JP7422143B2 - Cold rolled coated steel sheet and its manufacturing method - Google Patents

Description

The present invention relates to a cold rolled coated steel sheet suitable for use as an automotive steel sheet.

Automotive parts are required to satisfy two contradictory requirements: ease of molding and strength, but in recent years, from the perspective of consideration for the global environment, automobiles have been given a third requirement: improved fuel efficiency. is also given. Thus, automotive parts now need to be made from materials with high formability in order to meet criteria for ease of installation into complex car assemblies, while at the same time reducing the weight of the car. There is a need to improve the strength of automobiles for crashworthiness and durability while improving fuel efficiency.

Therefore, intense research and development efforts are underway to reduce the amount of materials used in automobiles by increasing the strength of the materials. Conversely, an increase in the strength of a steel sheet reduces its formability, so it is necessary to develop a material that has both high strength and high formability.

Previous research and development in the field of high strength and high formability steel sheets has led to several methods for producing high strength and high formability steel sheets, some of which were finally understood by the present invention. Listed herein to do so.

US20140234657 is a patent application that claims a hot-dip galvanized steel sheet having a microstructure containing one or two of martensite and bainite in a total volume fraction of 20% to 99%, and the remaining structure is ferrite. and one or two of retained austenite in a volume fraction of less than 8%, and pearlite in a volume fraction of 10% or less. Further, US20140234657 reaches a tensile strength of 980 MPa, but cannot reach an elongation of 25%.

No. 8,657,969 claims a high strength galvanized steel sheet with a tensile strength of more than 590 MPa and excellent workability. The component composition is mass%: C: 0.05 to 0.3%, Si: 0.7 to 2.7%, Mn: 0.5 to 2.8%, P: 0.1% or less, S : 0.01% or less, Al: 0.1% or less, and N: 0.008% or less, and the remainder is Fe or unavoidable impurities. The microstructure includes, in terms of area ratio, ferrite phase: 30% to 90%, bainite phase: 3% to 30%, and martensite phase: 5% to 40%. The site phase is present at a ratio of 30% or more.

US Patent Application Publication No. 2014/0234657 US Patent No. 8,657,969

The aim of the present invention is to solve these problems by making available a cold-rolled steel and a coated plate that at the same time has:

- ultimate tensile strength of more than 600 MPa, preferably more than 620 MPa,
- a total elongation of more than 31%, preferably more than 33%.

In a preferred embodiment, the steel plate of the present invention can also exhibit a yield strength of 320 MPa or more.

In preferred embodiments, the steel sheets of the invention may also exhibit a yield strength to tensile strength ratio of 0.6 or more.

Preferably, such steels may also have good suitability for forming, especially rolling, with good weldability and coatability.

Another object of the invention is also to make available a method for manufacturing these plates that is robust to changes in manufacturing parameters, while being compatible with conventional industrial applications.

The cold rolled and heat treated steel sheets of the present invention can optionally be coated with zinc or zinc alloys, or aluminum or aluminum alloys to improve its corrosion resistance.

Carbon is present in the steel between 0.13% and 0.18%. Carbon is a necessary element to increase the strength of steel sheets by generating low-temperature transformation phases such as bainite, and carbon also plays a vital role in stabilizing austenite, thus ensuring retained austenite. It is a necessary element. Carbon therefore plays two very important roles. One role is in increasing strength and the other is in retaining austenite and imparting ductility. However, carbon contents below 0.13% do not stabilize the austenite in adequate amounts required for the steels of the present invention. On the other hand, when the carbon content exceeds 0.18%, the steel exhibits poor spot weldability, limiting its application to automotive parts.

The manganese content of the steel of the invention is between 1.1% and 1.8%. This element is gammageneus. The purpose of adding manganese is to obtain a structure that essentially contains austenite and to impart strength to the steel. An amount of manganese of at least 1.1% by weight has been found to stabilize the austenite and provide strength and hardenability of the steel sheet. However, if the manganese content exceeds 1.8%, harmful effects such as retardation of the transformation from austenite to bainite occur during over-aging retention for bainite transformation. In addition, manganese content greater than 1.8% also reduces ductility and deteriorates the weldability of the steel, so elongation goals may not be achieved. The preferred content according to the invention can be kept between 1.2% and 1.8%, even more preferably between 1.3% and 1.7%.

