MX2012005738A - Stainless steel for molds having a lower delta-ferrite content. - Google Patents

Abstract

A stainless steel for molds having a lower delta-ferrite content is composed of alloying elements that, in percentages by mass, consist essentially of 0.01 to 0.20 carbon; 0.01 to 0.07 nitrogen; 2.0 to 4.0 manganese; 0.01 to 1.0 nickel; 11.0 to 13.0 chromium; less than 1.0 molybdenum and tungsten taken together; 0.01 to 1.5 copper; 0.01 to 1.0 vanadium; 0.01 to 0.20 sulphur; no more than 0.01 calcium, less than 0.50 aluminium; less than 1.0 silicon, the remainder consisting essentially of Fe and inevitable impurities.

Description

STAINLESS STEEL MOLD WITH REDUCED CONTENT OF FERRITA DELTA Field of the Invention This invention is a stainless steel for general applications in molds that form plastic, particularly, but not limited to, hot chamber molds. Its main characteristic is the combination of properties related to the manufacture of molds, such as machinability, weldability and low cost (associated with low content of nickel (Ni) and pro be easy for the process, in terms of control of undesirable microstructural phase called ferrite delta Due to these manufacturing advantages of mold and steel, this invention allows a considerable reduction of the cost of the mold.

Background of the Invention The tools and molds are usually operated to form other materials, either thermoplastic polymeric materials (commonly known as plastic materials) or metallic materials. Depending on the properties of the material used to make the tools, they are used in processes at high or ambient temperatures, around 700 ° C. The steel of this invention is especially applied to molds or mold devices, which are exposed to room temperature or Ref. : 230842 temperatures below 500 ° C and must be resistant to corrosion. A typical example of such applications are hot chambers used in molds that form plastics, which do not exceed 300 ° C. For such cases, the combined water / temperature cooling effect can lead to corrosion, which explains the need for stainless steels. And, due to the high content of machined material, the machinability property must be optimized.

In addition to these two characteristics, resistance to corrosion and machinability, welding is often applied to mold steels, minor repairs and mold modifications. However, conventional martensitic stainless steels with high chromium content (12 to 17%) and average carbon content (approximately 0.4%) have an extremely high durability causing significant hardness and potential cracking in welded areas (see Table 1). In this way, the development of a low carbon alloy is desirable.

Table 1: Typical chemical composition of traditional steels approximate in the state of the art. The approximate hardness of martensite is shown to highlight the difficulty of weldability caused by the high carbon content. The content in mass percentage and Fe equilibrium.

* Typical values; not specified by the standard In addition to these metallurgical properties, cost emissions have become even more critical. The strong competitiveness, especially considering low cost molds available around the world, makes the mold manufacturers look for low cost options. Under these conditions, a negative metallurgical factor is microstructural stability in terms of absence of delta ferrite. Carbon and nickel are the most important elements to promote the austenitic phase and the elimination of delta ferrite in martensitic steels. However, there is a limitation for carbon, as mentioned above, with respect to solderability problems. And, in the case of nickel, the cost limitation is significant. At higher carbon content, the need for nickel is lower and, thus, the cost of the alloy is higher.

New developments are under way to solve such problem. For example, patents US 6,358,334 and US 6,893,608 B2 serve the production of stainless steels low in nickel and carbon using high levels of copper and nitrogen (see Table 2). However, the appearance of ferrite delta is significant for both, with levels of up to 10% being common. On the other hand, the ferrite delta control in these alloys has an influence on the alloy slab and rolling temperatures. Table 2 shows the equilibrium temperature calculated by the "Thermocalc" thermodynamic calculation software for these alloys. When combined with high sulfur content, low temperatures can easily create cracking or excessive energy in the molding equipment (usually a forging press or rolling mill). Thus, considering all these points, there are some steels of the state of the art low carbon and nickel, but the processing of them is not an easy task, which results in more expensive processes and consequent increase in the cost of the alloy.

Therefore, the need for a stainless steel with high machinability, low nickel and carbon content and increased processing capacity is evident. To allow the reduction of the cost of the steelmaking process, the molding temperatures of the material must be significantly higher than those of steel of the state of the art.

