JPH02217419A - Production of hot rolled steel plate for working and heat treatment for worked product thereof - Google Patents

Abstract

PURPOSE:To obtain a soft hot rolled steel plate having superior workability by subjecting a carbon steel slab or a low alloy steel slab each having a specific composition to continuous hot rolling under prescribed conditions. CONSTITUTION:A slab of a carbon steel having a composition containing, by weight, <=0.20% C, <=0.008% sol.Al, and 0.003-0.02% N or a slab of a low alloy steel having a composition in which one or more components selected from one or more groups among a first group consisting of >=0.01%, in total, of Ti or Nb satisfying the relation in [0.003<=N-(14/48Ti+14/93Nb], a second group consisting of 0.5-3.0% Cu, and a third group consisting of 0.002-0.10% rare earth elements, 0.01-0.10% Zr, and 0.002-0.01% Ca are further incorporated to the above composition is prepared. The above slab is subjected to continuous hot rolling in which finish rolling-finishing temp. is regulated to >=Ar3 transformation point, and the resulting hot rolled plate is wound up at >=300 deg.C. It necessary, after the above winding of the above hot rolled steel plate at 1300 deg.C, the hot rolled steel plate is cooled slowly through the temp. from about 250 down to 150 deg.C in >= about 10hr.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention is a steel plate suitable for high-strength parts for automobiles or industrial equipment, specifically, a steel plate that is processed at a relatively low strength until it is subjected to forming processing. The present invention relates to a method for manufacturing a hot-rolled steel sheet for processing, which can be easily strengthened by an appropriate heat treatment after processing, and a method for heat-treating the processed product.

(Prior Art) Steel plates manufactured by continuous hot rolling, so-called hot-rolled steel plates, are widely used as relatively inexpensive structural materials in various industrial equipment including the above-mentioned automobiles. Many of these applications involve parts that are formed by press working, and therefore hot-rolled steel sheets are often required to have excellent workability, while as structural members they are also required to have high strength. However, it is usually difficult to achieve both high strength and excellent workability.

Therefore, various materials have been developed that have low strength and good workability at the raw material stage before processing (but can be made high in strength through appropriate heat treatment after processing). High-strength steel plates that harden and increase their yield strength during the baking process after processing, so-called bake-hardened high-strength steel plates, have already been put into practical use. Studies are also underway, and patents related to this have also been filed.

For example, Japanese Patent Publication No. Sho 62-180021 discloses a method of rapidly cooling steel with a specific chemical composition containing a large amount of N after hot rolling, as a method for manufacturing high-strength hot-rolled steel sheets of which baking is hardening. There is. This method uses the strain aging of solid solution N to obtain hardening properties, but according to the experimental results of the inventors of this application, the baking properties obtained by this method are similar to hardening type high-strength hot rolling. Although the yield strength of the steel plate significantly increased after baking, the increase in tensile strength was small.

It has been reported that when the increase in tensile strength is small, the improvement in fatigue properties is small; fatigue properties have a strong correlation with tensile strength, and fatigue properties increase as tensile strength increases (manual "Sumitomo Metals vol. 1.33 (198
1) Na4P, 121).

Therefore, if the increase in tensile strength is small, the improvement in fatigue properties required for high-strength parts for automobiles and industrial equipment, which are the main uses of these steel sheets, will be small and the practical value will be poor. .

(Problem to be solved by the invention) The present invention! The subject is a method for producing hot-rolled steel sheets for processing, which are soft before processing and whose tensile strength, which is effective in improving fatigue properties, is significantly increased by reheating at a relatively low temperature after processing, and The object of the present invention is to provide an optimal heat treatment method for processed products made from steel plates.

