RU2795542C1 - Hot-rolled and heat treated steel sheet and method for its manufacture - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: hot-rolled and heat-treated steel sheet used in the automotive industry. The sheet is made of steel having a composition comprising in wt.%: С: 0.12-0.25, Mn: 3.0-8.0, Si: 0.7-1.5, Al: 0.3-1.2, V: 0.0002-0.004, S ≤ 0.010, R ≤ 0.020, N ≤ 0.008 and optionally one or more of the following: Mo ≤ 0.5, V ≤ 0.2, Nb ≤ 0.06 and Ti ≤ 0.05, the rest is iron and inevitable impurities resulting from smelting. The sheet has a microstructure consisting in surface fractions of: 5-45% ferrite, 25-85% redistribution martensite, the indicated redistribution martensite has a carbide density of less than 2×10⁶/mm², 10-30% residual austenite, less than 8% fresh martensite. A portion of said fresh martensite is bonded to retained austenite in the form of martensite-austenite (MA) islands with a total surface fraction of less than 10%, and the cake-like index, defined as the ratio of original austenite grain size in the rolling direction (PAGS_roll) to the original austenite grain size in the direction perpendicular to the rolling direction (PAGS_norm), is less than 5.

EFFECT: required mechanical properties and machinability.

10 cl, 5 tbl

Description

The present invention relates to a hot-rolled and heat-treated high-strength steel sheet having high ductility and a method for producing such a steel sheet.

It is known that sheets made from DP (double phase) or TRIP (transformation ductility) steels are used for the manufacture of various products such as body structures and body panels of automobiles.

One of the main tasks in the automotive industry is to reduce the weight of vehicles in order to improve their fuel efficiency, taking into account the global preservation of the environment, without neglecting safety requirements. To meet these requirements, new high-strength steels are constantly being developed in the steel industry to provide sheets with improved flow and tensile strength, as well as suitable ductility and formability.

WO 2019123245 describes a process for producing a cold rolled steel sheet with high strength and high formability, yield strength YS in the range of 1000 to 1300 MPa, tensile strength TS in the range of 1200 to 1600 MPa, uniform elongation UE of at least 10%, opening expansion ratio HER of at least 20% through a hardening and redistribution (Q&P) process. The microstructure of the cold-rolled steel sheet consists in the surface fractions: 10 - 45% ferrite, having an average grain size of not more than 1.3 μm, the product of the ferrite surface fraction by the average grain size of ferrite is not more than 35 μm%, 8 - 30% of residual austenite, while said retained austenite has a Mn content of more than 1.1⋅Mn%, where Mn% denotes the Mn content of the steel, maximum 8% fresh martensite, maximum 2.5% cementite, and martensite obtained from the redistribution process in the Q&P process (hereinafter martensite redistribution). In order to achieve such mechanical properties and obtain this microstructure, the hot rolled steel sheet must first be subjected to annealing, cold rolling and secondary annealing prior to the quenching and redistribution steps. These processes, and in particular the second annealing, make it possible to control the content of Mn in the retained austenite, obtain a combination of high ductility and high strength, but complicate the manufacturing process. Thus, it is an object of the invention to provide a hot-rolled steel sheet with YS greater than 950 MPa, tensile strength TS greater than 1180 MPa, uniform elongation UE greater than 10%, and a hole expansion ratio HER greater than 25%, and easy to be machined under conventional processing conditions.

The object of the present invention is achieved by providing a steel sheet according to claim 1. The steel sheet may also include characteristics according to any one of paragraphs. 2-8. Another object is achieved by proposing the method according to claim 9. Another object of the invention is achieved by proposing a steel sheet according to claim 10.

The invention will now be described in detail and illustrated by non-limiting examples.

Hereinafter, Ae1 denotes the equilibrium transformation temperature, below which austenite is completely unstable, Ae3 denotes the equilibrium transformation temperature, above which austenite is completely stable, Ms denotes the temperature at which martensite transformation begins, i.e. the temperature at which austenite begins to transform into martensite upon cooling; and Tnr is the non-recrystallization temperature. These temperatures can be calculated from the formula based on the weight percentages of the respective elements:

Ae1=670 + 15⋅%Si – 13⋅%Mn + 18⋅%Al

Ae3 = 890 – 20 ⋅ √%C + 20 ⋅%Si – 30 ⋅%Mn + 130 ⋅%Al

Ms= 560-(30⋅%Mn+13⋅%Si-15⋅%Al+12⋅%Mo)-600⋅ (1-exp(-0.96*%C))

Tnr= 825+2300⋅⋅%Nb+710⋅%Ti+150⋅%Mo+120⋅%V+8⋅%Mn

The composition of the steel according to the invention is indicated in mass percent.

