KR20230100737A - Coated steel sheet and high-strength press-hardening steel parts and manufacturing method thereof - Google Patents

Abstract

In the present invention, by weight, 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 Coated steel sheets and press-hardened steel parts having a composition containing ≤ 0.020%, S ≤ 0.010%, N ≤ 0.010%, the balance being iron and unavoidable impurities due to smelting are covered. The press-hardened steel component comprises a bulk with a microstructure containing, in surface fraction, martensite and less than 5% bainite, a coating layer on the surface of the steel component, and a ferrite interdiffusion layer between the coating layer and the bulk. and the ratio between the ferrite grain width GW _int in the interdiffusion layer and the prior austenite grain size PAGS _bulk in the bulk satisfies the following equation: (GW _int / PAGS _bulk ) -1 ≥ 30%.

Description

Coated steel sheet and high-strength press-hardening steel parts and manufacturing method thereof

The present invention relates to a high-strength press-hardened steel part with good bendability and a coated steel sheet.

High-strength press-hardened parts can be used as structural elements in automobiles for intrusion prevention or energy absorption functions. In these types of applications, it is desirable to produce steel parts that combine high mechanical strength, high impact resistance and good corrosion resistance. Moreover, one of the major challenges of the automobile industry is to reduce the weight of vehicles to improve their fuel economy in terms of global environmental conservation without neglecting safety requirements.

This weight reduction can be achieved in particular thanks to the use of steel components with a martensitic or bainite-martensitic microstructure.

Publication WO2016104881 discloses a hot press-formed part used as a structural part of an automobile or the like, which requires impact resistance properties, and more specifically has a tensile strength of 1300 MPa or more, and heats a steel material to a temperature at which an austenite single phase can be formed, It relates to a method for producing it by quenching and hot forming using a mold. To obtain these properties, the base steel sheet must contain a thin ferrite layer of less than 50 μm on the surface, and the carbide size and density must be controlled. This ferrite layer in the substrate makes it possible to suppress propagation of microcracks formed in the plating layer to the base, but results in low bendability with a bending angle of less than 70°.

Publication WO2018179839 relates to a hot-pressed part obtained by hot-pressing a steel sheet having a microstructure varying in the thickness direction, comprising a soft layer composed of at least 90% of ferrite, a transitional layer composed of ferrite and martensite, and mainly martensitic It has a hard layer and has both high strength and high bendability. To obtain these properties, the cold-rolled steel sheet is annealed in an atmosphere with a dew point temperature of 50°C to 90°C, which may be detrimental to the aluminum alloy coating.

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

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

The object of the present invention is achieved by providing the steel sheet according to claim 1. Another object is achieved by providing a method according to claim 2 . Another object of the present invention is achieved by providing a press-hardened steel component according to claim 3 . The steel component can also comprise the features according to any one of claims 4 to 6 . Another object is achieved by providing a method according to claim 7 .

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will now be described in detail and demonstrated by way of example, without introducing any limitation, with reference to the accompanying drawings.

1A shows a schematic cross-section of a coated steel sheet of Test 4 not according to the present invention.
1b shows a schematic cross-section of a press-hardened steel part from test 4 not according to the invention.
Figure 2a shows a schematic cross-section of a coated steel sheet of test 3 not according to the present invention.
Figure 2b shows a schematic cross-section of a press-hardened steel part from test 3 not according to the invention.
Figure 3a shows a schematic cross-section of a coated steel sheet of test 2, according to the present invention.
Figure 3b shows a schematic cross-section of a press-hardened steel part from test 2 according to the invention.
4A shows a schematic cross-section of a coated steel sheet of Test 1 according to the present invention.
Figure 4b shows a schematic cross-section of a press-hardened steel part from test 1 according to the invention.
5A shows a schematic cross-section of a coated steel sheet of Test 5 not according to the present invention.
Figure 5b shows a schematic cross-section of a press-hardened steel part from test 5 not according to the invention.

Now, the composition of the steel according to the present invention is described, the content being expressed in weight percent.

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

The manganese content is between 0.5% and 1.8%. If added in excess of 1.8%, the risk of center segregation increases, impairing bendability. If it is less than 0.5%, the hardenability of the steel sheet is reduced. Preferably, the manganese content is between 0.5% and 1.3%.

