A low-temper brittle carbon steel

Abstract

A low-temper brittle carbon steel has the following composition by weight percentage: C≤0.50%, Si0.01~1.00%, Mn0.20~3.50%, Cr≤0.40%, Ni≤0.45%, P≤0.045%, S≤0.05%, As≤0.030%, Sb≤0.01%, Sn≤0.02%, with the remainder being Fe and other unavoidable impurities; and it must simultaneously satisfy: MEF≥5.85%, MEF=Mn / (C+2.1P+5.0As+7.6Sn+7.2Sb). This invention, based on controlling the content of each element harmful to temper brittleness such as As, Sb, Sn, and P, designs a temper brittleness sensitivity coefficient (MEF) through the coupling effect of these harmful elements. It also appropriately relaxes the residual content of Cr and Ni, especially those that can cause grain boundary segregation of As, Sb, Sn, and P, to improve or avoid the temper brittleness of carbon steel, thereby obtaining high-quality carbon steel with low temper brittleness, reducing the fluctuation and dispersion of tensile properties, and ensuring the stability of tensile properties and low-temperature impact resistance.

Description

Technical Field

[0001] This invention belongs to the field of ferrous metallurgical production, and in particular to a low-temper brittle carbon steel. Background Technology

[0002] Temper brittleness is a common defect in steel. It manifests as follows: although the hardness and strength of steel decrease and the plasticity and toughness increase with the increase of tempering temperature, the impact value decreases significantly when tempered within a certain temperature range.

[0003] Based on the different temperatures and characteristics of temper embrittlement, it can be roughly divided into two categories: temper embrittlement occurring in the range of 250°C to 400°C is called type I temper embrittlement, also known as irreversible temper embrittlement or low-temperature temper embrittlement. Its characteristic is intergranular brittle fracture, which is independent of the cooling rate after tempering. Temper embrittlement occurring in the range of 500°C to 650°C is called type II temper embrittlement. It is reversible and related to the cooling rate after tempering. It occurs with slow cooling and does not occur with rapid cooling. Therefore, after embrittlement occurs, it can be eliminated by reheating and then rapid cooling.

[0004] The second type of temper embrittlement is also affected by chemical composition, increasing with the increase of impurity elements. The segregation of impurity elements such as Sb, Sn, and P towards the original amorphous grain boundaries is the main reason for the second type of temper embrittlement. Ni and Cr not only promote the segregation of impurity elements, but also segregate themselves, thereby reducing the fracture strength of the grain boundaries and causing temper embrittlement.

[0005] Therefore, the conventional measures to reduce or avoid type II temper brittleness are to increase the purity of the steel, minimize impurities, or add appropriate amounts of beneficial alloying elements such as Mo and W. However, for carbon steel systems, firstly, adding beneficial elements such as Mo and W is impossible; secondly, harmful impurity elements such as As, Sb, Sn, and P objectively exist and cannot be completely eliminated. For example, literature shows the influence of trace harmful elements on the surface hot brittleness of steel ingots during forging. As, Sb, and Sn have low partial pressures and cannot be removed even under high vacuum by extending the smelting time. In other words, only by smelting carbon steel with high-purity raw materials that do not contain harmful elements such as As, Sb, and Sn can the temper brittleness of carbon steel be reduced or avoided. However, the cost is unaffordable for conventional industries such as machinery and chemicals, which are vital to national economy and people's livelihood. Therefore, this method is not very practical.

[0006] Chinese patent CN201611094718.7 proposes a thermomechanical treatment method for medium carbon steel that can replace quenching and tempering. By optimizing the number of rolling passes and heat treatment, the product process and mechanical properties are not affected by temper brittleness.

[0007] Chinese patent CN106624449 discloses a flux-cored welding wire for heat treatment of thick plates in marine engineering, its preparation method and application. By rationally designing the composition ratio of the flux-cored welding wire, without containing harmful elements such as As, Sb, Sn, and P, the generation of second-type temper brittleness is suppressed.

[0008] Japanese Patent JP201410059352.4 proposes a corrosion-resistant high-carbon steel alloy material and its preparation method. It is made by using a combination of raw materials such as Nb and Al, and by setting a reasonable ratio and production process, and by setting a reasonable order of addition. The resulting alloy material has extremely high low temper brittleness and does not involve harmful elements such as As, Sb, Sn, and P that are well known to affect temper brittleness.

