RU2802328C1 - Hot-rolled and heat treated steel sheet and method for its manufacture - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to a high-strength steel sheet with suitable weldability. Hot-rolled and heat-treated steel sheet made of steel having a composition containing, wt.%: C 0.03-0.18, Mn 6.0-11.0, Mo 0.05-0.5, B 0.0005-0.005, S ≤ 0.010, P ≤ 0.020, N ≤ 0.008, and optionally containing one or more of the following: Al < 3, Si ≤ 1.20, Ti ≤ 0.050, Nb ≤ 0.050, Cr ≤ 0.5, the rest of the composition is iron and inevitable impurities resulting from smelting. The sheet has a microstructure that includes, in fractions of the surface: 10 - 60% retained austenite, 40 - 90% ferrite, 0 to less than 5% martensite, 0 to less than 0.8% carbides, and an inhomogeneous distribution of manganese with areas below and above the nominal value of manganese. A weld obtained by resistance spot welding of two parts made of hot-rolled and heat-treated steel sheet of the composition mentioned. The weld has α, which is not less than 30 daN/mm², where α is the ratio of the value of the transverse tensile strength to the product of the diameter of the weld point and the thickness of the substrate.

EFFECT: obtaining a steel sheet with high impact strength and weldability.

10 cl, 2 dwg, 4 tbl, 36 ex

Description

The present invention relates to a high strength steel sheet having 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 parts 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 conservation of the environment, while not leaving safety requirements out of the question. To meet these requirements, the steel industry is constantly developing new high-strength steels to have sheets with improved yield strength and tensile strength, as well as with 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 the disadvantage of these steels is brittleness. To solve this problem, elements such as boron are added. These boron-containing chemicals make the steel very tough during the hot rolling stage, but the hot strip is too hard for further processing. Batch annealing is the most effective way to soften a hot strip, but this results in a loss of toughness.

In addition to these mechanical requirements, such steel sheets must exhibit suitable resistance to liquid metal embrittlement (LME). Zinc or zinc alloy coated steel sheets are very effective in corrosion protection and are therefore widely used in the automotive industry. However, experience shows that arc or resistance welding of some steels can cause individual cracks due to a phenomenon called liquid metal embrittlement ("LME") or liquid metal action cracking ("LMAC"). This phenomenon is characterized by the penetration of liquid Zn along the grain boundaries of the underlying steel substrate under the action of applied stresses or internal stresses resulting from fastening, thermal expansion or phase transformations. The addition of elements such as carbon or silicon is known to adversely affect LME resistance.

The automotive industry typically evaluates this durability by limiting the upper value of the so-called LME index, calculated according to the following equation:

LME Index = C% + Si%/4,

where C% and Si% mean, respectively, the content in mass percent of carbon and silicon in the steel.

The object of the invention is to provide a hot-rolled and annealed steel sheet having a high impact strength with a Charpy fracture energy at 20°C above 0.4 J/mm², as well as suitable weldability.

Preferably, the hot rolled and annealed steel sheet according to the invention has an LME index of 0.36 or less.

Preferably the hot rolled and annealed steel according to the invention has a Ceq carbon equivalent below 0.4, the carbon equivalent being defined as Ceq = C%+Si%/55+Cr%/20+Mn%/19-Al%/18+2.2P %-3.24B%-0.133*Mn%*Mo%

with the content of elements, expressed in mass percent.

The object of the present invention is achieved by offering a steel sheet according to claim 1. The steel sheet can also have the characteristics of claims. 2 - 9.

Another object of the invention is resistance spot welding of two steel parts according to claim 10.

In the following, the invention will be described in detail and exemplified without limitation.

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

According to the invention, the carbon content is 0.03 - 0.18% to ensure satisfactory strength and suitable weldability. When the carbon content is higher than 0.18%, the weldability of the steel sheet and the LME resistance may be lowered. The holding temperature depends in particular on the carbon content: the higher the carbon content, the lower the holding temperature for stabilizing the austenite. If the carbon content is lower than 0.03%, the austenite fraction is not sufficiently stabilized to obtain the required tensile strength and elongation after curing. In a preferred embodiment, the carbon content is 0.05-0.15%. In another preferred embodiment, the carbon content is 0.07-0.12%.

