JP7701973B2 - Gigapascal-grade bainite steel with ultra-high yield ratio and method for producing same - Google Patents

Description

The present invention relates to a type of steel and its manufacturing method, in particular, gigapascal-grade bainite steel and its manufacturing method.

Under the new era of "green safety," the demand for strength in structural and safety components of automobiles is increasing day by day, and gigapascal-class high-strength steel is becoming one of the automotive structural materials that major automobile manufacturers are paying the most attention to.

In recent years, to ensure the normal use of automobiles and the safety of passengers, an increasing number of automobile structural components (e.g., body and frame parts) are required to have "zero deformation" within their service life, which places high demands on material performance, and the higher the yield strength or yield ratio, the better. Currently, materials with high yield strength or high yield ratio are attracting more and more attention from automobile manufacturers, and the market demand for gigapascal steel with high yield strength or high yield ratio is also growing day by day.

Gigapascal-class high-strength steels described in previous invention patents generally do not have high yield ratios, and the yield strength of dual-phase steels, which account for 90% of the market share of gigapascal-class automotive high-strength steels, is only 0.6 to 0.75. Some other products, such as martensitic steels, tempered and quenched steels (Q&P steels), and multi-phase steels, have slightly higher yield ratios, but are still only around 0.75 to 0.85.

For example: Chinese patent document (Patent Publication CN103361577A, disclosed on October 23, 2013, titled "High-strength steel plate with excellent formability and high yield ratio") discloses a high-strength steel plate with a high yield ratio, the microstructure of which is mainly composed of ferrite, martensite, tempered martensite and bainite, and the tensile strength can reach 980 MPa or more, but the yield ratio is only ≧0.68, which still does not meet the latest requirements of the automotive parts market for high-yield-ratio gigapascal-class steel plate.

For example, the Chinese patent document (Patent Publication CN106170574A, disclosed on November 30, 2016, entitled "High-strength cold-rolled steel plate with high yield ratio and manufacturing method thereof") discloses a high-strength cold-rolled steel plate with high yield ratio and manufacturing method thereof, in which the steel plate structure mainly contains ferrite, retained austenite, martensite and traces of bainite and tempered ferrite, and the tensile strength can reach 980 MPa or more, but the yield ratio is only ≧0.75, and at most does not exceed 0.8, so it still does not meet the market demand for gigapascal-grade high-strength steel with a yield ratio of 0.9 or more.

On the other hand, although some patent documents disclose high yield ratio steel plates with a yield ratio of 0.9 or more and manufacturing methods thereof, the tensile strength of the steel plates described in these patent documents does not reach the 980 MPa level.

For example: A Chinese patent document (Patent Publication CN102719736A, disclosed on October 10, 2012, entitled "Ultrafine grained steel for runways with a yield ratio of 0.9 or more and a method for producing the same") discloses a steel plate with a yield ratio of 0.9 or more obtained by utilizing an ultrafine grain structure, but its tensile strength is only in the 700 MPa range.

Thus, at the current stage, the tensile strength of steel plate reaching gigapascal levels and a yield ratio of 0.9 or more are two mutually contradictory technical indicators. The technical problem behind this contradiction is that the microstructural control technology required to achieve an ultra-high yield ratio of 0.9 or more is extremely difficult.

First, to achieve a high yield ratio, the matrix structure in the steel plate must be relatively uniform, for example, the matrix is composed of simple bainite or simple martensite. Multi-phase or complex matrix structures, for example, steel plates whose matrix structure simultaneously contains ferrite, retained austenite, tempered martensite, and martensite, are unlikely to achieve a high yield ratio. In order for the strength of the steel plate to reach gigapascal levels, a combination of multi-phase structures is required, for example, typical ferrite/martensite dual-phase steels and advanced high-strength steels containing retained austenite with the TRIP effect introduced. This is the first layer of technical contradiction.

Even if the structure is purely bainite or purely martensite, the yield ratio of the steel plate is unlikely to exceed 0.9 due to dislocation movement and work hardening caused by processing strain, and typically the yield ratio of steel plate with a purely martensite or bainite matrix is approximately 0.8 to 0.9.

Therefore, in order to obtain a steel plate with a super-high yield ratio, a complex component process design is required to prevent the movement of dislocations in order to increase the yield strength of the material. For example: Chinese patent document (Patent Publication CN101910436A, disclosed on December 8, 2010, titled "High-strength cold-rolled steel plate with excellent weather resistance and its preparation method" discloses a method of increasing the yield strength of the material by introducing a large amount of expensive solid-solution alloys such as Cr, Zr, Co, W, etc. However, considering the complex preparation process and relatively high alloy addition amount in conventional gigapascal-class ultra-high strength steel, there is a doubt as to whether the above-mentioned complex process technology or the addition of expensive alloys to further increase the yield ratio is suitable for introduction into conventional gigapascal-class ultra-high strength steel, whose structure is already very complex. This is the second layer of technical contradiction.

Therefore, in order to obtain gigapascal-class ultra-high strength steel with a yield ratio of 0.9 or more, it is necessary to overcome numerous technical difficulties, including the first and second layer technical inconsistencies mentioned above, which has not yet been realized with conventional patented technologies.

