SURFACE-TREATED TOOL STEEL FOR METAL DIE CASTING

Abstract

A tool steel contains iron (Fe); carbon (C) in a range of 0.001 wt.% to 0.1 wt.%; copper (Cu) in a range of 0.2 wt.% to 2.0 wt.%; nickel (Ni) in a range of 3 wt.% to 10 wt.%; aluminum (Al) in a range of 0.5 wt.% to 3 wt.%; manganese (Mn) in a range of 0.2 wt.% to 1.5 wt.%; chromium (Cr) in a range of 0 wt.% to 1.5 wt.%; molybdenum (Mo) in a range of 0 wt.% to 1.5 wt.%; tungsten (W) in a range of 0 wt.% to 1.5 wt.%; and niobium (Nb) in a range of 0 wt.% to 0.2 wt.%. A transition layer is applied to the steel substrate. The metal of the transition layer contains nickel in a range of 5 wt.% to 20 wt.% and copper in a range of 1 wt.% to 5 wt.%. An oxide layer or an oxide / nitride layer is present on the transition layer.

Description

INTRODUCTION

[0001] The information provided in this section is intended to provide a general context for the disclosure. The work of the presently named inventors, to the extent described in this section, as well as those aspects of the description that do not otherwise qualify as prior art at the time of filing, are neither expressly nor implicitly admitted as prior art against the present disclosure.

[0002] The present disclosure relates to die casting tools and, more particularly, to tool steel and surface-treated tool steel for die casting metals.

[0003] Molten metal die casting uses tools, including dies made of tool steel. A lubricant is typically applied to the die surfaces that come into contact with the molten metal. The lubricant is used between cycles to lower the die temperature and prevent molten metal, such as aluminum, from adhering to the die during die casting. It takes time for the lubricant to spray onto the die surfaces, reducing productivity. Furthermore, the lubricant has both adverse economic and environmental impacts.

[0004] The dies are typically water-cooled. When the molten metal is poured into a die cavity, the mold is typically exposed to large thermal gradients, which lead to thermal fatigue of the mold. SUMMARY

[0005] A tool steel for a die for casting metal contains iron (Fe); carbon (C) in a range of 0.001 wt% to 0.1 wt%; copper (Cu) in a range of 0.2 wt% to 2.0 wt%; nickel (Ni) in a range of 3 wt% to 10 wt%; aluminum (Al) in a range of 0.5 wt% to 3 wt%; manganese (Mn) in a range of 0.2 wt% to 1.5 wt%; chromium (Cr) in a range of 0 wt% to 1.5 wt%; molybdenum (Mo) in a range of 0 wt% to 1.5 wt%; tungsten (W) in a range of 0 wt% to 1.5 wt%; and niobium (Nb) in a range of 0 wt% to 0.2 wt%. A transition layer with a thickness in a range of 0.1 µm to 3 µm is arranged on the steel substrate, wherein the metal of the transition layer contains Ni in a range of 5 wt% to 20 wt% and copper in a range of 1 wt% to 5 wt%.An oxide layer or an oxide / nitride layer with a thickness in a range of 1 µm to 10 µm is arranged on the transition layer.

[0006] Other features include a Ni:Al ratio of greater than or equal to 2. The chromium (Cr) content ranges from 0.01 wt% to 1.5 wt%. The molybdenum (Mo) content ranges from 0.01 wt% to 1.5 wt%. The tungsten (W) content ranges from 0.01 wt% to 1.5 wt%. The niobium (Nb) content ranges from 0.01 wt% to 0.2 wt%.

[0007] In other features, the microstructure of the tool steel after austenitization is a mixture of martensite, bainite and ferrite and nanoprecipitation with a size of less than 20 nm and a density of more than 10 24 / m 3 and an austenite content of less than 5 vol.%. The hardness of the tool steel after quenching from austenitization is less than 30 HRC.

[0008] In other characteristics, the hardness of the tool steel after hardening and oxidation is at least 42 HRC. The hardness of the tool steel after tempering and oxynitriding is at least 49 HRC. After tempering and surface treatment, the thermal conductivity is more than 35 W / mK.

