Low carbon high expandable steel and method for producing the same

Abstract

The application discloses a preparation method of low-carbon high-hole-expanding steel, provides an initial steel billet; the initial steel billet is subjected to hot rolling to obtain a hot-rolled plate, the average equivalent circle diameter size of the hot-rolled plate is between 2 microns and 5 microns, and the hot-rolled plate comprises a nano precipitated phase; the hot-rolled plate is cooled to a coiling temperature to obtain a second steel plate, the microstructure of the second steel plate comprises ferrite, bainite and a nano precipitated phase; the second steel plate is subjected to post-coiling cooling to obtain the low-carbon high-hole-expanding steel; wherein the components of the initial steel billet comprise: carbon 0.03-0.06wt%; manganese 1.55-1.8wt%; chromium 0.55-0.8wt%; titanium 0.08-0.10wt%; niobium 0.02-0.05wt%; vanadium 0.02-0.06wt%; molybdenum 0.02-0.05wt%; aluminum; silicon; the balance is Fe, and the components of the initial steel billet satisfy the following relationship: 0.12wt% <= W Si + Al <= 0.35wt%; wherein W Si represents the content of silicon, and W Al represents the content of aluminum. The application further discloses a low-carbon high-hole-expanding steel.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of advanced high-hole expansion steel preparation, and particularly relates to a low-carbon high-hole expansion steel and a preparation method thereof. BACKGROUND

[0002] With the rapid development of the automobile industry, people have higher and higher requirements for the safety, energy saving and emission of automobiles, and the application proportion of high-strength steel in the field of automobile parts is also increasing. However, it is found in the application process that high-strength steel has high edge cracking sensitivity and low hole expansion rate, and is prone to cracking when making parts with large hole expansion and large flanging, thereby hindering the design and stamping technology of some parts. Therefore, more and more products require steel to have good hole expansion performance and flange extension under the premise of ensuring high strength.

[0003] In the related art, a high-hole expansion steel plate and a manufacturing process thereof are disclosed, and although a high-hole expansion steel with high hole expansion rate can be obtained, the corresponding tensile strength is low. In addition, in the process of preparing the high-hole expansion steel, high content of Si is added to obtain steel with better performance, which will lead to the decline of the surface quality of the steel plate. In addition, the high-hole expansion steel prepared by the existing technology around the controlled rolling and controlled cooling process has generally low strength. SUMMARY

[0004] The present application aims to solve one of the problems in the related art to some extent, and provides a low-carbon high-hole expansion steel and a preparation method thereof.

[0005] As a first aspect of the present application, a preparation method of a low-carbon high-hole expansion steel is provided, comprising:

[0006] providing an initial steel blank;

[0007] hot rolling the initial steel blank to obtain a hot-rolled plate, the average equivalent circle diameter size of the grains of the hot-rolled plate being between 2 μm and 5 μm, and the hot-rolled plate comprising a nano precipitate phase;

[0008] cooling the hot-rolled plate to a coiling temperature to obtain a second steel plate, the microstructure of the second steel plate comprising ferrite, bainite and a nano precipitate phase;

[0009] cooling the second steel plate after coiling to obtain the low-carbon high-hole expansion steel;

[0010] wherein the components of the initial steel blank comprise:

[0011]

[0012] aluminum; silicon; the balance being Fe, and the components of the initial steel blank satisfy the following relationship:

[0013] 0.12wt%≤WSi +W Al ≤0.35wt%;

[0014] wherein, W Si represents the content of silicon, W Al represents the content of aluminum.

[0015] Further, in the step of hot rolling the initial steel billet, the initial steel billet is hot rolled for at least 3 passes, and the last two passes have a reduction rate between 60% and 70%.

[0016] Further, in the step of hot rolling the initial steel billet, the initial steel billet is hot rolled for at least 3 passes, and the last two passes have a reduction rate between 60% and 70%.

[0017] Further, the hot-rolled plate is cooled to a coiling temperature to obtain a second steel plate, comprising:

[0018] to a first cooling rate to obtain a first steel plate;

[0019] cooling the first steel plate to a coiling temperature at a second cooling rate to obtain a second steel plate; wherein the first cooling rate is greater than the second cooling rate, and the coiling temperature is between 580℃ and 620℃.

