JP2023553672A - Coated steel plate, high-strength press-hardened steel parts, and manufacturing method thereof - Google Patents

Abstract

The present invention has the following weight percentages: C0.26-0.40%, Mn0.5-1.8%, Si0.1-1.25%, Al0.01-0.1%, Cr0.1-0. 1.0%, Ti0.01-0.1%, B0.001-0.004%, P≦0.020%, S≦0.010%, N≦0.010%. The present invention relates to coated steel sheets and press-hardened steel parts in which the remainder of the composition is iron and unavoidable impurities resulting from smelting. Press-hardened steel parts include a bulk having a microstructure containing, in surface fraction, more than 95% martensite and less than 5% bainite, a coating layer on the surface of the steel part, and ferrite between the coating layer and the bulk. The ratio of the width GWint of the ferrite grains in the interdiffusion layer to the prior austenite grain size PAGSbulk in the bulk satisfies the following formula: (GWint/PAGSbulk)-1≧30%.

Description

The present invention relates to coated steel sheets and high strength press hardened steel parts with good bending properties.

High strength press hardened parts can be used as structural elements in motor vehicles for anti-intrusion or energy absorption functions.

For these types of applications, it is desirable to produce steel parts that combine high mechanical strength, high impact resistance and good corrosion resistance. Furthermore, one of the main challenges in the automotive industry is to reduce the weight of vehicles in order to improve their fuel efficiency from the perspective of protecting the global environment, without neglecting safety requirements.

This weight reduction can be achieved in particular by the use of steel parts with martensitic or bainitic/martensitic microstructures.

The publication, International Publication No. 2016104881, describes hot press-formed parts and manufacturing methods that are used as structural parts of vehicles and require impact resistance, more specifically, have a tensile strength of 1300 MPa or more. The present invention also relates to a method of heating a steel material to a temperature at which a single phase of austenite can be formed, and quenching and hot forming the steel material using a mold. In order to obtain such properties, the base steel sheet must contain a thin ferrite layer of less than 50 μm on the surface, and the size and density of carbides must be controlled. This ferrite layer in the substrate makes it possible to suppress the propagation of microscopic cracks formed on the plating layer to the base plate, but results in low bendability with a bending angle of less than 70°.

International Publication No. 2018179839 discloses a hot-pressed part obtained by hot-pressing a steel plate having a microstructure that changes in the thickness direction, comprising a soft layer made of at least 90% ferrite and a soft layer made of ferrite and marten. The present invention relates to a hot-pressed part having both high strength and high bendability, including a transition layer made of martensite and a hard layer mainly composed of martensite. To obtain such properties, cold rolled steel sheets are annealed in an atmosphere containing dew point temperatures of 50° C. to 90° C., which can be detrimental to the aluminum alloy coating.

International Publication No. 2016/104881 International Publication No. 2018/179839

The aim of the present invention is therefore to solve the above-mentioned problems and to provide a press-hardened steel part with a combination of high mechanical properties, with a tensile strength TS of more than 1500 MPa and a bending angle of more than 70°. Preferably, the press hardened steel component according to the invention has a yield strength YS of 1250 MPa or more.

Another object of the invention is to obtain a coated steel sheet that can be transformed into such a press-hardened steel part by hot forming.

The object of the invention is achieved by providing a steel plate according to claim 1. Another object is achieved by providing a method according to claim 2. Another object of the invention is achieved by providing a press hardened steel part according to claim 3. The steel part may also include the characteristics according to any one of claims 4-6. Another object is achieved by providing a method according to claim 7.

The invention will now be described in detail and illustrated by examples without introducing limitations, with reference to the accompanying drawings, in which: FIG.

