1470 MPa-grade high-hole-expansion steel plate for cold stamping and preparation method thereof
1. A1470 MPa-level high-hole-expansion steel plate for cold stamping is characterized by comprising the following components in percentage by weight: c: 0.22-0.28%, Si: 1.5% -1.8%, Mn: 2.5% -3.0%, Cr: 0.1-0.5%, Ti + Nb: 0.03-0.05%, Al: 0.015-0.05%, P is less than or equal to 0.02%, S is less than or equal to 0.005%, and the balance is FeAnd inevitable impurities.
2. The 1470 MPa-grade high-hole-expansion steel plate for cold stamping according to claim 1, further comprising V not more than 0.05%, Ni: less than or equal to 0.5 percent, Mo: not more than 0.5%, Cu：≤0.5％，Ca≤0.005％，B≤0.005％。
3. The 1470 MPa-grade high-hole-expansion steel plate for cold stamping as claimed in claim 2, wherein the steel plate contains Ni + Mo + Cu 0.3% or more and 0.6% or less.
4. The 1470MPa grade high-hole-expansion steel plate for cold stamping according to claim 1, wherein the microstructure of the steel plate is ferrite, tempered martensite, bainite, residual austenite; wherein; the steel plate structure comprises the following components in percentage by volume: ferrite is less than or equal to 10 percent, tempered martensite is 75 to 80 percent, and retained austenite is 9 to 13 percent.
5. The 1470 MPa-grade high-hole-expansion steel plate for cold stamping according to claim 1, wherein the yield strength of the steel plate is 1100-1270 MPa or more, the tensile strength is 1470MPa or more, the elongation is more than 8%, and the hole expansion rate is more than 30%.
6. A method for manufacturing a 1470MPa grade high-hole-expansion steel plate for cold stamping according to any one of claims 1 to 5, comprising smelting, hot rolling, bell annealing, pickling, cold rolling, continuous annealing; the method is characterized in that:
(1) hot rolling: the heating temperature is 1250-1280 ℃, the initial rolling temperature is 1100-1150 ℃, and the final rolling temperature is above 900 ℃; the coiling temperature is 450-550 ℃, U-shaped coiling is adopted, and the head and tail temperature is 600 +/-30 ℃;
(2) cover annealing: then, the steel plate is subjected to stress relief annealing through a bell-type furnace, the annealing temperature is 500 +/-20 ℃, and the annealing time is 15-30 hours;
(3) cold rolling: the rolling reduction rate is 50-58%;
(4) and (3) continuous annealing: firstly, heating a cold-rolled steel plate to 850-880 ℃ at the speed of 2-10 ℃/s, and keeping the temperature constant for 120-240 s; then, slowly cooling the steel plate to 750-780 ℃ at a cooling speed of 0.5-4 ℃/s; and then quenching the steel plate to 220-280 ℃ at a cooling speed of 30-35 ℃/s in a rapid cooling stage, and then heating to an overaging temperature of 360-420 ℃ at a heating speed of 10-20 ℃/s, wherein the overaging time is 280-410 s.
7. The method for preparing 1470MPa grade high-hole-expansion steel plate for cold stamping according to claim 6, characterized by: in the hot rolling in the step (1), the steel plate structure after coiling is ferrite, pearlite, bainite and unidentified phases; wherein, according to the volume percentage: 40-50% of ferrite, 20-30% of pearlite and 20-30% of bainite.
8. The method for preparing 1470MPa grade high-hole-expansion steel plate for cold stamping according to claim 6, characterized by: in the continuous annealing in the step (6), the microstructure of the steel plate after the rapid cooling stage contains more than or equal to 70 percent of martensite and more than or equal to 20 percent of super-cooled austenite.