[0020] Further, in the step of cooling the hot-rolled plate to a first cooling rate to a ferrite transformation temperature range, the first cooling rate is between 30℃ / s and 50℃ / s, and the ferrite transformation temperature range is between 720℃ and 750℃.

[0021] Further, in the step of cooling the first steel plate to a second cooling rate to a set temperature, the second cooling rate is between 10℃ / s and 15℃ / s.

[0022] Further, before cooling the first steel plate to a second cooling rate to a set temperature, the method further comprises slowly cooling the first steel plate in air for 4s to 8s.

[0023] Further, in the step of cooling the second steel plate after coiling, the coiling temperature is between 580℃ and 620℃, and the cooling uses a holding pit.

[0024] Further, before hot rolling the initial steel billet, the preparation method further comprises:

[0025] smelting and ingot casting the steel material to obtain an ingot;

[0026] forging the ingot to obtain a forged billet, wherein the final forging temperature is greater than or equal to 970℃.

[0027] heating the forging blank to 1200-1260°C, with a heating rate of 150-200°C / h and a holding time of 1.5-2h, to obtain the initial steel blank.

[0028] As a second aspect of the present application, a low-carbon high-expansion steel is provided, which is obtained by the above preparation method.

[0029] Further, the low-carbon high-expansion steel has a yield strength of greater than or equal to 650 MPa, a tensile strength of greater than or equal to 800 MPa, an elongation of greater than or equal to 20%, and an expansion rate of greater than or equal to 75%, and the low-carbon high-expansion steel has a room-temperature microstructure comprising alpha ferrite, bainite, and nanometer precipitates.

[0030] Compared with the prior art, the present application has the following beneficial effects:

[0031] (1) A more reasonable composition design is adopted to improve the strength while ensuring more excellent expansion performance. Low-carbon design C: 0.03-0.06%, carbon plays a role of solid solution strengthening in the steel. In the present application, the carbon content is ensured to be greater than or equal to 0.03% to obtain sufficient amount of ferrite and higher strength. However, the carbon content should not be higher than 0.06%, otherwise pearlite structure is easily formed during hot rolling and coiling, which is not conducive to the expansion performance. Therefore, the carbon content should be controlled to be between 0.03% and 0.06%, which can significantly improve the expansion rate and strength.

[0032] Low-silicon aluminum design: ensure the total content of Si and Al to be between 0.12% and 0.35%, Si plays a role of solid solution strengthening in the steel, and appropriate addition can improve the strength, but with the increase of Si content, the surface quality of the steel material significantly decreases. Controlling the Si content within a certain range ensures that the steel material has better surface quality and strength. At the same time, Al can increase the complete austenitizing temperature and expand the process window, but excessive Al can lead to an increase in ferrite phase, which is not conducive to the improvement of strength. Therefore, controlling the total content of Si and Al to be between 0.12% and 0.35% is more ideal for improving the performance of the steel material.

[0033] At the same time, micro amounts of Ti, Nb, V, Mo, and Cr micro-alloys are added, and during rolling, cooling, and coiling, nanometer carbide phases of (Ti, Nb, V, Mo, Cr) are complexly precipitated, which plays a role of fine-grain and precipitation strengthening.

[0034] (2) The low-carbon high-expansion steel preparation method provided by the application adopts a low-temperature large reduction hot rolling process, which can not only increase more dislocation accumulation to provide strength, but also produce sufficient deformation to refine the grain size, and can also obtain strain-induced nano precipitates, the nano precipitates can hinder the further growth of the grain, the nano precipitates are uniformly distributed in the structure, and the grain is broken and homogenized under the action of rolling force and nano precipitates, so that the existence of excessive mixed crystals is avoided, stress concentration in the subsequent strain or expansion deformation process is reduced, the early generation of cracks is prevented, the hot-rolled plate has small and uniform grain size after hot rolling, and exists nano precipitates and high dislocation density, thereby creating a precursor microstructure with small grain size, nano precipitates and high dislocation density for subsequent preparation processes.

