Normalizing rolled low-alloy structural steel and preparation method thereof

Abstract

The invention discloses normalizing rolled low-alloy structural steel and a preparation method thereof, and belongs to the technical field of steel rolling, and the normalizing rolled low-alloy structural steel comprises the following chemical components in percentage by mass: 0.10%-0.15% of C, 0.20%-0.24% of Si, 1.35%-1.45% of Mn, less than or equal to 0.02% of P, less than or equal to 0.015% of S, 0.03%-0.04% of V, 0.01%-0.05% of Nb and the balance of Fe and inevitable impurities. By reasonably controlling the content of Nb, V and other elements, the prepared structural steel has high strength and stable impact toughness, meanwhile, the normalizing rolling technology is adopted, so that the product performance meets the requirements of normalized steel, and the production cost is reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of steel rolling, and particularly relates to a normalized rolling low-alloy structural steel and a preparation method thereof. Background Art

[0002] With the national standards of low-alloy structural steel being in line with international standards, normalized rolling, as a new steel delivery condition in domestic standards, can be used to replace the normalized delivery condition in many steel structure projects. Normalized rolling can effectively save energy consumption costs and speed up the production cycle of steel plates. Therefore, it is necessary to study the process of normalized rolling.

[0003] With the rapid development of oil and gas exploitation, marine resource development, and building structures in cold regions, the service safety requirements for materials in major engineering construction in these fields are increasing day by day, and higher performance requirements are put forward for low-temperature thick steel structure materials with high strength, high toughness, and excellent welding performance. Q355ND steel is a typical normalized rolling low-temperature structural steel with good low-temperature toughness. Usually, it needs to be delivered after normalizing treatment, resulting in high manufacturing costs and long supply cycles of products, which seriously restricts its application in the steel structure field. Therefore, it is necessary to study its production process of rolling at normalizing temperature to achieve short-process manufacturing of high-quality steel and reduce production costs. Summary of the Invention

[0004] The purpose of the present invention is to provide a normalized rolling low-alloy structural steel and a preparation method thereof, which are mainly achieved through the following technical solutions.

[0005] In one aspect of the present invention, a normalized rolling low-alloy structural steel is provided, and its chemical composition is calculated by mass percentage as follows: C 0.10% - 0.15%, Si 0.20% - 0.24%, Mn 1.35% - 1.45%, P ≤ 0.02%, S ≤ 0.015%, V 0.03% - 0.04%, Nb 0.01% - 0.05%, and the rest is Fe and unavoidable impurities.

[0006] In some embodiments, the chemical composition of the normalized rolling low-alloy structural steel is calculated by mass percentage as follows: C 0.10% - 0.14%, Si 0.20% - 0.24%, Mn 1.35% - 1.45%, P ≤ 0.015%, S ≤ 0.015%, V 0.032% - 0.039%, Nb 0.01% - 0.03%, and the rest is Fe and unavoidable impurities.

[0007] In some embodiments, the chemical composition of the normalized rolling low-alloy structural steel is as follows by mass percentage: C 0.10%, Si 0.21%, Mn 1.35%, P 0.015%, S 0.015%, V 0.032%, Nb 0.01%, and the balance is Fe and unavoidable impurities.

[0008] In some embodiments, the chemical composition of the normalized rolling low-alloy structural steel is as follows by mass percentage: C 0.12%, Si 0.22%, Mn 1.40%, P 0.012%, S 0.012%, V 0.039%, Nb 0.02%, and the balance is Fe and unavoidable impurities.

[0009] In some embodiments, the chemical composition of the normalized rolling low-alloy structural steel is as follows by mass percentage: C 0.13%, Si 0.20%, Mn 1.42%, P 0.011%, S 0.011%, V 0.037%, Nb 0.01%, and the balance is Fe and unavoidable impurities.

[0010] In some embodiments, the chemical composition of the normalized rolling low-alloy structural steel is as follows by mass percentage: C 0.14%, Si 0.24%, Mn 1.45%, P 0.009%, S 0.014%, V 0.033%, Nb 0.03%, and the balance is Fe and unavoidable impurities.

