Wear-resistant steel and its manufacturing method

Abstract

This invention relates to a wear-resistant steel and a method for manufacturing the same. In addition to containing more than 90% Fe and unavoidable impurities, the steel also contains, by mass percentage, the following chemical elements: C: 0.10–0.20%, Si: 0.10–0.80%, Mn: 0.80–2.00%, Al: 0.010–0.080%, Ca: 0.001–0.008%, and one or more selected from Nb: 0.001–0.080%, V: 0.01–0.20%, Ti: 0.001–0.20%, Cr: 0.01–0.80%, Mo: 0.01–0.80%, Ni: 0.01–0.80%, and rare earth elements RE: 0.01–0.10%. This invention, through the adoption of a reasonable chemical composition design and optimized process, can effectively improve the hardness, strength, toughness, machinability such as bending performance and weldability of steel while controlling production costs, thereby obtaining wear-resistant steel with good mechanical properties and weldability.

Description

Technical Field

[0001] This invention relates to the field of steel, and more particularly to an impact-resistant, easy-to-weld wear-resistant steel and its manufacturing method. Background Technology

[0002] Wear-resistant steel is characterized by high strength and high wear resistance. Its performance is quite excellent, and it can be effectively used in mining, agriculture, cement production, ports, power and metallurgy. It can be used to manufacture mechanical products such as bulldozers, loaders, excavators, dump trucks, grab buckets and stacker-reclaimers, and has broad application prospects.

[0003] In current technologies, wear-resistant steel is mainly produced by adding appropriate amounts of carbon and alloying elements, combined with offline quenching and tempering heat treatment processes. The most widely used type is martensitic wear-resistant steel. This type of wear-resistant steel improves its mechanical properties by increasing the carbon content and adding appropriate amounts of alloying elements such as chromium, molybdenum, nickel, vanadium, and boron, and by fully utilizing phase transformation strengthening methods after heat treatment.

[0004] However, this type of wear-resistant steel often suffers from poor machinability, formability, impact resistance, and weldability due to its high hardness. This is a common and significant problem with low-alloy, high-strength wear-resistant steels. For thicker wear-resistant steels (80mm–100mm), more carbon and alloying elements are often added to ensure good mechanical properties. This results in even worse weldability, formability, and impact resistance, placing higher demands on processing equipment for machining, drilling, and bending, thus posing significant challenges for users.

[0005] For example, Chinese patent application CN112195397A, published on January 8, 2021, entitled "A Thick, Low-Carbon Equivalent, High-Toughness Wear-Resistant Steel Plate and Its Manufacturing Method," discloses a thick, low-carbon equivalent, high-toughness wear-resistant steel plate with the following chemical composition by mass percentage: C: 0.15%–0.17%, Si: 0.20%–0.40%, Mn: 0.90%–1.10%, P≤0.012%, S≤0.002%, Cr: 0.60%–0.80%, Mo: 0.30%–0.50%, Ni: 0.50%–0.70%, T… i: 0.008%~0.020%, Nb≤0.050%, V≤0.020%, B: 0.0010%~0.0020%, Alt: 0.04%~0.07%, N≤0.0040%, H≤0.0002%, with the remainder being Fe and unavoidable impurities. The offline heat treatment conditions were: quenching temperature controlled at 900℃, heating rate at 1.55min / mm, and holding time at 40min; tempering conditions were: tempering temperature at 185℃, heating rate at 40℃ / h, and holding time at 490min; after tempering, the material was removed from the furnace and air-cooled to obtain a tempered martensitic microstructure.

[0006] Chinese patent application CN102021492A, published on April 20, 2011, entitled "A Low-Carbon Low-Alloy Wear-Resistant Steel and Its Production Method," discloses a low-carbon low-alloy wear-resistant steel with the following chemical composition and mass percentage content: C: 0.13%–0.17%, Si: 0.35%–0.50%, Mn: 1.05%–1.25%, P≤0.020%, S≤0.010%, Ni: 0.25%–0.35%, Cr: 0.40%–0.50%, Mo: 0. The composition is as follows: 30%–0.40%, Nb: 0.03%–0.05%, V: 0.04%–0.06%, Ti: 0.01%–0.025%, B≤0.003%, Als: 0.005%–0.04%, with the remainder being Fe and unavoidable impurities. It is manufactured by offline quenching and tempering heat treatment process. The quenching temperature is controlled at 930±10℃, and the holding time is 2–2.5 min / mm. The tempering temperature is 300±10℃, and the holding time is 4.5–5.5 min / mm. Finally, tempered martensite structure is obtained.