[0009] A method for surface treatment of tool steel comprises providing tool steel containing iron (Fe), carbon (C) in a range of 0.001 wt% to 0.1 wt%, copper (Cu) in a range of 0.2 wt% to 2 wt%, nickel (Ni) in a range of 3 wt% to 10 wt%, aluminum (Al) in a range of 0.5 wt% to 3 wt%, manganese (Mn) in a range of 0.2 wt% to 1.5 wt%, chromium (Cr) in a range of 0 wt% to 1.5 wt%, molybdenum (Mo) in a range of 0 wt% to 1.5 wt%, tungsten (W) in a range of 0 wt% to 1.5 wt%, niobium (Nb) in a range of 0 wt% to 0.2 wt%.-%; austenitizing the tool steel at a first temperature in a range from 900 °C to 950 °C for a first time period in a range from 0.5 h to 24 h; creating a geometry of a die using the tool steel; and one of the following process steps: oxidizing the tool steel at a second temperature in a range from 380 °C to 600 °C for a second time period in a range from 0.1 h to 40 h; oxynitriding the tool steel at a third temperature in a range from 380 °C to 600 °C for a third time period in a range from 0.1 h to 48 h; oxidizing the tool steel after hardening at a fourth temperature in a range from 380 °C to 600 °C for a fourth time period in a range from 0.1 h to 20 h; and oxynitriding the tool steel after hardening at a fifth temperature in a range of 380 °C to 600 °C for a fifth time period in a range of 0.1 h to 24 h.

[0010] In other features, the method includes using the die to cast aluminum or an aluminum alloy. The tool steel is oxidized, the second temperature is in a range of 450°C to 550°C, and the second time is in a range of 2 hours to 30 hours. In other features, the tool steel is nitrided and oxidized, the third temperature is in a range of 450°C to 550°C, and the third time is in a range of 2 hours to 36 hours.

[0011] In other characteristics, the tool steel is oxidized, the fourth temperature is in a range of 450 °C to 550 °C, and the fourth time is in a range of 1 h to 10 h. The tool steel is nitrided and oxidized, the fifth temperature is in a range of 450 °C to 550 °C, and the fifth time is in a range of 1 h to 12 h.

[0012] In other features, the tool steel after oxidation or oxidation and hardening comprises an oxide layer having a thickness in a range of 1 µm to 10 µm, a transition layer having a thickness in a range of 0.1 µm to 3 µm, and a transition layer metal containing Ni in a range of 5 wt% to 20 wt% and Cu in a range of 1 wt% to 5 wt%.

[0013] In other features, the tool steel after oxynitriding or oxynitriding and hardening comprises a nitride layer having a thickness in a range of 30 µm to 100 µm; an oxide layer having a thickness in a range of 1 µm to 10 µm; and a transition layer having a thickness in a range of 0.1 µm to 3 µm and a transition layer metal containing Ni in a range of 5 wt% to 20 wt% and Cu in a range of 1 wt% to 5 wt%.

[0014] Further areas of applicability of the present disclosure will become apparent from the detailed description, claims, and drawings. The detailed description and specific examples are for purposes of illustration only and are not intended to limit the scope of the disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The present disclosure will be more fully understood from the detailed description and the accompanying drawings, in which: Fig. 1A to 1E are side cross-sectional views of an example of a tool including dies made of conventional tool steel such as H13 for casting light metal such as aluminum; Fig. 2A is a side cross-sectional view of an example of a tool having dies made of a tool steel with a surface treatment according to the present disclosure; Fig. 2B and Fig. 2C are side cross-sectional views illustrating examples of surface treatment, including oxidizing and oxynitriding, of the tool steel according to the present disclosure; Fig. 3A and Fig. 3B are enlarged views illustrating H13 tool steel and the tool steel with the surface treatment according to the present disclosure, respectively; Fig. 4 is a graph illustrating the thermal conductivity of H13 tool steel and the tool steel with the surface treatment according to the present disclosure; Fig. 5 is a side cross-sectional view of a die with one or more cooling channels during heating by the molten metal; Fig. 6 is a graph showing an example of tool steel temperatures as a function of time during austenitizing, geometric forming, hardening, and oxidizing (or oxynitriding) according to the present disclosure; Fig. 7A is a cross-sectional view showing the cohesion of the surface layers after oxidizing (or oxynitriding) the tool steel with the surface treatment according to the present disclosure; and Fig. 7B is a cross-sectional view showing the cohesion of surface layers after immersing the hardened tool steel with the surface treatment according to the present disclosure in molten aluminum.

