CN109609835B

CN109609835B - High-toughness wear-resistant nodular cast iron and preparation process and application thereof

Info

Publication number: CN109609835B
Application number: CN201910030522.9A
Authority: CN
Inventors: 龚剑
Original assignee: SUZHOU QINMEIDA PRECISION MACHINERY CO Ltd
Current assignee: SUZHOU QINMEIDA PRECISION MACHINERY CO Ltd
Priority date: 2019-01-14
Filing date: 2019-01-14
Publication date: 2021-04-30
Anticipated expiration: 2039-01-14
Also published as: CN109609835A

Abstract

The invention discloses high-strength and high-toughness wear-resistant nodular cast iron and a preparation process and application thereof, wherein the content of C, Si, Mn, P, S and other components is reasonably controlled in the preparation process, a proper amount of inoculant and nodulizer are added at proper time, the content of final Si is controlled to be 2.7-2.9%, the graphite is in an A type shape, the size is 7-8 grades, the graphite is uniformly distributed, the nodulizing rate is more than 90%, the base structure is more compact, the tensile strength of a casting is optimized to be more than 700MPa, and the high-strength wear-resistant nodular cast iron is better suitable for a high-performance crankshaft.

Description

High-toughness wear-resistant nodular cast iron and preparation process and application thereof

Technical Field

The invention relates to graphite cast iron, in particular to high-strength and high-toughness wear-resistant nodular cast iron and a preparation process and application thereof; belongs to the technical field of graphite cast iron.

Background

With the increasing demand of people on the service performance of automobiles, the power of the automobiles is also increased, and the rotation of a crankshaft is a power source of an engine. The crankshaft is used as an important part for converting the up-and-down (reciprocating) motion of a connecting rod into a circulating (rotating) motion, the crankshaft material in the prior art is made of carbon structural steel or nodular cast iron and has two important parts, namely a main journal and a connecting rod journal, the main journal is arranged on a cylinder body, the connecting rod journal is connected with a connecting rod big end hole, and the connecting rod small end hole is connected with a cylinder piston, so that the crankshaft is a typical crank block mechanism. With the enhancement of automobile power, the crankshaft is required to bear larger alternating load, so that higher requirements on the toughness and the reliability of the crankshaft material are also provided.

According to the morphology of graphite in cast iron, gray cast iron can be divided into: ordinary gray cast iron, nodular cast iron, malleable cast iron, and vermicular cast iron. The ductile iron is a high-strength cast iron material developed in the 20 th century and the fifties, the comprehensive performance of the ductile iron is close to that of steel, and the ductile iron is successfully used for casting parts which are complex in stress and high in requirements on strength, toughness and wear resistance based on the excellent performance of the ductile iron. Spheroidal graphite cast iron has been rapidly developed into a cast iron material with wide application, namely, the spheroidal graphite cast iron is mainly referred to as 'iron instead of steel'.

The nodular cast iron is spheroidized and inoculated to obtain spheroidal graphite, so that the mechanical properties of the cast iron are effectively improved, and particularly, the plasticity and toughness are improved, so that the strength of the cast iron is higher than that of carbon steel. The graphite morphology and the size grade of graphite nodules in the graphite structure have a crucial influence on the mechanical properties of the material, so that the basic structure of the nodular cast iron needs to be optimized in order to obtain a material with better properties.

Disclosure of Invention

In order to solve the defects of the prior art, the invention aims at providing the high-strength and high-toughness wear-resistant nodular cast iron, and also aims at providing a preparation process and application of the high-strength and high-toughness wear-resistant nodular cast iron.

In order to achieve the above object, the present invention adopts the following technical solutions:

a preparation process of high-strength-toughness wear-resistant nodular cast iron comprises the following steps:

s1, melting: adding the ductile iron return material and the scrap steel into a frequency conversion furnace, heating and smelting to melt the material in the frequency conversion furnace into molten iron, detecting the component content in the molten iron by using a carbon-silicon instrument, detecting the content of other components by using a spectroscopic spectrometer, powering off after the analysis is finished, carrying out first deslagging operation, adding SiC and other alloy ingredients according to a target value, and adjusting the component content until the component content of the molten iron detected by the carbon-silicon instrument and the spectroscopic spectrometer reaches the target value;

s2, soup discharging: transferring the ladle to the front of a furnace to carry out soup discharging operation, controlling the soup discharging temperature, adding an inoculant and a nodulizer into the ladle to carry out nodulizing treatment, adding an inoculation block into a sand mold before molding and pouring, and finally, the Si content is 2.7-2.9%, and casting to obtain a high-strength, high-toughness and wear-resistant nodular cast iron casting with the graphite grade of 7-8.