Background
In the competition between the cold forming steel and the hot forming steel at present, the hot forming is almost completely occupied in the market above 1500MPa, the main competition is that the part forming is difficult to realize by the cold forming mode of the steel plate above 1500MPa, the service life of a die is greatly influenced even if the part forming can be realized, and the main reason is that the cold stamping forming of 1500MPa cold stamping products is limited. As is known, the tensile mechanical properties of high-strength steel often represent the quality of the cold stamping performance to a certain extent, for example, the elongation mainly corresponds to the performance of the tensile performance, and the hole expansion rate corresponds to the flanging performance. At present, the cold rolled product of 1500MPa level only has Martensite Steel (MS), while the plasticity of MS1500 is only about 5 percent, and the cold rolled product is hardly suitable for cold stamping forming. In addition, the TWIP steel can also reach more than 1500MPa level by relying on the austenite twin crystal induced transformation mechanism, and good plasticity (not less than 30%) is also considered. However. The low yield strength of the alloy makes it difficult to complete flanging of most parts. Therefore, the improvement of the plasticity and the hole expansion rate of the 1500 MPa-grade steel plate is a key means for realizing the application of the grade steel plate.
Prior to the present invention, there has been little description of cold formable products at the 1500MPa class.
In patent document "Cold-rolled high-Strength Steel with Excellent formability at tensile Strength of 1500MPa or more and method for producing the same" (publication No. CN108018484B), a cold-rolled annealed steel sheet with tensile Strength of 1500MPa or more and elongation of more than 12% is produced by using a martensite + Austenite structure by a quenching division process. The product does not indicate the hole-expanding performance, and most of the products show low-yield high-tensile characteristics by referring to the yield strength in the embodiment, so that the higher hole-expanding performance is difficult to ensure.
The patent document "a low-carbon medium-manganese high-residual austenitic high-toughness steel and a heat treatment method thereof" (publication number: CN112063931A) discloses that the alloy components of the high-toughness steel are C: 0.10-0.25%, Mn: 4.0% -8.0%, Al: 1.0 to 2.5 percent, and the balance of Fe and inevitable impurities. The steel plate realizes the strengthening of work hardening by means of TRIP effect provided by a large amount of austenite in the deformation process, and then the 1500 MPa-grade steel plate is obtained. However, it is worth noting that the steel plate is added with a large amount of Mn (more than 4%) and Al (more than 1%), which seriously improves the smelting difficulty of the product, is only suitable for laboratory research, and has no industrialization prospect under the current conditions; meanwhile, the tensile curve has an obvious luders band, which seriously affects the surface quality of the steel plate and is not beneficial to stamping application.
Disclosure of Invention
The invention aims to overcome the problems and the defects and provides a 1470 MPa-grade high-hole-expansion steel plate for cold stamping, which has the yield strength of 1100-1270 MPa, the tensile strength of 1470MPa or more, the elongation of 8% and the hole expansion rate of more than 30%, and a preparation method thereof.
The purpose of the invention is realized as follows:
a1470 MPa-grade high-hole-expansion steel plate for cold stamping comprises the following components in percentage by weight: c: 0.22-0.28%, Si: 1.5% -1.8%, Mn: 2.5% -3.0%, Cr: 0.1-0.5%, Ti + Nb: 0.03-0.05%, Al: 0.015-0.05%, P less than or equal to 0.02%, S less than or equal to 0.005%, and the balance of Fe and inevitable impurities.
Further, the steel plate also contains V which is less than or equal to 0.05 percent, Ni: less than or equal to 0.5 percent, Mo: less than or equal to 0.5 percent, Cu: less than or equal to 0.5 percent, less than or equal to 0.005 percent of Ca and less than or equal to 0.005 percent of B. Preferably, the content of Ni, Mo and Cu in the steel plate is more than or equal to 0.3 percent and less than or equal to 0.6 percent.
The microstructure of the steel plate is ferrite, tempered martensite, bainite and residual austenite; the steel plate structure comprises the following components in percentage by volume: ferrite is less than or equal to 10 percent, tempered martensite is 75 to 80 percent, residual austenite is 9 to 13 percent, and the balance is bainite.
The yield strength of the steel plate is more than 1100-1270 MPa, the tensile strength is more than 1470MPa, the elongation is more than 8%, and the hole expansion rate is more than 30%.
The invention has the following design reasons:
c: as interstitial atoms in steel, it is very important to improve the strength of experimental steel. Too low C content cannot ensure sufficient C distribution in the annealing process of the critical zone of the experimental steel, so that the phase stability of austenite in the critical zone is reduced, and even the phase stability of residual austenite at room temperature is influenced. The residual austenite with too much C content being too stable can not perform TRIP effect in the subsequent deformation process, so that the work hardening behavior of the experimental steel is damaged, and the strength of the experimental steel is reduced, therefore, the invention has the advantages that: 0.22 to 0.28 percent.