According to the invention, the carbon content is 0.12 - 0.25%. The addition of carbon above 0.25% may reduce the weldability of the steel sheet. If the carbon content is lower than 0.12%, the retained austenite fraction is not sufficiently stabilized to obtain sufficient elongation. In a preferred embodiment, the carbon content is 0.15-0.25%.

According to the invention, the manganese content is 3.0 to 8.0% to obtain sufficient elongation while stabilizing the austenite. Above 8.0% addition increases the risk of center segregation at the expense of yield strength and tensile strength. Below 3.0%, the final structure contains an insufficient proportion of retained austenite so that the desired combination of ductility and strength is not achieved. In a preferred embodiment, the manganese content is 3.0-4.4%. In another preferred embodiment, the manganese content is 3.0-4.3%. In another preferred implementation, the manganese content is 3.0 - 4.2%. In another preferred embodiment, the manganese content is 3.0-4.1%. In another preferred implementation, the manganese content is 3.0 - 4.0%.

The silicon content according to the invention is 0.7 - 1.5%. The addition of silicon in an amount of at least 0.7% helps to stabilize a sufficient amount of retained austenite. Above 1.5%, silicon oxides are formed on the surface, which impairs the ability of the steel to be coated. In a preferred embodiment, the silicon content is 0.8-1.3%.

The aluminum content is 0.3 - 1.2%. Aluminum is a very effective element for deoxidizing steel in the liquid phase during processing. The aluminum content does not exceed 1.2% to avoid inclusions and oxidation problems. In a preferred embodiment, the aluminum content is 0.3-0.8%.

According to the invention, the boron content is 0.0002 - 0.004% to increase the hardenability of the steel and improve the weldability.

Optionally, some elements may be added to the composition of the steel according to the invention.

Niobium can be further added up to 0.06% to refine the austenite grains during hot rolling and provide precipitation strengthening. Preferably, the minimum amount of niobium added is 0.0010%. At a content above 0.06%, the yield strength and relative elongation are not provided at the required level.

Molybdenum can be additionally added up to 0.5%. Molybdenum stabilizes retained austenite, thereby reducing austenite decay during redistribution (during P&Q). The addition of molybdenum above 0.5% is costly and inefficient in terms of the required properties.

Vanadium can be added up to 0.2% to provide precipitation strengthening.

Titanium can be added up to 0.05% to provide precipitation strengthening. If the titanium content is higher than or equal to 0.05%, the yield strength and elongation are not provided at the desired level. Preferably, at least 0.01% titanium is added in addition to boron to protect against BN formation.

The rest of the steel composition is made up of iron and impurities formed as a result of smelting. In this regard, at least P, S and N are considered residual elements, which are unavoidable impurities. The content of S is less than 0.010%, P is less than 0.020%, and N is less than 0.008%.

The microstructure of the hot-rolled and heat-treated steel sheet according to the invention will now be described.

The hot-rolled and heat-treated steel sheet has a microstructure consisting in surface fractions of 5-45% ferrite, 25-85% redistribution martensite, said redistribution martensite having a carbide density of less than 2×10 ⁶ /mm ² , 10-30% retained austenite, less than 8% fresh martensite, part of the fresh martensite combines with residual austenite to form martensite-austenite (MA) islands with a total surface fraction of less than 10% and a pancake index below 5.

The microstructure of hot rolled and heat treated steel sheet includes 5 - 45% ferrite. This ferrite is formed during annealing between (Ae1+Ae3)/2 and Ae3. When the ferrite content is less than 5%, the uniform elongation does not reach 10%. If the proportion of ferrite is higher than 45%, the tensile strength of 1180 MPa and the yield strength of 950 MPa are not achieved. Preferably, the microstructure includes 10% or more ferrite. More preferably, the microstructure includes 15% or more ferrite.

The microstructure of hot rolled and heat treated steel sheet includes 25 - 85% redistribution martensite to ensure high ductility of the steel. Redistribution martensite is a martensite formed by cooling after annealing, and then carbon is redistributed in the redistribution step. Said redistribution martensite has a carbide density of less than 2×10 ⁶ /mm ² . The low density of carbides within the redistribution martensite provides a combination of a suitable level of tensile strength and elongation.

The microstructure of hot rolled and heat treated steel sheet includes 10% to 30% retained austenite to ensure high ductility of the steel and less than 8% fresh martensite. Fresh martensite is formed when hot rolled and heat treated steel sheet is cooled to room temperature.