[0011] On the other hand, the present invention provides a method for preparing a normalized rolling low-alloy structural steel, which includes the following process steps: blast furnace hot metal; hot metal pretreatment; converter smelting; LF refining; special-shaped billet continuous casting; charging; walking beam reheating furnace; high-pressure water descaling; BD blooming; CCS universal rolling; hot sawing; walking beam cooling bed cooling; wherein:

[0012] In the process step of the walking beam reheating furnace, the temperature of the preheating section is controlled at ≤1000 °C, the temperature of the first heating section is controlled at ≤1150 °C, the temperature of the second heating section is controlled at 1100 - 1250 °C, the temperature of the soaking section is controlled at 1100 - 1250 °C, and the total heating duration of the slab is controlled at 3 - 5 h;

[0013] In the process step of the BD blooming, before the slab enters BD1, high-pressure water descaling is carried out to prevent the generation of surface defects of the finished H-beam caused by the pressing-in of iron oxide slabs, and controlled rolling is not carried out in the blooming area;

[0014] In the process step of the CCS universal rolling, the rolling start temperature is controlled at 1170 - 1180 °C, and the finishing rolling temperature is controlled at 940 - 950 °C;

[0015] In the process step of the walking beam cooling bed cooling, close-packed cooling is carried out.

[0016] Compared with the prior art, the beneficial technical effects of the present invention are as follows: By reasonably controlling the contents of elements such as Nb and V, the structural steel prepared has high strength and stable impact toughness. At the same time, the normalizing rolling process (the starting rolling temperature is controlled at 1170 - 1180 °C, and the finishing rolling temperature is controlled at 940 - 950 °C) is adopted, so that the product performance meets the requirements of normalized steel, and the production cost is reduced. Specific Embodiments

[0017] In some embodiments, the present invention provides a normalized rolling low-alloy structural steel, and its chemical composition by mass percentage is: C 0.10% - 0.15%, Si 0.20% - 0.24%, Mn 1.35% - 1.45%, P ≤ 0.02%, S ≤ 0.015%, V 0.03% - 0.04%, Nb 0.01% - 0.05%, and the rest are Fe and inevitable impurities.

[0018] The reasons for limiting the main chemical components of the present invention are as follows:

[0019] C: The C element is the most effective element for improving the strength of steel. The increase in the C content can improve the tensile strength and yield strength of the steel, but the elongation and impact toughness will decrease to some extent. To ensure that the normalized rolling H-beam with low temperature resistance obtains good comprehensive performance, the C element content of the steel in the present invention is designed to be 0.10 - 0.15%.

[0020] Mn: Mn is an important element for strengthening and toughening, and the cost is low. With the increase in the manganese content, the strength of the steel is significantly improved, and the processing performance of the steel is improved, while the ductile-brittle transition temperature hardly changes. However, if the manganese content is too high, it will inhibit the transformation of ferrite, affect the yield strength of the steel, and is not conducive to the control of the yield ratio. The Mn element content of the steel in the present invention is designed to be 1.35 - 1.45%.

[0021] Si: Si can improve the strength of steel. By increasing the Si element, the strength of the steel can be improved to a certain extent. However, as the mass percentage of Si further increases, it is easy to form martensite structure in the steel. Therefore, for the normalized rolling low-alloy structural steel described in the present invention, the mass percentage of Si is controlled at 0.20 - 0.24%.

[0022] V: V can play the roles of precipitation strengthening, grain refinement strengthening and grain boundary strengthening in steel, and can reduce the ductile-brittle transition temperature. The carbonitrides of V can effectively refine the ferrite grains. In low-temperature steel, a small amount of V makes the grains refined and the toughness increased. The V element content of the steel in the present invention is designed to be 0.03% - 0.04%.

