JPH06299236A - Manufacture of high strength hot-rolled steel sheet having excellent workability - Google Patents

Abstract

PURPOSE:To obtain a hot-rolled steel sheet having tensile strength exceeding a specific value and excellent ductility and stretchflanging property. CONSTITUTION:Steel is composed of 0.02-0.08wt.% C, 0.3-0.8wt.% Si, 1.0-2.0wt.% Mn, 0.01-0.03wt.% Nb and if necessary, 0.01-0.05wt.% Ti and regulated to <=0.02wt.% P and <=0.005wt.% S. Further, in the case of using CV ( deg.C/sec) for average cooling velocity till 600 deg.C and CT for coiling temp. ( deg.C) the steel slab adjusted to the composition satisfying 32XC%+ 2.1XMn%-0.9XSi%+115/CV+CT /195<=9.5 is used as the base stock. After hot-rolling the base stock at the temp. range of 1000-850 deg.C as finish rolling temp., this rolled steel sheet is cooled at the average cooling velocity of >=40 deg.C/sec in >=600 deg.C and <=30 deg.C/sec in <=600 deg.C and coiled at temp. range of 400-550 deg.C.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used as undercarriage parts for automobiles and the like, and has excellent ductility and stretch flangeability.
It relates to a method for producing a high-strength hot-rolled steel sheet exhibiting a tensile strength of more than 0 N / mm ² .

[0002]

2. Description of the Related Art In recent years, research and development have been carried out to reduce the weight of a vehicle body in order to improve fuel efficiency. With regard to steel materials as well, they have the same press formability as conventional products and exhibit excellent strength. Material is required. As materials that meet this requirement, dual-face steel sheets having a ferrite + martensite structure and high-strength steel sheets using retained austenite have been proposed. Dual face steel sheets have been introduced in, for example, Japanese Patent Publication No. 57-42127, Japanese Patent Publication No. 61-10009, Japanese Patent Publication No. 61-11291, and Japanese Patent Publication No. 57-143433. Many studies have been reported on high-strength steel sheets using retained austenite, and various methods for obtaining steel materials having an excellent strength-ductility balance have been proposed (Japanese Patent Laid-Open Nos. 62-196336 and 62-196336). 63-4017, JP-A-1-79345, etc.).

[0003]

In the conventional dual face steel sheet, since the low temperature transformation phase martensite is used, softening of the heat affected zone is unavoidable when welding is performed. In addition, since the structure has a mixture of soft ferrite and hard martensite, when stretch-flanged, stress concentrates at the phase interface and becomes the starting point of cracking.
Defects such as cracks and fractures are likely to occur in the processed product. That is, the dual face steel sheet is a material having poor stretch-flangeability, and it is in the current situation that a product having a target shape cannot be produced with a high yield.

On the other hand, in order to obtain a desired retained austenite in a high-strength steel sheet using retained austenite, strict control of cooling conditions after rolling, winding temperature, etc. is required. Therefore, the steel plate manufacturing process becomes complicated, and many problems remain in the manufacturing stability and material stability in the hot rolling line. The present invention has been devised to solve such a problem, and by the alloy design and the control of hot rolling conditions, a structure in which pearlite or cementite is finely dispersed in a fine ferrite matrix is created, Without using low temperature transformation phase such as bainite and martensite and retained austenite, excellent material stability, good workability such as ductility and stretch flangeability, and high strength with tensile strength of 440 N / mm ² or more. The purpose is to stably manufacture a hot rolled steel sheet.