Based on this, in order to solve the above problems, gigapascal-grade bainite steel with an ultra-high yield ratio is expected to be developed. This gigapascal-grade bainite steel simultaneously has an ultra-high yield ratio, ultra-high strength, and excellent hole expansion and bending performance, and can be used to manufacture automobile structural components and realize the new design concept of "green-safe" automobiles.

One objective of the present invention is to provide a gigapascal-grade bainite steel with an ultra-high yield ratio. Through rational chemical composition design, the present invention can obtain a gigapascal-grade bainite steel with an ultra-high yield ratio, which has a tensile strength of ≥ 980 MPa, a yield strength of ≥ 900 MPa, a yield ratio of ≥ 0.9, and a hole expansion ratio of ≥ 55%. Since it simultaneously has an ultra-high yield ratio, ultra-high strength, and excellent hole expansion and bending performance, it can be used to manufacture automotive structural components, and has good general prospects and application value.

To achieve the above-mentioned objectives, the present invention proposes a gigapascal-grade bainitic steel with ultra-high yield ratio, which, besides Fe and unavoidable impurities, further contains the following chemical elements in mass percent content:
C: 0.12-0.24%;
Si: 0.2-0.5%;
Mn: 1.3-2.0%;
B: 0.001-0.004%;
Al: 0.01-0.05%;
At least one of Cr, Nb, Ti, and Mo, provided that Cr≦0.4%, Nb≦0.06%, Ti≦0.1%, and Mo≦0.4%.

Furthermore, in the gigapascal grade bainite steel with ultra-high yield ratio according to the present invention, the mass percent content of each chemical element therein is:
C: 0.12-0.24%;
Si: 0.2-0.5%;
Mn: 1.3-2.0%;
B: 0.001-0.004%;
Al: 0.01-0.05%;
At least one of Cr, Nb, Ti, and Mo, provided that Cr≦0.4%, Nb≦0.06%, Ti≦0.1%, and Mo≦0.4%;
The balance is Fe and other unavoidable impurities.

In the technical solution of the present invention, the specific design principles of each chemical element are as follows:
C: In the gigapascal-grade bainite steel with ultra-high yield ratio according to the present invention, the C element is one of the important elements for controlling the microstructural phase transformation in carbon steel. At the same time, the C element also has a great influence on the strength of the steel plate. The C element can form alloy carbides together with other alloy elements to increase the strength of the steel plate. If the C element content in the steel is less than 0.12%, the strength of the steel does not meet the target requirements; if the C element content in the steel is more than 0.24%, martensite structure and coarse cementite are easily generated, which deteriorates the performance of the steel plate. Based on this, in the gigapascal-grade bainite steel with ultra-high yield ratio according to the present invention, the mass percentage of C is between 0.12% and 0.24%.

Of course, in some preferred embodiments, the mass percentage of C element may be between 0.15% and 0.20% to obtain better performance.

Si: In the gigapascal-grade bainite steel with ultra-high yield ratio according to the present invention, the Si element is an essential element for deoxidation in steelmaking, which not only has a certain solid solution strengthening effect, but also has a certain effect on the formation of bainite (the higher the B element content in the steel, the easier it is to form carbon-free bainite). It should be explained that if the Si element content in the steel is less than 0.2%, it is difficult to obtain a sufficient deoxidation effect; and if the Si element content in the steel exceeds 0.5%, it is easy to form an iron oxide film or a tiger pattern color difference, which is disadvantageous to the surface quality of automotive steel sheets. Based on this, in the gigapascal-grade bainite steel with ultra-high yield ratio according to the present invention, the mass percentage of Si is between 0.2 and 0.5%.

Mn: In the gigapascal-grade bainite steel with ultra-high yield ratio according to the present invention, Mn is the main additive element and is one of the important elements for controlling the structural phase transition in the steel. It should be explained that Mn is not only an effective element for increasing the strength of steel due to its low cost, but also a relatively important solid solution strengthening element. However, it should be noted that the Mn content in the steel should not be too high. If the Mn content in the steel is too high, the corrosion resistance and welding performance will deteriorate, and at the same time, the crystal grains will tend to become coarse, and the plasticity and toughness of the steel will decrease. Based on this, in the gigapascal-grade bainite steel with ultra-high yield ratio according to the present invention, the mass percentage of Mn is between 1.3% and 2.0%.

Of course, in some preferred embodiments, the mass percentage of Mn element may be between 1.6% and 2.0% to obtain better performance.

B: In the gigapascal-grade bainite steel with ultra-high yield ratio according to the present invention, the B element is not only favorable for the formation of bainite in the steel, but also has a great influence on the strength and hardness of the steel plate. It should be noted that if the content of the B element in the steel is less than 0.001%, the strength of the steel does not meet the target requirements; and if the content of the B element in the steel is more than 0.004%, brittle borides are easily formed, which affects the hole expansion and bending performance of the steel plate. Based on this, in the gigapascal-grade bainite steel with ultra-high yield ratio according to the present invention, the mass percentage of B is between 0.001 and 0.004%.

