JP6763876B2 - Duplex stainless steel pipe manufacturing method - Google Patents

Description

The present disclosure relates to duplex stainless steels, particularly methods of making duplex stainless steel tubes suitable for use in fuel injection systems for injecting fuel into the combustion chambers of internal combustion engines.

In connection with the design of gasoline direct injection (GDI) systems for the automotive industry, it has been proposed to use duplex stainless steel for the rails used to conduct fuel into the combustion chamber of internal combustion engines.

There are several requirements for tubes used as GDI rails, which need to be considered when designing duplex stainless steels for such applications. Thus, in combination with a properly selected tube manufacturing process, a given austenite / ferrite ratio, required corrosion resistance (resistant to general corrosion and pitting corrosion), substantially intermediate phase, especially sigma phase and chromium nitride. It is important to select the chemical composition of duplex stainless steels that provide a pitting corrosion, a given impact toughness, a given tensile strength, and a given duplex strength. In addition, the mechanical properties of this duplex steel are such that the resulting tube is sufficiently high for the intended application, even if the tube wall thickness is relatively small, to a given burst pressure (ie, to breakage). It must be something that indicates (internal pressure), thereby enabling GDI rails that require small size and light weight. Corrosion fatigue properties must ensure the durability of the pipe over time.

Therefore, the process of designing duplex stainless steel and manufacturing duplex stainless steel tubing that is expected to meet the requirements of GDI rails is a complex task. The selected chemical composition and manufacturing process parameters must be coordinated with each other. As a result, once the nominal chemical composition of duplex stainless steel is determined, the manufacturing process parameters associated with it must also be selected. The chemical composition of duplex stainless steel must also facilitate a cost-effective manufacturing process. In other words, the chemical composition should not be such that it requires an overly complex, energy- or time-consuming manufacturing process.

Aspects of the present disclosure are methods of producing duplex stainless steel tubes, which include high requirements for corrosion resistance (resistant to general corrosion and pitting corrosion) and predetermined conditions by producing the duplex stainless steel tubes. Provided is a method by which a tube becomes exhibiting properties suitable for an application such that impact toughness, predetermined tensile strength and predetermined fatigue strength are present.

One such application is a GDI rail for conducting fuel injected into the combustion chamber of an internal combustion engine. The duplex stainless steel of the tube must exhibit a microstructure that is substantially free of intermediate phases, especially the sigma phase and chromium nitride. The chemical composition of duplex stainless steel must enable the cost-effective manufacture of the tubing in view of facilitating the use of cost-effective process steps.

The above aspect has the following composition:
C maximum 0.06;
Cr 21 to 24.5;
Ni 2.0-5.5;
Si maximum 1.5;
Mo 0.01-1.0;
Cu 0.01-1.0;
Mn maximum 2.0;
N 0.05-0.3;
P maximum 0.04;
A method for producing a duplex stainless steel pipe containing S maximum 0.03; and the remaining Fe and unavoidable impurities in% by weight (wt%) and having a PRE value of at least 23.0.
The following steps:
a) Providing a duplex stainless steel melt;
b) Casting a duplex stainless steel body from the melt;
c) Forming a rod of the body;
d) Forming a tube of rods by creating holes in it;
e) Reduce tube diameter and / or wall thickness by hot extrusion;
f) Further reducing the diameter and / or wall thickness of the tube by cold deformation; and g) including annealing the cold deformed tube.
After step g), the two-phase stainless steel of the resulting tube consists of 40-60% austenite and 40-60% ferrite, where step g) puts the tube in the range of 950 ° C to 1060 ° C. Provided is a method comprising subjecting to a temperature of 0.3 to 10 minutes and an atmosphere consisting of a gas mixture containing 1 to 6 vol% nitrogen gas and a residue of H ₂ or an inert gas. Achieved by this disclosure.

Therefore, the annealing temperature, annealing time and annealing atmosphere to reach the optimum material properties were found. The annealing temperature should be in the range of 950 to 1060 ° C. and the atmosphere should contain a gas mixture consisting of 1-6 vol% nitrogen and a balance selected from H ₂ or an inert gas, annealing. Was found to have to be carried out in 0.3-10 minutes.