Part of the fresh martensite combines with residual austenite to form martensitic-austenite (MA) islands with a total surface fraction of less than 10%. In a preferred embodiment, these (MA) islands have an aspect ratio of less than or equal to 2, the aspect ratio being defined as the ratio of the maximum grain length to the maximum grain width measured at 90° to the indicated maximum length. The size of fresh martensite and (M-A) islands is less than 0.7 µm.

The microstructure of the hot rolled and heat treated steel sheet has a pancake index below 5. The pancake index is defined as the ratio of the original austenite grain size in the rolling direction PAGS _roll to the original austenite grain size in the direction perpendicular to the rolling direction PAGS _norm . PAGS _roll is the maximum length of the former austenite grain in the rolling direction. PAGS _norm is the maximum length of the former austenite grain in the direction perpendicular to the rolling direction. When the pancake index is higher than 5, the target opening expansion ratio cannot be reached.

The steel sheet according to the invention can be made by any suitable manufacturing method and can be determined by a person skilled in the art. However, it is preferable to use the method according to the invention, comprising the following steps.

The semi-finished product suitable for further hot rolling has the steel composition described above. The semi-finished product is heated to a temperature T _reheat of 1150 to 1300° C. so that hot rolling can be facilitated, with a hot rolling end temperature FRT of (Tnr-100) to 950° C. to obtain a hot rolled steel sheet. The maximum value of FRT is chosen so as to avoid coarsening of austenite grains and so that the product of PAGS _roll and PAGS _norm is below 1000 µm². When PAGS _roll times PAGS _norm exceeds 1000 µm², the target strength level cannot be reached.

FRT above (Tnr-100)°C is needed to create a microstructure with a pancake index of the original austenite grain below 5, with the pancake index being defined as the ratio of PAGS _roll to PAGS _norm . When the pancake index is higher than 5, the target opening expansion ratio cannot be reached.

Then the hot rolled steel is cooled and wound into a coil at a temperature T _coil 20 - 700°C. Preferably the winding temperature is 20-550°C.

After winding, the sheet can be pickled to remove oxidation products. After winding and cooling to room temperature, the microstructure of hot rolled and coiled steel sheet includes martensite and bainite, the sum of which is more than 80%, strictly less than 20% ferrite, and strictly less than 20% of the sum of martensitic-austenitic (MA) islands and carbides, and the PAGSroll product is The PAGSnorm is less than 1000 µm² and the pancake index is below 5. Preferably, the microstructure after winding and cooling includes less than 10% ferrite, and more preferably no ferrite. Preferably, the microstructure after winding and cooling includes less than 10% of the sum of M-A islands and carbides.

Martensite M-A islands is a fresh martensite formed during the final cooling. Martensite, contained in the sum of martensite and bainite more than 80%, is a self-tempering martensite. Determination of the type of martensite and its quantification can be performed using a scanning electron microscope.

Then the hot rolled steel sheet goes through a quenching and redistribution (Q&P) process. The hardening and redistribution process includes the following steps:

- reheating the annealed steel sheet to a temperature of TA1 strictly below Ae3 and above (Ae1 + Ae3) / 2 and holding at the specified annealing temperature TA1 for a holding time tA1 of 3-1000 s to obtain a heat-treated steel sheet to obtain austenitic and ferritic structure .

- quenching the heat-treated steel sheet to a quenching temperature TQ below (Ms-50°C) to obtain a hardened steel sheet. During this quenching step, the austenite partially transforms into martensite. If the quenching temperature is higher (Ms-50°C), the proportion of tempered martensite in the final structure is too small, resulting in a final proportion of fresh martensite above 8%, and negatively affects the overall elongation of the steel.

reheating the hardened steel to a redistribution temperature TP of 350 to 550°C and holding at said redistribution temperature for a redistribution time of 1 to 1000 seconds before cooling to room temperature to obtain a hot-rolled and heat-treated steel sheet.

The hot rolled and heat treated steel sheet according to the invention has a tensile strength TS above 1180 MPa, a yield strength YS above 950 MPa, a uniform elongation UE above 10% and a hole expansion ratio HER above 25%. TS, YS, UE and total elongation TE are measured in accordance with ISO 6892-1. HER is measured in accordance with ISO 16630. In a preferred embodiment, the hot rolled and heat treated steel sheet according to the invention has TS and YS expressed in MPa, UE, TE and HER expressed in % and satisfies the following formula: YS⋅UE+TS ⋅TE+TS⋅HER>65000. Preferably the total elongation TE is greater than 14%.

The invention will now be illustrated by the following non-limiting examples.