The silicon content of the steel of the invention is between 0.5 and 0.9%. The presence of silicon stabilizes carbon-rich austenite at room temperature, since silicon is a component that can retard the precipitation of carbides during overaging. Furthermore, due to the low solubility of silicon in carbides, silicon effectively inhibits or retards the formation of carbides and thus the formation of the bainitic structure which is also required according to the invention to impart its essential characteristics to the steel. promote. However, an unbalanced content of silicon does not produce the above effects and leads to problems such as temper embrittlement. Therefore, its concentration is controlled within the upper limit of 0.9%. The preferred content for the present invention can be kept between 0.6% and 0.8%.

Aluminum is an essential element and is present in steel between 0.6% and 1%. Aluminum is alphagenic and imparts total elongation to the steel of the present invention. A minimum of 0.6% aluminum is required to have minimal ferrite and thereby impart elongation to the steel of the invention. Aluminum is also used to clean the steel of the present invention, to remove oxygen from the steel in the molten state, and also to prevent oxygen from forming a gas phase. However, the preferred range for the presence of aluminum is between 0.6% and 0.8%, since if it exceeds 1% it will form AlN which is detrimental to the steel of the invention.

The phosphorus composition of the steel of the invention is between 0.002% and 0.02%. In particular, phosphorus reduces spot weldability and hot ductility because it tends to segregate at grain boundaries or co-segregate with manganese. For these reasons, its content is limited to 0.02%, preferably lower than 0.014%.

Although sulfur is not an essential element, it may be present as an impurity in steel. From the viewpoint of the present invention, it is preferable to keep the sulfur content as low as possible, but from the viewpoint of manufacturing cost, it is 0.003% or less. Furthermore, if higher sulfur is present in the steel, the sulfur will combine, especially with manganese, to form sulfides, reducing its beneficial effect on the steel of the invention.

Nitrogen is limited to 0.007% to prevent aging of the material and also to minimize the precipitation of nitrides during solidification that are detrimental to the mechanical properties of the steel.

Chromium is an optional element of the present invention. The chromium content may be present in the steel of the invention between 0.05% and 1%. Chromium is an essential element that gives strength and hardening to steel, but when used in excess of 1% it impairs the surface finish of the steel. Furthermore, a chromium content of less than 1% coarsens the dispersion pattern of carbides in the bainite structure, thus keeping the density of carbides in the bainite low.

Molybdenum is an optional element that constitutes 0.001% to 0.5% of the steel of the present invention. Molybdenum plays an effective role in determining hardenability and hardness, delays the appearance of bainite, and avoids the precipitation of carbides in bainite. However, since the addition of molybdenum excessively increases the cost of adding alloying elements, its content is limited to 0.5% for economic reasons.

Niobium is an optional element in the present invention. The niobium content can be present in the steel of the invention between 0.001 and 0.1% and is added to the steel of the invention to form carbonitrides and harden the steel of the invention by precipitation hardening. Gives the strength of steel. Niobium also affects the size of microstructural components through its precipitation as carbonitrides and by retarding recrystallization during heat treatment. Thus, at the end of the holding temperature and after completion of the annealing leading to hardening of the steel of the invention, a finer microstructure is formed. However, niobium contents above 0.1% are not economically interesting since saturation effects of their influence are observed. This means that the additional amount of niobium does not result in any strength improvement of the product.

Titanium is an optional element and can be added to the steel of the present invention in amounts between 0.001% and 0.1%. Like niobium, titanium participates in carbonitride formation and therefore plays a role in the quenching of the steel of the present invention. In addition, titanium also forms titanium nitrides, which appear during solidification of cast products. The amount of titanium is limited to 0.1% to avoid the formation of coarse titanium nitride, which is detrimental to formability. When the titanium content is less than 0.001%, it has no effect on the steel of the present invention.