A schematic cross-sectional view of the coated steel plate of Test 4 is shown. This is not according to the invention. Figure 4 represents a schematic cross-sectional view of a press hardened steel part from Test 4. This is not according to the invention. A schematic cross-sectional view of the coated steel plate of Test 3 is shown. This is not according to the invention. Figure 3 represents a schematic cross-sectional view of a press hardened steel part from Test 3. This is not according to the invention. A schematic cross-sectional view of the coated steel plate of Test 2 is shown. This is according to the invention. Figure 2 represents a schematic cross-sectional view of a press hardened steel part from Test 2. This is according to the invention. A schematic cross-sectional view of the coated steel plate of Test 1 is shown. This is according to the invention. Figure 2 represents a schematic cross-sectional view of the press-hardened steel part from Test 1. This is according to the invention. A schematic cross-sectional view of the coated steel plate of Test 5 is shown. This is not according to the invention. Figure 5 represents a schematic cross-sectional view of a press hardened steel part from Test 5. This is not according to the invention.

Next, the composition of the steel according to the invention will be explained, the content being expressed in weight percent.

According to the invention, the carbon content comprises 0.26% to 0.40% to ensure satisfactory strength. If the carbon content exceeds 0.40%, the weldability and bendability of the steel plate may deteriorate. If the carbon content is less than 0.26%, the tensile strength will not reach the target value.

Manganese content includes 0.5% to 1.8%. If the amount added exceeds 1.8%, the risk of center segregation increases and bendability is impaired. If it is less than 0.5%, the hardenability of the steel sheet will decrease. Preferably, the manganese content comprises 0.5% to 1.3%.

According to the invention, the silicon content comprises between 0.1% and 1.25%. Silicon is an element that participates in hardening in solid solution. Silicon is added to limit carbide formation. If it exceeds 1.25%, silicon oxide is formed on the surface, impairing the coating properties of the steel. Furthermore, the weldability of the steel plate may be reduced. Preferably the silicon content is between 0.2% and 1.25%. More preferably, the silicon content is between 0.3% and 1.25%. More preferably, the silicon content is between 0.3% and 1%.

The aluminum content comprises 0.01% to 0.1% since it is a very effective element for deoxidizing the steel in the liquid phase during elaboration. Aluminum can protect boron if titanium content is not sufficient. Aluminum content is less than 0.1% to avoid oxidation problems and ferrite formation during press hardening. Preferably the aluminum content comprises 0.01% to 0.05%.

According to the invention, the chromium content comprises between 0.1% and 1.0%. Chromium is an element involved in hardening in solid solution and must be higher than 0.1%. Chromium content is less than 1.0% to limit processability problems and costs.

The titanium content includes 0.01% to 0.1% to protect the boron from BN formation. Titanium content is limited to 0.1% to avoid TiN formation.

According to the invention, the boron content comprises 0.001% to 0.004%. Boron improves the hardenability of steel. The boron content is below 0.004% to avoid the risk of slab breakage during continuous casting.

Some elements can be optionally added.

Nickel can be added as an optional element up to 0.5% since it can substantially reduce the sensitivity to delayed fracture.

Molybdenum content can be optionally added up to a maximum of 0.40%. Like boron, molybdenum improves the hardenability of steel. Molybdenum is below 0.40% to limit cost.

According to the invention, niobium can be optionally added up to 0.08% to improve the ductility of the steel. If the amount added exceeds 0.08%, the risk of forming NbC or Nb(C,N) carbides increases and bendability is impaired. Preferably the niobium content is 0.05% or less.

Calcium can also be added as an optional element up to 0.1%. Addition of Ca in the liquid stage allows the formation of fine oxides that promote castability in continuous casting.

The remainder of the steel composition is iron and impurities resulting from smelting. In this respect, P, S and N are considered at least as residual elements that are unavoidable impurities. Their content is less than 0.010% for S, less than 0.020% for P, and less than 0.010% for N.

Next, the microstructure of the coated steel sheet according to the present invention will be explained.

A cross-section of a coated steel sheet according to the invention is schematically represented in Figures 3a and 4a. The coated steel plate is covered on top with a decarburized layer (3) comprising a ferrite layer (4) with a thickness of 1 μm to 100 μm and comprises a coating layer (1) and a bulk (2). Preferably, the thickness of the ferrite layer comprises between 20 μm and 100 μm. More preferably, the thickness of the ferrite layer is between 25 μm and 100 μm. More preferably, the thickness of the ferrite layer is between 30 μm and 80 μm.

The bulk (2) of the coated steel sheet has a microstructure containing ferrite with a surface fraction of 60% to 90%, with the remainder consisting of island martensite-austenite, pearlite or bainite.

