JPH10121192A - Steel sheet excellent in formability and hardenability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a soft steel sheet and a high tensile strength steel sheet, combining press formability capable of meeting the recent demand for complicated shape with high strength-increasing capacity by means of heating by high energy density beam such as laser beam and rapid cooling. SOLUTION: These steel sheets have a composition consisting of, by weight, 0.0005-0.005% C, 0-1% Si, 0.05-2% Mn, <=0.15% P, <=0.01% S, 0.005-0.08% Al, 0.005-0.025% N, 48×(N/14) to 48×[(N/14)+(S/32)]% Ti, 93×(C/12) to [93×(C/12)+0.05]% Nb, 0.0003-0.003% B, 0-3% Cr, and the balance Fe with inevitable impurities and also have a crystalline structure composed of ferrite single phase structure.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel sheet excellent in formability and capable of partially increasing the strength by being rapidly cooled after being melted by a high energy density beam such as a laser beam.

[0002]

2. Description of the Related Art Efforts have been made to increase the strength and rigidity of components (for example, front rails) related to automobile crash safety as part of measures to enhance protection of occupants of automobiles.

However, high-strength steel sheets having a tensile strength of 340 to 440 MPa class have already been used for body structural parts of automobiles. There are many. For this reason, instead of processing from a single material, there are cases where it is supported by methods such as dividing into multiple parts and molding, adding parts and reinforcing, etc. Is pointed out as an issue. It is very useful if only the necessary parts can be strengthened after press molding.

JP-A-6-73438, JP-A-6-73440, JP-A-6-73441, JP-A-6-73442 and JP-A-6-73443 disclose 0.02 to 0.3% by weight. A steel sheet containing C and a large amount of Si or Mn and having the property of increasing the strength by irradiation with a high-density energy source is disclosed. However, all of these have a high C content in steel and are not sufficiently moldable.

Japanese Patent Application Laid-Open No. 6-73439 discloses that the C content in steel is 0.02% or less, Si, Mn and other elements are added in a large amount, and the high strength property by irradiation with a high density energy source is excellent. Steel sheets are disclosed. However, in this method, for example, to increase the yield stress by 20 MPa or more, 0.002
% Or more of C in steel is required. Furthermore, as can be seen from the examples, in order to increase the yield stress by 50 MPa or more, the C content in the steel must be made 0.005% or more. When C in the steel increases, a large amount of carbides is liable to precipitate, not only deteriorating the ductility of the steel sheet but also hindering the formation of a preferable recrystallization texture.
The value becomes low, and the deep drawability is impaired.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide a press formability capable of coping with a complicated shape and a high strength increasing ability (burning) by heating and rapidly cooling with a high energy density beam such as a laser beam. To provide a steel sheet having both the above-mentioned properties.

[0007]

The gist of the present invention is as follows.
It is in a steel sheet excellent in formability and hardenability.

"C: 0.0005-0.005% by weight, Si
: 0.005 to 1%, Mn: 0.05 to 2%, P: 0.15% or less, S: 0.01% or less, Al: 0.005 to 0.08%, N: 0.00
5 to 0.025%, Ti: 48 × (N / 14) to 48 × ｛(N / 1
4) + (S / 32)｝, Nb: 93 × (C / 12) ~ ｛93 ×
(C / 12) + 0.05 °, B: 0.0003-0.003%, Cr: 0
A steel sheet containing up to 3%, the balance being Fe and inevitable impurities, and having a crystal structure of a ferrite single phase structure and excellent in formability and hardenability. In order to enhance the ductility and the deep drawability represented by the r value, an extremely low C IF steel (Interstitial Free steel) in which C and N in the steel are completely precipitated as carbides and nitrides is used. The steel is hot-rolled, and further cold-rolled and annealed to form a recrystallized texture favorable for deep drawability. In a normal quenching operation, a steel sheet is heated to an austenite region and then rapidly cooled to make a crystal structure of the steel sheet a quenched structure. In this case, the hardness of the quenched structure is proportional to the amount of C in the steel sheet in supersaturation. The IF steel with extremely low C has insufficient hardenability due to the low C content. Since carbides re-dissolve in the austenite region, it is possible to increase the amount of carbides in the IF steel to provide hardenability. Is easily damaged.

