JPH09263885A - High strength hot rolled steel plate excellent in workability and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a high strength hot rolled steel plate of (50 to 60)kgf/mm<2> class tensile strength, which gas respective characteristics of conventional precipitation strengthened steel and structure strengthened steel and in which the defects of the known techniques for these steels are advantageously removed. SOLUTION: This steel plate has a composition consisting of, by weight, <=0.10% C, 0.5-2.5% Si, 0.5-1.3% Mn, <=0.05% P, <=0.02% S, 0.01-0.1% Al, 0.02-0.10% Ti, and the balance essentially Fe and satisfying C(%)>=0.03+Ti/4. Further, the steel plate has a structure composed of dual-phase structure consisting of ferrite and martensite, on which Ti carbide is precipitated.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a high-strength hot-rolled steel sheet having a tensile strength of 50 to 60 kgf / mm ² class, which is suitable for use as an inner plate of automobiles, underbody parts and strength members, and the like. The present invention relates to a manufacturing method.

[0002]

2. Description of the Related Art Conventionally, high strength steel plates have been widely used as steel plates for automobile inner plates, suspension parts, strength members and the like from the viewpoint of reducing the weight of the vehicle body. Such high-strength steel sheets for automobiles are required to have a certain level of strength to ensure safety, and at the same time have good formability represented by press working. Conditions such as good fatigue resistance of the are required. For steel sheets for automobiles,
In addition to the above characteristics, in order to reduce the cost of the steel sheet used, the use ratio of the hot-rolled steel sheet is gradually increasing even for the materials that have been mostly cold-rolled steel sheets in the past. Against the background described above, the tensile strength (TS) of materials that used to be ordinary steel sheets is 50 to 60 kgf.
/ mm ² grade tends to be used.

By the way, as a conventional strengthening method for a steel sheet, solid solution strengthening and precipitation strengthening are considered, but these methods have not yet been able to obtain good formability like ordinary steel sheets. That is, the solid solution strengthened steel is inferior to other strengthened steels in stretch flangeability represented by hole expandability. Further, although precipitation-strengthened steel is advantageous in terms of weldability and stable manufacturability, there is a problem that springback after press forming is very large because of poor elongation properties and high yield ratio.

On the other hand, there is a series of so-called structural strengthened steels. This type of steel has the advantage that it is easy to achieve both high strength and low yield ratio, and it is also called Dual Phase steel, which is a ferrite / martensite two-phase mixed structure steel (for example, Japanese Patent Publication No. 61-15128). In the case of the reference), there is an advantage that the elongation property and the fatigue resistance property are remarkably superior to those of precipitation strengthened steel. However, such a structure-reinforced steel has a large strength difference between the parent phase and the second phase, and deformation due to working mainly occurs in the parent phase, which is a soft phase. Is inferior to precipitation strengthened steel.

[0005]

As described above, there has been no high-strength hot-rolled steel sheet having various working characteristics, especially cold working characteristics balanced at a high level, and its development has been desired. . The present invention was developed in view of the above circumstances, while having the respective characteristics of conventional precipitation-strengthened steels and microstructure-strengthened steels, yet advantageously eliminating the drawbacks of these known techniques, the tensile strength is 50 ~ The purpose is to propose a 60 kgf / mm ² grade high strength hot rolled steel sheet together with its advantageous manufacturing method.

[0006]

[Means for Solving the Problems] As a result of intensive studies to achieve the above-mentioned object, the inventors have found that C is appropriate in relation to Ti and Nb based on conventional precipitation-strengthened steel. In addition to adjusting the amount, by adding an appropriate amount of Si and performing hot rolling under optimal conditions, (1) precipitation strengthening occurs simultaneously with the γ → α transformation after rolling. ,
(2) C discharged from the ferrite grains is concentrated in the untransformed austenite grains, and finally the precipitation-strengthened ferrite phase becomes the main component and a composite structure having a martensite phase as the second phase is obtained. , We have found that the intended purpose can be achieved advantageously.

Unlike the conventional Dual Phase steel, the steel of the present invention aims to improve the strength by precipitating and strengthening a soft ferrite phase, while the martensite required for obtaining the same strength is obtained. By lowering the phase ratio than that of the conventional one, the increase in C equivalent is suppressed. As described above, the steel of the present invention maintains the consistency between the hard second phase and the ferrite grains as compared with the conventional precipitation-strengthened steel, and therefore not only excels in the strength-ductility balance but also propagates fatigue cracks. Since the second phase blocks, the fatigue resistance is also improved. In addition, the conventional Dual P
Since the strength difference between the ferrite grains and the second phase is smaller than that of the conventional hase steel, the concentration of local deformation on the ferrite grains is relaxed, resulting in the difficulty of this type of reinforced steel. The local deformability is also improved.

