KR20220005572A - Cold-rolled martensitic steel sheet and manufacturing method thereof - Google Patents

Abstract

The present invention is a cold rolled martensitic steel sheet comprising: 0.3% ≤ C ≤ 0.4%; 0.5% ≤ Mn ≤ 1%; 0.2% ≤ Si ≤ 0.6%; 0.1% ≤ Cr ≤ 1%; 0.01% ≤ Al ≤ 1%; 0.01% ≤ Mo ≤ 0.5%; 0.001% ≤ Ti ≤ 0.1%; 0% ≤ S ≤ 0.09%; 0% ≤ P ≤ 0.09%; 0% ≤ N ≤ 0.09%; 0% ≤ Nb ≤ 0.1%; 0% ≤ V ≤ 0.1%; 0% ≤ Ni ≤ 1%; 0% ≤ Cu ≤ 1%; 0% ≤ B ≤ 0.05%; 0.001% ≤ Ca ≤ 0.01%; 0% ≤ Sn ≤ 0.1%; 0% ≤ Pb ≤ 0.1%; 0% ≤ Sb ≤ 0.1%; wherein the balance composition consists of iron and unavoidable impurities due to processing, and the microstructure of the steel is, in area percentage, at least 95% martensite, 1% to 5% cumulative amount of ferrite and bainite, and 0% to 2% of retained austenite in any amount.

Description

Cold-rolled martensitic steel and method of the martensitic steel

The present invention relates to a method for producing a cold-rolled martensitic steel suitable for the automobile industry, in particular to a martensitic steel having a tensile strength of 1700 MPa or more.

Automobile parts are required to satisfy two contradictory needs, namely, ease of forming and strength, but recently a third requirement is imposed on improvement of fuel consumption for automobiles in view of global environmental problems. Therefore, automobile parts now have to be made of materials with high formability in order to meet the standards for ease of fitting in complex automobile assemblies, and at the same time reduce vehicle weight to improve fuel efficiency, while reducing vehicle impact and durability. strength should be improved.

Accordingly, intensive research and development efforts are being made to reduce the amount of material used in automobiles by increasing the strength of the material. Conversely, the increase in the strength of the steel sheet decreases the formability, and thus the development of materials having both high strength and high formability has become necessary.

Early research and developments in the field of high strength and high formability steel sheet have resulted in several methods for manufacturing high strength and high formability steel sheet, some of which are listed here for a clear understanding of the present invention.

The steel sheet of WO2017/065371 contains C 0.08 to 0.30 wt%, Si 0.01 to 2.0 wt%, Mn 0.30 to 3.0 wt%, P 0.05 wt% or less, S 0.05 wt% or less, the balance being Fe and other unavoidable impurities rapidly heating the phosphorus steel sheet above the Ac3 transformation point for 3 to 60 seconds and holding the steel sheet; rapidly cooling the heated steel sheet to 100° C./s or more with water or oil; and rapid tempering to 500° C. to A1 transformation point for 3 to 60 seconds, including heating and holding time. However, the steel of WO2017/065371 cannot provide a hole expansion ratio of 22% while having a tensile strength of 1700 MPa.

It is an object of the present invention to solve these problems by providing a sheet of cold rolled martensitic steel having at the same time:

- ultimate tensile strength of at least 1700 MPa, preferably at least 1750 MPa,

- yield strength of at least 1500 MPa, preferably at least 1550 MPa,

- Hole expansion ratio of 22% or more, preferably 25% or more.

Preferably, such steel may also have good formability for rolling with good weldability and coatability.

Another object of the present invention is to make available a method for manufacturing such a steel sheet which is also compatible with conventional industrial applications while being robust to manufacturing parameter shifts.

The foregoing objects and other advantages of the present invention will become more apparent by describing preferred embodiments of the present invention in detail.

