CN111041336A

CN111041336A - Low-temperature impact-resistant high-strength as-cast nodular cast iron and production method thereof

Info

Publication number: CN111041336A
Application number: CN201911409637.5A
Authority: CN
Inventors: 陈淦; 孔庆柱; 李兆霞; 刘清良; 许文青; 胥京国; 吕海峰
Original assignee: SHANDONG HUIJIN CO Ltd
Current assignee: SHANDONG HUIJIN CO Ltd
Priority date: 2019-12-31
Filing date: 2019-12-31
Publication date: 2020-04-21

Abstract

The invention discloses low-temperature impact-resistant high-strength as-cast nodular cast iron and a production method thereof, and relates to the field of new materials, wherein the nodular cast iron comprises the following chemical components: w (C) 3.5% to 3.9%, w (Si) 1.8% to 2.1%, and CE +1/3(Si) 4.3% to 4.6%; w (Mn) is less than or equal to 0.2 percent; w (P) is less than or equal to 0.03 percent; w (S) 0.006% -0.015%; w (Cu) 0.15-0.25%; w (Mg) 0.025-0.045%, w (RE) 0.01-0.02%; the spheroidization coefficient K of the rest microelements is less than or equal to 1 +/-0.1 percent. The cast produced by the invention is not added with noble metals such as Ni for tempering and is not subjected to heat treatment, so the cost is lower. The cast-state material has higher strength and toughness and excellent low-temperature impact property, so that the cast-state material is suitable for a low-temperature working environment with higher mechanical property requirement and the service life is prolonged.

Description

Low-temperature impact-resistant high-strength as-cast nodular cast iron and production method thereof

Technical Field

The invention relates to the field of materials, in particular to low-temperature impact resistant high-strength as-cast nodular cast iron and a production method thereof.

Background

Nodular cast iron is an important metal structural material and is increasingly widely applied in the fields of automobiles, machinery, ships and the like. Some mechanical components, such as automotive crankshafts, require not only higher strength but also higher toughness. At present, the spheroidal graphite cast iron with high strength and high toughness is mainly obtained by heat treatment, and the method has high cost, complex process and low yield, so that the production of the spheroidal graphite cast iron with high strength and high toughness in an as-cast state has great practical significance. Some machines and parts work in high and cold environments for a long time, such as gearboxes, hubs and bases of wind power equipment, railway and subway accessories, locomotive and vehicle accessories, accessories of petroleum and petrochemical equipment and the like, and the nodular iron castings are required to have good low-temperature performance at the same time. At present, the market demand of low-temperature high-toughness ductile iron castings is increasing.

In the national standard GB1348-2009, the low-temperature impact performance requirements are only stipulated for two low-grade ductile iron materials QT350-22L, QT 400-18L. As the grade is increased, the toughness of the nodular cast iron is reduced, and the low-temperature toughness is lower, so that the application requirement cannot be met. Therefore, the low-temperature impact performance requirements of the domestic and foreign nodular cast iron standards on the grades above QT450-10 are not specified.

Disclosure of Invention

The invention aims to provide low-temperature impact-resistant high-strength as-cast nodular cast iron and a production method thereof, and aims to solve the problems that the toughness of the nodular cast iron is reduced along with the increase of the grade and the low-temperature toughness is lower so as not to meet the application requirements in the background art.

In order to achieve the purpose, the invention adopts the following technical scheme:

the low-temperature impact resistant high-strength as-cast nodular cast iron comprises the following chemical components: w (C) 3.5% to 3.9%, w (Si) 1.8% to 2.1%, and CE +1/3(Si) 4.3% to 4.6%; w (Mn) is less than or equal to 0.2 percent; w (P) is less than or equal to 0.03 percent; w (S) 0.006% -0.015%; w (Cu) 0.15-0.25%; w (Mg) 0.025-0.045%, w (RE) 0.01-0.02%; the spheroidization coefficient of the rest microelements is less than or equal to 1 +/-0.1 percent.

A method for producing nodular cast iron comprises the following steps: the method comprises the following steps:

the method comprises the following steps: preparing pure raw materials

The high-purity pig iron is pure and rust-free, and comprises the following chemical components: w (C) is more than or equal to 3.3 percent, w (Si) is 0.4-0.7 percent, w (Mn) is 0.05-0.10 percent, w (P) is less than or equal to 0.03 percent, w (S) is less than or equal to 0.02 percent, w (Ti) is less than or equal to 0.025 percent; 11 trace elements sigma-w (Cr + V + Mo + Sn + Sb + Pb + Bi + Te + As + B + Al) are less than or equal to 0.07 percent, and the anti-spheroidizing coefficient

