High Hole Expansion Steel and Method for Manufacturing Therefor

The present invention relates to a steel and a method for manufacturing therefor, in particular to a high hole expansion steel and a method for manufacturing therefor.

Vehicle plays a very important role in the development of national economy. Many parts in passenger vehicle, especially some parts of the chassis and body, often require hot-rolled pickled products. Lightweight of passenger vehicle is a development trend in the automotive industry. The high strength and weight reduction of vehicles require higher grades of steel, and the structure of the chassis also needs to be improved, resulting in more complex chassis parts and higher requirements for material properties, surface conditions, and forming technologies, such as hydroforming, hot stamping, laser welding, etc., thus transforming into performance requirements for materials such as high strength, stamping, flanging, rebound and fatigue performance.

At present, the high hole expansion steel used by domestic vehicle parts companies is basically high-strength steel with a tensile strength of 600 MPa or less. There is considerable competition for high hole expansion steel with a tensile strength of 540 MPa or less. At the same time, high hole expansion steel with a tensile strength of 780 MPa is gradually being used in batches in China. However, the elongation and hole expansion ratio, two important indicators in the forming process, are also demanding, while the requirements for performance stability are becoming more stringent.

In order to reduce process costs, passenger vehicle companies have further improved the performance requirements of the materials. For example, when producing vehicle chassis parts, in order to reduce the steps of the stamping process, the material is required to have high strength and high plasticity as well as a high hole expansion ratio. For example, the hole expansion ratio of 780 MPa grade high hole expansion steel is required to be guaranteed to be ≥50%, preferably further increase to 70% or even 80% or more. Existing high hole expansion steel, especially 780 MPa grade high hole expansion steel, mostly have hot-rolled bainite structures and are strengthened with precipitated phases at the same time. Most of the process paths used are medium-temperature coiling, but the temperature control accuracy is not high and the uniformity of finished product structure is poor, resulting in ununiform properties, such as hole expansion ratio, of the obtained hot-rolled steel, and stamping cracking is prone to occur during subsequent processing.

Prior arts of 780 MPa grade pickled high hole expansion steel are listed below:

Chinese patent CN103602895A discloses a low carbon Nb—Ti micro-alloyed high hole expansion steel, which adopts a composition design of low carbon, high silicon in combination with Nb—Ti micro-alloying, which can ensure a hole expansion ratio of ≥50%. However, the design of high silicon composition usually results in red scale on the surface of the steel plate. In addition, the coiling temperature required to form bainite is usually around 500° C., which makes it difficult to control the temperature on full length of the steel coil, resulting in large fluctuations in the performance of the entire length of the steel coil.

Chinese patent CN105821301A discloses an 800 MPa grade hot-rolled high-strength high hole expansion steel, which also adopts a composition design of low carbon and high silicon in combination with Nb—Ti micro-alloying. The Ti content in the steel is very high, ranging from 0.15 to 0.18%. In the actual production process, this composition design will lead to defects such as red scale on the surface of the strip steel. On the other hand, due to the high Ti content, coarse TIN is easily formed in the steel, which is detrimental to the stability of the hole expansion ratio.

Chinese patent CN108570604A discloses a 780 MPa grade hot-rolled pickled high hole expansion steel, which adopts a composition design of low carbon, high aluminum and high chromium, and adopts a three-stage cooling process in the process design. Although there is no red scale on the surface of the strip steel, the high aluminum composition design can easily cause clogging of the casting nozzle during the actual production process. On the other hand, the production process of this steel is complex, especially the three-stage cooling process is difficult to control, resulting in a low hole expansion ratio of the steel.

Chinese patent CN114107792A discloses a 780 MPa grade hot-rolled pickled high hole expansion steel, which adopts a composition design of low carbon and high titanium and an appropriate amount of molybdenum is added to the steel. Since the phase transformation process of molybdenum-containing steel is relatively slow, the phase transformation process mainly occurs after coiling. Therefore, in the actual production process, there are problems such as low strength of the inner and outer rings of steel coils.

In view of the above-mentioned defects in the prior arts, an objective of the present invention is to provide a steel and a method for manufacturing therefor. The steel of the present invention has high strength, high plasticity and high hole expansion ratio, and these properties are well-matched with each other, and can be used in passenger vehicle chassis parts needing high strength and thickness reduction such as a control arm and a subframe.

To achieve the above objective, the present invention adopts the following technical solution:

In order to meet user demands for the matching of higher surface quality, better performance stability, plasticity and hole expansion property, conventional high hole expansion steel needs to be improved.

