Low-cost nickel-free weathering steel copper brittleness control method

Abstract

The invention provides a low-cost nickel-free weathering steel copper brittleness control method which comprises the steps that the heating temperature of a heating section is controlled, so that the actual slab temperature is lower than the melting point 1083 DEG C of copper, the existence time of liquid copper is shortened, and aggregation of the liquid copper at an austenite grain boundary during heating is reduced; the heating temperature of the second heating section is controlled to be less than or equal to 1100 The adverse effect of copper brittleness of the low-cost nickel-free weathering steel is overcome, the production technology on the premise of the low-cost nickel-free weathering steel is finally formed, and the surface and performance of the product are qualified.

Description

Technical Field

[0001] The present invention relates to the field of iron and steel production and manufacturing, and particularly relates to a manufacturing method of nickel-free weathering steel, and more particularly to a method for controlling copper embrittlement of low-cost nickel-free weathering steel. Background Art

[0002] Weathering steel is a kind of steel resistant to atmospheric corrosion. By adding trace elements such as phosphorus, copper, chromium, and nickel to ordinary steel, a dense and strongly adherent protective film is formed on the surface of the steel. Although weathering steel will also rust, after a stable and dense protective rust layer is formed on the weathering steel, compared with the rust layer of ordinary steel, the pores of this rust layer are much fewer, which can hinder the inward diffusion and development of rust, protect the matrix under the protective rust layer, and slow down its corrosion rate.

[0003] Almost all weathering steels have a lower limit requirement for the content of copper and chromium elements, while there is often no lower limit requirement for the content of phosphorus and nickel elements. However, due to the relatively low melting point of copper, only 1083 °C, which is much lower than the billet heating temperature, the precipitated copper aggregates in a liquid state at the austenite grain boundaries. When the content of the precipitated copper reaches a certain level, cracks are likely to occur during heating or hot rolling, and finally copper embrittlement is formed on the surface of the steel plate. Moreover, generally, the thicker the finished product thickness, the more likely the copper embrittlement defect will appear.

[0004] To solve the problem of copper embrittlement in weathering steel, a certain amount of Ni is usually added to the Cu-containing steel. On the one hand, it can increase the solid solubility of Cu in the steel, reduce the precipitation of Cu and its intrusion into the grain boundaries. On the other hand, it can increase the melting point of the Cu-Ni alloy, reduce the intrusion of the liquid-phase Cu alloy into the matrix. As the mass fraction ratio of Ni / Cu increases, the incidence rate of copper embrittlement defects during the rolling process decreases significantly.

[0005] However, Ni belongs to precious alloys. Adding Ni will cause a significant increase in cost, thus greatly reducing the competitiveness of the product. Therefore, how to ensure the surface quality of weathering steel under the premise of low Ni or even no Ni is a difficult problem faced by many steel mills.

[0006] In summary, the following problems exist in the prior art: how to overcome the adverse effects of copper embrittlement in nickel-free weathering steel. Summary of the Invention

[0007] The present invention provides a method for controlling copper embrittlement of low-cost nickel-free weathering steel. Through the optimized design of the heating and rolling processes, the adverse effects of copper embrittlement of low-cost nickel-free weathering steel are overcome, and finally a production technology based on low-cost nickel-free weathering steel is formed.

[0008] For this reason, the present invention proposes a method for controlling copper embrittlement of low-cost nickel-free weathering steel, and the method for controlling copper embrittlement of low-cost nickel-free weathering steel includes:

[0009] Control the heating temperature of a heating section so that the actual slab temperature is lower than the melting point of copper, 1083 °C, shorten the existence time of liquid copper, and reduce the aggregation of liquid copper at the austenite grain boundaries during heating;

[0010] Control the heating temperature of the second heating section ≤ 1100 °C.

[0011] The present invention also provides a low-cost nickel-free weathering steel. The component weight percentages Wt% of the low-cost nickel-free weathering steel are as follows: C: 0.061% - 0.081%; Si: 0.04% - 0.28%; Mn: 0.55% - 0.90%; P: 0.010% - 0.104%; S: 0.0008% - 0.0041%; Alt: 0.034% - 0.037%; Nb: 0.002% - 0.0321%; Ti: 0.025% - 0.075%; Cr: 0.34% - 1.58%; Cu: 0.31% - 0.37%. The steel plate has a product thickness range of 1.5 - 8.0 mm, a product yield strength of 355 - 650 MPa, a tensile strength of 500 - 800 MPa, an elongation A / % of 20 - 35, a metallographic structure of ferrite + pearlite, a grain size of 9 - 10 grades, and the product surface and properties are all qualified, without cracks and without the adverse effects of copper embrittlement.

