CN110640088B

CN110640088B - Process control method for shrinkage porosity defect of iron casting

Info

Publication number: CN110640088B
Application number: CN201910951310.4A
Authority: CN
Inventors: 刘宇; 张宝权; 崔振雷; 徐海涛
Original assignee: Faw Jiefang Dalian Diesel Engine Co ltd; FAW Jiefang Automotive Co Ltd
Current assignee: Faw Jiefang Dalian Diesel Engine Co ltd; FAW Jiefang Automotive Co Ltd
Priority date: 2019-10-08
Filing date: 2019-10-08
Publication date: 2021-04-13
Anticipated expiration: 2039-10-08
Also published as: CN110640088A

Abstract

The invention belongs to the technical field of new materials and casting, and discloses a process control method for shrinkage porosity defects of iron castings. The method adopts the combination of vertical pouring and step pouring, and a flat pouring gate and a pouring system with the position far away from an isolated hot phase area; placing internal chill in the sand core assembly; adopting scrap steel and return iron to carry out high-temperature smelting according to the furnace charge proportion; adding carburant and a first inoculant along with the flow in the tapping process, and adjusting the content of Cr and Mo alloy elements; and adding a second inoculant along with the flow during pouring. The invention controls the shrinkage porosity tendency of the area with difficult shrinkage filling at a lower level while the casting obtains higher strength, realizes the synchronous reduction of the shrinkage porosity of the casting with complex structure and the rejection rate caused by the defect of air holes, and the shrinkage porosity leakage rejection rate is below 1 percent.

Description

Process control method for shrinkage porosity defect of iron casting

Technical Field

The invention relates to the technical field of new materials and casting, in particular to a process control method for shrinkage porosity defects of iron castings.

Background

High-strength cast iron is a trend of development of the casting industry, but for thin-wall iron castings with poor wall thickness uniformity, the high casting rejection rate becomes a main problem which hinders technical popularization. For this reason, a great deal of research has been conducted in all countries, and the american foundry association AFS has established a specialized thin-walled cast iron group (TWIG) for technical research; the high strength thin-walled gray cast iron technology in japan for the 21 st century has received government financial support. China also carries out a great deal of research on high-strength cast iron in the 'seventy-five' period, but the HT200-HT250 is mainly used, the attention degree on the high-strength cast iron above HT300 is insufficient, and especially for thin-wall parts with particularly complex structures, many enterprises cannot stably and efficiently produce thin-wall and complex-structure products above HT300 at present due to complex casting process and high technical content.

At present, the demand of the mechanical industry for high-strength thin-wall castings is increasingly urgent, and even some companies introduce or develop advanced products, the popularization and the use of the products are influenced because the process for producing the high-strength iron castings in batches is immature and the rejection rate is high. For example, after the national emission standard, a four-valve cylinder cover with higher overall strength and more complex structure is required, and because the high-strength alloy cast iron with the strength of more than HT300 is adopted, the carbon equivalent is low, and the contraction tendency is large in the casting process; alloying elements are added for improving the strength, so that the casting performance of molten iron is poorer; meanwhile, the cylinder cover is integrated with multiple functions, complex in structure, multiple in thin-wall structure and poor in wall thickness uniformity, so that local heat is saved, the heat is dispersed, feeding is not facilitated, and due to the combined action of the factors, the casting shrinkage porosity defect tendency is very large, the product shrinkage porosity rejection rate is very high, the production efficiency is seriously influenced, and great economic loss is caused.

Therefore, a casting process method which can greatly improve the integral mechanical property of the casting and simultaneously ensure that the shrinkage porosity rejection rate of the casting is low is developed by innovating a casting component formula and a casting process method, comprehensively utilizing and optimizing matching, and becomes a problem to be solved urgently in the mechanical industry.

Disclosure of Invention

The invention aims to provide a process control method for shrinkage defect of iron castings, which aims to solve the problem of high rejection rate caused by the shrinkage defect of the existing iron castings.