[0016] In the drawings, reference symbols may be used multiple times to identify similar and / or identical elements. DETAILED DESCRIPTION

[0017] The present disclosure relates to a tool comprising one or more tool steel dies and a surface treatment for the tool steel used to manufacture the dies. The tool is used for casting components made of light metal such as aluminum or aluminum alloys. The tool steel contains a higher content of nickel (Ni), aluminum (Al), and copper (Cu) (to improve the hardness of the tool steel and the cohesion of the iron oxide layer) and a lower carbon (C) content (to improve thermal conductivity).

[0018] In some examples, the surface treatment involves oxidizing (or oxynitriding) the tool steel to form an oxide layer or an oxynitride layer. During the oxidation (oxynitriding) process, a transition layer (containing metal with higher Ni and Cu contents than the tool steel) forms beneath the oxide layer. The surface treatment reduces the adhesion of the cast metal to the dies and increases the wear resistance of the dies.

[0019] The tool according to the present disclosure can be used to cast lightweight metal components without the need for die lubricants (or optionally with significantly less lubricant). For example, the typical cycle time for conventional casting of a component is approximately 10-20 seconds for spraying and blowing when using a die lubricant, and a total cycle time of approximately 40-90 seconds. For example, the typical cycle time for a casting is less than 0-10 seconds for spraying and blowing. In some cases, spraying / blowing may be omitted. In some cases, shorter spraying / blowing times may be used. Die lubrication is optional and does not affect the cycle time. For larger castings, the cycle time may exceed 40 seconds. In this example, omitting or reducing die lubrication between casting cycles reduces the cycle time by approximately 5-30%.This results in lower process costs, less environmental impact from the die lubricant and a longer tool life as the thermal fatigue associated with quenching is avoided.

[0020] In Fig. 1A through 1E, an example of a tool 30 is shown, including a first die section 42A, a second die section 42B, and a die fill section 51 defining a cavity 58. The first die section 42A and the second die section 42B define a die cavity 44 and a sprue 48 in fluid communication with the cavity 58. A mold 56 is used to pour molten metal 60 through an opening 54 into the cavity 58 of the die fill section 51. The tool 30 includes a core pin 68 for ejecting the casting.

[0021] After the molten metal has been added to the cavity 58, a piston 64 moves through the cavity 58 to force the molten metal into the die cavity 44, as shown in the Fig. 1B and Fig. 1C. The piston 64 causes the die cavity 44 to be filled with the molten metal and exerts pressure. The molten metal cools and solidifies, and the casting 70 can be ejected by moving the first die section 42A and the second die section 42B apart, as shown in Fig. 1D. In Fig. 1E, a spray system 80 sprays air and / or lubricant onto the surfaces of the tool 30 and blows water away from them.

[0022] In Fig. 2A through 2B, a die 150 is made from a tool steel 152 with a surface treatment 156 according to the present disclosure. As described below, the tool steel 152 contains higher levels of nickel (Ni), aluminum (Al), and copper (Cu), and lower levels of carbon (C) than conventional tool steel for light metal casting. The die 150 further includes a surface treatment 156, which includes oxidation (or oxynitriding) to reduce adhesion to the casting and increase the wear resistance of the die 150.

[0023] In Fig. Figure 2B shows the surface treatment 156 after austenitizing, hardening, and oxidizing. The tool steel 152 comprises a steel substrate 162, a transition layer 166 containing iron oxides and metals such as nickel (Ni) and copper (Cu), and an oxide layer 160.

[0024] In Fig. Figure 2C shows the surface treatment 156 after austenitizing, hardening, and oxynitriding. The tool steel 152 comprises the steel substrate 162, a nitrogen diffusion layer / zone 164, the transition layer 166, and the oxide layer 160.