Copper may be added as an optional element in an amount of 0.01% to 2% to increase the strength of the steel and improve its corrosion resistance. A minimum of 0.01% copper is required to achieve this effect. However, if its content exceeds 2%, the surface morphology may deteriorate.

Nickel can be added as an optional element in amounts of 0.01-3% to increase the strength of the steel and improve its toughness. A minimum of 0.01% is required to produce such an effect. However, when its content exceeds 3%, nickel causes a decrease in ductility.

The calcium content in the steel of the invention is between 0.0001% and 0.005%. Calcium is added as an optional element to the steel of the invention, especially during inclusion treatment. Calcium becomes spherical and captures harmful sulfur components, suppressing the harmful effects of sulfur and contributing to the refining of steel.

Vanadium is effective in forming carbides or carbonitrides to increase the strength of steel, and for economic reasons the upper limit is 0.1%. Other elements such as cerium, boron, magnesium or zirconium can be added individually or in combination in the following weight ratios: That is, cerium≦0.1%, boron≦0.003%, magnesium≦0.010%, and zirconium≦0.010%. Up to the maximum content levels indicated, these elements make it possible to refine the grains during solidification. The remainder of the steel composition consists of unavoidable impurities resulting from the steel and processing.

The microstructure of the steel plate includes the following:

Ferrite constitutes, in area fraction, 60% to 75% of the microstructure for the steels of the invention. Ferrite constitutes the main phase of steel as a matrix. In the present invention, the ferrite cumulatively includes polygonal ferrite and acicular ferrite, which imparts elongation and high strength to the steel of the present invention. In order to ensure an elongation of 31% or more, preferably 33% or more, it is necessary to have 60% ferrite. In the steel of the present invention, ferrite is formed during cooling after annealing. However, whenever a ferrite content of more than 75% is present in the steel of the invention, said strength is not achieved.

Bainite constitutes 20% to 30% of the microstructure for the steels of the invention in terms of area fraction. In the present invention, bainite cumulatively consists of lath bainite and granular bainite. In order to ensure a tensile strength of 620 MPa or more, preferably 630 MPa or more, it is necessary to have 20% bainite. Bainite is formed during overaging.

Retained austenite constitutes 10% to 15% of the steel in terms of area fraction. Retained austenite is known to have a higher solubility of carbon than bainite and thus acts as an effective carbon trap, thus retarding the formation of carbides in bainite. The proportion of carbon within the retained austenite of the present invention is preferably higher than 0.9%, preferably lower than 1.1%. The retained austenite in the steel according to the invention increases its ductility.

Martensite constitutes between 0% and 5% of the microstructure in area fraction and is found in trace amounts. The martensite of the present invention includes both fresh martensite and tempered martensite. In the present invention, martensite is formed by cooling after annealing, and is tempered during over-aging. Fresh martensite is also generated during cooling of cold-rolled steel sheets after coating. At less than 5% martensite, martensite imparts ductility and strength to the steel of the invention. More than 5% martensite provides excessive strength but reduces elongation beyond acceptable limits. Preferred limits for martensite are between 0% and 3%.

The total amount of ferrite and retained austenite must always be between 70% and 80% to have a total elongation of 31%, ensuring a total elongation of more than 31% while having a tensile strength of 600 MPa In order to achieve this, a minimum of 70% is required. Ferrite and retained austenite are soft phases compared to martensite and bainite, thus imparting elongation and ductility, but whenever their cumulative presence exceeds 80%, strength decreases beyond acceptable limits.

In addition to the microstructures mentioned above, the microstructure of cold-rolled and heat-treated steel sheets is free of microstructural components such as pearlite and cementite, without impairing the mechanical properties of the steel sheets.

The steel plate of the present invention can be produced by any suitable method. A preferred method consists of providing a semi-finished casting of steel having a chemical composition according to the invention. Casting can be carried out continuously in ingots or in the form of thin slabs or thin strips. ie, with thicknesses ranging from about 220 mm for slabs to several tens of mm for thin strips.