[0035] (3) The low-carbon high-expansion steel preparation method provided by the application adopts a cooling and coiling process to further optimize the structure of the hot-rolled plate in the previous step, create a two-phase structure of ferrite and bainite in a specific ratio, and also have more nano precipitates to play a strengthening role, so that the low-carbon high-expansion steel with better strength, plasticity and expansion performance can be obtained through the cooperation of ferrite, bainite and nano precipitates. BRIEF DESCRIPTION OF DRAWINGS

[0036] In order to more clearly illustrate the technical solutions in the embodiments of the application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or the prior art description. Obviously, the drawings in the following description only constitute some embodiments of the application, and for those skilled in the art, other drawings can also be obtained according to the structures shown in the drawings without creative labor.

[0037] Figure 1 is a flow chart of one embodiment of the low-carbon high-expansion steel preparation method provided by the application;

[0038] Figure 2 is a flow chart of one embodiment of the second steel plate preparation method provided by the application;

[0039] Figure 3 is a flow chart of another embodiment of the low-carbon high-expansion steel preparation method provided by the application;

[0040] Figure 4 is a flow chart of one embodiment of the initial steel billet preparation method provided by the application;

[0041] Figure 5 is a microstructure scanning electron microscope photo of embodiment 1 of the application;

[0042] Figure 6 is a transmission electron microscope photo of the nano precipitate distribution of embodiment 1 of the application;

[0043] Figure 7is an electron backscatter diffraction (EBSD) map of the hole expansion crack of Example 1 of the present application.

[0044] Reference numerals

[0045] 1: ferrite

[0046] 2: bainite

[0047] 3: nano precipitates

[0048] 4: hole expansion crack DETAILED DESCRIPTION

[0049] Embodiments of the present application are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. Based on the embodiments in the embodiments, it is intended to explain the present application, and cannot be understood as a limitation of the present application.

[0050] In this specification, "one embodiment" or "an embodiment" or "example" or "exemplary" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the application. The appearances of the phrase "in one embodiment" in various places in the specification are not necessarily all referring to the same embodiment.

[0051] As a first aspect of the present application, an embodiment flow chart of a method for preparing a low-carbon high-hole expansion steel is provided, as shown in Figure 1 , comprising:

[0052] In step S110, an initial steel billet is provided;

[0053] In step S120, the initial steel billet is hot-rolled to obtain a hot-rolled plate, the average equivalent circle diameter size of the hot-rolled plate is between 2 μm and 5 μm, and the hot-rolled plate includes nano precipitates;

[0054] In step S130, the hot-rolled plate is cooled to a coiling temperature to obtain a second steel plate, the microstructure of the second steel plate includes ferrite, bainite and nano precipitates;

[0055] In step S140, the second steel plate is subjected to post-coiling cooling to obtain a low-carbon high-hole expansion steel;

[0056] The composition of the initial steel billet includes:

[0057] Carbon 0.03-0.06wt%;

[0058] Manganese 1.55-1.8wt%;

[0059] Chromium 0.55-0.8wt%;

[0060] Ti 0.08-0.10wt%;

[0061] Nb 0.02-0.05wt%;

[0062] V 0.02-0.06wt%;

[0063] Mo 0.02-0.05wt%;

[0064] Al; Si; balance Fe, and the components of the initial steel billet satisfy the following relationship:

[0065] 0.12wt%≤W Si +W Al ≤0.35wt%;

[0066] wherein W Si represents the content of silicon, and W Al represents the content of aluminum.

[0067] In step S110, the initial steel billet designed in components is used to improve the strength while ensuring more excellent hole expansion performance. Low carbon design C: 0.03-0.06%, carbon plays a role of solid solution strengthening in the steel. In the present application, the carbon content is ensured to be above 0.03% to obtain sufficient amount of ferrite and higher strength. However, the carbon content cannot be higher than 0.06%, otherwise pearlite structure is easily formed during hot rolling and coiling, which is not conducive to the hole expansion performance. Therefore, the carbon content should be controlled between 0.03% and 0.06%, which can significantly improve the hole expansion rate and strength.