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

the invention provides a process control method for shrinkage porosity defects of iron castings, which comprises two parts of adjusting the material composition of castings and a pouring process, and specifically comprises the following steps:

the method comprises the following steps that firstly, a pouring system combining vertical pouring and step pouring is adopted, a flat pouring gate is adopted in the pouring system, and the position of the flat pouring gate is far away from an isolated hot phase area;

secondly, placing internal chill at the part with serious shrinkage porosity tendency in the sand core assembling process;

thirdly, the furnace burden is prepared by 65-75 percent of scrap steel and 25-35 percent of return iron and is smelted at high temperature;

fourthly, adding a carburant and a first inoculant along with the flow in the tapping process, and adjusting the content of Cr and Mo alloy elements;

and fifthly, casting, namely adding a secondary inoculant along with the flow to obtain the iron casting.

Preferably, the flat gate is formed by compressing one side of the round gate to a direction far away from the isolated hot phase region; the flat gate is in an oval or strip shape.

Preferably, the internal chill is made of tin-plated A3 steel, and is processed into an open cylinder by using a steel plate with the thickness of 0.8-1.2 mm.

Preferably, the ingredient content of the furnace burden in the third step is as follows: 3.10-3.30% of C, 1.80-1.90% of Si, 0.5-0.6% of Mn, less than or equal to 0.06% of P, less than or equal to 0.12% of S, 0.85-0.95% of Cu, 0.10-0.25% of Mo, 0.08-0.09% of Sn, 0.05-0.20% of Cr and the balance of Fe.

Preferably, the high-temperature smelting adopts a medium-frequency coreless induction furnace or a cupola-electric furnace duplex smelting electric furnace, and the smelting temperature is 1500-.

Preferably, the recarburizing agent is a graphite recarburizing agent, and the recarburizing amount is controlled to be 0.05-0.10%.

Preferably, the content of the alloy elements Cr and Mo satisfies the content of 0.25-0.30% Cr and 0.20-0.25% Mo in the final composition of the iron casting.

Preferably, the first inoculant is selected from a large-particle SiSr inoculant with the particle size of 0.7-6.0mm, wherein the Sr content is 0.6-1.0%, and the second inoculant is selected from a small-particle SiSr inoculant with the particle size of 0.2-0.6mm, wherein the Sr content is 0.6-1.0%.

Preferably, the first inoculant is added in an amount of 0.15-0.25% and the second inoculant is added in an amount of 0.10-0.15%.

The invention also provides a cylinder cover, which adopts the process control method for the shrinkage porosity defect of the iron casting, wherein the carbon equivalent omega (CE) of the final components of the cylinder cover is 3.8-3.9 percent, and the Si/C is 0.6-0.65 percent.

The invention has the beneficial effects that:

the invention combines the two aspects of material components and pouring process, and controls the shrinkage porosity and leakage rejection rate to be below 1 percent while obtaining higher strength of the casting.

(1) The invention comprehensively utilizes the process means of furnace burden proportioning, adding carburant and inoculant in the tapping process, adjusting the content of Cr and Mo elements and the like, and controls the shrinkage porosity tendency of a region with difficult shrinkage at a lower level while obtaining higher strength of the casting.

(2) The invention sets up a two-time inoculation process of ladle-to-ladle inoculation and stream inoculation during pouring, and has the advantages of less inoculant consumption, high efficiency and good effect.

(3) The invention adopts a pouring process combining vertical pouring and step pouring, and realizes the synchronous reduction of the rejection rate caused by shrinkage porosity and air hole defects of the casting with a complex structure.

(4) The invention adopts the flat gate far away from the isolated hot phase area, so that the molten iron injection is dispersed as far as possible and far away from the isolated hot phase area, and the shrinkage porosity defect is reduced to the maximum extent.

(5) The invention adopts the chilling technology of the internal chill aiming at the parts with serious shrinkage porosity tendency, fully plays the roles of chilling and shielding by reasonably designing the internal chill, and greatly reduces the defect of the shrinkage porosity of the casting.