[0007] Chinese patent application CN101748333A, published on June 23, 2010, entitled "A Low-Carbon Equivalent High-Strength Wear-Resistant Steel Plate and Its Production Method," discloses a low-carbon equivalent high-strength wear-resistant steel plate prepared from the following components by weight percentage: C: 0.15-0.18%, Si: 0.20-0.40%, Mn: 1.30-1.40%, P≤0.015%, S≤0.005%, V: 0.040-0.050%, Nb: 0.017-0.0 The steel contains 30% Al, 0.020-0.040% Cr, 0.15-0.25% Mo, 0.18-0.28% Ti, 0.017-0.026% B, and the remainder is Fe and unavoidable impurities. It is manufactured using the same offline quenching and tempering heat treatment process. The quenching temperature is 925℃ and the holding time is 2.3 min / mm. The tempering temperature is 206℃ and the holding time is 4.5 min / mm. After tempering, the steel plate is air-cooled to obtain martensitic steel.

[0008] The above-mentioned solutions still have technical shortcomings, as they cannot produce steel that simultaneously possesses good mechanical properties (including strength, hardness, impact resistance, and bending performance) and weldability. Therefore, the art seeks to obtain an impact-resistant and easily weldable wear-resistant steel that has better weldability, bending performance, and impact resistance than existing wear-resistant steels, making it suitable for equipment such as road rollers. Summary of the Invention

[0009] One of the objectives of this invention is to provide an impact-resistant and easily weldable wear-resistant steel. This steel not only has excellent comprehensive properties (including excellent hardness, weldability, bending performance and impact resistance), but also has low production costs. It is also environmentally friendly and has very good prospects for promotion and application value.

[0010] Through dedicated research, the inventors unexpectedly discovered that by fully utilizing the grain-refining and strengthening properties of microalloying elements such as Nb, Ti, V, and / or rare earth elements (RE), it is possible to ensure that steel possesses good mechanical properties and excellent weldability while reducing the content of carbon and alloying elements such as Cr, M, and Ni. Furthermore, by replacing the offline quenching and tempering heat treatment in existing technologies with the online quenching defined in this invention, the production process can be effectively shortened, production efficiency improved, and since only the upper surface layer of the steel is quenched, the hardness distribution within the steel can be further optimized, thereby increasing the steel's impact resistance and weldability.

[0011] This invention provides a steel that, in addition to containing more than 90% Fe and unavoidable impurities, also contains the following chemical elements in mass percentage:

[0012] C: 0.10–0.20%, Si: 0.10–0.80%, Mn: 0.80–2.00%, Al: 0.010–0.080%, Ca: 0.001–0.008%, and one or more of the following: Nb: 0.001–0.080%, V: 0.01–0.20%, Ti: 0.001–0.20%, Cr: 0.01–0.80%, Mo: 0.01–0.80%, Ni: 0.01–0.80%, and rare earth element RE: 0.01–0.10%.

[0013] The present invention also provides a steel comprising the following chemical elements in a mass percentage:

[0014] C: 0.10–0.20%, Si: 0.10–0.80%, Mn: 0.80–2.00%, Al: 0.010–0.080%, Ca: 0.001–0.008%, and one or more of Nb: 0.001–0.080%, V: 0.01–0.20%, Ti: 0.001–0.20%, Cr: 0.01–0.80%, Mo: 0.01–0.80%, Ni: 0.01–0.80%, and rare earth element RE: 0.01–0.10%, with the balance being Fe and unavoidable impurities.

[0015] Preferably, the chemical elements in the steel of the present invention, by mass percentage, satisfy the following: C: 0.12-0.19%, Si: 0.15-0.70%, Mn: 1.00-1.80%, Al: 0.010-0.070%.