The manganese content is 6.0 - 11.0%. If more than 11.0% is added, the weldability of the steel sheet may be reduced and the assembly performance of the parts may be reduced. In addition, the risk of axial segregation with deterioration in mechanical properties increases. Since the holding temperature also depends on the manganese content, the minimum manganese content is determined to stabilize austenite, to obtain a given microstructure and properties after holding. Preferably the manganese content is 6-9.0%.

According to the invention, the aluminum content is less than 3% to reduce manganese segregation during casting. Aluminum is a very effective element for deoxidizing steel in the liquid phase during processing. Adding more than 3% can reduce the weldability of the steel sheet as well as the casting properties. Moreover, the higher the aluminum content, the higher the holding temperature for stabilizing the austenite. Aluminum is preferably added up to at least 0.2% to increase the strength of the product due to the expansion of the intercritical range and to improve weldability. In addition, aluminum can be added to avoid inclusions and oxidation problems. In a preferred embodiment, the aluminum content is 0.2-2.5% and more preferably 0.5-2.2%.

Molybdenum content is 0.05 - 0.5% to reduce manganese segregation during casting. In addition, the addition of at least 0.05% molybdenum provides resistance to brittleness. The addition of molybdenum above 0.5% is costly and inefficient in terms of the required properties. In a preferred embodiment, the molybdenum content is 0.1-0.3%.

According to the invention, the boron content is 0.0005 to 0.005% to improve the toughness and spot weldability of the hot rolled steel sheet. Above 0.005%, it promotes the formation of boron carbides at the boundaries of the former austenite grains, which makes the steel more brittle. In a preferred embodiment, the boron content is 0.001-0.003%.

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

The maximum silicon content is limited to 1.20% to improve LME resistance. In addition, this low silicon content makes it possible to simplify the process by eliminating the pickling step of the hot-rolled steel sheet before annealing in the hot zone. Preferably, the maximum silicon content is 1.0%.

Titanium can be added up to 0.050% to provide precipitation strengthening.

Preferably, a minimum of 0.010% titanium is added to the boron to protect the boron from BN formation.

Optionally, up to 0.050% niobium can be added to refine the austenite grains during hot rolling and provide precipitation strengthening. Preferably, the minimum amount of added niobium is 0.010%.

Chromium and vanadium may optionally be added up to 0.5% and 0.2%, respectively, to provide increased strength.

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

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

A hot rolled steel sheet with a composition according to the invention is subjected to intercritical annealing, during which a microstructure is formed, including 10-60% austenite, 40-90% ferrite, less than 0.8% carbides and less than 5% martensite.

The austenite formed during such intercritical annealing is enriched in carbon and manganese. More precisely, regions with a manganese content above the nominal value and regions with a manganese content below the nominal value are formed, creating a non-uniform distribution of manganese. Accordingly, carbon co-segregates with manganese. This manganese inhomogeneity is measured by the slope of the manganese distribution curve, which must be greater than or equal to -40, as shown in FIG. 2 and explained below.

Ferrite is present in an amount of 40 - 90% and is formed during intercritical annealing.

Fresh martensite can be formed upon cooling after intercritical annealing as a result of destabilization of the part of austenite that is less rich in carbon and manganese. However, this component of the microstructure is undesirable and its share should remain below 5%.

Finally, hot-rolled steel and heat-treated steel sheet contain less than 0.8% carbides, resulting in a suitable impact strength, i.e. Charpy fracture energy at 20°C above 0.4 J/mm², measured in accordance with ISO 148-1 : 2006 (F) and ISO 148-1:2017 (F). In a preferred embodiment, the amount of carbides is 0.6% or less, or even preferably 0.5% or less.

According to the invention, the hot rolled and annealed steel sheet has a Ceq below 0.4 to improve weldability. The carbon equivalent is defined as Ceq = C%+Si%/55+Cr%/20+Mn%/19-Al%/18+2.2P%-3.24B%-0.133*Mn%*Mo%, where the content of elements expressed in mass percent.

The hot rolled steel sheets 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 - 1300°C, to facilitate hot rolling, with a final temperature of hot rolling KTP 800 - 1000°C. Preferably, the CTP is 850 - 950°C.

Then the hot rolled steel is cooled and wound into a coil at a temperature T _coil 20 - 600°C. Then, the hot-rolled steel sheet is cooled to room temperature and pickling is carried out.

Then, the hot-rolled steel sheet is heated to an annealing temperature T _HBA between Tc and 680°C. Tc is calculated using the formula below, which is applicable for manganese content above 4% wt.

