JPS58221263A - Special steel with superior workability and heat treatability and its manufcture - Google Patents

Abstract

PURPOSE:To obtain a special steel with superior workability, especially bendability and superior heat treatability, especially carburizability, by specifying the amount of alloying elements solubilized in the ferrite in a special steel having a specified composition and the ideal critical diameter (hardenability) of the steel. CONSTITUTION:This steel consists of <=0.35% C, <=0.25% Si, 0.30-1.50% Mn, <=0.60% Cr, <=0.020% S, Nb by <=4/10C%, <=0.07% sol. Al, <=200ppm N+O, Ti by (N%+O%)X4-10, 0.0005-0.009% B and the balance Fe or further contains <=0.5% Mo or <=200ppm Ca. The composition of the steel is regulated so as to adjust the amount Fs of alloying elements solubilized in the ferrite according to equation I to <=1.3% and the ideal critical diameter D1 according to equation IIto 0.6-4.5 in. In the equations, C, Si, Mn and Cr are the contents (%) of them in the steel.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a special steel with excellent workability, especially bendability, and excellent heat treatability, especially carburizing property. The present invention also relates to a special steel that further improves carburizing steel, which has excellent carburizability, to improve bendability and toughness by refining the austenite grain size during heat treatment, and a method for producing the same.

For example, for parts that require high strength and toughness, as well as wear resistance and spring properties, such as automobile parts, office machine parts, or general machine parts, steel is carburized and quenched, or quenched and tempered. It is common practice to impart the necessary properties by Among these, the steel used for carburizing treatment is carbon steel for mechanical structures with a relatively low carbon content, hot-rolled mild steel (ordinary steel) with a particularly low carbon content, or low carbon steel containing Nj, Or, Mo, etc. Alloy case-hardened steel was used. For members that need to be processed into complex shapes, carbon steel for machine structures and common steel with a low carbon content are often used in view of their workability, such as bendability.

However, many of these steels generally do not have good carburizability. In other words, it is difficult to obtain a deep hardened layer in the carburizing and quenching treatment after machining, and in order to obtain a deep hardened layer, a long time carburizing treatment or a high temperature carburizing treatment is required, which increases the carburizing cost and causes large heat treatment distortion. Problems such as outbreaks occur. In other words, it is virtually impossible to achieve both workability and carburizability with conventional steel.
It was necessary to sacrifice one or the other or to carry out excessive carburizing treatment.

The purpose of the present invention is to provide a steel that is excellent in both workability and carburizability, and for this purpose, the present inventors have conducted various studies, and as a result, the amount of solid solution of alloying elements in ferrite and We discovered that there is a certain relationship between cold formability and a good relationship between carburizing properties and hardenability of steel, and developed a steel that has both workability and carburizing properties. We were able to. That is, the present invention has the following properties: C: 0.35% or less, Si: 0.25'% or less, Mn: 0.306- to 1.50%, Cr: 0. (50% or less, S;
0.020% or less, Nb; (0%) (7) 4/
10 or less, So'1. A1; 0.07% or less, N
-1-0; 200 ppI'11 or less, T]; 4 to 10 times the (N% + 0) in steel, B; 0.0005 to
0.009%, Mo; Q% or 0.5% or less, the balance consists of Fe and unavoidable impurities, and O111 (However, C, Sj, Mn, Or, Mo are high contents of steel components ) The amount of alloying element solid solution in ferrite (F”B) according to the formula is 1.3- or less, and 1))0.n) = ((0,40+0.06)X
(0,78i + 1) X (3,33Mn+1)
X (2A6Cr + 1) X (3,0Mo +
The ideal critical diameter (D, ) according to the formula 1 x (1 + 1.5 (0,9 - C) 1 (C, Si, Mn, Cr, Mo is the content of steel) is 6 to 4. .5
The purpose of the present invention is to provide a special steel with excellent workability and heat treatability, whose composition has been adjusted so that it is in. As a steel with further improved bendability, VC2o is added to the above steel components.
To provide an improved steel in which the amount of A-based inclusions in a hot rolled sheet is controlled to 0.05% or less in area ratio by adding Ca in an amount of less than oppn, and as a method for producing this special steel, the above-mentioned Steel with the composition - 1100 pieces of improved steel
After heating to above ℃, it is hot rolled at a finishing temperature of 800 to 950℃, then cooled to 670℃ at a rate of 15℃/sec or more, and coiled at a temperature of 550 to 670℃. It provides a manufacturing method.

According to the present invention, compared to the steel described in Japanese Patent Application No. 55-172,379, the austenite grains are significantly finer even after carburizing treatment, resulting in better mechanical properties.

The present invention will be explained in detail below.

