UA129224C2 - High strength cold rolled and galvannealed steel sheet and manufacturing process thereof - Google Patents

Abstract

The invention deals with a cold rolled and galvannealed steel sheet having a composition comprising, by weight percent: C 0.15-0.25, Mn 2.4-3.5, Si 0.30-0.90, Cr 0.30-0.70, Mo 0.05-0.35, Al 0.001-0.09, Ti 0.01-0.06, B 0.0010-0.0040, Nb 0.01-0.05, P≤0.020, S≤0.010 and N≤0.008, the remainder of the composition being iron and unavoidable impurities resulting from the smelting, and having a microstructure consisting of, in surface fraction, between 80 % and 90 % of martensite, the balance being ferrite and bainite.

Description

This invention relates to a high-strength cold-rolled and annealed galvanized steel sheet and a method for producing such a steel sheet.

Reducing the weight of vehicles to reduce CO2 emissions is a major challenge in the automotive industry. This weight saving must be combined with safety requirements. To meet these requirements, the increased demand for high-strength steels with tensile strengths above 1450 MPa has led the steel industry to continuously develop new grades.

These steels are usually coated with a metal coating to improve properties such as corrosion resistance. Metal coatings can be applied by hot-dip galvanizing after annealing the steel sheets. To improve spot welding, after hot-dip coating, alloying treatment can be carried out to obtain a galvanized annealed steel sheet, so that the iron in the steel sheet diffuses into the zinc coating to obtain a zinc-iron alloy on the steel sheet.

UMO publication 2019188190 refers to a high-strength galvanized or annealed galvanized steel sheet, which has a tensile strength higher than 1470 MPa. To obtain such a level of tensile strength, the carbon content in the steel sheet is 0.200-0.280 95 wt., which can reduce the weldability of the steel sheet. In addition, the formation of ferrite and bainite is prevented, the total amount of which in the sum with pearlite does not exceed 2 95, which ensures a good level of tensile strength. For this, the aging stage after cold rolling must be carried out at a temperature higher than Ac3.

Publication MUO 2016199922 refers to a high-strength galvanized annealed steel sheet with a tensile strength above 1470 MPa. The large amount of carbon 0.25-0.70 95 allows to obtain such a high level of tensile strength. But the weldability of the steel sheet may be reduced. After the alloying stage, the steel sheet must be cooled in a controlled manner so that at the end of the cooling more than 1095 of retained austenite is obtained. After this cooling stage, the galvanized annealed steel sheet undergoes a tempering stage to obtain tempered martensite, stimulate bainite transformation and concentrate carbon in the retained austenite to obtain the final microstructure: 10-60 95 retained austenite, less than 595 high-temperature tempering martensite, less than 5595 low-temperature tempering martensite, less than 10 95 fresh martensite, less than 1595 ferrite, less than 10 95 pearlite, the rest is bainite. These controlled cooling and tempering stages complicate the production process.

Therefore, the aim of the invention is to solve the above-mentioned problem and to provide a galvanized annealed steel sheet which has a tensile strength greater than or equal to 1450 MPa and is easily processed by a conventional process route.

In a preferred embodiment of the invention, the yield strength 5 is greater than or equal to 1050 MPa.

The object of the present invention is achieved by providing a steel sheet according to claim 1. The steel sheet may also have the characteristics of claims 2-5. Another object is achieved by providing a method according to claim 6. The method may also include the characteristics of any of claims 7-8.

The invention will be described in detail below and illustrated by examples without introducing limitations.

Here and below, Ac3 denotes the temperature above which the microstructure is fully austenitic, Ac! denotes the temperature above which austenite formation begins.

The composition of the steel according to the invention, expressed in mass percent, will now be described.

The carbon content is 0.15-0.25 95 to ensure satisfactory strength. If the carbon content is too high, the weldability of the steel sheet is insufficient. A carbon content level below 0.15 95 cannot achieve a sufficient tensile strength.

The manganese content is 2.4-3.5 96 to ensure satisfactory strength and limit bainite transformation. When added above 3.5 95, the risk of axial dislocation increases due to loss of plasticity. A manganese content of at least 2.495 is required to ensure strength and annealing of the steel sheet, as well as to stabilize austenite. Preferably, the manganese content is 2.5-3.2 9.

According to the invention, the silicon content is 0.30-0.90 95. Silicon is an element involved in solid solution hardening. Adding silicon, at least 0.30 95 allows for sufficient hardening of ferrite and bainite. At a content above 0.90 95, silicon oxides are formed on the surface, which impairs the suitability of the steel for coating. In addition, silicon can impair weldability.

In a preferred embodiment, the silicon content is 0.30-0.70 956. In another preferred embodiment, the silicon content is 0.30-0.50 9".