[0005]

In order to achieve the object of the method for producing a high strength hot rolled steel sheet according to the present invention, C: 0.02 to
0.08% by weight, Si: 0.3 to 0.8% by weight, Mn:
1.0 to 2.0 wt% and Nb: 0.01 to 0.03 wt%, P and S are regulated to P: 0.02 wt% or less and S: 0.005 wt% or less, respectively. And 60
When the average cooling rate up to 0 ° C. is C _V (° C./sec) and the winding temperature (° C.) is C _T , 32 × C% + 2.1 × M
n% −0.9 × Si% + 115 / C _V + C _T / 195 ≦
A steel slab adjusted to have a composition satisfying 9.5 is used as a raw material, and after hot rolling at a finish rolling temperature in a temperature range of 1000 to 850 ° C., up to 600 ° C., 40 ° C./sec or more, 600
It is characterized in that it is cooled at an average cooling rate of 30 ° C./second or less and is wound in a temperature range of 400 to 550 ° C. The steel slab used may also contain Ti: 0.01 to 0.05% by weight.

[0006]

In the present invention, the alloy design and manufacturing conditions aiming at forming fine ferrite + finely dispersed pearlite or cementite structure without using low-temperature transformation phases such as bainite and martensite and retained austenite are adopted. is doing. The term "ferrite" used in the present invention includes polygonal ferrite and acicular ferrite.
The hot rolled steel sheet produced according to the present invention has a structure in which pearlite or cementite is finely dispersed in a fine ferrite matrix, and is stably produced without requiring strict control of hot rolling conditions. In this structure, pearlite, which is relatively soft as compared to bainite and martensite, is finely dispersed to improve stretch flangeability.

The structure in which pearlite or cementite is finely dispersed in a fine ferrite matrix is a structure in which ferrite having a small grain size is dispersed in the matrix, and as a result, pearlite or cementite generated after ferrite transformation is fine and dispersed. Among them, since pearlite is in a dispersed state without being continuously formed in a band shape, a steel material exhibiting good stretch flangeability can be obtained. Such a structure is formed by defining the compositional design of the alloy, particularly the content of Nb effective for the formation of fine ferrite, and controlling the cooling and coiling after hot rolling in relation to the steel composition. . Also, the X value (= 32 × C% + 2.1
× Mn% -0.9 × Si% + 115 / C V + C T / 19
5) is related to the hole expansion ratio λ, as shown in FIG.
Maintaining 5 or less improves the hole expandability.
This is because when X ≦ 9.5, generation of pearlite band in the central portion of the plate thickness is suppressed.

The alloy components and manufacturing conditions specified in the present invention will be described below. C: It is an effective alloying element for ensuring the strength, and 0.02% by weight or more is necessary for obtaining the target strength of the tensile strength of 440 N / mm ² or more. However, when a large amount of C exceeding 0.08% by weight is contained, the formability of the steel material is significantly deteriorated. Therefore, in the present invention, 0.0
The C content was set in the range of 2 to 0.08% by weight. Si: An alloying element that improves strength by solid solution strengthening and improves ductility and stretch flangeability. Si
The property improvement due to becomes remarkable when the content is 0.3% by weight or more. However, at a Si content above 0.8 wt%,
The surface quality of the hot rolled sheet deteriorates. Therefore, in the present invention, the Si content is set in the range of 0.3 to 0.8% by weight. Mn: An alloying element that improves the strength of steel by solid solution strengthening, and must be added in an amount of 1.0% by weight or more. However, when a large amount of Mn is contained in excess of 2.0% by weight, center segregation is promoted in the slab and a band structure is formed. As a result, the stretch flangeability of the plate material deteriorates. Also, the weldability is reduced. Therefore, in the present invention, the Mn content is set in the range of 1.0 to 2.0% by weight.

Nb: An alloying element that acts as a precipitation strengthening element and is effective in improving strength. Further, it suppresses recrystallization of austenite during rolling, refines ferrite grains, and facilitates the production of polygonal ferrite effective for improving ductility and stretch flangeability. In order to secure these effects, it is necessary to contain 0.01% by weight or more of Nb. However, a large amount of Nb exceeding 0.03% by weight
If the alloy contains, the strength is excessively increased due to the precipitation strengthening, and the ductility is remarkably reduced. Therefore, in the present invention, the Nb content is set in the range of 0.01 to 0.03% by weight.