Mn is also one of the most important elements in the 1000MPa grade low Mn dual-partition cold rolled steel sheet of the invention. Mn is an important element for expanding an austenite phase region, reduces the critical quenching speed of the experimental steel and delays the transformation from austenite to pearlite; meanwhile, the Ms point (martensite start temperature) in the experimental steel can be reduced, the austenite is stabilized, and the proper phase stability of the retained austenite is ensured. Too low a Mn content is insufficient to stabilize a sufficient content of austenite in the critical region and reduces the phase stability of the retained austenite at room temperature, resulting in poor work hardening behavior of the experimental steel. Mn segregation is easily generated due to excessively high Mn content, so that the continuous casting billet is hot cracked, and the production efficiency is not improved; secondly, the higher Mn content can improve the carbon equivalent of the steel plate and seriously affect the welding performance; therefore, the Mn: 2.5 to 3.0 percent.
Si: properly adding Si to promote ferrite generation elements and simultaneously avoiding carbon from being precipitated in the form of carbide in the distribution process, thereby providing conditions for diffusion of carbon atoms in the distribution process and promoting local enrichment of carbon. However, addition of too much Si reduces the surface quality of the steel; thus, the present invention Si: 1.5 to 1.8 percent.
Al can inhibit carbide precipitation and is beneficial to retaining residual austenite, but excessive Al addition can bring difficulty to smelting and can reduce the strength of the steel plate. Thus, the Al: 0.015 to 0.05 percent.
Ti + Nb: ti is one of the important elements in the invention; ti combines with impurity element N of steel grade to form TiN, free N atoms in the steel are present in the steel grade to deteriorate the toughness of the steel plate, so the formation of TiN plays a role in fixing N; in addition, Ti still forms Ti (C, N) with C, N, and plays a role in refining prior austenite grains. However, the Ti content is too high, which results in too large size of TiN and deterioration of the steel sheet properties. The Nb element is used in the present invention as an alternative element to the Ti element, and when the Ti addition is low or not, an appropriate amount of the Nb element may be added to form NbC or Nb (C, N) by strain-induced precipitation behavior of Nb at the hot rolling stage, which plays a role of refining the prior austenite grains. Therefore, in the present invention, Ti + Nb: 0.03 to 0.05 percent;
cr: the Cr element is a supplementary element of the Mn element, when the Mn content is added to be low, a certain amount of the Cr element needs to be added to ensure the hardenability of the steel plate, however, the excessive Cr content can improve the martensite content in the quenching stage, and the retained austenite is influenced. 0.1 to 0.5 percent of Cr.
P element is impurity element in steel, is easy to be partially gathered in crystal boundary, and is easy to form Fe when P content in steel is high2P particles, which reduce the plasticity and toughness of the steel, are preferably contained in a lower amount. In the invention, the content of the P element is controlled to be less than or equal to 0.02 percent.
The S element is an impurity element in steel, and is easily combined with Mn to form MnS inclusions, thereby deteriorating the plasticity of the steel plate, so that the lower the content is, the better the content is. In the invention, the content of the S element is controlled to be less than or equal to 0.005 percent.
Mo is a solid solution strengthening element and can improve the hardenability of the material, Ni improves the corrosion resistance of the steel plate to a certain extent, and the Cu element is dissolved in austenite in a solid solution manner and can improve the strength of the steel plate. In addition, Ni and Cu are austenite stabilizing elements and can promote retention of retained austenite; thus Ni + Mo + Cu: ni, Mo and Cu are elements for supplementing the stability of austenite, and are added in combination with Mn and Cr. Ni: less than or equal to 0.5 percent, Mo: less than or equal to 0.5 percent, Cu: less than or equal to 0.5 percent. Mo, Ni and Cu are all more noble alloys, and the total addition content is controlled to be lower than 0.6 percent in consideration of the alloy cost.
V is less than or equal to 0.05 percent, and the element V is properly added in the invention to strengthen the precipitation strengthening effect in the coiling stage, inhibit the dislocation self-recovery phenomenon in the cold rolling process, improve the retention of deformation energy storage and promote the recrystallization behavior in the continuous annealing stage; meanwhile, VC is precipitated in ferrite in the continuous annealing isothermal stage, and plays a role in precipitation strengthening.