By irradiating a steel sheet with a beam (eg, a laser beam) having a high energy density, the steel sheet can be heated to a much higher temperature than ordinary quenching heat treatment. According to this method,
It is also possible to make the steel sheet partially molten. In the present invention, the steel is partially heated to a high temperature to dissociate nitrides in the steel, and this is rapidly cooled to impart hardenability to the IF steel. The idea is summarized below and in the section.

Improvement of hardenability: Nitride in steel is dissociated in a molten portion generated by laser beam irradiation, and N that has formed nitride is dissolved in the steel sheet. Subsequently, by rapid cooling, N becomes a state of supersaturated solid solution in the steel sheet. If the N content in steel is increased in advance, the strength of the steel sheet after quenching can be increased.

Ensuring the formability: Even if the N content in the steel is increased, by adjusting the Ti content, the dissolved N can be precipitated as stable TiN at the completion of hot rolling. If this is cold-rolled and annealed, a recrystallized texture favorable for deep drawability is formed. TiN precipitates more coarsely than carbides. For this reason, the local elongation slightly decreases as the TiN precipitation increases, but the uniform elongation hardly deteriorates.

Based on the above-mentioned concept, the invention of a steel sheet having both press formability capable of forming into a complicated shape and strength increasing ability by laser beam irradiation has been completed.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The chemical composition and metal structure of the steel sheet of the present invention will be described below. The percentages described below mean weight%.

(1) Chemical composition of steel sheet C: The smaller the number, the better, because the formability is impaired. But,
With current technology, it is difficult to stably and economically make C less than 0.0005%. In the present invention, Nb is contained, and solute C is precipitated as NbC to render it harmless. When irradiated with a laser beam, NbC dissociates, and the C forms a solid solution in the steel sheet to increase the strength of the steel sheet. However, the C content is 0.005%
If N exceeds 3, the added amount of Nb necessary for fixing C increases, thereby impairing the economy, and the amount of NbC precipitated increases, and the ductility deteriorates. Therefore, the upper limit of the C content is 0.005%. In order to further improve the moldability, the C content is 0.002.
% Is preferable.

Si: Used as a deoxidizing agent for steel. In order to obtain a deoxidizing effect, the content must be 0.005% or more. Since Si also has the effect of increasing the strength of the steel sheet, Si may be contained for the purpose of increasing the strength of the steel. But S
When the i content is increased, formability is deteriorated, and scale is generated on the surface of the hot-rolled steel sheet, thereby impairing the surface appearance. Therefore, the upper limit of the content is 1%.

Mn: It has an effect of forming MnS by combining with S in steel to prevent hot embrittlement. In order to secure this effect, the content is made 0.05% or more. Mn also has the effect of increasing the strength and hardenability of the steel. However, if Mn is excessively contained, the ductility and deep drawability of the steel sheet are impaired, so the upper limit is made 2%. It is preferably at most 1.2%. From the viewpoint of ensuring deep drawability, Mn is preferably added in combination with P or Si.

P: P is an element inevitably contained, but has an effect of increasing the strength of the steel sheet, and is added as necessary. When P is used for this purpose, it is preferable to contain 0.02% or more. However, if it is excessively contained, the steel sheet becomes brittle, and even when B is added, the secondary working brittleness cannot be suppressed. Therefore, the content of P is set to 0.15% or less.

S: It is an unavoidable impurity. When the S content increases, nonmetallic inclusions increase and the formability is impaired. Therefore, the upper limit is set to 0.01%.

Al: Used as a deoxidizing agent for steel. If the content is less than 0.005%, the effect is not obtained, and if it exceeds 0.08%, the effect as a deoxidizing agent is saturated and the economy is impaired, so the upper limit is set.