This ferrite is formed during transformation zone annealing of cold rolled steel sheets. The remainder of the microstructure is austenite at the end of soaking, which transforms into island martensite-austenite, pearlite or bainite during cooling of the steel plate.

The decarburized layer present at the top of the bulk is obtained during annealing of cold rolled steel sheets by controlling the atmosphere in the furnace so that the dew point temperature is strictly set above -10°C and below 20°C.

The coated steel sheet according to the invention can be manufactured by any suitable manufacturing method, which can be defined by a person skilled in the art. However, it is preferable to use the method according to the invention, which comprises the following steps:
A semi-finished product which can be further hot rolled is provided with the above-mentioned steel composition. The semi-finished product is reheated at a temperature comprised between 1150°C and 1300°C.

The steel plate is then hot rolled at a finish hot rolling temperature comprising 800°C to 950°C.

The hot rolled steel is then cooled and coiled at a temperature Tcoil below 670°C and optionally pickled to remove oxidation.

Then, the rolled steel plate is optionally cold rolled to obtain a cold rolled steel plate. The cold rolling reduction preferably comprises 20% to 80%. If it is less than 20%, recrystallization during subsequent heat treatment is undesirable and may impair the ductility of the steel sheet. If it exceeds 80%, there is a risk that end cracks will occur during cold rolling.

The steel plate is then annealed in an HNx atmosphere containing 0% to 15% H2 to an annealing temperature T _A comprising 700° C. to 850° C. and for a holding time t _A comprising 10 seconds to 1200 seconds. _A is maintained to obtain an annealed steel plate. If the temperature is less than 700°C, the formation rate of the decarburized layer is too slow to form a ferrite layer on top of the decarburized layer. The holding time _tA is at least 10 seconds to allow the formation of a ferrite layer and at most 1200 seconds to limit the thickness of this ferrite layer.

During this annealing, the atmosphere in the furnace is controlled to have a dew point temperature T _DP1 strictly above -10° C. and below +20° C. in order to form a decarburized layer according to the invention. When T _DP1 is −10° C. or lower, the formation of a decarburized layer is delayed and a ferrite layer is not formed on top of the decarburized layer. The bendability of the steel parts becomes too low. If T _DP1 is higher than 20° C., the surface of the steel sheet may be completely oxidized, which may impair the coverage and mechanical properties of the steel sheet.

In one embodiment of the invention, the annealed steel plate is heated to an annealing temperature T2 comprising 700°C to 850°C and maintained at said temperature T2 for a holding time t2 comprising 10 seconds to 1200 seconds, and the atmosphere is strictly - It has a dew point T _DP2 higher than 10°C and lower than +20°C.

The steel plate is then coated with an aluminum alloy coating.

Next, the microstructure of the press hardened steel component according to the present invention will be explained. A cross-section of a press-hardened steel part is schematically represented in Figures 3b and 4b.

Steel parts are continuously processed from the bulk of the steel part to the surface as follows:
- a bulk (7) with a microstructure comprising, in surface fraction, more than 95% martensite and less than 5% bainite;
- ferrite interdiffusion layer (6),
- Aluminum-based coating layer (5)
including.

During heating of the steel blank cut from the steel sheet according to the invention, all the microstructural elements of the bulk are transformed into austenite, and the ferrite of the decarburized layer is transformed into austenite with a wider grain size than the austenite of the bulk. After hot forming, the steel part is then die quenched. Since the interdiffused layer grows from the former wide grain size austenite layer, it has a grain width larger than the prior austenite grain size in the bulk. In order to improve the bendability of the steel sheet without reducing the mechanical properties, the ratio of the width GW _int of the ferrite grains in the interdiffusion layer to the prior austenite grain size PAGS _bulk in the bulk is determined by the following formula (GW _int /PAGS _bulk )-1≧30%
satisfy.

This is to improve the bending workability of the steel sheet without deteriorating its mechanical properties. The width of the ferrite grains is the average distance between two parallel grain boundaries of the interdiffusion layer, and the grain boundaries are oriented in the thickness direction of the steel sheet. The combination of annealing temperature T _A , annealing time t _A and dew point temperature T _DP1 according to the invention promotes the formation of a large grain width GW _int in the interdiffusion layer. Additionally, heat treatment of the steel blank before stamping controls austenite grain growth and thus PAGS in the bulk.

