Carbide-free bainite steel with tensile strength larger than or equal to 1300MPa and preparation method of carbide-free bainite steel

Abstract

The invention discloses carbide-free bainite steel with tensile strength larger than or equal to 1300 MPa. The carbide-free bainite steel comprises, by weight, 0.35%-0.45% of C, 1.25%-1.65% of Si, 1.95%-2.45% of Mn, less than or equal to 0.05% of P, less than or equal to 0.05% of S, the preparation method comprises the following steps: heating a hot-rolled medium-thickness raw material plate; cooling is performed; applying pressure to the steel plate by using a pressure device; and air-cooling to room temperature after unloading the pressure. According to the invention, the tensile strength is 1300-1700 MPa, the elongation is 8-14%, the bainite phase change time is shortened to not more than 20 min, the production efficiency is obviously improved, the added elements are simple, only C, Si, Mn and the balance are iron, and the market demand is completely met.

Description

Technical Field

[0001] The invention relates to high-strength steel for engineering machinery and a preparation method thereof, in particular to carbide-free bainite steel with a tensile strength of ≥1300MPa and a preparation method thereof. Background Art

[0002] Carbide-free bainite steel is an advanced steel material with both high strength and high toughness. It has a wide range of applications in engineering machinery, rail transportation, automobile manufacturing and other fields. The microstructure of this steel is composed of bainite, residual austenite and martensite. Due to the addition of more Si elements, brittle carbides are difficult to precipitate from austenite, so carbide-free bainite is obtained.

[0003] At present, the tensile strength of carbide-free bainitic steel can generally reach 1300MPa and above. The main measures to ensure that carbide-free bainitic steel has ultra-high strength include: increasing the C content in the steel (i.e. high carbon + low alloy design), or increasing the addition of alloying elements such as Cr, Mo, and Ni (i.e. low carbon + high alloy design), or increasing the content of C and other alloying elements at the same time (i.e. high carbon + high alloy design). First, for high carbon + low alloy design, the increase in C content will significantly prolong the bainite phase transformation time, making the phase transformation time reach several hours or even several days, resulting in a long preparation cycle and low production efficiency. Secondly, for low carbon + high alloy design, reducing the C content can significantly shorten the bainite phase transformation time. For example, reducing the C mass fraction to below 0.25%, the bainite phase transformation time can be shortened to within 30min, but the strength will be greatly reduced. In order to ensure ultra-high strength (≥1300MPa), it is necessary to increase the content of alloying elements such as Cr, Mo, and Ni, resulting in a significant increase in alloy cost. This is the current situation: when alloy costs are met, the phase change time is long, production efficiency is low, and it is always in a state where requirements cannot be met at the same time.

[0004] After searching:

[0005] Chinese invention patent CN107630123A discloses a carbide-free bainitic steel with a C mass fraction of 0.5-0.8% and a method for preparing the same. The preparation process requires repeated isothermal treatment in the temperature range above and below the Ms point, and the isothermal treatment time is 4-15 hours. Although a higher hardness is obtained, there are problems of complex process and long isothermal time. Similarly, the document with Chinese patent publication number CN118814091A discloses "A high-carbon bainitic steel (C mass fraction of 0.65-0.90%) and its preparation method". They introduced rare earth oxides to accelerate the bainitic phase transformation and obtained a bainitic steel with a tensile strength of more than 1500MPa, but the bainitic phase transformation time still requires at least 6 hours.

[0006] The document with Chinese patent publication number CN117926130A discloses "A bainitic steel with a C mass fraction of 0.25-0.29% and a preparation method thereof", which adopts preheating, forging, hot rolling, primary cold rolling, primary annealing, secondary cold rolling, secondary continuous annealing and other processes to prepare high-strength bainitic steel with a tensile strength of 1500MPa. It does not require long-term isothermal bainite phase transformation, but the amount of precious alloy elements such as Nb and Mo added is relatively large, and the alloy cost is relatively high.

[0007] The document with Chinese patent publication number CN103397273A discloses "Fe-C-Si-Mn-Cr-Mo-Ni-Ti-B Bainitic Steel", whose C mass fraction is 0.3-0.4%, and bainitic steel with a tensile strength exceeding 1400MPa is prepared by air cooling / wind cooling + tempering heat treatment. The preparation process is relatively simple, but there is also the problem of adding too much alloy elements. Summary of the invention

[0008] The purpose of the present invention is to overcome the mismatch between alloy cost and bainite phase transformation time in the prior art, that is, the alloy cost is low and the bainite phase transformation time is long, otherwise, the insufficiency of meeting market demand is caused, and to provide a carbide-free bainite steel with a tensile strength of ≥1300MPa and an elongation of 8-14%, and a bainite phase transformation time shortened to no more than 20min, thereby significantly improving production efficiency and meeting market demand, and a preparation method thereof.