In general, the elongation of a material is inversely proportional to the hole expansion ratio, that is, the higher the elongation, the lower the hole expansion ratio: conversely, the lower the elongation, the higher the hole expansion ratio. Under the same or similar strengthening mechanism, the higher the strength of the material, the lower the hole expansion ratio. In order to obtain steel with both good plasticity and hole expansion flanging performance, a better balance between the two is needed. In order to achieve a good matching between strength, plasticity and hole expansion performance, the addition of higher amounts of silicon elements seems to be indispensable for high-strength high-plasticity high hole expansion steel. However, the compositional design of high silicon usually leads to poor surface quality of steel plates. Specifically, the red scale defects formed during the hot rolling process are difficult to be completely removed in the subsequent pickling process, resulting in appearance of stripe-shaped red scale on the surface of the pickled high-strength steel, which seriously affects the surface quality.

The present invention optimizes the chemical composition of the existing hot-rolled steel by adopting a low carbon and high vanadium element design without intentionally adding silicon element to the steel, and by adding the V element to improve the strength and plasticity of the steel by taking advantage of the nano-sized vanadium carbide it forms. A bainitic precipitation-strengthened high-strength steel with uniform structure and properties can be obtained without changing the existing hot continuous rolling production line.

Specifically, the steel according to the present invention comprises the following components in percentage by mass: C: 0.01-0.10%; Si≤0.2%; Mn: 0.5-2.0%; P≤0.02%; S≤0.003%; Al: 0.01-0.08%; N≤0.004%; V: 0.10-0.50%; O≤0.003%; and the balance of Fe and inevitable impurities.

Preferably, the steel further comprises Ti, with an upper limit of 0.2%, preferably 0.18%, more preferably 0.015%, and a lower limit of 0.05%, preferably 0.08% in percentage by mass.

Preferably, the steel further comprises 0.1-0.5%, more preferably 0.20-0.40%, still more preferably 0.2-0.3% of Mo in percentage by mass.

Preferably, the steel further comprises one or more components selected from Nb≤0.1%, Cu≤0.5%, Ni≤0.5%, Cr≤0.5%, and B≤0.002%, wherein Cu is more preferably 0.3% or less: Ni is more preferably 0.3% or less: Cr is more preferably 0.3% or less: Nb is more preferably 0.06% or less: B is more preferably 0.002% or less, still more preferably 0.001% or less.

Preferably, the composition of the steel satisfies one or more of the following: C: 0.03-0.07%; Si≤0.10%; Mn: 0.8-1.6%; S≤0.0018%; Al: 0.02-0.05%; N≤0.003%: O≤0.002%.

In the composition design of the steel according to the present invention:

Ca is a basic element in steel and one of the important elements in the present invention. C can expand the austenite phase area and stabilize the austenite. As an interstitial atom in steel, C plays a very important role in improving the strength of steel, among which it has the greatest impact on the yield strength and tensile strength of steel. In the present invention, since the structure to be obtained during the hot rolling stage is low carbon bainite, in order to obtain high hole expansion steel with final tensile strength reaching different strength levels, the C content must be 0.01% or more; at the same time, the C content must not be higher than 0.10%. If the C content is too high, low carbon martensite is easily formed during low-temperature coiling. Therefore, the present invention controls the C content to 0.01-0.10%, preferably 0.03-0.07%.

Si is a basic element in steel. As mentioned before, in order to meet user demands for high strength, high plasticity and high hole expansion ratio, a higher amount of Si is usually added in the composition design. However, the composition design of high silicon brings about a reduction in the surface quality of the steel plate, which has higher amount of red scale defects. In the present invention, in order to ensure good surface quality, the Si content should be strictly controlled during component design. In other words, Si is an impurity element in the present invention. Considering that Si—Mn is needed for deoxidation in actual steelmaking, it seems difficult to completely avoid the addition of Si. According to a large amount of statistical data from actual production, when the Si content is 0.2% or less, the surface red scale defects can be avoided during the hot rolling process. Usually, when the Si content is 0.10% or less, it is guaranteed that no red scale will appear. Therefore, the Si content in the steel of the present invention is controlled within 0.2%, preferably within 0.10%.

Mn is also the most basic element in steel and one of the most important elements in the present invention. It is well known that Mn is an important element in expanding the austenite phase area. It can reduce the critical quenching speed of steel, stabilize austenite, refine grains, and postpone the transformation of austenite to pearlite. In the present invention, in order to ensure the strength and grain refinement effect of the steel plate, the Mn content is usually controlled at 0.5% or more; at the same time, the Mn content should not generally exceed 2.0%, otherwise Mn segregation will easily occur during steelmaking, and hot cracking is also prone to occur during continuous casting of slabs. Therefore, the Mn content in the steel of the present invention is controlled at 0.5-2.0%, preferably 0.8-1.6%.

