A steel for an automobile engine fuel injection nozzle and a method for manufacturing the same

Abstract

The present application relates to a kind of automobile engine fuel injection nozzle steel and its manufacturing method, belong to automobile steel technical field, the chemical composition mass percentage of the automobile engine fuel injection nozzle steel is C:0.47~0.54%, Si:0.10~0.37%, Mn:0.55~0.85%, Cr:1.00~1.30%, P:≤0.020%, S:0.009~0.015%, Ni:≤0.10%, Mo:0.16~0.26%, Al:0.012~0.038%, N:0.0025~0.0065%, Ti:≤0.0020%, the balance is Fe and inevitable impurities.By converter-refining-vacuum degassing-continuous casting-water tank quenching-heating-rolling-rolling heat preservation-on-line cooling and other process steps, obtain a kind of automobile engine fuel injection nozzle steel, steel non-metallic inclusion A fine ≤20um, A fine long / width ≤8;A coarse ≤25um, A coarse long / width ≤5, steel TiN size ≤5um.Any pearlite structure volume size is 300-450um 3 , evenly distributed between ferrite structure.

Description

Technical Field

[0001] This invention relates to the technical field of special steel smelting, rolling and heat treatment, specifically to a steel for automobile engine fuel injectors and its manufacturing method. Background Technology

[0002] The working principle of a car engine fuel injector: When the solenoid coil is energized, it generates electromagnetic attraction, which lifts the needle valve, opening the injection orifice. Fuel is then sprayed at high speed through the annular gap between the needle head of the needle valve and the injection orifice, forming a mist that promotes complete combustion. The internal pressure of the engine fuel injector can reach up to approximately 200 MPa during operation, therefore requiring extremely high pressure resistance of the steel. The corresponding steel must possess high purity and a highly uniform microstructure. Summary of the Invention

[0003] To meet the requirements of automotive engine fuel injectors, a steel for automotive engine fuel injectors and its manufacturing method have been invented. The steel's non-metallic inclusions are: fine A ≤ 20 μm, fine A length / width ≤ 8; coarse A ≤ 25 μm, coarse A length / width ≤ 5; the maximum length between any two points of TiN inclusions in the steel is ≤ 5 μm. The steel hardness is 200-240 HBW, and the hardness variation within the same batch of steel is ≤ 15 HBW.

[0004] The technical solution adopted by this invention to solve the above problems is: a steel for automobile engine fuel injectors, the metallographic structure of which is pearlite and ferrite, and the volume of any pearlite structure is 300-450 μm. 3 The pearlite structure is uniformly distributed among the ferrite structure; the chemical composition of the steel by weight percentage is C: 0.47-0.54%, Si: 0.10-0.37%, Mn: 0.55-0.85%, Cr: 1.00-1.30%, P: ≤0.020%, S: 0.009-0.015%, Ni: ≤0.10%, Mo: 0.16-0.26%, Al: 0.012-0.038%, N: 0.0025-0.0065%, Ti: ≤0.0020%, with the balance being Fe and unavoidable impurities.

[0005] The main functions and design basis of each chemical element in the steel of this invention are as follows:

[0006] C: Carbon is the most important element for improving the strength and high pressure resistance of steel. To ensure that the steel of this invention has sufficient strength and high pressure resistance, this invention adopts a medium carbon design, with the C content selected in the range of 0.47 to 0.54%.

[0007] Si: Silicon is a commonly used deoxidizer in the smelting of killed steel, playing a certain deoxidation role. In this invention, the Si content is selected in the range of 0.10% to 0.37%.

[0008] Mn: Manganese plays a role in improving the strength of steel. Manganese can also combine with sulfur to form long strips of manganese sulfide. After being heated at high temperature in a steel rolling furnace and then rolled in multiple passes with high pressure, the long strips of manganese sulfide continuously break, deform and elongate, and break again, eventually forming short and flat manganese sulfide.