Drawings

FIG. 1 is a flow chart of a process control method for shrinkage defect of iron castings according to the present invention;

FIG. 2A is a schematic diagram of the distribution of isolated hot phase regions using round gate cooling 600s in an embodiment of the present invention;

FIG. 2B is a schematic view of a distribution of more than 70% of isolated hot phase regions using a round gate in an embodiment of the present invention;

FIG. 2C is a schematic diagram of the distribution of isolated hot phase regions using flat gate cooling 600s in an embodiment of the present invention;

FIG. 2D is a schematic view of a distribution of over 70% of isolated hot phase regions using flat gates in an embodiment of the present invention;

FIG. 3A is a schematic diagram of the distribution of isolated hot phase regions without chill cooling 600s in an embodiment of the present invention;

FIG. 3B is a schematic diagram of the distribution of isolated hot phase regions with chill cooling 600s according to an embodiment of the present invention;

FIG. 3C is a schematic diagram showing the distribution of isolated hot phase regions without adding more than 70% of chill in an embodiment of the present invention;

FIG. 3D is a schematic illustration of the distribution of isolated hot phase regions above 70% chill chilled with internal chill in an embodiment of the present invention;

FIGS. 4A to 4C are schematic views of the inner chill employed in the embodiment of the present invention in three states of opening, pressing, and closing after installation, respectively;

fig. 5 is a structural picture of a chill region on the surface of a cylinder head according to an embodiment of the present invention.

Detailed Description

The present invention will be described in further detail with reference to the accompanying drawings and examples. It is to be understood that the specific embodiments described herein are merely illustrative of the invention and are not limiting of the invention. It should be further noted that, for the convenience of description, only some of the structures related to the present invention are shown in the drawings, not all of the structures. Unless otherwise specified, the percentages (%) of the respective substance components referred to in the present invention are mass percentages.

The invention provides a process control method for shrinkage porosity defects of iron castings, which comprises two parts of adjusting material components of castings and a pouring process, wherein the adjusting of the material components of the castings comprises furnace burden proportioning and adjusting the content of Cr and Mo alloy elements, a carburant is added, and the process of twice inoculation is carried out, and the pouring comprises the improvement of a pouring mode and an inner cooling mode. The iron casting is high-strength alloy cast iron with the weight of HT300 or more and is used for manufacturing the cylinder cover. Specifically, the process control method for shrinkage porosity defects of iron castings, as shown in the flow chart of fig. 1, comprises the following steps:

the first step is as follows: manufacturing a pouring system: vertical pouring and step pouring are combined, and the flat pouring gate and the position are far away from an isolated hot phase area.

The shape and the position of the pouring gate determine a flow channel and flow rate of molten iron, great influence is exerted on a hot spot, particularly an isolated hot phase area, and compared with a traditional round pouring gate, the special-shaped pouring gate can avoid concentrated injection of the molten iron and enable the pouring gate to be arranged far away from the isolated hot phase area. The invention utilizes the casting process simulation software to analyze the characteristics of the cooling speed and the isolated hot phase region, as shown in figures 2A and 3A, the hot spots of areas with larger wall thickness, such as cylinder cover bolt holes, guide rod holes, oil nozzle holes and the like, in the temperature field of the iron casting cooled for more than 600s are more, and the hot spots are staggered with each other in the middle of the cylinder cover which is more intensively distributed, so that a large-area cooling lag region is formed. As shown in fig. 2B and 3C, a larger area of isolated hot phase region is formed at the gate position in the liquid phase region of 70% or more, particularly in the vicinity of the lower gate. The test results show that the flat gate design has faster cooling rates and smaller isolated hot phase regions. In order to reduce the area of an isolated hot phase region, the invention sets a design scheme that the shape of a flat sprue is far away from the isolated hot phase region and the chilling of the internal chill.

The shape of the flat gate adopted by the invention is an ellipse or a strip, and specifically, one side of the traditional round gate is compressed to the side far away from the isolated hot phase area to form the ellipse or strip-shaped flat gate.

And secondly, adding internal chill in the sand core molding process, and locally chilling the main feeding defective area by using the internal chill in the tin-plated A3 steel open column barrel.