In one embodiment, the press hardened steel part may further include a martensitic layer with a carbon gradient between the bulk and the interdiffusion layer, as represented by (8) in Figure 4b. During heating of the steel blank, carbon diffuses from the bulk to the surface. The ferrite top of the decarburized layer then transforms into a layer of austenite with a carbon gradient. During die quenching, this austenite layer with a carbon gradient transforms into a layer of martensite with a carbon gradient.

The press-hardened steel parts according to the invention have a tensile strength TS of more than 1500 MPa and a bending angle of more than 70°. Bending angles have been determined in press hardened parts according to the VDA238-100 bending standard (normalized to 1.5 mm thickness).

In a preferred embodiment of the invention, the yield strength YS is 1250 MPa or more. TS and YS are measured according to the ISO standard ISO 6892-1.

The press-hardened steel parts according to the invention can be manufactured by any suitable manufacturing method, which can be defined by a person skilled in the art. However, it is preferable to use the method according to the invention, which comprises the following steps:
A steel blank is obtained by cutting the coated steel plate according to the present invention into a predetermined shape. The steel blank is then heated to a temperature comprising 880° C. to 950° C. for 10 seconds to 900 seconds to obtain a heated steel blank. The heated blank is then transferred to a forming press before hot forming and die quenching.

The present invention will now be illustrated by the following examples, which are in no way limiting.

The six grades whose compositions are summarized in Table 1 were cast into semifinished products and processed into steel sheets and then into steel parts according to the processing parameters summarized in Table 2.

Table 1 - Composition The compositions tested are summarized in the table below, with elemental content expressed in weight percent.

Table 2 - Processing parameters Cast steel semi-finished products are reheated at 1200°C, hot rolled at finishing hot rolling temperatures including 800-950°C, coiled at 550°C and cold rolled at a rolling reduction of 60%. Rolled. The steel plate is then heated to a temperature T _A and maintained at said temperature for a holding time t _A in an HNx atmosphere containing 5% H ₂ with a controlled dew point. The steel plate was then cooled to a temperature of 560-700° C. and then hot-dipped coated with an aluminum-silicon coating containing 10% silicon.

Samples 1, 2, 5 and 6 were subjected to a second annealing at a temperature T ₂ before coating, and the steel plates were annealed for a holding time t ₂ in an HNx atmosphere with 5% H ₂ and a controlled dew point. Maintained at _T2 temperature. Applying the following specific conditions:

The coated steel plate is analyzed and the corresponding properties of the decarburized layer are summarized in Table 3.

Table 3 - Characteristics of decarburized layer of coated steel sheet

Next, the coated steel plate was cut to obtain a steel blank, which was heated at 900° C. for 6 minutes and hot-formed. The steel parts are analyzed and the corresponding microstructures, the width of the ferrite grains in the interdiffused layer GW _int and the prior austenite grain size in the bulk PAGS _bulk are summarized in Table 4. The mechanical properties are summarized in Table 5.

Table 4 - Microstructure of press hardened steel parts

The surface fraction, the width of the ferrite grains in the interdiffused layer and the PAGS are determined by the following method: specimens are cut from press-hardened steel parts, polished and etched with reagents known per se to form fine particles. Reveal the structure. Thereafter, using an optical microscope or a scanning electron microscope, for example a scanning electron microscope equipped with a field emission electron gun ("FEG-SEM") with a magnification of more than 5000 times in combination with a BSE (backscattered electron) device, Inspect the cross section.

Table 5 - Mechanical properties of press hardened steel parts The mechanical properties of the tested samples were measured and summarized in the table below:

The examples show that the steel parts according to the invention, namely Examples 1-2, are the only steel parts that exhibit all the target properties thanks to their specific composition and microstructure.

FIG. 3a represents a schematic cross-sectional view of the coated steel plate of Test 2. The combination of the processing parameters of the present invention, annealing temperature _TA , annealing time _tA , and dew point temperature _TDP1 , makes it possible to obtain a decarburized layer (3) on which a ferrite layer (4) is formed.