[0009] Cr: In medium carbon steel, it reacts with carbon to form a large number of granular Cr3C metallic compounds. After the steel is heated at high temperature, rolled in multiple passes with high pressure, the granular Cr3C metallic compounds are dispersed in the steel matrix, thereby greatly improving the strength and high pressure resistance of the steel. The Cr content in this invention is selected in the range of 1.00 to 1.30%.

[0010] P: Phosphorus can significantly reduce the low-temperature toughness of steel. The phosphorus content in this invention is selected within the range of P≤0.020%.

[0011] S: Sulfur exists in the steel of this invention in the form of manganese sulfide (MnS). This invention only requires the addition of a small amount of sulfur to the steel, combined with a reasonable rolling heating and rolling process, to ultimately form dispersed, short, and flat manganese sulfide, which can significantly improve the machinability of the steel. The S content in this invention is selected in the range of 0.009–0.015%.

[0012] Ni: Nickel is a precious metal and exists as a residual element in the steel of this invention. The selected range for Ni content in this invention is Ni: ≤0.10%.

[0013] Mo (Mo) can improve the stability of the annealed structure of steel and reduce the variation in hardness after annealing. The Mo content in this invention is selected in the range of 0.16% to 0.26%.

[0014] Al: Aluminum is a strong deoxidizer and plays a very good role in deoxidation of steel. The oxygen remaining after deoxidation combines with nitrogen in the steel to form AlN particles, which pin grain boundaries and refine the steel grains.

[0015] N: The steel grade of this invention is subsequently processed by medium-frequency induction hardening and tempering. Therefore, only a small amount of AlN is needed to pin the grain boundaries of the steel. If there are a large number of AlN particles in the steel, it will reduce the surface quality of the continuously cast billet. Therefore, the N content of the steel of this invention is selected in the range of 0.0025% to 0.0065%.

[0016] Ti: The steel grade of this invention has strict requirements for steel purity. Ti and N have a strong bonding force, forming angular quadrilateral TiN, which reduces the fatigue life of the steel used for fuel injectors. This invention uses a reasonable steelmaking and continuous casting process to reduce the titanium content in the steel and control the size of TiN. The titanium content in the steel of this invention is ≤0.0020%, and the size of TiN is ≤5µm.

[0017] The manufacturing method of the steel used for automobile engine fuel injectors includes the following steps:

[0018] Steel smelting

[0019] The steelmaking stage includes converter oxygen blowing decarburization, refining, and vacuum degassing.

[0020] Continuous casting

[0021] During the continuous casting stage, the high-temperature billet is directly quenched in a water bath after exiting the crystallizer. When entering the water bath, the thickness of the solid shell on the surface of the billet is 15-20 mm. The temperature of the liquid steel inside the billet is >1455℃, and the temperature of the billet exiting the water bath is ≤200℃. All the solid austenite in the steel is transformed into martensite. TiN inclusions will not precipitate when the liquidus temperature is above 1455℃. When the temperature of the steel is ≤1455℃, Ti and N elements in the solid austenitic steel begin to precipitate at the solidification front and gradually accumulate and grow, forming large-sized TiN inclusions in the steel. When the billet temperature is ≤200℃, all the solid austenite in the steel is transformed into martensite, and Ti and N elements in the steel can no longer precipitate. The continuous casting billet is directly quenched after leaving the crystallizer, which effectively shortens the existence time of the solid austenite structure in the steel, reduces the precipitation time of Ti and N elements in the steel, reduces the precipitation amount of Ti and N elements, and thus reduces the size of TiN. The size of TiN is ≤5um.

[0022] Heated rolling

[0023] Before rolling, the continuously cast billet is heated to a temperature of over 1200℃ across its entire cross-section. The heated billet then undergoes a large reduction, multi-pass rolling process, with a reduction rate ≥15% per pass, ≥10 rolling passes, and a billet rolling temperature ≥1050℃ per pass. This application not only observed the length of Class A billets but also evaluated and limited the length-to-width ratio of Class A billets.