K＝4.4Ti+1.6Al+2.0As+2.3Sn+5.0Sb+290Pb+370Bi≤0.36；

Step two: the same material is used for returning iron, and shot blasting is performed:

the high-quality crystal carburant for high-temperature graphitization treatment comprises the following chemical components:

w (C) 99.5-99.8%, w (S) 0.015-0.05%, w (N) 0.001-0.003%; the absorption rate is more than or equal to 95 percent; the granularity is 0.5-5 mm;

step three: before spheroidization, the components of molten iron smelted in the electric furnace are adjusted as follows: w (C) 3.75-4.15%, w (Si) 0.8-1.1%, and CE +1/3(Si) 4.32-4.52%; w (Mn) is less than or equal to 0.2 percent; w (P) is less than or equal to 0.03 percent; w (S) is less than or equal to 0.025 percent; w (Cu) 0.15-0.25%;

step four: spheroidizing and primary inoculation are carried out on the molten iron:

the spheroidization temperature is 1510 +/-10 ℃. The nodulizer adopts Mg6RE1, and comprises the following chemical components: w (Mg) 5.5-6.5%, w (RE) 0.5-1.5% (Ce/RE is more than or equal to 46%), w (Ca) 1.5-3.0%, w (Si) 35-45%, w (Mn) is less than or equal to 1.0%, w (Ti) is less than or equal to 0.5%, w (MgO) is less than or equal to 0.65%, w (Al) is less than or equal to 0.5%, and the balance is Fe; the granularity is 5-30 mm, and the adding amount is 1.1-1.4% of the weight of the processed iron liquid;

the inoculant adopts Si-Ba alloy inoculant and comprises the following chemical components: w (Si) 65-70%, w (Ba) 4-6%, granularity 2-5 mm, and addition amount 0.3-0.5%.

Step five: secondary inoculation

Namely ladle to ladle inoculation: adding a secondary inoculant to the bottom of the pouring ladle, and pouring the spheroidized molten iron into the pouring ladle for secondary inoculation. The inoculant adopts Si-Ba inoculant, and the components are the same as those of the first time; the granularity is 1-2 mm; the adding amount is 0.08 to 0.3 percent;

step six: pouring and three-time inoculation:

pouring the spheroidized and secondarily inoculated molten iron into a sand mold at a certain pouring temperature;

during the pouring process, the third inoculation is instantaneous inoculation along with the flow, the stream inoculant is Si-Bi alloy, and the chemical components are as follows: w (Si) 68-75%, w (Bi) 0.8-1.2%, w (Ca) less than 2.0%, w (RE) less than 1.2%, w (Al) less than 1.5%; the granularity is 0.2-0.7 mm; the adding amount is 0.08-0.15% of the amount of the cast iron liquid.

Step seven: shakeout:

and solidifying and forming the poured molten iron in a sand mold, and carrying out sand shakeout along with mold cooling to below 500 ℃ to form the nodular cast iron.

Further, the high-quality medium and low-carbon waste steel in the step one is pure and free of rust; the chemical components are as follows: w (C) is less than or equal to 0.5 percent, w (Si) is less than or equal to 0.4 percent, w (Mn) is less than or equal to 0.3 percent, w (P) is less than or equal to 0.03 percent, w (S) is less than or equal to 0.02 percent, w (Cr) is less than or equal to 0.1 percent, w (Cu) is less than or equal to 0.3 percent, and w (Ni) is less than or equal to 0.3 percent.

Furthermore, the smelting temperature of the electric furnace is 1530-1550 ℃.

And further, standing the molten iron obtained in the third step at a high temperature for 5-10 minutes, and removing slag.

Further, the spheroidizing method in the fourth step is a cover-ladle method and a dam-type spheroidizing ladle, the height of the dam is 30-50mm higher than the height of the charging material, the ladle cover and the ladle top are compacted by a sealing refractory material, the depth-diameter ratio is 1.5-2, and the ladle is preheated to be more than 800 ℃ before use; and (3) drying a nodulizer, an inoculant and the small steel sheets before use.

Further, in the step four, when spheroidizing, the coverage needs to be strengthened: and tightly covering by using a small steel sheet. The small steel sheet is made of stainless low-carbon steel, and the addition amount of the small steel sheet is 1.2 to 1.3 percent of the weight of the treated iron liquid; sequentially feeding a nodulizer, an inoculant and a small steel sheet and tamping layer by layer; the feeding height is lower than the top surface of the dam.

Further, the takt time from tapping and spheroidizing to the end of pouring the whole ladle of molten iron does not exceed 10 minutes.

Has the advantages that: the cast produced by the invention does not add noble metals such as Ni and the like, and does not carry out heat treatment, so the cost is lower. The cast-state material has higher strength and toughness and excellent low-temperature impact property, so that the cast-state material is suitable for a low-temperature working environment with higher mechanical property requirement and the service life is prolonged.