The steel of this invention will completely cover all these needs.

Table 2: Steels of the state of the art developed more recently than the steels shown in Table 1. Content in percentage in raisin and Fe equilibrium. The hardness of the martensite in these alloys, due to the low carbon content, is approximately 35. HRC.

* For AISI 420 steel, the molding temperature can reach up to 1260 ° C.

Brief Description of the Invention The stainless steel for molds, proposed by this invention, can be produced with a lower content of delta ferrite and at temperatures about 30 ° C higher during the rolling or forging processes. Its chemical composition also lacks high-cost elements such as nickel and molybdenum, but the chromium content is sufficient to ensure the inoxidability. And, as previously mentioned, weldability requirements can be achieved due to the lower carbon content.

To satisfy the conditions mentioned above, the alloys of this invention have a composition of alloying elements, which, in mass percentage, consist of: * Carbon: between 0.01 and 0.2, preferably, and between 0.03 and 0.10, typically 0.05.

* Nitrogen: between 0.01 and 0.07, preferably between 0.03 and 0.06, typically 0.055.

* Manganese: between 2.0 and 4.0, preferably between 2.2 and 3.0, typically 2.5 * Nickel: between 0.01 and 1.0, preferably between 0. 1 and 0.5, typically 0.3 * Chrome: between 11.0 and 13.0, preferably between 11.5 and 12.5, typically 12.0 * Molybdenum and Tungsten: the sum should be below 1.0, preferably below 0.5, typically below 0.2.

* Copper: between 0.01 and 1.5, preferably between 0.1 and 0.8, typically 0.55.

* Vanadium: between 0.01 and 1.0, preferably between 0.02 and 0.10, typically 0.05.

* Sulfur: between 0.01 and 0.20, preferably between 0.05 and 0.14, typically 0.09.

* Calcium: below 0.010, preferably between 0.001 and 0.003, typically 0.002.

* Aluminum: below 0.50, typically below 0.10, preferably below 0.050.

* Silicon: below 0.1, preferably below 0.05, typically between 0.1 and 0.6.

Balance by Fe and metallic or non-metallic impurities is inevitable to the steelmaking process.

Then, the relations of the specification of the composition of the new material and a description of the effect of each of the alloying elements are presented. The percentages listed refer to percentages in mass.

C: carbon is the main responsible for the response to heat treatment, and also for the hardness of martensite produced by rapid cooling. Due to the intense heating and rapid cooling, the welding process can be considered similar to rapid cooling. In this way, the carbon content controls the final hardness created in the welded zone of the steel of this invention. Therefore, to achieve the required hardness, the carbon content must be at least 0.01%, preferably above 0.03%. However, the carbon content must be below 0.2%, preferably below 0.1%, so that the hardness in the welded zones is below 40 HRC to prevent cracking and facilitate the machining process.

N: Nitrogen is necessary in the alloy of this invention because it is a powerful austenitizer and reduces the amount of delta ferrite. However, nitrogen increases the resistance to pitting corrosion. On the other hand, an excess of nitrogen can generate gases, since ferrite delta is the first solid phase in the steel of this invention, considering the limited solubility of nitrogen. In this way, the nitrogen content should fall between 0.01% and 0.08%, preferably between 0.02% and 0.06%, typically around 0.05%.

Mn: since Mn is not a more expensive element, but it is a powerful austenitizer, it should be employed at high levels in the steel of this invention. Therefore, its content must be above 2.0%, preferably above 2.2%, typically 2.5%. However, when used in excess, manganese increases the content of retained austenite, as well as the coefficient of hardening of material, reducing machinability, in addition to increasing the solubility of hydrogen and promoting the formation of flakes; in this way, the manganese content should not exceed 4.0%, preferably below 3.0%.

Ni: Nickel is a powerful austenitizer, but it makes the alloy more expensive. To obtain both aspects under control, the nickel content should remain between 0.01 and 1.0%, preferably between 0.10 and 0.50%, and typically, 0.30%.