Al: In the gigapascal-grade bainite steel with ultra-high yield ratio according to the present invention, the Al element is added to the steel only as a deoxidizing element, which can remove the O element in the steel and ensure the performance and quality of the steel. Therefore, in the gigapascal-grade bainite steel with ultra-high yield ratio according to the present invention, the mass percentage of Al is between 0.01% and 0.05%. In the prior art, the Al element is added to the steel in large amounts (≧0.1%) as a ferrite forming element and an inhibitor of carbide precipitation to bring about solid solution strengthening, or the addition of Al changes the phase transition temperature, bainite formation kinetics and carbide precipitation kinetics to change the phase transition of the steel, form retained austenite or carbon-free bainite, and finally increase the strength of the steel, but the composition control and process adjustment of the present invention can already obtain a gigapascal-grade bainite steel with ultra-high yield ratio, so there is no need to add a large amount of the Al element, and a significant increase in cost and difficulty in steelmaking can be avoided.

Ti, Cr, Nb and Mo: In the gigapascal-grade bainite steel with ultra-high yield ratio according to the present invention, Ti, Cr, Nb and Mo can be added as optional alloying elements to the steel to form fine and dispersed carbide second phase precipitation, further increasing the strength and yield ratio of the steel plate. It should also be noted that Cr and Mo elements can prolong the pearlite and ferrite growth period in the CCT curve and inhibit the formation of ferrite in pearlite, making it easier to obtain a bainite structure during cooling and easier to increase the hole expansion ratio of the steel.

Thus, the above four alloying elements have an effect on the structural control of the steel sheet and the corresponding annealing process, which in turn affect the formation of carbides, directly affecting the proportion and shape of carbide formation. Based on this, in the gigapascal-grade bainite steel with ultra-high yield ratio according to the present invention, the mass percentages of Cr, Nb, Ti and Mo are Cr≦0.4%, Nb≦0.06%, Ti≦0.1%, and Mo≦0.4%, respectively.

In addition, since the addition of the above alloying elements increases the cost of the material, in consideration of the overall performance and cost management, the technical solution according to the present invention preferably includes at least one of Cr, Nb, Ti, and Mo in the steel. In some preferred embodiments, the gigapascal-class bainite steel with an ultra-high yield ratio according to the present invention contains at least 0.1-0.4% Cr. In some preferred embodiments, the gigapascal-class bainite steel with an ultra-high yield ratio according to the present invention contains at least 0.1-0.4% Mo. In some preferred embodiments, the gigapascal-class bainite steel with an ultra-high yield ratio according to the present invention contains at least one or both of Cr and Mo. In some preferred embodiments, the gigapascal-class bainite steel with an ultra-high yield ratio according to the present invention contains at least 0.1-0.4% Cr and 0.1-0.4% Mo.

Furthermore, in the gigapascal grade bainitic steel with ultra-high yield ratio according to the present invention, the mass percentage of each chemical element satisfies at least one of the following:
C: 0.15-0.20%,
Mn: 1.6-2.0%.

Furthermore, in the gigapascal-grade bainite steel with ultra-high yield ratio according to the present invention, among other unavoidable impurities, P≦0.015% and/or S≦0.004%.

In the above technical proposals, P and S are both impurity elements in steel, and as far as technically possible, the content of impurity elements in steel should be reduced as much as possible to obtain tempered steel with better performance and better quality.

Furthermore, in the gigapascal grade bainite steel with ultra-high yield ratio according to the present invention, at least one of the following chemical elements is further contained:
0<Cu≦0.2%, 0<Ni≦0.2%, 0<V≦0.2%, 0<Ce≦0.2%.

In the technical solution of the present invention, the above-mentioned Cu, Ni, V and Ce elements can all further improve the performance of the gigapascal-grade bainite steel with ultra-high yield ratio of the present invention.

Furthermore, the gigapascal grade bainite steel with ultra-high yield ratio according to the present invention satisfies 0.18≦M≦0.27, where M=Cr/2.5+Ti+V/5+Nb/1.7+Mo/1.7, where Cr, V, Nb, Ti and Mo represent the numerical values before the percent sign of the mass percent content of each chemical element.

In the above technical solution, it should be explained that in the gigapascal-grade bainite steel with ultra-high yield ratio according to the present invention, in order to obtain a gigapascal-grade bainite steel with better performance and quality in controlling the mass percentage of a single chemical element, it is preferable to set M to 0.18≦M≦0.27, where M=Cr/2.5+Ti+V/5+Nb/1.7+Mo/1.7, and Cr, V, Nb, Ti and Mo represent the numerical values before the percent symbol of the mass percentage content of each chemical element.

It should be explained that in the present invention, if M is too high, coarse carbides are likely to form, and the hole expansion ratio and bending performance of the steel deteriorate; and if M is too low, a sufficient carbide precipitation phase cannot be formed, and the strength and yield ratio of the steel become insufficient. Therefore, in the present invention, in order to ensure that nano-order, sub-micro-order or micro-order granular carbides are dispersed and precipitated in the steel and the diameter size of the maximum granular carbide precipitation phase is secured, M may be set to 0.18≦M≦0.27.

Furthermore, the gigapascal-grade bainite steel with ultra-high yield ratio according to the present invention satisfies 0.20≦C _b ≦0.27, where the equivalent bainite carbon element content C _b = C-(Mo+Nb)/8-(Ti+V)/4-Cr/12+Ni/10+Mn/20+B×10, and each element in the formula represents the numerical value before the percent sign of the mass percent content of that element.