[0016] Preferably, the chemical elements in the steel of the present invention, by mass percentage, satisfy one or more of the following: C: 0.13-0.18%; Si: 0.15-0.65%; Mn: 1.00-1.60%; Al: 0.010-0.060%; Nb: 0.008-0.050%; V: 0.015-0.12%; Ti: 0.001-0.060%; Cr: 0.05-0.50%; Mo: 0.05-0.50%; Ni: 0.05-0.50%; rare earth element RE: 0.01-0.05%.

[0017] The steel of this invention mainly uses medium-to-high carbon, manganese, and vanadium as the main additive elements, which can effectively reduce the amount of precious metals such as Cr, Mo, and Ni, thereby effectively reducing the cost of production and manufacturing.

[0018] Furthermore, the steel of this invention is mainly low alloy, which makes full use of the refinement and strengthening characteristics of micro-alloying elements such as Nb and Ti. It can reduce carbon and precious metal alloying elements such as Cr, Mo and Ni, while ensuring that the steel has good mechanical properties and excellent weldability.

[0019] The design principles of each chemical element in the steel described in this invention are as follows:

[0020] C: C is the most basic and important element in steel. Adding an appropriate amount of C can improve the strength and hardness of steel, thereby improving its wear resistance. However, it should be noted that C can also have an adverse effect on the toughness and weldability of steel. Therefore, the C content in steel should be controlled at 0.10-0.20%, preferably 0.12-0.19%, and more preferably 0.13-0.18%.

[0021] Si: Si can be dissolved in ferrite and austenite, thereby increasing their hardness and strength. However, excessive Si content can lead to a sharp decrease in the toughness of steel. Furthermore, considering that silicon has a stronger affinity for oxygen than iron, it easily forms low-melting-point silicates during welding, increasing the fluidity of slag and molten metal and affecting weld quality. Therefore, the Si content in steel is controlled at 0.10–0.80%, preferably 0.15–0.70%, and more preferably 0.15–0.65%.

[0022] Mn: Adding an appropriate amount of Mn can significantly increase the hardenability of steel, reduce its transformation temperature, and decrease its critical cooling rate. However, it is important to note that the Mn content in steel should not be too high. Excessive Mn content not only tends to coarsen grains but also increases the steel's temper brittleness sensitivity and easily leads to segregation and cracks in the cast billet, reducing the performance of the steel plate. Therefore, the Mn content in steel should be controlled between 0.80% and 2.00%, preferably 1.00% to 1.80%, and more preferably 1.00% to 1.60%.

[0023] Al: Al can combine with nitrogen (N) in steel to form fine, insoluble AlN particles, refining the steel's grain structure. Therefore, adding an appropriate amount of Al to steel can effectively refine the grain structure, fix N and O in the steel, reduce the steel's sensitivity to notches, reduce or eliminate aging phenomena, and improve the steel's toughness. Therefore, the Al content in steel is controlled at 0.010–0.080%, preferably 0.010–0.070%, and more preferably 0.010–0.060%.

[0024] Ti: Ti is one of the strong carbide-forming elements. Ti can combine with C to form fine TiC particles. These small TiC particles can be distributed at grain boundaries, thus refining the grain size. Furthermore, TiC particles are relatively hard, which can improve the wear resistance of steel. Therefore, the Ti content in steel is controlled at 0.001–0.20%, preferably 0.001–0.060%.

[0025] Nitrogen (Nb): Nitrogen (Nb) significantly contributes to improving the strength and toughness of materials through its grain refinement and precipitation strengthening effects. As a strong C and N compound forming element, Nb strongly inhibits austenite grain growth. Nb can improve the strength and toughness of steel through grain refinement, primarily by enhancing steel properties through precipitation strengthening and phase transformation strengthening. Nb has been recognized as one of the most effective strengthening agents in high-strength low-alloy structural steels. Therefore, the Nb content in steel is controlled between 0.001% and 0.080%, preferably between 0.008% and 0.050%.

[0026] V: Adding an appropriate amount of vanadium (V) can refine the grains, ensuring that the austenite grains in the billet do not grow too coarse during the heating stage. This allows for further grain refinement during subsequent multi-pass rolling, improving the steel's strength and toughness. Therefore, the V content in the steel is controlled at 0.01–0.20%, preferably 0.015–0.12%.