Preferably, the alloy ingredients can be flexibly selected according to actual requirements, and include: one or more of Mn, P, S, Cu, Cr, Sn, Sb and Mg.

More preferably, in the foregoing step S1, the target value of the molten iron content is: by mass percentage, 3.3-3.5% of C, 1.6-1.8% of Si, 0.3-0.5% of Mn, 0.1% of P, 0.05% of S, 0.4-0.7% of Cu, 0.1% of Cr, Sn: 0.03 to 0.06%, Sb: 0.003-0.006 percent of the total weight of the alloy, 0.03-0.05 percent of Mg0.003-0.006 percent of the total weight of the alloy, and the balance of Fe and incidental impurities.

C (carbon) is an element that becomes a graphite structure, and its content has an important influence on the base structure of the material and the graphite morphology. Therefore, in the present invention, the content of C is 3.3 to 3.5%.

Si is an element for promoting graphite crystallization, if the content of Si is not enough, the shape stability of the high-toughness wear-resistant nodular cast iron is not facilitated, if the content of Si is too much, the graphite shape becomes larger, the ferrite content in the base structure is increased, and the improvement of the mechanical property of the high-toughness wear-resistant nodular cast iron is not facilitated, so the content of Si in the stock solution is 1.6-1.8%.

The Mn has the function of stabilizing the structure of pearlite, the preferable range is 0.3% -0.5%, the chilling trend is avoided, and the hardness and the wear resistance are effectively improved.

The content of P should not exceed 0.1% otherwise it would precipitate iron phosphide, a hard substance, in the base structure, leading to premature wear of the product used in conjunction with the cast iron product.

The content of S needs to be strictly controlled, the S prevents graphite from spheroidizing, and if the content of S is too high, the graphite is easy to flake, and stable high-toughness wear-resistant nodular cast iron cannot be obtained, so the preferable range is not more than 0.05%.

Cu is a stabilizing element of a pearlite structure, has an influence on mechanical properties such as tensile strength, and the like, and the content thereof is 0.4 to 0.7%, so that the cost is high and the amount of use needs to be controlled.

The content of Cr is not more than 0.1%, and the Cr is combined with carbon in the cast iron base material to precipitate carbide, so that the mechanical property of the cast iron is optimized through precipitation strengthening of the base material.

The content of Sn is 0.03 to 0.06%, which can alleviate graphite segregation and prevent graphite oxidation and exfoliation due to internal oxidation.

The content of Sb is 0.003-0.006%, which is a harmful element inevitably present in the spheroidal graphite cast iron, and therefore, it is advantageous to refine the graphite by controlling it within a safe ratio (less than 0.006%).

The content of Mg is 0.03-0.05%, which is an important factor influencing the internal defects of the castings, and the control of the residual magnesium amount has direct influence on the shapes and the thicknesses of the castings.

Preferably, in the step S2, the inoculant is 0.5% of the molten iron by mass.

More preferably, in the step S2, the inoculation block accounts for 0.3% of the molten iron by mass.

Still preferably, the chemical composition of the inoculant and inoculant block is the same, and the inoculant and inoculant block comprise, in mass percent (total mass of inoculant and inoculant block is taken as 100%): 70-75% of Si, 0.5-1.5% of Ba, 1-3% of Ca, less than or equal to 2.5% of Al, and the balance of Fe and incidental impurities.

More preferably, the nodulizer is generally about 0.1% by mass of the molten iron, and the amount of magnesium actually remaining in the molten iron is corrected by detecting the amount of magnesium actually remaining in the molten iron in accordance with the requirement of mg0.03 to 0.05% in actual production, wherein the nodulizer comprises, in terms of mass percentage (the total mass of the nodulizer is 100%): 42-47% of Si, 13-17% of Mg, 4-5% of Ca, 2.7-3.1% of RE and the balance of Fe, wherein RE represents the total of lanthanide rare earth elements.

