JP2023539649A - High strength low carbon martensitic high hole expandability steel and its manufacturing method - Google Patents

Abstract

Its chemical composition is in weight percentage: C 0.03-0.10%, Si 0.5-2.0%, Mn 1.0-2.0%, P≦0.02%, S≦0.003 %, Al 0.02-0.08%, N≦0.004%, Mo 0.1-0.5%, Ti 0.01-0.05%, O≦0.0030%, the balance is Fe. A low carbon martensitic high hole expandability steel with a tensile strength of 980 MPa or more and a method for manufacturing the same, which are other unavoidable impurities. The high hole expandability steel according to the present invention has yield strength≧800MPa, tensile strength≧980MPa, lateral elongation A50≧8%, hole expansion rate≧30%, and cold bending performance test (d≦4a, 180°). It has passed the test and can be used for passenger car chassis parts that require high strength and thin walls, such as control arms and subframes.

Description

TECHNICAL FIELD The present invention belongs to the field of high-strength steel, and particularly relates to a high-strength, low-carbon martensitic, high-hole expandability steel and a method for producing the same.

With the development of the national economy, the number of automobiles produced has increased significantly, and the amount of plate materials used has also continued to increase. China's domestic automobile industry produces automobile chassis parts, torsion beams, sedan subframes, wheel spokes and rims, front and rear axle assemblies, body structural parts, seats, clutches, seat belts, truck box panels, and protective nets. The original design of many car parts, such as car beams, requires the use of hot-rolled or pickled plates. Among these, chassis steel accounts for 24-34% of the total amount of steel used in automobiles.

Reducing the weight of passenger cars is not only a trend in the automotive industry, but also a regulatory requirement. Legislation stipulates fuel efficiency, but this is essentially a requirement for lighter bodies, and when reflected in materials, it becomes a requirement for higher strength, thinner walls, and lighter weight. Higher strength and lighter weight are inevitable requirements for future new cars, which will inevitably lead to increased levels of steel usage and changes in chassis structure: e.g., increased component complexity, resulting in lower material performance and Advances in surface requirements and forming techniques such as hydroforming, hot stamping, and laser welding have resulted in materials with higher strength, stamping, flanging, rebound, and fatigue performance.

The development of high-strength, high-hole-expandable steel in China not only has a relatively low strength level but also poor performance stability compared to overseas steels. For example, most of the high hole expandability steels used by automobile parts manufacturers in China are high hardness steels with a hardness of 600 MPa or less, and competition for high hole expandability steels with a level of 440 MPa or less is intensifying. Currently, high hole expandability steel with a tensile strength of 780 MPa level is gradually being mass-produced, but the requirements for elongation and hole expansion rate, which are two important indicators of forming, are also increasing. On the other hand, steel with high hole expandability of 980 MPa or more is still in the research, development and certification stage, and has not yet reached the stage of mass production. High-strength, high-hole expandability steels at higher strength levels, such as 1180 MPa and above, have not yet been developed by manufacturers. However, 980 MPa high hole expandability steel with higher strength and higher hole expansion rate will inevitably be the direction of future development. In order to meet the potential needs of future users, there is a need to develop steel with high hole expandability of 980 MPa or more that has good hole expandability.

There is almost no literature regarding high hole expandability steels at the 980 MPa level, and there is rather a blank slate regarding high hole expandability steels at the 1180 MPa level. Most of the existing patent documents related to this relate to high hole expandability steels at a level of 780 MPa or less. There is almost no literature regarding high hole expandability steel of 980 MPa or higher. Chinese patent application CN106119702A has a microstructure of granular bainite and a small amount of martensite, and a low carbon V-Ti microalloying design with trace amounts of Nb and Cr added as the main feature of its composition design at 980 MPa level. A hot rolled high expandability steel is disclosed. This invention is significantly different from the present invention in terms of components, processes, organization, etc.

According to the literature, the elongation of a material is usually inversely related to its hole expansion rate, ie, the higher the elongation, the lower the hole expansion rate, and conversely, the lower the elongation, the higher the hole expansion rate. Therefore, it is extremely difficult to obtain a high hole expandability steel that has high elongation and high hole expandability, and also has high strength. Furthermore, if the reinforcement mechanism is the same or similar, the higher the strength of the material, the lower the hole expansion rate.

In order to obtain a steel material with excellent plasticity and hole expansion/flangeability, it is necessary to maintain a better balance between the two. Of course, the hole expansion rate of a material is closely related to many factors, but among them, the homogeneity of the structure, the control level of inclusions and segregation, the type of structure, and the measurement of the hole expansion rate are the most important factors. It is. Generally, a single, homogeneous structure is advantageous in improving the hole expansion rate, but a two-phase or multiphase structure is disadvantageous in improving the hole expansion rate.

An object of the present invention is to provide a low carbon martensitic high hole expandability steel having a tensile strength of 980 MPa or more, and a method for producing the same. The elongation A ₅₀ is ≧8%, the hole expansion rate is ≧30%, preferably ≧50%, and it can be used for passenger car chassis parts such as control arms and subframes that require high strength and thin walls. In some embodiments, the high hole expandability steel has a yield strength ≧900 MPa, a tensile strength ≧1180 MPa, a lateral elongation A ₅₀ ≧10%, and a hole expansion rate ≧30%. In some embodiments, the high hole expandability steel of the present invention passes a cold bend performance (d≦4a, 180°) test.

In order to achieve the above purpose, the technical solution of the present invention is:
According to the composition design of the present invention, the C content is designed to be low in order to ensure excellent weldability during use by the user, and good hole expandability and impact toughness of the obtained martensitic structure; In order to improve the plasticity of the material by obtaining more retained austenite, the Si content is designed to be high; at the same time, it contributes to lowering the non-recrystallization temperature of the steel, resulting in a wider rolling finish temperature range. The Si content is designed to be high in order to complete the dynamic recrystallization process in the steel, thereby refining the size of the austenite crystal grains and the final martensite crystal grains, and improving the plasticity and hole expansion rate. .

Specifically, the low carbon martensitic high hole expandability steel according to the present invention having a tensile strength of 980 MPa or more has a chemical composition in weight percentages of 0.03 to 0.10% C and 0.5 to 2.0% Si. 0%, Mn 1.0-2.0%, P≦0.02%, S≦0.003%, Al 0.02-0.08%, N≦0.004%, Mo 0.1-0 .5%, Ti 0.01-0.05%, O≦0.0030%, the remainder being Fe and other unavoidable impurities.

Cr≦0.5%, B≦0.002%, Ca≦0.005%, Nb≦0.06%, V≦0.05%, Cu≦0.5%, Ni≦0.5% It further contains one or more elements.

In some embodiments, the low carbon martensitic high hole expandability steel having a tensile strength of 980 MPa or more according to the present invention has a chemical composition in weight percentage of C 0.03 to 0.10%, Si 0.5 to 2.0%, Mn 1.0-2.0%, P≦0.02%, S≦0.003%, Al 0.02-0.08%, N≦0.004%, Mo 0.1 ~0.5%, Ti 0.01~0.05%, O≦0.0030%, Cr≦0.5%, B≦0.002%, Ca≦0.005%, Nb≦0.06% , V≦0.05%, Cu≦0.5%, Ni≦0.5%, the remainder being Fe and other unavoidable impurities; Preferably, the low carbon martensitic high hole expandability steel contains Cr, Contains at least one of B, Ca, Nb, V, Cu, and Ni. In some preferred embodiments, the low carbon martensitic high hole expandability steel contains at least Ni, preferably the Ni content is 0.1 to 0.5%, more preferably 0.1 to 0.5%. It is 0.3%. In some preferred embodiments, the low carbon martensitic high hole expandability steel contains at least Cr and/or B, preferably the content of Cr is 0.1-0.5%, preferably 0. The content of B is preferably 0.0005 to 0.002%.

In some embodiments, the preferable content of Cr is 0.2 to 0.4%; the preferable content of B is 0.0005 to 0.0015%; the preferable content of Ca is ≦0.002%; the preferable contents of Nb and V are each ≦0.03%; the preferable contents of Cu and Ni are ≦0.3%, respectively.

Furthermore, the microstructure of the low carbon martensitic high hole expandability steel having a tensile strength of 980 MPa or more according to the present invention is martensite. In some embodiments, the microstructure is martensite or tempered martensite. Preferably, by volume, the content of retained austenite in the microstructure of the low carbon martensitic high hole expandability steel is ≦5%. In some embodiments, the austenite content is 0.5-5%.