Lower annealing temperatures can result in the formation of unwanted precipitates such as intermediate phases. In addition, recrystallization is slowed down, and thus the immersion time to complete the recrystallization must be increased, which adversely affects productivity.
As a general rule, the upper limit temperature of the annealing step is set to the temperature at which the duplex stainless steel begins to melt. However, there are also practical reasons for further limiting the annealing temperature. At temperatures higher than the specified spacing, duplex stainless steel will be softer and will have an increased risk of damage during the annealing process. Also, at high temperatures, increased grain growth makes it more difficult to obtain good processes and particle size adjustments.

It is also very important to use an annealing temperature that balances the phase fraction, too low the ferrite content too low and too high the ferrite content too high. In addition, since the temperature of the annealing process also affects the chemical composition of the ferrite and austenite phases, it is necessary to balance the chemical composition and the annealing temperature together to ensure that both phases have good corrosion resistance. ..

The time the tube is exposed to the annealing temperature is 0.3 to 10 minutes, for example 0.3 to 5 minutes, for example 0.3 to 2.5 minutes. This time needs to be long enough to guarantee complete recrystallization. However, if the time is too long, the resulting tube will have a coarse structure that adversely affects the mechanical properties. The thicker the tube wall, the longer the annealing time. A wall thickness of about 1 mm to about 5 mm can be considered.

In addition, the atmosphere of the annealing process is also very important. The nitrogen-containing atmosphere affects the nitrogen content of the surface of duplex stainless steel. Therefore, the role of nitrogen in the atmosphere is to maintain the nitrogen content on the surface of the material. At the annealing temperature of the method, nitrogen diffuses into and out of the material. The nitrogen content should be selected so that the surface nitrogen content is maintained. Too low a nitrogen content in the atmosphere in which the annealing takes place can lead to net loss of nitrogen on the surface, adversely affecting the corrosion resistance and mechanical properties of the duplex stainless steels described above or below. I found out. If the nitrogen level in the atmosphere in which the annealing is performed is too high, it causes an increase in nitrogen on the surface of the material during annealing, and since nitrogen is a strong austenite forming agent, changes in the nitrogen content can affect the phase balance. Therefore, when the nitrogen content in the atmosphere is high, austenite is formed on the surface. The nitrogen content of the material surface also affects the structural stability to the susceptibility to precipitate formation such as chromium nitride. The formation of precipitates adversely affects the corrosion resistance of the duplex stainless steels described above or below.

The pitting corrosion index PRE is PRE = Cr (wt%) + 3.3Mo (wt%) + 16N (wt%).
Is defined as. A PRE of at least about 23.0, although all three of chromium, molybdenum and nitrogen cannot be minimized at the same time in the above composition, it is necessary to combine them to obtain this specified PRE value. Shown. According to another embodiment, the PRE value is at least about 24.0. The term "about" used above and below refers to +/- 10% of an integer.

According to one embodiment, the temperature range of the annealing step (step g) is from 970 ° C to 1040 ° C. According to yet another embodiment, the temperature range is 1000 ° C to 1040 ° C.

According to one embodiment, the annealing step comprises subjecting the tube to said temperature for 0.5-5 minutes, eg 0.5-1.5 minutes.

According to one embodiment, the inert gas is argon or helium or a mixture thereof.

According to one embodiment, the content of nitrogen gas in the gas mixture is 4 vol% or less. According to another embodiment, the content of nitrogen gas in the gas mixture is 3 vol% or less. Further according to one embodiment, the content of nitrogen gas in the gas mixture is 1.5 vol% or more.

According to one embodiment, the hot extrusion step (step e) involves subjecting the tube to hot extrusion at a temperature in the range of 1100 ° C. to 1200 ° C. and reducing its cross-sectional area by 92-98%. Including. According to one embodiment, the hot extrusion step (step e) involves subjecting the tube to hot extrusion at a temperature in the range of 1100 ° C to 1170 ° C and a reduction of 92-98% in its cross-sectional area. Including. The reduction in cross-section is defined as (tube) (pre-extrusion cross-section-post-extrusion cross-section) / (pre-extrusion cross-section). The extrusion temperature and the degree of deformation of the duplex stainless steel are such that the microstructure of the duplex stainless steel is not adversely affected, or cracks that are harmful to the mechanical properties of the final product are not generated therein. Selected in relation to chemical composition.