Drawings

FIG. 1 is a dimensional chart of an impact test block of the present invention;

FIG. 2 is a schematic diagram showing the relative positions of a sample and a pendulum impact tester support and an anvil block according to the present invention;

FIG. 3 is a graph of tensile specimen dimensions in accordance with the present invention;

FIG. 4 is a schematic diagram of a method of sampling a tensile specimen and an impact specimen in accordance with the present invention.

Detailed Description

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments.

Referring to fig. 1-4, a method of producing ductile iron: the method comprises the following steps:

the method comprises the following steps: preparing pure raw materials

K＝4.4Ti+1.6Al+2.0As+2.3Sn+5.0Sb+290Pb+370Bi≤0.36；

Step five: secondary inoculation

step six: pouring and three-time inoculation:

Step seven: shakeout:

The high-quality medium and low carbon waste steel in the step one is pure and free of rust; the chemical components are as follows: w (C) is less than or equal to 0.5 percent, w (Si) is less than or equal to 0.4 percent, w (Mn) is less than or equal to 0.3 percent, w (P) is less than or equal to 0.03 percent, w (S) is less than or equal to 0.02 percent, w (Cr) is less than or equal to 0.1 percent, w (Cu) is less than or equal to 0.3 percent, and w (Ni) is less than or equal to 0.3 percent.

The smelting temperature of the electric furnace is 1530-1550 ℃.

And step three, standing the iron liquid at high temperature for 5-10 minutes before tapping, and removing slag.

The spheroidizing method in the fourth step is a cover-ladle method and a dam-type spheroidizing ladle, the height of the dam is 30-50mm higher than the height of the charging material, the ladle cover and the ladle top are compacted by a sealing refractory material, the depth-diameter ratio is 1.5-2, and the ladle is preheated to more than 800 ℃ before use; and (3) drying a nodulizer, an inoculant and the small steel sheets before use.

In the fourth step, when spheroidizing, the coverage needs to be strengthened: and tightly covering by using a small steel sheet. The small steel sheet is made of stainless low-carbon steel, and the addition amount of the small steel sheet is 1.2 to 1.3 percent of the weight of the treated iron liquid; sequentially feeding a nodulizer, an inoculant and a small steel sheet and tamping layer by layer; the feeding height is lower than the top surface of the dam.

The takt time from tapping and spheroidizing to the end of pouring the whole ladle of molten iron is not more than 10 minutes.

No Ni is added in the material manufacturing process, no heat treatment is needed, the cast tensile strength Rm is more than or equal to 450MPa, and the elongation A is more than or equal to 18 percent. The average value of three samples of the impact energy of a Charpy (V-notch) single-cast sample at the low temperature of minus 20 ℃ is not less than 12J, and the individual value is not less than 9J; the average value of the impact energy of the single cast sample with Charpy (V-notch) at the low temperature of minus 40 ℃ is not less than 9J, and the individual value is not less than 6J. The normal temperature performance and the low temperature performance are both superior to the standard mark QT 400-18L.

The low-temperature impact resistant high-strength and high-toughness as-cast nodular cast iron produced by the method takes high-purity pig iron and pure medium-low carbon steel scrap as raw materials, adopts high-quality high-efficiency nodulizer and inoculant for modification treatment, and ensures that the casting has high nodulizing rate, fine structure, good mechanical property and good low-temperature resistance through means of component optimization, process improvement, beat production and the like.

According to GB/T1348-2009, the material designation is QT 450-18L. The method is suitable for wind power equipment, engineering machinery, railways and other nodular iron castings which work in low-temperature environments with high mechanical property requirements.

The mechanical characteristics of QT450-18L and related materials are shown in table 1, the impact energy of the single-cast sample with the V-shaped notch is shown in table 2, and the metallographic characteristics of QT450-18L are shown in table 3.

TABLE 1 QT450-18L and mechanical Properties of related materials

TABLE 2 impact energy of QT450-18L and associated Material V notch Single cast test specimens

TABLE 3 QT450-18L metallographic requirements

Nodularity of spheroidization	Size of graphite nodule	Agglomerate graphite	Phosphorus eutectic	Cementite	Pearlite
						≥90％	6 to 8 grades	≤20％	Is free of	Is free of	≤25％

The production process and the method optimize the components of the casting, improve the as-cast structure and improve the normal-temperature mechanical property and the low-temperature impact property.

One, improve the nodularity and the amount of the spherical graphite

(1) High-purity pig iron is used, the purity of raw materials is improved, the contents of S and other spheroidizing elements are strictly controlled, good spheroidization is ensured, and the addition amount of a nodulizer and Mg in a casting are reduced_{Disabled person}Content, the white taste tendency is reduced.

(2) Improve the coverage of the nodulizer. The nodulizer, the inoculant and the small steel sheet are sequentially added, each layer is flattened and tamped by using a flat hammer, and the iron chips on the upper covering layer are changed into the small steel sheets, so that the melting point of the covering layer is improved to delay the spheroidization reaction time, the spheroidization reaction is ensured to be carried out under the condition of a certain iron liquid pressure head, the oxidation burning loss of Mg is reduced, and the absorption rate of the Mg reaches more than 70%. Final Mg_{Disabled person}The spheroidization effect of Mg is fully exerted by keeping the content between 0.025 percent and 0.045 percent.