Furthermore, the low carbon martensitic high hole expandability steel having a tensile strength of 980 MPa or more according to the present invention has a yield strength ≧800 MPa, preferably ≧900 MPa, a tensile strength ≧980 MPa, preferably ≧1180 MPa, and a lateral elongation A ₅₀ The hole expansion rate is ≧8%, preferably ≧10%, and the hole expansion rate is ≧30%, preferably ≧50%.

Preferably, the low carbon martensitic high hole expandability steel having a tensile strength of 980 MPa or more according to the present invention has a -40°C impact toughness of ≧60J, preferably ≧70J. In some embodiments, the low carbon martensitic high hole expandability steel with a tensile strength of 980 MPa or more has a -40°C impact toughness ≧140J, preferably ≧150J, more preferably ≧160J.

Preferably, the low carbon martensitic high hole expandability steel having a tensile strength of 980 MPa or more according to the present invention passes a cold bending performance test (d≦4a, 180°).

In some embodiments, the present invention provides chemical compositions, in weight percentages, of C 0.03-0.06%, Si 0.5-2.0%, Mn 1.0-2.0%, P ≦0.02%, S≦0.003%, Al 0.02-0.08%, N≦0.004%, Mo 0.1-0.5%, Ti 0.01-0.05%, The present invention provides an ultra-low carbon martensitic high hole expandability steel with a tensile strength of 980 MPa or more, in which O≦0.0030% and the balance being Fe and other unavoidable impurities.

Furthermore, the ultra-low carbon martensitic high hole expandability steel with a tensile strength of 980 MPa or more has Cr≦0.5%, B≦0.002%, Ca≦0.005%, Nb≦0.06%, V It further contains one or more elements selected from ≦0.05%, Cu≦0.5%, and Ni≦0.5%.

In some embodiments, the ultra-low carbon martensitic high hole expandability steel having a tensile strength of 980 MPa or more has a chemical composition in weight percentages of 0.03 to 0.06% C, 0.5 to 2% Si. .0%, Mn 1.0-2.0%, P≦0.02%, S≦0.003%, Al 0.02-0.08%, N≦0.004%, Mo 0.1- 0.5%, Ti 0.01-0.05%, O≦0.0030%, Cr≦0.5%, B≦0.002%, Ca≦0.005%, Nb≦0.06%, V≦0.05%, Cu≦0.5%, Ni≦0.5%, and the remainder is Fe and other unavoidable impurities. Preferably, the ultra-low carbon martensitic high hole expandability steel having a tensile strength of 980 MPa or more contains at least Cr and/or B, preferably, the content of Cr is 0.1 to 0.5%, preferably is 0.2 to 0.4%, and preferably the B content is 0.0005 to 0.002%.

In some embodiments, the preferable content of Cr is 0.2 to 0.4%; the preferable content of B is 0.0005 to 0.0015%; the preferable content of Ca is ≦0.002%; the preferable contents of Nb and V are each ≦0.03%; the preferable contents of Cu and Ni are ≦0.3%, respectively.

In a preferred embodiment, the microstructure of the ultra-low carbon martensitic high hole expandability steel having a tensile strength of 980 MPa or more is martensite or tempered martensite. In some embodiments, the microstructure of the ultra-low carbon martensitic high hole expandability steel having a tensile strength of 980 MPa or more further includes a small amount of retained austenite. Preferably, by volume, the content of retained austenite in the microstructure of the ultra-low carbon martensitic high hole expandability steel is ≦5%. In some embodiments, the austenite content is 0.5-5%.

In a preferred embodiment, the ultra-low carbon martensitic high hole expandability steel having a tensile strength of 980 MPa or more has a yield strength ≧800 MPa, preferably ≧820 MPa, a tensile strength ≧980 MPa, preferably ≧1000 MPa, and a lateral elongation. A ₅₀ ≧8%, preferably ≧10%, hole expansion rate ≧50%, preferably ≧55%, and passes the cold bending performance test (d≦4a, 180°). In a preferred embodiment, the ultra-low carbon martensitic high hole expandability steel having a tensile strength of 980 MPa or more has a -40°C impact toughness of ≧140 J, preferably ≧150 J, more preferably ≧160 J. In a preferred embodiment, the ultra-low carbon martensitic high hole expandability steel with a tensile strength of 980 MPa or more has a yield strength of 800 to 890 MPa, a tensile strength of 980 to 1150 MPa, and a lateral elongation _A50 of 8 to 13%. The hole expansion rate is 50 to 85%, the -40°C impact toughness is 140 to 185 J, and it passes the cold bending performance test (d≦4a, 180°). Preferably, the microstructure of the ultra-low carbon martensitic high hole expandability steel with a tensile strength of 980 MPa or more is martensite and retained austenite, provided that the volume percentage of retained austenite in the microstructure is ≦5 as described above. %.

In some embodiments, the low carbon martensitic high hole expandability steel with a tensile strength of 980 MPa or more according to the present invention is a high plasticity high hole expandability steel with a tensile strength of 1180 MPa or more, and the chemical composition thereof in weight percentage is C 0.06-0.10%, Si 0.8-2.0%, Mn 1.5-2.0%, P≦0.02%, S≦0.003%, Al 0.02-0 .08%, N≦0.004%, Mo 0.1-0.5%, Ti 0.01-0.05%, O≦0.0030%, the remainder being Fe and other unavoidable impurities.

Further, the high plasticity and high hole expandability steel having a tensile strength of 1180 MPa or more has Cr≦0.5%, B≦0.002%, Ca≦0.005%, Nb≦0.06%, V≦0. 05%, Cu≦0.5%, and Ni≦0.5%.

In some embodiments, the high plasticity and high hole expandability steel having a tensile strength of 1180 MPa or more has a chemical composition in weight percentage of 0.06 to 0.10% C and 0.8 to 2.0% Si. , Mn 1.5-2.0%, P≦0.02%, S≦0.003%, Al 0.02-0.08%, N≦0.004%, Mo 0.1-0.5 %, Ti 0.01-0.05%, O≦0.0030%, Cr≦0.5%, B≦0.002%, Ca≦0.005%, Nb≦0.06%, V≦0 .05%, Cu≦0.5%, Ni≦0.5%, the remainder being Fe and other unavoidable impurities. Preferably, in some embodiments, the high plasticity and high hole expandability steel having a tensile strength of 1180 MPa or more contains at least Ni, and preferably the Ni content is 0.1 to 0.3%. Preferably, the high plasticity and high hole expandability steel having a tensile strength of 1180 MPa or more contains at least Cr and/or B, preferably the content of Cr is 0.1 to 0.5%, preferably 0.5%. The content of B is 2 to 0.4%, preferably 0.0005 to 0.002%.

In some embodiments, the preferred content of Cr is 0.2 to 0.4%; the preferred content of Cu and Ni is each ≦0.3%; the preferred content of Nb and V is The amounts are each ≦0.03%; the preferable content of B is 0.0005-0.0015%; the preferable content of Ca is ≦0.002%.

In a preferred embodiment, the microstructure of the high plasticity and high hole expandability steel having a tensile strength of 1180 MPa or more is tempered martensite. In some embodiments, the microstructure of the high plasticity, high hole expandability steel having a tensile strength of 1180 MPa or more further includes a small amount of retained austenite. Preferably, by volume, the content of retained austenite in the microstructure is ≦5%. In some embodiments, the austenite content is 2-5%.

In a preferred embodiment, the high plasticity and high hole expandability steel having a tensile strength of 1180 MPa or more has a yield strength ≧900 MPa, preferably ≧930 MPa, more preferably ≧950 MPa, and has a tensile strength ≧1180 MPa, preferably ≧1200 MPa, or more. It is preferably ≧1220 MPa, the lateral elongation A ₅₀ ≧10%, and the hole expansion rate ≧30%, preferably ≧35%. In a preferred embodiment, the ultra-low carbon martensitic high hole expandability steel having a tensile strength of 980 MPa or more has a -40°C impact toughness of ≧60 J, preferably ≧70 J, more preferably ≧80 J. Preferably, the high plasticity and high hole expandability steel with a tensile strength of 1180 MPa or more passes a cold bending performance test (d≦4a, 180°).

In a preferred embodiment, the high plasticity and high hole expandability steel having a tensile strength of 1180 MPa or more has a yield strength of 900 to 1000 MPa, a tensile strength of 1200 to 1280 MPa, a lateral elongation of 10 to 13%, and a hole expansion rate of 10 to 13%. 30-50%, -40°C impact toughness is 60-100J. Preferably, the microstructure of the high plasticity and high hole expandability steel having a tensile strength of 1180 MPa or more is tempered martensite and retained austenite, provided that the volume percentage of retained austenite in the microstructure is ≦5% as described above. be. Preferably, the high plasticity and high hole expandability steel with a tensile strength of 1180 MPa or more passes a cold bending performance test (d≦4a, 180°).