[0068] Low silicon aluminum design: ensure the total content of Si and Al between 0.12% and 0.35%, Si plays a role of solid solution strengthening in the steel, and appropriate addition can improve the strength, but with the increase of Si content, the surface quality of the steel material decreases significantly. Controlling the Si content within a certain range ensures that the steel material has better surface quality and strength; at the same time, Al can increase the complete austenitizing temperature and expand the process window, but excessive Al can lead to the increase of ferrite phase, which is not conducive to the improvement of strength, therefore, controlling the total content of Si and Al between 0.12% and 0.35% is more ideal for the improvement of the performance of the steel material.

[0069] At the same time, micro amounts of Ti, Nb, V, Mo, and Cr micro-alloys are added, and during rolling, cooling and coiling, the nanometer carbide phase of (Ti, Nb, V, Mo, Cr) is compounded and precipitated, which plays a role of fine grain strengthening and precipitation strengthening.

[0070] In step S120, the lower temperature and high reduction hot rolling process can not only increase the accumulation of dislocations to provide strength, but also produce sufficient deformation to refine the grains, while ensuring that the grains are uniformly broken down under the rolling force and the combined nano precipitation effect, avoiding the existence of excessive mixed crystals, reducing stress concentration in the subsequent strain or hole expansion deformation process, preventing the premature initiation of cracks, and the hot-rolled plate has a small and uniform grain size, with an average equivalent circle diameter size of 2-5 μm, high dislocation density, and strain-induced precipitation of nano precipitates, which creates an ideal precursor microstructure for subsequent preparation processes.

[0071] As an optional embodiment, in the step of hot rolling the initial steel billet, the initial steel billet is hot rolled for at least 3 passes, preferably 5 passes, and the reduction rate of the last two passes is between 60% and 70%. The last two passes are hot rolled at a relatively lower temperature but a relatively higher reduction rate. The use of lower temperature and high reduction can not only further increase the accumulation of dislocations to provide strength, but also ensure that the grains are further broken down and homogenized under the rolling force and the combined nano precipitation effect, avoiding the existence of more mixed crystals, reducing stress concentration in the subsequent strain or hole expansion deformation process, and preventing the premature initiation of cracks.

[0072] As an optional embodiment, the hot rolling starting temperature is in the range of 1000-1030°C, and the hot rolling starting temperature is in the fully austenitic temperature range. Multiple hot rolling can sufficiently refine the original austenite grain size.

[0073] As an optional embodiment, the final hot rolling temperature is in the range of 850-890°C, and the gradual reduction of the hot rolling temperature will phase out a small amount of alpha ferrite, and also strain-induced precipitation of a large amount of micro metal carbide at this temperature. This precipitated carbide can further refine the grains and has a precipitation strengthening effect. In addition, the lower hot rolling temperature and the larger reduction rate ensure more accumulation of dislocations, creating a precursor of fine-grained, precipitated and high-density dislocation organization, which is inherited by the subsequent hot forming process.

[0074] As an optional embodiment, the total reduction rate of hot rolling is between 90% and 95%, which can fully deform the initial steel billet during the hot rolling stage, sufficiently refine the original austenite grain size in the steel billet, and have a high accumulation of dislocations. The average equivalent circle diameter size of the hot-rolled grains is between 2 μm and 5 μm.

[0075] In steps S130 and S140, the hot-rolled plate is cooled to the coiling temperature to obtain a second steel plate, and the microstructure of the second steel plate includes ferrite, bainite and nano precipitates. The second steel plate is then coiled and cooled to obtain a low-carbon high-hole expansion steel.

[0076] The microstructure of the hot-rolled plate is further optimized by adopting a cooling and coiling process, so as to create a two-phase microstructure of ferrite and bainite in a specific proportion, and more nano precipitates are precipitated to play a strengthening role. Through the cooperation of ferrite, bainite and nano precipitates, a low-carbon high-expansion steel with better strength, plasticity and hole expansion performance is obtained.