In the above technical solution, in the gigapascal-grade bainite steel with ultra-high yield ratio according to the present invention, the alloying elements and M value affect the precipitation of carbides, and therefore indirectly affect the content _Cb of the equivalent bainite carbon element in the steel. It should be noted that in the present invention, if _Cb is too low, sufficient single bainite matrix structure cannot be formed; and if _Cb is too high, the hardness of bainite becomes too large, which deteriorates the bending and hole expansion performance of the steel. Therefore, in the present invention, in order to effectively ensure that the phase proportion of acicular lower bainite in the steel is more than 90% while controlling the mass percentage of single chemical elements, it is preferable that _Cb is 0.20≦ _Cb ≦0.27.

Furthermore, in the gigapascal-grade bainite steel with ultra-high yield ratio according to the present invention, the microstructure is mainly acicular lower bainite, and the phase proportion of acicular lower bainite is 90% or more.

Furthermore, in the gigapascal-class bainite steel with an ultra-high yield ratio according to the present invention, the microstructure further contains a dispersed and precipitated nano-order, sub-micro-order or micro-order granular carbide precipitation phase, and the total phase proportion of the granular carbide precipitation phase + acicular lower bainite is 99% or more.

Furthermore, in the gigapascal-grade bainite steel with ultra-high yield ratio according to the present invention, the diameter of the maximum granular carbide precipitate phase is 2 μm or less.

Furthermore, the gigapascal-grade bainite steel with ultra-high yield ratio according to the present invention has a tensile strength of ≧980 MPa, preferably ≧1000 MPa, a yield strength of ≧900 MPa, preferably ≧950 MPa, a yield ratio of ≧0.9, preferably ≧0.95, and a hole expansion ratio of ≧55%, preferably ≧60%. In a preferred embodiment, the gigapascal-grade bainite steel with ultra-high yield ratio according to the present invention has a tensile strength of ≧1000 MPa, a yield strength of ≧910 MPa, a yield ratio of ≧0.9, and a hole expansion ratio of ≧55%.

Furthermore, the gigapascal-grade bainite steel with ultra-high yield ratio according to the present invention has a yield strength of ≥ 950 MPa and a yield ratio of ≥ 0.95; more preferably, its tensile strength is ≥ 1000 MPa and hole expansion ratio is ≥ 60%.

Furthermore, the gigapascal-grade bainite steel with ultra-high yield ratio according to the present invention has an elongation of ≧9.0%. Another object of the present invention is to provide an annealing process for the gigapascal-grade bainite steel with the above-mentioned ultra-high yield ratio. This annealing process has an important effect on the performance of the steel, and by rational process design and control of related process parameters, gigapascal-grade bainite steel with ultra-high yield ratio can be obtained.

In order to achieve the above-mentioned objectives, the present invention proposes an annealing process for the above-mentioned gigapascal-grade bainite steel with ultra-high yield ratio, which includes the following steps:
(a) in a heating step, heating to a soaking temperature Ts at a heating rate of 50°C/s or less, where Ts is 840-900°C;
(b) In the soaking stage, the temperature is kept at Ts for 5 min or less;
(c) In a slow cooling step, cooling is performed at a first cooling rate of 15°C/s or less to (Ts-80) to (Ts-140)°C;
(d) in a quenching step, cooling at a second cooling rate of (130-Q)°C/s or less to (Ts-490) to (Ts-440)°C; where Q=C x 180 + Si x 10 + Mn x 30 + Ni x 50 + Cr x 15 + Mo x 15 + B x 2000, each element in the formula representing the numerical value before the percent symbol of the mass percent content of that element;
(e) In the self-recovery temperature control cooling stage, cooling is performed at a third cooling rate for 10 to 40 s, where [(Q-80)/12] ° C./s ≦ third cooling rate ≦ [(Q-80)/8] ° C./s ;
(f) Finally, in an air cooling stage, the strip is air cooled to room temperature.

In the technical solution according to the present invention, it must be explained that the above-mentioned annealing process includes a heating stage, a soaking stage, a slow cooling stage, a quenching stage, a self-recovery temperature control cooling stage and an air cooling stage, which play an important role in the performance of the gigapascal-grade bainite steel according to the present invention.

In step (a), the heating stage requires heating to a soaking temperature Ts of 840-900°C at a heating rate of 50°C/s or less, preferably to a soaking temperature of 840-870°C. However, the heating rate in the heating stage should not be too high, otherwise the uniformity of the strip steel structure will decrease. It should also be noted that if the soaking temperature Ts is lower than the above-mentioned soaking temperature design range, the strip steel will not have an acicular lower bainite structure of 90% or more; and if the soaking temperature Ts exceeds the above-mentioned soaking temperature design range, the grains of the strip steel will become coarse, and the forming performance of the steel will deteriorate. In some embodiments, the heating rate in step (a) is 5-45°C/s.

In step (b), the incubation time is preferably 1 minute or more. For example, the incubation time is 1 minute to 4.5 minutes.

In step (c), in the slow cooling stage, it is necessary to cool to (Ts-80) to (Ts-140) °C at a first cooling rate of 15 °C/s or less. However, it is better not to have a too high first cooling rate in the slow cooling stage. Otherwise, not only will energy be wasted, but the strip steel structure will become non-uniform. It should also be explained that if the slow cooling temperature is lower than the slow cooling design temperature range described above, the strip steel will not have a bainite structure of 90% or more; and if the slow cooling temperature is higher than the slow cooling design temperature range described above, the subsequent quenching stage requires higher cooling capacity and higher temperature accuracy control capacity, so that the structural uniformity of the strip steel is likely to deteriorate due to insufficient cooling capacity or temperature accuracy control capacity, and the performance of the product will deteriorate. Preferably, the first cooling rate in step (c) is 5-15 °C/s, preferably 5-12 °C/s.