In another preferred embodiment, the high plasticity and high hole expandability steel having a tensile strength of 1180 MPa or more has a yield strength of 940 to 1000 MPa, a tensile strength of 1210 to 1300 MPa, a lateral elongation of 10 to 13%, and a hole expansion property of 1180 MPa or more. It has a -40°C impact toughness of 80 to 110 J, and passes the cold bending performance test (d≦4a, 180°). Preferably, the microstructure of the high plasticity and high hole expandability steel having a tensile strength of 1180 MPa or more is tempered martensite and retained austenite, provided that the volume percentage of retained austenite in the microstructure is ≦5% as described above. be.

In the compositional design of the high hole expandability steel according to the present invention:
Carbon is a basic element in steel and is also one of the important elements for the present invention. Carbon expands the austenite phase region and stabilizes austenite. As an interstitial atom in steel, carbon plays a very important role in improving the strength of steel, and has the greatest influence on the yield strength and tensile strength of steel. In the present invention, the structure to be obtained is low carbon or ultra-low carbon martensite, so in order to obtain high strength steel that can reach the tensile strength level of 980 MPa, the carbon content must be maintained at 0.03% or more. Otherwise, if the carbon content is less than 0.03%, its tensile strength cannot reach 980 MPa even if it is completely quenched to room temperature. However, it is also not permissible for the carbon content to exceed 0.10%, and if the carbon content is too high, the strength of the low carbon martensite formed will be too high, and the elongation and hole expansion rate will be low. Therefore, the carbon content should be controlled between 0.03 and 0.10%. In some embodiments, a preferred range of C is between 0.04 and 0.055%. In some other embodiments, the preferred range for C is 0.07-0.09%.

Silicon is a basic element in steel and is also one of the important elements for the present invention. Increasing the Si content can not only improve the solid solution strengthening effect, but more importantly, can also play the following two roles. One is to significantly lower the unrecrystallized temperature of the steel, allowing the steel to complete dynamic recrystallization within a very low temperature range. Then, in the actual rolling process, rolling can be carried out within a relatively wide rolling end temperature range, for example, 800 to 900°C, and the anisotropy of the structure can be significantly improved. , can reduce the anisotropy of the final martensitic structure, which contributes to improving the strength and plasticity, and also contributes to obtaining a good hole expansion rate; another important role of Si. This suppresses the precipitation of cementite, and under appropriate rolling process conditions, especially when obtaining a structure mainly composed of martensite, retained austenite can be retained in a predetermined amount, contributing to improved elongation. As is well known, the elongation of martensite is usually the lowest at the same strength level, but retaining a certain amount of stable retained austenite is one of the important measures to improve the elongation of martensite. This role of Si generally starts to appear when its content is 0.5% or more, but too much Si content is also not allowed, otherwise the rolling force load will be affected in the actual rolling process. becomes excessive, which is disadvantageous to stable product production. Therefore, in steel, the Si content is usually controlled between 0.5 and 2.0%, with a preferred range between 0.8 and 1.4%. In some embodiments, Si content is controlled between 1.0 and 1.4%.

Manganese is the most basic element in steel and is also one of the most important elements for the present invention. As is well known, Mn is an important element that expands the austenite phase region, reduces the critical quenching rate of steel, stabilizes austenite, refines grains, and delays the transformation from austenite to pearlite. can be done. In the present invention, in order to ensure the strength of the steel sheet and stabilize retained austenite, the Mn content is usually controlled to 1.0% or more, but the Mn content may exceed 2.0%. This is normally not allowed, otherwise Mn segregation is likely to occur during steel manufacturing, and hot cracking is also likely to occur during continuous slab casting. Therefore, in steel, the Mn content is usually controlled to 1.0 to 2.0%, with a preferred range of 1.4 to 1.8%. In some embodiments, Mn content is controlled between 1.6-1.9%.

Phosphorus is an impurity element in steel. P is extremely likely to be unevenly distributed at grain boundaries, and when the P content in steel is high (≧0.1%), it forms Fe ₂ P and precipitates around the grains, reducing the plasticity and toughness of the steel. Therefore, the lower the content, the better, and it is generally preferable to control it within 0.02%, and the steel manufacturing cost does not increase.

Sulfur is an impurity element in steel. S in steel usually combines with Mn to form MnS inclusions, especially when both S and Mn contents are high, a lot of MnS is formed in steel, but MnS itself contains some MnS has plasticity and deforms along the rolling direction during the subsequent rolling process, which not only reduces the transverse plasticity of the steel sheet but also increases the anisotropy of the structure, which is detrimental to hole expandability. be. Therefore, the lower the S content in steel, the better. Considering that the Mn content in the present invention must be at a high level, the S content must be strictly controlled in order to reduce the MnS content. , it is necessary to control the S content within 0.003%, and the preferred range is 0.0015% or less.

Aluminum primarily plays the role of deoxidation and nitrogen fixation in steel. Assuming the presence of strong carbide-forming elements such as Ti, Nb, and V, Al mainly plays the role of deoxidation and crystal grain refinement. In the present invention, Al is a general deoxidizing element and grain refining element, and its content may be normally controlled to 0.02 to 0.08%. If the Al content is less than 0.02%, it cannot contribute to grain refinement, and similarly, if the Al content is 0.08% or more, the grain refinement effect is saturated. Therefore, in steel, the Al content should normally be controlled between 0.02 and 0.08%, but the preferred range is between 0.02 and 0.05%.

Nitrogen belongs to an impurity element in the present invention, and the lower the content, the better. However, nitrogen is an unavoidable element in the steelmaking process. Although the content is small, when combined with strong carbide-forming elements such as Ti, the formed TiN particles have a very negative effect on the performance of the steel, especially the hole expandability. In addition, since TiN has a square shape, there is a large stress concentration between its sharp corners and the substrate, and during the hole expansion process, cracks are likely to occur due to stress concentration between the TiN and the substrate. , greatly reduces hole expandability. On the premise that the nitrogen content is controlled as much as possible, the content of strong carbide-forming elements such as Ti is preferably as low as possible. In the present invention, by adding a small amount of Ti to fix nitrogen, the adverse effects of TiN are reduced as much as possible. Therefore, the nitrogen content should be controlled to 0.004% or less, and the preferred range is 0.003% or less.

Titanium is one of the important elements for the present invention. Ti plays two main roles in the present invention: one, it combines with the impurity element N in the steel to form TiN, and plays the role of some "nitrogen fixation"; The purpose is to form a certain number of dispersed fine TiN in the subsequent welding process, suppress the size of austenite crystal grains, refine the structure, and improve low-temperature toughness. Therefore, in steel, the range of Ti content is controlled to be 0.01 to 0.05%, and the preferred range is 0.01 to 0.03%.

Molybdenum is one of the important elements for the present invention. Adding molybdenum to steel can significantly retard the transformation of ferrite and pearlite. This role of molybdenum is advantageous in adjusting various processes in the actual rolling process. For example, at the end of rolling, stepwise cooling may be performed, or air cooling may be followed by water cooling. In the present invention, even if a process of air cooling followed by water cooling or a process of directly water cooling after rolling is adopted, by adding molybdenum, structures such as ferrite and pearlite will not be formed during the air cooling process. At the same time, dynamic recovery of deformed austenite occurs during the air cooling process, contributing to improving the homogeneity of the structure; molybdenum has strong welding softening resistance. The main purpose of the present invention is to obtain a structure consisting of a single low carbon martensite and a small amount of retained austenite. However, since low carbon martensite tends to soften after welding, molybdenum is added in a predetermined amount. By doing so, the degree of weld softening can be effectively reduced. Therefore, the molybdenum content should be controlled between 0.1 and 0.5%, with a preferred range of 0.15 and 0.35%.

Chromium is one of the elements that can be added to the present invention. The addition of a small amount of chromium does not improve the hardenability of the steel, but rather combines with the B phase and contributes to the formation of an acicular ferrite structure in the weld heat affected zone after welding, which reduces the welding heat. This is to significantly improve the low-temperature toughness of the affected zone. Since the final applied part according to the present invention is a chassis product for a passenger car, the low-temperature toughness of the weld heat affected zone is an important index. In addition to ensuring that the strength of the weld heat-affected zone does not decrease too much, the low-temperature toughness of the weld heat-affected zone must also meet certain requirements. In addition, chromium itself has some welding softening resistance effect. Therefore, in steel, the amount of chromium added is usually ≦0.5%, with a preferred range of 0.2 to 0.4%.