According to the invention, the silicon content is between 0.1% and 1.25%. Silicon is an element that participates in solid solution hardening. Silicon is added to limit carbide formation. Above 1.25%, silicon oxide is formed on the surface, which impairs the coatability of the steel. Moreover, the weldability of the steel sheet 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%.

Since aluminum is a very effective element for deoxidizing liquid steel during elaboration, the aluminum content is between 0.01% and 0.1%. Aluminum can protect boron if the titanium content is insufficient. The aluminum content is less than 0.1% to avoid ferrite formation and oxidation problems during press hardening. Preferably, the aluminum content is between 0.01% and 0.05%.

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

The titanium content is between 0.01% and 0.1% to protect boron from formation of BN. The titanium content is limited to 0.1% to avoid TiN formation.

According to the present invention, the boron content is between 0.001% and 0.004%. Boron improves the hardenability of steel. To avoid the risk of slab fracture during continuous casting, the boron content is below 0.004%.

Some elements may optionally be added.

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

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

According to the present invention, niobium may optionally be added up to 0.08% to improve the ductility of the steel. If added in excess of 0.08%, the risk of formation of NbC or Nb(C,N) carbides increases, impairing bendability. Preferably, the niobium content is 0.05% or less.

Calcium may also be added up to 0.1% as an optional element. The addition of Ca in the liquid phase makes it possible to generate fine oxides that promote the castability of continuous casting.

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

Now, the microstructure of the coated steel sheet according to the present invention will be described.

A cross section of the coated steel sheet of the present invention is schematically shown in FIGS. 3A and 4A. The coated steel sheet includes a bulk 2 covered with a decarburized layer 3 comprising a ferrite layer 4 and a coating layer 1 having a thickness of 1 μm to 100 μm thereon. Preferably, the thickness of the ferrite layer is 20 μm to 100 μm. More preferably, the thickness of the ferrite layer is 25 μm to 100 μm. More preferably, the thickness of the ferrite layer is 30 μm to 80 μm.

The bulk of the coated steel sheet 2 has a microstructure containing 60% to 90% of ferrite as a surface fraction, the balance being martensitic-austenite islands, pearlite or bainite.

This ferrite is formed during intercritical annealing of cold rolled steel sheet. The remainder of the microstructure is austenite at the end of soaking, which is transformed to martensite-austenite islands, pearlite or bainite during cooling of the steel sheet.

The decarburization layer present on the bulk is obtained during annealing of the cold-rolled steel sheet thanks to the control of the atmosphere in the furnace so as to set the dew point temperature strictly above -10°C and below 20°C.

The coated steel sheet according to the present invention can be produced by any suitable manufacturing method, and a person skilled in the art can define it. However, preference is given to using the method according to the invention comprising the following steps:

A further hot-rollable semifinished product having the above-mentioned steel composition is provided. The semi-finished product is reheated at a temperature of 1150 ° C to 1300 ° C.

Then, the steel sheet is hot rolled at a finish hot rolling temperature of 800°C to 950°C.

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

Then, the coiled steel sheet is selectively cold-rolled to obtain a cold-rolled steel sheet. The cold reduction ratio is preferably 20% to 80%. At less than 20%, recrystallization during subsequent heat treatment is undesirable, which may impair the ductility of the steel sheet. Above 80%, there is a risk of edge cracking during cold rolling.

Then, the steel sheet is annealed at an annealing temperature T _A of 700 ° C to 850 ° C in an HNx atmosphere having 0% to 15% H ₂ , and maintained at the annealing temperature T _A for a holding time t _A of 10 s to 1200 s, annealed get a steel plate Below 700 DEG C, the rate of formation of the decarburized layer is too slow to obtain a ferrite layer thereon. The holding time t _A is 10 s or more so that a ferrite layer is formed, and 1200 s or less to limit the thickness of this ferrite layer.

During this annealing, the atmosphere in the furnace is strictly controlled to have a dew point temperature T _DP1 above -10°C and below +20°C to form the decarburized layer according to the present invention. When T _DP1 is -10°C or lower, formation of the decarburization layer is slowed, and no ferrite layer is formed thereon. The bendability of the steel part will be too low. If T _DP1 is higher than 20°C, the surface of the steel sheet is completely oxidized, and the plating properties and mechanical properties of the steel sheet may be impaired.