[0008] The present invention is based on the above findings. That is, the gist configuration of the present invention is as follows. 1) C: 0.10 wt% or less, Si: 0.5 to 2.5 wt%, Mn: 0.
5 to 1.3 wt%, P: 0.05 wt% or less, S: 0.02 wt% or less,
Al: 0.01 to 0.1 wt% and Ti: 0.02 to 0.10 wt% are contained in a range satisfying C (%) ≧ 0.03 + Ti (%) / 4, and the balance is substantially Fe composition, and A high-strength hot-rolled steel sheet having excellent workability, characterized in that its structure is composed of a two-phase structure of ferrite in which Ti carbide is precipitated and martensite.

2) C: 0.10 wt% or less, Si: 0.5 to 2.5
wt%, Mn: 0.5 to 1.3 wt%, P: 0.05 wt% or less, S: 0.
02 wt% or less, Al: 0.01 to 0.1 wt% and Nb: 0.01 to 0.
10 wt% in the range satisfying C (%) ≧ 0.03 + Nb (%) / 8, the balance is substantially Fe composition, and the structure is ferrite and martensite in which Nb carbide precipitates. A high-strength hot-rolled steel sheet having excellent workability, characterized by having a two-phase structure with a site.

3) C: 0.10 wt% or less, Si: 0.5 to 2.5
wt%, Mn: 0.5 to 1.3 wt%, P: 0.05 wt% or less, S: 0.
02wt% or less, Al: 0.01-0.1wt%, Ti: 0.02-0.10wt
% And Nb: 0.01 to 0.10 wt% in a range satisfying C (%) ≧ 0.03 + (Ti / 4 + Nb / 8), and the balance is substantially Fe composition, and its structure is A high-strength hot-rolled steel sheet having excellent workability, characterized by having a two-phase structure of ferrite and martensite in which carbides of Ti and Nb are precipitated.

4) C: 0.10 wt% or less, Si: 0.5 to 2.5
wt%, Mn: 0.5 to 1.3 wt%, P: 0.05 wt% or less, S: 0.
02 wt% or less, Al: 0.01 to 0.1 wt% and Ti: 0.02 to 0.
A steel slab containing 10 wt% in a range satisfying C (%) ≥ 0.03 + Ti (%) / 4, the balance being substantially Fe composition, was hot-rolled at a rolling end temperature of 820 ° C or higher. Is completed, and after it is retained in the temperature range of 820 to 720 ℃ for 10 seconds or more, it is cooled at a cooling rate of 10 ℃ / s or more, and then wound at a temperature of 500 to 350 ℃, A method for producing a high-strength hot-rolled steel sheet having excellent workability.

5) C: 0.10 wt% or less, Si: 0.5 to 2.5
wt%, Mn: 0.5 to 1.3 wt%, P: 0.05 wt% or less, S: 0.
02 wt% or less, Al: 0.01 to 0.1 wt% and Nb: 0.01 to 0.
A steel slab containing 10 wt% in a range satisfying C (%) ≥ 0.03 + Nb (%) / 8, the balance being substantially Fe composition, was hot-rolled at a rolling end temperature of 820 ° C or higher. Is completed, and after it is retained in the temperature range of 820 to 720 ℃ for 10 seconds or more, it is cooled at a cooling rate of 10 ℃ / s or more, and then wound at a temperature of 500 to 350 ℃, Manufacturing method of high strength hot rolled steel sheet.

6) C: 0.10 wt% or less, Si: 0.5 to 2.5
wt%, Mn: 0.5 to 1.3 wt%, P: 0.05 wt% or less, S: 0.
02wt% or less, Al: 0.01-0.1wt%, Ti: 0.02-0.10wt
% And Nb: 0.01 to 0.10 wt% in a range satisfying C (%) ≧ 0.03 + (Ti / 4 + Nb / 8), and the balance being substantially Fe composition. Temperature: Finish hot rolling at 820 ℃ or more, and then retain it in the temperature range of 820 to 720 ℃ for 10 seconds or more, then cool it at a cooling rate of 10 ℃ / s or more, then 500 to 350 ℃ A method for producing a high-strength hot-rolled steel sheet, which comprises winding the steel sheet in a roll.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The steel of the present invention is a conventional precipitation strengthened steel.
It is a steel containing an appropriate amount of Si, and this Si changes the Ar ₃ transformation point and expands the (γ + α) 2 phase region.
Since it has the effect of promoting two-phase separation during the α-transformation, it is considered to have a great influence on the precipitation reaction of carbides in the ferrite grains and the C concentration process in the untransformed austenite phase. As a result, a high-strength hot-rolled steel sheet composed of ferrite and martensite in which carbide is precipitated can be obtained.