Boron is one of the elements that can be added to the present invention. The role of boron in steel is mainly to suppress the formation of pro-eutectoid ferrite by being unevenly distributed in prior austenite grain boundaries; adding boron to steel can also greatly improve the hardenability of steel. . However, in the present invention, the main purpose of adding a trace amount of boron is not to improve hardenability, but to combine with the chromium phase, improve the structure of the weld heat affected zone, and achieve excellent low-temperature toughness. This is to obtain an acicular ferrite structure. The boron element added to steel is usually controlled to 0.002% or less, and the preferred range is between 0.0005 and 0.0015%.

Calcium is an element that can be added to the present invention. Calcium improves the morphology of sulfides such as MnS and changes long striped sulfides such as MnS to spherical CaS, contributing to the improvement of the morphology of inclusions, thereby changing the long striped sulfides to spherical CaS. Although the negative effect on hole expandability can be reduced, if too much calcium is added, the number of calcium oxides increases, which is disadvantageous for hole expandability. Therefore, in steel, the amount of calcium added is usually ≦0.005%, with a preferred range being ≦0.002%.

Oxygen is an unavoidable element in the steelmaking process, and for the present invention, the O content in the steel can normally reach 30 ppm or less after deoxidation, without any obvious negative impact on the performance of the steel plate. Therefore, in steel, the O content may be controlled within 30 ppm.

Niobium is one of the elements that can be added to the present invention. Niobium, like titanium, is a strong carbide-forming element in steel, and adding niobium to steel can significantly increase the non-recrystallization temperature of steel, resulting in deformed austenite with higher dislocation density in the finish rolling stage. can be obtained, and the final metamorphosed structure can be refined in the subsequent metamorphosis process. However, the amount of niobium added should not be too large; on the other hand, if the amount of niobium added exceeds 0.06%, it tends to form relatively coarse niobium carbon nitride in the structure, and some of the carbon atoms is consumed, reducing the precipitation strengthening effect of carbides. In addition, when the niobium content is high, anisotropy tends to occur in the austenite structure in the hot rolled state, which is carried over to the final structure in the subsequent cooling transformation process, which is disadvantageous for hole expandability. Therefore, in steel, the niobium content is usually controlled to ≦0.06%, with a preferred range being ≦0.03%.

Vanadium is an element that can be added to the present invention. Vanadium, like titanium and niobium, is a strong carbide-forming element. However, vanadium carbide has a low solid solution temperature and low precipitation temperature, and is usually completely dissolved in austenite during the finish rolling stage. As the temperature drops and transformation begins, vanadium begins to form in the ferrite. In the present invention, the purpose of adding vanadium is mainly to improve the softening resistance of the weld heat affected zone together with molybdenum. In terms of weld softening resistance effect, molybdenum and vanadium have the strongest effect, but when molybdenum is contained, vanadium may be selectively added. Thus, in steel, the amount of vanadium added is usually ≦0.05%, with a preferred range of ≦0.03%.

Copper is one of the elements that can be added to the present invention. By adding copper to steel, the corrosion resistance of steel can be improved, and when added together with P element, the corrosion resistance is even better; when the amount of Cu added exceeds 1%, under certain conditions It can form an ε-Cu precipitate phase and exhibit a strong precipitation strengthening effect. However, the addition of Cu tends to cause "Cu embrittlement" phenomenon during the rolling process, and in some application environments, in order to fully utilize the corrosion resistance improvement effect of Cu without significantly causing "Cu embrittlement" The elemental content is usually controlled within 0.5%, with a preferred range being within 0.3%.

Nickel is one of the elements that can be added to the present invention. Adding nickel to steel provides a certain degree of corrosion resistance, but the corrosion resistance effect is weaker than that of copper. Adding nickel to steel does not affect the tensile performance of the steel much, but it improves the structure and precipitate phase of the steel. The low-temperature toughness of the steel can be greatly improved by making it fine; and by adding a small amount of nickel to steel to which copper has been added, the occurrence of "Cu embrittlement" can be suppressed. The addition of large amounts of nickel has no obvious negative effect on the performance of the steel itself. Adding copper and nickel at the same time not only improves corrosion resistance, but also refines the structure and precipitated phases of steel, significantly improving low-temperature toughness. However, since both copper and nickel are relatively expensive alloying elements, to minimize the cost of the alloy, the nickel addition is typically ≦0.5%, with a preferred range of ≦0.3%.

The method for manufacturing low carbon martensitic high hole expandability steel having a tensile strength of 980 MPa or more according to the present invention includes the following steps:
1) Smelting and casting Smelting in a converter or electric furnace according to the stated composition, secondary smelting in a vacuum furnace, and then casting into billets or ingots;
2) Reheat the billet or ingot at a heating temperature of 1100 to 1200°C and a heat retention time of 1 to 2 hours;
3) Hot rolling The main purpose is to refine the austenite crystal grains, the rolling start temperature is 950 to 1100°C, 3 to 5 passes are performed at 950°C or higher under heavy reduction, and the cumulative amount of deformation is ≧50%. Optionally, after setting the temperature of the intermediate billet to 900-950°C, final rolling is performed for 3-7 passes, and the cumulative amount of deformation is ≧70%; the rolling end temperature is 800-950°C;
4) Cooling First, perform air cooling for 0 to 10 seconds to ensure dynamic recovery and dynamic recrystallization, then cool at a cooling rate of 30°C/s to a certain temperature below the Ms point (between room temperature and Ms point). The steel strip is water-cooled, coiled, and then cooled to room temperature (preferably at a cooling rate of ≦20°C/h); alternatively, it is first air-cooled for 0-10 seconds and then directly to room temperature≧30°C. The steel strip is water-cooled and coiled at a cooling rate of /s; alternatively, it is first air-cooled for 0 to 10 seconds, and then cooled at a rate of ≥30°C/s to a certain temperature below the Ms point, which is the starting point of martensitic transformation. Water-cool the steel plate at a cooling rate, coil it, and then slowly cool it to room temperature (preferably at a cooling rate of ≦20°C/h);
5) Pickling Adjust the pickling speed of the steel strip in the range of 30 to 100 m/min, control the pickling temperature between 75 to 85°C, and adjust the tensile straightening rate to reduce the elongation loss of the steel strip. ≦2%, then rinse, dry the strip surface and apply oil.

Preferably, after pickling in step 5), the steel strip is rinsed at a temperature range of 35-50° C. to ensure the surface quality, and the surface is dried at a temperature of 120-140° C. and coated with oil.

In some embodiments, between steps 4) and 5), further step 4-1): annealing with bell-shaped annealing, the heating rate is ≧20 ° C / h, and the bell-shaped annealing temperature is 100 ~ The temperature is 300° C., and the bell-type annealing time is 12 to 48 h; the steel plate is cooled to ≦100° C. and tapped at a cooling rate of ≦50° C./h.

In some embodiments, the method of manufacturing a low carbon martensitic high hole expandability steel having a tensile strength of 980 MPa or more according to the present invention includes the following steps:
1) Smelting and casting Smelting in a converter or electric furnace according to the stated composition, secondary smelting in a vacuum furnace, and then casting into billets or ingots;
2) Reheat the billet or ingot at a heating temperature of 1100 to 1200°C and a heat retention time of 1 to 2 hours;
3) Hot rolling The main purpose is to refine the austenite crystal grains, the rolling start temperature is set at 950 to 1100°C, and 3 to 5 passes are performed at 950°C or higher under heavy reduction, and the cumulative deformation is ≧50%, preferably. is ≧60%; Next, after setting the temperature of the intermediate billet to 920 to 950°C, rolling is finally performed for 3 to 5 passes, and the cumulative amount of deformation is ≧70%, preferably ≧85%; The rolling end temperature is 800 to 920°C;
4) Cooling First, perform air cooling for 0 to 10 seconds to allow dynamic recovery and dynamic recrystallization, then cool to a certain temperature below the Ms point (between room temperature and the Ms point) at ≥50°C/s, preferably at 50°C/s. Water-cooling the steel strip at a cooling rate of ~85°C/s, coiling, and then cooling to room temperature (preferably at a cooling rate of ≦20°C/h);
5) Pickling Adjust the pickling speed of the steel strip in the range of 30 to 100 m/min, control the pickling temperature between 75 to 85°C, and adjust the tensile straightening rate to reduce the elongation loss of the steel strip. ≦2%, then rinse, dry the strip surface and apply oil.