In one embodiment of the present invention, the annealed steel sheet is heated to an annealing temperature T2 of 700°C to 850°C, maintained at the temperature T2 for a holding time t2 of 10s to 1200s, and the atmosphere is strictly above -10°C and + It has a dew point T _DP2 of 20°C or less. The steel sheet is then coated with an aluminum alloy coating.

Now, the microstructure of the press-hardened steel component according to the present invention will be described. A cross-section of a press-hardened steel part is schematically shown in FIGS. 3b and 4b.

The steel part continuously from the bulk to the surface of the steel part contains:

- bulk (7) with a microstructure comprising, in surface fraction, martensite greater than 95% and bainite less than 5%;

- a ferrite interdiffusion layer (6);

- a coating layer (5) based on aluminum.

During heating of a steel blank cut from a steel sheet according to the invention, all microstructural elements of the bulk are transformed to austenite, and the ferrite of the decarburized layer is transformed to austenite having a grain size larger than that of the austenite of the bulk. After hot forming, the steel part is then die-quenched. The interdiffusion layer grows from the previous wide grain size austenite layer and has a grain width greater than the old austenite grain size in the bulk. The ratio between the ferrite grain width GW _int in the interdiffusion layer and the prior austenite grain size PAGS _bulk in the bulk, in order to improve the bendability of the steel sheet without degrading the mechanical properties, satisfies the following equation: ( GW _int / PAGS _bulk ) -1 ≥ 30%.

The ferrite grain width is the average distance between two parallel grain boundaries of the interdiffusion layer, and the grain boundaries are oriented in the direction of the steel sheet thickness. The combination of the annealing temperature T _A , the annealing time t _A and the dew point temperature T _DP1 according to the present invention promotes the formation of a large grain width GW _int in the interdiffusion layer. Moreover, heat treatment of steel blanks before press forming dominates austenite grain growth and thus PAGS in bulk.

In one embodiment, the press-hardened steel component may further include a martensite layer having a carbon gradient between the bulk and the interdiffusion layer, as indicated by (8) in FIG. 4B. During heating of the steel blank, carbon diffuses from the bulk to the surface. Subsequently, the ferrite top of the decarburized layer is transformed in an austenite layer with a carbon gradient. During die-quenching, this austenite layer with a carbon gradient is transformed into a martensite layer with a carbon gradient.

The press-hardened steel component according to the present invention has a tensile strength TS of 1500 MPa or more and a bending angle of more than 70°. Bending angles were determined on press-hardened parts according to the method VDA238-100 bending standard (normalized to a thickness of 1.5 mm).

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

The press-hardened steel parts according to the present invention can be produced by any suitable manufacturing method, and can be defined by those skilled in the art. However, preference is given to using the method according to the invention comprising the following steps:

The coated steel sheet according to the present invention is cut into a predetermined shape to obtain a steel blank. Then, the steel blank is heated to a temperature of 880° C. to 950° C. for 10 s to 900 s to obtain a heated steel blank. The heated blank is then transferred to a forming press prior to hot forming and die-quenching.

Now, the present invention is illustrated by the following examples, which are by no means limiting.

yes

Six grades (the composition of which is collected in Table 1) were cast into semi-finished products and processed into steel sheets, and then processed into steel parts according to the process parameters collected in Table 2.

Table 1 - Composition

The compositions tested are compiled in the table below, with the element content expressed in weight percent:

Steels A-D are according to the present invention.

Underlined values: not applicable to the present invention

Table 2 - Process Parameters

The steel semi-finished product in the as-cast state was reheated at 1200°C, hot rolled at a finish hot rolling temperature of 800 to 950°C, coiled at 550°C, and cold rolled at a reduction ratio of 60%. The steel sheet was then heated to a temperature T _A and held at this temperature for a holding time t _A in an HNx atmosphere with a controlled dew point and 5% of H ₂ . Then, the steel sheet was cooled to a temperature of 560 to 700° C., and then hot-dip plated 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 prior to coating, and the steel sheet was maintained at the temperature T ₂ for a holding time t ₂ in an HNx atmosphere with 5% H ₂ and a controlled dew point. . The following specific conditions apply:

The coated steel sheet was analyzed, and the corresponding properties of the decarburization layer were collected in Table 3.

