JPS60184630A - Manufacture of hot-rolled high-tension steel sheet having superior workability - Google Patents

Abstract

PURPOSE:To manufacture a hot-rolled high-tension steel sheet having superior workability by adding a very small amount of Nb to a steel similar in composition to a C-Si-Mn steel for popular use. CONSTITUTION:The composition of a steel billet is composed of, by weight, 0.02-0.2% C, 0.3-1.5% Mn, <=1.5% Si, <=0.02% P, <=0.01% S, 0.008-0.025% Nb and the balance Fe with inevitable impurities. The billet is heated and subjected to continuous hot finish rolling at >=80% total draft and (Ar3+50 deg.C) final rolling pass temp. The resulting steel sheet is coiled at 300-550 deg.C. A hot-rolled high- tension steel sheet having superior workability is obtd. The steel sheet has a fine bainite-base composite structure consisting of bainite and ferrite.

Description

Detailed Description of the Invention [Field of Application in Industry-1-J The present invention uses C,-S]-1 steel or C-Mn steel with a trace amount of Nb added, Through process improvements, stretch flangeability is better than conventional precipitation-strengthened high-strength hot-rolled sheets with special elements added≦1q. The present invention relates to a method for manufacturing rolled steel sheets.

[Prior art]

In recent years, in the automobile industry, one of the measures to improve the fuel efficiency of automobiles is to make the car body lighter, and there is an increasing demand for thinner steel plates and high-strength steel plates with excellent workability in terms of safety. .

Conventionally, in order to obtain hot rolled steel sheets with excellent workability, Nb.

A manufacturing method that improves strength through solid solution hardening and precipitation strengthening through carbonitride formation by adding special elements such as Ti, V, etc., and Dual Phase (which has recently begun to be manufactured)
Dp) There is a manufacturing method that improves the strength-ductility balance by forming a stuff plate. The former is Nb, Tj
Additive elements such as , V, etc. are expensive and cause an increase in cost, and resource constraints are expected in the future. The latter steel plate has a good balance of strength and ductility, but there are problems when applying it to wheels, which are automobile parts.
Due to poor stretch flangeability, cracks are likely to occur when forming a hub six by burring, for example, when processing a wheel disk. ■Wheel rim and mold correction++ due to large softening phenomenon in the weld heat affected zone; There are two points in 7: there is a large decrease in the thickness of the welding heat affected zone and no improvement in fatigue properties is observed.

A conceivable way to solve these problems is to manufacture a high-tensile hot-rolled steel sheet using C-C-5j steel having a composite structure of ferrite and Boehni 1. For example, Japanese Patent Publication No. 5714596
As shown in No. 5, Ferrai 1-
A high-strength hot-rolled steel sheet with a composite structure of bainite and bainite and its production have been demonstrated, and it is also inexpensive in terms of wheel disc processing.] At one point, cracking caused by forming hub holes by burring has also been improved. is not enough. In addition, there are some automobile suspension parts that particularly require stretch flangeability, and cracks have occurred. Steel is not the same when it comes to the softening phenomenon of the weld heat affected zone, but a decrease in the meat Pi is observed during wheel rif type 1 straightening, which poses a problem in fatigue properties.

Therefore, when applying to wheels, which are automobile parts, it is necessary to improve the reduction in wall thickness due to the softening phenomenon of the weld heat affected zone.

[Purpose of the invention]

In view of the current situation, the present invention has been developed to develop a general-purpose C-5j-M
By adding a small amount of Nb to a steel with similar composition to N-series steel, we can produce high-strength hot-rolled steel sheets at low cost that have good stretch flangeability and excellent workability with less wall thickness reduction in the weld heat affected zone. This provides a new manufacturing method for

[Structure and operation of the invention]

The basic components of the present invention are C: 0.02 to 0.20%,
Mn=0.3-1.5%, Sj: 1.5% or less,
P: 0.02% or less, S: 0.01% or less,
Nb: A steel piece containing a limited content of 0.008 to 0.025%, with the balance consisting of Fe and unavoidable elements is used.