(Means for Solving the Problems) The present inventors have discovered that by using carbon steel slabs or low alloy steel slabs with a specified chemical composition as raw materials, and subjecting them to continuous hot rolling under specific conditions, they become soft. A hot rolled steel sheet can be obtained, and
It has been discovered that the strength can be increased by processing and forming this hot-rolled steel plate into a predetermined shape and then subjecting it to appropriate heat treatment. That is, ■ Low Aj2 high N carbon steel billet or low alloy steel billet is rolled at above the transformation point and coiled at 300°C or above, or is rolled at 300°C or above and then slowly cooled. If solid solution N is precipitated as iron nitride, a soft hot-rolled steel sheet with excellent workability can be produced.

■Then, after processing and forming this steel plate into a predetermined shape,
If the iron nitride is dissolved in solid solution by heating to a temperature of 0 to 750° C., and the solid solution N produced by the solid solution of iron nitride is allowed to remain by rapid cooling after heating, the strength will be significantly increased.

The present invention was completed based on such knowledge, and
The gist is in (i) to (ij) below.

(i) In weight%, C: 0.20% or less, Sol, Al
: 0.008% or less, N: 0.003 to 0.02
% or C: 0.20% or less, So
l, Al: 0.008% or less, N: 0.0
03 to 0.02% and further contains one or more components selected from at least one of the following groups 1, 2, and 3. ＾
r, continuous hot rolling to a temperature higher than the transformation point, and then 3
A method for producing a hot-rolled steel sheet for processing, which comprises winding at a temperature of 00°C or higher.

[Group 1] The total amount is 0.01% or more, and [0.003≦N(
14/48Tl + 14/93Nb)) Ti or Nh.

[Second group] 0.5-3.0% Cu.

[Group 3] 0.002-0.10% rare earth elements, 0.002-0
．． 01% Ca, 0.01-0.10% Zr.

(ii) The carbon steel billet or low alloy steel billet of (i) above is subjected to continuous hot rolling with a finish rolling finish temperature of at least the H3 transformation point, then coiled at a temperature of 300°C or higher, and rolled at a temperature of 250°C to
A method for producing a hot-rolled steel sheet for processing, characterized by slow cooling in a temperature range of 150° C. for 10 hours or more.

(iii) After processing and forming the hot-rolled steel plate obtained by the above manufacturing methods (i) and (ii) into a predetermined shape,
Heat to a temperature of 400-750°C for 5 seconds or more, then 4
20 at a cooling rate of 10°C/S or more from a temperature of 00°C or higher
A method for heat treatment of processed products, characterized by cooling to below 0°C.

(Function) The excellent properties of the hot-rolled steel sheet of the present invention, namely, the property that it is easy to work with relatively low strength until it is subjected to forming processing, and that the strength is increased by reheating after processing is mainly due to solid solution Due to the action of N. If solid solution N is precipitated as iron nitride to soften it as much as possible during the production of hot-rolled steel sheets, it will be easier to form it in that state, and during the reheating process after processing, iron nitride will be removed. If the solid solution N obtained by dissolving the substance in solid solution remains, the tensile strength will significantly increase. In order to fully obtain such effects, C%Soj! is required for carbon steel pieces, as described later. , ^2, It is important to adjust the content of N and the content of these components and other alloy components in a low alloy steel billet.

Below, the material steel piece C, Soj! , A1, N, and other alloy components are limited as described above, together with their effects.

C: C is a preferable element that increases the strength of the steel plate.

However, if the content exceeds 0.20%, weldability deteriorates, so the content is set to 0.20% or less.

Al = If A2 is contained in an amount exceeding 0.008%, it will combine with N,
It precipitates as ^2N and does not dissolve into solid solution even during reheating after processing, inhibiting strengthening by solid solution N, which will be described later. Therefore, A
ffi is Sof, Ai! , the content shall be 0.008% or less.

N: N is an important element in the present invention, and is present as iron nitride before forming, and is dissolved in solid solution during reheating after processing and forming to improve tensile strength without impeding workability. However, if the content is less than 0.003%, the desired high strength cannot be expected after reheating, and if the content exceeds 0.02%, aging deterioration will become significant.
% content.