[0024] Post-rolling cooling

[0025] After rolling, the high-temperature steel is subjected to water quenching in a water tank to rapidly reduce its temperature to 650°C. After quenching, the steel is held in a holding furnace at 650°C for 1 hour. The high-temperature steel initially has an austenitic structure. After quenching, some of the austenitic structure rapidly transforms into a ferrite structure with lower hardness. When the steel cools to 650°C, the remaining austenitic structure begins to transform into a pearlite structure with higher hardness in the holding furnace at a constant temperature (650°C). To ensure that the pearlite structure has a relatively uniform size after transformation and to guarantee the uniformity of the pearlite structure distribution, the steel needs to be held in a holding furnace at a constant temperature (650°C) for 1 hour after quenching to 650°C. After holding, the steel is cooled to room temperature with the furnace. Both the heat preservation and cooling processes need to ensure the uniformity of the furnace temperature. The maximum temperature difference in the furnace should be ≤5℃. Small temperature fluctuations in the furnace are conducive to the transformation of the remaining austenite structure in the steel into a pearlite structure with a more uniform volume. At the same time, it is also conducive to reducing the fluctuation range of the steel hardness. The hardness of the produced steel can be stably controlled at 200-240HBW, and the hardness difference of the same batch of steel should be ≤15HBW.

[0026] Preferably, during the steelmaking stage, oxygen blowing in the converter ensures the oxidizing properties of the molten steel, controlling the final carbon content to 0.02-0.05%; ferrosilicon, ferrochrome, ferromolybdenum, and manganese nitrogen wires are added as early as possible during the refining process, allowing the residual titanium in the alloy to fully react with the oxygen in the molten steel, forming titanium dioxide that floats to the surface of the molten steel and is adsorbed by the steel slag, reducing the titanium content in the steel to ≤0.0020%; after the titanium removal from the molten steel is completed, aluminum wire is fed in and aluminum particles are added for deep deoxidation; after vacuum degassing of the molten steel, it is allowed to stand, and bottom blowing argon is used during the standing process for ≥25 minutes to remove the deoxidation product aluminum oxide from the steel.

[0027] Preferably, during the heated rolling stage, the sulfides in the steel continuously fracture, deform, elongate, and fracture again under high temperature and multiple passes of high-pressure rolling. The final microstructure contains non-metallic inclusions with Afine ≤ 20 μm and Afine length / width ≤ 8 μm; Acoarse ≤ 25 μm and Acoarse length / width ≤ 5 μm.

[0028] Preferably, during the post-rolling cooling stage, the maximum temperature difference within the furnace during the heat preservation and furnace cooling processes is ≤5℃.

[0029] Preferably, during the heating and rolling stage, the continuously cast billet is heated to 1220-1260°C before rolling.

[0030] Compared with the prior art, the advantages of the present invention are as follows:

[0031] (1) The composition adopts a medium carbon design, with the addition of appropriate amounts of aluminum and small amounts of sulfur and nitrogen elements, and the titanium content is strictly controlled. A reasonable smelting process is used to control the titanium content and the size of titanium nitride in the steel, and a reasonable rolling process is used to control the size and morphology of sulfides in the steel.

[0032] (2) The steelmaking process adopts converter-refining-RH furnace vacuum degassing, converter oxygen blowing for deep decarburization, and the converter controls the final carbon at a low level of 0.02-0.05% to ensure that the molten steel has strong oxidizing properties; in the refining process, silicon manganese, ferrochrome, ferromolybdenum and nitrogen manganese wire are added as early as possible to allow the residual titanium in the alloy to fully react with the oxygen in the molten steel to form titanium dioxide that floats to the surface of the molten steel and is adsorbed by the steel slag, thereby reducing the titanium content in the steel to ≤0.0020%; after the molten steel is detitanized, aluminum wire is fed in and aluminum particles are added for deep deoxidation; after the RH furnace vacuum degassing is completed, the molten steel is held in place by bottom blowing argon for ≥25min to remove large-sized deoxidation products aluminum oxide in the steel; in the continuous casting process, full-process protective casting is adopted, and the high-temperature billet is directly quenched in the water tank after exiting the crystallizer to reduce the size of TiN, and the size of TiN is ≤5um.