In some embodiments, the method for manufacturing high plasticity and high hole expandability steel with a tensile strength of 1180 MPa or more according to the present invention includes the following steps:
1) Smelting and casting Smelting in a converter or electric furnace according to the stated composition, secondary smelting in a vacuum furnace, and then casting into billets or ingots;
2) Reheat the billet or ingot at a heating temperature of 1100 to 1200°C and a heat retention time of 1 to 2 hours;
3) Hot rolling The rolling start temperature is 950 to 1100°C, and 3 to 5 passes are performed at 950°C or higher under heavy reduction, and the cumulative deformation is ≧50%, preferably ≧60%; then, rolling is carried out. Perform 3 to 7 passes, and set the cumulative deformation amount to ≧70%, preferably ≧85%; set the rolling end temperature to 800 to 950°C;
4) Cooling First air cooling for 0-10 seconds, then water-cooling the steel strip to room temperature at a cooling rate of ≧30°C/s, preferably 30-65°C/s, and winding;
5) Annealing Anneal using bell-shaped annealing, heating rate ≧20°C/h, preferably 20-40°C/h, bell-shaped annealing temperature 100-300°C, and bell-shaped annealing time 12-48h. ; Cool the steel plate to 100°C or less at a cooling rate of ≦50°C/h, preferably 15 to 50°C/h, and tap the steel plate;
6) Pickling Adjust the pickling speed of the steel strip in the range of 30 to 90 m/min, control the pickling temperature between 75 to 85°C, and control the tensile straightening rate to ≦1.5%. Rinse at a temperature range of 35-50°C, dry the surface at a temperature of 120-140°C, and apply oil.

Preferably, after the pickling in step 6), the surface is rinsed at a temperature of 35 to 50°C, dried at a temperature of 120 to 140°C, and coated with oil.

In some other embodiments, the method for manufacturing high plasticity and high hole expandability steel with a tensile strength of 1180 MPa or more according to the present invention includes the following steps:
1) Smelting and casting Smelting in a converter or electric furnace according to the stated composition, secondary smelting in a vacuum furnace, and then casting into billets or ingots;
2) Reheat the billet or ingot at a heating temperature of 1100 to 1200°C and a heat retention time of 1 to 2 hours;
3) Hot rolling The rolling start temperature is 950 to 1100°C, and 3 to 5 passes are performed at 950°C or higher and under large reduction, and the cumulative deformation is ≧50%, preferably ≧60%; Next, the intermediate billet After setting the temperature to 900 to 950°C, perform 3 to 7 passes of rolling, and make the cumulative amount of deformation ≧70%, preferably ≧85%; the rolling end temperature should be 800 to 900°C;
4) Cooling First, air cooling is performed for 0 to 10 seconds, and then the steel plate is water cooled at a cooling rate of ≧30°C/s, preferably 30 to 70°C/s, to a certain temperature below the Ms point, which is the starting point of martensitic transformation. and then winding it up and slowly cooling it to room temperature (preferably at a cooling rate of ≦20°C/h);
5) Annealing Anneal using bell-shaped annealing, heating rate ≧20°C/h, preferably 20-50°C/h, bell-shaped annealing temperature 100-300°C, and bell-shaped annealing time 12-48h. ; Cool the steel plate to ≦100°C at a cooling rate of ≦50°C/h, preferably 20 to 50°C/h, and tap the steel plate;
6) Pickling Adjust the pickling speed of the steel strip in the range of 30 to 90 m/min, control the pickling temperature between 75 to 85°C, and adjust the tensile straightening rate to reduce the elongation loss of the steel strip. ≦1.5%, then rinse, dry the strip surface and apply oil.

Preferably, after the pickling in step 6), the surface is rinsed at a temperature of 35 to 50°C, dried at a temperature of 120 to 140°C, and coated with oil.

The innovations of the present invention are as follows.
According to the component design of the present invention, the C content is designed to be low in order to ensure excellent weldability during use by the user, and good hole expandability and impact toughness of the obtained martensitic structure. In the case of tensile strength≧1180MPa, the lower the carbon content, the better in satisfying the tensile strength≧1180MPa. In line with the process, the Si content is designed higher to obtain more retained austenite and improve the plasticity of the material; at the same time, it contributes to lowering the unrecrystallized temperature of the steel, resulting in a wider final The Si content is increased to allow the steel to complete a dynamic recrystallization process within the rolling temperature range, thereby refining the size of the austenite grains and the final martensite grains and improving the plasticity and hole expansion rate. design. In addition, in the process of bell-shaped annealing, by removing part of the quenching stress, the purpose of improving the homogeneity of the structure and improving the plasticity and hole expansion rate can be achieved.

In composition design, we adopt the design concept of low carbon martensite and add more silicon to suppress and reduce the formation of cementite, while lowering the non-recrystallization temperature and improving rolling and post-rolling air cooling. By carrying out the rolling in a relatively wide rolling end temperature range, it is possible to obtain homogeneous and equiaxed prior austenite grains with fine crystal grains, and finally to obtain a homogeneous structure of martensite and retained austenite structure. Retained austenite gives steel sheets high plasticity and cold bending performance, martensite gives them high strength, and a homogeneous, fine structure gives them better hole expandability and low-temperature toughness.

In the design of the rolling process, the rolling process should be completed as quickly as possible in the rough rolling and finishing rolling stages. After rolling, air cooling is first performed for a predetermined period of time. The main purpose of air cooling is the composition design, which is rich in manganese and molybdenum, manganese is an element that stabilizes austenite, and molybdenum significantly retards ferrite and pearlite transformation. Therefore, during the process of air cooling for a predetermined period of time, the rolled deformed austenite undergoes dynamic recrystallization and relaxation processes without being transformed, that is, without forming a ferrite structure. When deformed austenite undergoes dynamic recrystallization, it can form austenite with a homogeneous structure and quasi-equiaxed structure, and after relaxation, the dislocations inside the austenite grains are significantly reduced, and the combination of both makes it possible to form a quasi-equiaxed austenite with a homogeneous structure. Martensite with a homogeneous structure is obtained during the quenching process. In order to obtain a martensitic structure, the water cooling rate should exceed the critical cooling rate of low carbon martensite. It is necessary to set it to 30 degrees C/s or more.

Since the microstructure according to the present invention is low carbon or ultra-low carbon martensite, after rolling, the steel strip may be cooled to below Ms, which is the martensitic transformation starting point, at a cooling rate exceeding the critical cooling rate. The content of retained austenite at room temperature also differs depending on the cooling stop temperature. There is usually an optimum quenching/cooling stop temperature range, which varies depending on the alloy composition, but is generally between 150 and 350°C. In order to obtain high-strength steel with good plasticity and hole expansion rate, it is necessary to quench the steel strip to a certain temperature range below the Ms point, but according to theoretical calculations and actual tests, the steel strip must be quenched to When quenched to 400°C, a structure with excellent overall performance can be obtained. Further, when the quenching temperature is 400° C., although there is a large number of retained austenite, a bainite structure is generated in the structure, making it impossible to satisfy the strength requirement of 980 MPa or more. Due to the above reasons, it is necessary to control the winding temperature to ≦400°C. Based on these innovative ingredients and process design concepts, the present invention has developed an ultra-low carbon martensitic high hole expandability steel at the 980 MPa level that has excellent properties such as strength, plasticity, toughness, cold bending, and hole expandability. can be obtained.

In some embodiments, the microstructure according to the present invention is low carbon tempered martensite, so that after rolling, the steel strip may be cooled to room temperature at a cooling rate exceeding the critical cooling rate, and the subsequent bell annealing may be performed. By controlling the bell-shaped annealing temperature and time within a predetermined range during the process, it is possible to obtain ultra-high strength steel with uniform properties such as strength, plasticity, and hole expandability.

In the process of bell-type annealing, the steel coil is first heated to 100 to 300 °C at a rate of 20 °C/s or more, and kept in that temperature range for a long time of 12 to 48 hours, thereby making the temperature of the entire steel coil uniform. , contributes to stabilization of organization and performance. The lower the insulating temperature, the longer the insulating time will be; conversely, the higher the insulating temperature, the shorter the insulating time will be. Finally, the steel coil may be cooled to 100° C. or less at a cooling rate of ≦50° C./s, tapped from a bell-shaped annealing furnace, and allowed to cool naturally.

