RU2798523C1 - High-strength hot-rolled steel sheet and method for its manufacturing - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention is related to hot-rolled steel sheet used in the automotive industry. The sheet is made of steel having the following composition, in wt.%: С: 0.10-0.25, Мn: 3.5-5.0, Si: 0.80-1.60 V: 0.0003-0.004, S ≤ 0.010, P ≤ 0.020, N ≤ 0.008, if necessary, at least one of the following elements: Ti ≤ 0.04, Nb ≤ 0.05, Mo ≤ 0.3, Al ≤ 0.90 и Cr ≤ 0.80, the rest is iron and inevitable impurities resulting from smelting. The steel sheet has a microstructure that includes, in fractions of the surface: 50-80% lath bainite with a size ratio greater than or equal to 3, less than 30% granular bainite with a size ratio less than 3, the rest are martensite and martensite-austenite M-A pockets having a size ratio less than or equal to 2, and austenite films, moreover, the total amount of martensite, M-A pockets and austenitic films is 15-35%. Less than 20% of said martensite and said M-A packets have a maximum grain length L_max times maximum grain width W_max greater than 1 mcm².

EFFECT: steel sheet has high mechanical properties, the required level of machinability and weldability.

8 cl, 5 tbl, 1 ex

Description

The present invention relates to a high strength steel sheet having high toughness and suitable weldability, and to 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 to manufacture various products such as body structures and body panels for 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, the steel industry is constantly developing new high-strength steels to have sheets with improved deformability and tensile strength, as well as suitable ductility and formability.

One of the developments aimed at improving the mechanical properties is to increase the manganese content in steels. The presence of manganese contributes to an increase in the ductility of steels due to the stabilization of austenite. But these medium manganese steels have the disadvantages of brittleness.

WO 2007101921 describes a method for producing hot-rolled sheets from "multi-phase" steels, in particular with a manganese content of 1-3%. The microstructure consists of at least 75% bainite, retained austenite greater than or equal to 5%, and martensite greater than or equal to 2%. To achieve a Charpy V-notch fracture energy above 28 J (corresponding to 0.52 J/mm²) and a target microstructure, it is necessary to control the cooling of the hot rolled steel sheet. Two stages of cooling are actually required to obtain the required properties, which complicates the manufacturing process.

Thus, the object of the invention is to solve the above problem and to provide a steel sheet having a high Charpy impact strength at 20°C above 0.50 J/mm², a tensile strength TS greater than or equal to 1450 MPa, high uniform elongation greater than or equal to 5%, which is easily processed by conventional technology. Another object of the invention is to provide a steel sheet having suitable weldability.

The purpose of the present invention is achieved by offering a steel sheet according to claim 1. The steel sheet may also have the characteristics of any one of paragraphs. 2-6. Another object is achieved by offering the method according to claim 7. Another object of the invention is achieved by offering a steel sheet having the characteristics according to claim 8.

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

Here and below, Ms denotes the temperature at which martensite begins to form, i.e., the temperature at which austenite begins to transform into martensite upon cooling. This temperature can be calculated from the formula, based on the mass percentages of the relevant elements:

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

Next will be described the composition of the steel according to the invention, the content of which is expressed in mass percent.

The carbon content is 0.10 - 0.25%. If the carbon content is too high, the weldability of the steel sheet is insufficient. If the carbon content is below 0.10%, the austenite fraction is not sufficiently stabilized to obtain the desired properties. In a preferred embodiment of the invention, the carbon content is 0.15 - 0.20%.

The manganese content is 3.5 - 5.0%. Above 5.0% addition increases the risk of center segregation at the expense of viscosity. Below 3.5%, the final structure contains an insufficient proportion of retained austenite to obtain the required properties. In a preferred embodiment of the invention, the manganese content is 3.5 to 4.5%.