[0077] As an optional embodiment, as shown in Figure 2 The hot-rolled plate is cooled to a coiling temperature to obtain a second steel plate, including:

[0078] In step S131, the hot-rolled plate is cooled to a ferrite transformation temperature range at a first cooling rate to obtain a first steel plate.

[0079] In step S132, the first steel plate is cooled to a coiling temperature at a second cooling rate to obtain a second steel plate, wherein the first cooling rate is greater than the second cooling rate, and the coiling temperature is between 580°C and 620°C.

[0080] In step S131, the hot-rolled plate is cooled to a ferrite transformation temperature range at a first cooling rate, the first cooling rate is relatively fast, on the one hand, to ensure the growth of sufficient ferrite, to create a certain proportion of two-phase microstructure of ferrite and bainite, and on the other hand, to further precipitate microalloy and carbide to play a precipitation strengthening effect. As an optional embodiment, the first cooling rate is between 30°C / s and 50°C / s, and the ferrite transformation temperature range is between 720°C and 750°C.

[0081] In step S132, the first steel plate is cooled to a coiling temperature at a second cooling rate to obtain a second steel plate, the second cooling rate is relatively slow, to ensure that a sufficient proportion of ferrite and bainite is formed in the common transformation range of ferrite and bainite, and to create a certain proportion of two-phase microstructure of ferrite and bainite together with the first cooling, and also to ensure that a sufficient amount of microalloy and carbide is precipitated to further play a strengthening role.

[0082] In step S131, the coiling temperature is between 580°C and 620°C, a higher coiling temperature is adopted to ensure the combined effect of the complex precipitation of various microalloy carbides and grain recovery recrystallization, to promote the interaction of precipitation strengthening and dislocation.

[0083] As an optional embodiment, the second cooling rate is between 10°C / s and 15°C / s.

[0084] As an optional embodiment, after the end of the first cooling stage, the first steel plate is slowly cooled in air for 4s to 8s and then subjected to a second cooling.

[0085] As an optional implementation, the second steel plate is rolled at a temperature in a range of 580-620℃, and is placed in a holding pit for slow cooling, and is taken out after 24 hours to obtain the low-carbon high-hole-expanding steel.

[0086] As shown in Figure 3 , the present application provides a flow chart of an embodiment of a method for preparing a low-carbon high-hole-expanding steel, wherein the method comprises:

[0087] In step S210, an initial steel billet is provided;

[0088] In step S220, the initial steel billet is hot-rolled for at least 3 passes, and the reduction rate of the last two passes is in a range of 60-70%, to obtain a hot-rolled plate;

[0089] In step S230, the hot-rolled plate is cooled at a cooling rate in a range of 30-50℃ / s to a temperature in a range of 720-750℃, to obtain a first steel plate;

[0090] In step S240, the first steel plate is cooled at a cooling rate in a range of 10-15℃ / s to a temperature in a range of 580-620℃, to obtain a second steel plate;

[0091] In step S250, the second steel plate is rolled and placed in a holding pit for slow cooling, and is taken out after 24 hours to obtain the low-carbon high-hole-expanding steel.

[0092] In the embodiment of the present application, how to obtain the initial steel billet is not specially limited. For example, the initial steel billet can be obtained by outsourcing. For another example, the initial steel billet can be obtained by directly using a steel material after smelting, calcining and heat treatment.

[0093] As an optional implementation, as shown in Figure 4 , before the initial steel billet is hot-rolled, the method further comprises:

[0094] In step S111, the steel material is smelted and cast into an ingot to obtain a steel ingot;

[0095] In step S112, the steel ingot is heated to a temperature in a range of 1150-1250℃ for holding, to obtain a first steel ingot, wherein the heating rate is in a range of 150-200℃ / h, and the holding time is in a range of 2.6-3.6h, and this step is a conventional homogenization treatment;

[0096] In step S113, the first steel ingot is forged to obtain a forged billet, wherein the final forging temperature is greater than or equal to 960℃;

[0097] In step S114, the above-mentioned forging blank is heated to 1150-1250℃ and kept for 1.6-2.6h, wherein the heating rate is 200-300℃ / h, to obtain the above-mentioned initial steel blank.