In step (d), in the quenching stage, it is necessary to cool to (Ts-490) to (Ts-440) ° C. at a second cooling rate of (130-Q) ° C./s or more; where Q = C x 180 + Si x 10 + Mn x 30 + Ni x 50 + Cr x 15 + Mo x 15 + B x 2000. However, if the second cooling rate in the quenching stage is insufficient or the cooling temperature exceeds (Ts-440) ° C., the phase transformation of bainite will occur earlier than expected, and high-temperature bainite structure (such as upper bainite or equiaxed bainite) will be generated, which will not only fail to ensure that the phase proportion of acicular lower bainite in the steel is 90% or more, but also greatly reduce the latent heat of the phase transformation, making it impossible to realize the subsequent self-return temperature control cooling, resulting in abnormal material structure, and making it impossible to obtain an ultra-high yield ratio for the steel plate and steel strip. Furthermore, if the cooling temperature in the quenching stage is less than (Ts-490)°C, martensite structure will be generated, reducing the hole expansion ratio and bending performance of the steel.

In step (e), in the self-return temperature controlled cooling stage, if the steel strip is quenched according to the design parameters, the temperature self-return phenomenon is realized by the massive release of latent heat of the steel strip's phase transformation, and the self-return temperature can rapidly, uniformly and efficiently increase the steel strip's temperature by 50-120°C, thus promoting the uniform and dispersed precipitation of carbides. In order to fully precipitate the carbides and keep the precipitate size fine, the steel strip temperature should be controlled and cooled at the third cooling rate for 10-40 s, where [(Q-80)/12] °C/s ≦third cooling rate≦[(Q-80)/8] °C/s .

It should be noted that if the third cooling rate in the self-recovery temperature controlled cooling stage is too low or the controlled cooling time is too long and does not meet the above-mentioned design requirements of the present invention, the carbide precipitation is likely to become rough, and the hole expansion ratio and bending performance will deteriorate; and if the third cooling rate is too high or the controlled cooling time is too short, the carbide precipitation is likely to be insufficient, and the steel will not achieve ultra-high yield ratio performance of a yield ratio of 0.9 or more.

Another object of the present invention is to provide a method for producing the gigapascal-class bainite steel having the above-mentioned ultra-high yield ratio. This manufacturing method makes it possible to effectively produce the gigapascal-class bainite steel having the ultra-high yield ratio according to the present invention.

In order to achieve the above-mentioned objectives, the present invention proposes a method for producing gigapascal-grade bainitic steel with ultra-high yield ratio, which includes the following steps:
(1) Smelting and foundry;
(2) hot rolling;
(3) Post-rolling cooling and coiling;
(4) Pickling and cold rolling;
(5) The annealing process described above.

In the technical solution of the present invention, in the above-mentioned manufacturing method, the operation steps in steps (1) to (4) of the pre-annealing process are mainly performed to obtain a steel plate or strip with uniform composition and initial structure, and to ensure uniform and stable structure and performance when the subsequent annealing process is carried out. It is the annealing process in step (5) that has an important effect on the performance of the steel plate.

Furthermore, in the manufacturing method according to the present invention, in step (2), the heating temperature is set to 1150 to 1260°C; the finish rolling start temperature is set to 1100 to 1220°C, and the finish rolling end temperature is set to 900 to 950°C.

Furthermore, in the manufacturing method according to the present invention, in step (3), the cooling rate is set to 30 to 150°C/s and the coiling temperature is set to 450 to 580°C.

Furthermore, in the manufacturing method according to the present invention, in step (4), the cold rolling reduction is set to 50% or less.

Furthermore, in the manufacturing method according to the present invention, the gigapascal-class bainite steel having an ultra-high yield ratio is a gigapascal-class bainite steel having an ultra-high yield ratio according to any one of the embodiments of the present invention.

Compared with the prior art, the gigapascal-grade bainite steel with ultra-high yield ratio and its manufacturing method according to the present invention have the following advantages and beneficial effects:
The present invention is based on the premise that the chemical element composition and process are relatively simple and controllable, and by optimizing the combination of alloying elements and innovatively adjusting the annealing process, it ensures that the matrix structure of the steel sheet is a simple and uniform bainite structure, and introduces the release of latent heat of phase transformation to realize the self-restoring temperature of the steel strip, which not only reduces energy consumption, but also realizes fast, uniform and efficient control of the restoring temperature of the strip, induces fine dispersed precipitation of second phase, and obtains gigapascal-grade bainite steel with ultra-high yield ratio and good forming performance.

By rationally designing the chemical components, the present invention can obtain gigapascal-grade bainite steel with an ultra-high yield ratio, i.e., tensile strength ≧980 MPa, yield strength ≧900 MPa, yield ratio ≧0.9, and hole expansion ratio ≧55%. This gigapascal-grade bainite steel simultaneously combines an ultra-high yield ratio, ultra-high strength, and excellent hole expansion and bending performance, and can be used to manufacture automotive structural components, meeting the new design concept of "green-safe" for automobiles, and has good general prospects and application value.