[0012] A low-cost nickel-free weathering steel according to Example 1 of the present invention has a finished product thickness of 6 mm, a carbon content of 0.081%, a silicon content of 0.25%, a manganese content of 0.55%, a phosphorus content of 0.104%, a sulfur content of 0.0041%, an aluminum content of 0.034%, a niobium content of 0.002%, a titanium content of 0.025%, a chromium content of 0.34%, and a copper content of 0.37%.

[0013] A low-cost nickel-free weathering steel according to Example 2 of the present invention has a finished product thickness of 4 mm, a C content of 0.061%, a Si content of 0.04%, a Mn content of 0.74%, a P content of 0.011%, a S content of 0.0035%, an Alt content of 0.037%, a Nb content of 0.002%, a Ti content of 0.060%, a Cr content of 0.35%, and a Cu content of 0.36%.

[0014] A low-cost nickel-free weathering steel according to Example 3 of the present invention has a finished product thickness of 5 mm, a C content of 0.065%, a Si content of 0.05%, a Mn content of 0.90%, a P content of 0.010%, a S content of 0.0021%, an Alt content of 0.036%, a Nb content of 0.002%, a Ti content of 0.075%, a Cr content of 0.34%, and a Cu content of 0.35%.

[0015] A low-cost nickel-free weathering steel according to Example 4 of the present invention, with a finished thickness of 8 mm, a C content of 0.075%, a Si content of 0.28%, a Mn content of 0.85%, a P content of 0.085%, an S content of 0.0008%, an Alt content of 0.034%, an Nb content of 0.0321%, a Ti content of 0.061%, a Cr content of 1.58%, and a Cu content of 0.31%.

[0016] Furthermore, for the low-cost nickel-free weathering steel plate with a thickness of more than 6 mm of the present invention, the Charpy impact energy at -20°C is above 85 J, and the Charpy impact energy of the 8-mm-thick low-cost nickel-free weathering steel plate at -20°C can reach 121 J.

[0017] Effect

[0018] The method for controlling copper embrittlement of the low-cost nickel-free weathering steel obtained in the present invention overcomes the adverse effects of copper embrittlement of the low-cost nickel-free weathering steel through the optimized design of the heating and rolling processes, and finally forms a production technology based on the low-cost nickel-free weathering steel. The strength of the product involves the 355-600 MPa grade, the thickness range is 1.5-8.0 mm, and the surface and performance of the product are all qualified. Description of the Drawings

[0019] Figure 1 It is a metallographic structure photograph of Example 1 of the present invention. Detailed Embodiments

[0020] In order to have a clearer understanding of the technical features, objectives, and effects of the present invention, the present invention will now be described.

[0021] The inventive concept of the present invention:

[0022] Since the copper content of weathering steel is relatively high, and the melting point of copper is relatively low, only 1083°C, which is much lower than the steel billet heating temperature. When the heating temperature is higher than the melting point of copper, with the progress of oxidation, copper in the steel enriches at the interface between the matrix and the oxide layer to form a liquid copper phase, and the liquid copper phase penetrates into the matrix along the austenite grain boundaries. The precipitated copper accumulates in a liquid state at the austenite grain boundaries. When the precipitated copper content reaches a certain level, cracks are likely to occur during heating or hot rolling, and finally copper embrittlement is formed on the steel plate surface.

[0023] The applicant believes that: without adding Ni, it is required to minimize the accumulation of liquid copper at the austenite grain boundaries during heating. The penetration of the liquid copper phase in weathering steel along the austenite grain boundaries into the matrix is closely related to the heating process.