According to one embodiment, the cold deformation step (step f) comprises subjecting the tube to cold deformation without preheating the tube. According to one embodiment, the cold deformation step (step f) comprises reducing the cross section of the tube by 50-90%. The reduction in cross-section is defined as (tube) (pre-pilgaring cross-section-post-pilgaring cross-section) / (pre-pilgaring cross-section). The chemical composition of duplex stainless steel is selected to allow such cold deformation of the stainless steel without unwanted cracking in the material or harmful negative effects on the microstructure of the material.

According to one embodiment of the above or below method, the cold deformation is either pilgaring or cold drawing.

According to one embodiment, when the cold deformation is pilgaring, the relationship between the wall thickness reduction and the outer diameter reduction of the pipe is expressed as the following Q value.
Q value = (Wallh-Wallt) * (Odh-Wallh) / Wallh ((Odh-Wallh)-(Odt-Wallt))
During the above ceremony
Wallh = Hollow wall = Wall thickness before Pilgaring Wallt = Tube wall = Wall thickness after Pilgaring Odh = Hollow OD = Diameter of tube before Pilgaring Odt = Tube OD = Diameter of tube after Pilgaring, Q is 0. It is in the range of 5 to 2.5. If the area reduction is too large, the force may be too strong and the material may crack.

According to yet another embodiment, Q is in the range 0.9-1.1.

According to one embodiment, the duplex stainless steel has the following composition (unit: weight%).
C 0.01-0.025;
Si 0.35-0.6;
Mn 0.8-1.5;
Cr 21 to 23.5;
Ni 3.0-5.5;
Mo 0.10 to 1.0;
Cu 0.15 to 0.70;
N 0.090 to 0.25;
P 0.035 or less;
P 0.003 or less;
The remaining Fe and unavoidable impurities.

Duplex stainless steels with this chemical composition are particularly suitable for use in the process steps described above using the process parameters described above. In other words, the process steps and parameters described above or below are particularly suitable for duplex stainless steels with this chemical composition and fuel for injecting fuel into the combustion chamber of an internal combustion engine. It is selected to provide a tube with particularly suitable properties for GDI rail applications for fuel conduction in injection systems.

According to another embodiment, this pipe is a pipe for conducting fuel in a fuel injection system for injecting fuel into the combustion chamber of an internal combustion engine. Alternatively, the present disclosure can be defined as a method of manufacturing a fuel conductor in a fuel injection system for injecting fuel into the combustion chamber of an internal combustion engine, the method for making a duplex stainless steel tube. , And / or the methods described below. Such a method involves attaching a duplex stainless steel tube to a further structural member of the internal combustion engine by brazing. Further structural members can be metal, typically austenite or two-phase steel. Methods for making tubes, including selection of the chemical composition of two-phase stainless steel, also for favorable brazing properties, especially for liquid metal induced embrittlement (LMIE) caused by liquid metal penetration. The aim is to achieve low sensitivity. Brazing optionally includes copper brazing in a continuous furnace at temperatures in the range 1100 ° C to 1140 ° C.

According to one embodiment, the tube has an outer diameter in the range of 15-35 mm after the Pilgaring step. According to one embodiment, the pipe is used as a GDI rail in a fuel injection system for conducting fuel injected into the combustion chamber of an internal combustion engine.

According to another embodiment, the tube has an outer diameter of 7-10 mm after the Pilgaring step. According to one embodiment, the tube is used as a fuel line in a fuel injection system for conducting fuel injected into the combustion chamber of an internal combustion engine.

The functions and effects of the essential alloying elements of duplex stainless steels described above and below are shown in the following paragraphs. The list of functions and effects of each alloying element should not be considered complete and additional functions and effects may exist. However, the list includes duplex stainless steel and duplex stainless steel tubes, especially duplex stainless steel tubes intended for fuel conduction within a fuel injection system for injecting fuel into the combustion chamber of an internal combustion engine. It provides an overview of the basic knowledge to consider when designing process parameters for manufacturing methods.