Cr: chromium confers stainless steel to this invention, being the most important element in terms of this property (due to the low content of Mo and Ni in this alloy). Thus, the chromium content should be above 11.0%, typically above 12.0%. However, chromium is also a primary ferritizer, contributing to increase the delta ferrite content and to reduce the austenitic field. To counterbalance such effects, the Cr content must be less than 13.0%, preferably below 12.5%.

Molybdenum and Tungsten: when combined, the total content must be below 1.0% because they increase the cost of the alloy and the ferrite content. Preferably, the sum should be below 0.5%, typically below 0.2%.

Copper: it is an austenitizer and also promotes the hardening of precipitation required for the response to heat treatment. However, if used excessively, copper can have a negative effect on cost and is a major scrap contaminant. Thus, the copper content should fall between 0.01% and 1.5%, preferably between 0.1% and 0.8%, and typically, 0.55%.

Vanadium: vanadium plays an important role in secondary hardening which, because it is not intense in the steel of this invention, is essential to achieve the post-temper hardness required at high temperature. However, since vanadium is also a ferritizer and has a negative impact on the cost of the alloy, its content must be controlled. Thus, the vanadium content should fall between 0.01% and 1.0%, preferably between 0.05% and 0.50%, typically around 0.1%.

S: in the steel of this invention, the sulfur forms manganese sulfide inclusions (MnS) which become elongated through the hot forming process. As the inclusions become malleable at temperatures developed in the machining process, they facilitate the fragment breaking process and lubricate the cutting tool, thereby improving machinability. To produce this effect, the sulfur content must be greater than 0.01%, preferably above 0.05%, typically above 0.09%. Because it is beneficial to the machining process, inclusions of MnS have a negative effect on the mechanical properties, especially corrosion resistance and toughness. Therefore, the sulfur content should be limited to 0.20%, preferably below 0.15%.

Ca: calcium also has an effect on the inclusions by modifying the hard alumina inclusions that hamper the machinability and reducing the size (spheroidal) of the inclusions in general. This effect is mainly important for the control of inclusions of MnS, making them more distributed and less elongated, thus favoring the machining process and the mechanical properties. However, controlling the calcium content is quite complex due to its high reactivity. In this way, the use of calcium can be considered optional for those cases in which high machinability and polishing capacity are required. If used, the calcium content should not exceed 100 ppm (0.01%) because its solubility in the molten metal and high reactivity (when in contact with refractories) limits the higher values. Preferably, the Ca content should fall between 10 and 30 ppm (0.001 and 0.003%), typically 20 ppm (0.002%).

Al: due to the formation of hard alumina inclusions, the Al content should not be excessively high to hamper machinability. It should be below 0.5%, typically below 0.1%, preferably below 0.05%.

Yes: silicon is used as a deoxidizer, an important agent in situations of low Al content, which is the case of the steel of this invention. However, this element is a ferritizer and if used excessively, favors the formation of delta ferrite. In this way, the silicon content should remain between 0.1% and 1.0%, preferably between 0.2% and 0.7%, typically 0.40%.

Brief Description of the Figures The figures enclosed in this have been referenced in the description of the experiments carried out, and their contents are listed below: Figure 1 shows the increase in the amount of delta ferrite for alloy 1 and PI 1 and PI 2 alloys of the state of the art of this invention. Representative microstructures have also been added.

Figure 2 shows the tempering curves obtained for the three alloys, alloy 1, PI 1 and PI 2 - the hardness of the alloy is low after rapid cooling, changing from 30 to 34 HCR after quenching.

Figures 3a-3b show a comparison of the microstructure of PI 1 and PI 2 alloys for two sulfur contents - note that the increase in the number of inclusions is directly proportional to the increase in the sulfur content.

Figure 4 shows delta ferrite content measured in raw cast samples for the three alloys of Table 4 Detailed description of the invention EXAMPLE 1: The "Thermo-calc" software was used to simulate the effect of N and Mn on the increase of the delta ferrite formation temperature to allow to define the steel composition of this invention. Simulations 1 to 4 show the strong effect of nitrogen, to a composition equivalent to that of document US 6358334. However, the extremely high N content, above 0.06%, already anticipates gas formation during the solidification stage, which generates gaps in the billets, making their use unfeasible. On the other hand, for simulation 5, the effect of Mn associated with a safe upper content of N can be analyzed. In this alloy steel, it is estimated that there is a gain of 30 to 90 ° C at the maximum formation temperature with relation to the alloys of the state of the art. This indicates the possibility of better hot formation and delta ferrite removal, (as mentioned above, reducing corrosion and mechanical resistance).