According to the tests conducted by the present inventors, S dissolved in ferrite
When any element such as i, Mn, Or, etc. is added within a range that does not create special carbides (carbides other than Fe5C), the amount of these elements dissolved in ferrite expressed by the following formula Fs (
%), i.e. 6.78i Mn Cr1″8
(9))-(2°-〇°4)+6.7-C! +1+1.
40 +1+40 (however, C, Sj, Mn, Cr, Mo are the steel content) and the mechanical properties and processing properties are determined by the amount of alloying element solid solution in ferrite Fs□□□) It was found that the lower the value, the better the ductility and bendability.

On the other hand, for applications that require high surface pressure such as automobile parts and mechanical parts, the hardening depth after carburizing is often required to be 0.3 or more, and in order to satisfy this condition, carburizing The subsequent surface hardness must be HRO40 or higher, but as a result of various studies on chemical components that satisfy these conditions, the hardenability of the material, i.e., the ideal critical diameter DI (
j, n), DI(in,) = ((o, 4c + 0.0
6) x (0,7si +1) X (3,33Mn
+1) X (2,16Or + January X (1+1.
5(0,9-C)1 or DI (in,) = +(Skin AC+0.06) X
(0,7Si +1) X (3,33Mn+1)
x (2,16cr+1)X (3,0MO+1 month×(
1+1.5(0,9-C)) (However, D, Sj, Mn, Or, Mo are high contents of steel components), there is a certain relationship between ~9- and the surface hardness after carburizing. It was found that it is important to have a DI of a certain value or more in order to obtain a deep hardened layer.

The steel of the present invention has successfully achieved both workability and carburizability by making use of this knowledge and by appropriately balancing the chemical components of the steel, and has significantly reduced austenite grains during carburization. It is made finer and has good mechanical properties. The reasons for limiting the chemical composition values of the steel of the present invention are summarized as follows.

(1) If the content of C exceeds 35%, the workability, especially the bendability, is markedly reduced, so the upper limit was set at 0.35%.

(2) Si is an element that dissolves only in ferrite and significantly hardens the ferrite matrix, and is preferably as low as possible from the viewpoint of bendability.

Furthermore, since the toughness after heat treatment is improved by lowering the Sj content, the Si content was limited to 0.25% or less.

(3) Mn is an element that significantly improves the hardenability of materials, but an increase in the amount of Mn increases the number of Mn-based nonmetallic inclusions, and the anisotropy of steel materials due to the occurrence of Vanblad strafe. The upper limit was set at 1.2% because it becomes too strong and is not favorable for press formability.

On the other hand, if the Mn content is less than 60%, the hardenability will decrease significantly.
Since the hardening depth after carburizing treatment could not be guaranteed, the range of Mn content was limited to 0.30 to 1.50% VC.

(4) Or significantly improves the hardenability of the steel material and increases the hardness of the surface layer after carburizing treatment, so it is a favorable element for carburizing steel. However, if the amount of (~, C]- exceeds 60 inches, the growth of spherical carbides during annealing is suppressed and is unfavorable for press formability, so the amount of Or was limited to 0.60% or less.

(5) Nb provides the effect of suppressing coarsening of austenite crystal grains during heat treatment in the steel of the present invention.

It is particularly effective when the heating time is long, such as in carburizing.8 However, even if the amount added exceeds 4/10 of 0%, the effect does not change, and the effect of this addition is saturated. Therefore, it is not economical to add more than this. Therefore, the amount added was limited to 4/10 of 0% or less.

(6) MO is effective in suppressing abnormal structures in the carburized layer, preventing grain boundary oxidation, improving material properties of the carburized layer, and improving hardenability. However, even if it is added in an amount exceeding 0.5%, the effect does not change, and the spheroidization of carbides during short annealing is suppressed, so the amount added should be 0.5% or less, including from an economical point of view. limited to.

(7) S affects the amount of nonmetallic inclusions in the steel, increases the anisotropy of the steel material, and worsens press formability, so the amount of S is reduced to 0.01.
It was limited to 5% or less.

(8) N and 0 are gas components in steel, and 20%
If it exceeds 0TllTf7+, the amount of T added in order to stably obtain the hardenability effect of B becomes large and the toughness decreases, so the upper limit of N+0 was limited to 200 ppm.

(9) B is one of the important elements for the steel of the present invention, since it greatly improves the hardenability of the steel material even with the addition of a small amount, and no solid solution hardening phenomenon of the ferrite matrix is observed. However, if the amount added is less than 0.0005%, it has no effect on hardenability, and if it exceeds 0.009%, the hardenability deteriorates, so it was limited to a range of 0.0005 to 0.009%.