Ca: the inclusion morphology can be controlled by adding a proper amount of Ca, so that the quality of a casting blank steel plate is improved; therefore, Ca is less than or equal to 0.005 percent.
B: the addition of B in the present invention can supplement the hardenability of the steel sheet, and the formation of martensite at the rapid cooling stage in the continuous annealing process. Too much B will increase the brittleness of the steel plate and deteriorate the workability of the steel plate; therefore, the B content of the invention is less than or equal to 0.005 percent.
The second technical scheme of the invention is to provide a preparation method of 1470MPa grade high-hole-expansion steel plate for cold stamping, which comprises smelting, hot rolling, cover annealing, acid washing, cold rolling and continuous annealing,
(1) smelting: smelting, refining and casting the components into a wedge-shaped casting blank.
(2) Hot rolling: the heating temperature is 1250-1280 ℃, the initial rolling temperature is 1100-1150 ℃, and the final rolling temperature is above 900 ℃.
Heating temperature: for high-strength steel containing Ti, the heating temperature is more important to control, and in order to achieve a more ideal N fixation effect, the heating temperature is usually controlled to 1250-1280 ℃. Thereby ensuring that fine TiN or Ti (C, N) is separated out in the heating and recrystallization zone rolling stages, and playing the effects of pinning grain boundaries and refining original austenite grains.
The initial rolling and final rolling temperature: the combination of the initial rolling temperature and the final rolling temperature ensures the rolling pass of the steel in a sufficient recrystallization region, and ensures the strain-induced precipitation behavior of microalloys such as Nb and Ti, and the like, thereby playing the roles of pinning grain boundaries and refining original austenite grains.
The coiling temperature is 450-550 ℃, U-shaped coiling is adopted, and the head and tail temperature is 600 +/-30 ℃; the steel plate is coiled at a temperature of 450-550 ℃. The steel of the invention adds more than 1.5 percent of Si for preventing cementite precipitation in the overaging stage, and the addition of a large amount of Si element can cause the formation of Si and Mn oxides in a coiling interval above 550 ℃, and the oxides grow below an iron scale and are distributed close to a subsurface layer. After the low-temperature coiling is adopted, the load of a coiling machine is increased due to the high alloy addition, and the limitation of 'coil fixation' is easy to occur, so that the head and tail temperature is increased to 600 +/-30 ℃ by adopting the U-shaped coiling, and the smooth coiling is ensured.
The steel plate structure after coiling is ferrite, pearlite, bainite and unidentified phases; wherein, according to the volume percentage: 40-50% of ferrite, 20-30% of pearlite and 20-30% of bainite.
(3) Cover annealing: and then the steel plate is subjected to stress relief annealing through a bell-type furnace, the annealing temperature is 500 +/-20 ℃, and the annealing time is 15-30 hours.
After low-temperature coiling, a large amount of bainite structures are formed, and the cold rolling load is greatly improved, so that cover type stress relief annealing is carried out before cold rolling, and the cold rolling difficulty is reduced.
(4) Acid washing: FeO and Fe exist on the surface of the hot rolled and coiled steel2O3、Fe3O4And removing iron oxide in different existing forms after acid pickling.
(5) Cold rolling: the specification of a cold-rolled product is kept to be 1.2-1.8 mm thick, the thickness of a target automobile part corresponding to the product is controlled to be 50% -58% corresponding to the thickness (1.2mm, 1.4mm, 1.6mm and 1.8mm) of the product, and the low rolling reduction rate cannot ensure enough cold-rolling deformation energy storage, so that the insufficient ferrite recrystallization effect in the continuous annealing stage is caused; the load of the cold rolling mill is greatly increased under the condition of overhigh rolling reduction, and the realization of the target thickness cannot be ensured.