Examples

4 samples, the composition of which is presented in table 1, are cast into semi-finished products and processed into steel sheets according to the technological parameters presented in table 2.

The tested composition is presented in the following table, in which the content of the elements is expressed in mass percent.

Table 1. Compositions

Steels A-D according to the invention

The cast steel semi-finished products were subjected to reheating, hot rolling and coiling before the quenching and tempering process. Samples 2 and 5 are annealed after coiling at T2 before cold rolling with a reduction ratio of 50%. The following special conditions apply:

Table 2. Process parameters

*: samples according to the invention.

Underlined values: do not correspond to the invention

The annealed sheets are then analyzed and the respective microstructural elements before Q&P, after Q&P and mechanical properties after Q&P are respectively presented in Tables 3, 4 and 5.

The microstructure of hot rolled and coiled steel sheets is determined before the Q&P process, the results are shown in the following table:

Table 3. Microstructure of steel sheets before Q&P process

* : samples according to the invention.

Underlined values: not in accordance with the invention

B: denotes the proportion of the bainite surface.

F: stands for the proportion of ferrite surface.

M: denotes the proportion of martensite surface.

MA: denotes the proportion of the surface of martensitic-austenite islands.

The surface fractions are determined by the following method: a sample is cut from a cold-rolled and heat-treated steel sheet, polished and etched with a reagent known in the art to reveal the microstructure. The section is then examined with an optical or scanning electron microscope, such as a field emission gun scanning electron microscope ("FEG-SEM") at a magnification of more than 5000×, connected to a BSE (backscattered electrons) device.

The determination of the surface fraction of each component is performed by image analysis using a known method. The proportion of retained austenite is determined, for example, by X-ray diffraction (XRD).

PAGS in the rolling direction (RD) PAGS _roll and in the direction perpendicular to the rolling direction (ND) PAGS _norm are determined by the following method: a sample is cut from a hot rolled sheet, polished and etched with a known reagent to reveal the microstructure, especially the former austenitic grain boundaries. The cross section in the RD-ND plane is then examined in an optical or scanning electron microscope, for example, using a scanning electron microscope at a magnification of 1000 - 5000x. Measure the maximum length of the former austenite grains in RD and ND.

The microstructure of the test samples is determined and is presented in the following table.

Table 4 Microstructure of steel sheet after Q&P process

*: samples according to the invention.

Underlined values: do not correspond to the invention

n.a.: values were not evaluated.

γ: denotes the surface proportion of retained austenite.

PM: denotes the redistribution martensite surface proportion.

FM: denotes the surface fraction of fresh martensite.

B: denotes the proportion of the bainite surface.

F: stands for the proportion of ferrite surface.

MA: denotes the proportion of the surface of martensitic-austenite islands.

The mechanical properties of the test specimens are determined and presented in the following table.

Table 5 Mechanical Properties of P&Q Steel Sheet

* : samples according to the invention / Underlined values: not according to the invention.

Examples 1 and 3, according to the invention, have all the desired properties due to their specific composition and microstructure. In the case of Sample 2, the steel sheet is annealed and cold rolled before the Q&P process. The microstructure before Q&P results in 80% ferrite, resulting in a high content of fresh martensite after Q&P. This high proportion of large size fresh martensite results in a hole expansion ratio of less than 25%.

In the case of Sample 4, the steel sheet is hot rolled with a lower FRT (Tnr-100), resulting in a pancake index greater than 5 before and after Q&P. Therefore, the expansion ratio of the hole does not meet the target value.

In the case of Sample 5, the steel sheet is annealed and cold rolled before the Q&P process. The microstructure before Q&P results in 97% ferrite, resulting in a high fresh martensite size after Q&P. This coarse fresh martensite results in a hole expansion ratio of less than 25%.

Claims (60)