The annealing process of the present invention has an important effect on the performance of the steel. This annealing process includes a heating stage, a soaking stage, a slow cooling stage, a quenching stage, a self-recovery temperature-controlled cooling stage and an air cooling stage, and through a rational process design and control of the relevant process parameters, a gigapascal-grade bainite steel with an ultra-high yield ratio can be obtained.

In addition, the production process of the manufacturing method according to the present invention is unique, and it adopts the above-mentioned annealing process to ensure the performance of the resulting gigapascal-grade bainite steel. The resulting gigapascal-grade bainite steel not only has ultra-high strength and yield ratio, but also has excellent hole expansion and bending performance.

FIG. 1 is a microstructure photograph of the gigapascal-grade bainite steel of Example 1, magnified 3000 times. FIG. 2 is a micrograph of the comparative steel of Comparative Example 7, magnified 3000 times. FIG. 2 is a micrograph of the comparative steel of Comparative Example 8, enlarged 1000 times.

The following provides a more detailed interpretation and explanation of the gigapascal-grade bainite steel with an ultra-high yield ratio and its manufacturing method according to the present invention, based on the drawings and specific examples, but the interpretation and explanation are not intended to inappropriately limit the technical solution of the present invention.

Examples 1-14 and Comparative Examples 1-10
The gigapascal grade bainite steels having ultra-high yield ratios of Examples 1-14 were made by the following steps:
(1) Smelting and casting were carried out with the chemical composition shown in Table 1.

(2) Hot rolling: The heating temperature was 1150-1260°C; the finish rolling start temperature was 1100-1220°C, and the finish rolling end temperature was 900-950°C.

(3) Cooling after rolling and coiling : The cooling rate was 30 to 150°C/s, and the coiling temperature was 450 to 580°C.

(4) Pickling and cold rolling: The cold rolling reduction was set to 50% or more.
(5) Annealing.

It should be noted that in step (5), the annealing process includes the following steps:
(a) In the heating step, the sample was heated to a soaking temperature Ts at a heating rate of 50° C./s or less, where Ts was 840 to 900° C.

(b) In the soaking stage, the temperature was maintained at Ts for 5 minutes or less.
(c) In the slow cooling stage, the material was cooled to (Ts-80) to (Ts-140)° C. at a first cooling rate of 15° C./s or less.

(d) In the quenching stage, the material was cooled to (Ts-490) to (Ts-440)°C at a second cooling rate of (130-Q)°C/s or more; where Q=C x 180 + Si x 10 + Mn x 30 + Ni x 50 + Cr x 15 + Mo x 15 + B x 2000.

(e) In the self-recovery temperature controlled cooling stage, cooling was performed at the third cooling rate for 10 to 40 s, where [(Q-80)/12] °C/s ≦ the third cooling rate ≦ [(Q-80)/8] °C/s .

(f) Finally, in an air cooling stage, the strip was air cooled to room temperature.
It should also be noted that the gigapascal-grade bainite steels with ultra-high yield ratios of Examples 1-14 according to the present invention are all prepared by the above steps, and their chemical compositions and related process parameters all meet the design criteria control requirements of the present invention.

Similarly, the comparative steels of Comparative Examples 1-10 were also produced by the steps of smelting and casting, hot rolling, post-rolling cooling and coiling , pickling and cold rolling, and annealing. However, the chemical compositions and related process parameters of Comparative Examples 1-6 do not meet the required parameters designed in the present invention; the chemical compositions of Comparative Examples 7-14 meet the design requirements of the present invention, but there are process parameters that do not meet the design requirements of the present invention.

However, in the examples and comparative examples of the present invention, the chemical element components of Comparative Example 7 and Example 1 are the same, the chemical element components of Comparative Example 8 and Example 2 are the same, the chemical element components of Comparative Example 9 and Example 6 are the same, and the chemical element components of Comparative Example 10 and Example 11 are the same.

Table 1 shows the mass percent composition ratio (%) of each chemical element in the gigapascal-grade bainite steel with ultra-high yield ratio of Example 1-14 and the comparative steel of Comparative Example 1-10.

Note: In the above table, Cb = C - (Mo + Nb)/8 - (Ti + V)/4 - Cr/12 + Ni/10 + Mn/20 + B x 10, where each element in the formula represents the numerical value before the percent sign of the mass percent content of that element; M = Cr/2.5 + Ti + V/5 + Nb/1.7 + Mo/1.7, where Cr, V, Nb, Ti and Mo represent the numerical value before the percent sign of the mass percent content of each chemical element.

Tables 2-1 and 2-2 show the specific process parameters for the gigapascal-grade bainite steel with ultra-high yield ratio of Example 1-14 and the comparative steel of Comparative Example 1-10.

(Continuation of Table 2-2)

Note: In the above table, Q = C x 180 + Si x 10 + Mn x 30 + Ni x 50 + Cr x 15 + Mo x 15 + B x 2000, and each element in the formula represents the mass percent content of that element, preceded by the percent symbol.