Generally, the bell annealing temperature and the bell annealing time are inversely proportional; the lower the bell annealing temperature, the longer the bell annealing time; conversely, the higher the bell annealing temperature, the longer the bell annealing time. Becomes shorter. If the bell-shaped annealing temperature is less than 100℃, the strength will be high, but the hole expansion rate will be low, and it will not be possible to reach a hole expansion rate of 30% or more; if the bell-shaped annealing temperature exceeds 300℃, the strength will be low. It becomes difficult to satisfy the requirement of ≧1180 MPa. Therefore, the bell-shaped annealing temperature is set between 100 and 300°C. Because we adopted a high-silicon composition design concept, silicon can effectively suppress the formation of cementite in steel during the low-temperature bell annealing process, and also contribute to the diffusion of carbon atoms from martensite to retained austenite. , further improves the stability of retained austenite, giving the steel strip higher elongation and better forming performance than high-strength steels at the same strength level.

The advantageous effect of the present invention is that
(1) By adopting a relatively economical component design concept and an innovative cooling process route, the 980 MPa level provides excellent performance in terms of strength, plasticity, toughness, cold bending, hole expandability, etc. A steel with high hole expandability was obtained;
(2) The steel coil or steel plate has excellent strength, compatibility between plasticity and toughness, and also has good cold bending performance, hole expansion and flanging performance, and its yield strength is ≧800 MPa and tensile strength is ≧980 MPa, and also has good elongation (lateral A ₅₀ ≧8%), passing the cold bending performance (d≦4a, 180°) test and hole expandability (hole expansion rate≧30%). It can be used to manufacture parts such as automobile chassis and subframes that require high strength, thin walls, hole expansion, and flanging, and is expected to have a wide range of applications.

FIG. 1 is a process flowchart of a method for manufacturing ultra-low carbon martensitic high hole expandability steel of 980 MPa level according to the present invention. FIG. 2 is a conceptual diagram of the rolling process in the method for manufacturing ultra-low carbon martensitic high hole expandability steel of 980 MPa level according to the present invention. FIG. 3 is a conceptual diagram of the cooling process in the method for manufacturing ultra-low carbon martensitic high hole expandability steel of 980 MPa level according to the present invention. FIG. 4 is a process flowchart of a method for manufacturing high plasticity and high hole expandability steel of 1180 MPa level according to Preparation Examples II and III of the present invention. FIG. 5 is a conceptual diagram of a rolling process in a method for manufacturing a high plasticity and high hole expandability steel at a level of 1180 MPa according to Preparation Example II of the present invention. FIG. 6 is a conceptual diagram of the cooling process in the method for producing high plasticity and high hole expandability steel at the 1180 MPa level according to Preparation Example II of the present invention. FIG. 7 is a conceptual diagram of the bell-shaped annealing process in the method for manufacturing high plasticity and high hole expandability steel at the 1180 MPa level according to Preparation Examples II and III of the present invention. FIG. 8 is a typical metallographic photograph of Example 10 of the high hole expandability steel according to the present invention. FIG. 9 is a typical metallographic photograph of Example 12 of the high hole expandability steel according to the present invention. FIG. 10 is a typical metallographic photograph of Example 14 of the high hole expandability steel according to the present invention. FIG. 11 is a typical metallographic photograph of Example 16 of the high hole expandability steel according to the present invention. FIG. 12 is a conceptual diagram of the rolling process in the method for manufacturing high plasticity and high hole expandability steel at the 1180 MPa level according to Preparation Example III of the present invention. FIG. 13 is a conceptual diagram of a cooling process in a method for manufacturing a high plasticity and high hole expandability steel at a level of 1180 MPa according to Preparation Example III of the present invention.

In each example below, tensile performance (yield strength, tensile strength, elongation) was measured based on the ISO 6892-2-2018 international standard, and hole expansion rate was measured based on the ISO 16630-2017 international standard. , -40°C impact toughness was measured based on the ISO 14556-2015 international standard, and cold bending performance was measured based on the ISO 7438-2005 international standard.

Preparation example I
Referring to FIGS. 1 to 3, the method for manufacturing ultra-low carbon martensitic high hole expandability steel at 980 MPa level according to the present invention includes the following steps:
1) Smelting and casting Smelting in a converter or electric furnace according to the stated composition, secondary smelting in a vacuum furnace, and then casting into billets or ingots;
2) Reheat the billet or ingot at a heating temperature of 1100 to 1200°C and a heat retention time of 1 to 2 hours;
3) Hot rolling The rolling start temperature is 950 to 1100°C, and 3 to 5 passes are performed at 950°C or higher under large reduction, and the cumulative amount of deformation is ≧50%; Next, the temperature of the intermediate billet is set to 920 to 950°C. ℃, finally perform 3 to 5 passes of rolling, and make the cumulative amount of deformation ≧70%; set the rolling end temperature to 800 to 920 ℃;
4) Cooling First, perform air cooling for 0 to 10 seconds to ensure dynamic recovery and dynamic recrystallization, then cool at a cooling rate of ≥50°C/s to a certain temperature below the Ms point (between room temperature and Ms point). The steel strip is water-cooled and coiled, and then cooled to room temperature (with a cooling rate of ≦20°C/h);
5) Pickling Adjust the pickling execution speed of the strip steel in the range of 30 to 100 m/min, control the pickling temperature in the range of 75 to 85°C, control the tensile straightening rate to ≦2%, and Rinse in a temperature range of 50°C, dry the surface of the steel strip between 120-140°C, and apply oil.

The composition of the example of the high hole expandability steel according to this preparation example is shown in Table 1, and the production process parameters of the example of the steel according to the invention are shown in Tables 2 and 3. The thickness is 120 mm; the mechanical performance of the steel plate according to the example of the present invention is shown in Table 4.

As can be seen from Table 4, the steel coils all have yield strength ≧800MPa, tensile strength ≧980MPa, elongation is usually between 8-13%, impact energy is relatively stable, - The 40°C low-temperature impact energy is stable at 140-180J, the retained austenite content varies between 1.5-5% overall depending on the winding temperature, and the hole expansion rate satisfies ≧50%.

As can be seen from the above examples, the 980 MPa high-strength steel according to the present invention has good strength, plasticity, toughness, and hole expandability, and is particularly suitable for high strength, thinning, hole expansion, etc., such as control arms. It is suitable for manufacturing parts that require flanging, such as automobile chassis, and can also be applied to parts that require hole flanging, such as wheels, and is expected to have a wide range of applications.

Preparation example II
Referring to FIGS. 4 to 7, the method for manufacturing high plasticity and high hole expandability steel at 1180 MPa level according to the present invention includes the following steps:
1) Smelting and casting Smelting in a converter or electric furnace according to the stated composition, secondary smelting in a vacuum furnace, and then casting into billets or ingots;
2) Reheat the billet or ingot at a heating temperature of 1100 to 1200°C and a heat retention time of 1 to 2 hours;
3) Hot rolling The rolling start temperature is 950 to 1100°C, and 3 to 5 passes are performed at 950°C or higher and under large reduction, and the cumulative amount of deformation is ≧50%; Next, 3 to 7 passes of rolling are performed, And the cumulative amount of deformation is ≧70%; the rolling end temperature is 800 to 950°C;
4) Cooling First, air cool for 0 to 10 seconds, then water cool the steel strip to room temperature at a cooling rate of ≧30°C/s and wind it;
5) Annealing Anneal by bell-type annealing, heating rate ≧20℃/h, bell-type annealing temperature 100-300℃, bell-type annealing time 12-48h; cooling rate ≦50℃/h. Cool the steel plate to below 100℃ and tap it;
6) Pickling Adjust the pickling speed of the steel strip in the range of 30 to 90 m/min, control the pickling temperature between 75 to 85°C, and control the tensile straightening rate to ≦1.5%. Rinse at a temperature range of 35-50°C, dry the surface at a temperature of 120-140°C, and apply oil.

The composition of the example of the high hole expandability steel according to this preparation example is shown in Table 5, and the production process parameters of the example of the steel according to the invention are shown in Tables 6 and 7. The thickness is 120 mm; the mechanical performance of the steel plate according to the example of the present invention is shown in Table 8.

As can be seen from Table 8, the steel coils all have yield strength ≧900MPa, tensile strength ≧1180MPa, elongation is usually between 10-13%, impact energy is relatively stable, - The low-temperature impact energy at 40°C is stable at 60 to 100 J, the retained austenite content changes depending on the winding temperature, and the hole expansion rate satisfies ≧30%.