Table 3 - Characteristics of decarburized layer of coated steel sheet

Then, the coated steel sheet was cut to obtain a steel blank, heated at 900 DEG C for 6 minutes, and hot formed. The steel parts were analyzed and the corresponding microstructure, ferrite grain width in the interdiffusion layer GW _int , and prior austenite grain size PAGS _bulk in the bulk are collected in Table 4. Mechanical properties are collected in Table 5.

Table 4 - Microstructure of press-hardened steel parts

The surface fraction, the ferrite grain width in the interdiffusion layer and the PAGS are determined through the following methods: specimens are cut from press-hardened steel parts, polished to reveal the microstructure and etched with reagents known per se. The cross-section is then examined through an optical or scanning electron microscope, such as a "FEG-SEM" (Scanning Electron Microscope with a Field Emission Gun) at a magnification of 5000x or greater, coupled to a Back Scattered Electron (BSE) device.

Table 5 - Mechanical properties of press-hardened steel parts

The mechanical properties of the tested samples were determined and compiled in the table below:

The examples show that the steel components according to the invention, examples 1-2, exhibit all the targeted properties thanks to their specific composition and microstructure.

Figure 3a shows a schematic cross section of the coated steel sheet of test 2. The combination of the process parameters of the present invention, the annealing temperature T _A , the annealing time t _A and the dew point temperature T _DP1 make it possible to obtain a decarburized layer 3 , in which a ferrite layer is formed on top 4 .

Then, the coated steel sheet is hot formed. Figure 3b shows a schematic cross-section of a press-hardened steel part of test 2.

The grain width of the ferrite formed in the interdiffusion layer 5 is a legacy of the pure ferrite layer in which austenite formation occurs upon heating, and the grain size is larger. Interdiffusion layers grow from these large austenite grain sizes. And, the ferrite grain width in the interdiffusion layer 6 is larger than that of the prior austenite grain size in the bulk 7, leading to good bendability with a bending angle higher than 70°.

4A shows a schematic cross section of the coated steel sheet of Test 1. The combination of the process parameters of the present invention, the annealing temperature T _A , the annealing time t _A and the dew point temperature T _DP1 make it possible to obtain a decarburized layer 3 in which a ferrite layer is formed on top 4 and a higher It is thicker than in Test 1 due to the C content.

Then, the coated steel sheet is hot formed. Figure 4b shows a schematic cross-section of a press-hardened steel part of test 1.

The grain width of the ferrite formed in the interdiffusion layer 6 is a legacy of the pure ferrite layer in which austenite formation occurs upon heating, and the grain size is larger. Interdiffusion layers grow from these large austenite grain sizes. And, the ferrite grain width in the interdiffusion layer 6 is larger than that of the prior austenite grain size in the bulk 7, leading to good bendability with a bending angle higher than 70°. Also, due to the thick ferrite layer 4 of the coated steel sheet, a martensite layer with a carbon gradient is formed between the bulk and the interdiffusion layer of the press-hardened steel part, resulting in a tensile strength higher than 1500 MPa.

In Test 3, the coated steel sheet had a decarburized layer without a ferrite layer thereon, as schematically shown 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 decarburization kinetics.

Then, the coated steel sheet is hot formed. Figure 2b shows a schematic cross-section of a press-hardened steel part from test 3. Due to the absence of the ferrite layer, the ferrite grain width in the interdiffusion layer 6 is equal 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 the absence of a decarburized layer and a ferrite layer in the coated steel sheet.

1A shows a schematic cross-section of a coated steel sheet of this test having a coating layer (1) and a bulk (2).

Then, the coated steel sheet is hot formed. 1b shows a schematic cross-section of a press-hardened steel part from test 4. Due to the absence of the ferrite layer, the ferrite grain width in the interdiffusion layer 6 is equal 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 sheet is kept at the soaking temperature for 10800 s, which forms a thicker ferrite layer in the decarburized layer than in the previous tests in the coated steel sheet. Figure 5a shows a schematic cross-section of a coated steel sheet of test 5, having a coating layer (1), a decarburized layer (3), a thicker ferrite layer with a coarser grain size (4), and a bulk (2). .