Relevant mechanical performance measurements were performed on the gigapascal-grade bainite steels having ultra-high yield ratios of Examples 1-14 and the comparative steels of Comparative Examples 1-10, and the mechanical performance measurement results of each of the Examples and Comparative Examples are listed in Table 3. The relevant performance measurement means are as follows:

Samples were taken from the gigapascal-grade bainite steel with ultra-high yield ratio of Example 1-14 and the comparative steel of Comparative Example 1-10, and the yield strength and tensile strength of the steel were measured using JIS 5# tensile samples along the transverse direction, and the hole expansion ratio and bending performance of the steel were measured in the central region of the plate.

However, the hole expansion ratio of the steel was measured by a hole expansion test, where a male sample with a hole in the center was pressed into a female mold, and the central hole of the sample was expanded until necking or through-hole cracks appeared at the edge of the plate hole. Because the method of making the initial hole in the center of the sample and the quality of the corresponding initial hole edge have a large effect on the hole expansion ratio measurement result, the test and measurement method was performed using the hole expansion ratio measurement method specified in the ISO/DIS16630 standard, and the central initial hole was of the punch hole type (corresponding to the processing method with the worst quality of the initial hole edge). The 180° bending test was performed using the bending performance measurement method specified in the GB/T232-2010 standard (bending diameter d = 1a).

Table 3 shows the results of measuring the mechanical properties of gigapascal-grade bainite steels with ultra-high yield ratios in Examples 1-14 and comparative steels in Comparative Examples 1-10.

As can be seen from Table 3, the gigapascal-grade bainite steel with an ultra-high yield ratio of Example 1-14 of the present invention has clearly superior mechanical properties compared to the comparative steel of Comparative Example 1-10.

The gigapascal-grade bainite steel with ultra-high yield ratio of Example 1-14 of the present invention simultaneously has an ultra-high yield ratio, ultra-high strength, and excellent hole expansion performance and bending performance, with all tensile strengths of ≥980 MPa, all yield strengths of ≥900 MPa, all yield ratios of ≥0.9, and all hole expansion ratios of ≥55%.

In a particular preferred embodiment, for example in Example 1, the gigapascal-grade bainite steel with ultra-high yield ratio of Example 1 has a yield strength of ≧950 MPa and a yield ratio of ≧0.95, and has an ultra-high yield ratio and an ultra-high yield strength.

Figure 1 is a micrograph of the gigapascal-grade bainite steel of Example 1, magnified 3,000 times.

As shown in Figure 1, the gigapascal-grade bainite steel of Example 1 was cooled to the lower bainite phase region (the quenching cooling temperature satisfies the invention requirements) at a sufficiently fast cooling rate in the quenching stage (the second cooling rate satisfies the invention requirements), so the matrix of its microstructure is acicular lower bainite, and the cooling rate in the self-return temperature controlled cooling stage is appropriate (the third cooling rate satisfies the invention requirements), so the structure contains fine and dispersedly precipitated nano-, sub-micro- or micro-order granular carbide precipitation phases. However, the phase proportion of acicular lower bainite is 90% or more, the sum of the phase proportions of the granular carbide precipitation phase + acicular lower bainite is 99% or more, and the maximum diameter of the granular carbide precipitation phase is 2 μm or less.

FIG. 2 is a micrograph of the comparative steel of Comparative Example 7, magnified 3000 times.
As shown in FIG. 2 , in the comparative steel of Comparative Example 7, the cooling rate during the quenching step was insufficient (the second cooling rate did not satisfy the requirements of the invention). Therefore, in the comparative steel, the bainite phase transition occurred when the steel was not completely cooled to the lower bainite phase region, that is, at a relatively high temperature. Although the steel was finally cooled to an appropriate lower bainite phase region temperature, the microstructure mainly contained bulk equiaxed bainite and almost no acicular lower bainite, and the carbide precipitation was not sufficiently fine and uniform.

FIG. 3 is a micrograph of the comparative steel of Comparative Example 8, enlarged 1000 times.
As shown in FIG. 3 , in the comparative steel of Comparative Example 8, the cooling rate in the quenching step is appropriate (the second cooling rate meets the requirements of the invention), but the quenching temperature is too high (the cooling temperature in the quenching step does not meet the requirements of the invention). As a result, the microstructure is almost entirely a bulk equiaxed bainite structure, with almost no acicular lower bainite, and the carbide precipitation is not sufficiently fine and uniform.

As can be seen from the above, the present invention makes it possible to obtain gigapascal-grade bainite steel with an ultra-high yield ratio, i.e., tensile strength ≧980 MPa, yield strength ≧900 MPa, yield ratio ≧0.9, and hole expansion ratio ≧55%, through rational chemical composition design. This gigapascal-grade bainite steel simultaneously combines ultra-high yield ratio, ultra-high strength, and excellent hole expansion and bending performance, and can be used to manufacture automotive structural components, meeting the new design concept of "green-safe" for automobiles, and has good general prospects and application value.

The annealing process of the present invention has an important effect on the performance of the steel. This annealing process includes a heating stage, a soaking stage, a slow cooling stage, a quenching stage, a self-recovery temperature-controlled cooling stage and an air cooling stage, and through a rational process design and control of the relevant process parameters, a gigapascal-grade bainite steel with an ultra-high yield ratio can be obtained.

In addition, the production process of the manufacturing method according to the present invention is unique, and it adopts the above-mentioned annealing process to ensure the performance of the resulting gigapascal-grade bainite steel. The resulting gigapascal-grade bainite steel not only has ultra-high strength and yield ratio, but also has excellent hole expansion and bending performance.