Because the material and the pouring process for integrally improving the mechanical property of the casting are adopted, the casting (cylinder cover) has a complex structure, thin wall thickness and large variation, and shrinkage porosity is easily generated in an area with poor shrinkage filling, a local chilling process is required to be adopted for a part with serious shrinkage porosity, local cooling is accelerated, and a thermal junction in the casting is shielded. The scheme of tinned A3 steel open-ended cylindrical internal chill is adopted, the internal chill and cast iron have good fusion and self elasticity, the defective feeding area is locally chilled, the manufacturing and assembly are simple, the manufacturability is good, the effect of shielding the internal heat section of the casting is good, and the fusion and chilling stability of the casting is good.

The inner chill is made of pure low-carbon steel A3 steel with the wall thickness of 0.8-1.2mm, and the surface is plated with tin; the internal chill comprises the following chemical components in percentage by mass: c: 3.2 ± 0.05%, Si: 1.6% -1.8%, Cr: 0.25 to 0.30 percent of Mo, 0.20 to 0.25 percent of Mo and the balance of Fe.

And cutting the steel plate into a rectangle according to the size of the placement area of the internal chill, wherein the length is the depth of a main shrinkage porosity risk area hole of the placement area, and the width is the perimeter of the cross section of the placement area. And (3) rolling the steel plate into an open column barrel as shown in fig. 4A by using a plate rolling machine, then manually extruding the column barrel to reduce the overlapping of the opening as shown in fig. 4B, putting the extruded column barrel into a position to be chilled, opening the internal chill by elasticity after releasing the hand, attaching the internal chill to the surface of the sand core at the position to be chilled, and closing the opening of the column barrel just as shown in the circled area in fig. 4C.

For casting of the cylinder cover, the placement position of the internal chill is selected to be a bolt hole or a conduit hole to be chilled. According to the distribution of the isolated hot phase region, 1-3 bolt holes or conduit holes or a row of bolt holes which are closest to the isolated hot phase region and have the largest influence on shrinkage porosity are selected for placement.

Thirdly, the furnace burden is prepared by 65-75 percent of scrap steel and 25-35 percent of return iron; adopting a medium-frequency coreless induction furnace or a cupola-electric furnace duplex smelting electric furnace to carry out high-temperature smelting at the temperature of 1500-;

the furnace burden comprises the following components in percentage by mass: 3.10-3.30% of C%, 1.80-1.90% of Si%, 0.5-0.6% of Mn%, not more than 0.06% of P%, not more than 0.12% of S%, 0.85-0.95% of Cu%, 0.10-0.25% of Mo%, 0.08-0.09% of Sn%, 0.05-0.20% of Cr% and the balance of Fe. Among them, the content of C element is more preferably 3.18 to 3.25%.

And fourthly, adding a carburant and a first inoculant along with the flow in the tapping process, and adjusting the content of Cr and Mo alloy elements.

The recarburizing agent is a graphite recarburizing agent, the recarburizing amount is controlled to be 0.05-0.10% (the recarburizing agent accounts for the percentage content of the total mass of the casting), and the recarburizing agent is used for adjusting the carbon equivalent and the Si/C value in the tapping process.

In the invention, the carbon equivalent omega (CE) is controlled to be 3.8-3.9 percent, the Si/C is controlled to be 0.6-0.65 percent, wherein, the content of the C element omega (C) is 3.10-3.30 percent;

the adjustment of the content of the Cr and Mo alloy elements means that the Cr and Mo alloy elements are properly added according to the content of the Cr and Mo alloy elements in furnace materials to meet the final component content of the iron casting, wherein the Cr is 0.25-0.30 percent, and the Mo is 0.20-0.25 percent;

the first inoculant adopts large-particle SiSr inoculant, and the transition ladle is poured into the ladle and 0.15-0.25% of the first inoculant is added; the particle size of the large-particle SiSr inoculant is 0.7-6.0mm, and the Sr content is 0.6-1.0%.

And fifthly, pouring.

The molten iron is poured at the temperature of 1420-. The second inoculant is small-particle SiSr inoculant with particle size of 0.2-0.6mm, wherein the Sr content is 0.6-1.0%.

The iron casting prepared by the method finally controls the carbon equivalent omega (CE) to be 3.8-3.9 percent and the Si/C to be 0.6-0.65 percent. The shrinkage porosity and leakage rejection rate of the iron castings is below 1%.