[0098] The process route of the present application is simple, and the heat treatment cycle is greatly shortened, which is beneficial to realize low-cost and high-efficiency production.

[0099] The low-carbon high-expansion steel in the present application adopts the above-mentioned component designed initial steel blank and is obtained by the above-mentioned preparation method. The low-carbon high-expansion steel has a tensile strength greater than or equal to 800MPa, a yield strength greater than or equal to 650MPa, an elongation greater than or equal to 20.0%, and an expansion rate greater than or equal to 75%. The obtained low-carbon high-expansion steel has a room temperature microstructure including alpha ferrite, bainite and nanometer precipitated phase.

[0100] The present application will be further described below through specific examples and comparative examples.

[0101] Examples

[0102] In the following examples of the present application, the components of the steel blank used are as follows:

[0103]

[0104]

[0105] Aluminum; silicon; the balance is Fe, and the components of the initial steel blank satisfy the following relationships:

[0106] W Si +W Al 0.26wt%;

[0107] Wherein, W Si represents the content of silicon, W Al represents the content of aluminum.

[0108] Example 1

[0109] A low-carbon high-expansion steel and a preparation method thereof, comprising:

[0110] First step: smelting and casting the steel blank to obtain a cast blank;

[0111] Second step: forging the cast blank to obtain a forging blank, wherein the final forging temperature is maintained at greater than or equal to 970℃;

[0112] Third step: heating the forging blank to 1200℃ at a heating rate of 200℃ / h and keeping for 1.8h to obtain an initial steel blank;

[0113] Fourth step: hot rolling the initial billet to obtain a hot-rolled plate, wherein the hot rolling opening rolling temperature is 1000℃, the final rolling temperature is 870℃, the total reduction of 5 passes is 92%, and the cumulative reduction of the last two passes is 60%;

[0114] Fifth step: cooling the hot-rolled plate to 750℃ at a cooling rate of 45℃ / s, slowly cooling in air for 4s, and then cooling to 600℃ at a cooling rate of 15℃ / s;

[0115] Sixth step: coiling the hot-rolled plate cooled in the previous step at a coiling temperature of 600℃, and slowly cooling the coiled steel strip in the holding pit, taking it out after 24h to obtain sample 1.

[0116] The microstructure is shown in Figure 5 , the room temperature structure contains about 40% ferrite and about 60% bainite by volume fraction, and the nanometer second phase precipitation is shown in Figure 6 , the hole expansion crack EBSD map is shown in Figure 7 , and other related process parameters and mechanical properties are shown in Tables 1 and 2.

[0117] Example 2

[0118] An 800MPa grade low-carbon high-hole expansion steel was prepared by the method provided in Example 1, except that the total reduction of the last two passes of the hot rolling process of Example 2 was 68%, and the coiling temperature was 620℃, to obtain sample 2, and the specific process parameters are shown in Table 1.

[0119] Example 3

[0120] An 800MPa grade low-carbon high-hole expansion steel was prepared by the method provided in Example 1, except that in the cooling process of Example 3, the hot-rolled plate was first cooled to 725℃ at a cooling rate of 30℃ / s, slowly cooled in air for 7s, and then cooled to 580℃ at a cooling rate of 10℃ / s, to obtain sample 3, and the specific process parameters are shown in Table 1.

[0121] Comparative Example 1

[0122] An 800MPa grade low-carbon high-hole expansion steel was prepared by the method provided in Example 1, except that the total reduction of the last two passes of the hot rolling process of Comparative Example 1 was 40%, to obtain sample 4, and the specific process parameters are shown in Table 1.

[0123] Comparative Example 2

[0124] An 800MPa grade low-carbon high-hole expansion steel was prepared by the method provided in Example 1, except that in the cooling process of Comparative Example 2, the hot-rolled plate was cooled to 600℃ at a cooling rate of 45℃ / s for coiling, to obtain sample 5, and the specific process parameters are shown in Table 1.