For example, a slab with the above chemical composition is produced by continuous casting, where the slab is cast in order to avoid center segregation and to keep the local carbon to nominal carbon ratio below 1.10. During the continuous casting process, it was optionally subjected to direct light reduction casting. The slabs provided by the continuous casting process can be used directly at elevated temperatures after continuous casting or can be first cooled to room temperature and then reheated for hot rolling.

The temperature of the slab subjected to hot rolling must be at least 1150°C and less than 1280°C. If the temperature of the slab is lower than 1150°C, an excessive load is applied to the rolling mill. For this reason, the temperature of the slab is adjusted such that hot rolling can be completed in the temperature range between Ac1+50°C and Ac1+250°C, preferably between Ac1+50°C and Ac1+200°C, while the final rolling temperature always remains above Ac1+50°C. Preferably, it is sufficiently high. Reheating at temperatures above 1280° C. is industrially expensive and must be avoided.

A final rolling temperature range between Ac1+50°C and Ac1+250°C is preferred to have a structure favorable to recrystallization and rolling. Below this temperature, the steel plate exhibits a significant decrease in rollability, so the final rolling pass must be performed at a temperature higher than Ac1+50°C. The plate obtained in this way is then cooled at a cooling rate of more than 30° C./sec to the coiling temperature, which must be below 625° C. Preferably, the cooling rate is 200° C./second or less.

The hot rolled steel sheet is then coiled at a coiling temperature below 625°C to avoid ovalization, preferably below 600°C to avoid scale formation. The preferred range of such winding temperature is between 350°C and 600°C. The wound hot rolled steel sheet may be cooled to room temperature before being subjected to any hot zone annealing.

The hot rolled steel sheet may be subjected to an optional descaling step to remove scale formed during hot rolling prior to any hot zone annealing. The hot rolled plate may then be subjected to an optional hot zone annealing at a temperature between 400° C. and 750° C. for at least 12 hours and no more than 96 hours, where the temperature The temperature remains below 750° C. in order not to partially alter the structure and thus lose the homogeneity of the microstructure. An optional descaling step of this hot rolled steel plate can then be carried out, for example by pickling such a plate. This hot-rolled steel plate is subjected to cold rolling to obtain a cold-rolled steel plate with a rolling reduction of between 35% and 90%. The cold rolled steel sheet obtained from the cold rolling process is then annealed to impart microstructure and mechanical properties to the steel of the invention.

Ac1 and Ac3 of this steel, which is a cold rolled steel plate subjected to two steps of heating to reach a soaking temperature between Ac1+30°C and Ac3 during annealing, are calculated by the following formula.

Ac1=723-10.7[Mn]-16[Ni]+29.1[Si]+16.9[Cr]+6.38[W]+290[As]
Ac3=910-203[C]^(1/2)-15.2[Ni]+44.7[Si]+104[V]+31.5[Mo]+13.1[W]-30[Mn]-11 [Cr]-20[Cu]+700[P]+400[Al]+120[As]+400[Ti]
In the formula, the elemental content is expressed in weight percent.

In step 1, the cold rolled steel plate is heated to a temperature range between 550 and 650°C at a heating rate between 10°C/sec and 40°C/sec. Thereafter, in the subsequent second heating step, the cold rolled steel plate is heated to the soaking temperature for annealing at a heating rate of between 1° C./sec and 5° C./sec.

The cold rolled steel sheet is then held at a soaking temperature for 10 to 500 seconds to ensure at least 30% transformation of the initial hard work hardened structure to an austenitic microstructure. Next, the cold rolled steel plate is cooled to the overaging holding temperature by two-step cooling. In step 1 cooling, the cold rolled steel sheet is cooled in a temperature range between 600°C and 720°C, preferably between 625°C and 720°C, with a cooling rate of less than 5°C/s, preferably less than 3°C/s. cooled down to During this step 1 cooling, the ferrite matrix of the present invention is formed. Thereafter, in a subsequent second cooling step, the cold rolled steel sheet is cooled to an overaging temperature range of between 250° C. and 470° C. at a cooling rate between 10° C./sec and 100° C./sec. The cold rolled steel plate is then held in the overaging temperature range for 5 to 500 seconds. Next, the cold rolled steel sheet is brought to a temperature in the coating bath temperature range of 400° C. to 480° C. to facilitate coating of the cold rolled steel sheet. The cold rolled steel sheet is then coated by any of the known industrial methods such as electrogalvanizing, JVD, PVD, hot dip galvanizing (GI).