N: One of the important constituent factors of the present invention, N is used to enhance hardenability. Since a necessary amount of Ti is added to the steel sheet, N in the steel sheet is fixed as TiN. However, TiN is dissociated in a portion heated to a high temperature by irradiation with a laser beam or the like. By being rapidly quenched, the dissociated N forms a supersaturated solid solution in the steel sheet and increases the strength of the steel sheet. If the N content is less than 0.005%, sufficient strength cannot be secured. Also, if it exceeds 0.025%, Ti
The deposition amount of N 2 increases, the ductility is deteriorated, and a defect called a blowhole is generated in a molten portion caused by laser beam irradiation. Therefore, the range of the N content is 0.005%
To 0.025%.

Ti: One of the important constituent factors of the present invention, N in the steel sheet is fixed as TiN to improve the formability. To fix N, Ti is set to 48 × (N / 14)%
It is necessary to contain the above. However, if Ti is excessively contained, N dissociated by laser beam irradiation tends to be reprecipitated as TiN, and the strength cannot be secured. Therefore, the upper limit of the Ti content is 48 ×, which is the range in which TiN is formed and a small amount of TiS is formed.
{(N / 14) + (S / 32)}% or less. Here, the symbol of the element indicates the content (% by weight) of each element.

Nb: C in the steel plate is fixed as NbC. Therefore, the lower limit is set to 93 × (C / 12)%. Excessive addition suppresses recrystallization and impairs moldability. Therefore, the upper limit is set to 93 × (C / 12 + 0.05)%.

B: has the effect of securing hardenability and suppressing embrittlement of secondary working by P. If the amount is less than 0.0003%, these effects are not obtained, and if the amount exceeds 0.003%, these effects are saturated. Therefore, the range is 0.0003% to 0.00
3%.

Cr: Since it has the effect of increasing the strength of the steel sheet and improving the hardenability, Cr is added as necessary. In order to exhibit these effects, it is preferable to contain 0.1% or more. However, if it is contained excessively, the deep drawability of the steel sheet is impaired. Therefore, even when Cr is contained, the upper limit is 3%.
And

Crystal structure of steel sheet: Using IF steel of extremely low C,
Even if a recrystallization texture favorable for deep drawability is formed by cold rolling and annealing, deep drawability may be impaired if a hard second phase such as martensite is present in the crystal structure. It is presumed that this is because the stress state in a microscopic range changes during press forming, and a deformation behavior different from the case of the ferrite structure occurs. In order to avoid this and secure deep drawability, the crystal structure of the steel sheet of the present invention is a structure consisting of only ferrite.

The steel sheet of the present invention is produced by melting a steel having the above-mentioned chemical composition and hot rolling the steel in a conventional manner. When a cold-rolled steel sheet is required, the above-mentioned hot-rolled steel sheet is used as a raw material, through pickling, cold rolling, annealing, etc.,
Further, if necessary, temper rolling is performed to manufacture.

The steel sheet of the present invention may be used as a thick or thin hot-rolled steel sheet or a cold-rolled steel sheet, depending on the application. A hot-rolled steel sheet has a lower r-value than a cold-rolled steel sheet, but has extremely low carbon and therefore has good ductility and excellent economic efficiency. If necessary, these steel sheets may be used as a base material, and the surface thereof may be subjected to surface treatment such as Zn-based or Al-based hot-dip plating or electroplating.

In order to ensure the deep drawability of the cold-rolled steel sheet, after hot rolling, the steel sheet is rolled at a temperature of 700 ° C. or less so that the crystal grains are not coarsened, and at a rolling reduction of 50% or more. Cold rolling is preferred. The recrystallization annealing may be performed by any method such as box annealing, continuous annealing, and continuous galvanizing equipment.

[0029]

【Example】

[Example 1] Steel ingots having the chemical compositions shown in Table 1 were produced using an experimental vacuum melting furnace.