As can be seen from the above examples, the 1180 MPa high hole expandability steel according to the present invention has good strength, plasticity, toughness and hole expandability, and is particularly suitable for high strength, thin wall and hole expandability, such as control arms. It is suitable for manufacturing parts such as automobile chassis that require expansion and flanging, and can also be applied to parts such as wheels that require hole flanging, and is expected to have a wide range of applications.

FIGS. 8 to 11 show typical metallographic structures of steel plates according to Examples 10#, 12#, 14#, and 16#, respectively. As can be seen from the metallographic photographs, the structure is single-phase low-carbon martensite and contains a certain amount of retained austenite, expressing higher elongation and hole expansion rate at the same strength level.

Preparation Example III
Referring to FIGS. 4, 7, 12 and 13, the method for producing 1180 MPa level high plasticity high hole expandability steel according to the present invention includes the following steps:
1) Smelting and casting Smelting in a converter or electric furnace according to the stated composition, secondary smelting in a vacuum furnace, and then casting into billets or ingots;
2) Reheat the billet or ingot at a heating temperature of 1100 to 1200°C and a heat retention time of 1 to 2 hours;
3) Hot rolling The rolling start temperature is 950 to 1100°C, and 3 to 5 passes are performed at 950°C or higher under large reduction, and the cumulative amount of deformation is ≧50%; Next, the temperature of the intermediate billet is set to 900 to 950°C. ℃, perform 3 to 7 passes of rolling, and make the cumulative amount of deformation ≧70%; set the rolling end temperature to 800 to 900 ℃;
4) Cooling First, the steel plate is air cooled for 0 to 10 seconds, then water cooled at a cooling rate of ≧30℃/s to a certain temperature below the Ms point, rolled up, and then cooled to room temperature (≦20℃/h). slow cooling (at cooling rate);
5) Annealing Anneal by bell-type annealing, heating rate ≧20℃/h, bell-type annealing temperature 100-300℃, bell-type annealing time 12-48h; cooling rate ≦50℃/h. Cool the steel plate to ≦100℃ and tap it;
6) Pickling Adjust the pickling speed of the steel strip in the range of 30 to 90 m/min, control the pickling temperature between 75 to 85°C, and control the tensile straightening rate to ≦1.5%. Rinse at a temperature range of 35-50°C, dry the surface of the steel strip at a temperature of 120-140°C, and apply oil.

The composition of the example of the high hole expandability steel according to this preparation example is shown in Table 9, and the production process parameters of the example of the steel according to the invention are shown in Tables 10 and 11. The thickness is 120 mm; the mechanical performance of the steel plate according to the example of the present invention is shown in Table 12.

As can be seen from Table 12, all steel coils have yield strength ≧900MPa, tensile strength ≧1180MPa, elongation is usually between 10-13%, impact energy is relatively stable, - The low-temperature impact energy at 40°C is stable at 80 to 110 J, the retained austenite content changes depending on the winding temperature, and the hole expansion rate satisfies ≧30%.

As can be seen from the above examples, the 1180 MPa high-strength steel according to the present invention has good strength, plasticity, toughness, and hole expandability, and is particularly suitable for high strength, thinning, hole expansion, etc., such as control arms. It is suitable for manufacturing parts that require flanging, such as automobile chassis, and can also be applied to parts that require hole flanging, such as wheels, and is expected to have a wide range of applications.

Claims (18)