(3) The RE content is reduced. The Rare Earth (RE) has the function of counteracting the adverse effect of the counter-spheroidizing element on graphite spheroidization, meanwhile, the rare earth element (mainly Ce) is easy to promote the graphite to form a cluster shape and a flaky shape, and the roundness of graphite nodules is influenced by too much RE content, so that the quantity of the spherical graphite is further increased, and the RE content is properly low. Because the content of the spheroidizing trace elements in the high-purity pig iron is strictly controlled, the low-RE spheroidizing agent is selected, the content of the RE in the spheroidizing agent is selected to be 0.5-1.5 percent, and the content of the RE in the nodular graphite is not more than 20 percent.

(4) In order to improve the absorptivity of magnesium, the content of magnesium is determined to be about 6 percent under the production condition of an electric furnace and when the RE content in a nodulizer is 1 percent.

(5) In order to increase the content of available magnesium in the nodulizer and improve the nodulizing effect, the content of MgO in the nodulizer is regulated to be not more than 1/10 of the content of magnesium.

By combining the factors, the nodulizer Mg6RE1 is finally determined, and the components of w (Mg) 5.5-6.5%, w (RE) 0.5-less than 1.5%, w (Ca) 1.5-3.0%, w (Si) 35-45%, w (Mn) less than or equal to 1.0%, w (Ti) less than or equal to 0.5%, w (MgO) less than or equal to 0.65%, and w (Al) less than or equal to 0.5%.

(6) Slag is removed in time after spheroidizing, so that the phenomenon of resulfurization is reduced.

(7) And no more than 8 minutes are spent from the completion of the spheroidization treatment to the completion of the casting.

By fine batching and strict process operation, the nodulizing rate is improved to more than 2 grade, and the agglomerated graphite is lower than 20 percent.

And secondly, the purity of the molten iron is improved.

(1) Clean raw materials are used, impurities are reduced from the source, and strict control standards are established for chemical components and physical states of furnace materials.

(2) The high-temperature standing treatment enables the iron liquid to carry out self-deoxidation reaction at the moment, so that the iron liquid has greatly reduced oxidation and slag inclusion tendency, and becomes high-quality iron liquid with high-temperature low oxidation.

(3) Because the absorption rate of Mg is improved, the content of effective Mg in the nodulizer is improved, and the content of the spheroidizing elements in the high-purity pig iron is reduced, the addition amount of the nodulizer is reduced, and the slagging amount is reduced.

(4) By using the patent cover in our factory, the spheroidization is carried out in a relatively closed bag, the contact with the outside air is reduced, and the oxidation slag inclusion is reduced.

(5) The original iron liquid is subjected to slag skimming before spheroidization, and the slag is timely and fully skived after the spheroidization is finished.

Thirdly, the inoculation effect is enhanced. By means of spheroidizing ladle bottom Si-Ba inoculation, ladle-down Si-Ba inoculation and stream-following Si-Bi three-time inoculation, inoculation recession is effectively prevented. The Si-Ba alloy long-acting inoculant prolongs the recession time, and the Si-Bi alloy inoculant is adopted in stream inoculation, so that the number of graphite balls is greatly increased, the solidification quality is improved, the comprehensive performance of the ductile iron is correspondingly improved, and the shrinkage cavity, shrinkage porosity and white tendency are reduced.

And fourthly, optimizing the components, including reasonable chemical components and matrix tissues. According to the influence of each chemical element on the structure and the performance of the casting, through repeated development, the reasonable chemical components of the casting are determined, and the production scheme is controlled in a narrow range determined according to the wall thickness of the casting and the pouring temperature, so that the matrix structure of QT450-18L in Table 3, the mechanical property in Table 1 and the low-temperature property in Table 2 are ensured. The chemical composition scheme is set forth in the following technical scheme.

And fifthly, improving the raw materials. The raw materials are optimized, the influence of trace interfering spheroidizing elements is fully considered, and the K value of the spheroidizing elements is strictly controlled to be less than 1 +/-0.1; spheroidizing and inoculating agent are improved, alloy type Si-Bi composite inoculating agent is selected for stream inoculation, and the alloy type Si-Bi composite inoculating agent is cooperated with a material factory to develop a spheroidizing agent Mg6RE1, so that the components are optimized and adjusted; the furnace burden is purified and is clean and rustless; and C is regulated by using a crystal carburant subjected to high-temperature graphitization treatment.