[0023] P: P has a strong solution strengthening and cold work hardening effect in steel. P acts on ferrite. Although it can improve the strength and hardness of steel, the biggest harm is that segregation is serious, temper brittleness is increased, and the plasticity and toughness of steel are significantly increased, resulting in the phenomenon that the steel is prone to brittle fracture during cold working, that is, the so-called "cold brittleness" phenomenon. Therefore, the P element content of the steel of the present invention is designed to be ≤0.020%.

[0024] S: S is an element of hot brittleness and easy machinability. It is known that the cutting performance improves with the increase of the mass percentage of sulfur, but the hot workability deteriorates with the increase of sulfur content. Therefore, in the preparation method of the normalized rolling low-alloy structural steel described in the present invention, the mass percentage of S is limited to S≤0.015%.

[0025] Nb: Adding trace alloying element Nb to steel has a strong effect of preventing the growth of austenite grains and is easy to obtain a fine-grained structure. Niobium will produce significant grain refinement and moderate precipitation strengthening during controlled rolling. The Nb element content of the steel of the present invention is designed to be 0.01% - 0.05%.

[0026] In some embodiments, the present invention also provides a preparation method of a normalized rolling low-alloy structural steel, which includes: blast furnace hot metal; hot metal pretreatment; converter smelting; LF refining; special-shaped billet continuous casting; charging; walking beam reheating furnace; high-pressure water descaling; BD blooming; CCS universal rolling; hot sawing; walking beam cooling bed cooling; straightening; sawing; wherein:

[0027] The technological steps of the blast furnace hot metal, hot metal pretreatment, converter smelting, LF refining, special-shaped billet continuous casting, and charging can be carried out according to the technological parameters of preparing Q355ND steel in the prior art;

[0028] In the walking beam reheating furnace technological step, the temperature of the preheating section is controlled at ≤1000°C, the temperature of the first heating section is controlled at ≤1150°C, the temperature of the second heating section is controlled at 1100 - 1250°C, the temperature of the soaking section is controlled at 1100 - 1250°C, and the total heating time of the slab is controlled at 3 - 5h;

[0029] In the BD blooming technological step, before the slab enters BD1, high-pressure water descaling must be carried out to prevent the generation of surface defects of the finished H-beam caused by the pressing-in of iron oxide slabs, and controlled rolling is not carried out in the blooming area;

[0030] In the CCS universal rolling technological step, the rolling start temperature is controlled at 1170 - 1180°C, and the final rolling temperature is controlled at 940 - 950°C (i.e., the normalized temperature rolling process, which refers to the rolling process in which the final deformation is carried out within a certain temperature range to make the steel reach a normalized state so that the specified mechanical property values can be achieved even after normalization);

[0031] In the step-cooling process of the walking beam cooling bed, close-packed cooling is carried out.

[0032] The content of the present invention will be described in detail through specific embodiments below. The embodiments are intended to facilitate the understanding of the present invention and are not intended to limit the content of the present invention.

[0033] Table 1 is a table of the chemical compositions and contents of the steel grades of each embodiment. Table 2 is a table for controlling the rolling start temperature and final rolling temperature of each example. Table 3 is a table of the mechanical property test results of the structural steel prepared in each embodiment. Table 4 is a table of the mechanical property test results of the structural steel in each embodiment after normalizing heat treatment (air cooling after holding at 940°C for 0.5 h).

[0034] Table 1: Chemical Compositions (wt%) of Embodiments 1-4 of the Present Invention

[0035] Example C Si Mn P S V Nb 1 0.10 0.21 1.35 0.015 0.015 0.032 0.01 2 0.12 0.22 1.40 0.012 0.012 0.039 0.02 3 0.13 0.20 1.42 0.011 0.011 0.037 0.01 4 0.14 0.24 1.45 0.009 0.014 0.033 0.03

[0036] Table 2: Control of Rolling Start Temperature and Final Rolling Temperature of Embodiments 1-4 of the Present Invention

[0037] Example Initial rolling temperature (°C) Final rolling temperature (°C) 1 1170 940 2 1174 943 3 1175 947 4 1180 950