[0025] During oxidation or oxynitriding, Ni and Cu react less with oxygen than iron. In other words, during oxidation, the atmosphere reacts with an external surface of the tool steel (i.e., oxides are formed with Fe and / or other active elements such as Al and Cr). Ni and Cu do not form oxides and are ejected from the surface oxides into the steel substrate 162. Furthermore, since the diffusion coefficient of Ni and Cu in iron is low, Ni and Cu do not diffuse quickly into the core, and their content becomes much higher at the interface between the core and the oxide layer. As a result, a transition layer 166 forms (in which a large island of iron oxides is interspersed in a matrix of Ni+Cu-rich metal). This creates a strong mechanical oxide-metal bond between layer 160 and the steel substrate 162.

[0026] During oxynitriding, the atmosphere reacts with an external surface of the tool steel (i.e., oxides / nitrides containing Fe and / or other active elements such as Al and Cr form). The transition layer 166 (enriched with Ni and Cu) forms between layer 160 and the nitrogen diffusion layer / zone 164. The molten metal contacts the die surface and promotes the diffusion of oxygen atoms in the molten metal (e.g., Al) into the steel substrate. The energy required to eject Ni+Cu metal in the transition layer into the core reduces the driving force for internal oxidation, slowing internal oxidation and resulting in fewer voids and better coherence.

[0027] In Fig. 3A and Fig. Figure 3B shows differences between conventional tool steel such as H13 and tool steel with the surface treatment according to the present disclosure. Conventional tool steel such as H13 is a medium-carbon steel with a carbon content of approximately 0.4%. The reinforcement is based on martensite with a high density of defects and a certain carbides content. However, the thermal conductivity decreases due to the defects.

[0028] In Fig. 3B, tool steel according to the present disclosure has a carbon content ranging from 0.001 wt% to 0.1 wt%. The reinforcement is based on Ni (Fe, Al) and Cu nanoprecipitation. The tool steel according to the present disclosure exhibits a much lower density of microstructural defects and higher thermal conductivity.

[0029] In Fig. 4, the thermal conductivity of the tool steel according to the present disclosure (at 182) is approximately 40% higher than the thermal conductivity of the H13 tool steel (at 180) at all manufacturing temperatures. Both have the same hardness.

[0030] In Fig. 5, a die 190 typically includes one or more cooling channels 194 for receiving coolant. There is a large temperature gradient when the molten metal, e.g., aluminum, comes into contact with a surface of the die (the heat of the molten metal is shown at 196). Because the die is made of tool steel with a very low carbon content, it has a high thermal conductivity. The high thermal conductivity allows for more even heat distribution throughout the die, resulting in a lower overall die surface temperature. The lower surface temperature suppresses the reaction between the tool steel and the molten metal (e.g., Al) to prevent sticking / brazing of the metal to the tool steel.

[0031] The tool steel according to the present disclosure has the following composition: carbon (C) in a range of 0.001 wt% to 0.1 wt%, copper (Cu) in a range of 0.2 wt% to 2 wt%, nickel (Ni) in a range of 3 wt% to 10 wt%, aluminum (Al) in a range of 0.5 wt% to 3 wt%, manganese (Mn) in a range of 0.2 wt% to 1.5 wt%, chromium (Cr) in a range of 0 wt% to 1.5 wt%, molybdenum (Mo) in a range of 0 wt% to 1.5 wt%, tungsten (W) in a range of 0 wt% to 1.5 wt%, niobium (Nb) in a range of 0 wt% to 0.2 wt%, the balance being iron (Fe) and other components.

[0032] In some examples, a Ni:Al ratio greater than or equal to 2 is used to ensure complete precipitation of Al and reduce interstitial Al, which causes lattice distortion and reduces thermal conductivity.

[0033] In some examples, the chromium (Cr) content ranges from 0.01 wt% to 1.5 wt%. In some examples, the molybdenum (Mo) content ranges from 0.01 wt% to 1.5 wt%. In some examples, the tungsten (W) content ranges from 0.01 wt% to 1.5 wt%. In some examples, the tungsten (Nb) content ranges from 0.01 wt% to 0.2 wt%.

[0034] In some examples, the microstructure of the tool steel is a mixture of martensite, bainite and ferrite and nanoprecipitation with a size of less than 20 nm and a density of more than 10 24 / m 3 with unavoidable austenite (less than 5 vol%).