C and Mn are essential elements for securing the necessary strength and obtaining a bainite-ferrite composite structure mainly composed of bainite (composite structure including the Boehni 1-structure and the Acidcuturf 1941- structure). (,: less than 0.02%, Mn
: If C: less than 0.3%, it is difficult to obtain a steel sheet with a bainite-ferrite composite structure mainly composed of bainite by cooling at a normal hot rolling process;
．． If it exceeds 5%, the ductility deteriorates significantly and the weldability is also impaired. C: 0.02 to 0.20%, Mn: 0, : 1% to
It is set at 1.5%. Sj improves the strength of the steel plate as a solid solution hardening element. SL: If it exceeds 1.5%, solid solution hardening will become saturated, which is economically disadvantageous, and weldability will be impaired, so Sj: should be 1.5% or less.

1] may impair weldability, so P: 0.02% or less. Since S forms MnS-based inclusions and reduces stretch-flangeability, S: is set to 0.01% or less in order to reduce MnS-based inclusions and improve stretch-flangeability. Ca has the function of controlling the shape of inclusions to form fine spheres and improves stretch flangeability, so it is preferably contained up to Ct+: 0.01°X1. Nb is normally used as an element to increase strength through solid solution hardening and precipitation strengthening, but here it is used to suppress the softening phenomenon of the weld heat-affected zone rather than to increase strength. The purpose is to prevent thickness reduction, and as shown in Fig. 1, the reduction in wall thickness during welding is effective from o, oog% or more, and is saturated at 0.025%. Below 1-, the cost becomes high, and the -1- limit is set at 0.025%.

It is thought that the effect of Nb is to prevent the tempering softening of Behnai 1 through precipitation hardening. The heating temperature of the steel plate during hot rolling may be 1100° C. or lower. This is because there is no need to form a solid solution of Nb, so if the heating temperature is high, the rolling speed will be reduced to ensure the final rolling pass temperature, or there will be an hour's delay on the entry side of the finish rolling, which will reduce productivity. It is. The reason why the total rolling reduction ratio in finish rolling (reduction ratio before and after second rolling) is set to 80% or more is to reduce the austenite grain size and form fine bainite to prevent deterioration in elongation. The finishing temperature is over (Ar3 +50), cooling starts from the γ region and the cooling rate is 45℃/S or higher.
-], temperature at the end of cooling before and after OO℃/S 300 to 55
This is the cooling rate and temperature required to obtain a '-2 structure.

The characteristics of the hot-rolled steel sheet with a composite structure of bainite and ferrite mainly composed of bohnite 1 produced by the method described above are that in terms of strength-ductility balance,
Although it is inferior to bainite composite structure hot-rolled steel sheets, it has excellent properties in terms of six expansion ratios, which is one of the evaluation criteria for stretch flangeability.

The reason for this is that the bainite phase of a hot-rolled steel sheet with a composite structure of bainite and "noelai 1", in which bainiite 1-- is the main body, is obtained by rapid cooling from γ1, so a two-phase deformation takes place. The reason why the stretch flangeability of I).1'' steel is poor is that the punched end face is hardened to a large extent and the hard martensite is destroyed or voids are generated from the interface with ferrite.

Figure 2 compares and organizes the strength-ductility balance of these hot rolled steel sheets. Bainite of the present invention
The strength-ductility balance of a hot-rolled steel sheet with a composite structure of bainite and ferrite 1 made into an E body is inferior to that of a hot-rolled steel sheet with a composite structure of polygonal fine-grained ferrite and bainite, but it is equivalent to a precipitation-strengthened high-strength hot-rolled steel sheet. be. Fig. 3 is a comparison of these hole-expanding skewers for hot-rolled steel sheets. The hot-rolled steel sheet of the present invention having a bainite-ferrite composite structure mainly composed of bainite is superior in hole expansion rate to the hot-rolled steel sheet having a composite structure of ferrite 1- and baini 1-. Figure 1 compares and organizes the wall thickness reduction rate of the weld heat affected zone of these hot rolled steel sheets. D-P hot-rolled steel sheets show a decrease in wall thickness to the extent that it becomes a problem in actual use, but the hot-rolled steel sheets of the present invention having a bainitic (--ferrite) composite structure mainly composed of bainite are precipitation-strengthened high-tensile hot-rolled steel sheets. As with steel sheets, there is no decrease in wall thickness.From this, by changing the martensitic structure to a Boehni 1~ structure, tempering softening due to the heat effect of flash butt welding can be reduced.
Precipitation hardening by adding a small amount of Nb was able to prevent the decrease in wall thickness.

