High-strength gray cast iron and smelting method thereof

Abstract

The invention relates to gray cast iron and a smelting method thereof, in particular to high-strength gray cast iron and a smelting method thereof. According to the high-strength gray cast iron and the smelting method thereof, the defect that in the prior art, alloying is adopted for improving the strength of the gray cast iron, and consequently the texture of the gray cast iron is poor is overcome, the tensile strength of an attached casting test bar of the high-strength gray cast iron is larger than 300 MPa, and the hardness is larger than 190 HB; the metallographic structure of the high-strength gray cast iron in an as-cast state can achieve more than 98% of pearlite, less than 1% of cementite and more than 90% of A-type graphite, and the strength and the metallographic structure of the alloyed gray cast iron are effectively improved.

Description

TECHNICAL FIELD

[0001] The present application relates to gray cast iron and its smelting method, in particular to a high-strength gray cast iron and its smelting method. BACKGROUND

[0002] The traditional gray cast iron production mainly uses pig iron, which is medium and high silicon cast pig iron. In order to ensure that the gray cast iron reaches the mechanical properties of medium and high strength, a certain amount of alloying elements need to be added for alloying treatment. The alloying elements have a certain undercooling tendency, which causes the metallographic structure of the thin-walled part to be poor, the undercooled graphite to be more, and the undercooled graphite to account for more than 10%, thereby causing the mechanical properties to be poor. SUMMARY

[0003] In view of the problem that in the prior art, the undercooling tendency of alloying elements is not well controlled in the smelting process of gray cast iron, the metallographic structure of the thin-walled part is poor, the undercooled graphite is more, and the undercooled graphite accounts for more than 10%, thereby causing the mechanical properties to be poor, a high-strength gray cast iron and its smelting method are provided. The purpose of the present application is achieved by a high-strength gray cast iron, the weight percentage of the elements contained in the gray cast iron is: carbon 3.1%-3.4%; silicon 1.7%-2.1%; manganese 0.6%-1.0%; phosphorus ≤0.05%; sulfur 0.06%-0.12%; copper 0.5%-1.0%; chromium 0.15%-0.4%; nickel 0.3%-0.7%, titanium 0.015%-0.025%, and the rest is iron.

[0004] The smelting method of the high-strength gray cast iron comprises the following steps:

[0005] S1: using a medium-frequency electric furnace to smelt, adding raw materials, the melting temperature is less than 1380℃, after melting, adding alloying elements;

[0006] S2: after melting, sampling and detecting, the sampling temperature is controlled between 1440℃-1460℃, and the overheating temperature is ensured between 1490℃-1510℃;

[0007] S3: using a nitrogen-hydrogen-oxygen instrument to sample and detect the nitrogen content, the nitrogen content meets the standard, and the titanium content in the furnace is adjusted according to the titanium content in front of the furnace;

[0008] S4: after selecting an inoculant for inoculation treatment, pouring is completed at a preset time.

[0009] In one embodiment, the raw materials are pig iron, scrap steel, machine iron and silicon iron, and carbon additive.

[0010] In one embodiment, during feeding, the scrap steel, machine iron and pig iron are weighed according to the proportion and added in sequence, and then the carbon additive and silicon iron are added.

[0011] In one embodiment, the carbon raiser is a high-nitrogen carbon raiser with a nitrogen content between 5000 and 8000 ppm.

[0012] In one embodiment, the particle size of the inoculant is 3mm-7mm.

[0013] In one embodiment, the amount of the progesterone used is 0.4%-0.6%.

[0014] In one embodiment, the inoculant is a barium silicon inoculant.

[0015] This invention overcomes the defect of poor microstructure in gray cast iron caused by alloying to improve its strength in existing technologies. It provides a high-strength gray cast iron and its smelting method. The cast test bar of the high-strength gray cast iron has a tensile strength greater than 300 MPa and a hardness greater than 190 HB. The microstructure of the high-strength gray cast iron in the as-cast state can reach more than 98% pearlite, less than 1% cementite, and more than 90% type A graphite, effectively improving the strength and microstructure of alloyed gray cast iron. Attached Figure Description

[0016] Appendix Figure 1 Metallographic features of attached test block 1 Figure 1 ;

[0017] Appendix Figure 2 Metallographic features of attached test block 1 Figure 2 ;

[0018] Appendix Figure 3 Attached is a metallographic image of the corroded test block 1;

[0019] Appendix Figure 4 Attached is a 500x metallographic image of the cast test block 1 after corrosion. Detailed Implementation

[0020] To facilitate understanding of the present invention, a more complete description will be given below with reference to the accompanying drawings. Preferred embodiments of the invention are shown in the drawings. However, the invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of the invention.