[0027] Cr: Cr can lower the critical cooling rate and improve the hardenability of steel. Cr can form (Fe,Cr)3C, (Fe,Cr)7C3, and (Fe,Cr) in steel. 23 Various carbides, such as C7, can effectively improve the strength and hardness of steel. Furthermore, it should be noted that adding an appropriate amount of Cr to steel can prevent or slow down the precipitation and aggregation of carbides during tempering, thereby improving the tempering stability of the steel. Therefore, the Cr content in steel should be controlled between 0.01% and 0.80%, preferably between 0.05% and 0.50%.

[0028] Mo: Adding an appropriate amount of Mo can effectively refine the grain size and improve the strength and toughness of steel. Mo exists in both the solid solution phase and the carbide phase in steel; therefore, Mo has both solid solution strengthening and carbide dispersion strengthening effects. Furthermore, Mo is an element that reduces temper brittleness; adding an appropriate amount of Mo to steel can also improve the tempering stability of the material. Therefore, the Mo content in steel should be controlled between 0.01% and 0.80%, preferably between 0.05% and 0.50%.

[0029] Ni: Ni is miscible with iron in any proportion. It can improve the low-temperature toughness of steel by refining ferrite grains and has a significant effect on reducing the brittle-to-ductile transition temperature. However, it is important to note that the Ni content in steel should not be too high. When the Ni content is too high, it can make it difficult to remove the oxide scale from the steel plate surface, significantly increasing production costs. Therefore, the Ni content in steel should be controlled between 0.01% and 0.80%, preferably between 0.05% and 0.50%.

[0030] Rare earth elements (RE): Adding appropriate amounts of rare earth elements (RE) can reduce the segregation of elements such as sulfur and phosphorus, improve the shape, size, and distribution of non-metallic inclusions, and refine grains, thereby increasing hardness. Furthermore, rare earth elements can improve the yield strength ratio, which is beneficial for improving the strength and toughness of low-alloy high-strength steel and enhancing the thermal stability of steel plates. However, it is important to note that the content of rare earth elements in steel should not be excessive, otherwise severe segregation will occur, thus reducing the quality and mechanical properties of the cast billet. Therefore, the content of RE elements in steel is controlled at 0.01–0.10%, preferably 0.01–0.05%. Rare earth elements include the lanthanides (atomic numbers 57 to 71) in the periodic table, as well as scandium and yttrium. In this invention, any rare earth element can be selected, and no limitation is made herein.

[0031] Ca: In the impact-resistant, weldable, and wear-resistant steel described in this invention, Ca plays a significant role in the modification of inclusions in the cast steel. Adding an appropriate amount of Ca to the cast steel can transform elongated sulfide inclusions into spherical CaS or (Ca,Mn) sulfide inclusions. The oxides and sulfide inclusions formed by Ca have low density and are easily floated and removed. Furthermore, Ca can significantly reduce the segregation of S at grain boundaries, all of which are beneficial to improving the quality of the cast steel and thus its performance. Therefore, the Ca content in the molten steel is controlled at 0.001–0.008%.

[0032] Furthermore, unavoidable impurities in this invention include P, S, N, O, and H. P and S are both harmful elements, while N, O, and H are also detrimental to the quality of steel. Excessive N, O, and H in steel are particularly detrimental to its properties, especially weldability, impact toughness, and crack resistance, reducing the quality and service life of the steel plate. Therefore, to ensure steel quality, the lower the content of impurity elements in the steel, the better, provided conditions permit. Generally, considering the control of production costs, the impurity element content in the steel of the present invention, by mass percentage, preferably satisfies the following: P ≤ 0.030%, preferably P ≤ 0.015%, more preferably P ≤ 0.012%; S ≤ 0.010%, preferably S ≤ 0.005%, more preferably S ≤ 0.003%; N ≤ 0.0080%, preferably N ≤ 0.0060%, more preferably N ≤ 0.0050%; O ≤ 0.0080%, preferably O ≤ 0.0060%, more preferably O ≤ 0.0050%; H ≤ 0.0004%, preferably H ≤ 0.0003%, more preferably H ≤ 0.0002%.

[0033] Preferably, the steel comprises an upper surface layer, a core, and a lower surface layer in sequence. The microstructure of the upper surface layer is bainite, martensite, and retained austenite, and the microstructure of the core and the lower surface layer is ferrite and pearlite.