Next, the coated steel plate is hot formed. Figure 3b represents a schematic cross-sectional view of the press-hardened steel part of Test 2.

The width of the ferrite grains formed in the interdiffused layer (5) is a legacy of the pure ferrite layer where austenite formation occurs during heating and has a larger grain size. An interdiffusion layer grows with this large austenite grain size. The width of the ferrite grains in the interdiffused layer (6) is larger than the prior austenite grain size in the bulk (7) and has a bending angle greater than 70°, resulting in good bendability.

FIG. 4a represents a schematic cross-sectional view of the coated steel plate of Test 1. The combination of the inventive processing parameters, annealing temperature _TA , annealing time _tA and dew point temperature _TDP1 , resulted in the formation of a layer of ferrite on top (4), thicker than in test 1 due to the higher C content. It becomes possible to obtain a decarburized layer (3).

Next, the coated steel plate is hot formed. FIG. 4b represents a schematic cross-sectional view of the press-hardened steel part of Test 1.

The width of the ferrite grains formed in the interdiffused layer (6) is a legacy of the pure ferrite layer where austenite formation occurs during heating and has a larger grain size. An interdiffusion layer grows with this large austenite grain size. The width of the ferrite grains in the interdiffused layer (6) is larger than the prior austenite grain size in the bulk (7) and has a bending angle greater than 70°, resulting in good bendability. Furthermore, due to the thick ferrite layer (4) in the coated steel sheet, a layer of martensite with carbon gradient is formed between the bulk and the interdiffusion layer in the press-hardened steel part, resulting in a tensile strength higher than 1500 MPa.

In test 3, the coated steel plate has a decarburized layer without a ferrite layer on top of it, as schematically represented in Fig. 2a. The absence of a ferrite layer is due to the low dew point temperature T _DP1 of −10° C., which slows down the rate of decarburization.

Next, the coated steel plate is hot formed. Figure 2b represents a schematic cross-sectional view of the press-hardened steel part from Test 3. Due to the absence of a ferrite layer, the width of the ferrite grains in the interdiffused layer (6) is comparable to the prior austenite grain size in the bulk (7), resulting in a low bending angle of less than 70°.

In test 4, the low dew point temperature T _DP1 of −40° C. suggests that there is no decarburized layer and ferrite layer in the coated steel sheet.

FIG. 1a represents a schematic cross-sectional view of the coated steel plate of this test, with the coating layer (1) and the bulk (2).

Next, the coated steel plate is hot formed. FIG. 1b represents a schematic cross-sectional view of the press-hardened steel part from Test 4. Due to the absence of a ferrite layer, the width of the ferrite grains in the interdiffused layer (6) is comparable to the prior austenite grain size in the bulk (7), resulting in a low bending angle of less than 70°.

In test 5, the steel plate is held at the soaking temperature for 10800 seconds, which results in the formation of a thicker ferrite layer on the coated steel plate with a decarburized layer than in the previous tests. Figure 5a represents a schematic cross-section of the coated steel plate of test 5, with a coating layer (1), a decarburized layer (3), a thicker ferrite layer with coarser grain size (4) and the bulk (2).

The coated steel plate was then hot formed and Figure 5b represents a schematic cross-sectional view of the press hardened steel part from test 5. During heating of the steel part, the bulk microstructure is austenitic and the thick ferrite layer transforms into a layer of austenite with a carbon gradient. However, due to the thickness of the ferrite layer greater than 100 μm, a layer of ferrite remains between the interdiffusion layer and the layer of austenite with carbon gradient.

During die quenching of steel parts, the ferrite layer is still present and the layer of austenite with carbon gradient transforms into the martensitic layer with carbon gradient, resulting in a multiphase layer. This causes a decrease in yield strength.

In test 6, the steel plate has a low carbon level of 0.21%. This low carbon content, combined with the processing parameters, results in a decarburized layer of the coated steel sheet with a ferrite layer. Nevertheless, yield strength and tensile strength of press hardened steel parts are not achieved due to low carbon levels.