Toge-me*=-i=-a-i<--5=-y ζ7-htoz3-□・de=)10-"Ma=Yo=B□i-Ki11FI
-111-41 analysis to strong V-high force to 111 #J plate, - and 47-s? Examples Examples of the present invention are shown in Table 1. Inventive Examples 1 to 13 are obtained by hot rolling steel having predetermined components according to the present invention. Comparative Example 14 was obtained by hot rolling a component system with a high C content in accordance with the present invention, and the strength-ductility balance was low, with a particularly large decrease in elongation. Comparative Example 15, which has a very low hole expansion, was hot-rolled in accordance with the present invention with a low Nb composition, but thinning of the weld heat-affected zone becomes a serious problem. Comparative Example 16 has the prescribed ingredients but the winding temperature is too high, resulting in Ferai 1- and Barai 1.
~Organized and its strength is reduced. Comparative Example 17 has a predetermined component, but the cooling rate is low and has a ferrite/pearlite structure, resulting in a decrease in strength.

[Brief explanation of the drawing]

Figure 1 shows the relationship between the Nb content and the amount of wall thinning (local strain at 5% tension) in the weld heat affected zone, Figure 2 shows the ratio axis of strength-ductility balance, and Figure 3 is a bow showing a comparison of hole expansion ratios. Agent Patent Attorney Yasuyoshi Yoshijima (x) ibttmaminets Director General Shiga Gakudon1, Indication of the case 1982 Patent application No. 037454' 29 Name of the invention Method for manufacturing high tensile strength hot-rolled steel sheets with excellent workability 3, Relationship with the ministry case for amendment Patent applicant address 2-6-3 Otemachi, Chiyoda-ku, Tokyo Name (6
65) Nippon Steel Corporation Representative Takeshi 1) Yutaka 4, Agent Address: 105 Tit (503) 4877 Address: 8th floor, Daiichi Mori Building, 1-12-1 Nishi-Shinbashi, Minato-ku, Tokyo Claims of Seiyoi 1. Detailed Description of the Invention Column 7, Contents of Amendment (1) The scope of claims is amended as shown in the attached sheet. (2) On page 3, line 7 of the specification, the word “Haritori” is
"Corrected as "Rewind J." (3) Similarly, on page 6, line 11, r(Ar3+50
) J is corrected to r (A'r, +50°C). (4) Also in page 7, line 3, the phrase "two-phase quality" is corrected to "softer than the bainite phase obtained by rapid cooling from the two-phase separated state, and homogeneous up to the high deformation region." . (5) Similarly, on page 7, line 7, the phrase "interface with" is corrected to "interface with." (6) Similarly, on page 7, line 13, the phrase "fine grain nucleus" is corrected to "fine grain." (7) Similarly, on page 9, line 12, the word "low" is corrected to "low." (8) Similarly, on page 9, line 16, the word "winding up" is corrected to "rolling up." (9) Similarly, in Table 1 on page 10, the phrase "F winding temperature within the framework of hot rolling conditions" is corrected to "coiling temperature." (10) Similarly, in Table 1 on page 10, delete the components (wt%) on the right side of the table, and all of the Ca and numbers in the vertical frame. (11) Also in Table 1 (continued) on page 11, N
C), ``86'' in ■R (%) of 5 is corrected to ``80''. Attachment Claims co, oz ~0.20%, MnO,: 3-1.
5%, SiS2.5%, P≦0.02%, S≦0.01
%, NbO, 0.08-0.025%, with the remainder consisting of Fe and unavoidable elements, is heated, the total reduction rate of continuous hot finish rolling is set to 80% or more, and the temperature of the final rolling pass is ('Ar, +50℃), cooled at a cooling rate of 45℃/S or more after completion, and cooled to 300~550''
Method for manufacturing a high tensile strength hot-rolled steel sheet with excellent workability and having a complex structure of bainite and ferrite, mainly composed of fine bainite, characterized by winding at C 6

Claims (1)

[Claims] C0.02-0.20%, MnO, 3-1.5%
, Si≦1.5%, 1″≦0.02%, S≦
A steel billet containing 0.01%, NbO, 008 to 0.025% and the remainder consisting of Fe and unavoidable elements is heated, and the 1-1 tal reduction rate of continuous hot finish rolling is set to 80% or more. The temperature of the final rolling bath was set to (Ar3+50℃), and the final rolling bath was
Cool at a cooling rate of 45 degrees (2 to 1-1), then cool to 3 degrees from 0 to 5
4. A method for producing a high tensile strength hot-rolled steel sheet having a composite structure of fine bainite and ferrite, which is characterized by winding at 50°C and has excellent formability.