[0024] Research shows that when the temperature of the heated slab is lower than the melting point of copper (1083 °C), the copper-rich phase in weathering steel is distributed in the form of solid particles at the interface between the matrix and the oxide layer and in the oxide layer near the interface, forming an internal oxide layer between the outer oxide layer and the matrix interface; when the temperature of the heated slab is 1100 °C, the wettability of copper is the strongest, and at this temperature, the penetration of copper along the austenite grain boundaries is the easiest; when the temperature of the heated slab exceeds 1100 °C, with the increase of the heating temperature, the enrichment and penetration of copper decrease, and the copper-rich phase at the interface between the matrix and the oxide layer decreases. This is mainly determined by the high-temperature diffusion of copper. When heated at 1300 °C, the liquid copper phase disappears.

[0025] The applicant also found that: in addition, when the finished product thickness is thicker, the copper embrittlement defect is more likely to occur. The reduction ratio of thin specifications is larger. With the increase of the reduction ratio, a higher density of dislocations is formed after finish rolling, providing a large number of nucleation sites and increasing the nucleation rate. Not only does it refine the grains, but even if a slight Cu-rich phase is generated at high temperatures, with the refinement of dislocations and grain rolling flattening during the steel rolling process, the relative area of grain boundaries increases, increasing the allowable Cu-rich degree of grain boundaries and reducing the tendency of copper embrittlement during rolling. Therefore, for thick specifications, in addition to heating process control, corresponding control of the slab charging temperature and the thickness of the intermediate slab should also be matched. Reducing the slab charging temperature can effectively reduce the adverse effect of high-temperature liquid copper on copper embrittlement during continuous casting. In addition, by increasing the thickness of the intermediate slab and matching and optimizing the reduction ratio of the rolling line, etc., certain inhibitory effects can be exerted on the copper embrittlement of thick specifications.

[0026] Combining the above situations, the present invention provides a method for controlling copper embrittlement of low-cost nickel-free weathering steel, which is as follows.

[0027] 3.1 Scheduling requirements:

[0028] (1) For slabs with a rolling thickness h ≤ 5.0 mm, the required charging temperature range is 0 - 800 °C.

[0029] (2) For slabs with a rolling thickness h > 5.0 mm, production scheduling can be organized 9 hours after slab cutting and charging can be carried out 10 hours after cutting. The required charging temperature range is 0 - 450 °C.

[0030] (3) When scheduling non-Ni steel grades, they should not be scheduled together with high-temperature steels before and after to prevent copper embrittlement caused by too long heating time.

[0031] 3.2 Heating process

[0032] (1) The heating section of the heating furnace of the present invention includes: a first heating section, a second heating section, a third heating section, and a soaking section. In the present invention, the actual slab temperature in the low-temperature heating section is strictly controlled to be lower than the melting point of copper, 1083 °C, so as to shorten the existence time of liquid copper and reduce the aggregation of liquid copper at the austenite grain boundaries during heating. Considering the difference between the furnace gas temperature and the slab temperature and the capacity of the heating furnace, the heating temperature before the second heating section is significantly reduced by 100 - 150 °C. The target temperature of the furnace gas temperature in the second heating section is set to ≤1100 °C, and the temperature of the first heating section is correspondingly reduced to ≤1060 °C. The typical temperature comparison between normal heating and low-temperature heating is shown in Table 1.

[0033] Table 1 Temperature comparison between the low-temperature heating process of the present invention and the existing normal heating process

[0034] Heating process First heating section Second heating section Third heating section Soaking section 15 Low-temperature heating 1060 1100 1260 1240 Normal heating 1160 1240 1260 1240

[0035] (2) In the high-temperature section after the second heating section, a fast heating strategy (heating rate ≥ 5 °C / min) is adopted to shorten the time in the heating section where the wettability of copper is the strongest and reduce the aggregation of copper along the austenite grain boundaries at this temperature. The weathering steel is quickly heated to the high-temperature third heating section, and the temperature and time of the third heating section and the subsequent soaking section are the same as those in normal heating.

[0036] (3) A weakly reducing heating furnace atmosphere can reduce the enrichment amount of copper elements at the interface, and reduce the copper embrittlement hazard of the liquid copper-rich phase during the subsequent rolling process. However, a weakly reducing atmosphere will cause incomplete combustion and increase energy consumption. Considering comprehensively, the high-temperature third heating section and the soaking section are set to have a weakly reducing atmosphere by controlling the air excess coefficient. The air excess coefficients of each heating section are specifically shown in Table 2.

