel, particularly its resistance to deformation, impact resistance, corrosion resistance, and fatigue performance. To address these issues, this invention utilizes nitrogen (N) as a precipitation strengthening and grain refinement strengthening element to enhance the performance of cutting tool steel. N in steel has three main functions: first, it increases the absolute content of dissolved N and combined N in the steel; second, it increases the ratio of dissolved N to total N, promoting N dissolution; and third, it increases the N content in AlN and BN precipitates, as well as the ratio of N content in AlN and BN precipitates to total N, promoting the precipitation of combined N in the form of AlN and BN phases. By rationally adding N, high-nitrogen cutting tool steel with excellent corrosion resistance, high fatigue life, and tensile strength ≥1800 MPa can be produced, while also reducing the substrate thickness and achieving lightweighting.

[0004] Currently, the main methods for increasing nitrogen in steel include nitrogen blowing and alloy nitrogen addition, but these methods have the following problems: 1) The nitrogen addition methods are complex and difficult to implement; 2) The nitrogen yield is low and fluctuates greatly; 3) The nitrogen addition process is accompanied by an increase in inclusions.

[0005] Chinese patent application CN113025780A discloses a "method for producing ultra-high nitrogen tinplate," which involves smelting molten iron to obtain molten steel; refining the molten steel by blowing nitrogen and simultaneously vacuuming it to a vacuum level controlled at 2-15 kPa; then adding a nitrogen-containing alloy for alloying; circulating the mixture and breaking the vacuum to obtain refined molten steel; finally, the refined molten steel is killed and continuously cast to obtain tinplate billets with a nitrogen content ≥150 ppm. This production process is relatively complex and requires precise control during production, which is not conducive to its widespread application.

[0006] Chinese patent application CN117467826A discloses "a tin-plated sheet for easy-open lids and its manufacturing method." The chemical composition of the tin-plated sheet, by weight percentage, is: C 0.060–0.080%, Si ≤0.03%, Mn 0.15–0.25%, P ≤0.015%, S ≤0.010%, Al 0.04–0.06%, N 0.0060–0.0090%, with the balance being Fe and other unavoidable impurities. Its chemical composition design uses conventional methods and still suffers from the aforementioned problems.

[0007] Chinese patent application CN117587330A discloses "a high-nitrogen, high-purity tinplate and its production method." The chemical composition of the tinplate, by weight percentage, is: C 0.06–0.08%, Si ≤0.03%, Mn 0.20–0.30%, P ≤0.020%, S ≤0.010%, Al 0.03–0.06%, N 0.008–0.01%, As ≤0.03%, Cr and Pb total ≤0.01%, with the balance being Fe and unavoidable impurities. Its high C content and high levels of residual elements As, Cr, and Pb in the steel are detrimental to the formability and corrosion resistance of the steel plate.

[0008] Chinese patent application CN111041153A discloses a "method and system for smelting high-nitrogen tinplate steel". The method involves measuring the nitrogen content of the molten steel in a bottom-blowing furnace to obtain a first nitrogen content; determining the bottom-blowing nitrogen time t based on the target nitrogen content, the first nitrogen content, and the nitrogen blowing rate of the bottom-blowing furnace; controlling the bottom-blowing furnace to switch to bottom-blowing nitrogen; and continuously blowing nitrogen into the molten steel for a time t according to the nitrogen blowing rate and pressure to obtain the first target molten steel. However, this process is complex, difficult to implement in production, and not conducive to widespread application. Summary of the Invention

[0009] This invention provides a high-nitrogen cutting tool steel and its preparation method, which solves the problems of poor working stability, uneven performance, and easy breakage and tooth chipping that are common in existing cutting tool steels. It reduces the segregation of cutting tool steel components and improves the surface quality of cutting tool steel. The prepared high-nitrogen cutting tool steel has high strength, high toughness and long fatigue life, and also has good wear resistance, deformation resistance, impact resistance and weldability.