The coated steel sheet was then hot-formed, and FIG. 5B shows a schematic cross-section of the press-hardened steel part from Test 5. During heating of the steel component, the microstructure of the bulk is austenitic and the thick ferrite layer is transformed into an austenite layer with a carbon gradient. However, due to the thickness of the ferrite layer higher than 100 μm, the ferrite layer remains between the austenite layer with carbon gradient and the interdiffusion layer.

During die quenching of the steel component, the ferrite layer is still present, and the austenite layer with carbon gradient is transformed into a martensite layer with carbon gradient, resulting in a multiphase layer. This triggers a decrease in yield strength.

In test 6, the steel sheet has a low carbon level of 0.21%. This low carbon content, coupled to process parameters, results in a decarburization layer in coated steel sheets with a ferrite layer. Nevertheless, the yield strength and tensile strength of press-hardened steel parts are not obtained because of the low carbon level.

Claims (7)

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, in weight percent,
Ni ≤ 0.5%
Mo ≤ 0.40%
Nb ≤ 0.08%
Ca ≤ 0.1%
A coated steel sheet made of steel having a composition optionally containing one or more of,
The remainder of the composition is iron and unavoidable impurities due to smelting;
The coated steel sheet, from the bulk to the surface of the coated steel sheet,
- a bulk with a microstructure containing between 60% and 90% ferrite as a surface fraction, the remainder being martensitic-austenite islands, perlite or bainite;
- a decarburization layer covering the bulk, comprising a ferrite layer having a thickness of 1 μm to 100 μm on top, the decarburization layer,
- Coating layer made of aluminum or aluminum alloy
Including, coated steel sheet. As a method for producing a coated steel sheet, the following successive steps:
- casting a steel having a composition according to claim 1 to obtain a slab;
- reheating the slab to a temperature T _reheat of 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 sheet at a coiling temperature T _coil of less than 670° C. to obtain a coiled steel sheet;
- optionally pickling the coiled steel sheet,
- optionally cold rolling the coiled steel sheet to obtain a cold rolled steel sheet;
- heating a hot-rolled steel sheet or cold-rolled steel sheet to an annealing temperature T _A of 700° C. to 850° C., and holding the steel sheet at the temperature T _A for a holding time t _A of 10 s to 1200 s to obtain an annealed steel sheet; obtaining the annealed steel sheet, wherein the atmosphere contains 0% to 15% of H2 and has a dew point T _DP1 strictly above -10°C and below +20°C;
- cooling the annealed steel sheet to a temperature range of 560°C to 700°C,
- coating the annealed steel sheet with an aluminum or aluminum alloy coating;
- cooling the coated steel sheet to room temperature
Including, the manufacturing method of the coated steel sheet. A press-hardening steel part, the steel part 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, in weight percent,
Ni ≤ 0.5%
Mo ≤ 0.40%
Nb ≤ 0.08%
Ca ≤ 0.1%
Has a composition optionally containing one or more of,
The remainder of the composition is iron and unavoidable impurities due to smelting;
The steel part is continuously from the bulk to the surface of the steel part,
- a bulk with a microstructure comprising a surface fraction of more than 95% martensite and less than 5% bainite,
- ferrite interdiffusion layer,
- comprising a coating layer based on aluminum,
The ratio between the ferrite grain width GW _int in the interdiffusion layer and the prior austenite grain size PAGS _bulk in the bulk is:
(GW _int / PAGS _bulk ) -1 ≥ 30%
, press-hardened steel parts that satisfy According to claim 3,
The press-hardening steel component comprises a martensitic layer with a carbon gradient between the bulk and the ferrite interdiffusion layer. According to claim 3 or 4,
The press-hardening steel part has a tensile strength TS of 1500 MPa or more and a bending angle of more than 70°. According to claim 5,
The press-hardening steel part has a yield strength YS of 1250 MPa or more. Process for producing a press-hardened steel component according to claim 3 , comprising the following successive steps:
- providing a steel having a composition according to claim 1 or a steel sheet produced by the method according to claim 2,
- cutting the steel sheet into a predetermined shape to obtain a steel blank;
- heating the steel blank to a temperature of 880 ° C to 950 ° C for 10 s to 900 s to obtain a heated steel blank,
- conveying the heated steel blank to a forming press;
- hot forming the heated steel blank in the forming press to obtain a molded part;
- die-quenching the molded part
Method of manufacturing a press-hardened steel part comprising a.

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