[0009] In the welding method for Cr-Mo steel disclosed in Japanese Patent JPS58221678, measures to prevent temper brittleness involve harmful elements such as As, Sb, Sn, and P, and require 10P+5Sb+4Sn+As / 100<15. However, this empirical formula is proposed for Cr-Mo alloy steel, and the elements Cr and Mo are not applicable to carbon steel, because adding appropriate amounts of alloying elements such as Mo can improve resistance to temper brittleness.

[0010] In summary, there are very few patents related to the temper brittleness of steel, especially none that improve the temper brittleness of carbon steel by controlling harmful elements such as As, Sb, Sn, and P. Summary of the Invention

[0011] The purpose of this invention is to design a carbon steel with low temper brittleness, which not only reduces the sensitivity to temper brittleness, but also ensures low-temperature impact resistance, while taking into account manufacturability and economy.

[0012] To achieve the above objectives, the technical solution of the present invention is as follows:

[0013] This invention, based on controlling the content of each element harmful to temper brittleness such as As, Sb, Sn, and P, designs a temper brittleness sensitivity coefficient (MEF) through the coupling effect of these harmful elements. It also appropriately relaxes the residual content of Cr and Ni, especially those that can cause grain boundary segregation of As, Sb, Sn, and P, to improve or avoid the temper brittleness of carbon steel, thereby obtaining high-quality carbon steel with low temper brittleness, reducing the fluctuation and dispersion of tensile properties, and ensuring the stability of tensile properties and low-temperature impact resistance.

[0014] Specifically, the low temper brittle carbon steel of this invention has the following composition by weight percentage: C≤0.50%, Si: 0.01~1.00%, Mn: 0.20~3.50%, Cr≤0.40%, Ni≤0.45%, P≤0.045%, S≤0.05%, As≤0.030%, Sb≤0.01%, Sn≤0.02%, with the remainder being Fe and other unavoidable impurities; and it must simultaneously satisfy: MEF≥5.85%, MEF=Mn / (C+2.1P+5.0As+7.6Sn+7.2Sb).

[0015] The low-tempered brittle carbon steel of the present invention has a tensile strength ≥415MPa, a yield strength (0.2% offset) ≥240MPa, an elongation after fracture EL longitudinally ≥24%, and an impact Kv2 ≥50J at -60℃.

[0016] In the composition design of the low temper embrittlement carbon described in this invention:

[0017] Carbon (C) is one of the most common inherent elements in steel. Because its oxidation potential is close to that of iron (Fe), it is partially retained in molten steel, meaning it is an objective presence and cannot be completely avoided. This invention has found that C is also an element detrimental to temper brittleness, therefore its content should be kept as low as possible. Furthermore, C is the most economical element for ensuring strength; depending on the application, the C content of steel varies from 0.02% to 2.11%. However, a high carbon content inevitably leads to a higher tendency and content of carbides, resulting not only in increased sensitivity to temper brittleness but also negatively impact resistance and corrosion resistance. Therefore, the C content of temper brittle-resistant carbon steel should be ≤0.50%, preferably ≤0.35%.

[0018] Si has a higher oxidation potential than Fe and is rarely retained in molten steel, but it is also one of the most common inherent elements in steel. The Si content in steel is usually 0.01 to 1.00%, with preferred Si content being 0.05 to 0.80%.

[0019] Mn (metallic minerals) is beneficial for improving the temper brittleness resistance of carbon steel, but its oxidation potential is close to that of Fe, resulting in partial retention in molten steel. It is also one of the most common inherent elements in steel. Mn is beneficial for mechanical properties such as strength and impact resistance, but excessive amounts are detrimental to manufacturability. This invention has found that Mn can improve the temper brittleness resistance of carbon steel; therefore, the optimal Mn content is 0.20–3.50%, preferably 1.25–3.2%.

[0020] Phosphorus (P) causes undesirable temper brittleness, but its oxidation potential is close to that of Fe, and it is partially retained in molten steel. It is also one of the inherent harmful residual elements in conventional steel, and the lower the value, the better. This invention has found that manganese (Mn) can suppress the adverse effect of P on temper brittleness. Therefore, although conventional requirements ≤ 0.035%, this invention ≤ 0.045%, preferably ≤ 0.040%.

[0021] S causes unfavorable temper brittleness, but its oxidation potential is close to that of Fe, so it is partially retained in molten steel. It is also one of the inherent harmful residual elements in conventional steel. The lower the value, the better. In this invention, S ≤ 0.050%, preferably S ≤ 0.045%.