(6) And (3) continuous annealing: firstly, heating a cold-rolled steel plate to 850-880 ℃ at the speed of 2-10 ℃/s, and keeping the temperature constant for 120-240 s; then, cooling the steel plate to 750-780 ℃ at the cooling temperature of 0.5-4 ℃/s; and then quenching the steel plate to 220-280 ℃ at a cooling speed of 30-35 ℃/s in a rapid cooling stage, then raising the temperature to an overaging temperature of 360-420 ℃ at a reheating speed of 10-20 ℃/s, wherein the overaging time is 280-410 s, and key parameters are described as follows:
heating at 850-880 ℃ for 120-240 s, and carrying out full austenite annealing, so that at least more than 70% of martensite is obtained in the quenching stage, and the part of martensite is transformed into a tempered martensite structure after tempering, so that the strength of the steel is ensured; meanwhile, austenite grains are prevented from being coarsened due to overhigh heating temperature, and the final strength is prevented from being influenced. Isothermal time of heating stage: an excessively short isothermal time is detrimental to the completion of the all-austenite, and an excessively long isothermal time leads to coarsening of austenite grains.
The slow cooling temperature is 750-780 ℃: according to the invention, a small amount of ferrite grains need to be introduced into the steel to play a role in coordinating the structure deformation, the introduction of a ferrite phase needs to be realized by separating out a small amount of oriented epiferrite in a slow cooling stage for the full austenitizing annealing, the ferrite cannot be formed due to the excessively high slow cooling temperature, and the ferrite content is excessive due to the excessively low slow cooling temperature.
And (3) a quick cooling stage: firstly, the cooling speed is 30-35 ℃/s to prevent the formation of excessive oriented epiphyte ferrite in the rapid cooling stage from influencing the performance of the steel plate; secondly, the quenching temperature is 220-280 ℃, the quenching temperature is one of the very important parameters in the invention, the final structure state of the invention is ferrite, tempered martensite, bainite and retained austenite, the selection of the quenching temperature directly influences the proportion of the tempered martensite, the bainite and the retained austenite in the final configuration, the quenching temperature of 220-280 ℃ in the invention ensures that the proportion of the martensite is more than or equal to 70% and more than or equal to 20% in the quenching state, and further ensures that the tempered martensite content is 75-80% and the retained austenite content is 9-13% in the final structure. If the quenching temperature is too low, the content of super-cooled austenite in a quenching state is reduced, so that the proportion of subsequent bainite and residual austenite is too low, and the plasticity of the steel plate is influenced; if the quenching temperature is too high, the martensite content in the quenched state is reduced, so that the tempered martensite content in the final structure is reduced, and the strength of the steel plate is influenced.
The overaging temperature is 360-420 ℃, and the isothermal time is 280-410 s: the main mechanism of action in this stage is to temper and soften martensite and to distribute martensite to austenite; if the overaging temperature is too high, martensite is tempered and softened, and then cementite is formed, so that the performance of the steel plate is influenced; if the overaging temperature is too low, the C distribution is slow, the content of residual austenite is reduced, and the plasticity of the steel plate is affected. The length of the overaging time also affects the tempering resistance of the tempered martensite, and too long overaging time can cause cementite in the tempered martensite to be separated out, so that the mechanical property of the steel plate is deteriorated; too short overaging isothermal time will affect the C partitioning behavior, affect the C-enrichment of the retained austenite, cause the stability of the retained austenite to decrease, and affect the plasticity of the steel sheet.
The microstructure of the final steel plate under a scanning electron microscope is that ferrite is less than or equal to 10 percent, tempered martensite is 75 to 80 percent, residual austenite is 9 to 13 percent, and the balance is bainite phase.
The invention has the beneficial effects that:
(1) the 1470 MPa-level steel plate provided by the invention has the advantages that precious alloys such as V, Cr, Mo and Cu are less or not added in alloy components, and the alloy cost is well controlled.
(2) According to the invention, through reasonable component and process design, the strength of the steel plate is greatly improved by adopting the design of a tempered martensite matrix, the residual austenite is obtained by adopting the idea of quenching distribution, and the plasticity of the steel plate is improved by depending on the TRIP effect of 10% of the residual austenite.
(3) The yield strength of the steel plate is 1100-1270 MPa, the tensile strength is over 1470MPa, the elongation is over 8 percent, the hole expansion rate is over 30 percent, and the cold-forming high-strength steel which is over 1470MPa and can be punched and expanded is prepared.
Detailed Description
The present invention is further illustrated by the following examples.