Tc = Tec - (Mn% - 4)/C%

Tec is the equilibrium dissolution temperature of carbides, which can be determined from thermodynamic calculations done using software such as Thermo-Calc®, and Mn% and C% are the nominal mass content of manganese and carbon in the steel.

Preferably the temperature T _HVA is 580 - 680°C. The steel sheet is held at the specified temperature T _HBA for a holding time t _HBA of 0.1 to 120 hours to promote the diffusion of manganese and the formation of a non-uniform distribution of manganese.

T _HBA is chosen to obtain after cooling 10-60% austenite, 40-90% ferrite and less than 5% martensite, while the proportion of carbides is kept below 0.8%. In particular, the selection of a suitable time and temperature for such intercritical annealing must take into account the maximum carbide fractions that are allowed according to the invention, bearing in mind that an increase in T _HBA limits the precipitation of carbide.

With regard to the chemical composition, the higher the content of carbon and aluminum in the steel, the higher the concentration of carbides at a given temperature. This means that for carbon and aluminum content in the upper part of the claimed ranges, it is necessary to increase T _HBA in order to adequately limit the release of carbides.

The lower the amount of manganese in the steel, the higher the concentration of carbide for a given temperature. This means that for manganese content in the lower part of the claimed range, it is necessary to increase T _HBA in order to limit the precipitation of carbides accordingly.

The hot rolled and heat treated steel sheet is then cooled to room temperature and may be pickled to remove oxidation.

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

Examples

Eleven steel grades, the compositions of which are given in table 1, are cast into semi-finished products and processed into steel sheets.

Table 1. Compositions

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

Temperatures Ae1, Ae3 and Tec are determined by thermodynamic calculations performed using software such as Thermo-Calc®.

Table 2. Technological parameters of hot-rolled and heat-treated steel sheet

Steel semi-finished products in cast form are reheated to 1200°C, subjected to hot rolling and then wound into coils. The hot rolled and coiled steel sheets are then heat treated at a temperature T _HBA and held at that temperature for a holding time t _HBA . To obtain hot-rolled and heat-treated steel sheet, the following special conditions apply:

Underlined values: Parameters that don't get the target properties

Hot-rolled and heat-treated steel sheets are analyzed and the corresponding properties are shown in Table 3.

Table 3 Microstructure and Properties of Hot Rolled and Heat Treated Steel Sheet

The proportions of the surface of the phases in the microstructure are determined by the following method: a sample is taken from a hot-rolled and annealed steel sheet, polished and etched with a known reagent to reveal the microstructure. The section is then examined using a scanning electron microscope, such as a field emission gun scanning electron microscope ("FEG-SEM") at a magnification of more than 5000 times in secondary electron mode.

The determination of the surface fraction of ferrite is performed by SEM analysis after etching with Nital or Picral/Nital reagent.

Determination of the volume fraction of residual austenite is performed using X-ray diffraction.

The carbide fraction is determined by examining a section of the sheet using a scanning electron microscope with a field emission gun ("FEG-SEM") and image analysis at a magnification of more than 15000x.

Underlined values: do not match the target values.

Determine the slope of the distribution curve of manganese and energy destruction Charpy at 20°C.

Charpy fracture energy is measured in accordance with ISO 148-1:2006 (F) and ISO 148-1:2017 (F).

Heat treatment of hot rolled steel sheet allows manganese to diffuse into austenite: the redistribution of manganese is non-uniform with areas of low manganese content and areas of high manganese content. This heterogeneity of manganese helps to achieve mechanical properties and can be measured by the distribution of manganese.

Fig. 1 is a cross section of the hot rolled and heat treated steel sheet in test 1 and 4. The black area corresponds to the area with less manganese, the gray area corresponds to more manganese.

This figure is obtained by the following method: a sample of 1/4 thickness is cut from a hot-rolled and heat-treated steel sheet and polished.

The cross section is then analyzed with a field emission gun (“FEG”) electron probe microanalyzer at more than 10,000 times magnification to determine the manganese content. Get three maps 10 µm * 10 µm of different parts of the section. These maps consist of 0.01 µm ² pixels. The amount of manganese in mass percent is calculated in each pixel and then plotted on a curve representing the total area fraction of three maps depending on the content of manganese.

This curve is shown in Fig. 2 for tests 1 and 4: 100% of the sheet section contains more than 1% manganese. In test 1, 20% of the sheet section contains more than 10% manganese.