The following tests, examples, figurative illustrations and tables presented herein are not limiting in nature and should be considered for illustrative purposes only, and indicate advantageous features of the invention.

Steel plates made of steels with different compositions are summarized in Table 1, and each steel plate is manufactured according to the process parameters specified in Table 2. Thereafter, Table 3 summarizes the microstructures of the steel sheets obtained during the test, and Table 4 summarizes the evaluation results of the properties obtained.

<Table 2>
Table 2 summarizes the annealing process parameters performed on the steels in Table 1. Steel compositions A and B are useful for producing plates according to the invention. The table also specifies the reference steels designated C and D in the table. Further, Table 2 shows a list of Ac1 and Ac3. These Ac1 and Ac3 are defined as follows for the steel of the present invention and the reference steel.

Ac1=723-10.7[Mn]-16[Ni]+29.1[Si]+16.9[Cr]+6.38[W]+290[As]
Ac3=910-203[C]^(1/2)-15.2[Ni]+44.7[Si]+104[V]+31.5[Mo]+13.1[W]-30[Mn]-11 [Cr]-20[Cu]+700[P]+400[Al]+120[As]+400[Ti]
In the formula, the elemental content is expressed in weight percent.

All plates were cooled after hot rolling at a cooling rate of 34°C/sec and finally brought to a temperature of 460°C before coating. All plates have a cold reduction of 65%.

Table 2 is as follows.

<Table 3>
Table 3 illustrates the results of tests carried out according to standards on different microscopes, such as scanning electron microscopes, for determining the microstructure of both the inventive steel and the reference steel.

Specify the results as follows.

<Table 4>
Table 4 illustrates the mechanical properties of both the inventive steel and the reference steel. In order to determine tensile strength, yield strength, and total elongation, a tensile test is conducted according to the JIS Z2241 standard.

The results of various mechanical tests carried out according to said standards are summarized.

Claims (19)