[0021] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0022] A high-strength gray cast iron, comprising the following elements in percentage by weight: carbon 3.1%-3.4%; silicon 1.7%-2.1%; manganese 0.6%-1.0%; phosphorus ≤0.05%; sulfur 0.06%-0.12%; copper 0.5%-1.0%; chromium 0.15%-0.4%; nickel 0.3%-0.7%, titanium 0.015%-0.025%, and the rest is iron.

[0023] A smelting method of the high-strength gray cast iron, comprising the following steps:

[0024] S1: smelting by using a medium-frequency electric furnace, adding raw materials, melting temperature less than 1380℃, and adding alloying elements after melting; specifically, smelting by using a medium-frequency electric furnace, weighing pig iron, scrap steel, machine iron, carbon additive and ferrosilicon according to the proportion of 5% pig iron, 77% scrap steel, 15% machine iron and 0.8% ferrosilicon by using an electronic scale, adding the materials in the order of scrap steel-machine iron-pig iron, adding the carbon additive and ferrosilicon with the second batch of scrap steel to ensure stable material quality, melting temperature less than 1380℃, and adding alloying elements after melting of the pig iron.

[0025] S2: sampling and detecting after completion of melting, sampling temperature controlled between 1440℃-1460℃, and overheating temperature ensured between 1490℃-1510℃;

[0026] S3: sampling and detecting nitrogen content by using a nitrogen-hydrogen-oxygen instrument, nitrogen content meeting the standard, and adjusting titanium content in the furnace according to titanium content in front of the furnace; preferably, nitrogen content meeting the standard between 80-120ppm, and adjusting titanium content in the furnace to titanium 0.015%-0.025% by using ferrotitanium according to Ti content in front of the furnace.

[0027] S4: selecting an inoculant for inoculation treatment, and completing pouring at a preset time after inoculation, inoculation time accounting for more than 80% of tapping time to ensure uniform inoculation, and completing pouring within 10 minutes after treatment to avoid spheroidization inoculation recession.

[0028] The pig iron can be Z18. The carbon additive is a high-nitrogen carbon additive, wherein nitrogen content is between 5000-8000ppm. The alloying elements are added in the amount of copper 0.5%-1.0%, chromium 0.15%-0.4%, and nickel 0.3%-0.7%. Ferrotitanium is added to adjust titanium content in the furnace to titanium 0.015%-0.025%. The inoculant has a particle size of 3mm-7mm. The inoculant is used in the amount of 0.4%-0.6%. The inoculant is a silicon-barium inoculant.

[0029] The high-strength gray cast iron of the present application is smelted with carbon equivalent controlled at 3.1%-3.4%, and excessive carbon equivalent will make graphite thick, increase the volume fraction, and result in the decrease of the strength and hardness of the gray cast iron, and low carbon equivalent can refine graphite and increase the amount of pro-eutectic austenite, but tends to increase the eutectic transformation undercooling degree, thus creating favorable conditions for the formation of cementite eutectic, and the casting and processing performance are poor. Manganese can strongly reduce the eutectic and eutectoid transformation temperature of the cast iron, expand the austenite zone, reduce the activity of carbon, and make the cast iron undergo eutectic transformation and eutectoid transformation at a lower temperature. Manganese can promote and stabilize pearlite, and increase the strength of the gray iron part. The manganese content in the test is 0.6%-1.0%. Phosphorus increases the brittleness of the cast iron, so the content in the gray cast iron part is preferably lower, and the content of phosphorus in the present application is ≤0.05%. Sulfur hinders graphitization on one hand, and forms compounds with manganese on the other hand, and can be used as the heterogeneous core of graphite, which is beneficial to the precipitation of graphite. Nickel can promote graphitization and inhibit the formation of carbide, and the content of sulfur in the present application is 0.06%-0.12%. The higher the content of nickel, the lower the strength of the cast gray iron with pearlite as the matrix, and the strength is obviously increased with the increase of the content of nickel under the austenitizing condition. Nickel has completely different effects when the matrix is pearlite and austenite. Under the conventional condition, nickel forms solid solution in ferrite, but the brittle phase cementite cannot absorb nickel. In the austenitized cast iron, nickel exists in the austenite and ferrite phases, although the content is different, but the matrix strength is increased. Preferably, the content of nickel in the present application is 0.3%-0.7%. Copper increases the stable system eutectic transformation temperature and reduces the metastable system transformation temperature during eutectic transformation, and has a weak graphitization effect. However, copper is usually not used alone as an element to promote graphitization, because the size of the cast iron eutectic group increases after the addition of copper, and graphite tends to be thick. Nickel promotes the formation of ferrite and D-type graphite, and is a strong carbide-forming element. In addition, iron has a strong affinity with oxygen and nitrogen, and can form corresponding extremely stable compounds with them. Titanium can resist the harmful effect of nitrogen.