The invention also discloses high-strength and high-toughness wear-resistant nodular cast iron which is prepared by adopting the preparation process, the tensile strength of the cast iron reaches 700MPa, the graphite is in the form of A type, the graphite size is 7-8 grades, and the nodularity is more than 90%.

The application of the high-strength and high-toughness wear-resistant nodular cast iron on the crankshaft specifically comprises the following steps: the nodular cast iron is used as a raw material, and a casting is obtained through pouring, sand washing, grinding and cleaning.

The technical scheme of the invention has the following advantages: the method reasonably controls the content of each component such as C, Si, Mn, P, S and the like, and adds a proper amount of inoculants and nodulizers at proper time, so that the final Si content is controlled to be 2.7-2.9%, the graphite is in an A shape, the graphite is 7-8 grades in size, the graphite is uniformly distributed, the nodulizing rate is more than 90%, the base structure is more compact, the tensile strength of the casting is optimized to be more than 700MPa, and the method is better suitable for high-performance crankshafts.

The material of the invention achieves the high requirement of the customer on the mechanical performance by improving the material, and can adopt a green sand production line with lower cost, thereby reducing the production cost and the purchasing cost of the customer and improving the market competitiveness. In the traditional crankshaft processing, in order to achieve the high-strength, high-toughness and high-wear-resistance performance similar to the product of the invention, the performance requirement of the casting is generally achieved by adopting an iron mold sand-coated casting process and heat treatment. At least hundreds of thousands of investment in the aspect of mold cost is required, the mold cost is far higher than 10 thousands of mold cost of the process, and the cost in the aspect of casting process is higher than 2000 yuan/ton of green sand casting process.

Drawings

FIG. 1 is a metallographic picture of a common prior art ductile iron;

FIG. 2 is a metallographic diagram of the high-toughness wear-resistant ductile iron of the present invention.

Detailed Description

The invention is described in detail below with reference to the figures and the embodiments.

Comparative examples 1 to 2 and example 1 are the target values of the respective components in step S1 in the preparation process of each test piece:

TABLE 1 chemical composition of comparative examples 1 to 2 and example 1

TABLE 2 chemical composition of inoculant/inoculant blocks in comparative examples 1-2 and example 1

TABLE 3 chemical composition of nodulizer in comparative examples 1-2 and example 1

The nodular cast iron of the present invention was prepared according to the chemical compositions of tables 1 to 3 and cast into a block-shaped test material for performance testing, and comparative examples 1 to 2 were the same as the preparation process of example 1 except that the control of the components in the preparation process was different, and they all included the following steps:

s1, melting: adding ductile iron return materials and scrap steel into a frequency conversion furnace, heating and smelting to melt materials in the frequency conversion furnace into molten iron, detecting the component content of the molten iron by using a carbon-silicon instrument, detecting the content of other components by using a spectroscopic spectrometer, carrying out first deslagging operation after analysis is finished and adding SiC and other alloy ingredients to adjust the component content according to the target value (shown in table 1) of each embodiment until the content of the molten iron component detected by the carbon-silicon instrument and the spectroscopic spectrometer reaches the target value shown in table 1;

s2, soup discharging: the ladle is transferred to the front of a furnace for soup discharging operation, the soup discharging temperature is controlled, an inoculant and a nodulizer are added into the ladle for nodulizing, an inoculation block is added into a sand mold after molding and before pouring, special attention needs to be paid to the fact that the final Si content of the embodiment 3 is 2.7-2.9% (detected by a carbon-silicon instrument), and a nodular cast iron casting is obtained through casting.

In step S1, the components are strictly controlled and detected, and in step S2, an inoculant block and a nodulizer are added to adjust the base structure and optimize the material performance.