[0034] Preferably, the upper surface layer contains 70-95% bainite, 0-20% martensite, and 5-10% retained austenite by volume; the core contains 70-80% ferrite and 20-30% pearlite by volume; and the lower surface layer contains 80-85% ferrite and 15-20% pearlite by volume.

[0035] Preferably, in the steel of the present invention, the Brinell hardness of the upper surface layer is 360-440 HBW, the Brinell hardness of the core is 150-250 HBW, and the Brinell hardness of the lower surface layer is 170-270 HBW.

[0036] Preferably, in the steel of the present invention, the Charpy V-shaped longitudinal impact energy of the upper surface layer, the core and the lower surface layer are 38-69J, 225-275J and 210-220J, respectively.

[0037] This invention sets different Brinell hardnesses in the upper surface layer, core, and lower surface layer of its steel. The upper surface layer has a higher hardness, which enhances the steel's wear resistance; the core has a relatively lower hardness, which increases toughness and thus improves the steel's impact resistance; the ferrite and pearlite in the lower surface layer enhance the steel's weldability. Therefore, the steel of this invention simultaneously possesses high strength, superior toughness, uniform hardness, and excellent machinability and thermal stability.

[0038] The present invention also provides a method for manufacturing the above-mentioned steel, the method comprising the following steps:

[0039] 1) The molten steel is smelted and cast to obtain a billet;

[0040] 2) Heating the billet;

[0041] 3) Rolling to obtain steel plates;

[0042] 4) Online quenching.

[0043] Preferably, in step 4), the upper surface layer of the steel plate is water-cooled to 350-500°C and then air-cooled to room temperature, wherein the water cooling rate is ≥15°C / s; more preferably, in step 4), the upper surface layer of the steel plate is water-cooled to 360-480°C and then air-cooled to room temperature, wherein the water cooling rate is 20-45°C / s; even more preferably, in step 4), the upper surface layer of the steel plate is water-cooled to 380-480°C and then air-cooled to room temperature, wherein the water cooling rate is 25-45°C / s.

[0044] While fully refining the microstructure of the steel after rolling, online water cooling can eliminate the need for offline heat treatment, thereby shortening the production process, improving production efficiency, reducing production costs, and enabling the upper surface layer of the steel of the present invention to produce a microstructure different from other layers, thereby adjusting the hardness at different locations of the steel and giving the obtained steel excellent mechanical properties, machinability, and weldability.

[0045] This invention reduces the hardenability of steel, prevents steel plate cracking, ensures the hardenability and red hardness of steel, and guarantees toughness by performing online quenching under the above conditions after rolling.

[0046] Preferably, in step (2), the heating temperature is 1000-1200℃, more preferably 1000-1150℃, even more preferably 1000-1130℃, and most preferably 1000-1110℃; the holding time is 1-3 hours. Heating under these conditions can...

[0047] To improve production efficiency and prevent excessive growth of austenite grains and severe oxidation of the billet surface.

[0048] Preferably, step 3) includes rough rolling and finish rolling. The rough rolling temperature is 900-1150℃, preferably 900-1100℃, more preferably 900-1080℃, and most preferably 910-1080℃. The finish rolling temperature is 820-920℃, preferably 830-910℃, more preferably 830-900℃, and most preferably 830-905℃.

[0049] Preferably, the rolling reduction rate of the roughing mill is ≥20%, more preferably ≥25%, and even more preferably ≥28%; the rolling reduction rate of the finishing mill is ≥40%, more preferably ≥45%, and even more preferably ≥50%.

[0050] Preferably, the method of the present invention further includes step 5) tempering after step 4): the tempering heating temperature is 400-600℃, and the holding time is 30-120 min. More preferably, the tempering heating temperature is 420-580℃, and the holding time is 30-100 min. Tempering can further adjust the mechanical properties and microstructure of the steel, so as to better achieve the beneficial effects of the present invention.

[0051] Compared with the prior art, the steel and its manufacturing method described in this invention have the following advantages and beneficial effects:

[0052] (1) From the perspective of chemical composition, the present invention has fully optimized the alloy composition in the steel, which is mainly low carbon and low alloy. By making full use of the characteristics of micro-alloying elements such as Nb, Ti, V and / or rare earth elements RE to refine and strengthen the grains, the steel has good mechanical properties and excellent weldability while reducing the content of carbon and alloying elements Cr, Mo and / or Ni.