[0010] To achieve the above objectives, the present invention employs the following technical solution:

[0011] A high-nitrogen cutting tool steel, wherein the chemical composition of the steel, by mass percentage, is: C 0.75%–0.85%, Si 0.10%–0.20%, Mn 1.50%–1.70%, P≤0.015%, S 0.08%–0.10%, Cr 0.60%–0.80%, Ti 0.12%–0.15%, Alt 0.015%–0.055%, N 0.026%–0.030%, B 0.0040%–0.0060%, Nd 0.0020%–0.0030%, with the balance being Fe and unavoidable impurities.

[0012] Furthermore, the microstructure of the finished steel plate is tempered sorbite; the properties of the finished steel plate are: yield strength ≥1600MPa, tensile strength ≥1800MPa, and elongation after fracture A50 ≥20%.

[0013] Furthermore, the thickness of the finished steel plate is 1.5 to 20.0 mm.

[0014] A method for preparing high-nitrogen cutlery steel, the process flow including steelmaking, continuous casting, heating, hot continuous rolling, laminar flow cooling, coiling, blanking, forming, welding and heat treatment; wherein the following processes are controlled:

[0015] 1) Steelmaking;

[0016] Converter smelting: Top and bottom blowing converter smelting is adopted, the ratio of molten iron to scrap steel is (80%~90%): (10%~20%), and nitrogen is blown from the bottom throughout the process; lime and lightly calcined dolomite are used for slag making, and the oxygen blowing time is 12~16 minutes; ferrochrome and ferromanganese are added during the tapping process, and the tapping temperature is controlled at 1700℃~1720℃;

[0017] LF refining: Fluorite is added to make white slag, molten steel is poured into the LF refining furnace, and deoxidizer is added at the same time to deoxidize. The Mn content in the steel is finely adjusted to the set range. Ferroboron, ferrochrome and ferrotitanium are added for alloying. Nd alloy is added to modify the inclusions. Soft blowing argon stirring time is ≥10min and the calming time is ≥14min.

[0018] RH vacuum treatment: After the vacuum is broken, the molten steel is fed with wire to increase nitrogen. The feeding rate of manganese nitride cored wire is 4.4 to 5.5 m / t steel, and the feeding speed is 2.6 to 3.0 m / s. The bottom blowing device of the ladle is turned off during the wire feeding process.

[0019] 2) Continuous casting;

[0020] A straight arc continuous casting machine is used for continuous casting, and the superheat of continuous casting is controlled at 10-20℃.

[0021] 3) Heating;

[0022] The hot charging and hot delivery process is adopted, with the continuous casting billet heating temperature at 1220-1260℃ and the soaking time at 40-50min.

[0023] 4) Hot continuous rolling;

[0024] Rough rolling: A 3+3 pass rolling mode is adopted, and the rough rolling start temperature is 1140~1180℃;

[0025] Finishing rolling: 7-stand hot continuous rolling is adopted, with cooling water introduced between stands, and the final rolling temperature is 855~885℃;

[0026] 5) Winding up;

[0027] The coiling temperature is 450-480℃; the coiled steel plate is placed in a slow cooling pit for slow cooling treatment, and the slow cooling time is not less than 72 hours.

[0028] 6) Heat treatment;

[0029] The quenching temperature is 820-850℃, and the tempering temperature is 200-260℃.

[0030] Furthermore, during the furnace smelting process, the amount of slag discharged during tapping is controlled to be ≤5kg / ton of steel.

[0031] Furthermore, in the LF refining process, aluminum-manganese-iron deoxidizer is used, and the addition amount is 400-600 kg / furnace of steel; the thickness of the top slag in the molten steel is maintained between 30 mm and 50 mm.

[0032] Furthermore, during the RH vacuum treatment process, the nitrogen content in the manganese nitride cored wire is 7% to 8%, the manganese content is 85% to 90%, and the remainder is impurities.