[0022] As causes temper brittleness, but its oxidation potential is lower than that of Fe, so it is completely retained in molten steel. Therefore, it is one of the inherent harmful residual elements in steel. The lower the value, the better. In this invention, As ≤ 0.030%, preferably As ≤ 0.020%.

[0023] Sb causes temper brittleness, but its oxidation potential is lower than that of Fe, so it is completely retained in molten steel. Therefore, it is one of the inherent harmful residual elements in steel. The lower the value, the better. In this invention, Sb ≤ 0.01%, preferably Sb ≤ 0.008%.

[0024] Sn causes temper brittleness, but its oxidation potential is lower than that of Fe, so it is completely retained in molten steel. Therefore, it is one of the inherent harmful residual elements in steel. The lower the value, the better. In this invention, Sn ≤ 0.02%, preferably Sn ≤ 0.01%.

[0025] Cr can cause grain boundary segregation of tempering brittle elements such as P, As, Sn, and Sb. As a residual element in carbon steel, the conventional requirement is Cr ≤ 0.25%, while in this invention, Cr ≤ 0.40% is preferred, and Cr ≤ 0.35% is even more preferred.

[0026] Ni can cause grain boundary segregation of tempering brittle elements such as P, As, Sn, and Sb. In carbon steel, as a residual element, the conventional requirement is Ni ≤ 0.30%, while in this invention, Ni ≤ 0.45%, preferably Ni ≤ 0.40%.

[0027] As for Mo and W, which can suppress temper brittleness, they are not specifically added to suppress temper brittleness. Therefore, the present invention does not limit the content of Mo and W, but only treats them as residual elements.

[0028] CEF = P + 2.4As + 3.6Sn + 8.2Sb is a commonly used empirical formula for temper brittleness sensitivity coefficient, requiring CEF ≤ 0.1%. However, because it is an empirical formula and criterion for Cr-Mo alloy steel, it is not applicable to carbon steel. Considering the influence of major elements such as C, Si, Mn, P, and S, and harmful residual elements such as As, Sb, and Sn on the mechanical properties of carbon steel, a fitted MEF = Mn / (C + 2.1P + 5.0As + 7.6Sn + 7.2Sb) is proposed, with MEF ≥ 5.85% as the criterion for low temper brittleness.

[0029] This invention reveals that carbon (C) causes temper brittleness, while manganese (Mn) can suppress it. Although Mn cannot inhibit the segregation of harmful elements such as asperformant (As), scalp (Sb), snip (Sn), and p (P) at grain boundaries, it can offset the adverse effects of these elements on temper brittleness through the coupling effect of Mn, C, asperformant (As), snip (Sb), and p. Therefore, Mn can suppress the adverse effects of C, p, asperformant (As), snip (Sb), and p on temper brittleness and improve low-temperature impact resistance. This invention proposes a metal-enhanced metal flow (MEF) model to ensure reduced impact of temper brittleness on performance, such as the fluctuation and dispersion of tensile and impact mechanical properties.

[0030] The beneficial effects of this invention are:

[0031] The low temper brittleness carbon steel described in this invention is based on the effect of chemical elements on the temper brittleness of steel, especially carbon steel. Without adding beneficial alloying elements such as Mo and W, it focuses solely on controlling the content of harmful elements such as As, Sb, Sn, and P, and limiting the coupling effect of these elements. Even with a certain amount of elements such as Cr and Ni that can cause enrichment of As, Sb, Sn, and P, the temper brittleness of carbon steel can be reduced or even avoided. The temper brittleness sensitivity coefficient MEF proposed in this invention is not limited to conventional elements such as As, Sb, Sn, and P and their coupling effects, but also includes elements such as Mn and C and their coupling effects with elements such as As, Sb, Sn, and P. This not only helps reduce temper brittleness sensitivity but also better ensures low-temperature impact resistance; moreover, it takes into account feasibility and economy. Detailed Implementation

[0032] The present invention will be further illustrated by the following embodiments, but the specific embodiments and related descriptions do not constitute an improper limitation on the technical solution of the present invention.

[0033] The composition and MEF coefficient of Examples 1-13 of this invention are shown in Table 1. The balance in the composition is Fe and other unavoidable impurities. The mechanical properties of the products after tempering at 690℃ for 2 hours are shown in Table 2. When MEF ≥ 5.85%, the mechanical properties meet the standard and there is no temper brittleness.

[0034] The composition and MEF coefficient of Comparative Examples 1 to 13 are shown in Table 3, and the mechanical properties of their tempered products are shown in Table 4. Although the CEF of the comparative examples is ≤0.1%, the mechanical properties do not meet the standard when the MEF is <5.85%, and there is temper brittleness.