[0035] In some examples, layer 160 has a thickness in a range of 1.0 µm to 10 µm. In some examples, transition layer 166 has a thickness in a range of 0.1 µm to 3 µm. The metal of transition layer 166 contains Ni in a range of 5 wt% to 20 wt% and copper in a range of 1 wt% to 5 wt%. In some examples, nitrogen diffusion layer / zone 164 has a thickness in a range of 30 µm to 100 µm after oxynitriding.

[0036] The hardness of the tool steel after austenitizing is less than 30 HRC. After hardening and oxidizing, the hardness of the tool steel is at least 42 HRC. After hardening and oxynitriding, the hardness of the nitrogen diffusion layer / zone is at least 49 HRC. The thermal conductivity after surface treatment is more than 35 W / mK.

[0037] In Fig. 6, the tool steel in 320 is austenitized (e.g., after forging or rolling) at a temperature in the range of 900°C to 950°C for a first predetermined time period in the range of 0.5 h to 24 h. After austenitizing, the die geometry in 330 is created by any suitable process. After the die geometry has been created in 330, the die is oxidized or oxynitrided, while in 340 it is hardened at a temperature in the range of 380°C to 600°C for a second predetermined time period in the range of 0.1 h to 48 h. An oxidizing or oxynitriding treatment can also be carried out separately after austenitizing and hardening at a temperature in the range of 400°C to 600°C for a third predetermined time period in the range of 0.1 h to 30 h. Oxidization is carried out by exposing the tool steel to a gas atmosphere containing oxygen.Nitriding is carried out by exposing the tool steel to a gas atmosphere containing nitrogen.

[0038] In some examples, quenching and tempering is combined with oxidizing or oxynitriding to improve die manufacturing efficiency and reduce costs. Austenitizing (also called solution treatment for this material) dissolves Ni, Al, and Cu in the steel matrix, thereby reducing the steel's hardness. Then, the quenching and tempering treatment causes Ni (Fe, Al) and Cu nanoprecipitation and improves hardness. The temperature / time range of quenching and tempering and oxidizing / oxynitriding largely overlap in this disclosure, so the process time is not significantly increased. Optionally, another quenching and tempering treatment can be performed before or after oxidizing or oxynitriding to achieve the desired hardness (usually harder is better).

[0039] The method for manufacturing a die according to the present disclosure reduces processing steps by combining surface treatment with heat treatment and shortens the time required to combine surface treatment by rapid oxidation or oxynitriding. This reduces the cost and lead time for die manufacturing.

[0040] If oxidation is performed after austenitizing, the die is oxidized at a second temperature within a second temperature range of 380°C to 600°C for a second predetermined time period within a range of 0.1 h to 40 h. In some examples, the second temperature range is from 450°C to 550°C. In some examples, the second predetermined time period is within a range of 2 h to 30 h. The die is then cooled to room temperature.

[0041] If oxynitriding is performed after austenitizing, the die is oxynitrided at a second temperature within a second temperature range of 380°C to 600°C for a second predetermined time period ranging from 0.1 h to 48 h. In some examples, the second temperature range is from 450°C to 550°C. In some examples, the second predetermined time period is in a range of 2 h to 36 h. The die is then cooled to room temperature.

[0042] If oxidation is performed after austenitizing and hardening, the die is oxidized at a third temperature within a third temperature range of 380°C to 600°C for a third predetermined time period within a range of 0.1 h to 20 h. In some examples, the third temperature range is from 450°C to 550°C. In some examples, the third predetermined time period is within a range of 1 h to 10 h. The die is then cooled to room temperature.

[0043] If oxynitriding is performed after austenitizing and hardening, the die is oxynitrided at a third temperature within a third temperature range of 380°C to 600°C for a third specified time period ranging from 0.1 h to 24 h. In some examples, the third temperature range is from 450°C to 550°C. In some examples, the third specified time period is in a range of 1 h to 12 h. The die is then cooled to room temperature.

[0044] With reference to Fig. Figure 7A shows a transition layer with the penetration of the oxide and the metal after surface treatment according to the present disclosure. This thin layer contains iron oxide and a Ni+Cu metal content. As shown in Fig.As can be seen in Figure 7B, the cohesion of the surface oxide layers after immersion in Al melt is shown; due to the suppression of internal oxidation by the chemistry of the transition layer of the tool die (e.g., higher Ni and Cu contents), fewer and smaller voids are formed.