According to the invention, the silicon content is 0.80 - 1.60%. The addition of at least 0.80% silicon helps to stabilize a sufficient amount of retained austenite. The silicon content above 1.60% deteriorates the toughness. In addition, silicon oxides are formed on the surface, which impairs the ability of the steel to be coated. In a preferred embodiment of the invention, the silicon content is 1.00 - 1.60%.

According to the invention, the boron content is 0.0003 - 0.004%. The presence of boron delays the bainite transformation to a lower temperature, and bainite formed at low temperature has a lath morphology that increases toughness. In addition, boron improves the weldability of the steel sheet. The content above 0.004% promotes the formation of boron carbides at the boundaries of the former austenite grains, which makes the steel more brittle. At a content below 0.0003%, the concentration of free B, which is released at the boundaries of the former austenite grains, is not enough to increase the toughness of the steel. In a preferred embodiment of the invention, the boron content is 0.001 - 0.003%.

Certain elements may optionally be added to the composition of the steel according to the invention.

Titanium may optionally be added up to 0.04% to provide precipitation strengthening. Preferably, at least 0.01% titanium is added in addition to boron to protect against BN formation.

Niobium can be added up to 0.05% to refine the austenite grains during hot rolling and provide precipitation strengthening. Preferably, the minimum amount of niobium added is 0.0010%.

Optionally, molybdenum may be added, but not more than 0.3%. Molybdenum stabilizes the austenite and increases the toughness of the steel. In addition, molybdenum improves the weldability of the steel sheet. The addition of molybdenum above 0.3% is costly and inefficient in terms of the required properties.

Optionally aluminum can be added up to 0.90% as it is a very effective element for deoxidizing the steel in the liquid phase during processing. In addition, aluminum improves the weldability of the steel sheet. The aluminum content is less than 0.90% to avoid inclusions and oxidation problems. Preferably the aluminum content is 0.10 - 0.90%. More preferably, the aluminum content is 0.20-0.90%. More preferably the aluminum content is 0.30-0.90%, more preferably 0.40-0.90%.

According to the invention, a maximum chromium content of 0.80% is allowed. The effect of saturation is noted above, and the addition of chromium is both useless and expensive.

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. Their content is less than 0.010% S, less than 0.020% P and less than 0.008% N.

In particular, phosphorus is released at the grain boundary, and when the phosphorus content is higher than 0.020%, the toughness of the steel decreases.

The microstructure of the hot rolled steel sheet according to the invention will now be described. The aspect ratio indicated below represents the ratio of the maximum grain length L _max to the maximum grain width W _max measured at an angle of 90° to the indicated maximum length.

Hot-rolled steel sheet has a microstructure consisting of 50-80% lath bainite in the surface, less than 30% granular bainite, and the rest is martensite, martensite-austenitic islands (MA) and austenite films, the sum of which is 15-35%. Moreover, less than 20% of the martensite and M-A islands have a maximum grain length L _max times a maximum grain width W _max greater than 1 µm².

The lath morphology of bainite is obtained due to the presence of boron, which slows down the bainite transformation, and due to low-temperature winding. In accordance with the present invention, lath bainite is bainite having an aspect ratio greater than or equal to 3. The presence of 50% to 80% lath bainite is beneficial to the toughness of the hot rolled steel. Granular bainite has an aspect ratio below 3.

The rest of the microstructure consists of martensite, M-A islands and austenitic films, the sum of which is 15 - 35% to ensure a uniform elongation above 5%. If the sum of martensite, M-A islands and austenite films is above 35%, then the austenite in M-A islands and austenite films becomes unstable and turns into martensite, which leads to a decrease in relative elongation.

Less than 20% of the martensite fraction and M-A islands have an L _max x W _max product greater than 1 µm². Above 20%, the M-A islands turn into fresh martensite, resulting in poor elongation.

Martensitic-Austenitic (M-A) islands have an aspect ratio less than or equal to 2. These M-A islands are formed during winding. Part of the austenite is converted to lath bainite as described above. Part of the austenite turns into martensite, forming M-A islands during winding. The last part of the austenite remains in the final microstructure. The austenite films are austenite between bainite laths with an aspect ratio greater than or equal to 2. Both M-A islands and austenite films improve the toughness of the hot rolled steel sheet.