Carbon (C) provides an austenite stabilizing effect and prevents the transformation of austenite into a martensitic structure during deformation of duplex stainless steel. C has a positive effect on the strength of duplex stainless steel. Therefore, the content of C is set to 0.01 wt% or more. However, at levels that are too high, carbon tends to form unwanted carbides with other alloying elements. Therefore, the content of C should not exceed 0.06 wt%. According to one embodiment, the C content should not exceed 0.025 wt%.

Chromium (Cr) has a strong effect on the corrosion resistance of duplex stainless steel, especially pitting corrosion. According to the present disclosure, the PRE value is greater than 23.0. In addition, Cr improves yield strength and prevents the transformation of austenite structure to martensite structure during deformation of duplex stainless steel. Therefore, the Cr content is set to 21.0 wt% or more. At high levels, as the Cr content increases, the temperature rises due to the undesired stable sigma phase and the sigma phase is formed more rapidly. Therefore, the Cr content is 24.5 wt% or less. Cr also imparts a ferrite stabilizing effect to duplex stainless steel. According to one embodiment, the Cr content is 23.5 wt% or less.

Nickel (Ni) has a positive effect on resistance to general corrosion. In addition, Ni gives a strong austenite stabilizing effect and prevents the transformation of austenite into a martensite structure when the two-phase stainless steel is deformed. Therefore, the Ni content is 2.0 wt% or more. According to another embodiment, the Ni content is 3.5 wt% or more. The austenite stabilizing effect of Ni can be balanced to some extent by adjusting the Cr content. However, the Ni content should not be more than 5.5 wt%.

Silicon (Si) is often present in duplex stainless steel because it may have been used to deoxidize molten steel. Si is a ferrite stabilizer, but it also prevents the transformation of austenite to martensite, which is associated with the deformation of two-phase stainless steel. Si may also have improved corrosion resistance in some environments. However, Si reduces the solubility of nitrogen and carbon and can form unwanted silicides if present at too high a level. Therefore, according to one embodiment, the content of Si in duplex stainless steel is 1.5 wt% or less. According to one embodiment, the content of Si in duplex stainless steel is 0.6 wt% or less. According to one embodiment, the Si content may be about 0 wt%. According to one embodiment, the Si content is 0.35 wt% or more.

Molybdenum (Mo) has a strong effect on the corrosion resistance of duplex stainless steel. It has a great influence on the PRE. Mo is added in an amount of 0.01 wt% or more. Mo also imparts a ferrite stabilizing effect to duplex stainless steel. According to one embodiment, the Mo content is greater than 0.10 wt%. Mo also raises the temperature at which the unwanted sigma phase is stable and accelerates its formation rate. Mo is also a relatively expensive alloying element. Therefore, the Mo content is 1.0 wt% or less.

Copper (Cu) has a positive effect on corrosion resistance. Cu is also duplex stainless steel. Prevents the transformation from austenite to martensite during the transformation of. Therefore, the intentional addition of Cu to duplex stainless steel is optional. Cu is often present in scrap used for the production of steel, and it is permissible to stay in steel at moderate levels. According to one embodiment, the Cu content can be 0.01 wt% or more. According to another embodiment, the Cu content is 0.15 wt% or more. According to one embodiment, the Cu content is 1.0 wt% or less. According to another embodiment, the Cu content is 0.7 wt% or less.

Manganese (Mn) imparts a duplex hardening effect to the duplex stainless steel and prevents the transformation of austenite to a martensite structure during duplex stainless steel. Mn also has an austenite stabilizing effect. According to one embodiment, the Mn content in duplex stainless steel is 0.8 wt% or more. However, Mn adversely affects the corrosion resistance in the acid and chloride-containing environment and increases the tendency to form the intermediate phase. Therefore, the maximum content of Mn must not exceed 2.0 wt%. According to one embodiment, the Mn content is 1.0 wt% or less.