[0033] Compared with the prior art, the beneficial effects of the present invention are:

[0034] This method solves the common problems of poor working stability, uneven performance, and easy breakage and tooth chipping in existing cutting tool steels. It reduces the segregation of cutting tool steel components and improves the surface quality of cutting tool steel. The high-nitrogen cutting tool steel prepared has high strength, high toughness and long fatigue life, as well as good wear resistance, deformation resistance, impact resistance and weldability. Attached Figure Description

[0035] Figure 1 This is a metallographic photograph of the high-nitrogen cutting tool steel produced in Example 1 of the present invention.

[0036] Figure 2 This is a morphological diagram of the inclusions in the high-nitrogen cutting tool steel produced in Example 1 of the present invention.

[0037] Figure 3 This is a metallographic photograph of the tool steel produced by Comparative Example 1 of the present invention. Detailed Implementation

[0038] This invention provides a high-nitrogen cutting tool steel with high fatigue life, comprising the following chemical composition by mass percentage: C 0.75%–0.85%, Si 0.10%–0.20%, Mn 1.50%–1.70%, P ≤0.015%, S 0.08%–0.10%, Cr 0.60%–0.80%, Ti 0.12%–0.15%, Alt 0.015%–0.055%, N 0.026%–0.030%, B 0.0040%–0.0060%, Nd 0.0020%–0.0030%, with the balance being Fe and unavoidable impurities. After heat treatment, the high-nitrogen cutting tool steel plate exhibits a yield strength ≥1600 MPa, a tensile strength ≥1800 MPa, and an elongation at fracture (A50) ≥20%. Compared with ordinary cutlery steel plates, high-nitrogen cutlery steel plates have a fatigue life that increases by more than 800,000 cycles and a corrosion resistance that is improved by more than 80%. The microstructure of high-nitrogen cutlery steel is tempered sorbite, which has good formability.

[0039] The reasons for selecting the chemical elements in the high-nitrogen cutting tool steel described in this invention and their working principles are as follows:

[0040] Carbon (C) exists in cutter steel in the form of cementite, TiC, and interstitial solid solution, and is the most important strengthening element in steel. C plays a beneficial role in improving the strength and hardness of cutter steel, while nano-sized TiC is beneficial for improving fatigue performance and wear resistance. In addition, a high C content is advantageous for heat treatment processes. This invention controls the C content to be between 0.75% and 0.85%.

[0041] Si plays a role in calming and deoxidizing in tool steel, but excessive Si content can adversely affect the surface quality of the tool steel. Therefore, this invention controls the Si content to be 0.10%–0.20%.

[0042] Mn is a commonly used solid solution strengthening element in steel. In order to ensure the strength of tool steel, the Mn content is controlled at 1.50% to 1.70% in this invention.

[0043] P: When it aggregates and grows at grain boundaries, it reduces the formability and toughness of tool steel, causing brittleness. Therefore, this invention controls the P content to be ≤0.015%.

[0044] Sulfur (S) is a harmful element in steel, and its content should be as low as possible. However, in this invention, S generates a large amount of Mn-SBN-Nd compounds, as well as NdS and Nd2O2S inclusions in the steel, which can effectively improve machinability. Therefore, the S content is controlled at 0.08% to 0.10% in this invention.

[0045] Ti is relatively reactive in steel, playing a role in precipitation strengthening. On one hand, Ti combines with C and N to form TiN, TiC, and Ti(C,N), among which TiC and Ti(C,N) are beneficial to the strength of steel. On the other hand, TiC can improve the wear resistance of tool steel, effectively enhancing its fatigue resistance. In this invention, the Ti content is controlled at 0.12% to 0.15%.

[0046] Nd: Its main function is to modify inclusions, playing a role in oxide metallurgy, forming spherical and dot-like Mn-SOBN-Nd inclusions as well as NdS, NdLa2O3, and Nd2O2S inclusions, thereby improving the fatigue resistance and wear resistance of tool steel. Therefore, the present invention controls the Nd content at 0.0020% to 0.0030%.