After this evidence of the strong effects of N and Mn, two compositions have been produced for pilot scale billets and compared with the alloy of US 6358334, later called alloy 1. The alloys of the present invention will be called PI 1 and PI 2. The chemical compositions of the billets are shown in Table 4. The main variables in terms of matrix stability in relation to ferrite formation are the contents of Mn and N; however, the S content of the alloys is also varied, and the respective effects will be discussed later.

Table 3: Equilibrium temperature required to produce 10% by volume of delta ferrite, in various alloys of the state of the art and those proposed by this invention, calculated via "Thermo-calc".

* N2 gas formation during solidification The results of the delta ferrite content measured in raw cast samples for the three alloys of Table 4 are shown in Table 5 ND Figure 4. The increase in the proposed N content results in significant gain (compare alloy 1 against alloy PI 1 ) in terms of the temperature increase required to form 10% delta ferrite. However, the strongest effect takes place after combining the effect of N and Mn, with an even higher gain than that calculated by the thermodynamics software. Apart from the values in Table 4, it is also worth observing the evolution of delta ferrite content as a function of temperature. This is shown in Figure 1, with a clear reduction of the delta ferrite content of the alloy 1 when compared to the alloy PI 1 and, especially, when compared to the alloy PI 2.

Table 4: Chemical composition of pilot-scale billets containing the alloy of the state of the art of finida in patent US 6358334, called alloy 1, and two alloys investigated in the present invention (PI 1 and PI2). Values in percentage in mass and equilibrium by Fe.

Table 5: Fraction of delta ferrite volume in alloy 1 and PI 1 and PI 2 alloys calculated through quantitative metallography. The measurements were taken after 24 hours at the specified temperature.

In terms of the response to heat treatment as shown in Figure 2, the PI 1 and PI 2 alloys are both capable of reaching the 30 to 34 HRC levels required for the applications. It is also worth emphasizing that PI alloys 1 and PI 2 have fast post-curing hardness of approximately 35 to 40 HRC (value extracted from the graph, for rapid cooling temperature = 0 ° C), well below 55/65 HRC of the conventional steels of the state of the technique shown in Table 1.

The content of S of the PI 1 and PI alloys 2 is not the same, and this can be positive or negative for the application, and in this way, the content of S must be specified depending on the application. This problem was investigated for the billets shown in Table 4, but after hot forming for section sizes of 70 x 70 square mm (4x reduction per area). The low values are due to the low degree of reduction applied to the test billets.

The higher S content of the PI 2 alloy results in improved machinability but toughness and lower corrosion resistance. The results of such changes can be seen in Table 5 and, in microstructural terms, the different distribution of the S content of the PI 1 and PI 2 alloys can be seen in Figures 3a-3b. The higher amount of sulfides (dark gray in Figures 3a-3b) and their persistence explain the lower values obtained for corrosion resistance and toughness, respectively. And, in terms of machinability, the predominant factor is the higher sulfur content of the PI 2 alloy.

Therefore, for applications that demand high machinability and low corrosion and toughness requirements, high Si alloys are recommended (approximately 0.15%). For cases of more stringent requirements for toughness and corrosion, alloys with S content around 0.10% are more suitable.

Table 5: Values related to machine 1, corrosion resistance and toughness of PI 1 and PI 2 alloys. The differences observed are associated with the different S content of the alloys.

EXAMPLE 2: Due to the increased stability in terms of delta ferrite, the basic composition of the PI 2 alloy has been privileged and made on an industrial scale. However, due to the poorer corrosion and mechanical properties, the sulfur content PI 1 was applied to such an industrialized product. Table 6 shows the chemical composition of the alloy, called PI 3, and also the chemical composition of a conventional steel 420 whose machinability can be compared with the PI 3. The volume machined to the end of the tool's life cycle shows in the last row of Table 6; the higher machined volume of the PI 3 alloy is observed, indicating a significant gain in relation to the steel 420 of the state of the art.