Its chemical composition is in weight percentage: C 0.03-0.10%, Si 0.5-2.0%, Mn 1.0-2.0%, P≦0.02%, S≦0.003 %, Al 0.02-0.08%, N≦0.004%, Mo 0.1-0.5%, Ti 0.01-0.05%, O≦0.0030%, the balance is Fe. Low carbon martensitic high hole expandability steel with tensile strength of 980 MPa or more, other unavoidable impurities. Its chemical composition is in weight percentage: C 0.03-0.06%, Si 0.5-2.0%, Mn 1.0-2.0%, P≦0.02%, S≦0.003 %, Al 0.02-0.08%, N≦0.004%, Mo 0.1-0.5%, Ti 0.01-0.05%, O≦0.0030%, the balance is Fe. The low carbon martensitic high hole expandability steel having a tensile strength of 980 MPa or more according to claim 1, which is other unavoidable impurities. Its chemical composition is in weight percentage: C 0.06-0.10%, Si 0.8-2.0%, Mn 1.5-2.0%, P≦0.02%, S≦0.003 %, Al 0.02-0.08%, N≦0.004%, Mo 0.1-0.5%, Ti 0.01-0.05%, O≦0.0030%, the balance is Fe. The low carbon martensitic high hole expandability steel having a tensile strength of 980 MPa or more according to claim 1, which is other unavoidable impurities. Cr≦0.5%, B≦0.002%, Ca≦0.005%, Nb≦0.06%, V≦0.05%, Cu≦0.5%, Ni≦0.5% further comprising one or more elements, provided that the preferred content of Cr is 0.2 to 0.4%, and the preferred content of B is 0.0005 to 0.0015%, The preferable content of Ca is ≦0.002%, the preferable content of Nb and V is each ≦0.03%, and the preferable content of Cu and Ni is ≦0.3%, respectively. The low carbon martensitic high hole expandability steel having a tensile strength of 980 MPa or more according to any one of claims 1 to 3, characterized in that: The C content is 0.04 to 0.055%, the Si content is 0.8 to 1.4%, and the Mn content is 1.4 to 1.8%. , the S content is controlled to 0.0015% or less, the Al content is 0.02 to 0.05%, the N content is controlled to 0.003% or less, and the Ti The content of Mo is 0.01 to 0.03%, and the content of Mo is 0.15 to 0.35%, indicating that the Mo content has one or more characteristics selected from the group. The low carbon martensitic high hole expandability steel having a tensile strength of 980 MPa or more according to claim 2 or 4. The C content is 0.07 to 0.09%, the Si content is 1.0 to 1.4%, and the Mn content is 1.6 to 1.9%. , the S content is controlled to 0.0015% or less, the Al content is 0.02 to 0.05%, the N content is controlled to 0.003% or less, and the Ti The content of Mo is 0.01 to 0.03%, and the content of Mo is 0.15 to 0.35%, indicating that the Mo content has one or more characteristics selected from the group. The low carbon martensitic high hole expandability steel having a tensile strength of 980 MPa or more according to claim 3 or 4. Claims 1 to 6, characterized in that the microstructure of the high hole expandability steel is martensite or tempered martensite and retained austenite, provided that the volume percentage of retained austenite in the microstructure is ≦5%. The low carbon martensitic high hole expandability steel having a tensile strength of 980 MPa or more according to any one of the above. The high hole expandability steel has a yield strength ≧800 MPa, a tensile strength ≧980 MPa, a lateral elongation A ₅₀ ≧8%, and a hole expansion rate ≧30%. Preferably, the high hole expandability steel has a -40°C impact resistance. The low carbon martensitic high hole expandability steel having a tensile strength of 980 MPa or more according to any one of claims 1 to 7, characterized in that the toughness is 60 J or more. The high hole expandability steel has yield strength ≧800 MPa, tensile strength ≧980 MPa, lateral elongation A ₅₀ ≧8%, hole expansion rate ≧50%, and passes the cold bending performance test (d≦4a, 180°). and preferably the low carbon martensite high hole expandability steel having a tensile strength of 980 MPa or more according to claim 2 or 5, wherein the high hole expandability steel has a -40°C impact toughness ≧140J. Sex steel. The ultra-low carbon martensitic high hole expandability steel with a tensile strength of 980 MPa or more has a yield strength of 800 to 890 MPa, a tensile strength of 980 to 1150 MPa, a lateral elongation _A50 of 8 to 13%, and a hole expansion rate of 800 to 890 MPa. 50 to 85%, -40℃ impact toughness of 140 to 185 J, passed cold bending performance test (d≦4a, 180°), and ultra-low carbon martensite high hole with tensile strength of 980 MPa or more. A low tensile strength of 980 MPa or more according to claim 9, characterized in that the microstructure of the expandable steel is martensite and retained austenite, provided that the volume percentage of retained austenite in the microstructure is ≦5%. Carbon martensitic high hole expandability steel. The high hole expandability steel has a yield strength ≧900 MPa, a tensile strength ≧1180 MPa, a lateral elongation A ₅₀ ≧10%, and a hole expansion ratio ≧30%. Preferably, the high hole expandability steel has a -40°C impact resistance. The low carbon martensite with a tensile strength of 980 MPa or more according to claim 3 or 6, characterized in that the toughness is ≧60 J and preferably passes a cold bending performance test (d≦4a, 180°). High hole expandability steel. The high hole expandability steel has a yield strength of 900 to 1000 MPa, a tensile strength of 1200 to 1280 MPa, a lateral elongation of 10 to 13%, a hole expansion rate of 30 to 50%, and a -40°C impact toughness of 60. ~100J, and preferably, the microstructure of the high hole expandability steel is tempered martensite and retained austenite, provided that the volume percentage of retained austenite in the microstructure is ≦5% as described above, or The high hole expandability steel has a yield strength of 940 to 1000 MPa, a tensile strength of 1210 to 1300 MPa, a lateral elongation of 10 to 13%, a hole expansion rate of 30 to 50%, and a -40°C impact toughness. 80 to 110 J and passes a cold bending performance test (d≦4a, 180°), and preferably the microstructure of the high hole expandability steel is tempered martensite and retained austenite, provided that: The low carbon martensitic high hole expandability steel with a tensile strength of 980 MPa or more according to claim 11, characterized in that the volume percentage of retained austenite in the microstructure is ≦5% as described above. A method for producing a low carbon martensitic high hole expandability steel having a tensile strength of 980 MPa or more according to claims 1 to 12, which comprises the following steps.
1) Smelting and casting According to the compositions described in claims 1 to 6, smelting in a converter or electric furnace, secondary smelting in a vacuum furnace, and then casting into billets or ingots;
2) Reheat the billet or ingot at a heating temperature of 1100 to 1200°C and a heat retention time of 1 to 2 hours;
3) Hot rolling The rolling start temperature is 950 to 1100°C, and 3 to 5 passes are performed at 950°C or higher under heavy reduction, and the cumulative amount of deformation is ≧50%; Next, the final rolling is performed for 3 to 7 passes. and the cumulative amount of deformation is ≧70%; the rolling end temperature is 800 to 950°C; optionally, after performing 3 to 5 passes under the above-mentioned high pressure, the temperature of the intermediate billet is set to 900 to 950°C. After that, perform the above rolling for 3 to 7 passes;
4) Cooling First, air cool for 0 to 10 seconds, then water cool the steel strip at a cooling rate of ≧50°C/s between room temperature and Ms point, coil it, and then cool it to room temperature; Air cooling for 0-10 seconds, then water cooling the steel strip directly to room temperature at a cooling rate of ≧30 °C/s and winding; alternatively, first air cooling for 0-10 seconds, then martensitic transformation. The steel plate is water-cooled at a cooling rate of ≧30°C/s to a certain temperature below the Ms point, which is the starting point, and then coiled, and then slowly cooled to room temperature;
5) Pickling Adjust the pickling speed of the strip steel in the range of 30 to 100 m/min, control the pickling temperature between 75 to 85°C, control the tensile straightening rate to ≦2%, and then rinse. Wash, dry the strip surface and apply oil. The tensile material according to claim 13, characterized in that after the pickling in step 5), rinsing is carried out in a temperature range of 35 to 50 °C, and the surface is dried in a range of 120 to 140 °C and coated with oil. A method for producing low carbon martensitic high hole expandability steel having a strength of 980 MPa or more. Between steps 4) and 5), further step 4-1): annealing with bell-shaped annealing, with a heating rate of ≧20°C/h, a bell-shaped annealing temperature of 100 to 300°C, and bell-shaped annealing. The tensile strength according to claim 13 or 14, characterized in that the method includes: cooling the steel plate to ≦100°C at a cooling rate of ≦50°C/h and tapping the steel plate for a time of 12 to 48 hours. A method for producing low carbon martensitic high hole expandability steel of 980 MPa or more. The method for producing a low carbon martensitic high hole expandability steel having a tensile strength of 980 MPa or more according to claim 13 or 14, which comprises the following steps.
6) Smelting and casting According to the composition described in claim 2, 4 or 5, smelting in a converter or electric furnace, secondary smelting in a vacuum furnace, and then casting into billets or ingots;
7) Reheat the billet or ingot at a heating temperature of 1100 to 1200°C and a heat retention time of 1 to 2 hours;
8) Hot rolling The rolling start temperature is 950 to 1100°C, and 3 to 5 passes are performed at 950°C or higher and under large reduction, and the cumulative amount of deformation is ≧50%, preferably ≧60%; Next, the intermediate billet After setting the temperature to 920 to 950°C, finally perform 3 to 5 passes of rolling, and make the cumulative amount of deformation ≧70%, preferably ≧85%; set the rolling end temperature to 800 to 920°C;
9) Cooling First, perform air cooling for 0 to 10 seconds to ensure dynamic recovery and dynamic recrystallization, then cool to a certain temperature below the Ms point (between room temperature and the Ms point) at ≥50°C/s, preferably at 50°C/s. Water-cool the steel strip at a cooling rate of ~85°C/s, coil it, and then cool it to room temperature;
10) Pickling Adjust the pickling speed of the steel strip in the range of 30 to 100 m/min, control the pickling temperature between 75 to 85°C, and adjust the tensile straightening rate to reduce the elongation loss of the steel strip. ≦2%, then rinse, dry the strip surface and apply oil. The method for producing a low carbon martensitic high hole expandability steel having a tensile strength of 980 MPa or more according to claim 13 or 14, which comprises the following steps.
7) Smelting, casting According to the composition described in claim 3, 4 or 6, smelting in a converter or electric furnace, secondary refining in a vacuum furnace, and then casting into billets or ingots;
8) Reheat the billet or ingot at a heating temperature of 1100 to 1200°C and a heat retention time of 1 to 2 hours;
9) Hot rolling The rolling start temperature is 950 to 1100°C, and 3 to 5 passes are performed at 950°C or higher under heavy reduction, and the cumulative deformation is ≧50%, preferably ≧60%; then, rolling is carried out. Perform 3 to 7 passes, and set the cumulative deformation amount to ≧70%, preferably ≧85%; set the rolling end temperature to 800 to 950°C;
10) Cooling First air cooling for 0-10 seconds, then water-cooling the steel strip to room temperature at a cooling rate of ≧30°C/s, preferably 30-65°C/s, and winding;
11) Annealing Anneal using bell-shaped annealing, heating rate ≧20°C/h, preferably 20-40°C/h, bell-shaped annealing temperature 100-300°C, and bell-shaped annealing time 12-48h. ; Cool the steel plate to ≦100°C at a cooling rate of ≦50°C/h, preferably 15 to 50°C/h, and tap the steel plate;
12) Pickling Adjust the pickling speed of the steel strip in the range of 30 to 90 m/min, control the pickling temperature between 75 and 85°C, and control the tensile straightening rate to ≦1.5%. Rinse at a temperature range of 35-50°C, dry the surface at a temperature of 120-140°C, and apply oil. The method for producing a low carbon martensitic high hole expandability steel having a tensile strength of 980 MPa or more according to claim 13 or 14, which comprises the following steps.
7) Smelting, casting According to the composition described in claim 3, 4 or 6, smelting in a converter or electric furnace, secondary refining in a vacuum furnace, and then casting into billets or ingots;
8) Reheat the billet or ingot at a heating temperature of 1100 to 1200°C and a heat retention time of 1 to 2 hours;
9) Hot rolling The rolling start temperature is 950 to 1100°C, and 3 to 5 passes are performed at 950°C or higher under large reduction, and the cumulative amount of deformation is ≧50%, preferably ≧60%; Next, the intermediate billet After setting the temperature to 900 to 950°C, perform 3 to 7 passes of rolling, and make the cumulative amount of deformation ≧70%, preferably ≧85%; the rolling end temperature should be 800 to 900°C;
10) Cooling First, air cooling is performed for 0 to 10 seconds, and then the steel plate is water cooled at a cooling rate of ≧30°C/s, preferably 30 to 70°C/s, to a certain temperature below the Ms point, which is the starting point of martensitic transformation. Roll it up and slowly cool it to room temperature;
11) Annealing Anneal using bell-shaped annealing, heating rate ≧20°C/h, preferably 20-50°C/h, bell-shaped annealing temperature 100-300°C, and bell-shaped annealing time 12-48h. ; Cool the steel plate to ≦100°C at a cooling rate of ≦50°C/h, preferably 20 to 50°C/h, and tap the steel plate;
12) Pickling Adjust the pickling speed of the steel strip in the range of 30 to 90 m/min, control the pickling temperature between 75 to 85°C, and adjust the tensile straightening rate to reduce the elongation loss of the steel strip. ≦1.5%, then rinse, dry the strip surface and apply oil.

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