[0053] (2) From the perspective of production process, the steel described in this invention is produced by controlled rolling and controlled cooling (TMCP) process. By controlling the process parameters such as the initial rolling temperature, final rolling temperature, rolling deformation amount and / or cooling method and speed in the TMCP process, the microstructure of the steel is refined and strengthened, thereby reducing the content of carbon and alloying elements, and obtaining steel with excellent mechanical properties and welding performance.

[0054] (3) In terms of product performance, the steel of the present invention has a specific microstructure distribution. This microstructure effectively adjusts the hardness distribution inside the steel, which is beneficial to the good distribution of the steel's strength, hardness and toughness, thereby obtaining an impact-resistant and easy-to-weld wear-resistant steel that can simultaneously possess high strength, superior toughness, uniform hardness and excellent machinability and thermal stability.

[0055] In summary, it can be seen that the steel described in this invention, through reasonable chemical composition design and optimized process, can effectively improve the strength, toughness, machinability such as bending performance and weldability of steel while controlling production costs, and has excellent application value. Detailed Implementation

[0056] definition

[0057] In this invention, the "upper surface layer" refers to the surface of the steel located above the core, also known as the "upper surface of the steel plate," and its thickness can be less than about 30% of the overall thickness of the steel plate, for example, 25%. In the online quenching process described herein, only the upper surface layer of the steel is water-cooled, forming bainite, martensite, and retained austenite within it.

[0058] The “lower surface layer” is the surface located below the core and opposite to the upper surface layer. It can also be called the “lower surface of the steel plate”. Its thickness can be less than 30% of the overall thickness of the steel plate, for example, 25%.

[0059] The “core” is the steel substrate located between the “upper surface layer” and the “lower surface layer”, and its thickness can be more than 40% of the overall thickness of the steel plate, for example, 50%.

[0060] The "thickness wear-resistant steel" mentioned in this invention refers to wear-resistant steel with a thickness of 40-150mm, preferably 60-120mm.

[0061] It should be noted that directional adjectives such as "up" and "down" are for illustrative purposes only and are not intended to limit the steel used in this invention.

[0062] The following embodiments will provide a more detailed description of the present invention to make its content clearer and easier to understand. However, it should be noted that the embodiments disclosed below are merely examples of specific implementations of the present invention, and the present invention is not limited to these embodiments.

[0063] Examples 1-10 and Comparative Example 1

[0064] The steels of Examples 1-10 were obtained through the following steps:

[0065] (1) The molten steel was smelted and cast according to the formula shown in Table 1 to obtain a billet.

[0066] (2) Heating the billet: The heating temperature is 1000-1200℃, and the temperature is held for 1-3 hours;

[0067] (3) Rolling to obtain steel plates: Rolling includes rough rolling and finish rolling. The rough rolling temperature is 950-1150℃, and the rolling reduction rate in the rough rolling stage is greater than 20%. The finish rolling temperature is 820-910℃, and the rolling reduction rate in the finish rolling stage is greater than 40%.

[0068] (4) Online quenching: First, use water cooling to cool to 355-500℃, and then air cooling to room temperature. The cooling rate of water cooling is 20-45℃ / s.

[0069] The comparative example steel of the present invention is manufactured by the same steps described above, except that the chemical composition and / or process parameters of the comparative example steel are not within the scope of the present invention.

[0070] In embodiments 1-10 of the present invention, after completing the above steps (1)-(4), embodiments 1 and 8-10 further include performing the tempering operation in step (5).

[0071] During the tempering process in step (5), the tempering heating temperature is 435-555℃ and the holding time is 30-120min.

[0072] Table 1 lists the mass percentage of each chemical element in the steels of Examples 1-10 and the comparative examples.

[0073] Table 1. (wt%, balance Fe and other unavoidable impurities other than P, S, N, O, and H)

[0074]

[0075]

[0076] Tables 2-1 and 2-2 list the specific process parameters for Examples 1-10 and Comparative Example 1.

[0077] Table 2-1.

[0078]

[0079] Table 2-2.