And sixthly, producing at a beat. The production method also has the advantages that standardized management, takt time production and consistent shakeout temperature are required, the time from the completion of spheroidization to the end of pouring is ensured to be not more than 8 minutes, and the takt time from the tapping and spheroidization to the end of pouring the whole ladle of molten iron is not more than 10 minutes for facilitating the practical control of production.

The measures can greatly improve the comprehensive indexes of the casting: the spheroidization rate is more than or equal to 90 percent, and the bulk graphite is not more than 20 percent.

The cast produced by the invention is not added with noble metals such as Ni for tempering and is not subjected to heat treatment, so the cost is lower. The cast-state material has higher strength and toughness and excellent low-temperature impact property, so that the cast-state material is suitable for a low-temperature working environment with higher mechanical property requirement and the service life is prolonged.

The technical scheme adopted by the invention is as follows:

material chemical composition (mass fraction) requirement

C +1/3(Si) 4.3-4.6%, w (C) 3.5-3.9%, w (Si) 1.8-2.1%.

w(Mn)≤0.2％，w(Cu)0.15％～0.25％。

w(S)0.006％～0.015％，w(P)≤0.03％。

w(Mg)0.025％～0.045％,w(RE)0.01％～0.02％。

The spheroidization coefficient of the trace elements is less than 1 +/-0.1.

Raw material requirements

The pig iron for smelting adopts high-purity pig iron and comprises the following chemical components: w (C) is more than or equal to 3.3 percent, w (Si) is 0.4-0.7 percent, w (Mn) is 0.05-0.10 percent, w (P) is less than or equal to 0.03 percent, w (S) is less than or equal to 0.02 percent, w (Ti) is less than or equal to 0.025 percent; 11 trace elements sigma-w (Cr + V + Mo + Sn + Sb + Pb + Bi + Te + As + B + Al) are less than or equal to 0.07 percent, and the anti-spheroidizing coefficient

K＝4.4Ti+1.6Al+2.0As+2.3Sn+5.0Sb+290Pb+370Bi≤0.36.

The smelting scrap steel is medium and low carbon steel, and comprises the following chemical components: w (C) is less than or equal to 0.5 percent, w (Si) is less than or equal to 0.4 percent, w (Mn) is less than or equal to 0.3 percent, w (P) is less than or equal to 0.03 percent, w (S) is less than or equal to 0.02 percent, w (Cr) is less than or equal to 0.05 percent, w (Cu) is less than or equal to 0.3 percent, and w (Ni) is less than or equal to 0.1 percent.

The returned materials for smelting are made of the same material.

And (3) purifying pig iron, scrap steel and foundry returns, such as polishing, cleaning and derusting.

The smelting carburant is a high-quality crystal carburant subjected to high-temperature graphitization treatment, and comprises the following chemical components: w (C) 99.5-99.8%, w (S) 0.015-0.05%, w (N) 0.001-0.003%; the absorption rate is more than or equal to 95 percent; the particle size is 0.5-5 mm.

Electric furnace smelting

The smelting temperature is 1530-1550 ℃.

Adjusting the components of the original molten iron: w (C) 3.75-4.15%, w (Si) 0.8-1.1%, and CE (C + 1/3) (Si) 4.32-4.52% (C loss 0.25%, spheroidizing inoculation Si addition 1%, adjusted as the case may be); w (Mn) is less than or equal to 0.2 percent, P is less than or equal to 0.03 percent, w (S) is less than or equal to 0.025 percent; w (Cu) is less than or equal to 0.15-0.25%, and the total amount of trace elements is less than or equal to 0.1%.

Standing at high temperature for 5-10 minutes, and removing slag.

Spheroidizing inoculation

The iron liquid with the components is spheroidized, and the spheroidized ladle bottom is inoculated for one time.

Spheroidizing method-bag covering method

A dam-type spheroidizing bag is adopted, and the dam is 30-50mm higher than the feeding surface; the ladle cover and the top of the ladle body are tightly and compactly covered, and the depth-diameter ratio of the ladle body is 1.5-2.

The residual iron in the spheroidizing ladle is cleaned before use, dried and preheated to orange yellow (more than or equal to 800 ℃).

The nodulizer, the inoculant and the small steel sheet are dried before use, and are added on the opposite side of the molten iron injection side separated by the dam: feeding, flattening and tamping the nodulizer, the inoculant and the small steel sheets layer by layer in sequence.

Selection and addition of a nodulizer: the nodulizer adopts Mg6RE1, and the chemical composition is as follows: w (Mg) 5.5-6.5%, w (RE) 0.5-1.5% (Ce/RE is more than or equal to 46%), w (Ca) 1.5-3.0%, w (Si) 35-45%, w (Mn) is less than or equal to 1.0%, w (Ti) is less than or equal to 0.5%, w (MgO) is less than or equal to 0.65%, w (Al) is less than or equal to 0.5%, and the rest is Fe; the particle size is 5-30 mm.

The adding amount is 1.1-1.4% of the weight of the processed iron liquid.