The following provides an application example of the process control method for shrinkage porosity defect of iron casting in the preparation of cylinder cover.

Example 1: a process control method for shrinkage porosity defects of a high-strength TCD cylinder cover comprises the following steps:

the first step, a process combining vertical pouring and stepped pouring is adopted, pouring gates are arranged at the bottom and the middle of a casting mold, and the pouring gates and pouring channels are uniformly arranged according to structural characteristics. The gate is provided in an elliptical shape.

Step two, assembling a sand core: the internal chill is disposed in the area of the row of bolt holes closest to the isolated hot phase zone. The internal chill is in the shape of an open cylinder made of 1mm tin-plated A3 steel.

Thirdly, the furnace burden is prepared by 65 percent of scrap steel and 35 percent of return iron in proportion; the furnace burden comprises the following components in percentage by mass: 3.23 percent of C, 1.80 percent of Si, 0.5 percent of Mn, less than or equal to 0.06 percent of P, 0.11 percent of S, 0.85 percent of Cu, 0.10 percent of Mo, 0.08 percent of Sn, 0.05 percent of Cr and the balance of Fe.

High-temperature smelting is carried out at 1530 ℃ by adopting a medium-frequency coreless induction furnace.

The recarburizing agent is a graphite recarburizing agent, the recarburizing amount is controlled to be 0.09% (mass percentage based on the total mass of the prepared iron casting), and the recarburizing agent is used for adjusting the carbon equivalent and the Si/C value in the iron tapping process.

The alloy elements of Cr and Mo are added with the flow, so that the iron casting has the final component content of 0.25% of Cr and 0.25% of Mo.

0.20 percent of first inoculant is added into the transition ladle and the ladle, the particle diameter of the first inoculant is 0.7-6.0mm, and the Sr content is 0.6 percent.

And fifthly, pouring.

Pouring molten iron at the temperature of 1430 ℃, adding 0.10% of a second inoculant along with the flow during pouring, wherein the second inoculant is a small-particle SiSr inoculant with the particle size of 0.2-0.6mm, and the Sr content is 0.6%.

The finally prepared iron casting is a high-strength TCD cylinder cover, and comprises the following main components in percentage by mass: 3.24% of C, 1.98% of Si, 0.25% of Cr, and 0.25% of Mo.

The tensile strength of a top plate of the high-strength TCD cylinder cover is 300MPa, the central tensile strength of a combustion chamber of the bottom plate is 300MPa, the tensile strength of a stud part is 243-one-257 MPa, the Brinell hardness HBS of a compression area is 214-one 225, and the shrinkage porosity leakage rejection rate is 0.5-0.6%.

Comparing the arrangement mode of the flat gate with the isolated hot phase region of the round gate, as shown in fig. 2A-2D, the flat gate scheme reduces the cooling time of the hot spot part compared with the round gate scheme, especially the isolated hot phase region of more than 70%, and the flat gate scheme has certain improvement effect on reduction of shrinkage porosity rejection rate and has equivalent tensile strength and hardness.

The bolt hole area main body of the high-strength TCD cylinder cover adopts an inner-cooling iron chilling scheme, the inner-cooling iron is placed in a middle ring area with the dissection result shown in figure 5, and the inner-cooling iron and the iron casting body form a compact structure and are better fused. The effect of the chill on cylinder head performance and shrinkage porosity is shown in table 1.

TABLE 1 Effect of chill on Performance and shrinkage porosity

Tests show that the internal chill can be well fused with a cylinder cover body, the shrinkage porosity leakage problem of a hot spot part can be eliminated through chilling and shielding effects, the shrinkage porosity leakage rate is reduced from 15% -18% to 0.5% -1% after the internal chill is added, the shrinkage porosity rejection rate of the high-strength TCD cylinder cover is effectively reduced, and meanwhile, the strength and the hardness are not obviously changed.

Example 2

The invention provides a process control method for shrinkage porosity defects of an automobile cylinder cover, which comprises the following steps:

firstly, a vertical step pouring and pouring combined pouring process is adopted.