[0080]

[0081] Samples of the steels from Examples 1-10 and Comparative Example 1 were taken and analyzed by ESBD using a Hitachi SU70 thermal field emission scanning electron microscope to obtain the microstructure distribution of the samples. The results showed that the microstructure of the upper surface layer of the steels from Examples 1-10 of the present invention consisted of bainite, martensite, and retained austenite, with volume percentage contents of 70-95%, 0-20%, and 5-10%, respectively; while the microstructure of the core and lower surface layer consisted of ferrite and pearlite, with volume percentage contents of 70-80% and 20-30%, respectively. The microstructure of the steel in the comparative example was entirely martensite.

[0082] The mechanical properties and weldability of the steel samples of Examples 1-10 and Comparative Example 1 were then tested to obtain the mechanical properties (including cold bending properties, impact resistance and Brinell hardness) and weldability of each example and Comparative Example 1. The test results are listed in Tables 3 and 4 below.

[0083] The relevant testing methods are as follows:

[0084] 1) Brinell hardness test: The Brinell hardness was tested at room temperature using an SCL246 Brinell hardness tester according to GB / T 231.1 standard. The surface hardness of the steel samples from Examples 1-10 and Comparative Example 1 was tested to obtain the corresponding Brinell hardness.

[0085] 2) Impact test: The impact performance was tested at room temperature using an SCL186750J instrumented impact testing machine in accordance with GB / T 229 standard to obtain the Charpy V-type longitudinal impact energy Akv of the steel samples of Examples 1-10 and Comparative Example 1 at room temperature (25℃).

[0086] 3) Cold bending test: Take a steel plate sample and perform a 90° cold bending test. The steel plate sample with no cracks or breaks on the surface is qualified.

[0087] 4) Weldability Test: The weldability of the steel plate is characterized by carbon equivalent (CEV), the formula is C E =C+Mn / 6+(Ni+Cu) / 15+(Cr+Mo+V) / 5(%). C E The higher the temperature, the greater the hardenability of the steel, and the easier it is to produce cold cracks during welding.

[0088] Table 3 lists the Brinell hardness and cold bending test results of the steel samples of Examples 1-10 and Comparative Example 1.

[0089] Table 3

[0090]

[0091] Table 4 lists the test results of impact toughness and weldability of the steel samples of Examples 1-10 and Comparative Example 1 at room temperature.

[0092] Table 4

[0093]

[0094] As can be seen from Tables 3 and 4, the steels of Examples 1-10 of the present invention possess excellent mechanical properties (e.g., excellent impact resistance, bending performance, and Brinell hardness) as well as excellent weldability. The steel in Comparative Example 1, however, does not simultaneously possess both the aforementioned excellent mechanical properties and weldability.

[0095] The preferred embodiments of the present invention have been described in detail above. It should be understood that those skilled in the art can make various modifications and variations based on the concept of the present invention without creative effort. Therefore, all technical solutions that can be obtained by those skilled in the art based on the concept of the present invention through logical analysis, reasoning, or limited experimentation on the basis of existing technology should be within the scope of protection defined by the claims.

[0096] Furthermore, the combination of the technical features in this case is not limited to the combination methods described in the claims of this case or the combination methods described in the specific embodiments. All technical features described in this case can be freely combined or combined in any way, unless they contradict each other.

Claims

1. A type of steel, characterized in that, The steel has the following chemical elements by mass percentage: C: 0.10~0.20%, Si: 0.10~0.80%, Mn: 0.80~2.00%, Al: 0.010~0.080%, Ca: 0.001~0.008%, and one or more of the following: Nb: 0.001~0.080%, V: 0.01~0.20%, Ti: 0.001~0.20%, Cr: 0.01~0.80%, Mo: 0.01~0.80%, Ni: 0.01~0.80%, and rare earth element RE: 0.01~0.10%, with the balance being Fe and unavoidable impurities; The steel comprises an upper surface layer, a core, and a lower surface layer in sequence. The microstructure of the upper surface layer is bainite, martensite, and retained austenite, while the microstructure of the core and the lower surface layer is ferrite and pearlite.

2. The steel according to claim 1, characterized in that, The chemical elements in the steel, expressed as a percentage by mass, are as follows: C: 0.12~0.19%, Si: 0.15~0.70%, Mn: 1.00~1.80%, Al: 0.010~0.070%.