[0030] The high-strength gray cast iron produced by reasonable chemical composition control has a tensile strength of the attached cast test bar ≥300Mpa and a hardness ≥190HB, and the detection data of the hardness of the attached cast test block and the body of the high-strength gray cast iron material are shown in Table 1. The size of the attached cast test block is 30mm*200mm, the test block is placed at a position with a wall thickness of 40mm, and the body hardness is detected.

[0031] Table 1: Actual measured values of the attached cast test block and the body performance of the high-strength gray cast iron

[0032]

[0033] Reasonable inoculation process is adopted to make the metallographic structure of the attached cast test block good, the A-type graphite is >90%, and the carbide is <1%, and Table 2 is the metallographic structure of the attached cast test block.

[0034] Table 2 Metallographic structure of high-strength gray cast iron attached test block

[0035]

[0036] The present application overcomes the defect of poor structure of gray cast iron caused by using alloying to improve the strength of gray cast iron in the prior art, and provides a high-strength gray cast iron and a smelting method thereof, wherein the tensile strength of the attached test bar of the high-strength gray cast iron is greater than 300 MPa, and the hardness is greater than 190 HB, and the metallographic structure of the high-strength gray cast iron in the as-cast state can reach more than 98% pearlite, less than 1% cementite, and more than 90% A-type graphite, thereby effectively improving the strength and metallographic structure of the alloyed gray cast iron.

[0037] The technical features of the above-described embodiments can be combined in any manner, and to make the description concise, all possible combinations of the technical features in the above-described embodiments are not described, however, as long as the combinations of the technical features do not contradict each other, they should be considered as falling within the scope of the present application.

[0038] The above-described embodiments only express several implementation manners of the present application, and the description is relatively specific and detailed, but it should not be understood as a limitation on the scope of the patent of the present application. It should be pointed out that, for ordinary skilled persons in the art, several modifications and improvements can be made without departing from the concept of the present application, and these all fall within the protection scope of the present application. Therefore, the protection scope of the patent of the present application should be subject to the appended claims.

Claims

1. A high-strength gray cast iron and its smelting method, characterized in that, The smelting method includes the following steps: S1: Use a medium-frequency electric furnace for smelting, add raw materials, the melting temperature is less than 1380℃, and add alloying elements after melting; S2: After melting, take samples for testing. The sampling temperature is controlled between 1440℃ and 1460℃, and the overheating temperature is guaranteed to be between 1490℃ and 1510℃. S3: Use a nitrogen, hydrogen and oxygen analyzer to sample and test the nitrogen content. If the nitrogen content meets the standard, adjust the titanium content in the furnace according to the titanium content in front of the furnace. S4: After selecting an inoculant for inoculation treatment, pouring is completed within a preset time.

2. The high-strength gray cast iron smelting method according to claim 1, characterized in that, The raw materials are pig iron, scrap steel, machine-made iron and ferrosilicon, and carbon raisers.

3. The high-strength gray cast iron smelting method according to claim 2, characterized in that, When adding materials, weigh out scrap steel, machine iron, and pig iron in proportion and add them in order, followed by carbon raiser and ferrosilicon.

4. The high-strength gray cast iron smelting method according to claim 1, characterized in that, The particle size of the inoculant is 3mm-7mm.

5. The high-strength gray cast iron smelting method according to claim 1, characterized in that, The dosage of the inoculant is 0.4%-0.6%.

6. The high-strength gray cast iron smelting method according to claim 2, characterized in that, The carbon raiser is a high-nitrogen carbon raiser with a nitrogen content between 5000 and 8000 ppm.

7. The high-strength gray cast iron smelting method according to claim 1, characterized in that, The inoculant is a barium silicon inoculant.

8. A type of gray cast iron suitable for the high-strength gray cast iron smelting method according to any one of claims 1-7, characterized in that, It includes the following elements in weight percentage: carbon 3.1%-3.4%, silicon 1.7%-2.1%, manganese 0.6%-1.0%, phosphorus ≤0.05%, sulfur 0.06%-0.12%, copper 0.5%-1.0%, chromium 0.15%-0.4%, nickel 0.3%-0.7%, titanium 0.015%-0.025%, and the remainder is iron.

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