According to the invention, the chromium content is 0.30-0.70 95. Chromium is an element involved in solid solution hardening. A chromium content level below 0.30 95 does not allow obtaining a sufficient tensile strength. The chromium content should be lower than or equal to 0.70 95 to obtain a satisfactory elongation at break and limited costs.

According to the invention, the molybdenum content is 0.05-0.35 95. The addition of molybdenum in an amount of at least 0.05 95 improves the annealing of the steel and limits the bainite transformation before and during hot dip coating. The addition of molybdenum above 0.35 95 is expensive and ineffective in terms of the required properties. Preferably, the molybdenum content is 0.05-0.20 9".

According to the invention, the aluminum content is 0.001-0.0995, because it is a very effective element for deoxidizing steel in the liquid phase during processing. The aluminum content is below 0.0995 to avoid problems with oxidation and ferrite formation during cooling after intercritical holding. The preferred amount of aluminum is 0.001-0.06 Fo.

Titanium is added in an amount of 0.01-0.06 95 to provide dispersion strengthening and protect boron from the formation of VM.

According to the invention, the boron content is 0.0010-0.0040 95. Like molybdenum, boron improves the hardenability of steel.

The boron content is below 0.0040 95 to avoid the risk of slab fracture during continuous casting. Niobium is added in an amount of 0.01-0.05 95 to refine austenite grains during hot rolling and to provide dispersion strengthening.

The rest of the composition of the steel is made up of iron and impurities that were formed as a result of smelting. In this respect, P, S and M are at least considered residual elements that are inevitable impurities.

Their content does not exceed 0.010 95 for 5, 0.020 95 for P and 0.008 95 for M.

The microstructure of the cold-rolled and annealed galvanized steel sheet according to the invention will now be described.

After cold rolling, the cold-rolled steel sheet is heated to the holding temperature

The die is then held at the specified temperature for the duration of the annealing time, both parameters being selected to produce at the end of this intercritical holding period a steel sheet with a microstructure consisting of 85-95% austenite and 5-15% ferrite.

Some of the austenite transforms to bainite upon cooling after intercritical holding, during hot dip coating.

During the cooling stage at room temperature after the galvanic annealing stage, the austenite transforms into martensite. The cold-rolled and galvanized steel sheet has a final microstructure consisting of 80-90% martensite in the surface particles, the remainder being ferrite and bainite.

These 80-90 95 martensite provide a good level of ultimate tensile strength. This martensite includes self-tempering martensite and fresh martensite.

The sum of ferrite and bainite is 10-20 95 to ensure the successful annealing stage of the galvanized sheet.

In a preferred embodiment of the invention, the ferrite content is greater than or equal to 5 95. In another embodiment of the invention, the bainite content is greater than or equal to 5 95.

The cold-rolled and galvanized steel sheet according to the invention has a tensile strength T5 greater than or equal to 1450 MPa. In a preferred embodiment of the invention, the yield strength U5 is greater than or equal to 1050 MPa. T5 and U5 are measured in accordance with ISO 6892-1.

The steel sheet according to the invention can be produced by any suitable production method, and can be determined by a person skilled in the art. However, it is preferable to use a method according to the invention, which includes the following steps:

A semi-finished product suitable for hot rolling has the steel composition described above. The semi-finished product is heated to a temperature of 1150-1300 "C to facilitate hot rolling, with the final hot rolling temperature of the ECT being 850-950 76.

Then the hot-rolled steel is cooled and wound into a coil at a temperature of 250-650 "C.

After winding, the sheet is etched to remove oxidation products.

The steel sheet is annealed at an annealing temperature Ta, which is 500-650 "C, and held at the specified temperature TA for the holding time to improve its suitability for cold rolling.

After annealing, the sheet can be etched to remove oxidation products.

The steel sheet is then cold rolled with a compression ratio of 20-80 95 to obtain a cold rolled steel sheet having a thickness that may be, for example, 0.7-3 mm or even in the range of 0.8-2 mm. The compression ratio in cold rolling is preferably 20-80 95.

Below 20 95, recrystallization during subsequent heat treatment is not suitable, which may deteriorate the ductility of the cold-rolled and annealed galvanized steel sheet. Above 80 95, the force required for deformation during cold rolling will be too high.

Then the cold-rolled steel sheet is reheated to the holding temperature T-oage, which consists of Ac! and Ac3, and held at the specified temperature Tesak for the holding time

The holding time is 30-200 s to obtain at the end of this intercritical holding a microstructure containing 85-95% austenite and 5-15% ferrite.