The chemical composition of cold rolled martensitic steel consists of the following elements:

The carbon present in the steel of the present invention is 0.3% to 0.4%. Carbon is an element necessary to increase the strength of the steel of the present invention by generating a low-temperature transformation phase such as martensite. Therefore, carbon plays two pivotal roles, one of which is to increase strength. However, if the carbon content is less than 0.3%, it is impossible to impart tensile strength to the steel of the present invention. On the other hand, when the carbon content exceeds 0.4%, the steel exhibits poor spot weldability, limiting its application to automobile parts. A preferred content for the present invention can be maintained between 0.3% and 0.38%, more preferably between 0.3% and 0.36%.

The manganese content of the steel of the present invention is 0.5% to 1%. This element is gamma ray. Manganese aids martensite formation by providing solid solution strengthening, suppressing the ferrite transformation temperature, and reducing the ferrite transformation rate. An amount of at least 0.5% is needed to impart strength and aid in the formation of martensite. However, if the manganese content exceeds 1%, adverse effects such as delaying the transformation of austenite into martensite during cooling after annealing occur. If the manganese content exceeds 1%, excessive segregation may occur in the steel during solidification, and the homogeneity of the inside of the material may be impaired and surface cracks may occur during the hot working process. A preferred limit for the presence of manganese is between 0.5% and 0.9%, more preferably between 0.6% and 0.8%.

The silicon content of the steel of the present invention is 0.2% to 0.6%. Silicon is an element contributing to increasing strength by solid solution strengthening. Silicon is a component capable of retarding the precipitation of carbides during cooling after annealing, and thus silicon promotes the formation of martensite. However, silicon is also a ferrite former and increases the Ac3 transformation point to push the annealing temperature to a higher temperature range, so that the silicon content is maintained at a maximum of 0.6%. A silicon content of more than 0.6% can mitigate the brittleness and silicon impairs the coatability. A preferred limit for the presence of silicon is 0.2% to 0.5%, more preferably 0.25% to 0.45%.

The chromium content of the composite coil of the steel of the present invention is 0.1% to 1%. Chromium is an essential element that imparts strength to steel by solid solution strengthening, and at least 0.1% or more is required to impart strength, but when used in excess of 1%, the surface finish of the steel is impaired. A preferred limit for the presence of chromium is between 0.3% and 0.9%, more preferably between 0.4% and 0.8%.

The content of aluminum is 0.01% to 1% in the present invention. Aluminum removes oxygen present in the molten steel to prevent oxygen from forming a gaseous phase during the solidification process. Aluminum also fixes nitrogen in the steel to form aluminum nitride, reducing particle size. When the aluminum content is high, exceeding 1%, the Ac3 point increases at a high temperature, and the productivity decreases. A preferred limit for the presence of aluminum is between 0.01% and 0.5%.

Titanium is added to the steel of the present invention at 0.001% to 0.1%. This forms titanium nitride, which appears in the solidification process of the casting. The amount of titanium is limited to 0.1% to avoid formation of coarse titanium nitride which is detrimental to formability. A titanium content of less than 0.001% has no effect on the steel of the present invention.

Molybdenum is an essential element constituting 0.01% to 0.5% of the steel of the present invention; Molybdenum, particularly when added in an amount of 0.01% or more, plays an effective role in improving hardenability and hardness and delays the appearance of bainite to promote the formation of martensite. Molybdenum also facilitates the formation of ferrite and pearlite microstructures during cooling during hot rolling, which ferrite and pearlite microstructures facilitate cold rolling. However, since the addition of molybdenum excessively increases the addition cost of alloying elements, the content is limited to 0.5% for economic reasons. The preferred limit for molybdenum is 0.1% to 0.3%.

Sulfur is not an essential element, but may be included as an impurity in the steel in the context of the present invention. The content of sulfur is preferably as low as possible, but is 0.09% or less from the viewpoint of manufacturing cost. Also, if higher sulfur is present in the steel, it combines to form sulfides, particularly manganese, and reduces the beneficial effect on the present invention.

The phosphorus content of the steel of the present invention is 0% to 0.09%. Phosphorus reduces spot weldability and hot ductility, especially due to its tendency to segregate at grain boundaries or with manganese. For this reason, the content is limited to 0.09%, preferably less than 0.06%.

Nitrogen is limited to 0.09% to prevent aging of the material and to minimize precipitation of aluminum nitride during solidification, which is detrimental to the mechanical properties of the steel.