[0037] The reason is that Cu can easily migrate through the FeO grain boundaries and the matrix grain boundaries. There is good wettability between liquid copper and solid FeO. At the same time, Cu has a high solubility of Fe and O at high temperatures. Therefore, in a low-oxygen environment, the scale mainly consists of a single-layer structure of FeO, and the scale thickness is small. More Cu diffuses along the FeO grain boundaries to the surface of the scale and evaporates in the form of Cu gas. Therefore, oxidation in an atmosphere with a low O 2 concentration can reduce the enrichment of Cu at the scale / steel interface, that is, reheating at a lower O 2 concentration is beneficial to inhibiting the generation of hot cracks.

[0038] Table 2 Air excess coefficients of each heating section of the present invention

[0039] Heating temperature range First heating section Second heating section Third heating section Soaking section Excess air coefficient 1.00-1.15 1.00-1.15 0.90-1.00 0.90-1.00

[0040] 3.3 Rolling process

[0041] (1) The rough rolling stage is carried out in the austenite recrystallization zone, using reciprocating longitudinal rolling. The rough rolling exit temperature is 1060 - 1080 °C. Increase the reduction ratio per pass, and the reduction ratio of the last 2 - 3 passes > 30% (for example, the reduction ratio of the last 2 passes is 32% - 34%). The design of a lower rough rolling exit temperature can correspondingly increase the number of descaling passes in rough rolling. Multi-pass descaling (≥3 passes, for example, for a steel strip with a product specification of 4 mm, descaling is carried out 4 times) can remove the liquid copper-rich phase at the interface between the scale and the steel matrix together with the scale. Increasing the reduction ratio of the last three passes after rough rolling can reduce the harmful effect of the liquid copper-rich phase at the austenite grain boundaries on the steel matrix surface. The positive effect of the rough rolling exit temperature being 1060 °C to 1080 °C is that within this temperature range, it can ensure that the strip exit temperature after passing through the rough rolling mill is controlled below the melting point of the liquid copper-rich phase, which is 1085 °C.

[0042] (2) The thickness of the intermediate billet (the thickness of the steel plate at the rough rolling exit) is controlled at the upper limit. Increasing the thickness of the intermediate billet (from the previous 38 mm to ≥42 mm, the thickness of the continuous casting billet is 220 - 240 mm, for example, 230 mm) can increase the total reduction ratio in finish rolling and reduce the generation of copper embrittlement.

[0043] (3) The finish rolling stage is mainly carried out in the austenite non-recrystallization zone, avoiding rolling in the partial recrystallization zone or two-phase zone. The main purpose is to control the grain structure of the steel plate and avoid the appearance of coarse grains and mixed grains, which may cause surface quality problems or unqualified properties of the steel plate. The finish rolling final rolling temperature is 850 - 890 °C, and the reduction ratio of the last pass in finish rolling > 10% (for example, 11% - 13%). For thick specifications, the rolling temperature is appropriately reduced. Specific embodiments

[0045] A method for controlling copper embrittlement of a low-cost nickel-free weathering steel according to the present invention adopts the following component ratios and specific processes. Among them, Table 3 shows the components of the steel in each embodiment (by weight percentage). Tables 4 and 5 are the process parameters corresponding to the steel in the embodiments described in Table 3. Table 6 is the comprehensive performance corresponding to the steel with the components in each embodiment of Table 3.

[0046] Table 3 Chemical composition of the product (Wt%)

[0047]

[0048] Table 4 Specific heating process parameters of each embodiment

[0049]

[0050] Table 5 Specific rolling process parameters of each embodiment

[0051]

[0052] Table 6 Comprehensive performance obtained in each embodiment

[0053]

[0054] Figure 1 is the metallographic structure photo of Example 1. From Figure 1 it can be seen that the metallographic structure is ferrite + pearlite, and the grain size is grade 10.

[0055] The method for controlling copper embrittlement of the low-cost nickel-free weathering steel obtained by the present invention overcomes the adverse effects of copper embrittlement of the low-cost nickel-free weathering steel through the optimized design of the heating and rolling processes, and finally forms a production technology based on the low-cost nickel-free weathering steel. The strength of the product ranges from 355 to 600 MPa, the thickness range is 1.5 - 8.0 mm, the surface and properties of the product are qualified, without cracks and adverse effects of copper embrittlement.