Claims (7)

In weight percent, the following:
C: 0.26-0.40%
Mn: 0.5-1.8%
Si: 0.1-1.25%
Al: 0.01~0.1%
Cr: 0.1-1.0%
Ti: 0.01~0.1%
B: 0.001-0.004%
P≦0.020%
S≦0.010%
N≦0.010%
including;
and optionally, in percent by weight,
Ni≦0.5%
Mo≦0.40%
Nb≦0.08%
Ca≦0.1%
containing one or more of the elements of
A coated steel plate made of steel having a composition, the remainder of the composition being iron and unavoidable impurities resulting from smelting, and wherein the coated steel plate has the following from the bulk to the surface of the coated steel plate:
- a bulk with a microstructure containing 60% to 90% ferrite in terms of surface fraction, the remainder being island martensite - austenite, pearlite or bainite;
- such bulk is covered with a decarburized layer comprising a ferrite layer on top with a thickness of 1 μm to 100 μm;
- Coated steel sheets containing a coating layer made of aluminum or aluminum alloys. A method for manufacturing a coated steel sheet, the method comprising:
The following consecutive steps,
- casting a steel having a composition according to claim 1 to obtain a slab;
- reheating said slab at a temperature T _reheat comprising 1100°C to 1300°C;
hot rolling the reheated slab at a finish hot rolling temperature of -800°C to 950°C; coiling the hot rolled steel plate at a coiling temperature T _coil of less than -670°C to obtain a rolled steel plate; The process of obtaining
- optionally pickling the rolled steel sheet;
- optionally cold rolling said rolled steel sheet to obtain a cold rolled steel sheet;
- annealing a hot-rolled steel plate or a cold-rolled steel plate by heating it to an annealing temperature _TA comprising 700°C to 850°C and maintaining the steel plate at said temperature _TA for a holding time _tA comprising 10 seconds to 1200 seconds; obtaining a steel plate, the atmosphere containing 0% to 15% H2 and having a dew point T _DP1 strictly higher than -10°C and lower than +20°C;
- cooling the annealed steel plate to a temperature range of 560°C to 700°C;
A method comprising: - coating the annealed steel plate with an aluminum or aluminum alloy coating; - cooling the coated steel plate to room temperature. Press-hardened steel parts, in weight percent:
C: 0.26-0.40%
Mn: 0.5-1.8%
Si: 0.1-1.25%
Al: 0.01~0.1%
Cr: 0.1-1.0%
Ti: 0.01~0.1%
B: 0.001-0.004%
P≦0.020%
S≦0.010%
N≦0.010%
including;
and optionally, in percent by weight,
Ni≦0.5%
Mo≦0.40%
Nb≦0.08%
Ca≦0.1%
has a composition containing one or more of the elements,
The remainder of the composition is unavoidable impurities resulting from iron and smelting,
The steel part continuously comprises the following steps from the bulk to the surface of the steel part:
- a bulk with a microstructure comprising, in surface fraction, more than 95% martensite and less than 5% bainite,
- ferrite interdiffusion layer,
- a coating layer based on aluminum,
including;
The ratio of the width GW _int of the ferrite grains in the interdiffused layer to the prior austenite grain size PAGS _bulk in the bulk is expressed by the following formula,
(GW _int /PAGS _bulk )-1≧30%
meet, press hardened steel parts. 4. The press hardened steel component of claim 3, including a layer of martensite with a carbon gradient between the bulk and the ferrite interdiffusion layer. Press-hardened steel component according to claim 3 or 4, having a tensile strength TS of 1500 MPa or more and a bending angle of more than 70°. The press hardened steel component according to claim 5, having a yield strength YS of 1250 MPa or more. A method for manufacturing a press-hardened steel part according to any one of claims 3 to 6, comprising the following successive steps:
- providing a steel plate having the composition according to claim 1 or produced by the method according to claim 2;
- cutting said steel plate into a predetermined shape to obtain a steel blank;
- heating the steel blank to a temperature comprising 880°C to 950°C for 10 seconds to 900 seconds to obtain a heated steel blank;
- transferring the heated blank to a forming press;
- hot forming the heated blank in said forming press to obtain a formed part;
- A method comprising the step of die-quenching said molded part.

Images