The reason why the composition of the steel is limited to the above range in the present invention will be explained below. C: 0.10 wt% or less When C is contained in excess of 0.10 wt%, the second phase fraction increases and elongation and flangeability deteriorate. Therefore, the upper limit of the amount of C is basically 0.10 wt% or less. However, this C
Is C ≧ (0.03 + Ti / in relation to Ti and Nb described later.
4) wt%, C ≧ (0.03 + Nb / 8) wt% or C (%)
If the condition of ≧ 0.03 + (Ti / 4 + Nb / 8) is not satisfied, this C is preferentially consumed in the precipitation reaction of TiC and NbC during the γ → α transformation, and the C concentration in the untransformed γ grains is increased. As a result of insufficient conversion, the stability of the untransformed γ grains as austenite decreases, the second phase is less likely to become martensite, and good strength-ductility balance cannot be obtained. Therefore, the lower limit of C must be set to a value that satisfies the conditions of the above equation in relation to Ti and Nb.

Si: 0.5 to 2.5 wt%, Si is the most important element in the present invention. That is, it has both the action of promoting the precipitation of TiC and NbC in ferrite during the γ → α transformation and the action of converting the second phase to martensite. The effect of this Si addition is 0.5
It develops at addition of wt% or more. On the other hand, when the content exceeds 2.5 wt%, the effect is saturated, and further, the descaling property after hot rolling deteriorates and the cost increases, so the Si content is 0.5
The range is up to 2.5 wt%.

Mn: 0.5-1.3 wt% If Mn is less than 0.5 wt%, a desired composite structure cannot be obtained. On the other hand, if Mn is contained in excess of 1.3 wt%, the strength becomes high and the elongation and stretch-flangeability are improved. Is deteriorated, the Mn amount is
The range is 0.5 to 1.3 wt%.

P: 0.05 wt% or less P is 0.05 wt% or less in order to secure workability and weldability.

S: 0.02 wt% or less S reacts with Mn in steel to form A-type inclusions and deteriorates stretch flangeability, so there is no such possibility.
It was limited to wt% or less.

Al: 0.01 to 0.10 wt%, Al needs to be added at least 0.01 wt% for cleaning steel. Improvement of cleanliness is indispensable especially for the purpose of strengthening steel. However, addition of more than 0.10 wt% is not preferable because it causes surface defects due to alumina clusters, so the Al content was limited to the range of 0.01 to 0.10 wt%.

Ti: 0.02 to 0.10 wt%, Nb: 0.01 to 0.10 wt% Both Ti and Nb are elements that play an important role in the present invention. That is, Ti and Nb both precipitate as carbides in the α grains simultaneously with the γ → α transformation after hot rolling, and greatly contribute to the matrix strengthening. However, when the content of Ti and Nb is small, not only the precipitated particles become coarse and the precipitation strengthening ability is lost, but also the second phase ratio increases and the structure strengthens. On the other hand, if the content of Ti or Nb is too large, the amount of C required to form the second phase will be insufficient, resulting in a bias in the properties of the precipitation strengthened high strength steel. For this reason, the Ti content is 0.02 to 0.10 wt%, and Nb
The content was limited to the range of 0.01 to 0.10 wt%. When Ti and Nb are added in combination, the total amount (Ti + Nb) of them is preferably in the range of 0.02 to 0.10 wt%.

Next, the manufacturing conditions for the steel of the present invention will be described. At the beginning of hot rolling, the finish rolling temperature is
It must be 820 ℃ or higher. This is because if the hot rolling finish temperature is less than 820 ° C, the ductility after hot rolling will be significantly deteriorated. The hot rolling conditions of the present invention include, in addition to the case where the continuous casting slab is once cooled and then reheated to perform rough rolling, from the viewpoint of energy saving, immediately or without being kept warm after the continuous casting without lowering the temperature to 820 ° C or lower. The case where rough rolling is performed after the treatment may be performed.

In the present invention, after the above hot rolling is completed, 820
It is important to stay in the temperature range of ~ 720 ℃ for 10 seconds or more. If the residence time is less than 10 seconds, the γ → α transformation will be insufficient. That is, the precipitation of TiC in the transformed α-grains and the concentration of C in the untransformed γ-grains become insufficient, and the desired composite structure consisting of precipitation-strengthened ferrite and martensite cannot be finally obtained. Is.