The cold-rolled steel sheet is then cooled to a temperature of 440-480°C to bring the sheet to a temperature close to the temperature of the coating bath before coating by continuous immersion in a zinc bath at a temperature Tl of 450-480°C.

Then the hot-dip galvanized steel sheet is reheated to the annealing temperature Ted of the galvanized sheet, which is 510-550 "C, and held at the temperature Tsd for the holding time ts, which is 10-30 s.

The steel sheet is then cooled to room temperature to obtain a cold-rolled and annealed galvanized steel sheet.

In a preferred embodiment of the invention, the step of annealing the hot-rolled steel sheet is carried out periodically in an inert atmosphere at a heat treatment temperature Ta, which is 500-650 °C, and holding at said temperature Ti for a holding time yd, which is 1800-36000 s.

In another preferred embodiment of the invention, the annealing step of the hot-rolled steel sheet is carried out by continuous annealing at a heat treatment temperature TA of 550-650 and holding at said temperature TA for a holding time id of 30-100 s.

The invention will now be illustrated by the following examples, which are in no way intended to limit it.

Examples

Steels of 2 grades, the compositions of which are given in Table 1, are cast into semi-finished products and processed into steel sheets in accordance with the technological parameters presented in Table 2.

The compositions tested are listed in the following table, in which the element content is expressed in mass percentages.

Table 1

Warehouses

Steel S |Mp/| zi| st Mo! Adi | ti | in | mu RIY 5 | M GAs1 SS) | Asz SS) 0.0022 0.002. 0.004 in 0i15| 26 |0.4510,4510,0310,0110,0310,0020|10.013 0.002 (0.004

Steel A according to the invention. Steel B outside the scope of the invention

Underlined values: not in accordance with the invention

For each steel, Ac1 and Ac3 are measured using dilatometric tests and metallographic analysis.

Castings of steel semi-finished products are subjected to reheating to 1200 "C, hot rolling with a finish rolling temperature of ECT 910 "C, winding into a coil at a temperature of Tso 550 "C. Some steel sheets are first annealed at a temperature of TA 600 "C and maintained at the specified temperature.

And during the holding time Her before pickling. Then the steel sheets are subjected to cold rolling with a compression degree of 45595. The cold-rolled steel sheets are reheated to the holding temperature Tesak and held at the specified temperature for the holding time, and then coated by hot dipping in a zinc bath at a temperature Tllp 460 "C with subsequent annealing of the galvanized sheet at an annealing temperature of 5 "C and held at the specified temperature for 20 s. The following specific conditions are applied:

Table 2

Process parameters

Annealing after galvanizing 1111111111taso) Tvoak (C) Tea SS) 11111117 A 17771600 1790 | 180 | (B 50 KrKizhoYa2("S( 7771772 | A 17771600 | 790 | 1938 | щ-(хМ 520.Й.ЮКСИ 7717279 | A 17771600 | 843 | 1938 | щ-Ххм 520...:С 77717174... | A 17 1600 | 80 | 1938 | щ-Ххм 520.Й..ЮКСИ 25111 7в'ї 7777-1171 790 | 80 | (км 520.Й.ЙЙЮЮЮЮюЕЯЕКСН

Underlined values: not in accordance with the invention

Cold-rolled steel sheets are analyzed after aging and the corresponding microstructure elements are presented in Table 3.

Table C

Microstructure of cold-rolled steel sheets after aging

Samples Austenite (5) Ferrite (5) 11111116 a11111111941111111111111116 86111101 69111111

Underlined values: not in accordance with the invention

To quantify this microstructure at the end of aging, the steel sheets are quenched after aging to transform 100% of the austenite into martensite, austenite being unstable at room temperature. Thus, the amount of martensite corresponds to the amount of austenite at the end of aging. The amount of martensite and ferrite is then quantified by image analysis.

Then, the cold-rolled and annealed galvanized steel sheets are analyzed, and the corresponding microstructure elements and properties are listed in Tables 4 and 5, respectively.

Table 4

Microstructure of cold-rolled and annealed galvanized steel sheets

Samples Martensite (95) Ferrite «x Bainite (95) Ferrite(Uo Bainite (95) 228 7171111789 1111111 11111516 28 1111111798 11111112 | 0 HER 2 7471771111782 HER 11117187 1111110 18

Underlined values: not in accordance with the invention

The surface fraction is determined by the following method: a sample is cut from a cold-rolled and annealed galvanized steel sheet, polished and etched with a reagent (nital) to reveal the microstructure. The determination of the surface fraction of each component is performed by image analysis using an optical microscope: martensite has a darker shade than ferrite and bainite. Bainite is determined quantitatively by measuring the difference in the fraction of martensite in the sample hardened after aging and the sample cooled after annealing of the galvanized sheet. Bainite is identified by the carbide inside this bainite.