Selection and addition of a primary inoculant: adopts Si-Ba inoculant, and comprises 65-70% of chemical components w (Si) and 4-6% of w (Ba); the granularity is 1.5-3 mm; the addition amount is 0.3 to 0.5 percent.

Covering the small steel sheets: the small steel sheet is stainless low-carbon steel; the adding amount is 1.2 to 1.3 percent of the weight of the processed molten iron.

The spheroidizing temperature is 1510 +/-10 ℃.

Slagging off: and after the spheroidization is finished, removing slag for at least three times.

Pouring

And after the bag is turned over, the slag is removed for one time.

Secondary inoculation: namely ladle to ladle inoculation, the inoculation is carried out at the bottom of a pouring ladle, Si-Ba inoculant is adopted, and the chemical components are as follows: w (Si) 65% -70%, w (Ba) 4% -6%; the granularity is 1-2 mm; the addition amount is 0.1 to 0.3 percent.

The molten iron after spheroidizing and secondary inoculation is poured into a sand mold at a reasonable pouring temperature (1350 ℃ -1450 ℃) according to the key wall thickness of the casting.

And (3) tertiary inoculation: and carrying out third inoculation in the pouring process, namely instantaneous inoculation along with flow. The inoculant adopts Si-Bi alloy inoculant; the chemical components are as follows: w (Si) 68-75%, w (Bi) 0.8-1.2%, w (Ca) less than 2.0%, w (RE) less than 1.2%, w (Al) less than 1.5%; the granularity is 0.2-0.7 mm; the adding amount is 0.1-0.15% of the amount of the cast iron liquid.

And pouring a spectrum test block at the later casting stage of each spheroidizing ladle.

And casting a Y-shaped standard test block with the thickness of 25mm at the later casting stage of each spheroidizing ladle.

Shakeout

And (4) after casting, cooling to the temperature below 500 ℃ along with the mold, and shakeout.

Practice for production of as-cast QT450-18L

The material QT450-18L of a certain bracket, the weight 18kg and the critical wall thickness 30 mm.

The final chemical composition w (%) was determined as follows:

TABLE 3

Selecting high-quality raw materials

High-purity pig iron: clean and rust-free, chemical composition w (%) is as follows:

C	Si	Mn	P	S	Ti	sum of the remaining trace elements
							4.2	0.5	0.046	0.018	0.017	0.019	＜0.07％

TABLE 4

Scrap steel: clean and rust-free, chemical composition w (%) is as follows:

TABLE 5

C	Si	Mn	P	S	Cr	Cu	Ni
								0.16	0.18	0.21	0.02	0.01	0.03	0.027	0.057

The recycled iron is selected, and the chemical components w (%) are as follows:

TABLE 6

C	Si	Mn	P	S	Cr	Cu	Mg	RE
									3.8	2.0	0.15	0.02	0.011	0.03	0.15	0.030	0.018

Shot blasting the returned materials for 15 minutes.

The absorption rate of the crystal carburant is more than or equal to 95 percent; the particle size is 0.5-5 mm, and the chemical components w (%) are as follows:

TABLE 7

C	S	N
			99.6	0.03	0.002

Electric furnace smelting

The smelting temperature is 1530-1550 ℃.

Adjusting the chemical composition w (%) of the original molten iron in the electric furnace as follows:

TABLE 8

The P, S, Mn content in the components is mainly controlled by raw materials, and desulfurization treatment is carried out when the S content exceeds the standard; C. adding carburant, scrap steel or ferrosilicon alloy into the electric furnace according to C, Si analysis value before adjusting the Si content; adding cathode copper into the electric furnace for adjusting Cu according to the Cu analysis value before adjusting the original molten iron; the CE values for each protocol were controlled as the target values in the table. + -. 0.05%.

The adjustment needs to be held: the components of the original iron liquid are determined according to the final chemical components, the spheroidizing inoculation element increment and the process burning loss condition of the elements; the C loss is calculated by 0.25 percent,the loss of Si is 5 percent of the silicon amount, and the burning loss of each element is adjusted according to the actual condition; si content increased by spheroidizing inoculation_{Spheroidizing}+Si_{Inoculation I}+Si_{Inoculation II}+Si_{Inoculation III}≈1％。

Standing the adjusted molten iron at high temperature for 8 minutes, and completely removing slag.

Spheroidizing inoculation

The iron liquid of the components is spheroidized and the ladle bottom is inoculated for the first time, and the weight of the iron liquid treated for the first time is 700 Kg.

Spheroidizing-ladle covering method: the dam type spheroidizing bag is qualified after being repaired according to the technical requirements before use.