Furthermore, the combination methods of each technical feature in this application are not limited to the combination methods described in the claims or the combination methods described in the specific examples, and all technical features described in this application may be freely combined or combined in any manner as long as they are not inconsistent with each other.

Furthermore, it should be noted that the above-mentioned embodiments are merely specific embodiments of the present invention. The present invention is not limited to the above-mentioned embodiments, and similar changes and modifications can be directly obtained or easily conceived by a person skilled in the art from the disclosure of the present invention, and therefore, needless to say, fall within the scope of protection of the present invention.

Claims (12)

A gigapascal grade bainite steel plate having an ultra-high yield ratio, comprising the following chemical elements in mass percent content:
C: 0.12-0.24%;
Si: 0.2-0.5%;
Mn: 1.3-2.0%;
B: 0.001-0.004%;
Al: 0.01-0.05%;
At least one of Cr, Nb, Ti, and Mo, provided that Cr≦0.4%, Nb≦0.06%, Ti≦0.1%, and Mo≦0.4%;
The balance is Fe and other unavoidable impurities.
0.18≦M≦0.27
(wherein M=Cr/2.5+Ti+V/5+Nb/1.7+Mo/1.7, where Cr, V, Nb, Ti and Mo represent the numerical values before the percent sign of the mass percent content of each chemical element)
and,
0.20≦C _b ≦0.27
(wherein the equivalent bainite carbon element content C _b = C - (Mo + Nb) / 8 - (Ti + V) / 4 - Cr / 12 + Ni / 10 + Mn / 20 + B x 10, each element in the formula represents the value before the percent symbol of the mass percent content of that element)
Fulfilling
The microstructure of the gigapascal-grade bainite steel plate is mainly acicular lower bainite, and the proportion of acicular lower bainite is 90% or more;
The gigapascal-grade bainite steel plate having an ultra-high yield ratio is characterized in that the gigapascal-grade bainite steel plate has a tensile strength of ≧980 MPa, a yield strength of ≧900 MPa, a yield ratio of ≧0.9, and a hole expansion ratio of ≧55%. The mass percent content of each chemical element is as follows:
C: 0.15-0.20%,
Mn: 1.6-2.0%
The gigapascal-grade bainite steel plate having an ultra-high yield ratio according to claim 1, characterized in that at least one of the following is satisfied: The gigapascal-grade bainite steel plate having an ultra-high yield ratio according to claim 1, characterized in that, among other unavoidable impurities, P≦0.015% and/or S≦0.004%. The following chemical elements:
0<Cu≦0.2%, 0<Ni≦0.2%, 0<V≦0.2%, 0<Ce≦0.2%
The gigapascal-grade bainite steel plate having an ultra-high yield ratio according to claim 1, characterized in that it contains at least one of the following: The gigapascal-grade bainite steel plate with ultra-high yield ratio described in claim 1, characterized in that the microstructure further contains dispersedly precipitated nano-order, sub-micro-order or micro-order granular carbide precipitate phases, and the total phase proportion of the granular carbide precipitate phases + acicular lower bainite is 99% or more. A gigapascal-grade bainite steel plate having an ultra-high yield ratio according to claim 5, characterized in that the diameter of the maximum granular carbide precipitate phase is 2 μm or less. Gigapascal-grade bainite steel plate having an ultra-high yield ratio according to claim 1, characterized in that the yield strength is ≧950 MPa and the yield ratio is ≧0.95. An annealing process for gigapascal grade bainite steel sheet with ultra-high yield ratio according to any one of claims 1 to 7, characterized in that it comprises the following steps:
(a) a heating step, heating to a soaking temperature _Ts at a heating rate of 50°C/s or less, where _Ts is between 840 and 900°C;
(b) In the soaking stage, the temperature is kept at _Ts for 5 min or less;
(c) in a slow cooling step, cooling to (T _s -80) to (T _s -140) °C at a first cooling rate of 15 °C/s or less;
(d) in a quenching step, cooling at a second cooling rate of at least (130-Q)°C/s to ( _Ts -490) to ( _Ts- 440)°C;
(e) in a self-recovery temperature controlled cooling stage, cooling at a third cooling rate for 10 to 40 s, where [(Q-80)/12] ° C./s ≦ the third cooling rate ≦ [(Q-80)/8] ° C./s ;
(f) Finally, in an air cooling stage, the strip is air cooled to room temperature;
Here, Q = C x 180 + Si x 10 + Mn x 30 + Ni x 50 + Cr x 15 + Mo x 15 + B x 2000. Steps below:
(1) Smelting and foundry;
(2) hot rolling;
(3) Post-rolling cooling and coiling;
(4) pickling and cold rolling; and (5) the annealing process according to claim 8.
The gigapascal-class bainite steel plate having an ultra-high yield ratio is as described in any one of claims 1 to 7. A method for producing a gigapascal-class bainite steel plate having an ultra-high yield ratio. The manufacturing method described in claim 9, characterized in that in step (2), the heating temperature is 1150 to 1260°C; the finish rolling start temperature is 1100 to 1220°C, and the finish rolling end temperature is 900 to 950°C. The method according to claim 9, wherein in step (3), the cooling rate is 30 to 150° C./s and the coiling temperature is 450 to 580° C. The manufacturing method described in claim 9, characterized in that in step (4), the cold rolling reduction is 50% or more.

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