Table 5

Properties of cold-rolled and annealed galvanized steel sheet

Samples T5 (MPa) U5 (MPa) Positive CA result 1522 1095 1634 1055 1519 1163 None 1611 1096 None 1363

Underlined values: Insufficient T5 or U5, or unsuccessful annealing stage of galvanized sheet.

A positive result of annealing of galvanized sheet is checked by measuring the iron content in the coating. Galvanized steel is considered annealed if the iron content in the coating is 7-12 95.

The examples show that the steel sheet according to the invention, namely examples 1 and 2, are the only sheets that demonstrate all the target mechanical properties when successfully annealed after galvanizing due to their specific composition and microstructure. The mechanical properties are provided by a martensite content of 80-90 95. The annealing stage after galvanizing is provided by the presence of ferrite and bainite in a total amount of 10-20 95.

In tests 3 and 4, steel A is heated above the Tesak temperature, which provides 85-95% austenite and 5-15% ferrite at the end of the aging, resulting in too much austenite and not enough ferrite. This results in less than 10% of the sum of ferrite and bainite at the end of the hot dip coating, which complicates the annealing stage after galvanizing.

In test 5, the absence of molybdenum, which is a strengthening element that slows down the bainite transformation, leads to the formation of 25 95 of the sum of ferrite and bainite at the end of the hot dip coating. Then the martensite that forms in the last cooling stage is less than 80 95, which leads to low mechanical properties.

Claims (9)

1. Cold-rolled and annealed galvanized steel sheet, the chemical composition of which contains, by weight: C -0.15-0.25, Ma - 2.4-3.5, Vi - 0.30-0.90, St - 0.30-0.70, Mo - 0.05-0.35, A - 0.001-0.09, Ti - 0.01-0.06, B - 0.0010-0.0040, Mb - 0.01-0.05, R-F,020, 5-0.010, M-0.008, the rest of the composition is iron and inevitable impurities formed as a result of melting, and the specified steel sheet has a microstructure consisting, in surface fractions, of 3: 80-90 95 martensite, the rest is ferrite 1 bainite. 2. The steel sheet of claim 1, wherein the ferrite content is greater than or equal to 5 90. 3. The steel sheet of claim 1, wherein the bainite content is greater than or equal to 5 90. 4. Steel sheet according to any one of claims 1-3, in which the silicon content is 0.30-0.70 90. 5. The steel sheet according to any one of claims 1-4, wherein the ultimate tensile strength is greater than or equal to 1450 MPa. 6. A method for manufacturing a cold-rolled and annealed galvanized steel sheet, which includes the following sequential stages: pouring steel to obtain a semi-finished product having the composition according to claim 1, heating the slab to a temperature of 1150-1300 ° C, hot rolling the heated semi-finished product with a final rolling temperature of 850-950 ° C to obtain a hot-rolled steel sheet, then cooling the specified steel sheet to a coiling temperature Tso, which is 250-650 ° C, then coiling the steel sheet into a roll at the specified temperature Tso to obtain a coiled steel sheet, then pickling the steel sheet, annealing of the steel sheet at the annealing temperature TA 500-650 C 1 holding the steel sheet at the specified temperature TA for the holding time id, cold rolling of the hot-rolled steel sheet with a compression degree of 20-80 90 to obtain a cold-rolled steel sheet, heating the cold-rolled steel sheet to the holding temperature Tok from Asi to As3 1 holding the steel sheet at the specified temperature Toak for the holding time isoak, which is 30-200 s, to obtain 85-95 90 austenite 1 5-15 96 ferrite, cooling the steel sheet to a temperature of 440-480 C, coating the steel sheet by continuous immersion in a zinc bath at a temperature of T7p 450-480 2 C, reheating the steel sheet to the annealing and galvanizing temperature Tosad, which is 510-5502 C, 1 holding the steel sheet at the specified temperature Tc for a holding time ssl, which is 10-30 s; cooling the reheated steel sheet to room temperature to obtain a cold-rolled 1 annealed galvanized steel sheet. 7. The method according to claim 6, wherein after annealing to an annealing temperature Ta and holding the steel sheet at the specified temperature TA for a holding time (4), the steel sheet is subjected to pickling. 8. The method according to claim 6 or 7, wherein said annealing of the hot-rolled steel sheet is carried out periodically in an inert atmosphere at a heat treatment temperature Tl of 500-650 "C for a time A at said annealing temperature, which is 1800-36000 s. 9. The method according to claim 6 or 7, wherein said annealing of the hot-rolled steel sheet is carried out by continuous annealing at a heat treatment temperature TA of 550-650 "C for a time id at said annealing temperature of 30-100 s.