[0038] Table 3: Mechanical Properties of Structural Steel in Each Embodiment

[0039]

[0040] Table 4: Mechanical Properties of Structural Steel in Each Embodiment after Normalizing Heat Treatment (Air Cooling after Holding at 940°C for 0.5 h)

[0041]

[0042] As can be seen from the above examples, the structural steel in each embodiment has high strength and stable low-temperature impact toughness, and its various properties can still meet the standard requirements of normalized steel after normalizing heat treatment. The normalized rolling low-alloy structural steel of the present invention has simple chemical composition, and has the advantages of simple control, low manufacturing cost, and strong operability for steel production.

[0043] Finally, it should be noted that the above are only the preferred embodiments of the present invention and are not used to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions recorded in the foregoing embodiments or perform equivalent replacements for some of the technical features. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A normalized rolling low-alloy structural steel, the chemical composition of which is by mass percentage: C 0.10% - 0.15%, Si 0.20% - 0.24%, Mn 1.35% - 1.45%, P ≤ 0.02%, S ≤ 0.015%, V 0.03% - 0.04%, Nb 0.01% - 0.05%, and the rest is Fe and inevitable impurities.

2. The normalized rolling low-alloy structural steel according to claim 1, the chemical composition of which is by mass percentage: C 0.10% - 0.14%, Si 0.20% - 0.24%, Mn 1.35% - 1.45%, P ≤ 0.015%, S ≤ 0.015%, V 0.032% - 0.039%, Nb 0.01% - 0.03%, and the rest is Fe and inevitable impurities.

3. The normalized rolling low-alloy structural steel according to claim 1 or 2, the chemical composition of which is by mass percentage: C 0.10%, Si 0.21%, Mn 1.35%, P 0.015%, S 0.015%, V 0.032%, Nb 0.01%, and the rest is Fe and inevitable impurities.

4. The normalized rolling low-alloy structural steel according to claim 1 or 2, the chemical composition of which is by mass percentage: C 0.12%, Si 0.22%, Mn 1.40%, P 0.012%, S 0.012%, V 0.039%, Nb 0.02%, and the rest is Fe and inevitable impurities.

5. The normalized rolling low-alloy structural steel according to claim 1 or 2, the chemical composition of which is by mass percentage: C 0.13%, Si 0.20%, Mn 1.42%, P 0.011%, S 0.011%, V 0.037%, Nb 0.01%, and the rest is Fe and inevitable impurities.

6. The normalized rolling low-alloy structural steel according to claim 1 or 2, the chemical composition of which is by mass percentage: C 0.14%, Si 0.24%, Mn 1.45%, P 0.009%, S 0.014%, V 0.033%, Nb 0.03%, and the rest is Fe and inevitable impurities.

7. A preparation method of the normalized rolling low-alloy structural steel according to any one of claims 1 - 6, which includes the following process steps: blast furnace hot metal; hot metal pretreatment; converter smelting; LF refining; special-shaped billet continuous casting; charging; walking beam reheating furnace; high-pressure water descaling; BD blooming; CCS universal rolling; hot sawing; walking beam cooling bed cooling; wherein: In the process step of the walking beam reheating furnace, the temperature of the preheating section is controlled at ≤ 1000 °C, the temperature of the first heating section is controlled at ≤ 1150 °C, the temperature of the second heating section is controlled at 1100 - 1250 °C, the temperature of the soaking section is controlled at 1100 - 1250 °C, and the total heating duration of the slab is controlled at 3 - 5 h; In the BD blooming process step, before the billet enters BD1, high-pressure water descaling is carried out to prevent the generation of surface defects on the finished H-beam caused by the pressing-in of iron oxide billets, and controlled rolling is not carried out in the blooming area; In the CCS universal rolling process step, the rolling start temperature is controlled at 1170 - 1180 °C, and the final rolling temperature is controlled at 940 - 950 °C; In the walking beam cooling bed cooling process step, dense cooling is carried out.

Images