In the present invention, C, Sol, Al and N are added to the raw steel piece.
When using a carbon steel billet containing
Hot-rolled steel sheets are easy to process when hot-rolled, and hot-rolled steel sheets with high strength can be obtained by reheating after processing.

On the other hand, a similar hot-rolled steel sheet can be manufactured by using a low-alloy steel billet containing T1, Nb, Cu, rare earth elements, Cas Zr, etc. in place of the carbon steel billet.

The effects of these alloying elements are as follows.

First group of TI and Nb: Tt and Nb combine with C and N in the steel and precipitate as carbonitrides, thereby strengthening the steel plate.

However, 0.003% or more of N, which is not combined with T1 and Nb, is required in the steel for the above-mentioned reason.

N − (14/48Ti + 14/93Nb) is 0.
If it is less than 0.003, the N in the steel that is not combined with TI and Nb is less than 0.003% (also, TI,
If the content of Nb is less than 0.01% in one type or in total of two types, the above effect cannot be obtained. Therefore, Ti and Nb
The content of 0.003≦N − (14/48Ti +
14/93Nb), and type 1 or 2
The total amount of species must be 0.01% or more.

Second group of Cu: Cu precipitates independently without combining with C, and has the effect of strengthening the steel plate. If the content is less than 0.5%, the above-mentioned desired effect cannot be obtained, and if the content exceeds 3.0%, the effect becomes saturated, which is economically disadvantageous. Therefore,
The content should be 0.5 to 3.0%.

In addition, when Cu is contained, 1.5% is added together with Cu.
The following Ni is preferably added. Adding Ni can prevent cracks caused by Cu during hot rolling.

The third group of rare earth elements, Ca and z': These elements are added in 11ml or more to adjust the shape of inclusions and improve cold workability. :o, oo2% ungrooved, Ca: 0.002
% and Zr: less than 0.01%, the desired effect due to the above action cannot be obtained; on the other hand, rare earth elements:
O, XO%, Ca: 0.01% and Zr: 0.10%
If the content exceeds the above, on the contrary, there will be too many inclusions in the steel, resulting in poor cold workability.

One or more of these components from the first group to the third group can be selected and added from at least one group depending on the purpose.

The rope piece used in the present invention has C: 0°20% as described above.
Below, Sol, ^II! : 0.008% or less, N
carbon steel containing 0.003 to 0.02%; furthermore, low alloy steel containing at least one component of the first to third groups, but these carbon steels Elements other than the above that may be included in the low alloy steel include Sl, Ml, and Ni.

These desirable contents are: Si: 1.2% or less;
Mn: 2.0% or less, Ni: 1. ．． It is 5% or less.

If St and Mn are contained in excess, weldability deteriorates, but if they are contained in appropriate amounts, they have the effect of improving strength and ductility through solid solution strengthening. Ni is Cu during hot rolling.
This has the effect of suppressing cracking due to

Note that P and S are inevitable impurities. Since P has an adverse effect on weldability and S forms MnS-based inclusions and reduces workability, it is desirable that their content be 0.05% or less.

If carbon steel slabs and low-alloy steel slabs containing the above-mentioned components are hot rolled under the conditions described below, they will be easy to work with low strength as hot-rolled, and will have high strength when reheated after working. This makes it possible to obtain hot-rolled steel sheets.

The hot-rolled steel sheet manufacturing conditions and post-processing heat treatment conditions will be described below.

(Hot-rolled steel sheet manufacturing conditions) (AL continuous hot rolling The above-mentioned carbon steel slabs and low-alloy steel slabs to be subjected to hot rolling are hot slabs directly delivered from continuous casting or blooming process, or slabs that are once cooled. Any slab that has been rolled and then reheated can be used.However, hot rolling is
) It is important to finish finish rolling at a temperature above the transformation point.