[0047] Nitrogen (N): A certain amount of nitrogen can combine with Ti, B, and C to form Ti(C,N), BN, etc., playing a role in precipitation strengthening and grain refinement in steel. N has three main functions in steel: first, it increases the absolute content of dissolved nitrogen and combined nitrogen in the steel; second, it increases the ratio of dissolved nitrogen to total nitrogen in the steel, promoting nitrogen solubility; and third, it increases the nitrogen content in AlN and BN precipitates, as well as the ratio of nitrogen in these precipitates to total nitrogen, promoting the precipitation of combined nitrogen in the form of AlN and BN phases, thus improving the strength of tool steel. In this invention, the nitrogen content is controlled between 0.026% and 0.030%.

[0048] B: In tool steel, it combines with nitrogen to form BN particles. BN particles have good stability and are not easily dissolved at high temperatures, thus improving cutting performance. In addition, B segregation at austenite grain boundaries can suppress the precipitation of proeutectoid ferrite, allowing carbides to precipitate diffusely in the low-temperature region. In this invention, the B content is controlled at 0.0040% to 0.0060%.

[0049] Alt (AlN) reacts with nitrogen (N) to form AlN, which refines the grain size and inhibits grain growth. Combined with specific processes, it can effectively improve the strength of tool steel. This invention controls the Alt content to be 0.015%–0.055%.

[0050] It should be noted that obtaining high-nitrogen cutter steel with good strength-toughness matching, wear resistance of the cutting surface, excellent corrosion resistance and formability, which is beneficial for cutting tool thinning and improved fatigue life, is related not only to chemical composition design but also to the production process. Only through reasonable composition design combined with optimized production processes can the desired microstructure be obtained, thereby effectively improving the wear resistance, fatigue resistance and corrosion resistance of the cutter steel.

[0051] This invention also provides a method for producing high-nitrogen tool steel plates. The process includes: steelmaking → continuous casting → heating → hot continuous rolling → laminar flow cooling → coiling → blanking → forming → welding → heat treatment, etc. The specific steps are as follows:

[0052] 1. Converter smelting: Top-and-bottom blowing converter is used for smelting. The ratio of molten iron to scrap steel in the converter is (80%~90%):(10%~20%). Nitrogen is blown from the bottom throughout the process, and lime and lightly calcined dolomite are used for slag formation. The oxygen blowing time is 12~16 minutes. When the final composition and temperature meet the requirements for tapping, tapping begins. The tapping spout should be round. Ferrochrome and ferromanganese are added during the tapping process. The tapping temperature is controlled between 1700℃ and 1720℃. The amount of slag discharged during tapping is controlled to be ≤5kg / ton of steel.

[0053] 2. LF Refining: Fluorite is added to create white slag. Molten steel is then poured into the LF refining furnace, and a deoxidizer (aluminum-manganese ferrooxidizer) is added simultaneously at a rate of 400-600 kg per furnace. The Mn content in the steel is fine-tuned to within a set range. During refining, ferroboron, ferrochrome, and ferrotitanium are added for alloying treatment, followed by the addition of Nd alloy for inclusion modification treatment. Soft blowing argon stirring time is ≥10 min, and the settling time is ≥14 min. During refining, the thickness of the top slag in the molten steel furnace is maintained between 30 and 50 mm.

[0054] RH Vacuum Treatment: After RH vacuum treatment, the molten steel is fed with wire to increase nitrogen content. Manganese nitride cored wire is fed into the RH vacuum-treated steel using a wire feeder at a rate of 4.4–5.5 m / t of steel and a feeding speed of 2.6–3.0 m / s. The bottom blowing device of the ladle is shut off during the feeding process. The nitrogen content of the manganese nitride cored wire is 7%–8%, the manganese content is 85%–90%, and the remainder is impurities.

[0055] Continuous casting: A straight arc continuous casting machine is used for continuous casting, with the superheat controlled between 10 and 20°C, and the fixed length of the continuous casting billet is 10.7m.