Then, the cooling rate until the subsequent winding is
It should be 10 ° C / s or more. This is because if the cooling rate at this time is less than 10 ° C./s, pearlite will precipitate. Finally, the winding temperature should be below 500 ℃. This is because bainite precipitates when the coiling temperature exceeds 500 ° C. The lower limit of coiling temperature is 350 ℃. This is because the strength increases and the elongation and stretch-flangeability deteriorate when the winding temperature is lower than 350 ° C.

[0025]

Example 1 A steel slab adjusted to have 9 kinds of composition shown by A to I in Table 1 was hot-rolled under various conditions shown in Table 2, and a plate thickness: 2.6.
mm hot rolled sheet was produced. Table 2 shows the results of an examination of the tensile properties, hole expansion ratio and fatigue strength of the thus obtained hot rolled sheet. The tensile test is JIS5 in the L direction.
It carried out by the usual method using the No. test piece. In addition, the hole expansion ratio was set to 150 mmφ for the test piece size, and in accordance with the hole expansion test procedure shown in Fig. 1, the center of the test piece with a punched hole of diameter: 36 mmφ (D ₀ ) and the lower end radius: 50 mm. It was pushed up by a ball head punch and the diameter D ₁ when a minute crack was generated was measured and calculated from the following formula. Hole expansion rate (%) = (D ₁ −D ₀ ) / D ₀ × 100 Furthermore, the fatigue strength is the sample with the dimensions shown in Fig. 2 (90 mm
-15mm-30.4R) was used to perform a full double swing plane bending fatigue test.

[0026]

[Table 1]

[0027]

[Table 2]

As is apparent from Table 2, all of the products obtained according to the present invention have a tensile strength of 50 to 60 kgf / mm ² , and have a good strength-ductility balance, a good hole expansion ratio and high fatigue. Has strength. The structure of the steel of the present invention was composed of a two-phase structure of ferrite in which Ti carbide was precipitated and martensite. Further, in each of these invention steels, the strength of the spot-welded portion investigated separately was good.

On the other hand, none of the comparative examples which do not satisfy the requirements of the present invention have good properties in terms of tensile properties, hole expansion ratio and fatigue strength. In particular, C
In Test No. 9 whose amount was less than the lower limit of the present invention, although the hole expansion ratio was good, the strength-ductility balance was poor, the fatigue strength was low, and properties similar to so-called precipitation strengthened steel were exhibited. In addition, the test No. 10 in which the amount of C deviated from the upper limit was the strength-
Good ductility balance and high fatigue strength,
The hole expansion ratio was inferior and the properties were similar to those of so-called structurally strengthened steel, and the strength deterioration of the spot welds was large.

Example 2 Nine kinds of steel slabs (steels: A to I) shown in Table 3 were hot-rolled under various conditions shown in Table 4 to produce hot-rolled sheets having a plate thickness of 2.6 mm. Table 4 shows the results of an examination of the tensile properties, hole expansion ratio and fatigue strength of the hot rolled sheet thus obtained.

[0031]

[Table 3]

[0032]

[Table 4]

As is clear from Table 4, in all the examples of the present invention, the tensile strength was 50 to 60 kgf / mm ² and the good strength was-
It has ductility balance, good hole expansion ratio and high fatigue strength.

Example 3 A steel slab of the type (steel: A, B) shown in Table 5 was hot-rolled under various conditions shown in Table 6 to produce a hot rolled sheet having a plate thickness of 2.6 mm. Table 6 shows the results of an examination of the tensile properties, hole expansion ratio and fatigue strength of the thus obtained hot rolled sheet.

[0035]

[Table 5]

[0036]

[Table 6]

As is apparent from Table 6, even when Ti and Nb are added in combination, the tensile strength of the present invention is 50 to 50%.
It has 60 kgf / mm ² and has a good strength-ductility balance, a good hole expansion ratio and high fatigue strength.

[0038]

As described above, according to the present invention, each of the characteristics of the conventional precipitation strengthened steel and the structure strengthened steel is combined,
Moreover, the disadvantages of each of these steels have been advantageously eliminated, the workability is excellent, especially the cold workability, and the tensile strength is 50 to 60 kgf.
It is possible to easily obtain a high strength hot rolled steel sheet having a thickness of / mm ² . Moreover, the hot-rolled sheet obtained by the method of the present invention exhibits not only high strength but also a good strength-ductility balance, and further stretch flangeability represented by a hole expansion ratio, and further fatigue characteristics and spot weldability. It is also excellent, and it is extremely effective when used for automobile inner plates, suspension parts, and strength members.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a hole expansion test procedure.