Niobium is present in 0% to 0.1% in the steel of the present invention and is suitable for forming carbonitrides to impart strength to the steel of the present invention by precipitation hardening. Niobium also precipitates as carbonitrides and retards recrystallization during heating, affecting the size of microstructure components. Thus, the finer microstructure formed at the end of the holding temperature and consequently after complete annealing will lead to hardening of the product. However, a niobium content of more than 0.1% is not economically interesting because the saturation effect of the effect is observed, which means that the amount of niobium added does not lead to an improvement in the strength of the product.

Vanadium has the effect of increasing the strength of steel by forming carbides or carbonitrides, and the upper limit is 0.1% from an economical point of view.

Nickel may be added as an optional element in an amount of 0% to 1% to increase the strength and improve the toughness of the steel of the present invention. A minimum of 0.01% is preferred to achieve this effect. However, when the content exceeds 1%, nickel becomes a cause of ductility deterioration.

Copper may be added as an optional element in an amount of 0% to 1% to increase the strength of the steel of the present invention and improve corrosion resistance. A minimum of 0.01% is preferred to achieve this effect. However, if the content exceeds 1%, the surface aspect may be deteriorated.

Boron is an optional element for the steel of the present invention and may be present from 0% to 0.05%. When boron is added in an amount greater than 0.0001%, it forms boron nitride and imparts additional strength to the steel of the present invention.

Calcium can be added to the steel of the present invention at 0.001% to 0.01%%. Calcium is added to the steel of the present invention as an optional element, particularly during inclusion treatment. Calcium contributes to the refining of steels by delaying the harmful effects of sulfur by binding the harmful sulfur content into a spherical form.

Other elements such as Sn, Pb or Sb may be added individually or in combination in the following proportions: Sn ≤0.1%, Pb ≤0.1% and Sb ≤0.1%. The use of these elements up to the indicated maximum content level allows for purification of the particles during coagulation. The remainder of the steel composition consists of iron and unavoidable impurities resulting from processing.

The microstructure of the martensitic steel sheet is now described in detail, all percentages are area fractions.

Martensite constitutes more than 95% of the microstructure by area fraction. The martensite of the present invention may include both fresh and tempered martensite. However, fresh martensite is an optional minor component, preferably limited to an amount of 0% to 4%, preferably 0 to 2%, even more preferably 0% in the steel. Fresh martensite may form during cooling after tempering. Tempered martensite is formed from martensite formed after annealing, in particular below the Ms temperature, more particularly at Ms-10° C. to 20° C. during the second cooling step. This martensite is then tempered while maintaining at a tempering temperature (Ttemper) of 150°C to 300°C. The martensite of the present invention imparts ductility and strength to these steels. Preferably, the content of martensite is 96% to 99%, more preferably 97% to 99%.

The cumulative amount of ferrite and bainite represents 1% to 5% of the microstructure. The cumulative presence of bainite and ferrite does not adversely affect the present invention up to 5%, but exceeding 5% may adversely affect the mechanical properties. The preferred limits for cumulatively present ferrite and bainite are therefore maintained between 1% and 4%, more preferably between 1% and 3%.

Bainite is formed during reheating prior to tempering. In a preferred embodiment, the inventive steel contains 1-3% bainite. Bainite can impart formability to steel, but too much can adversely affect the tensile strength of the steel.

Ferrite can be formed in a first cooling step after annealing, but is not required as a microstructure component. Ferrite formation should be kept as low as possible, preferably less than 2% or even less than 1%.

Residual austenite is a selective microstructure that can be present in the steel from 0% to 2%.

In addition to the microstructure described above, the microstructure of the cold-rolled martensitic steel sheet is free from microstructure components such as pearlite and cementite.

The steel according to the invention may be produced by any suitable method. However, preference is given to using the method according to the invention, which will be described in detail by way of non-limiting example.

This preferred method consists in providing a semi-finished casting of a steel having the chemical composition of the prime steel according to the invention. Casting can be done in ingots or continuously in the form of thin slabs or thin strips, ie with a thickness of about 220 mm for slabs to several tens of millimeters for thin strips.