[0033] (3) High-temperature heating is used in steel rolling to ensure that the temperature of the entire cross-section of the continuously cast billet is heated to 1220-1260℃. After heating, the continuously cast billet is rolled with large reduction and multiple passes, with a reduction rate of ≥15% per pass, ≥10 rolling passes, and a billet rolling temperature of ≥1050℃ per pass. Under high temperature conditions, the sulfides in the steel continuously fracture, deform and elongate, and fracture again under multiple passes and large reduction rolling. The non-metallic inclusions in the steel are: Afine ≤20um, Afine length / width ≤8; Acoarse ≤25um, Acoarse length / width ≤5.

[0034] (4) The rolled high-temperature steel is immersed in a water tank for water treatment to quickly reduce the temperature of the steel to 650°C. After immersion, the steel is placed in a holding furnace for 1 hour with the furnace temperature set at 650°C. The steel is then cooled to room temperature with the furnace. Both the holding and cooling processes must ensure the uniformity of the furnace temperature, with a maximum temperature difference of ≤5°C.

[0035] (5) The steel for automotive engine fuel injectors produced according to this invention has the following characteristics: Fine A inclusions ≤20µm, fine A length / width ≤8µm; coarse A inclusions ≤25µm, coarse A length / width ≤5µm; TiN size ≤5µm. The steel hardness is 200-240 HBW, with a hardness variation of ≤15 HBW within the same batch. The volume of any pearlite structure in the steel is 300-450µm. 3 Pearlite is evenly distributed among ferrite. Attached Figure Description

[0036] Figure 1 The fine dimensions and morphology (x1000x) of non-metallic inclusion A in Embodiment 1 of the present invention;

[0037] Figure 2 The coarse dimensions and morphology (x1000x) of non-metallic inclusion A in Embodiment 1 of the present invention;

[0038] Figure 3 The fine dimensions and morphology (X1000x) of non-metallic inclusion A in Embodiment 2 of the present invention;

[0039] Figure 4 The coarse dimensions and morphology (X1000x) of non-metallic inclusion A in Embodiment 2 of the present invention. Detailed Implementation

[0040] The present invention will be further described in detail below with reference to the embodiments. The embodiments are exemplary and intended to explain the present invention, but should not be construed as limiting the present invention.

[0041] Example 1 and Example 2:

[0042] The two embodiments involve a method for manufacturing steel for automobile engine fuel injectors: 100t converter primary refining → 100t refining furnace refining → RH furnace vacuum degassing → continuous casting square billet (240mmX240mm) → water bath quenching → continuous casting billet high temperature heating → large reduction and multi-pass rolling → water cooling of rolled steel to 650℃ → steel placed in a holding furnace for holding and then cooled with the furnace.

[0043] The steelmaking process employs a converter-refining-RH furnace vacuum degassing process, with deep decarburization via oxygen blowing in the converter. The final carbon content in the converter is low, at 0.03% (Example 1) and 0.05% (Example 2), to ensure the molten steel has strong oxidizing properties. During the refining process, silicon manganese, ferrochrome, ferromolybdenum, and nitrogen manganese wire are added early to allow residual titanium in the alloy to fully react with oxygen in the molten steel, forming titanium dioxide that floats to the surface of the molten steel and is adsorbed by the steel slag. The titanium content in the steel is 0.0009% (Example 1) and 0.0020% (Example 2). After titanium removal from the molten steel, aluminum wire is fed in and aluminum granules are added for deep deoxidation. After vacuum degassing in the RH furnace, the steel... The liquid settling process employed bottom-blowing argon gas for 33 min (Example 1) and 26 min (Example 2) to remove large-sized deoxidation product aluminum oxide from the steel. The continuous casting process employed full-process protective casting. After exiting the crystallizer, the high-temperature continuously cast billet was directly immersed in a water bath for quenching. The thickness of the solid shell on the surface of the billet upon quenching was 15.3 mm (Example 1) and 19.8 mm (Example 2). The temperature of the liquid steel inside the billet was 1458℃ (Example 1) and 1463℃ (Example 2). The temperature of the billet exiting the water bath was 196℃ (Example 1) and 185℃ (Example 2). The maximum size of TiN in the steel was 2.8 μm (Example 1) and 4.1 μm (Example 2).