Nitrogen (N) has a positive effect on the corrosion resistance of duplex stainless steel and also contributes to deformation hardening. Nitrogen also has a strong effect on the pitting corrosion index PRE. Nitrogen is also added in an amount of 0.05 wt% or more to provide a strong austenite stabilizing effect and prevent the transformation of the austenite structure to the martensite structure during plastic deformation of the two-phase stainless steel. According to one embodiment, the N content is 0.090 wt%. That is all. At levels that are too high, N tends to form chromium nitride in duplex stainless steel, which has a negative effect on ductility and corrosion resistance and should be avoided. Therefore, the content of N is set to 0.3 wt% or less. According to one embodiment, the N content is 0.25 wt% or less.

Phosphorus (P) is an impurity contained in duplex stainless steel and is well known to have a negative effect on hot workability. Therefore, the content of P is set to 0.03 wt% or less.

Sulfur (S) is an impurity contained in austenitic stainless steel and deteriorates hot workability. Therefore, the acceptable content of S is 0.03 wt% or less, for example 0.005 wt% or less.

Duplex stainless steels described above or below are one or more elements selected from the group Al, V, Nb, Ti, O, Zr, Hf, Ta, Mg, Ca, La, Ce, Y and B. May be included in some cases. These elements are added during the manufacturing process, for example to enhance deacidification, corrosion resistance, hot ductility or machinability. However, as is known in the art, the addition of these elements must be limited by which elements are present. Therefore, the total content when these elements are added is 1.0 wt% or less.

The term "impurity" as used herein means raw materials such as ore and scrap, and other substances mixed due to various factors in the manufacturing process when producing duplex stainless steel industrially, and is described above or below. Mixing is allowed within a range that does not adversely affect the duplex stainless steel.

The present disclosure is further described by the following non-limiting examples.

その後、得られた溶融物を以下のように適宜処理加工した。
連続鋳造を用い、これらを本体へと鋳造した。
その後、鍛造によって丸棒を形成し、その中に穴を開けることによって管を形成した。その後、１１２０℃〜１１５０℃の範囲の温度で熱間押出しを用いることによって管の直径を小さくし、得られた管は、断面積が９６〜９８％減少していた。熱間押出しに続いてピックリングを行い、ガラスビーズを除去した。
ピルガリングによって直径をさらに小さくし、管の断面積を８０〜８６％の範囲で減少させた。
その後、窒素約２％と残部のアルゴンガスとを含むガス混合物からなる雰囲気中でピルガリングした管をアニールし、管を約１分間、約１０３０℃に温度に供した。 Examples Two melts having the following compositions were prepared (both Fe was the balance).

Then, the obtained melt was appropriately treated and processed as follows.
These were cast into the body using continuous casting.
After that, a round bar was formed by forging, and a tube was formed by making a hole in it. The diameter of the tube was then reduced by using hot extrusion at a temperature in the range of 1120 ° C to 1150 ° C, and the resulting tube had a 96-98% reduction in cross section. Hot extrusion was followed by pickling to remove the glass beads.
The diameter was further reduced by pilgaring and the cross section of the tube was reduced in the range of 80-86%.
Then, the pilgared tube was annealed in an atmosphere consisting of a gas mixture containing about 2% nitrogen and the balance of argon gas, and the tube was exposed to a temperature of about 1030 ° C. for about 1 minute.

In the pilgaring step, Q is about 1.0.

After annealing, the obtained tube was subjected to a straightening step. The straightening was performed with a roll straightening machine that combined bending and elliptical machining. A series of bending motions was applied to the pipe by rotating the pipe through a series of tilted rollers. During the correction, the yield strength was exceeded in order to obtain a permanent shape change to obtain a straight tube.

Since the obtained pipe has an outer diameter of 30 mm, the pipe can be used as a GDI rail in a fuel injection system for conducting fuel injected into the combustion chamber of an internal combustion engine.

One additional tube of melt 1 was also manufactured according to the method disclosed above. This tube had an outer diameter from 8 mm after the Pilgaring step. This tube was also used as a fuel line in a fuel injection system to conduct the fuel injected into the combustion chamber of an internal combustion engine.