Preheating the spheroidized ladle to be more than or equal to 800 ℃ before use;

the nodulizer, the inoculant and the small steel sheets are dried before use, the nodulizer, the inoculant and the small steel sheets are sequentially added layer by layer, and are leveled and tamped by a flat-headed hammer; the height of the dam is 30-50mm lower than that of the dam;

selection and addition of a nodulizer: the nodulizer is Mg6RE1, and the chemical composition w (%) is as follows:

TABLE 9

Mg	RE	Si	Others
				6	1.1	43	Qualified

The particle size is 5-30 mm.

The amounts added were calculated as follows:

according to our research experiment and production practice, w (Mg) can be controlled when the nodular cast iron is produced by using Mg6RE1 nodulizer and high-purity pig iron_{Disabled person}) Between 0.025 and 0.045. Due to Mg in nodular cast iron_{Disabled person}When the content of Mg exceeds 0.04%, the solidification range is sharply widened, and the tendency of shrinkage is increased, so that Mg is added on the premise of ensuring good spheroidization_{Disabled person}Should be as low as possible.

The formula for calculating the addition amount of the nodulizer is as follows:

wherein: lambda is the amount of nodulizer added, S_{Liquid iron}The content of S in the raw iron liquid before spheroidization, S_{After spheroidizing}Is the S content, Mg, in molten iron after spheroidization_{Disabled person}Is the Mg content in the casting, Mg_{Attenuation of}η is the absorption rate of Mg, the decrement of Mg from spheroidization to the end of casting_{Content (wt.)}Is the Mg content in the nodulizer.

The test results show that: amount of desulfurization (S)_{Liquid iron}-S_{After spheroidizing}) If < 0.01%, the amount of magnesium desulfurated is 0.76 (S)_{Liquid iron}-S_{After spheroidizing}) Less than 0.0076%; the magnesium has an attenuation rate of 0.001-0.003% per minute, the average value is 0.002%, the time from the completion of the spheroidization reaction to the end of pouring is less than or equal to 8 minutes, and the in-mold attenuation of 1 minute is increased according to the wall thickness of the casting, so that the Mg content_{Attenuation of}0.002% × (8+1) ═ 0.018%,% absorption of magnesium η ≥ 70%,% Mg_{Content (wt.)}According to the proportion of 6%; mg (magnesium)_{Disabled person}According to the proportion of 0.025%;

lambda < (0.0076% + 0.025% + 0.018%)/(70% × 6%), i.e. lambda < 1.2%, should meet the requirement of graphite spheroidization.

The addition of the nodulizer is determined to be 1.3 percent by combining the production experience, and the requirement of graphite spheroidization is completely met. The spheroidizing silicon content is as follows: si_{Spheroidizing}＝1.3％×43％×95％＝0.53％。

Selection and addition of a primary inoculant: the grain size of the Si-Ba inoculant is 1.5-3 mm, and the chemical components w (%) are as follows:

watch 10

Si	Ba
		69	5.5

The addition amount is 0.5 percent, the silicon content increased by the primary inoculation is as follows: si_{Inoculation I}＝0.5％×69％× 95％＝0.328％。

Covering the small steel sheets: the small steel sheets should be clean and rust-free, otherwise, the rust removal treatment is carried out. Chemical composition w (%) is as follows:

TABLE 11

C	Si	Mn	P	S	Cr	Cu	Ni
								0.21	0.17	0.25	0.012	0.01	0.04	0.013	0.028

The adding amount is 1.2 to 1.3 percent;

the spheroidizing temperature is 1510 +/-10 ℃; the weight of the cast iron is 700 +/-10 Kg.

Slagging off: and removing the slag for multiple times after the spheroidization is finished, and scattering a layer of slag conglomeration agent on the surface of the molten iron after each time of slag removal, and continuously and rapidly carrying out. The slag is removed for at least three times, and the time is not more than 2 minutes.

Pouring

Secondary inoculation: performing secondary inoculation when the spheroidized ladle is poured into a pouring ladle, and adopting a Si-Ba inoculant with the granularity of 1-2 mm; chemical composition w (%) is as follows:

TABLE 12

Si	Ba
		69	5

The addition amount is 0.15%, the secondary inoculation silicon increasing amount is as follows: si_{Inoculation II}＝0.15％×69％× 95％＝0.098％。

The pouring temperature is 1360-1400 ℃ determined according to the wall thickness of the casting, and the casting is poured into the clay green sand mold at a specified pouring speed by using an automatic pouring machine.

Instantaneous inoculation (tertiary inoculation) is carried out along with flow in the pouring process; the inoculant for the Si-Bi alloy is adopted, and the chemical components w (%) are as follows:

watch 13

Si	Bi	Ca	RE	Al
					72	0.85	1.3	0.83	1.21

The granularity is 0.2-0.7 mm; the adding amount is 0.12 percent of the amount of the poured iron liquid, and the third inoculation silicon increasing amount is as follows: si_{Inoculation III}＝0.12％×72％×95％＝0.082％。

Pouring time of the whole ladle of molten iron: and from the tapping spheroidization time to the end of the whole ladle of molten iron casting, the time is not more than 10 minutes.