[0125] Comparative Example 3

[0126] A low carbon high hole expansion steel of 800 MPa grade was prepared by the method provided in Example 1, except that the coiling temperature of Comparative Example 3 was 450 °C to obtain sample 6, and the specific process parameters are shown in Table 1.

[0127] Test Example

[0128] The microstructure photo of sample 1 was obtained by using Zeiss Gemini SEM 500 scanning electron microscope, as shown in Figure 5 The nano precipitate distribution photo of sample 1 was obtained by using transmission electron microscope FEI F20, as shown in Figure 6 The hole expansion crack propagation photo of sample 1 was obtained by using electron backscatter diffraction Symmetry S2, as shown in Figure 7 The mechanical properties and hole expansion rate of the sample were tested by using a universal testing machine (MTS E45.305) device, and the test data are shown in Table 2.

[0129] Figure 5 and Figure 6 It is shown that the microstructure of sample 1 includes ferrite 1, bainite 2 and nano precipitate 3, and specifically includes about 40% ferrite + 60% bainite + nano precipitate. Figure 7 It is shown that the grain distribution around the hole expansion crack 4 of sample 1 is uniform and fine, effectively preventing the further expansion of the crack and improving the hole expansion performance of sample 1. The yield strength of sample 1 is 670 MPa, the tensile strength is 825 MPa, the elongation is 20.5%, and the hole expansion rate is 78%.

[0130] Sample 2 increases the total reduction rate of the last two passes of hot rolling from 60% of Example 1 to 68% of Example 2, slightly increases the coiling temperature, and the microstructure is the same as that of sample 1, which is ferrite and bainite. Due to the increase of the reduction rate, the grain of sample 2 is further refined and the mechanical properties are improved. The yield strength of sample 2 is 702 MPa, the tensile strength is 844 MPa, the elongation is 21.2%, and the hole expansion rate is 75%;

[0131] Sample 3 reduces the cooling rate of the two times of cooling and the final temperature of the two times of cooling. Reducing the cooling rate gives more time for the formation and growth of ferrite structure, but also reduces the coiling temperature. The content of ferrite and bainite is basically the same as that of sample 1. The yield strength of sample 3 is 656 MPa, the tensile strength is 820 MPa, the elongation is 20.9%, and the hole expansion rate is 77%;

[0132] Sample 4 reduces the total reduction of the last two passes of hot rolling, from 60% reduction of Example 1 to 40% reduction of Comparative Example 1, the deformation is insufficient, the microstructure of sample 4 includes about 40% ferrite + 60% bainite + nano precipitates, the dislocation accumulation is less, the mixed crystal is more, the degree of dislocation accumulation, grain refinement and homogenization is not enough, the mechanical properties are obviously reduced, the yield strength of sample 4 is 645 MPa, the tensile strength is 799 MPa, the elongation is 18.7%, and the expansion rate is 65%;

[0133] Sample 5 is directly cooled to the coiling temperature at the first stage cooling rate, and there is not enough ferrite formed by transformation in the cooling stage, resulting in a decrease in ferrite content and an excessive amount of bainite, which is a hard phase and is not conducive to the expansion performance, and the expansion rate is low, the microstructure of sample 5 includes about 33% ferrite + 67% bainite + nano precipitates, the yield strength of sample 5 is 724 MPa, the tensile strength is 857 MPa, the elongation is 16.4%, and the expansion rate is 40%;

[0134] Sample 6 is coiled at a temperature of 450℃, and the bainite content is further increased, but the lower temperature causes the nano precipitates to precipitate significantly and the size of the precipitates to be large, and the mechanical properties and expansion performance are both poor, the microstructure of sample 6 includes about 27% ferrite + 73% bainite, and the precipitates are less or the size of the precipitates is large, the yield strength of sample 6 is 685 MPa, the tensile strength is 760 MPa, the elongation is 20.0%, and the expansion rate is 46%.