The hot rolled steel sheet according to the invention has a Charpy impact strength at 20°C strictly above 0.50 J/mm2, measured in accordance with ISO 148-1:2006 (F) and ISO 148-1:2017(F). The hot rolled steel sheet according to the invention has a tensile strength TS greater than or equal to 1450 MPa and a uniform elongation UE greater than or equal to 5%. Preferably, the hot rolled steel sheet according to the invention has a total elongation TE strictly above 7%. TS, UE and TE are measured according to ISO 6892-1.

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 of 1150 to 1300°C to facilitate hot rolling, and the hot rolling end temperature FRT is 750 to 900°C. Preferably the FRT is 800-900°C. When the FRT is above 900° C., the bainitic transformation kinetics slow down considerably during winding, resulting in a high proportion of martensite, M-A islands and austenite in the final microstructure. In addition, the presence of a large proportion of martensite and M-A islands with L _max ⋅ W _max above 1 μm ² leads to a deterioration in relative elongation.

The hot rolled steel is then cooled and coiled at a temperature T _coil between (Ms-100°C) and 550°C.

Then, the hot rolled steel sheet is cooled to room temperature.

After winding, the sheet can be pickled to remove oxidation products.

Another object of the invention is to provide a steel sheet having suitable weldability.

A welded product is made by fabricating two sheets of hot rolled steel and spot welding two steel parts.

Spot welding under the conditions of ISO 18278-2 is performed on hot rolled steel sheets.

In the test used, the samples consist of two identical sheets of steel welded crosswise. The force is applied so as to destroy the weld point. This force, known as the lateral tensile strength (CTS), is expressed in daN. It depends on the diameter of the weld point and the thickness of the metal, that is, the thickness of the steel and the metal coating. It allows you to calculate the coefficient α, which is the ratio of the CTS value to the product of the diameter of the weld point and the thickness of the substrate. This factor is expressed in daN/mm².

The ratio of the electrode diameter to the diameter of the weld point is equal to the diameter of the electrode divided by the diameter of the molten zone.

Spot electric welds connecting the first sheet with the second sheet are characterized by high resistance in transverse tensile testing, determined by the value of α not less than 50 daN/mm ² , and the ratio of the electrode diameter to the diameter of the welding spot is not less than 80%.

The invention will now be illustrated by the following examples, which in no way limit its scope.

Example 1

4 samples, the compositions of which are shown in table 1, are cast into semi-finished products and processed into steel sheets.

Table 1. Compositions

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

Steels A and B correspond to the invention, C and D do not correspond to the invention.

Table 2. Process parameters

Steel semi-finished products in cast form are reheated to 1200°C, hot rolled and coiled. The following specific conditions apply:

Underlined values: do not correspond to the invention

The hot rolled sheets are then analyzed and the respective microstructural elements, mechanical properties and weldability properties are presented in Tables 3, 4 and 5 respectively.

Table 3 Microstructure of Hot Rolled Steel Sheet

The percentage of phases of the microstructure of the obtained hot-rolled steel sheet is determined:

Underlined values: not in accordance with the invention

n.a.: value not evaluated

The surface fractions of phases in the microstructure are determined by the following method: a sample is cut out from hot-rolled steel, which is polished and etched with a reagent known in the prior art to reveal the microstructure. The section is then examined using a scanning electron microscope, for example, a field emission gun scanning electron microscope ("FEG-SEM") at a magnification of more than 5000 in secondary electron mode.

Determination of the surface fraction of austenitic films and M-A islands is performed by SEM after etching with nital or picral/nital reagents.

In accordance with the present invention, lath bainite will be bainite having an aspect ratio greater than or equal to 3. According to the invention, islands M-A have an aspect ratio less than or equal to 2.