[0009] Technical measures to achieve the above objectives

[0010] A carbide-free bainite steel with a tensile strength of ≥1300MPa, the components and weight percentages of which are: C: 0.35-0.45%, Si: 1.25-1.65%, Mn: 1.95-2.45%, and the rest are iron and other small amounts of inevitable impurities.

[0011] Preferably, the weight percentage of C is 0.38-0.42%.

[0012] Preferably, the weight percentage of Si is 1.35-1.55%.

[0013] Preferably, the weight percentage of Mn is 2.05-2.40%.

[0014] A method for preparing carbide-free bainite steel with a tensile strength of ≥1300 MPa, comprising the following steps:

[0015] 1) The hot-rolled medium-thick raw material plate is heated at a temperature of 850-900°C and kept at this temperature for 20-40 minutes;

[0016] 2) Cooling: cooling the steel plate to 330-370° C. at a cooling rate of 3-8° C. / s;

[0017] 3) Use a pressure device to pressurize the steel plate: Apply a pressure of 400-800MPa to the steel plate cooled to 330-370°C, and simultaneously keep the temperature and pressure at 330-370°C and 400-800MPa, and the time of keeping the temperature and pressure is controlled within 5-20min;

[0018] 4) After unloading the pressure, air cool to room temperature.

[0019] Preferably, the heating temperature of the hot-rolled medium-thick raw material plate is 855-895°C.

[0020] The reason is that the pressure applied to the steel plate is negatively correlated with the heat preservation and pressure holding time of the steel plate.

[0021] Preferably, when the applied pressure does not exceed 600 MPa, the heat preservation and pressure holding time is controlled within 6 to 20 minutes; when the applied pressure is greater than 600 MPa, the heat preservation and pressure holding time is controlled within 5 to 6 minutes.

[0022] Functions and principles of each component and main process of the present invention:

[0023] C can improve the stability of the parent phase austenite, postpone the ferrite phase transformation, and more easily obtain bainite structure, and C improves the strength of steel in the mode of solid solution strengthening, but the excessive C content will cause the bainite phase transformation time to be extended, so the C mass percentage is controlled at 0.35~0.45%. Si can not only play the effect of solid solution strengthening, but also can suppress the precipitation of brittle carbides in austenite, thereby obtaining carbide-free bainite structure, and after comprehensive consideration, its addition (mass percentage) is 1.25~1.65%. Mn also has the effect of solid solution strengthening, postpones the ferrite and pearlite phase transformation, but Mn also can prolong the bainite phase transformation time, and easily produces segregation, so the Mn mass percentage is controlled at 1.95~2.45%. Of course, the present invention uses S and P as harmful elements, and the lower the better it needs to be controlled.

[0024] The present invention controls the heating temperature of the hot-rolled medium-thick raw material plate to 850-900°C and keeps it at this temperature for 20-40 minutes because if the heating temperature is too low, the austenite phase transformation is insufficient, and if the heating temperature is too high, the austenite grains are coarse, so it is more appropriate to select the heating temperature of 850-900°C. Keeping it at this temperature for 20-40 minutes makes the temperature distribution of the steel plate uniform.

[0025] The reason why the present invention controls the cooling rate to cool the steel plate to 330-370°C at 3-8°C / s is that the cooling rate of 3-8°C / s can avoid the ferrite and pearlite formed at a higher temperature, thereby obtaining more bainite structure. If the bainite phase transformation temperature is too low, the transformation time is long, and if it is too high, the structure is coarse and the toughness is poor, so it is more appropriate to cool to 330-370°C for insulation.

[0026] The reason why the present invention applies a pressure of 400-800MPa to the steel plate cooled to 330-370°C and then carries out heat preservation and pressure holding for 5-20min at 330-370°C and 400-800MPa is that the compressive stress is applied during the bainite phase transformation and maintained until the end of the phase transformation. On the one hand, the 400-800MPa compressive stress is greater than the yield strength of the parent phase austenite, causing the austenite to deform and providing additional nucleation points for the bainite phase transformation; on the other hand, the continuously applied stress provides additional mechanical driving force for the bainite phase transformation. Both factors can accelerate the bainite phase transformation and ultimately shorten the phase transformation time of the bainite steel to about 5min at the fastest. If the applied pressure is too small, the acceleration of the bainite phase transformation and the improvement of the mechanical properties will be limited. If the pressure is too large, the above effects will reach saturation and may exceed the upper limit of the capacity of the pressure equipment. Therefore, it is more appropriate to apply a pressure of 400-800MPa. The pressure holding and heat preservation time are determined according to the bainite phase transformation time. As the pressure increases, the bainite phase transformation time is shortened, and the fastest time is shortened to 5 minutes, so the pressure holding and heat preservation time is set to 5 to 20 minutes.