If the finish rolling end temperature is lower than the transformation point, processed &1Iva will be mixed into the ferrite grains generated by transformation, degrading workability and increasing the amount of strain recovered during reheating after processing. As a result, the increase in tensile strength is small.

(b) Winding Winding is performed in a temperature range of 300° C. or higher. By doing so, the solid solution N can be reliably precipitated as iron nitride, and the steel sheet will be softened. However, the winding temperature is 300℃.
If heat treatment is carried out at a lower temperature, there will be a large amount of solid solute N in the hot rolled steel sheet, which will significantly harden and deteriorate workability, and an increase in tensile strength due to solid solute N after heat treatment cannot be expected. It is important that the winding is carried out at a temperature of 300° C. or higher so that solid solution N is sufficiently precipitated as iron nitride.

(C) Cooling after winding The steel plate after winding may be left to cool, or
It may be slowly cooled in a temperature range of 0° C. for 10 hours or more, preferably 15 hours or more.

If the slow cooling process is used, the solid IN remaining on the steel plate after coiling will be removed.
can be further precipitated as iron nitrides, so the steel sheet becomes softer through a decrease in solid solution N, and the tensile strength after reheating also increases.

For slow cooling, cover the rolled steel plate with a heat insulating cover, or
This can be done by inserting it into a slow cooling furnace.

Steel sheets manufactured under the above hot-rolled steel sheet manufacturing conditions have low strength and are easy to process because many iron nitrides precipitate before being processed (reheating after processing removes iron nitrides from solid solution). The tensile strength increases significantly.

In order to greatly increase the tensile strength, reheating after processing is preferably performed under the following heat treatment conditions.

[Heat treatment conditions after processing and forming]

After forming the hot-rolled steel sheet manufactured by the above method into a predetermined shape, the formed product is heated at a temperature of 400 to 750°C for 5 seconds or more, and then cooled from a temperature of 400°C or more at a rate of 10"C/s or more. It cools down to 200°C or less at a rapid rate.

If the heating temperature is less than 400℃, sufficient solid solution N cannot be obtained due to insufficient solid solution of iron nitride, and the increase in tensile strength will be small.On the other hand, if the heating temperature exceeds 750℃, heat will be applied to the processed product. Distortion occurs. Even if the heating temperature is 400 to 750°C, if the heating time is less than 5 seconds, the solid solution of iron nitride is insufficient and the solid solution N
Depending on the intended use of the processed product, heating may be done locally only in areas that require high strength, or the entire processed product may be heated.Local heating is easier to rapidly cool after heating. , the aim of the present invention can be easily achieved.

After heating, 10℃/10℃ from 400℃ or higher to 200℃ or lower.
If the steel is rapidly cooled to a temperature higher than S, the precipitation of iron nitrides dissolved in solid solution due to heating is suppressed, and the steel sheet is significantly strengthened by the solid solution N generated by solid solution of iron nitrides.

Next, the present invention will be further explained by examples.

(Example) Steel having the chemical composition shown in Table 1 is melted in a 50 kg vacuum melting furnace and hot forged into an ingot with a diameter of 300 am, or
Alternatively, a 60 mm thick slab was produced by directly casting into a mold, and hot rolled under the conditions shown in Table 2 to produce a 2 mm thick hot rolled steel plate.

The hot-rolled steel sheet was wound up and then allowed to cool or slowly cooled. In addition, the Ar and transformation points of the steel plates in the examples are all 9.
It is less than 00℃.

A test piece was taken from the obtained hot-rolled steel sheet and the tensile strength (TS) and elongation (EL) of the as-hot-rolled steel sheet were examined. Reheating treatment was performed under the conditions shown in , and the tensile strength after the reheating treatment was measured to examine the amount of increase in tensile strength (ΔTS). These results are shown in Table 3.