A key observation can be made with respect to the PI 3 alloy. The slab takes place at temperatures of 1200 ° C and, even so, the delta ferrite content remains below 10%.

Therefore, the two examples mentioned above show that the steel of the present invention, especially PI 3, is capable of meeting the requirements of weldability, machinability, corrosion resistance and toughness without creating processing problems, for allow higher temperatures of hot molding.

Table 6: Chemical composition of the steel of the present invention, produced on an industrial scale, and steel 420, subjected to the machinability test (both with 32 HRC) It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Claims (9)

CLAIMS Having described the invention as above, the content of the following claims is claimed as property:

1. Stainless steel mold with reduced delta ferite content, characterized in that one comprises a composition of alloying elements consisting essentially of, in mass percentage, Carbon between 0.01 and 0.20; Nitrogen between 0.01 and 0.07; Manganese between 2.0 and 4.0; Nickel between 0.01 and 1.0; Chrome between 11.0 and 13.0; Molybdenum + Bottom Tungsten 1.0; Copper between 0.01 and 1.5; Vanadium between 0.01 and 1.0; Sulfur between 0.01 and 0.2; Calcium at maximum 0.01; Aluminum less than 0.50; Lower silicon 1.0; the rest consists essentially of Faith and unavoidable impurities to the preparation process.

2. Stainless steel mold with reduced delta ferite content, according to claim 1, characterized in that it comprises a composition of alloy elements consisting essentially of, in mass percentage, Carbon between 0.03 and 0.10; Nitrogen between 0.03 and 0.06; Manganese between 2.2 and 3.0; Nickel between 0.10 and 0.5; Chrome between 11.0 and 13.0; Molybdenum + Tungsten lower than 0.5; Copper between 0.1 and 0.8; Vanadium between 0.02 and 0.10; Sulfur between 0.05 and 0.14; Calcium between 0.01 and 0.003; Aluminum lower than 0.10; Lower silicon 0.50; the rest consists essentially of Fe and inevitable impurities to the preparation process.

3. Stainless steel mold with reduced delta ferite content, according to claim 2, characterized in that it comprises a composition of alloy elements consisting essentially of, in mass percentage, Carbon between 0.03 and 0.08; Nitrogen between 0.03 and 0.06; Manganese between 2.2 and 2.8; Nickel between 0.10 and 0.50; Chrome between 11.5 and 12.5; Molybdenum + Tungsten lower than 0.1; Copper between 0.3 and 0.7; Vanadium between 0.03 and 0.08; Sulfur between 0.08 and 0.12; Calcium between 0.0015 and 0.0025; Aluminum lower than 0.05; Lower silicon 0.50; the rest consists essentially of Faith and unavoidable impurities to the preparation process.

4. Stainless steel mold with reduced delta ferite content, according to any of claims 1 to 3, characterized in that it replaces Vanadium with Niobium or Titanium in a ratio corresponding to IV: 2Nb and IV: 1 Ti.

5. Stainless steel mold with reduced content of delta ferite, according to any of claims 1 to 3, characterized in that: content of delta ferrite in the lower microstructure of 10%.

6. Stainless steel mold with reduced delta ferite content, according to any of claims 1 to 3, characterized in that it is homogenized, forged or hot rolled at temperatures above 1160 ° C, but with delta ferrite content in the lower microstructure from 10%.

7. Stainless steel mold with reduced delta ferite content, according to any of claims 1 to 3, characterized in that it is applicable to molds, dies and tools of multiple use, for the formation of solid or liquid materials, at room temperature or at temperatures up to 1300 ° C.

8. Stainless steel mold with reduced delta ferite content, according to claim 7, characterized in that it is applicable to plastic molds and plastic mold components.

9. Stainless steel mold with reduced delta ferite content according to claim 7, characterized in that it is applicable to hot chambers or other plastic mold devices, in which high resistance to corrosion and high machinability is required.