P is an impurity element in steel. P is very easy to segregate on the grain boundaries. When the P content in the steel is high (≥.1%), FezP will be formed and precipitated around the grains, reducing the plasticity and toughness of steel. Therefore, the lower its content, the better. The present invention controls the P content within 0.02%, and the steel obtained has better mechanical performances and does not increase the cost of steelmaking.

S is an impurity element in steel. S in steel usually combines with Mn to form MnS inclusions. Especially when the contents of S and Mn are both high, more MnS will be formed in the steel. MnS itself has a certain plasticity, it would deform along the rolling direction during the subsequent rolling process, which not only reduces the transverse plasticity of the steel, but also increases the anisotropy of the structure, which is detrimental to the hole expansion performance. In order to reduce the MnS content, the S content needs to be strictly controlled. The lower the S content in the steel, the better. In the present invention, the S content is controlled within 0.003%, preferably 0.0018% or less.

Al's main function in steel is to deoxidize and fix nitrogen. In the presence of strong carbide-forming elements such as Ti, the main function of Al is to deoxidize and refine grains. In the present invention, Al is a common deoxidizing element and grain refining element, and its content is usually controlled at 0.01-0.08%; when the Al content is less than 0.01%, it will not have the effect of refining grains; similarly, when the Al content is higher than 0.08%, its effect of refining grains will reach saturation. Therefore, the Al content in the steel of the present invention is controlled between 0.01-0.08%, preferably between 0.02-0.05%.

N is an impurity element in the present invention, and the lower its content, the better. However, N is an inevitable element in the steelmaking process. Although its content is low, in combination with strong carbide forming elements such as V, the VN particles formed adversely affect the performance of the steel, especially very detrimental to the hole expansion performance. Due to the square shape of VN, there is a large stress concentration between its sharp corners and the substrate. During the process of hole expansion deformation, the stress concentration between VN and the substrate can easily form crack sources, thus greatly reducing the hole expansion performance of the material. Since the present invention adopts a high vanadium design in the composition system, in order to minimize the adverse effect on hole expansion caused by VN, the present invention controls the N content to 0.004% or less, preferably 0.003% or less.

V is an important element in the present invention. Similar to Ti and Nb, V is also a strong carbide-forming element. However, vanadium carbides have low solid-solution or precipitation temperature and are usually fully solid-solutionized in austenite during the finishing rolling stage. Only when the temperature is lowered and phase transformation begins, V begins to form in the ferrite. In order to fully utilize the precipitation strengthening effect of V, the amount of V added to the steel should be at least 0.10% or more to have obvious precipitation strengthening effect: as the V content increases, the precipitation strengthening effect of V gradually increases. When the V content exceeds 0.50%, the precipitation strengthening effect of V is saturated and the size of the vanadium carbide formed is larger, which adversely reduces the contribution to the steel strength. Therefore, the amount of V added to the steel of the present invention is controlled to be ≤0.50%. Specifically, when the V content is 0.10-0.20%, a 590 MPa grade high hole expansion steel can be obtained: when the V content is 0.20-0.35%, a 780 MPa grade high hole expansion steel can be obtained: when the V content is 0.35-0.50%, a 980 MPa grade high hole expansion steel can be obtained.

Mo is one of the important elements in the present invention. The addition of Mo in steel can greatly postpone the phase transformation of ferrite and pearlite, which is conducive to obtaining a bainite structure. In addition, Mo has strong resistance to welding softening. Since the main objective of the present invention is to obtain a low carbon bainite structure, and low carbon bainite is prone to softening after welding, adding a certain amount of Mo can effectively reduce the degree of welding softening. Therefore, in the present invention, the Mo content is controlled at 0.10-0.50%, preferably 0.20-0.40%, more preferably 0.2-0.3%. When combined with the sectional cooling process, Mo plays a certain inhibiting role in the formation of ferrite during the sectional cooling process. When the Mo content is within the above range, its effect can be fully realized.

Nb is one of the addable elements in the present invention. Nb, similar to Ti, is a strong carbide element in steel. The addition of Nb in steel can greatly increase the non-recrystallization temperature of the steel, obtain deformed austenite with higher dislocation density during the finishing rolling stage, and refine the final phase change structure during the subsequent transformation process. However, the amount of Nb added should not be too much. On one hand, if the amount of Nb added exceeds 0.10%, it is easy to form relatively coarse niobium carbonitrides in the structure, consuming part of the carbon atoms and reducing the precipitation strengthening effect of carbides. At the same time, the higher Nb content also tends to cause anisotropy of the hot-rolled austenite structure, which is inherited to the final structure during the subsequent cooling phase transformation process, harming the hole expansion performance. Therefore, the Nb content in the steel of the present invention is controlled at ≤0.10%, preferably ≤0.06%.