Claims (15)

By weight%, the following composition:
C maximum 0.06;
Cr 21 to 24.5;
Ni 2.0-5.5;
Si maximum 1.5;
Mo 0.01-1.0;
Cu 0.01-1.0;
Mn maximum 2.0;
N 0.05-0.3;
P maximum 0.04;
S maximum 0.03;
Optionally, one or more elements 1.0 or less selected from the group consisting of Al, V, Nb, Ti, O, Zr, Hf, Ta, Mg, Ca, La, Ce, Y and B; Remaining Fe and unavoidable impurities
Consists of
A method of producing duplex stainless steel tubing with a PRE value of at least 23.0.
The following steps:
a) Providing a duplex stainless steel melt;
b) Casting a duplex stainless steel body from the melt;
c) Forming a rod of the body;
d) Forming a tube of rods by creating holes in it;
e) Reduce tube diameter and / or wall thickness by hot extrusion;
f) Further reducing the diameter and / or wall thickness of the tube by cold deformation; and g) including annealing the cold deformed tube.
After step g), the two-phase stainless steel of the resulting tube consists of 40-60% austenite and 40-60% ferrite, where step g) puts the tube in the range of 950 ° C to 1060 ° C. A method comprising subjecting to a temperature of 0.3 to 10 minutes and an atmosphere consisting of a gas mixture containing 1 to 6 vol% nitrogen gas and the balance being H ₂ or an inert gas. The method of claim 1, wherein the temperature range is 970 ° C to 1040 ° C. The method of claim 1, wherein the temperature range is 1000 ° C to 1040 ° C. The method according to any one of claims 1 to 3, wherein the annealing step comprises subjecting the tube to the temperature for 0.5 to 5 minutes. The method according to any one of claims 1 to 4, wherein the inert gas is argon or helium or a mixture thereof. The method according to any one of claims 1 to 5, wherein the content of nitrogen gas in the gas mixture is 4 vol% or less. The method according to any one of claims 1 to 6, wherein the content of nitrogen gas in the gas mixture is 1.5 vol% or more. One of claims 1 to 7, wherein step e comprises subjecting the tube to the hot extrusion at a temperature in the range of 1100 ° C. to 1200 ° C. and subjecting it to a 92-98% reduction in cross-sectional area. The method described in. The method according to any one of claims 1 to 8, wherein step f comprises subjecting the tube to cold deformation without preheating. The method of any one of claims 1-9, wherein step f comprises subjecting the tube to a cross-section reduction in the range of 50-95%. The method according to any one of claims 1 to 10, wherein the cold deformation is pilgaring. In the Pilgaring step, the relationship between the reduction in the wall thickness of the pipe and the reduction in the outer diameter is as follows:
Q value = (Wallh-Wallt) * (Odh-Wallh) / Wallh ((Odh-Wallh)-(Odt-Wallt))
[During the above ceremony,
Wallh = Hollow wall = Wall thickness before Pilgaring Wallt = Tube wall = Wall thickness after Pilgaring Odh = Hollow OD = Diameter of tube before Pilgaring Odt = Tube OD = Diameter of tube after Pilgaring]
The method of claim 11, wherein Q is in the range of 0.5 to 2.5. The method according to claim 12, wherein Q is in the range of 0.9 to 1.1. Duplex stainless steel by weight%
C 0.01-0.025;
Si 0.35-0.6;
Mn 0.8-1.5;
Cr 21 to 23.5;
Ni 3.0-5.5;
Mo 0.10 to 1.0;
Cu 0.15 to 0.70;
N 0.090 to 0.25;
P 0.035 or less;
S 0.003 or less;
Optionally, one or more elements 1.0 or less selected from the group consisting of Al, V, Nb, Ti, O, Zr, Hf, Ta, Mg, Ca, La, Ce, Y and B; Remaining Fe and unavoidable impurities
It consists method according to any one of claims 1 to 13. The method according to any one of claims 1 to 14, wherein the pipe is a pipe for conducting fuel in a fuel injection system for injecting fuel into a combustion chamber of an internal combustion engine.