[0056] Heating: The continuous casting billet is heated using a hot charging process. The heating temperature is 1220-1260℃ and the soaking time is 40-50 minutes to fully dissolve the alloying elements in the steel.

[0057] Hot continuous rolling: The roughing mill adopts a 3+3 pass rolling mode, with an initial rolling temperature of 1140~1180℃. The finishing mill adopts a 7-stand hot continuous rolling mode, with cooling water introduced between the stands, and a final rolling temperature of 855~885℃.

[0058] Coiling: The coiling temperature is 450-480℃. After coiling, the steel plate is placed in a slow cooling pit for slow cooling treatment. It can only be moved after slow cooling for more than 72 hours.

[0059] Forming: Cutting according to the set dimensions, such as cutting into round blanks of different diameters according to the dimensions of a metal saw and forming the saw back.

[0060] Welding: Welding is performed according to the set structure, such as welding the back of the saw to the saw teeth of a certain shape to form the required cutting tool.

[0061] Heat treatment: After quenching (temperature 820~850℃) + tempering (temperature 200~260℃), a high-nitrogen cutting tool steel with a certain strength and toughness matching and good wear resistance is obtained.

[0062] After a series of inspections and checks, the high-nitrogen cutting tool steel is obtained as a finished product.

[0063] The high-nitrogen cutting tool steel prepared by the method described in this invention has superior performance compared to ordinary cutting tool steel. On the basis of good strength and toughness matching, it further achieves the goals of cutting tool thinning, improved fatigue life, and excellent corrosion resistance.

[0064] The following embodiments are implemented based on the technical solution of the present invention, and provide detailed implementation methods and specific operation processes. However, the scope of protection of the present invention is not limited to the following embodiments.

[0065]

Example

[0066] The chemical composition of Examples 1-6 and Comparative Examples 1-2 is shown in Table 1.

[0067] Table 1 Chemical composition (mass fraction / %)

[0068]

[0069]

[0070] The steelmaking-coiling process parameters for Examples 1-6 and Comparative Examples 1-2 are shown in Table 2.

[0071] Table 2 Steelmaking-coiling process parameters

[0072]

[0073] The heat treatment process regimes of Examples 1-6 and Comparative Examples 1-2 are shown in Table 3.

[0074] Table 3 Heat Treatment Process Regulations

[0075]

[0076] The mechanical properties, fatigue resistance, and corrosion resistance of the finished products from Examples 1-6 and Comparative Examples 1-2 are shown in Table 4.

[0077] Table 4. Mechanical properties, fatigue resistance, and corrosion resistance of the finished product.

[0078]

[0079]

[0080] Note: In the table, the first parameter of the item "Improved Fatigue Performance" is compared with Comparative Example 1, and the second parameter is compared with Comparative Example 2.

[0081] In Examples 1-6 and Comparative Examples 1-2, the continuous casting speed was 1.1 m / min, and the thickness of the finished steel plate was 6.0 mm.

[0082] The metallographic structure and inclusion morphology of the high-nitrogen cutting tool steel plate prepared in Example 1 are as follows: Figure 1 , Figure 2 As shown in the photograph, the metallographic structure of the tool steel plate prepared in Comparative Example 1 is as follows: Figure 3 As shown. From Figure 1 , Figure 3 It can be seen that the metallographic structure of Example 1 is tempered martensite with a grain size of 11 to 11.5, while the metallographic structure of Comparative Example 1 is tempered martensite with a grain size of 9 to 9.5.

[0083] As can be seen from Table 3, the finished steel plates prepared in Examples 1 to 6 have a yield strength ≥1600MPa, a tensile strength ≥1800MPa, and an elongation after fracture A. 50 ≥20%. As a key parameter for evaluating the performance of cutting tool steel, compared with ordinary cutting tool steel, the finished steel plates prepared in Examples 1-6 have improved fatigue resistance by more than 800,000 cycles, corrosion resistance by more than 80%, and good formability.