Table 4 Mechanical Properties of Hot Rolled Steel Sheet

The mechanical properties of the tested samples are determined and presented in the following table:

Underlined values: do not meet target values

n.a.: value not evaluated

Table 5 Weldability of Hot Rolled Steel Sheet

Weldability properties of some samples are determined and presented in the following table:

Underlined values: do not meet target values

The examples show that the steel sheets according to the invention, namely examples 1-3, are the only sheets that exhibit all the desired properties due to their specific composition and microstructure.

In Test 4, the steel sheet was hot rolled at an FRT temperature of 910° C., resulting in a high proportion of martensite and MA islands. This results in a uniform elongation of less than 5%.

The absence of boron in steels C and D results in a low level of Charpy impact strength in tests 5 and 6 with the formation of more than 30% granular bainite, which reduces the fracture toughness of the steel. As for the weldability parameters, the absence of boron, molybdenum and aluminum negatively affects α and the ratio of the electrode diameter to the weld spot.

Claims (33)

1. Hot rolled steel sheet made from steel having a composition comprising, wt.%: C: 0.10-0.25; Mn: 3.5–5.0; Si: 0.80–1.60; B: 0.0003-0.004; S ≤ 0.010; P ≤ 0.020; N ≤ 0.008; at least one of the following, if necessary: Ti ≤ 0.04; Nb ≤ 0.05; Mo ≤ 0.3; Al ≤ 0.90; Cr ≤ 0.80; the rest is iron and inevitable impurities resulting from smelting, moreover, the specified steel sheet has a microstructure, including in fractions of the surface: 50-80% lath bainite with an aspect ratio greater than or equal to 3, less than 30% granular bainite with an aspect ratio of less than 3, the rest are martensite and martensitic-austenitic M-A islands having a size ratio of less than or equal to 2, and austenite films, with the total amount of martensite, M-A islands and austenitic films being 15-35%, and less than 20% of said martensite and said islands MA have a maximum grain length L _max times a maximum grain width W _max greater than 1 µm ² . 2. Hot rolled steel sheet according to claim 1, wherein the manganese content is 3.5-4.5% by weight. 3. Hot rolled steel sheet according to claim 1 or 2, wherein the silicon content is 1.00-1.60% by weight. 4. Hot rolled steel sheet according to any one of paragraphs. 1-3, which has a Charpy impact strength at 20°C strictly above 0.50 J/mm ² . 5. Hot rolled steel sheet according to any one of paragraphs. 1-4 which has a tensile strength TS greater than or equal to 1450 MPa. 6. Hot rolled steel sheet according to any one of paragraphs. 1-5 which has a uniform UE elongation greater than or equal to 5%. 7. A method of manufacturing a hot-rolled steel sheet according to any one of paragraphs. 1-6, including the following successive stages: casting steel to obtain a semi-finished product having a composition, including, wt.%: C: 0.10-0.25, Mn: 3.5-5.0, Si: 0.80-1.60, B: 0.0003 -0.004, S ≤ 0.010, P ≤ 0.020, N ≤ 0.008, optionally at least one of the following elements: Ti ≤ 0.04, Nb ≤ 0.05, Mo ≤ 0.3, Al ≤ 0.90, Cr ≤ 0.80, the rest is iron and unavoidable impurities resulting from smelting, reheating of the semi-finished product at T _reheat 1150-1300°C, hot rolling of a semi-finished product with a final hot rolling temperature of 750-900°C to obtain hot-rolled steel, cooling of hot rolled steel, winding hot rolled steel into a coil at a winding temperature T _coil between (Ms-100°C) and 550°C to obtain a coil of steel sheet, and cooling the steel sheet coil to room temperature to obtain a hot-rolled steel sheet. 8. Seam of resistance spot welding of two hot-rolled steel sheets according to any paragraph. 1-6 or hot-rolled steel sheets obtained by the method according to p. 7, and the resistance spot welding seam has a value of α, which is the ratio of the transverse tensile strength to the product of the diameter of the weld point and the thickness, which is not less than 50 daN/mm ² , and the ratio diameter of the electrode to the diameter of the welding point at least 80%.