Cold-rolled steel sheet containing the following elements expressed in weight percent: 0.13%≦carbon≦0.18%
1.1%≦manganese≦1.8%
0.5%≦Silicon≦0.9%
0.6%≦aluminum≦1%
0.002%≦phosphorus≦0.02%
0%≦sulfur≦0.003%
0%≦nitrogen≦0.007%
containing one or more of the following arbitrary elements, i.e. 0.05%≦Chromium≦1%
0.001%≦Molybdenum≦0.5%
0.001%≦niobium≦0.1%
0.001%≦Titanium≦0.1%
0.01%≦Copper≦2%
0.01%≦nickel≦3%
0.0001%≦Calcium≦0.005%
0%≦vanadium≦0.1%
0%≦Boron≦0.003%
0%≦Cerium≦0.1%
0%≦Magnesium≦0.010%
0%≦zirconium≦0.010%
The remainder of the composition is composed of iron and unavoidable impurities resulting from processing, and the microstructure of the steel sheet has an area fraction of 60 to 70 % ferrite, 20 to 70% ferrite. A steel plate comprising 30% bainite, 10-15% retained austenite, and 0-5% martensite, with a cumulative amount of retained austenite and ferrite between 70% and 80%. The cold rolled steel sheet of claim 1, wherein the composition includes 0.6% to 0.8% silicon. The cold rolled steel sheet according to claim 1 or 2, wherein the composition includes 0.14% to 0.18% carbon. Cold rolled steel sheet according to claim 3, wherein the composition comprises 0.6% to 0.8% aluminum. Cold rolled steel sheet according to any one of claims 1 to 4, wherein the composition comprises 1.2% to 1.8% manganese. The cold rolled steel sheet according to claim 5, wherein the composition includes 1.3% to 1.7% manganese. Cold rolled steel sheet according to any one of claims 1 to 6, wherein the cumulative amount of ferrite and retained austenite is between 73% and 80%, and the proportion of retained austenite is less than 13%. Cold rolled steel sheet according to any one of claims 1 to 7, wherein the amount of martensite is between 0% and 3%. Cold rolled steel sheet according to any one of claims 1 to 8, wherein the carbon content of the retained austenite is between 0.9 and 1.1%. The cold rolled steel plate according to any one of claims 1 to 9, wherein the steel plate has an ultimate tensile strength of 600 MPa or more and a total elongation of 31% or more. The cold rolled steel sheet according to claim 10, wherein the steel sheet has a yield strength of 320 MPa or more and a total elongation of 33% or more. The cold rolled steel plate according to any one of claims 1 to 11, wherein the steel plate is coated. A method for manufacturing a cold rolled steel plate, the method comprising:
Sequential steps of - providing a steel composition according to any one of claims 1 to 6;
- reheating the semi-finished product to a temperature between 1150°C and 1280°C;
- rolling the semi-finished product in the austenitic range such that the hot rolling finishing temperature is between Ac1+50°C and Ac1+250°C to obtain a hot rolled steel sheet;
- cooling the plate to a coiling temperature of less than 625°C at a cooling rate of greater than 30°C/sec and coiling the hot rolled plate;
- cooling the hot rolled sheet to room temperature;
- optionally carrying out a descaling treatment on the hot rolled steel sheet;
- optionally carrying out annealing of the hot rolled steel sheet at a temperature between 400°C and 750°C;
- optionally carrying out a descaling treatment on the hot rolled steel sheet;
- cold rolling the hot rolled steel sheet at a reduction rate between 35 and 90% to obtain a cold rolled steel sheet;
- Next, heating the cold rolled steel plate by two-stage heating to perform annealing at a soaking temperature between Ac1 + 30 ° C. and Ac3 for 10 to 500 seconds, where - In heating step 1, heating the cold rolled steel sheet to a temperature range between 550 and 650°C at a heating rate between 10°C/sec and 40°C/sec;
- Then, in heating step 2, the cold rolled steel plate is heated at a heating rate between 1°C/sec and 5°C/sec from a temperature range between 550 and 650°C to an annealing soaking temperature at which the steel plate is maintained. step,
- then cooling said cold-rolled steel sheet in a two-stage cooling, wherein: in cooling step 1, said cold-rolled steel sheet is cooled at a temperature range between 600°C and 720°C at a cooling rate of less than 5°C/sec; a step of cooling until
- Then, in a cooling step 2, cooling the plate from a temperature range between 600°C and 720°C to an overaging temperature at a cooling rate between 10 and 100°C/sec;
- then overaging the cold rolled steel sheet at a temperature range between 250 and 470° C. for 5 to 500 seconds;
- Then cooling to room temperature to obtain a cold-rolled steel sheet, and the microstructure of the obtained cold-rolled steel sheet has an area fraction of 60-70 % ferrite, 20-30% bainite, 10% ~15% retained austenite and 0-5% martensite, with a cumulative amount of retained austenite and ferrite between 70% and 80%;
Method. 14. The method of claim 13, wherein the winding temperature is less than 600<0>C. The method according to claim 13 or 14, wherein the finish rolling temperature is between Ac1+50°C and Ac1+200°C. The method according to any one of claims 13 to 15, wherein in cooling step 1, the cold rolled steel sheet is cooled to a temperature range of 625-720°C at a cooling rate of less than 3°C/sec. Any of claims 13 to 16, wherein the cold rolled steel sheet is annealed between Ac1+30°C and Ac3, the annealing temperature being selected to ensure that at least 30% austenite is present at the end of soaking. The method for producing a cold rolled steel plate according to item 1. The method for producing a cold rolled steel sheet according to any one of claims 13 to 17, wherein the cold rolled steel sheet is coated at a temperature in a temperature range of 400°C to 480°C. Use of a steel plate according to any one of claims 1 to 12 or a steel plate produced by the method according to any one of claims 13 to 18 for the production of structural parts or safety parts of vehicles.