The following performance tests were performed on the castings:

(1) metallographic structure: adopting metallographic microscope to detect, the brand: OLYMPUS, model: BX 41M;

(2) mechanical properties: adopt tensile test machine to detect, the brand: shimadzu, model number: AG-X-PLUS;

(3) hardness: adopt the detection of cloth formula hardness machine, the brand: jinjing Jingji, model BRINELL-BO 3;

(4) graphite grade: and (5) evaluating the alloy phase detection result graph.

The performance test results are shown in table 4:

TABLE 4 results of testing the properties of examples 1 to 3

The metallographic image of comparative example 1 is shown in FIG. 1, and the metallographic image of example 1 is shown in FIG. 2.

Comparing the metallographic structures (the proportions of fig. 1 and fig. 2) of the nodular cast iron shown in fig. 1 and fig. 2, it can be seen that the graphite form of the cast iron in example 1 of the invention is changed remarkably, the graphite form is a type a, the size is 7-8 grades, the graphite form is uniformly distributed, the spheroidization rate is more than 90%, the base structure is more compact, the tensile strength of the cast iron is optimized to be more than 700MPa, and the cast iron is better suitable for high-performance crankshafts. Therefore, the nodular cast iron is high-strength and high-toughness wear-resistant nodular cast iron, the tensile strength, the yield strength and the elongation are optimized, the wear resistance is obviously improved, and the nodular cast iron can be used as a raw material for casting crankshafts with stronger automobile power and higher requirements.

The foregoing illustrates and describes the principles, general features, and advantages of the present invention. It should be understood by those skilled in the art that the above embodiments do not limit the present invention in any way, and all technical solutions obtained by using equivalent alternatives or equivalent variations fall within the scope of the present invention.

Claims

1. A preparation process of high-strength and high-toughness wear-resistant nodular cast iron is characterized by comprising the following steps:

s1, melting: returning ductile iron and scrap steel into the frequency conversion furnace, heating and smelting to melt materials in the frequency conversion furnace into molten iron, detecting the component content by using a carbon-silicon instrument, detecting the content of other components by using a spectroscopic spectrometer, powering off to remove slag for the first time after the analysis is finished, adding SiC and other alloy ingredients according to a target value to adjust the component content, wherein the alloy ingredients comprise: one or more of Mn, P, S, Cu, Cr, Sn, Sb and Mg until the content of the molten iron components detected by a carbon-silicon instrument and a spectroscopic spectrometer reaches a target value, wherein the target value of the content of the molten iron components is as follows: by mass percentage, C3.4%, Si 1.68%, Mn 0.47%, P0.02%, S0.021%, Cu 0.57%, Cr 0.03%, Sn: 0.045%, Sb: 0.0043%, Mg0.037%, and the balance Fe and incidental impurities;

s2, soup discharging: transferring the ladle to the front of a furnace for soup discharging operation, controlling the soup discharging temperature, adding an inoculant during ladle transferring, and simultaneously adding an inoculation block into a cavity, wherein the inoculant and the inoculation block have the same chemical components and comprise the following components in percentage by mass: 70-75% of Si, 0.5-1.5% of Ba, 1-3% of Ca, less than or equal to 2.5% of Al, and the balance of Fe and accompanying impurities; and adding a nodulizer for nodulizing, wherein the nodulizer comprises the following components in percentage by mass: 42-47% of Si, 13-17% of Mg, 4-5% of Ca, 2.7-3.1% of RE and the balance of Fe, wherein RE represents the total of lanthanide rare earth elements, the final Si content is 2.7-2.9%, the graphite grade is 7-8 grade, and the graphite form is A type.

2. The process for preparing the high-strength and high-toughness wear-resistant nodular cast iron according to claim 1, wherein in the step S2, the inoculant accounts for 0.5 percent of the mass of the molten iron.

3. The process for preparing the high-toughness wear-resistant ductile iron according to claim 1, wherein in the step S2, the inoculation block accounts for 0.3% of the molten iron by mass.

4. The high-strength and high-toughness wear-resistant nodular cast iron is characterized by being prepared by the preparation process according to any one of claims 1 to 3, the tensile strength of the cast iron reaches 700MPa, the graphite is in the form of A, the graphite size is 7-8 grades, and the nodularity is more than 90%.

5. The use of the high-toughness wear-resistant ductile iron according to claim 4 in crankshafts.