FIG. 2 is a diagram showing the dimensions and shape of a fatigue test piece.

Claims (6)

[Claims] 1. C: 0.10 wt% or less, Si: 0.5 to 2.5 wt%, Mn: 0.5 to 1.3 wt%, P: 0.05 wt% or less, S: 0.02 wt% or less, Al: 0.01 to 0.1 wt% and Ti: 0.02 to 0.10 wt% in a range satisfying C (%) ≧ 0.03 + Ti / 4, the balance is substantially Fe composition, and the structure is ferrite in which Ti carbide is precipitated. A high-strength hot-rolled steel sheet having excellent workability, which is characterized by comprising a two-phase structure of a martensite and 2. C: 0.10 wt% or less, Si: 0.5 to 2.5 wt%, Mn: 0.5 to 1.3 wt%, P: 0.05 wt% or less, S: 0.02 wt% or less, Al: 0.01 to 0.1 wt% and Nb: 0.01 to 0.10 wt% is contained in a range satisfying C (%) ≧ 0.03 + Nb / 8, the balance is substantially Fe composition, and the structure is a ferrite in which Nb carbide is precipitated. A high-strength hot-rolled steel sheet having excellent workability, which is characterized by comprising a two-phase structure of a martensite and 3. C: 0.10 wt% or less, Si: 0.5 to 2.5 wt%, Mn: 0.5 to 1.3 wt%, P: 0.05 wt% or less, S: 0.02 wt% or less, Al: 0.01 to 0.1 wt%, Ti: 0.02 to 0.10 wt% and Nb: 0.01 to 0.10 wt% are contained in a range satisfying C (%) ≧ 0.03 + (Ti / 4 + Nb / 8), and the balance is substantially Fe composition. A high-strength hot-rolled steel sheet having excellent workability, characterized in that its structure is composed of a two-phase structure of ferrite and martensite in which Ti and Nb carbides are precipitated. 4. C: 0.10 wt% or less, Si: 0.5 to 2.5 wt%, Mn: 0.5 to 1.3 wt%, P: 0.05 wt% or less, S: 0.02 wt% or less, Al: 0.01 to 0.1 wt% and A steel slab containing Ti: 0.02 to 0.10 wt% in a range satisfying C (%) ≧ 0.03 + Ti (%) / 4, the balance being substantially Fe composition, rolling end temperature: 820 ° C The hot rolling is completed as described above, and then the material is allowed to stay in the temperature range of 820 to 720 ℃ for 10 seconds or more, then cooled at a cooling rate of 10 ℃ / s or more, and then wound at a temperature of 500 to 350 ℃. And a method for producing a high-strength hot-rolled steel sheet having excellent workability. 5. C: 0.10 wt% or less, Si: 0.5 to 2.5 wt%, Mn: 0.5 to 1.3 wt%, P: 0.05 wt% or less, S: 0.02 wt% or less, Al: 0.01 to 0.1 wt% and Nb: 0.01 to 0.10 wt% is contained in a range satisfying C (%) ≧ 0.03 + Nb (%) / 8, and the balance is a steel slab having a substantially Fe composition. Rolling end temperature: 820 ° C The hot rolling is completed as described above, and then the material is allowed to stay in the temperature range of 820 to 720 ℃ for 10 seconds or more, then cooled at a cooling rate of 10 ℃ / s or more, and then wound at a temperature of 500 to 350 ℃. And a method for producing a high-strength hot-rolled steel sheet having excellent workability. 6. C: 0.10 wt% or less, Si: 0.5 to 2.5 wt%, Mn: 0.5 to 1.3 wt%, P: 0.05 wt% or less, S: 0.02 wt% or less, Al: 0.01 to 0.1 wt%, Ti: 0.02 to 0.10 wt% and Nb: 0.01 to 0.10 wt% are contained in a range satisfying C (%) ≧ 0.03 + (Ti / 4 + Nb / 8), and the balance is substantially Fe composition. The steel slab is hot-rolled at a rolling end temperature of 820 ° C or higher, then allowed to stay in the temperature range of 820 to 720 ° C for 10 seconds or longer, and then cooled at a cooling rate of 10 ° C / s or higher. A method for producing a high-strength hot-rolled steel sheet having excellent workability, which comprises winding at a temperature of 500 to 350 ° C.