(io) In addition to adjusting the deoxidation of steel, Ti is necessary to stabilize and effectively harden the hardenability by adding L~ and B for denitrification. 4 for total cutting amount
If the amount added is less than double the amount, sufficient hardenability cannot be ensured, and the
If more than 10 times the amount of TiN is added, the amount of TiN produced increases and the press formability and toughness after heat treatment are deteriorated, so the addition range was limited to 4 to 10 times.

(11) Sol, A] is necessary for deoxidizing molten steel,
If the amount exceeds 0.07%, the surface defects of the product will increase and the product value will decrease, so it is limited to 0.07% or less.

(12) Solid solution amount of alloying elements in ferrite Fs (%)
is related to the mechanical properties and bendability of steel, and Fs is 1□
If it exceeds 3 inches, deterioration of the bendability due to the punched end face is observed, so the upper limit was limited to 1.3%.

(13) The ideal critical diameter (T) affects the surface hardness after carburizing, and if DJ is less than 0.6jn, it is difficult to obtain a hardness of HRC40 or higher with a short carburizing process. , on the other hand, if it exceeds 4.5 inches, the amount of solid solution of alloying elements in the ferrite will increase, resulting in poor material properties such as press formability.
- Since the properties deteriorate, the range is 0.6 to 4.5j, n,
Limited to VC.

(14) Ca treatment of molten steel (method of pouring Ca alloy into the molten steel flow during tapping, into a ladle, into a tundish,
A.
The sulfide inclusions in the system change in shape and composition into spherical CaO-A], O, system inclusions and CaS, and the ductility, bendability, and toughness in the direction perpendicular to the rolling direction (C direction) are significantly increased. The anisotropy of the material is improved. However, if the amount of A-based inclusions in the sheet after hot rolling exceeds 05% in area ratio, the effect of improving bendability and toughness will be small, and the anisotropy of the material will increase, so the amount should be reduced to 0. It was limited to .03% or less.

In addition, since the Ca content in the matrix corresponds to the amount of Ca-based inclusions, if the Ca amount exceeds 200]:ll'n, the amount of inclusions will increase and the ductility, bendability, and toughness will actually decrease.
The amount of a was limited to 200 ppn or less.

(15) The hot-rolling conditions of the steel strip are related to the hot-rolled microstructure during the hot-rolling process, and have a large effect on the mechanical properties and the spheroidization rate of carbides during annealing, and affect the press formability such as the bendability of the finished product. Affect.

In order to ensure this bendability, it is preferable that the structure after rolling has a large amount of ferrite and that the carbide has a fine pearlite structure.

In order to obtain such a structure, a steel slab is heated to 1100°C or higher, then hot rolled at a finishing temperature of 800 to 950'°C, and then rolled at a temperature of 15°C/sec or higher up to 670°C. If the cooling rate is lower than this cooling rate, the pearlite after transformation will produce coarse layered pearlite, which will have a negative effect on the mechanical properties and the spheroidization rate of the carbide. If the temperature exceeds 550°C, decarburization will occur on the surface of the steel strip.If the temperature is lower than 550°C, a bainitic structure will result and the hardness after annealing will be high, making it impossible to obtain good bendability, so it is better to produce hot rolled sheets under these conditions.

Next, examples will be given and explained in comparison with conventional steel.

EXAMPLE Steel having the chemical composition shown in Table 1 was melted and hot rolled to a thickness of 4.5 rrvn. During rolling, the steel of the present invention completes rolling at a finishing temperature of 850°C after heating the slab for 120 days, and continues rolling at 60°C/s up to a temperature of 660°C.
It was cooled at a rate of EC and wound up at 580°C.

After pickling, this was soaked at 710° C. for 20 hours to transform the carbide into a spherical cementite structure.

Table 2 shows the bendability of the obtained steel, as well as the surface hardness and austenite grain size after gas carburizing treatment (0, P; 0.9%) at 880° C. for 90 ml.

These results show that comparative steels Nα1 and Nα3 have good bendability, but Nα1 has low surface hardness after carburizing, and a deep hardened layer cannot be obtained by carburizing. The austenite crystal grains of shredded N13 become larger after carburization, and there is a concern that the toughness after carburization may decrease.

Furthermore, although comparative steel Nα2 has a high surface hardness after carburizing treatment and good carburizing properties, it has poor bendability and cracks occur at the punched end face, so that the object of the present invention cannot be achieved.

On the other hand, the steel of the present invention has a N[L of 4.5, and the steel of B has a large amount of ferrite produced by controlling the hot rolling conditions.
Moreover, since the carbides are finely and uniformly distributed, the spheroidization rate and distribution of the carbides after annealing are good, and good bendability is obtained, as well as good carburizing properties.