Ti is an optional element in the present invention. The addition of a small amount of Ti in steel can, on one hand, combine with N to form TiN during the high-temperature stage, which can fix N and help reduce the subsequent formation of VN: on the other hand, the excess Ti after combining with N can be combined with C to form nano-TiC during the subsequent process and improve the performance of the steel with the nano-VC together. When the Ti content is higher than 0.20%, coarser TiN is prone to be formed during the high-temperature stage and deteriorates the impact toughness of the steel. Therefore, the content of the optional element Ti in the steel of the present invention is within 0.20%, preferably within 0.18%, more preferably within 0.015%, most preferably within 0.10%. On the other hand, the Ti content is preferably 0.05% or more, more preferably 0.08% or more, which provides an excellent precipitation strengthening effect.

Cu is an optional element in the present invention. The addition of Cu in steel can improve the corrosion resistance of steel. When it is added together with the P element, the corrosion resistance performance is better: when the amount of Cu added exceeds 1%, under certain conditions, an E-Cu precipitated phase can be formed, causing a strong precipitation strengthening effect. However, the addition of Cu can easily cause the “Cu brittleness” phenomenon during the rolling process. In order to make full use of Cu's corrosion resistance improvement effect in certain applications without causing significant “Cu brittleness” phenomenon, the present invention controls the Cu content to within 0.5%, preferably within 0.3%.

Ni is an optional element in the present invention. The addition of Ni in steel has a certain degree of corrosion resistance, but the corrosion resistance effect is weaker than that of Cu. The addition of Ni in steel has little effect on the tensile performance of the steel, but can refine the structure and precipitated phases of the steel, greatly improving the low-temperature toughness of the steel: at the same time, in steel with added Cu element, adding a small amount of Ni can inhibit the occurrence of “Cu brittleness”. Adding a higher amount of Ni has no significant adverse effect on the performance of the steel itself. If Cu and Ni are added at the same time, it can not only improve the corrosion resistance, but also refine the structure and precipitated phases of the steel, greatly improving the low-temperature toughness. However, since both Cu and Ni are relatively expensive alloying elements, in order to minimize the cost of the alloy design, the amount of Ni added to the steel of the present invention is ≤0.5%, preferably ≤0.3%.

Cr is an optional element in the present invention. Cr is added to steel to improve the strength of steel mainly through solid solution strengthening or microstructure refinement. Since the structure of the steel in the present invention is fine bainitic ferrite plus nano-precipitated carbides, the ratio of yield strength and tensile strength of the steel, i.e., the yield ratio, is relatively high, and usually reaches 0.90 or more. Adding a small amount of Cr can appropriately reduce the yield strength of steel, thereby reducing the yield ratio. In addition, adding a small amount of Cr can also improve corrosion resistance. Usually, the amount of Cr added is ≤0.5%, preferably ≤0.3%.

B is an optional element in the present invention. B can greatly improve the hardenability of steel, promote bainite transformation, and promote lath bainite transformation during medium-temperature bainite phase transformation. Therefore, adding a trace amount of B to the steel is conducive to obtaining a fine lath bainite structure. However, the B content should not be too much. Adding too much B will lead to the formation of martensite and more M-A islands, which is unfavorable to plasticity and hole expansion. Therefore, the amount of B added in the steel of the present invention is controlled at ≤0.002%, preferably ≤0.001%.

O is an impurity element in the present invention. In order to obtain steel with better performance, the lower the O content in the steel, the better. However, a lower oxidation amount will increase the cost of steelmaking. In order to ensure the performance of the strip steel, the O content in the steel of the present invention is controlled to be within 0.003%, preferably within 0.002%.

Existing high hole expansion steels are usually designed with high titanium composition, in which the main purpose of adding micro-alloying element Ti is to refine the grains, and the addition amount is generally within 0.1%. The present invention adopts a high vanadium composition design, and Ti is present as an optional element in the steel of the present invention. The main purpose of adding V in the present invention is to combine it with C to form dispersively distributed nano-vanadium carbide for precipitation strengthening.

The steel of the present invention obtains a steel having both high tensile strength and hole expansion ratio by including a content of V up to 0.1-0.5%. When the V content in the steel is 0.10-0.20%, the tensile strength of the steel is 590 MPa, and the hole expansion ratio is ≥70%, preferably ≥80%; when the V content in the steel is 0.20-0.35%, the tensile strength of the steel is 780 MPa, and the hole expansion ratio is ≥50%; when the V content in the steel is 0.35-0.50%, the tensile strength of the steel is 980 MPa, and the hole expansion ratio is ≥30%, preferably ≥40%.