[0045] The foregoing description is merely illustrative in nature and is in no way intended to limit the disclosure, its application, or uses. The broad teachings of the disclosure may be implemented in various forms. Therefore, while this disclosure contains specific examples, the true scope of the disclosure should not be so limited because other modifications will become apparent upon a study of the drawings, the specification, and the following claims. It is to be understood that one or more steps within a method may be performed in a different order (or overlapping in time) without altering the principles of the present disclosure.Furthermore, although each of the embodiments has been described above with specific features, one or more of the features described with respect to any embodiment of the disclosure may be implemented and / or combined with its features in any of the further embodiments, even if this combination is not explicitly described. In other words, the described embodiments are not mutually exclusive, and permutations of one or more embodiments with each other remain within the scope of this disclosure.

[0046] Spatial and functional relationships between elements (e.g., between modules, circuit elements, semiconductor layers, etc.) are described using various terms, such as "connected," "engaged," "coupled," "adjacent," "beside," "on top of," "over," "below," and "disposed." Unless explicitly described as "direct," when the above-described disclosure describes a relationship between a first and a second element, that relationship may be a direct relationship, with no other intervening elements between the first and second elements, but it may also be an indirect relationship, with one or more intervening elements (either spatial or functional) between the first and second elements.As used herein, the phrase at least one of A, B, and C shall be construed to mean a logical (A OR B OR C) using a non-exclusive logical OR, and shall not be construed to mean "at least one of A, at least one of B, and at least one of C."

Claims

[1] Surface-treated tool steel for a die for casting metal, comprising: a steel substrate containing iron (Fe), carbon (C) in a range of 0.001 wt% to 0.1 wt%, copper (Cu) in a range of 0.2 wt% to 2.0 wt%, nickel (Ni) in a range of 3 wt% to 10 wt%, aluminum (Al) in a range of 0.5 wt% to 3 wt%, manganese (Mn) in a range of 0.2 wt% to 1.5 wt%, chromium (Cr) in a range of 0 wt% to 1.5 wt%, molybdenum (Mo) in a range of 0 wt% to 1.5 wt%, tungsten (W) in a range of 0 wt% to 1.5 wt% and niobium (Nb) in a range of 0 wt% to 0.2 wt%; a transition layer on the steel substrate having a thickness in the range of 0.1 µm to 3 µm, wherein the metal of the transition layer contains Ni in a range of 5 wt% to 20 wt% and copper in a range of 1 wt% to 5 wt%; and an oxide layer or an oxide / nitride layer on the transition layer with a thickness in a range of 1 µm to 10 µm. [2] Surface-treated tool steel according to claim 1, wherein a ratio of Ni:Al is greater than or equal to 2. [3] Surface-treated tool steel according to claim 1, wherein the chromium content (Cr) is in a range of 0.01 wt% to 1.5 wt%. [4] Surface-treated tool steel according to claim 1, wherein the molybdenum content (Mo) is in a range of 0.01 wt% to 1.5 wt%. [5] Surface-treated tool steel according to claim 1, wherein the tungsten content (W) is in a range of 0.01 wt% to 1.5 wt%. [6] Surface-treated tool steel according to claim 1, wherein the niobium content (Nb) is in a range of 0.01 wt% to 0.2 wt%. [7] Surface-treated tool steel according to claim 1, wherein the microstructure of the tool steel after austenitization is a mixture of martensite, bainite and ferrite and nanoprecipitation with a size of less than 20 nm and a density of more than 10 24 / m 3 and an austenite content of less than 5 vol.%. [8] Surface-treated tool steel according to claim 1, wherein the hardness of the tool steel after quenching from austenitization is less than 30 HRC. [9] Surface-treated tool steel according to claim 8, wherein one of the following applies: after hardening and oxidation, the hardness of the tool steel is at least 42 HRC; and After tempering and oxynitriding, the hardness of the tool steel is at least 49 HRC. [10] Surface-treated tool steel according to claim 1, wherein after tempering and surface treatment the thermal conductivity is more than 35 W / mK.

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