3. The steel according to claim 1, characterized in that, The chemical elements in the steel, expressed as a percentage by mass, satisfy one or more of the following: C: 0.13–0.18%; Si: 0.15–0.65%; Mn: 1.00–1.60%; Al: 0.010–0.060%; Nb: 0.008–0.050%. V：0.015～0.12%； Ti: 0.001~0.060%; Cr: 0.05~0.50%; Mo: 0.05~0.50%; Ni: 0.05~0.50%; Rare earth elements (RE): 0.01~0.05%.

4. The steel according to claim 1, characterized in that, The content of impurity elements, expressed as a percentage by mass, shall meet the following requirements: P≤0.030%; S≤0.010%; N≤0.0080%; O≤0.0080%; H≤0.0004%.

5. The steel according to claim 4, characterized in that, The content of impurity elements, expressed as a percentage by mass, shall meet the following requirements: P≤0.015%; S≤0.005%; N≤0.0060%; O≤0.0060%; H≤0.0003%.

6. The steel according to claim 4, characterized in that, The content of impurity elements, expressed as a percentage by mass, shall meet the following requirements: P≤0.012%; S≤0.003%, N≤0.0050%, O≤0.0050%, H≤0.0002%.

7. The steel according to claim 1, characterized in that, The upper surface layer contains 70-95% bainite, 0-20% martensite, and 5-10% retained austenite by volume; the core contains 70-80% ferrite and 20-30% pearlite by volume; and the lower surface layer contains 80-85% ferrite and 15-20% pearlite by volume.

8. The steel according to claim 1, characterized in that, The upper surface layer has a Brinell hardness of 360~440 HBW, the core has a Brinell hardness of 150~250 HBW, and the lower surface layer has a Brinell hardness of 170~270 HBW.

9. The steel according to claim 1, characterized in that, The Charpy V-shaped longitudinal impact energy of the upper surface layer, core, and lower surface layer are 38-69J, 225-275J, and 210-220J, respectively.

10. A method for manufacturing the steel according to any one of claims 1-9, characterized in that, The method includes the following steps: 1) The molten steel is smelted and cast to obtain a billet; 2) Heating the billet; 3) Rolling to obtain steel plates; 4) Online quenching: The upper surface layer of the steel plate is water-cooled to 350-500℃, and then air-cooled to room temperature. The water cooling rate is ≥15℃ / s.

11. The method according to claim 10, wherein, The method satisfies one or more of the following: In step (2), the heating temperature is 1000-1200℃ and the holding time is 1-3 hours; Step 3) includes rough rolling and finish rolling, with the rough rolling temperature being 900-1150℃ and the finish rolling temperature being 820-920℃; The rolling reduction rate of the roughing mill is ≥20%, and the rolling reduction rate of the finishing mill is ≥40%. as well as In step 4), the upper surface layer of the steel plate is water-cooled to 360-480°C and then air-cooled to room temperature, wherein the water cooling rate is 20-45°C / s.

12. The method according to claim 11, wherein, The method satisfies one or more of the following: In step (2), the heating temperature is 1000-1150℃; The roughing temperature is 900-1100℃, and the finishing temperature is 830-910℃; The roughing mill has a rolling reduction rate of ≥25%, and the finishing mill has a rolling reduction rate of ≥45%; and In step 4), the upper surface layer of the steel plate is water-cooled to 380-480°C and then air-cooled to room temperature, wherein the water cooling rate is 25~45°C / s.

13. The method according to claim 11, wherein, The method satisfies one or more of the following: In step (2), the heating temperature is 1000-1130℃; The roughing temperature is 900-1080℃, and the finishing temperature is 830-900℃; The rolling reduction rate of the roughing mill is ≥28%, and the rolling reduction rate of the finishing mill is ≥50%.

14. The method according to claim 11, wherein, The method satisfies one or more of the following: In step (2), the heating temperature is 1000-1110℃; The roughing temperature is 910-1080℃, and the finishing temperature is 830-905℃.

15. The method according to claim 10, characterized in that, The method further includes a tempering step 5) following step 4), wherein the tempering heating temperature is 400-600℃ and the holding time is 30-120min.

16. The method according to claim 15, characterized in that, The tempering heating temperature is 420-580℃, and the holding time is 30-100min.

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