For example, a slab having a chemical composition according to the present invention is produced by continuous casting, and the slab is produced during a continuous casting process to avoid central segregation and to ensure a local carbon to nominal carbon to nominal carbon ratio of less than 1.10. Direct soft reduction was optionally performed. The slab provided by the continuous casting process may be used directly at high temperature after continuous casting or may be initially cooled to room temperature and then reheated for hot rolling.

The temperature of the slab subjected to hot rolling should be not less than 1000℃ and less than 1280℃. When the temperature of the slab is lower than 1280 ° C, an excessive load is applied to the rolling mill, and furthermore, the temperature of the steel may be lowered to the ferrite transformation temperature during finish rolling, whereby the steel is rolled in a state in which the ferrite contained in the structure is deformed. . Therefore, the temperature of the slab must be sufficiently high so that the hot rolling can be completed in the temperature range of Ac3 to Ac3+100°C. Reheating at temperatures above 1280°C is industrially expensive and should be avoided.

The sheet obtained in this way is cooled with a cooling rate of at least 20 °C/s to the coiling temperature which should be less than 650 °C. Preferably, the cooling rate will be 200° C./s or less.

Then, the hot-rolled steel sheet is coiled at a coiling temperature of less than 650°C to avoid ellipsoidation, preferably 475°C to 625°C to avoid scale formation, a more preferred range for this coiling temperature is 500°C to 625°C. The coiled hot rolled steel sheet is then cooled to room temperature and then subjected to optional hot band annealing.

The hot rolled steel sheet may be subjected to an optional descaling step to remove scale formed during hot rolling prior to the optional hot band annealing. The hot rolled sheet may be subjected to an optional hot band annealing. In a preferred embodiment, this hot band annealing is performed at a temperature of 400° C. to 750° C., preferably at least 12 hours and up to 96 hours, and the temperature is preferably kept below 750° C. so that partial Avoid the possibility of deformation and thus loss of microstructure homogeneity. Thereafter, an optional descaling step of this hot-rolled steel sheet can be carried out, for example, via pickling of this steel sheet.

Then, this hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet having a thickness reduction ratio of 35 to 90%.

Thereafter, the cold rolled steel sheet is heat treated to impart the required mechanical properties and microstructure to the steel of the present invention.

The cold rolled steel sheet is then heated in a two-stage heating process, the first heating stage starting from room temperature, the cold-rolled steel sheet being heated to a temperature (HT1) in the range of 550 °C to 750 °C with a heating rate of at least 10 °C/s It is heated to (HR1). In a preferred embodiment, the heating rate (HR1) for this first heating step is at least 15°C/s, more preferably at least 18°C/s. The preferred temperature HT1 for this first heating step is 575° C. to 725° C.

In the second heating step, the cold-rolled steel sheet is obtained from HT1 at an annealing temperature (Tsoak) of from Ac3 to Ac3+100°C, preferably from Ac3+10°C to Ac3+100°C, from 1°C/s to 50°C/s. It is heated at a heating rate (HR2). In a preferred embodiment, the heating rate (HR2) for the second heating step is from 1 °C/s to 25 °C/s, more preferably from 1 °C/s to 20 °C/s, wherein Ac3 of the steel sheet is It is calculated using the formula:

Here the element content is expressed as a percentage by weight of the cold rolled steel sheet.

The cold rolled steel sheet is held at Tsoak for 10 to 500 seconds to ensure complete recrystallization of the strongly work-hardened initial structure and complete transformation to austenite.

The cold rolled steel sheet is cooled in a two stage cooling process in which the first cooling stage starts from Tsoak, the cold rolled steel sheet is 30 °C/s to 150 °C with a temperature T1 in the range of 630 °C to 750 °C. It is cooled at a cooling rate (CR1) of /s. In a preferred embodiment, the cooling rate CR1 for this first cooling step is between 30° C./s and 120° C./s. The preferred T1 temperature for this first step is 640°C to 725°C.

In the second cooling step, the cold rolled steel sheet is cooled from T1 to a temperature T2 of Ms-10°C to 20°C with a cooling rate (CR2) of at least 50°C/s. In a preferred embodiment, the cooling rate CR2 for the second cooling step is at least 100° C./s, more preferably at least 150° C./s. The preferred T2 temperature for this second step is Ms-50°C to 20°C.

The Ms of the steel sheet is calculated using the formula:

Thereafter, the cold-rolled steel sheet is subjected to a tempering temperature of 150°C to 300°C with a heating rate of 1°C/s or more, preferably 2°C/s or more, more preferably 5°C/s or more for 100 seconds to 600 seconds. It is reheated to (Ttemper). The preferred temperature range for tempering is 200° C. to 300° C., and the preferred holding time at Ttemper is 200 seconds to 500 seconds.

Thereafter, the cold-rolled steel sheet is cooled to room temperature to obtain a cold-rolled martensitic steel.

The cold-rolled martensitic steel sheet of the present invention may be optionally coated with zinc or a zinc alloy or coated with aluminum or an aluminum alloy to improve corrosion resistance.

Yes

The following tests, examples, figurative illustrations and tables presented herein are to be regarded as completely non-limiting and for illustrative purposes only, and will show advantageous features of the invention.

Steel sheets made of steels with different compositions are collected in Table 1, and the steel sheets are each manufactured according to the process parameters specified in Table 2. After that, Table 3 shows the microstructure of the steel sheet obtained during the test, and Table 4 shows the evaluation results of the obtained properties.

Table 1

Table 2

Table 2 collects the hot rolling and annealing process parameters implemented in the cold rolled steel sheet to give the steel of Table 1 the mechanical properties necessary to become the cold rolled martensitic steel.

Table 2 is as follows:

Table 3 illustrates the results of tests performed according to standards for different microscopes, such as scanning electron microscopy, to determine the microstructure of both the inventive and reference steels in terms of area fraction. The results are set out here:

Table 3

Table 4

The results of various mechanical tests performed according to the standard are collected. Test the ultimate tensile strength and yield strength according to JIS-Z2241. To estimate the hole expansion, a test called hole expansion is applied, in this test sample a hole of 10 mm is punched and deformed after deformation, the hole diameter is measured and HER% = 100*(Df-Di)/Di is calculated .

Claims (18)

A sheet of cold-rolled martensitic steel comprising, in weight percentages, the following elements:
0.3% ≤ C ≤ 0.4%;
0.5% ≤ Mn ≤ 1%;
0.2% ≤ Si ≤ 0.6%;
0.1% ≤ Cr ≤ 1%;
0.01% ≤ Al ≤ 1%;
0.01% ≤ Mo ≤ 0.5%;
0.001% ≤ Ti ≤ 0.1%;
0% ≤ S ≤ 0.09%;
0% ≤ P ≤ 0.09%;
0% ≤ N ≤ 0.09%;
has a composition comprising
any of the following elements
0% ≤ Nb ≤ 0.1%;
0% ≤ V ≤ 0.1%;
0% ≤ Ni ≤ 1%;
0% ≤ Cu ≤ 1%;
0% ≤ B ≤ 0.05%;
0.001% ≤ Ca ≤ 0.01%;
0% ≤ Sn ≤ 0.1%;
0% ≤ Pb ≤ 0.1%;
0% ≤ Sb ≤ 0.1%;
may include one or more of
The balance composition consists of iron and unavoidable impurities due to processing, the microstructure of the steel, in area percentage, of at least 95% martensite, cumulative amounts of 1% to 5% ferrite and bainite, and 0% to 2% A sheet of cold rolled martensitic steel comprising any amount of retained austenite. The method of claim 1,
The composition comprises 0.3% to 0.36% of carbon, cold-rolled martensitic steel sheet. 3. The method according to claim 1 or 2,
The composition comprises 0.3% to 0.38% of carbon, cold-rolled martensitic steel sheet. 4. The method according to any one of claims 1 to 3,
The composition comprises 0.01% to 0.5% of aluminum, cold-rolled martensitic steel sheet. 5. The method according to any one of claims 1 to 4,
The composition comprises 0.5% to 0.9% of manganese, cold-rolled martensitic steel sheet. 6. The method according to any one of claims 1 to 5,
The composition comprises 0.3% to 0.9% of chromium, cold-rolled martensitic steel sheet. 7. The method according to any one of claims 1 to 6,
A cold rolled martensitic steel sheet, wherein the amount of martensite is 96% to 99%. 8. The method according to any one of claims 1 to 7,
A sheet of cold rolled martensitic steel having a cumulative amount of ferrite and bainite of 1% to 4%. 9. The method according to any one of claims 1 to 8,
wherein the steel sheet has an ultimate tensile strength of 1700 MPa or more, and a yield strength of 1500 MPa or more. A method for producing a cold rolled martensitic steel sheet, comprising the following successive steps
- providing a steel composition according to any one of claims 1 to 6;
- reheating the semi-finished product to a temperature between 1000°C and 1280°C;
- obtaining a hot-rolled steel sheet by rolling the semi-finished product in an austenitic range at a hot-rolling finishing temperature of Ac3 to Ac3 + 100°C;
- cooling the steel sheet to a coiling temperature of less than 650 °C at a cooling rate of at least 20 °C/s and coiling the hot rolled steel sheet;
- cooling the hot rolled steel sheet to room temperature;
- optionally performing a descaling process on the hot rolled steel sheet;
- optionally performing annealing on the hot rolled steel sheet;
- optionally performing a descaling process on the hot rolled steel sheet;
- cold-rolling the hot-rolled steel sheet at a reduction ratio of 35 to 90% to obtain a cold-rolled steel sheet;
- After that,
o a first heating step of heating the cold rolled steel sheet from room temperature to a temperature (HT1) of 550°C to 750°C with a cooling rate (HR1) of at least 10°C/s;
o a second heating step of heating from HT1 to a temperature (Tsoak) from Ac3 to Ac3+100°C (Tsoak) with a heating rate (HR2) of 1°C/s to 50°C/s maintained for 10 to 500 seconds
heating the cold-rolled steel sheet with a two-step heating of;
- After that,
o a first cooling step of cooling the cold rolled steel sheet from Tsoak to a temperature (T1) of 630°C to 750°C at a cooling rate (CR1) of 30°C/s to 150°C/s;
o a second cooling step of cooling from T1 to a temperature (T2) of Ms-10°C to 20°C with a cooling rate (CR2) of at least 50°C/s
cooling the cold-rolled steel sheet by two-step cooling of;
- thereafter, reheating the cold rolled steel sheet at a rate of at least 1 °C/s to a tempering temperature (Ttemper) of 150 °C to 300 °C maintained for 100 to 600 seconds;
- A method for producing a cold-rolled martensitic steel sheet, which is then cooled to room temperature at a cooling rate of at least 1° C./s to obtain a cold-rolled martensitic steel sheet. 11. The method of claim 10,
The coiling temperature is 475 °C to 625 °C, a method for producing a cold-rolled martensitic steel sheet. 12. The method of claim 10 or 11,
A method for producing a cold-rolled martensitic steel sheet, wherein Tsoak is Ac3+10°C to Ac3+100°C. 13. The method according to any one of claims 10 to 12,
A method for producing a cold-rolled martensitic steel sheet, wherein CR1 is 30° C./s to 120° C./s. 14. The method according to any one of claims 10 to 13,
A method for producing a cold-rolled martensitic steel sheet, wherein T1 is 640° C. to 725° C. 15. The method according to any one of claims 10 to 14,
A method for producing a cold rolled martensitic steel sheet, wherein the CR2 is at least 100° C./s. 16. The method according to any one of claims 10 to 15,
A method for producing a cold-rolled martensitic steel sheet, wherein T2 is Ms-50°C to 20°C. 17. The method according to any one of claims 10 to 16,
A method for producing a cold-rolled martensitic steel sheet, wherein the Ttemper is 200° C. to 300° C. 18. Use of a steel sheet obtainable according to any one of claims 1 to 9 or a steel sheet produced according to the method according to any one of claims 10 to 17 for manufacturing structural parts of vehicles.