[0135] Table 1 Process parameters of the examples and comparative examples of the present application

[0136]

[0137] Table 2 Mechanical properties and microstructure of the examples and comparative examples of the present application

[0138]

[0139] From Tables 1 and 2, it can be seen that the low-carbon high-expansion steel with better comprehensive performance of strength, elongation and expansion rate can be obtained by using the component design and preparation method of the present application, and the low-carbon high-expansion steel of the present application has an ideal and appropriate proportion of ferrite, bainite and nano precipitates, and sufficient dislocation accumulation and grain refinement, which can obtain better performance.

[0140] The above description is only preferred embodiments of the present application and is not intended to limit the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A method for preparing low-carbon, high-hole-expansion steel, characterized in that, include: Provide initial steel billets; The initial steel billet is hot-rolled to obtain a hot-rolled plate, wherein the average equivalent circle diameter of the grains in the hot-rolled plate is between 2 μm and 5 μm, and the hot-rolled plate includes nano-precipitates; The hot-rolled plate is cooled to the coiling temperature to obtain a second steel plate: wherein the first steel plate is obtained by cooling at a first cooling rate to the ferrite transformation temperature range; The first steel plate is cooled to the coiling temperature at a second cooling rate to obtain the second steel plate. The first cooling rate is greater than the second cooling rate. The coiling temperature is between 580°C and 620°C. The microstructure of the second steel plate includes ferrite, bainite and nano-precipitates. The second steel plate is coiled and then cooled to obtain the low-carbon high-expansion steel; The initial steel billet comprises the following components: Carbon 0.03~0.06wt%; Manganese 1.55~1.8wt%; Chromium 0.55~0.8wt%; Titanium 0.08~0.10wt%; Niobium 0.02~0.05wt%; Vanadium 0.02~0.06wt%; Molybdenum 0.02~0.05wt%; Aluminum; silicon; balance Fe, and the composition of the initial steel billet satisfies the following relationship: 0.12wt%≤W Si +W Al ≤0.35wt%; Among them, W Si W indicates the silicon content. Al This indicates the aluminum content.

2. The preparation method according to claim 1, characterized in that, In the hot rolling step of the initial steel billet, the initial steel billet is hot rolled for at least 3 passes, and the reduction rate of the last two passes is between 60% and 70%.

3. The preparation method according to claim 2, characterized in that, In the step of hot rolling the initial steel billet multiple times, the initial rolling temperature is in the range of 1000℃ to 1030℃, the final rolling temperature is in the range of 850℃ to 890℃, and the total reduction rate of hot rolling is between 90% and 95%.

4. The preparation method according to claim 1, characterized in that, In the step of cooling the hot-rolled plate to a ferrite transformation temperature range at a first cooling rate, the first cooling rate is between 30°C / s and 50°C / s, and the ferrite transformation temperature range is between 720°C and 750°C.

5. The preparation method according to claim 1, characterized in that, In the step of cooling the first steel plate to a set temperature at a second cooling rate, the second cooling rate is between 10°C / s and 15°C / s.

6. The preparation method according to claim 5, characterized in that, Before cooling the first steel plate to the set temperature at the second cooling rate, the process also includes slowly cooling the first steel plate in air for 4 to 8 seconds.

7. The preparation method according to any one of claims 1 to 6, characterized in that, Before hot rolling the initial steel billet, the preparation method further includes: Steel is smelted and cast into ingots to obtain ingots; The ingot is forged to obtain a forging billet, wherein the final forging temperature is greater than or equal to 970°C; The forging billet is heated to between 1200°C and 1260°C and held at that temperature to obtain the initial steel billet, wherein the heating rate is between 150°C / h and 200°C / h and the holding time is between 1.5h and 2h.

8. A low-carbon, high-expansion-hole steel, characterized in that, The low-carbon high-expansion steel is obtained by the preparation method described in any one of claims 1 to 7.

9. The low-carbon, high-expansion-hole steel according to claim 8, characterized in that, The low-carbon high-expansion steel has a yield strength greater than or equal to 650 MPa, a tensile strength greater than or equal to 800 MPa, an elongation greater than or equal to 20%, and a porosity greater than or equal to 75%. The room-temperature microstructure of the low-carbon high-expansion steel includes α-ferrite, bainite, and nano-precipitates.

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