[0030]

[Table 1]

These ingots are hot forged to a thickness of 25 mm.
Then, it was heated in an electric furnace at 1250 ° C. for 1 hour, and rolled to a thickness of 8 mm by a three-pass rolling in a temperature range of 1150 ° C. to 930 ° C. in a hot rolling mill. Immediately after hot rolling, cool to 500 ° C by forced air cooling or water spray cooling.
After holding for 1 hour in an electric furnace maintained at 500 ° C,
The furnace was cooled at the speed of the time.

Next, each of the front and back surfaces was ground by 1 mm to a thickness of 6 mm, and was cold-rolled to obtain a cold-rolled plate having a thickness of 1.4 mm. The cold rolled sheet was heated to 850 ° C. at a heating rate of 10 ° C./sec by an infrared heating furnace, held for 40 seconds, and then cooled to room temperature. Cooling rate is 5 ℃ / sec up to 700 ℃, 50 ℃ / sec between 700 ℃ and 400 ℃, 400 ℃
The process was performed at 20 ° C / min between 350 ° C and 350 ° C, and at 10 ° C / sec between 350 ° C and room temperature. After cooling, temper rolling at an elongation of 1% was performed to obtain a cold-rolled steel sheet. A JIS No. 5 tensile test piece was sampled from this cold-rolled steel sheet and subjected to a tensile test.

Further, the cold-rolled steel sheet subjected to temper rolling was irradiated with a laser beam in parallel with the rolling direction. The laser beam irradiation was performed using a carbon dioxide gas laser generator having an output of 3 kW, and three beams were irradiated in the width direction of the steel sheet at intervals of 5 mm. The focal point was set at the center of the plate thickness, and scanning was performed at a speed of 3 m / min so that the molten portion slightly reached the back surface of the steel plate. From the position where the irradiation mark at the center of the beam is along the center line of the test piece, JIS
A No. 5 tensile test piece was collected and subjected to a tensile test.

Table 2 shows the results of the tensile test of the steel sheet after the temper rolling and the results of measuring the increase in the yield stress and the tensile stress of the steel sheet due to the irradiation of the laser beam.

[0035]

[Table 2]

FIG. 1 is a graph showing the effect of the N content of the steel sheet on the increase in the yield stress and the tensile stress (denoted by ΔYS and ΔTS, respectively) of the steel sheet by laser beam irradiation, obtained in the present embodiment. is there. As shown in Table 2 or FIG. 1, the steel sheet of the present invention has a yield strength increased by 100 MPa or more and a tensile strength increased by 50 MPa or more by laser beam irradiation.

Further, as shown in Table 2, the steel sheet of the present invention is excellent in ductility and r-value despite high hardenability and high tensile stress.

[0038]

The steel sheet of the present invention has both excellent press formability for forming a complicated shape and excellent strength increasing ability by laser beam irradiation. For this reason, it is useful as a material for improving fuel efficiency and collision safety of automobiles, and greatly contributes to improvement of global environmental problems.

[Brief description of the drawings]

FIG. 1 is a diagram showing the effect of the N content of a steel sheet on the amounts of increase in yield stress and tensile stress (denoted by ΔYS and ΔTS, respectively) of the steel sheet due to laser beam irradiation.

Claims (1)

[Claims] (1) C: 0.0005 to 0.005% by weight, Si:
0.005 to 1%, Mn: 0.05 to 2%, P: 0.15% or less,
S: 0.01% or less, Al: 0.005 to 0.08%, N: 0.005 to
0.025%, Ti: 48 × (N / 14) to 48 × ｛(N / 14) +
(S / 32)｝, Nb: 93 × (C / 12)-｛93 × (C / 1
2) + 0.05%, B: 0.0003-0.003%, Cr: 0-3%
A steel sheet comprising Fe and unavoidable impurities and having a crystal structure of a ferrite single phase structure and having excellent formability and hardenability.