[0035] CEF = P + 2.4As + 3.6Sn + 8.2Sb is a commonly used empirical formula for the temper brittleness sensitivity coefficient, requiring CEF ≤ 0.1%. However, because this empirical formula and criterion are for Cr-Mo alloy steels, it is not applicable to carbon steels. For example, taking the compositions in Tables 1 and 3 as examples, as shown in Table 5, the CEF values ​​in Tables 1 and 3 are 0.02–0.09% and 0.01–0.10% respectively, both not exceeding the upper limit of 0.1%, and there is a significant overlap between the two CEF values, approximately 0.02–0.09%. However, in terms of the corresponding tempering mechanical properties, the composition in Table 1 meets the standard, indicating no temper brittleness, as shown in Table 2; while the composition in Table 3 does not meet the standard, indicating temper brittleness, as shown in Table 4. Therefore, the CEF value needs to be corrected. Taking into account the influence of major elements such as C, Si, Mn, P, and S, as well as harmful residual elements such as As, Sb, and Sn on the mechanical properties of carbon steel, a fitting MEF = Mn / (C + 2.1P + 5.0As + 7.6Sn + 7.2Sb) is proposed. The MEF calculated according to the composition in Tables 1 and 3 are 5.85–9.23% and 1.20–6.22%, respectively, as detailed in Tables 1 and 3. MEF ≥ 5.85% is used as the criterion for low temper brittleness.

[0036] Looking at the mechanical properties corresponding to Tables 1 and 3, Tables 2 and 4 show that although the CEF values ​​of each component are all far ≤0.1%, the properties of the components in Table 3 corresponding to those in Table 4 do not meet the standards, meaning that the components in Table 3 exhibit temper brittleness, while the components in Table 1 do not. Therefore, CEF ≤0.1% is not applicable to carbon steel.

[0037] According to the present invention, if the MEF is ≥ 5.85%, the presence of temper brittleness in carbon steel can be completely predicted from the composition. For example, the MEF of the components in Table 1 that do not exhibit temper brittleness is ≥ 5.85%, while the MEF of the components in Table 3 that exhibit temper brittleness is < 5.85%. Therefore, a MEF ≥ 5.85% can ensure that the produced carbon steel is free from temper brittleness, even from the billet and the source of steelmaking.

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Claims

1. A low-temper brittle carbon steel, wherein the weight percentage composition is: C≤0.50%, Si: 0.01~1.00%, Mn: 0.20~3.50%, Cr≤0.40%, Ni≤0.45%, P≤0.045%, S≤0.05%, As≤0.030%, Sb≤0.01%, Sn≤0.01%, with the remainder being Fe and other unavoidable impurities; and it must simultaneously satisfy: MEF≥5.85%, MEF=Mn / (C+2.1P+5.0As+7.6Sn+7.2Sb).

2. The low-temper brittleness carbon steel as described in claim 1, characterized in that, The carbon steel contains residual elements Cr and / or Ni, wherein Cr ≤ 0.40% and Ni ≤ 0.45% by weight.

3. The low-temper brittleness carbon steel as described in claim 2, characterized in that, Cr ≤ 0.35% by weight.

4. The low-temper brittleness carbon steel as described in claim 2, characterized in that, Ni ≤ 0.40%, by weight percentage.

5. The low-temper brittleness carbon steel as described in claim 1, characterized in that, C ≤ 0.35%, by weight percentage.

6. The low temper brittle carbon steel as described in claim 1, characterized in that Si: 0.05-0.80%, by weight percentage.

7. The low temper brittle carbon steel as described in claim 1, characterized in that Mn: 1.25-3.2% by weight.

8. The low-temper brittleness carbon steel as described in claim 1, characterized in that, P≤0.040%, S≤0.045%, by weight percentage.

9. The low-temper brittleness carbon steel as described in claim 1, characterized in that, As ≤ 0.020%, by weight percentage.

10. The low-temper brittleness carbon steel as described in claim 1, characterized in that, Sb ≤ 0.008%, by weight percentage.

11. The low-temper brittleness carbon steel as described in claim 1, characterized in that, Sn ≤ 0.01% by weight.

12. The low-temper brittle carbon steel according to any one of claims 1 to 11, characterized in that, The low-tempered brittle carbon steel has a tensile strength ≥415MPa, a yield strength ≥240MPa, a longitudinal elongation after fracture EL ≥24%, and an impact strength Kv2 ≥50J at -60℃.

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