Shakeout: and cooling to below 500 ℃ for shakeout after casting.

Detection of

Chemical composition

And carrying out spectral analysis on the spectral test block.

TABLE 14 results of spectral analysis (trace elements not counted)

Metallographic structure

TABLE 15 metallographic examination results from Y-shaped test blocks

Mechanical properties

TABLE 15 mechanical Property measurements from Y-shaped test blocks

Low temperature impact energy

TABLE 16 Charpy impact test results for Charpy (V-notch) specimens from Y-type single cast test blocks

And (4) conclusion: the performance index of the nodular cast iron produced by the embodiment method reaches the QT450-18L requirement.

The above description is only for the purpose of illustrating the present invention, and the scope of the present invention is not limited thereto, and any person skilled in the art can substitute or change the technical solution of the present invention and the inventive concept within the technical scope of the present invention.

Claims

1. The low-temperature impact resistant high-strength as-cast nodular cast iron is characterized in that: the ductile cast iron comprises the following chemical components: w (C) 3.5% to 3.9%, w (Si) 1.8% to 2.1%, and CE +1/3(Si) 4.3% to 4.6%; w (Mn) is less than or equal to 0.2 percent; w (P) is less than or equal to 0.03 percent; w (S) 0.006% -0.015%; w (Cu) 0.15-0.25%; w (Mg) 0.025-0.045%, w (RE) 0.01-0.02%; the spheroidization coefficient K of the rest microelements is less than or equal to 1 +/-0.1 percent.

2. A method for producing nodular cast iron comprises the following steps: the method is characterized in that: the method comprises the following steps: the method comprises the following steps: preparing pure raw materials

K＝4.4Ti+1.6Al+2.0As+2.3Sn+5.0Sb+290Pb+370Bi≤0.36；

step three: before spheroidization, the components of molten iron smelted in the electric furnace are adjusted as follows: w (C) 3.75-4.15%, w (Si) 0.8-1.1%, and CE +1/3(Si) 4.32-4.52%; w (Mn) is less than or equal to 0.2 percent; w (P) is less than or equal to 0.03 percent; w (S) is less than or equal to 0.025 percent; w (Cu) 0.15-0.25%; the sum of the mass fractions of the other trace elements is less than or equal to 0.1 percent; the balance being Fe;

Step five: secondary inoculation

step six: pouring and three-time inoculation:

during the pouring process, the third inoculation is instantaneous inoculation along with the flow, the stream inoculant is Si-Bi alloy, and the chemical components are as follows: w (Si) 68-75%, w (Bi) 0.8-1.2%, w (Ca) less than 2.0%, w (RE) less than 1.2%, w (Al) less than 1.5%; the granularity is 0.2-0.7 mm; the adding amount is 0.08-0.15% of the amount of the cast iron liquid;

step seven: shakeout:

3. The method for producing nodular cast iron according to claim 2, wherein the high-quality medium-low carbon steel scrap in the first step is pure and free of rust; the chemical components are as follows: w (C) is less than or equal to 0.5 percent, w (Si) is less than or equal to 0.4 percent, w (Mn) is less than or equal to 0.3 percent, w (P) is less than or equal to 0.03 percent, w (S) is less than or equal to 0.02 percent, w (Cr) is less than or equal to 0.1 percent, w (Cu) is less than or equal to 0.3 percent, and w (Ni) is less than or equal to 0.3 percent.

4. The method for producing nodular cast iron according to claim 2, wherein the melting temperature of the electric furnace is 1530 to 1550 ℃.

5. The method for producing nodular cast iron according to claim 2, wherein the three steps of the liquid iron are performed with high-temperature standing for 5-10 minutes before slag removal.

6. The method for producing nodular cast iron according to claim 2, wherein the spheroidizing treatment in the fourth step is a ladle method, a dam-type spheroidizing ladle, the height of the dam is 30-50mm higher than the height of the charging material, the ladle cover and the ladle top are compacted by a sealing refractory material, the depth-diameter ratio is 1.5-2, and the spheroidizing ladle is preheated to more than 800 ℃ before use; and (3) drying a nodulizer, an inoculant and the small steel sheets before use.

7. A method for producing nodular cast iron according to claim 2, wherein the spheroidization in the fourth step requires the strengthening of the covering: and tightly covering by using a small steel sheet. The small steel sheet is made of stainless low-carbon steel, and the addition amount of the small steel sheet is 1.2 to 1.3 percent of the weight of the treated iron liquid; sequentially feeding a nodulizer, an inoculant and a small steel sheet and tamping layer by layer; the feeding height is lower than the top surface of the dam.

8. The method for producing ductile iron according to claim 2, wherein a tact time from the tapping and spheroidization to the end of the pouring of the whole ladle of molten iron is not more than 10 minutes.