(Hereafter, blank space) The hot-rolled steel sheets of Sho 1 to N113, t4 [L5 to N116, Sho 9 and Ma 11 to Sho 21 are manufactured using carbon steel pieces and allowed to cool after winding, Nn4 , No. 7, Nl18.
8I110 and No. 22 hot-rolled steel sheets are produced using low-alloy steel slabs and allowed to cool after winding.
~N+125, -27~Na28, N[L31 and 34 to -46 hot-rolled steel plates are manufactured using carbon steel slabs and slowly cooled after winding, H[L26, Na29 to 30 and The hot-rolled steel sheets No. 32 and No. 33 were produced using low-alloy steel slabs, which were similarly slowly cooled after winding.

The hot-rolled steel sheets of Magnetic 1-81112 and N11L23-35 manufactured by the method of the present invention and heat-treated by the method of the present invention after processing have low strength and are soft before processing, and the strength is 10 Kgf/am for heat treatment. ”In particular,
When the material is slowly cooled after winding, the strength increases significantly.

On the other hand, as shown in the comparative example, products manufactured and heat-treated under conditions other than those specified in the present invention have poor workability or a small increase in strength, that is, N
The hot-rolled steel sheets of I113 and Stone 36, Ih14 and Magnetic 37 with low coiling temperature, and Seed 38 with short annealing time have high strength as hot-rolled and have a small increase in strength after heat treatment.
N1116 and Magneto 40, which have a low maximum heating temperature, Sho 17 and Ashi 41, which have a short reheating holding time, N111B and N[L42, which have a slow cooling rate after reheating, 1t21 and Sho 45.5o11., which have a low N content. In the hot rolled steel sheets of L20 and Grade 44, which have a high N content, sufficient solid solution N is not obtained after heat treatment and the increase in strength is small.

In addition, magnet 15 and stone 39 with high maximum heating temperature for heat treatment
The hot-rolled steel sheets of 14[Li2 and 43, which have a high C content, have arc welding defects, and the hot-rolled steel sheets of 22 and N1146, which have a high N content, have thermal strain during heat treatment. , aging deterioration occurred.

(Effects of the Invention) As explained above, the hot-rolled steel sheet produced by the method of the present invention is soft and easy to work when hot-rolled, and its strength increases significantly when reheated after working. Therefore, this steel sheet is ideal as a material for automobiles and other industrial equipment that requires both excellent workability and high strength.

Claims (3)

[Claims] (1) In weight%, C: 0.20% or less, Sol, Al:
Carbon steel piece containing 0.008% or less, N: 0.003 to 0.02%, or C: 0.20% or less, Sol, Al: 0
．． 008% or less, containing N: 0.003 to 0.02%,
Further, a low-alloy steel billet containing one or more components selected from at least one of the following groups 1, 2, and 3 is continuously heated so that the finish rolling temperature is equal to or higher than the Ar_3 transformation point. A method for producing a hot-rolled steel sheet for processing, which comprises performing inter-rolling and then winding at a temperature of 300°C or higher. [Group 1] The total amount is 0.01% or more, and [0.003≦N-
(14/48Ti+14/93Nb)]
or Nb, [2nd group] 0.5-3.0% Cu, [3rd group] 0.002-0.10% rare earth elements, 0.002-0
．． 01% Ca, 0.01-0.10% Zr, (2) The carbon steel billet or low-alloy steel billet described in claim 1 is subjected to continuous hot rolling with a finish rolling finish temperature of Ar_3 transformation point or higher, and then coiled at a temperature of 300°C or higher. , a method for producing a hot-rolled steel sheet for processing, characterized by slow cooling in a temperature range of 250 to 150°C for 10 hours or more. (3) After processing and forming the hot-rolled steel sheet for processing obtained by the manufacturing method according to claim 1 or 2 into a predetermined shape, heating it to a temperature of 400 to 750°C for 5 seconds or more, A method for heat treating a processed product, characterized in that the product is then cooled from a temperature of 400°C or higher to 200°C or lower at a cooling rate of 10°C/s or higher.