The shape of the flat gate is a strip shape near the isolated hot phase zone, and the gate is realized by compressing one side of the round gate to the direction far away from the isolated hot phase zone, so that the shape of the gate is changed, but the perimeter and the area are not changed, but the distance between the whole gate and the isolated hot phase zone is increased, and the shape of the runner is changed accordingly.

Step two, assembling a sand core: and (3) locally chilling the three rows of bolt holes with the most serious feeding defects by adopting an open cylindrical internal chill made of 0.8mm tinned A3 steel.

Thirdly, the furnace burden is prepared by 75 percent of scrap steel and 25 percent of return iron, and the furnace burden comprises the following components: 3.18 percent of C, 1.89 percent of Si, 0.6 percent of Mn, less than or equal to 0.06 percent of P, 0.10 percent of S, 0.95 percent of Cu, 0.25 percent of Mo, 0.09 percent of Sn, 0.20 percent of Cr and the balance of Fe.

A cupola-electric furnace duplex smelting electric furnace is adopted, and high-temperature smelting is carried out at the temperature of 1530 ℃.

Fourthly, adding carburant, 0.15 percent of first inoculant and 0.1 percent of Cr alloy element along with the flow in the process of tapping. As the Mo alloy element in the charging materials reaches 0.25 percent, the Mo element does not need to be added in the tapping process. The recarburizing agent is a graphite recarburizing agent, and the recarburizing amount is controlled to be 0.05%.

The first inoculant is a large-particle SiSr inoculant with the particle size of 0.7-6mm, wherein the Sr content is 0.6%;

and fifthly, pouring.

And adding 0.15% of a secondary inoculant into the casting machine along with the flow, wherein the secondary inoculant adopts small-particle SiSr inoculant with the particle size of 0.2-0.6mm, and the Sr content is 0.6%. Pouring at the molten iron temperature of 1430 ℃.

The iron casting obtained by the preparation method is a cylinder cover, and the cylinder cover comprises the following main components in percentage by mass: : 3.19% of C, 2.06% of Si, 0.30% of Cr, and 0.25% of Mo.

The tensile strength of a top plate of the cylinder cover is 326Mpa, the tensile strength of a stud part is 262Mpa, the central tensile strength of a combustion chamber of the bottom plate is 312Mpa, and the Brinell hardness HBS of a compression region is 228.

Comparing the performance of the cylinder head with and without chill in this example, as shown in fig. 3A-3D, the solidification cooling time and isolated hot phase area were reduced after the chill was allowed to stand. Therefore, the internal chill has the chilling function, can shield the internal thermal junctions of the casting, reduces the mutual influence of the thermal junctions, and can solve the problem of local shrinkage porosity and leakage through the shielding function; meanwhile, the shrinkage porosity leakage rate of the cylinder cover is 0.7-0.1% by matching with the adopted flat gate design and material component adjustment.

Example 3

The invention provides a process control method for shrinkage porosity defects of an iron casting cylinder cover, which comprises the following steps:

firstly, a pouring system adopts a vertical step pouring process;

the shape of the flat gate is a strip shape, and the gate position is far away from the isolated hot phase area relative to the round gate.

Step two, assembling a sand core: a row of bolt holes with poor feeding are locally chilled by adopting 1.2mm tinned A3 steel open-ended cylindrical internal chill.

Thirdly, the furnace burden is prepared by 70 percent of scrap steel and 30 percent of return iron, and the furnace burden comprises the following components: 3.21 percent of C, 1.85 percent of Si, 0.6 percent of Mn, less than or equal to 0.06 percent of P, 0.11 percent of S, 0.90 percent of Cu, 0.15 percent of Mo, 0.08 percent of Sn, 0.10 percent of Cr and the balance of Fe.

Adopting a medium-frequency coreless induction furnace, and controlling the temperature at 1540 ℃ to carry out high-temperature smelting.

And fourthly, adding a carburant, a first inoculant, Mo and Cr alloy elements along with the flow in the tapping process. The recarburizing agent is a graphite recarburizing agent, and the recarburizing amount is controlled to be 0.07%.