P: Solid solution strengthening element, which effectively contributes to the improvement of strength. However, it promotes central segregation in the steel slab and forms a band structure, which reduces stretch flangeability. Therefore, in the present invention, P
The upper limit of the content is specified as 0.02% by weight. S: A harmful element that forms MnS and significantly deteriorates stretch flangeability. Therefore, in the present invention, the S content is restricted to 0.005% by weight or less, preferably 0.003% by weight or less. Ti: An alloying element added as necessary in the steel material of the present invention. Ti combines with S to form TiS and suppresses the generation of MnS which deteriorates stretch flangeability. In this respect, addition of 0.01% by weight or more of Ti is extremely effective in improving stretch flangeability. However,
The property improvement due to the addition of Ti is saturated at 0.05% by weight, and even if it is contained in a larger amount, it adversely affects the economical efficiency in steel production. Therefore, when Ti is contained, its content is set in the range of 0.01 to 0.05% by weight.

Finish rolling temperature: The hot rolling is performed so that the finish rolling temperature is in the temperature range of 1050 to 850 ° C. At finish rolling temperatures above 1050 ° C, recrystallization of austenite proceeds during hot rolling, and it becomes difficult to obtain ferrite stably after cooling. As a result, the ductility of the obtained hot rolled sheet deteriorates. On the contrary, at the finish rolling temperature below 850 ° C., in the case of the steel containing Nb as in the present invention, deformation resistance increases because rolling is performed in a non-recrystallized state,
The threadability deteriorates significantly. In addition, it causes deterioration of plate thickness accuracy and increase of electric power consumption. Therefore, the finish rolling temperature of hot rolling was set to a temperature range of 1050 to 850 ° C.

Cooling rate: Cooling after finish rolling is an important manufacturing condition for incorporating a target fine ferrite + pearlite or cementite structure into a hot-rolled sheet.
In the temperature range from finish rolling to 600 ° C, pearlite, which is harmful to stretch flangeability, is finely dispersed while suppressing transformation of ferrite and pearlite. Therefore, it is necessary to cool this temperature region at a cooling rate of 40 ° C./sec or more. On the other hand, in the temperature range of 600 ° C. or less, it is necessary to set the cooling rate to 30 ° C./second or less in order to generate fine ferrite while suppressing the bainite transformation.

Winding temperature: The inventors of the present invention have found from a number of experiments that the winding temperature of a hot-rolled sheet is 550 ° C. or less in order to sufficiently miniaturize ferrite under the condition of suppressing the formation of coarse pearlite. I found that it was necessary to However, at a winding temperature lower than 400 ° C., bainite is likely to be formed, which causes deterioration of ductility. Therefore, in the present invention, a winding temperature of 550 to 400 ° C. is adopted. The hot rolling conditions and the cooling conditions after hot rolling are controlled in relation to the composition of the raw material in order to suppress the pearlite band. That is, the average cooling rate to 600 ° C. C _V
(C / sec) and the winding temperature is C _T (C), X =
32 x C% + 2.1 x Mn% -0.9 x Si% + 115
The X value defined by / C _V + C _T / 195 is set to 9.5 or less. The X value is a pearlite band suppression index obtained from many experimental results by the present inventors. When X ≦ 9.5, the pearlite band is effectively suppressed, and good hole expandability of the obtained steel sheet is secured.

[0014]

EXAMPLES Various steels whose components are shown in Table 1 were melted. Group A steels in Table 1 are steels that satisfy the requirements for the components specified in the present invention. On the other hand, Group B steel
It is a steel that is out of the range specified in the present invention.