1. Hot rolled and heat treated steel sheet made from steel having a composition comprising, in wt %: C: 0.12-0.25 Mn: 3.0-8.0 Si: 0.7-1.5 Al: 0.3-1.2 B: 0.0002-0.004 S ≤ 0.010 P ≤ 0.020 N ≤ 0.008 and optionally one or more of the following in wt.%: Mo ≤ 0.5 V ≤ 0.2 Nb ≤ 0.06 Ti ≤ 0.05 the rest of the iron and inevitable impurities resulting from smelting, moreover, the specified steel sheet has a microstructure consisting in fractions of the surface of: - 5-45% ferrite, - 25-85% redistribution martensite, wherein said redistribution martensite has a carbide density of less than 2×10 ⁶ /mm ² , - 10-30% residual austenite, - less than 8% fresh martensite, at the same time, a part of the specified fresh martensite is connected with residual austenite in the form of martensitic-austenite (MA) islands with a total surface fraction of less than 10%, and the pancake index, defined as the ratio of the size of the original austenite grain in the rolling direction (PAGS _roll ) to the size of the original austenite grain in the direction perpendicular to the rolling direction (PAGS _norm ) is less than 5. 2. Hot-rolled and heat-treated steel sheet according to claim 1, in which the manganese content is 3.0-5.0 wt.%. 3. Hot-rolled and heat-treated steel sheet according to claim. 1 or 2, in which the silicon content is 0.8-1.3 wt.% 4. Hot-rolled and heat-treated steel sheet according to any one of paragraphs. 1-3, the yield strength of which exceeds 950 MPa. 5. Hot rolled and heat treated steel sheet according to any one of paragraphs. 1-4, the tensile strength of which exceeds 1180 MPa. 6. Hot rolled and heat treated steel sheet according to any one of paragraphs. 1-5, the uniform elongation of which exceeds 10%. 7. Hot-rolled and heat-treated steel sheet according to any one of paragraphs. 1-6, whose opening expansion ratio exceeds 25%. 8. Hot-rolled and heat-treated steel sheet according to any one of paragraphs. 1-7, in which the size of fresh martensite and martensite-austenite islands is less than 0.7 microns. 9. A method for manufacturing a hot-rolled and heat-treated steel sheet, including the following successive steps: - casting steel to obtain a semi-finished product having the composition specified in paragraph 1, - reheating of the semi-finished product at a temperature T _reheat 1150-1300°C, - hot rolling the reheated semi-finished product with a finishing rolling temperature FRT between (Tnr-100) and 950°C to obtain a hot-rolled steel sheet, where Tnr represents a non-recrystallization temperature defined as 825+2300⋅%Nb+710⋅%Ti+150⋅ %Mo+120⋅%V+8⋅%Mn, - winding the hot-rolled steel sheet into a coil at a winding temperature T _coil of 20-700°C, and cooling to room temperature to obtain a microstructure, including martensite and bainite, the sum of which is more than 80%, strictly less than 20% ferrite and strictly less than 20 % of the sum of martensitic-austenite (MA) islands and carbides, and having the product of the size of the original austenite grain in the rolling direction (PAGS _roll ) by the size of the original austenite grain in the direction perpendicular to the rolling direction (PAGS _norm ), which is less than 1000 μm ² , and a pancake index of less than 5, - reheating the hot-rolled steel sheet to a temperature TA1 strictly below Ae3 and above (Ae1 + Ae3)/2 and holding the steel sheet at the specified annealing temperature TA1 for a holding time tA1 of 3-1000 s, where the temperature Ae1 and Ae3 is defined as Ae1=670 + 15⋅%Si – 13⋅%Mn + 18⋅%Al Ae3 = 890 – 20⋅√%C + 20⋅%Si – 30⋅%Mn + 130⋅%Al, - quenching a hot rolled steel sheet at a quenching temperature TQ below (Ms-50°C) to obtain a hardened steel sheet, where Ms is defined as Ms=60-(30⋅%Mn+13⋅%Si-15⋅%Al+12⋅%Mo)-600⋅(1-exp(-0.96⋅%C)), reheating the hardened steel sheet to a redistribution temperature TP of 350-550°C and holding the hardened steel sheet at said redistribution temperature for a redistribution time of 1-1000 s, - cooling the steel sheet to room temperature to obtain a hot-rolled and heat-treated steel sheet. 10. Hot rolled and coiled steel sheet made from steel having a composition comprising, in wt %: C: 0.12-0.25 Mn: 3.0-8.0 Si: 0.70-1.50 Al: 0.3-1.2 B: 0.0002-0.004 S ≤ 0.010 P ≤ 0.020 N ≤ 0.008 and optionally one or more of the following in wt %: Mo ≤ 0.5 V ≤ 0.2 Nb ≤ 0.06 Ti ≤ 0.05 the rest of the iron and inevitable impurities resulting from smelting, moreover, the specified steel sheet has a microstructure consisting in fractions of the surface of: - martensite and bainite, the sum of which exceeds 80%, - strictly less than 20% ferrite, - strictly less than 20% of the sum of martensitic-austenitic (MA) islands and carbides, and has a product of the size of the original austenite grain in the rolling direction (PAGS _roll ) and the size of the original austenite grain in the direction perpendicular to the rolling direction (PAGS _norm ) of less than 1000 μm ² , and a pancake index of less than 5.