The first inoculant is a large-particle SiSr inoculant with the particle size of 0.7-6mm, wherein the Sr content is 1%; the addition amount of the first inoculant is 0.25%.

Adding Cr and Mo alloy elements to meet the final component content of the iron casting, wherein Cr is 0.30 percent and Mo is 0.20 percent;

and fifthly, pouring.

And adding 0.10% of a second inoculant into the casting machine along with the flow, wherein the second inoculant adopts small-particle SiSr inoculant with the particle size of 0.2-0.6mm, and the Sr content is 1%. Pouring at the molten iron temperature of 1430 ℃.

The iron casting obtained by the preparation method is a cylinder cover, and the cylinder cover comprises the following main components in percentage by mass: 3.22% of C, 2.05% of Si, 0.30% of Cr and 0.20% of Mo.

The tensile strength of a top plate of the cylinder cover is 320MPa, the tensile strength of a stud part is 250MPa, the central tensile strength of a bottom plate combustion chamber is 310MPa, and the Brinell hardness HBS of a compression region is 220.

In the cylinder cover, the internal chill and the iron casting body form a compact structure, so that the integration is better, and the problem that the hot spot part is loose and leaks is solved; the long strip flat gate is far away from the isolated hot phase region, so that the cooling time of the hot junction part is reduced, particularly the isolated hot phase region of more than 70 percent is greatly reduced, and the shrinkage porosity leakage rate of the cylinder cover is 2 percent.

In the high-strength cylinder heads prepared in the embodiments 1-3, the tensile strength of the top plate of the cylinder head is 300-.

By applying the process control method for shrinkage porosity defects of iron castings, the shrinkage porosity leakage rejection rate of the iron castings such as cylinder covers is reduced and stably controlled to be 0.5-1% from more than 60% before improvement under the condition of meeting high strength performance, and the shrinkage porosity defect control effect is very obvious. The process control method provided by the invention can be used for manufacturing alloy cast iron parts with high strength and complex structures, obtains an ideal effect of reducing the shrinkage porosity rejection rate, meets the development trend of high-strength thin-wall iron castings, ensures lower production cost and promotes the popularization and application of advanced technology.

Claims

1. A process control method for shrinkage porosity defects of iron castings is characterized by comprising the following steps:

secondly, placing internal chill at the part with serious shrinkage porosity tendency in the sand core assembling process, wherein the internal chill is attached to the surface of the sand core at the position to be chilled;

the furnace charge comprises the following components in percentage by weight: c% =3.10-3.30%, Si% =1.80-1.90%, Mn% =0.5-0.6%, P% ≦ 0.06%, S% ≦ 0.12%, Cu% =0.85-0.95%, Mo% =0.10-0.25%, Sn% =0.08-0.09%, Cr =0.05-0.20%, and the balance Fe;

fifthly, casting, namely adding a secondary inoculant along with the flow to obtain an iron casting;

the first inoculant is a large-particle SiSr inoculant with the particle size of 0.7-6.0mm, wherein the Sr content is 0.6-1.0%, the second inoculant is a small-particle SiSr inoculant with the particle size of 0.2-0.6mm, and the Sr content is 0.6-1.0%; the addition amount of the first inoculant is 0.15-0.25%, and the addition amount of the second inoculant is 0.10-0.15%;

the contents of the Cr and Mo alloy elements meet the final component contents of the iron casting, wherein Cr =0.25-0.30% and Mo = 0.20-0.25%.

2. The method as claimed in claim 1, wherein said flat gate is formed by compressing one side of a round gate away from an isolated hot phase zone; the flat gate is in an oval or strip shape.

3. The method as claimed in claim 1, wherein said internal chill is made of tin-plated a3 steel, and is made into open-ended cylindrical shell from steel plate with thickness of 0.8-1.2 mm.

4. The process control method for shrinkage porosity defects of iron castings according to claim 1, characterized in that the high-temperature smelting adopts a medium-frequency coreless induction furnace or a cupola-electric furnace duplex smelting furnace, and the smelting temperature is 1500-.

5. The method as claimed in claim 1, wherein said carburant is graphite carburant, and the carburant content is controlled at 0.05-0.10%.