[0027] Compared with the prior art, the present invention not only has a tensile strength of 1300-1700 MPa and an elongation of 8-14%, but also shortens the bainite phase transformation time to no more than 20 minutes, significantly improving production efficiency. The added elements are also simple, namely, only C, Si, Mn, and the rest is iron, which fully meets market demand. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is the metallographic structure diagram of the present invention. DETAILED DESCRIPTION

[0029] The present invention is further described in detail below

[0030] Table 1 is a list of chemical components of various embodiments and comparative examples of the present invention;

[0031] Table 2 is a list of main process parameters of various embodiments and comparative examples of the present invention;

[0032] Table 3 is a table of the effects of various embodiments and comparative examples of the present invention.

[0033] Each embodiment of the present invention is prepared according to the following steps:

[0034] 1) Heat the hot-rolled medium-thick raw material plate, control the heating temperature at 850-900°C, and keep it warm at this temperature

[0035] 20~40min;

[0036] 2) Cooling: cooling the steel plate to 330-370° C. at a cooling rate of 3-8° C. / s;

[0037] 3) Use a pressure device to pressurize the steel plate: Apply a pressure of 400-800MPa to the steel plate cooled to 330-370°C, and simultaneously keep the temperature and pressure at 330-370°C and 400-800MPa, and the time of keeping the temperature and pressure is controlled within 5-20min;

[0038] 4) After unloading the pressure, air cool to room temperature.

[0039] Table 1 Chemical composition list of various embodiments and comparative examples of the present invention (wt.%)

[0040] Example C Si Mn Cr Mo Ni 1 0.41 1.53 2.25 / / / 2 0.39 1.55 2.30 / / / 3 0.42 1.41 2.16 / / / 4 0.41 1.53 2.40 / / / 5 0.45 1.35 1.95 / / / 6 0.35 1.65 2.45 / / / Comparative Example 1 0.50 1.71 2.06 / / / Comparative Example 2 0.25 1.61 1.98 1.25 0.45 0.45

[0041] Table 2 List of main process parameters of various embodiments of the present invention and comparative examples

[0042]

[0043]

[0044] Table 3 Effect list of various embodiments of the present invention and comparative examples

[0045]

[0046] From the results in Table 3, it can be clearly seen that the steel plate using the composition design and production process of the present invention has higher tensile strength, lower alloy cost and shorter bainite phase transformation time, with a tensile strength of 1300-1700 MPa, a bainite phase transformation time of less than 20 min, and only C, Si, and Mn cheap alloy elements are added, so the alloy cost is relatively low. However, the alloy cost and bainite phase transformation time of Comparative Examples 1 and 2, which are not within the design range, are not as good as those of the embodiments, and cannot meet industrial needs.

[0047] This specific implementation is only the best example and is not a restrictive implementation of the technical solution of the present invention.

Claims

1. A carbide-free bainite steel with a tensile strength of ≥1300 MPa, wherein the components and weight percentages thereof are: C: 0.35-0.45%, Si: 1.25-1.65%, Mn: 1.95-2.45%, and the remainder is iron and other small amounts of inevitable impurities.

2. The carbide-free bainite steel strip with a tensile strength of ≥1300 MPa according to claim 1, characterized in that: The weight percentage of C is between 0.38% and 0.42%.

3. The carbide-free bainite steel with a tensile strength of ≥1300 MPa according to claim 1, characterized in that: The weight percentage of Si is 1.35-1.55%.

4. The carbide-free bainite steel with a tensile strength of ≥1300 MPa as claimed in claim 1, characterized in that: The weight percentage of Mn is 2.05-2.40%.

5. The method for preparing a carbide-free bainite steel with a tensile strength of ≥1300 MPa as claimed in claim 1, comprising the following steps: 1) The hot-rolled medium-thick raw material plate is heated at a temperature of 850-900°C and kept at this temperature for 20-40 minutes; 2) Cooling: cooling the steel plate to 330-370° C. at a cooling rate of 3-8° C. / s; 3) Use a pressure device to pressurize the steel plate: Apply a pressure of 400-800MPa to the steel plate cooled to 330-370°C, and simultaneously keep the temperature and pressure at 330-370°C and 400-800MPa, and the time of keeping the temperature and pressure is controlled within 5-20min; 4) After unloading the pressure, air cool to room temperature.

6. The method for preparing a carbide-free bainite steel with a tensile strength of ≥1300 MPa according to claim 5, characterized in that: The heating temperature of hot-rolled medium and thick raw material plates is between 855 and 895°C.

7. The method for preparing carbide-free bainite steel with a tensile strength of ≥1300 MPa according to claim 5, characterized in that: The pressure applied to the steel plate is negatively correlated with the insulation and pressure holding time of the steel plate.

8. The method for preparing a carbide-free bainite steel with a tensile strength of ≥1300 MPa according to claim 5 or 7, characterized in that: When the applied pressure does not exceed 600MPa, the heat preservation and pressure holding time is controlled within 6 to 20 minutes; when the applied pressure is greater than 600MPa, the heat preservation and pressure holding time is controlled within 5 to 6 minutes.

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