The slope of the resulting curve is then calculated between a point representing 80% of the total area of the fraction and a point representing 20% of the total area of the fraction. For test 1, this slope is greater than -40, indicating that the redistribution of manganese is non-uniform, with areas of low manganese content and areas of high manganese content.

In contrast, in test 4, the absence of heat treatment after hot rolling means that the redistribution of manganese is not heterogeneous, as can be seen from the magnitude of the slope of the distribution of manganese below -40.

Underlined values: do not match the target values.

In tests 6 and 7, the temperature T _HBA is significantly lower than Tc, which leads to the formation of too much carbides, which leads to an increase in strength, but a decrease in ductility. Moreover, a small volume fraction of austenite (<10%) is not enough to cover all of the grain boundaries, which are very rich in manganese, leading to grain boundary embrittlement and thus lower toughness.

In test 8, conducted at a relatively higher temperature T _HBA (but still lower than Tc), the carbides tend to grow along the boundaries of the former austenite grains, which deteriorates the toughness.

Tests 13, 14, 15, 19, 20, 21, 22, 27, 28, 33, 34, 35, 36 show similar trends and also include too many carbides.

Table 4 Weldability of Hot Rolled and Heat Treated Steel Sheet

The weldability of the obtained hot-rolled and heat-treated steel sheet was determined and presented in the following table.

Spot welding under the conditions of ISO 18278-2 is performed on hot-rolled and annealed steel plates that are pickled before welding.

In the test used, the samples consist of two identical sheets of steel welded crosswise. Apply such force 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/mm ² .

The spot welds connecting the first sheet to the second sheet are characterized by high transverse tensile strength, defined by an α value of at least 30 daN/mm ² .

Underlined values: do not match the target values.

LME Index = C% + Si%/4,

In tests 19, 20, 21, 33, 34, 35 and 36 the chemical composition does not allow to obtain the target weldability parameters according to the invention.

Claims (31)

1. Hot-rolled and heat-treated steel sheet made of steel having a composition containing, in wt.%: C 0.03 - 0.18 Mn 6.0 - 11.0 Mo 0.05 - 0.5 B 0.0005 - 0.005 S ≤ 0.010 P ≤ 0.020 N ≤ 0.008 and optionally containing one or more of the following in wt.%: Al < 3 Si ≤ 1.20 Ti ≤ 0.050 Nb ≤ 0.050 Cr ≤ 0.5, the rest of the composition is iron and inevitable impurities resulting from smelting, the specified steel sheet has a microstructure, including in fractions of the surface: 10 - 60% residual austenite, 40 - 90% ferrite, 0 to less than 5% martensite, 0 to less than 0.8% carbides, and non-uniform distribution of manganese with areas below and above the nominal value of manganese. 2. The steel sheet according to claim 1, wherein the microstructure of the steel has a carbide content of 0.6% or less. 3. The steel sheet according to claim 2, wherein the microstructure of the steel has a carbide content of 0.5% or less. 4. Steel sheet according to any one of paragraphs. 1 or 3, in which the carbon content is 0.05 - 0.15 wt.%. 5. Steel sheet according to any one of paragraphs. 1 - 4, in which the manganese content is 6.0 - 9.0 wt.%. 6. Steel sheet according to any one of paragraphs. 1 - 5, in which the aluminum content is 0.2 - 2.5 wt.%. 7. Steel sheet according to any one of paragraphs. 1-6, in which the Charpy fracture energy at 20° C. is more than 0.4 J/mm ² . 8. Steel sheet according to any one of paragraphs. 1 to 7, wherein the liquid metal embrittlement resistance LME index is 0.36 or less. 9. Steel sheet according to any one of paragraphs. 1 to 8, in which the steel has a Ceq carbon equivalent below 0.4, where the carbon equivalent is defined as Ceq = C% + Si%/55 + Cr%/20 + Mn%/19 - Al%/18 + 2.2P% - 3.24B% - 0.133xMn%xMo%, where the content of elements is expressed in mass percent. 10. A weld obtained by resistance spot welding of two parts made of hot-rolled and heat-treated steel sheet according to any one of paragraphs. 1 to 9, wherein said resistance spot weld has an α value of at least 30 daN/mm ² , where α is the ratio of the transverse tensile strength to the product of the weld point diameter and the substrate thickness.