[0084] Conclusion: The high-nitrogen cutting tool steel prepared according to the chemical composition and production process described in this invention has excellent wear resistance, fatigue resistance and corrosion resistance after heat treatment, and is particularly suitable for products with high fatigue performance requirements.

[0085] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing high-nitrogen cutting tool steel, characterized in that, The chemical composition of the steel, by mass percentage, is as follows: C 0.75%–0.85%, Si 0.10%–0.20%, Mn 1.50%–1.70%, P≤0.015%, S 0.08%–0.10%, Cr 0.60%–0.80%, Ti 0.12%–0.15%, Alt 0.015%–0.055%, N 0.026%–0.030%, B 0.0040%–0.0060%, Nd 0.0020%–0.0030%, with the balance being Fe and unavoidable impurities; The microstructure of the finished steel plate is tempered sorbite; the properties of the finished steel plate are: yield strength ≥1600MPa, tensile strength ≥1800MPa, elongation after fracture A50 ≥20%; the process flow includes steelmaking, continuous casting, heating, hot continuous rolling, laminar flow cooling, coiling, blanking, forming, welding and heat treatment; the following processes are controlled: 1) Steelmaking; Converter smelting: Top and bottom blowing converter smelting is adopted, the ratio of molten iron to scrap steel is (80%~90%): (10%~20%), and nitrogen is blown from the bottom throughout the process; lime and lightly calcined dolomite are used for slag making, and oxygen blowing time is 12~16 minutes; ferrochrome and ferromanganese are added during the tapping process, and the tapping temperature is controlled at 1700℃~1720℃; LF refining: Fluorite is added to make white slag, molten steel is poured into the LF refining furnace, and deoxidizer is added at the same time to deoxidize. The Mn content in the steel is finely adjusted to the set range. Ferroboron, ferrochrome and ferrotitanium are added for alloying. Nd alloy is added to modify the inclusions. Soft blowing argon stirring time is ≥10min and the calming time is ≥14min. RH vacuum treatment: After the vacuum is broken, the molten steel is fed with wire to increase nitrogen. The feeding rate of manganese nitride cored wire is 4.4 to 5.5 m / t steel, and the feeding speed is 2.6 to 3.0 m / s. The bottom blowing device of the ladle is turned off during the wire feeding process. 2) Continuous casting; A straight arc continuous casting machine is used for continuous casting, and the superheat of continuous casting is controlled at 10-20℃. 3) Heating; The hot charging and hot delivery process is adopted, with the continuous casting billet heating temperature at 1220-1260℃ and the soaking time at 40-50min. 4) Hot continuous rolling; Rough rolling: A 3+3 pass rolling mode is adopted, and the rough rolling start temperature is 1140~1180℃; Finishing rolling: 7-stand hot continuous rolling is adopted, with cooling water introduced between stands, and the final rolling temperature is 855~885℃; 5) Winding; The coiling temperature is 450-480℃; the coiled steel plate is placed in a slow cooling pit for slow cooling treatment, and the slow cooling time is not less than 72 hours. 6) Heat treatment; The quenching temperature is 820-850℃, and the tempering temperature is 200-260℃.

2. The method for preparing a high-nitrogen cutting tool steel according to claim 1, characterized in that, The thickness of the finished steel plate is 1.5 to 20.0 mm.

3. The method for preparing a high-nitrogen cutting tool steel according to claim 1, characterized in that, During the converter smelting process, the amount of slag discharged during tapping is controlled to be ≤5kg / ton of steel.

4. The method for preparing a high-nitrogen cutting tool steel according to claim 1, characterized in that, During the LF refining process, aluminum-manganese-iron deoxidizer is used, with an addition amount of 400-600 kg / furnace of steel; the thickness of the top slag in the molten steel is maintained between 30 mm and 50 mm.

5. The method for preparing a high-nitrogen cutting tool steel according to claim 1, characterized in that, During the RH vacuum treatment process, the nitrogen content in the manganese nitride cored wire is 7% to 8%, the manganese content is 85% to 90%, and the remainder is impurities.

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