According to the component proportion of the technical scheme, smelting, hot rolling, cover annealing, acid washing, cold rolling and continuous annealing are carried out;
(1) hot rolling: the heating temperature is 1250-1280 ℃, the initial rolling temperature is 1100-1150 ℃, and the final rolling temperature is above 850 ℃; the coiling temperature is 450-550 ℃, U-shaped coiling is adopted, and the head and tail temperature is 600 +/-30 ℃;
(2) cover annealing: then, the steel plate is subjected to stress relief annealing through a bell-type furnace, the annealing temperature is 500 +/-20 ℃, and the annealing time is 15-30 hours;
(3) cold rolling: the rolling reduction rate is 50-58%;
(4) and (3) continuous annealing: firstly, heating a cold-rolled steel plate to 850-880 ℃ at the speed of 2-10 ℃/s, and keeping the temperature constant for 120-240 s; then, reducing the cooling speed of the steel plate to 750-780 ℃ at 0.5-4 ℃/s; then, in a rapid cooling stage, quenching the steel plate to 220-280 ℃ at a cooling speed of 30-35 ℃/s, then heating to an overaging temperature of 360-420 ℃ at a heating speed of 10-20 ℃/s, and the overaging time is 280-410 s;
furthermore, in the hot rolling in the step (1), the steel plate structure after coiling comprises ferrite, pearlite, bainite and unidentified phases; wherein, according to the volume percentage: 40-50% of ferrite, 20-30% of pearlite and 20-30% of bainite.
Further, in the continuous annealing in the step (6), the microstructure of the steel plate after the rapid cooling stage contains more than or equal to 70% of martensite and more than or equal to 20% of super-cooled austenite.
The compositions of the steels of the examples of the invention are shown in table 1. The main process parameters of the rolling of the steel of the embodiment of the invention are shown in Table 2. The main process parameters of the steel continuous annealing of the embodiment of the invention are shown in Table 3. The microstructure of the steels of the examples of the invention is shown in Table 4. The properties of the steels of the examples of the invention are shown in Table 5.
TABLE 2 Main Process parameters for the Rolling of steels according to the examples of the invention
TABLE 3 Main Process parameters for continuous annealing of steels according to the invention examples
TABLE 4 microstructure of inventive example steels
Examples
Ferrite/% of
Tempered martensite/%)
Retained austenite/%
Bainite/% of
1
9.4
75.6
9.3
5.7
2
8.1
79.7
9.9
2.3
3
7.6
79.5
11.4
1.5
4
6.2
78.7
11.2
3.9
5
5.7
76.9
9.8
7.6
6
3.5
79.1
10.5
6.9
7
0
77.5
9.4
13.1
8
9.6
77.3
9.7
3.4
9
9.3
76
11.5
3.2
10
5.8
79.4
10.9
3.9
11
6.5
78.8
11.8
2.9
12
4.9
77.4
11.2
6.5
13
3.8
77.3
11.2
6.7
14
0
78.5
12.5
9.0
15
0
79.7
12.6
8.7
TABLE 5 Properties of steels of examples of the invention
Examples
Rp0.2/MPa
Rm/MPa
A80/％
λ/％
1
1123
1485
11.6
33.6
2
1145
1496
10.5
34.5
3
1163
1502
10.8
33.5
4
1174
1508
10.9
34.3
5
1162
1511
11.2
31.3
6
1178
1527
9.5
32.9
7
1231
1522
10.5
32.1
8
1121
1501
10.1
35.7
9
1182
1503
9.5
38.7
10
1142
1512
10.8
31.9
11
1167
1516
9.6
33.9
12
1193
1528
9.2
31.2
13
1223
1531
8.8
37.6
14
1256
1516
8.8
32.5
15
1247
1517
10.1
41.3
According to the invention, the yield strength of the steel plate is 1100-1270 MPa, the tensile strength is 1470MPa or more, the elongation is more than 8%, the hole expansion rate is more than 30%, and the cold-forming high-strength steel which can be punched and expanded at a high hole under 1470MPa is prepared.
In order to express the present invention, the above embodiments are properly and fully described by way of examples, and the above embodiments are only used for illustrating the present invention and not for limiting the present invention, and those skilled in the relevant art can make various changes and modifications without departing from the spirit and scope of the present invention, and any modifications, equivalent substitutions, improvements, etc. made by the persons skilled in the relevant art should be included in the protection scope of the present invention, and the protection scope of the present invention should be defined by the claims.