Most of the microstructures of existing high hole expansion steels are ferrite or ferrite plus bainite. In order to obtain higher strength, nano-titanium carbide is used for strengthening. When the high hole expansion steel of the present invention does not contain Ti, its microstructure is bainite and nano-vanadium carbide in bainite. Moreover, according to the different V content, high hole expansion steel of different strength grades can be obtained to meet the needs of downstream users for different strength grades of high hole expansion steels. When the steel of the present invention contains Ti, its microstructure is ferrite and bainite, wherein the ferrite contains nano-TiC and the bainite contains nano-VC.

The present invention also provides a method for manufacturing the steel of the present invention, including the following steps:

The above composition is smelted by a converter or an electric furnace, secondary refined by a vacuum furnace, and casted into billets or ingots.

Heating temperature≥1100° C., preferably 1200° C. or more; holding time: 1-2 hours.

As an embodiment of the method for manufacturing the present invention, preferably, in step 3) of the method for manufacturing the present invention, the initial rolling temperature is 1000-1100° C., and rough rolling of 3-5 passes is carried out under high pressure at 950° C. or more to a cumulative deformation of ≥50%, then the intermediate billet is air-cooled or water-cooled to 900-950° C., and finishing rolling of 7 passes is carried out to a cumulative deformation of ≥70%, completing the finishing rolling between 800-900° C., and obtaining a steel strip, thereafter, the steel strip is water-cooled to 400-550° C. at a cooling rate of ≥10° C./s and coiled, and slowly cooling to room temperature at a cooling rate of ≤20° C./s, obtaining a hot-rolled steel strip.

As another embodiment of the method for manufacturing the present invention, preferably, in step 3) of the method for manufacturing the present invention, the initial rolling temperature of the hot rolling is 1050-1150° C., and rough rolling of 3-5 passes is carried out under high pressure at 1050° C. or more to a cumulative deformation of ≥50%, then an intermediate billet is heated to 950-1000° C., and finishing rolling of 3-7 passes are carried out to a cumulative deformation of ≥70%, and the finishing rolling temperature is 800-950° C., and obtaining a steel strip:

Preferably, when the steel of the present invention contains Ti, it is carried out using the above-mentioned method including sectional cooling.

In the step 3) above, the initial rolling temperature of hot rolling is 1050-1150° C., and rough rolling of 3-5 passes is carried out under high pressure at 1050° C. or more to a cumulative deformation is ≥50%. The main purpose is to refine the austenite grains, and retain more solid solutionized Ti at the same time.

In the rough rolling and finishing rolling stages of step 3), rolling should be completed as quickly as possible to ensure that there is more solid solutionized Ti and V in the austenite. After the high-temperature finishing rolling, the steel strip is first cooled to 600-750° C. at a cooling rate of ≥30° C./s, then forming ferrite and nano-TiC inside the ferrite grain at the air-cooling stage. The steel strip is subsequently water-cooled to 400-550° C. at a cooling rate of ≥10° C./s to obtain bainite and nano-precipitated VC. Eventually, a microstructure dominated by ferrite and bainite and nano-precipitated TiC and VC inside ferrite and bainite is obtained.

When the steel contains both high titanium and high vanadium, the main purpose of adding more V is to make it combine with C to form dispersively distributed nano-VC for precipitation strengthening. The high titanium and high vanadium compositions together with the sectional cooling process results in the formation of nano-TiC inside the ferrite grains in the ferrite formation region, and nano-VC inside the bainite in the bainite formation region. Through the combination of compositions and processes, the excess Ti after combining with N can be formed with C into nano-TiC in the air-cooling stage after the first water-cooling stage, which serves to strengthen the ferrite. Introducing nano-TiC into ferrite can reduce the performance difference between ferrite and bainite, which is conducive to improve the hole expansion ratio: by controlling the strength of bainite through different V contents, high hole expansion steel of different strength grades can be obtained to meet the needs of downstream users for different strength grades of high hole expansion steels.

Preferably, the method further includes step 4) Pickling, wherein the pickling operating speed of the hot-rolled strip steel is 30-120 m/min, the pickling temperature is 75-85° C., the straightening rate is ≤3%, rinsing is carried out at 35-50° C., and the surface drying and oiling are carried out at 120-140° C. to obtain pickled high hole expansion steel.

In the method for manufacturing the steel of the present invention: