KR102708271B1 - A cold rolled martensitic steel sheet and a method of producing thereof - Google Patents

Abstract

The present invention relates to a cold rolled martensitic steel sheet, wherein the steel sheet contains 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%; 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%; and the remainder is iron and inevitable impurities resulting from processing, wherein the microstructure of the steel comprises, in area percentage, at least 95% martensite, a cumulative amount of ferrite and bainite of 1% to 5%, and an arbitrary amount of retained austenite of 0% to 2%.

Description

{A COLD ROLLED MARTENSITIC STEEL SHEET AND A METHOD OF PRODUCING THEREOF}

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

Automotive parts are required to satisfy two contradictory requirements, namely, ease of forming and strength, but recently, a third requirement for improved fuel consumption of automobiles has been imposed in view of global environmental issues. Therefore, automobile parts must now be manufactured from materials with high formability to meet the criteria for ease of fitting in complex automobile assemblies, and at the same time, to improve the strength for impact resistance and durability of the vehicle while reducing the weight of the vehicle to improve fuel efficiency.

Therefore, 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, increasing the strength of steel sheets reduces their formability, and thus the development of materials with both high strength and high formability has become necessary.

Early research and developments in the field of high strength and high formability steel sheets gave rise to several methods for manufacturing high strength and high formability steel sheets, some of which are enumerated herein for a thorough understanding of the present invention.

The steel sheet of WO2017/065371 is manufactured by the steps of rapidly heating a material steel sheet containing 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, and the remainder being Fe and other unavoidable impurities, to a temperature higher than the Ac3 transformation point for 3 to 60 seconds and maintaining the material steel sheet; rapidly cooling the heated steel sheet with water or oil at a rate of 100°C/s or more; and rapidly tempering the steel sheet to the A1 transformation point at 500°C for 3 to 60 seconds including the heating and maintaining time. However, the steel of WO2017/065371 has a tensile strength of 1700 MPa but cannot provide a hole expansion ratio of 22%.

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

- Ultimate tensile strength of 1700 MPa or more, preferably 1750 MPa or more,

- Yield strength of 1500 MPa or more, preferably 1550 MPa or more,

- 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 steel sheets which is robust against manufacturing parameter shifts while being compatible with conventional industrial applications.

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

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 for increasing the strength of the steel of the present invention by forming a low-temperature transformation phase such as martensite. Therefore, carbon plays two pivotal roles: one is to increase the strength. However, if the carbon content is less than 0.3%, the steel of the present invention cannot provide tensile strength. On the other hand, if the carbon content exceeds 0.4%, the steel exhibits poor spot weldability, which limits its application to automobile parts. The preferred content for the present invention can be maintained at 0.3% to 0.38%, more preferably 0.3% to 0.36%.

The manganese content of the steel of the present invention is 0.5% to 1%. This element is a gamma ray. Manganese provides solid solution strengthening, suppresses the ferrite transformation temperature, and reduces the ferrite transformation rate to help the formation of martensite. At least 0.5% is required to provide strength and help 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 within the material may be impaired, which may cause surface cracking during the hot working process. The preferred limit for the presence of manganese is 0.5% to 0.9%, more preferably 0.6% to 0.8%.

The silicon content of the steel of the present invention is 0.2% to 0.6%. Silicon is an element that contributes to increasing the strength by solid solution strengthening. Silicon is a component that can delay 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, thereby pushing the annealing temperature to a higher temperature range, so the silicon content is maintained at a maximum of 0.6%. A silicon content exceeding 0.6% can alleviate brittleness, and silicon also impairs the coatability. The 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 steel composite coil of the present invention is 0.1% to 1%. Chromium is an essential element that provides strength to steel by solid solution strengthening, and a minimum of 0.1% or more is required to provide strength, but when used in excess of 1%, it damages the surface finish of the steel. The preferred limit for the presence of chromium is 0.3% to 0.9%, more preferably 0.4% to 0.8%.

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

Titanium is added to the steel of the present invention in an amount of 0.001% to 0.1%. This forms titanium nitride which appears during the solidification process of the casting. The amount of titanium is limited to 0.1% to avoid the formation of coarse titanium nitride which is detrimental to the formability. A titanium content of less than 0.001% does not have any 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, especially 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, and these ferrite and pearlite microstructures facilitate cold rolling. However, since the addition of molybdenum excessively increases the addition cost of alloying elements, its content is limited to 0.5% for economic reasons. The preferable limit of molybdenum is 0.1% to 0.3%.

Sulfur is not an essential element, but may be included as an impurity in the steel from the viewpoint of the present invention. The content of sulfur is preferably as low as possible, but from the viewpoint of manufacturing cost, it is less than 0.09%. In addition, if a higher sulfur is present in the steel, it will combine to form sulfides, especially with manganese, and reduce the beneficial effects on the present invention.

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

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

Niobium is present in the steel of the present invention in an amount of 0% to 0.1% 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 delays recrystallization during the heating process, thereby influencing the size of the microstructural components. Therefore, a finer microstructure formed at the end of the holding temperature and consequently after complete annealing will lead to hardening of the product. However, niobium contents exceeding 0.1% are not economically interesting since a saturation effect of its influence is observed, which means that the addition of niobium 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 economic perspective.

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. To achieve this effect, a minimum of 0.01% is preferred. However, when the content exceeds 1%, nickel causes deterioration of ductility.

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. To achieve these effects, a minimum of 0.01% is preferred. However, if the content exceeds 1%, the surface appearance may be degraded.

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

Calcium may be added to the steel of the present invention in an amount of 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 the steel by binding harmful sulfur content in a spherical form and thereby delaying the harmful effects of sulfur.

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 levels allows for particle refinement during solidification. The remainder of the steel composition consists of iron and inevitable impurities resulting from processing.

The microstructure of martensitic steel sheets is now described in detail, with all percentages being 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 trace element, preferably limited to an amount of 0 to 4%, preferably 0 to 2%, and even more preferably 0% in the steel. Fresh martensite may be formed during cooling after tempering. Tempered martensite is formed from martensite formed during a second cooling step, especially below the Ms temperature, more especially at Ms-10°C to 20°C, after annealing. This martensite is then tempered while being held at a tempering temperature (Ttemper) of 150°C to 300°C. The martensite of the present invention imparts ductility and strength to the steel. Preferably, the martensite content 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 have a negative effect on the present invention up to 5%, but may have a negative effect on the mechanical properties if it exceeds 5%. Therefore, the preferred limit for the cumulative presence of ferrite and bainite is maintained at 1% to 4%, more preferably 1% to 3%.

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

Ferrite may form during the first cooling step after annealing, but is not required as a microstructural component. Ferrite formation should be kept as low as possible, preferably below 2% or even below 1%.

Retained austenite is an optional microstructure that can exist in steel at 0% to 2%.

In addition to the microstructure described above, the microstructure of the cold-rolled martensitic steel sheet does not contain microstructural components such as pearlite and cementite.

The steel according to the present invention can be manufactured by any suitable method. However, it is preferred to use the method according to the present invention, which will be described in detail as a 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 present invention. The casting can be carried out as an ingot or continuously in the form of thin slabs or thin strips, i.e. in a thickness ranging from 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 manufactured by continuous casting, the slab optionally subjected to direct soft reduction during the continuous casting process to avoid center segregation and to ensure that the local carbon to nominal carbon ratio is maintained below 1.10. The slab provided by the continuous casting process can be used directly at elevated temperature after continuous casting or can be initially cooled to room temperature and then reheated for hot rolling.

The temperature of the slab subjected to hot rolling should be 1000℃ or higher and less than 1280℃. If the temperature of the slab is lower than 1280℃, excessive load is applied to the rolling mill, and further, the temperature of the steel may be lowered to the ferrite transformation temperature during the finish rolling, so that the steel is rolled in a state where the ferrite contained in the structure is transformed. Therefore, the temperature of the slab should be sufficiently high so that the hot rolling can be completed in the temperature range of Ac3 to Ac3+100℃. Reheating at a temperature higher than 1280℃ should be avoided because it is industrially expensive.

The sheet obtained in this manner is cooled at a cooling rate of at least 20 °C/s to a 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 ovalization, 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 subjected to optional hot band annealing.

The hot rolled steel sheet may be subjected to an optional descaling step prior to the optional hot band annealing to remove scale formed during hot rolling. 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 from 400° C. to 750° C., preferably for at least 12 hours and not more than 96 hours, the temperature being preferably maintained below 750° C. to avoid partial deformation of the hot rolled structure and thus the possibility of loss of microstructural homogeneity. An optional descaling step of the hot rolled steel sheet may then be performed, for example, by pickling the steel sheet.

Afterwards, the 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 mechanical properties and microstructure required for the steel of the present invention.

The cold rolled steel sheet is then heated in a two-step heating process, wherein the first heating step is started from room temperature and the cold rolled steel sheet is heated to a temperature (HT1) in the range of 550°C to 750°C at a heating rate (HR1) of at least 10°C/s. 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. A preferred temperature HT1 for this first heating step is at least 575°C to 725°C.

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

Here, the element contents are expressed as weight percentages of the cold rolled steel sheet.

Cold rolled steel sheets are held in a 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 starting from Tsoak in the first cooling stage, wherein the cold rolled steel sheet is cooled to a temperature (T1) in the range of 630°C to 750°C at a cooling rate (CR1) of 30°C/s to 150°C/s. In a preferred embodiment, the cooling rate (CR1) for this first cooling stage is 30°C/s to 120°C/s. A preferred T1 temperature for this first stage 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 at 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. A preferred T2 temperature for this second step is Ms-50°C to 20°C.

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

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

Afterwards, the cold rolled steel sheet is cooled to room temperature to obtain cold rolled martensitic steel.

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

yes

The following tests, examples, figurative examples and tables presented herein are entirely non-limiting and should be considered for illustrative purposes only, and will illustrate the advantageous features of the present invention.

Steel sheets manufactured with different compositions are collected in Table 1, and the steel sheets are manufactured according to the process parameters specified in Table 2, respectively. Table 3 then shows the microstructure of the steel sheets 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 on the cold rolled steel sheet to impart the steel in Table 1 with the mechanical properties required to become a 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 microscope to determine the microstructure of both the invention steel and the reference steel in terms of area fraction. The results are set forth herein:

Table 3

Table 4

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

Claims (18)

A cold rolled martensitic steel sheet, comprising, in weight percentage, 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%;
having a composition including,
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 following:
The remainder of the composition is iron and inevitable impurities resulting from processing, and the microstructure of the steel comprises, in area percentage, at least 95% martensite, 1% to 5% cumulative amounts of ferrite and bainite, and 0% to 2% optional amounts of retained austenite,
The above steel sheet is a cold-rolled martensitic steel sheet having an ultimate tensile strength of 1700 MPa or more and a yield strength of 1500 MPa or more. In the first paragraph,
The above composition is a cold-rolled martensitic steel sheet containing 0.3% to 0.36% carbon. In the first paragraph,
The above composition is a cold-rolled martensitic steel sheet containing 0.3% to 0.38% carbon. In the first paragraph,
The above composition is a cold-rolled martensitic steel sheet containing 0.01% to 0.5% aluminum. In the first paragraph,
The above composition is a cold-rolled martensitic steel sheet containing 0.5% to 0.9% manganese. In the first paragraph,
The above composition is a cold-rolled martensitic steel sheet containing 0.3% to 0.9% chromium. In any one of claims 1 to 6,
Cold rolled martensitic steel sheet having a martensite content of 96% to 99%. In any one of claims 1 to 6,
Cold rolled martensitic steel sheet having a cumulative amount of ferrite and bainite of 1% to 4%. delete A method for manufacturing a cold-rolled martensitic steel sheet according to any one of claims 1 to 6, comprising the following sequential steps:
- A step of providing a steel composition having the above composition;
- A step of reheating the semi-finished product to a temperature of 1000℃ to 1280℃;
- A step of rolling the semi-finished product in the austenitic range at a hot-rolling finishing temperature of Ac3 to Ac3 + 100℃ to obtain a hot-rolled steel sheet;
- A step of 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;
- A step of cooling the hot-rolled steel sheet to room temperature;
- Optionally, a step of performing a scale removal process on the hot-rolled steel sheet;
- Optionally, a step of performing annealing on the hot-rolled steel sheet;
- Optionally, a step of performing a scale removal process on the hot-rolled steel sheet;
- A step of 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 at a heating rate (HR1) of 10°C/s or more;
A second heating step of heating from HT1 to a temperature (Tsoak) of Ac3 to Ac3+100°C at a heating rate (HR2) of 1°C/s to 50°C/s maintained for 10 to 500 seconds.
A step of heating the cold rolled steel sheet in a second heating process;
- 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℃ to 20℃ at a cooling rate (CR2) of at least 50℃/s.
A step of cooling the cold rolled steel sheet by a second cooling step;
- Thereafter, a step of reheating the cold-rolled steel sheet at a rate of at least 1 ℃/s to a tempering temperature (Ttemper) of 150 ℃ to 300 ℃ maintained for 100 to 600 seconds;
- A method for manufacturing a cold-rolled martensitic steel sheet, wherein the cold-rolled martensitic steel sheet is then cooled to room temperature at a cooling rate of at least 1 ℃/s to obtain a cold-rolled martensitic steel sheet. In Article 10,
A method for manufacturing a cold-rolled martensitic steel sheet, wherein the coiling temperature is 475°C to 625°C. In Article 10,
A method for manufacturing a cold-rolled martensitic steel sheet having a Tsoak of Ac3+10℃ to Ac3+100℃. In Article 10,
A method for manufacturing a cold-rolled martensitic steel sheet having a CR1 of 30°C/s to 120°C/s. In Article 10,
A method for manufacturing a cold-rolled martensitic steel sheet having a T1 temperature of 640°C to 725°C. In Article 10,
A method for manufacturing a cold-rolled martensitic steel sheet having a CR2 of at least 100°C/s. In Article 10,
A method for manufacturing a cold-rolled martensitic steel sheet having a T2 temperature of Ms-50℃ to 20℃. In Article 10,
A method for manufacturing a cold-rolled martensitic steel sheet having a Ttemper of 200°C to 300°C. In any one of claims 1 to 6,
The above steel sheet is a cold-rolled martensitic steel sheet used for manufacturing structural parts of vehicles.