[0044] The chemical composition of the steels obtained in Examples 1 and 2 is shown in Table 1.

[0045] Table 1 (wt%)

[0046] C Si Mn Cr P S Ni Mo Al N Ti Example 1 0.47 0.12 0.59 1.05 0.017 0.010 0.02 0.17 0.013 0.0028 0.0012 Example 2 0.53 0.35 0.83 1.28 0.015 0.015 0.03 0.25 0.035 0.0062 0.0019

[0047] The steel rolling process employed high-temperature heating. The temperature of the entire cross-section of the continuously cast billet was heated to 1227℃ (Example 1) and 1253℃ (Example 2). The heated billet underwent high-reduction, multi-pass rolling, with a reduction rate ≥15% per pass. The number of rolling passes was 10 (Example 1) and 12 (Example 2), and the billet rolling temperature per pass was ≥1050℃. Under high-temperature conditions and multiple passes with high reduction, sulfides in the steel continuously fractured, deformed, elongated, and fractured again. The non-metallic inclusion A had a slender length of 14.59 μm, a slender width of 2.05 μm, and a slenderness / width ratio of 7.1; the coarse length of A was 19.05 μm, and the coarse width was 4.43 μm, with a coarse length / width ratio of 4.3 (Example 1, see details). Figure 1 and Figure 2 Non-metallic inclusion A has a fine length of 15.13 μm, a fine width of 2.16 μm, and a fine length / width ratio of 7.0; and a coarse length of 13.77 μm, a coarse width of 4.21 μm, and a coarse length / width ratio of 3.3 (Example 2, see details). Figure 3 and Figure 4 ).

[0048] The rolled high-temperature steel is subjected to water immersion treatment in a water tank to rapidly reduce its temperature to 650°C. After immersion, the steel is placed in a holding furnace and held at 650°C for 1 hour. The steel is then cooled to room temperature in the furnace. Both the holding and cooling processes must ensure the uniformity of the furnace temperature, with a maximum temperature difference of 3°C (Example 1) and 2°C (Example 2).

[0049] The volumetric size of any pearlite structure in the steels obtained in Examples 1 and 2 is between 300-450 μm. 3 Pearlite is evenly distributed among ferrite.

[0050] The steel obtained in Example 1 has a hardness range of 209-221 HBW, with a hardness variation of 12 HBW within the same batch. The steel obtained in Example 2 has a hardness range of 224-235 HBW, with a hardness variation of 11 HBW within the same batch.

[0051] This invention relates to steel for automotive engine fuel injectors. It employs a medium-carbon composition with added aluminum and small amounts of sulfur and nitrogen. A rational smelting and continuous casting process controls the titanium content and titanium nitride size, while a rational rolling process controls the size and morphology of sulfides. The rolled high-temperature steel is then rapidly cooled in water and subsequently held and cooled in a heat-holding furnace with good temperature uniformity. Through this rational steelmaking, rolling, and post-rolling heat-holding and cooling process, a new type of steel for automotive engine fuel injectors has been successfully manufactured, filling a domestic gap in the market.