[Table 1]

Under various hot rolling conditions shown in Table 2, various steels had a plate thickness of 3
It was hot rolled into a hot rolled sheet of mm. A test piece was cut out from the hot-rolled sheet and subjected to a strength test with a JIS No. 5 tensile tester.
In addition, a punch hole is formed in the test piece with an initial hole diameter d ₀ of 10 mm, and the punch hole is formed with a conical punch having an apex angle of 60 degrees.
_The stretch-flangeability was investigated by a hole expansion test in which the test piece was expanded to ₁ and the limit hole expansion rate λ% [= (d ₁ −d ₀ ) / d ₀ × 100] at which no crack or breakage occurred in the test piece was measured. The results of the investigation are shown in Table 2 together with the manufacturing conditions.

[Table 2]

As is clear from Table 2 showing the test results,
The comparative examples of Group B were inferior in material strength, ductility, and hole expandability. This tendency was also found in the test pieces of Test Nos. 13 and 14 which were hot-rolled under the condition that the X value exceeded 9.5, although the constituents satisfy the requirements specified in the present invention. In particular, the test pieces of test numbers 13 and 14 show low values of the hole expansion ratio λ of 98% and 102%. On the other hand, when the test pieces of the A group are subjected to hot rolling under the conditions according to the present invention, excellent ductility and stretch flangeability are exhibited. Moreover, the tensile strength is at least 516 N / mm ² , and a material having sufficient strength is obtained. Thus, it has been found that when the alloy design and manufacturing conditions are combined under specific conditions, a high-strength hot-rolled steel sheet having excellent properties can be stably obtained.

[0017]

INDUSTRIAL APPLICABILITY As described above, in the present invention, a hot rolled sheet having a fine ferrite + pearlite or cementite structure is stably produced by specifying the hot rolling conditions in relation to the steel composition. is doing. This hot rolled sheet has excellent ductility and stretch flangeability, and
It has a tensile strength of 0 N / mm ² or more. Therefore, it is used as a lightweight and high-strength material in various fields, including parts for automobiles that have been reduced in weight.

[Brief description of drawings]

FIG. 1 is a graph showing the effect of X value on the hole expansion ratio λ.

─────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] August 17, 1993

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0011

[Correction method] Change

[Correction content]

Finish rolling temperature: The hot rolling is carried out so that the finish rolling temperature is in the temperature range of 1000 to 850 ° C. At finish rolling temperatures exceeding 1000 ° C., recrystallization of austenite proceeds during hot rolling, and it becomes difficult to obtain ferrite stably after cooling. As a result, the ductility of the obtained hot rolled sheet deteriorates. On the contrary, at the finish rolling temperature below 850 ° C., in the case of the steel containing Nb as in the present invention, deformation resistance increases because rolling is performed in a non-recrystallized state,
The threadability deteriorates significantly. In addition, it causes deterioration of plate thickness accuracy and increase of electric power consumption. Therefore, the finish rolling temperature of hot rolling was set to a temperature range of 1000 to 850 ° C.

Claims (2)

[Claims] 1. C: 0.02-0.08% by weight, Si:
0.3-0.8 wt%, Mn: 1.0-2.0 wt% and Nb: 0.01-0.03 wt%, P and S are P: 0.02 wt% or less and S: 0.005% by weight or less, and when the average cooling rate up to 600 ° C. is C _V (° C./sec) and the winding temperature (° C.) is C _T , 32 × C% + 2.1 XMn% -0.9xSi% +
115 / C _V + C _T /195≦9.5 The steel slab adjusted to have a composition satisfying 9.5 is used as a material, and the temperature range is from 1000 to 8
After hot rolling at a finish rolling temperature of 50 ° C, cool up to 600 ° C at an average cooling rate of 40 ° C / sec or more and 600 ° C or less at 30 ° C / sec or less, and wind in a temperature range of 400 to 550 ° C. And a method for producing a high-strength hot-rolled steel sheet having excellent workability. 2. The steel slab according to claim 1, further comprising T
i: A method for producing a high-strength hot-rolled steel sheet excellent in workability, which uses 0.01 to 0.05% by weight.