[0056] The above are only the schematic specific embodiments of the present invention, and are not intended to limit the scope of the present invention. The components of the present invention can be combined with each other under the condition of no conflict. Any equivalent changes and modifications made by those skilled in the art without departing from the concept and principle of the present invention shall fall within the scope of protection of the present invention.

[0057] The above are only the schematic specific embodiments of the present invention, and are not intended to limit the scope of the present invention. The components of the present invention can be combined with each other under the condition of no conflict. Any equivalent changes and modifications made by those skilled in the art without departing from the concept and principle of the present invention shall fall within the scope of protection of the present invention.

Claims

1. A low-cost nickel-free weathering steel copper embrittlement control method, characterized in that: The low-cost nickel-free weathering steel copper embrittlement control method comprises: Control the heating temperature of a heating section so that the actual slab temperature is lower than the melting point of copper, 1083°C, shortening the existence time of liquid copper and reducing the aggregation of liquid copper at the austenite grain boundary during heating; Control the heating temperature of the second heating section to be ≤1100℃.

2. The low-cost nickel-free weathering steel copper embrittlement control method according to claim 1, characterized in that: The low-cost nickel-free weathering steel copper embrittlement control method also includes: controlling the excess air coefficient, setting the three heating sections and the soaking section of the heating furnace to a weak reducing atmosphere to reduce the amount of copper element enrichment at the interface.

3. The low-cost nickel-free weathering steel copper embrittlement control method according to claim 2, characterized in that: Set the excess air coefficient of the three heating sections and the equalizing section to 0.90-1.

00.

4. The low-cost nickel-free weathering steel copper embrittlement control method according to claim 1, characterized in that: The low-cost nickel-free weathering steel copper embrittlement control method also includes: production scheduling control: (a) For rolling slabs with a thickness of h≤5.0 mm, the charging temperature range is required to be 0-800°C; (b) For slabs with a rolling thickness of h>5.0mm, production planning can be organized 9 hours after the slab is cut, and the furnace can be loaded 10 hours later. The charging temperature range is required to be 0-450℃.

5. The low-cost nickel-free weathering steel copper embrittlement control method according to claim 1, characterized in that: The rough rolling outlet temperature is controlled at 1060-1080°C to ensure that the outlet temperature of the strip after the rough rolling mill is controlled below the melting point of the liquid copper-rich phase of 1085°C.

6. The low-cost nickel-free weathering steel copper embrittlement control method according to claim 1, characterized in that: Control the reduction rate of the last 2-3 passes of rough rolling to be greater than 30%.

7. The low-cost nickel-free weathering steel copper embrittlement control method according to claim 1, characterized in that: Control the final rolling temperature at 850-890℃.

8. The low-cost nickel-free weathering steel copper embrittlement control method according to claim 1, characterized in that: Control the reduction rate of the last pass of finishing rolling to be >10%.

9. The low-cost nickel-free weathering steel copper embrittlement control method according to claim 1, characterized in that: The composition weight percentage Wt% of the low-cost nickel-free weathering steel is: C: 0.061%-0.081%; Si: 0.04%-0.28%; Mn: 0.55%-0.90%; P: 0.010%-0.104%; S：0.0008％-0.0041％； Alt: 0.034%-0.037%; Nb: 0.002%-0.0321%; Ti: 0.025%-0.075%; Cr: 0.34%-1.58%; Cu: 0.31%-0.37%, product thickness range is 1.5-8.0mm, intermediate billet thickness is 44-48mm, product yield strength is 355-650MPa; During production scheduling: (a) For slabs with a rolling thickness of h≤5.0mm, the steel charging temperature range is required to be 0-800℃; (b) For slabs with a rolling thickness of h>5.0mm, the production schedule can be organized 9 hours after the slab is cut, and the furnace can be loaded 10 hours later, and the steel charging temperature range is required to be 0-450℃; In the heating process: set the target temperature of the furnace gas temperature of the second heating section to ≤1100℃, and set the temperature of the first heating section to ≤1060℃. The high temperature section after the second heating section is kept warm for a long time by a weak reducing atmosphere, and the excess air coefficient is 0.9-1.

0. The rough rolling outlet temperature is 1060-1080℃; the single pass reduction rate is increased, and the reduction rate of the last 2-3 passes is greater than 30%; the finishing rolling temperature is 850-890℃, and the reduction rate of the last pass of finishing rolling is greater than 10%.

Images