Claims

1. A type of steel for automotive engine fuel injectors, characterized in that: The metallographic structure consists of pearlite and ferrite, with the volume of any pearlite structure ranging from 300 to 450 μm. 3 The pearlite structure is uniformly distributed among the ferrite structures; the chemical composition of the steel (by weight percentage) is: C: 0.47–0.54%, Si: 0.10–0.37%, Mn: 0.55–0.85%, Cr: 1.00–1.30%, P: ≤0.020%, S: 0.009–0.015%, Ni: ≤0.10%, Mo: 0.16–0.26%, Al: 0.012–0.038%, N: 0.00 25~0.0065%, Ti:≤0.0020%, balance is Fe and unavoidable impurities; steel hardness is 200-240HBW, hardness variation within the same batch of steel ≤15HBW; referring to GB / T10561—2005 Determination of Non-metallic Inclusions in Steel, the steel contains fine A ≤20um, fine A length / width ≤8; coarse A ≤25um, coarse A length / width ≤5, and the maximum length between any two points of TiN inclusions in the steel ≤5um; The method for preparing the steel for automobile engine fuel injectors includes: Steel smelting The steelmaking stage includes converter oxygen blowing decarburization, refining, and vacuum degassing; Continuous casting During the continuous casting stage, the high-temperature billet is directly placed into a water tank for quenching after exiting the crystallizer. When entering the water tank, the thickness of the solid phase shell on the surface of the continuous casting billet is 15-20mm, the temperature of the liquid phase molten steel inside the billet is >1455℃, and the temperature of the billet exiting the water tank is ≤200℃. All the solid phase austenite in the steel is transformed into martensite. Heated rolling Before rolling, the continuous casting billet is heated to a temperature of over 1200℃ across the entire cross section. The heated billet is then subjected to large reduction and multiple rolling passes, with a reduction rate of ≥15% per pass, ≥10 rolling passes, and a billet rolling temperature of ≥1050℃ per pass. Post-rolling cooling After rolling, the high-temperature steel is cooled by water to reduce the temperature to 650°C. After water cooling, the austenitic structure of the steel is rapidly transformed into a ferrite structure with lower hardness. Then, it is placed in a furnace for heat preservation at 650°C for more than 1 hour. The remaining austenitic structure of the steel is transformed into a pearlite structure with uniform volume and higher hardness. After heat preservation, the steel is cooled to room temperature with the furnace.

2. The method for manufacturing steel for automobile engine fuel injectors according to claim 1, characterized in that: During the steelmaking stage, oxygen blowing in the converter ensures the oxidizing properties of the molten steel, controlling the final carbon content to 0.02-0.05%. In the refining process, silicon manganese, ferrochrome, ferromolybdenum, and nitrogen manganese wires are added as early as possible to allow residual titanium in the alloy to fully react with oxygen in the molten steel, forming titanium dioxide that floats to the surface of the molten steel and is adsorbed by the steel slag, reducing the titanium content in the steel to ≤0.0020%. After titanium removal from the molten steel, aluminum wire is fed in and aluminum particles are added for deep deoxidation. After vacuum degassing of the molten steel, it is allowed to stand. During the standing process, bottom blowing argon is used for ≥25 minutes to remove the deoxidation product aluminum oxide from the steel.

3. The method for manufacturing steel for automobile engine fuel injectors according to claim 1, characterized in that: During the heated rolling stage, the sulfides in the steel continuously fracture, deform, elongate, and fracture again under high temperature and multiple passes of rolling with high pressure. The final microstructure contains non-metallic inclusions with Afine ≤ 20 μm and Afine length / width ≤ 8 μm; Acoarse ≤ 25 μm and Acoarse length / width ≤ 5 μm.

4. The method for manufacturing steel for automobile engine fuel injectors according to claim 1, characterized in that: During the post-rolling cooling stage, the maximum temperature difference inside the furnace during the heat preservation and furnace cooling processes is ≤5℃.

5. The method for manufacturing steel for automobile engine fuel injectors according to claim 1, characterized in that: During the heating and rolling stage, the continuously cast billet is heated to 1220-1260℃ before rolling.

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