In addition, steel with NIL 6 has a solid solution amount F of alloying elements in ferrite.
Although θ was as high as 1.26%, the Ca treatment significantly improved the bendability.

388-

Claims (1)

[Claims] (110; 0.35% or less, Sj; 0.25%
Below, Mn: 0.30-1.50%, Cr: 0.
60% or less, S; 0,020% or less, Nb; (C
H) 4/10 or less, SO], A]; 0.07- or less, N-1-0; 200 pPn or less, Tt;
4 to 10 times of N% 10Ochi), B; 0.0005 to 0
．． 0119chi, the remainder consists of Fe and unavoidable impurities, and 6.7 S, l Mn Fs (%) - (2C - 0,4)
-1j〒 (where C, Si, Mn, Or are the Q contents of the steel components) The amount of alloying element solid solution (Fs) in the ferrite is 1.3 Q or less, and DX (in,) = ( (0,40+0.06)
(0,7Si +1) X (3,33Mn +1)
X (2,16 Or + January X41 + 1.5 (0,9-
C) The ideal critical diameter (DI) according to the formula 1 (C, Si, Mn, Or is the percentage content of steel components) is 0.6 to 4.5.
A special steel with excellent workability and heat treatability whose composition has been adjusted so that it is in. (21C!; 0.35% or less, Si; 0.25
% or less, Mn; 0.30 to 1.50%, Cr; 0.
60% or less, S; 0.020% or less, Nb; 4/10 or less of (C), Sol,, A]; 0.07 or less, N+O; 200p or less, T]; N%
+0%) 4 to 10 times, B; 0.0005 to 0.0
Section 09, Mo: 0.5'% or less, the remainder consisting of Fe and inevitable impurities, and 6.7 Fs (%) - (2C"4)10F,7-C"1-1-1.
40+1+4゜H-positive side (however, C, Si, Mn, Cr, MoVi content of steel components) The amount of alloying element solid solution (Fs) in ferrite is 1.3% or less, and D□ (i, n,) = ((0,4(,+0.06)
X (0,78i +1) X (3,33Mn +1
)x (2,16Or + 1)X (3,0Mo
+ January
A special steel with excellent workability and heat treatability. (310: 0.35% or less, Sj: 0.25%)
Below, Mn: 0.30-1.50%, Cr: 0
．． 60% or less, S; 0,020% or less, Nb; (
4/10 or less of 0%), Sol, A]; 0.07 or less, N+O; 200 pP1 or less, T1;
4-10 times of N%+0chi), B; 0.0005-0
．． 009%, Ca; 200ppn or less, balance Fe
and unavoidable impurities, and 6,7Sj Fe(%)=(2C”4)+6.7−C+1+1,4o
+l-4-40 (however, C, Si, Mn, Cr are the steel content). n, ) = ((0,40+0.06)X
(o, 7s], + 1) x (ro, 33Mn + 1
)X (2,16Or + JanuaryX (1+1.5(0
, 9-〇)) (where C, Sj, , Mn, Cr are the chromium contents of the steel components), the ideal critical diameter (Dr) is 0.
．． A special steel with excellent workability and heat treatability whose composition has been adjusted to give a thickness of 6 to 4.5 inches. □ (41C; 0.35% or less, Si; 0.25% or less, Mn; 0.30-1.50%, Or; 0.60
% or less, s; o, o2o f6 or less, Nb; (
c%) 4/10 or less, Sol, A]; 0.07%
Hereinafter, N+0; 200 pIXn or less, T]; 4 to 10 times that of (10%) in steel, B; 0.0005 to
0.009%, and in some cases Mo; 0.5
% or less, the remainder consists of Fe and unavoidable impurities, and 6.76i Mn 0rF
s(%)-(2C"4)+6.7-C+1+1.4C+
1-H6+L+(E=(However, Cr S 1+ Mn
+ O"+ ('1: Ti content of steel components) M. The amount of alloying element solid solution in ferrite (Fs) according to the formula is 1.3% or less, and DI (], n, ) = ((0 ,40+o,006)x
(0,7Si+1)x(ro,ro3Mn+1)x (2,
160r + 1)x (3, OMo+1□))
(1+1.5(0,9-C) 1 (However, C, S'', M
If the ideal critical diameter (D,) according to the formula O is 0.6~4,5i
After heating a steel piece of steel having a component composition adjusted so as to have a Ca treatment of 200p11)m or less in some cases to 1100℃ or higher, a finishing temperature of 800~
Hot rolling at 950°C, then 15°C/15°C up to 670°C.
A method for producing special steel with excellent workability and heat treatability, which comprises cooling at a rate of 1.5 seconds or more and winding at a temperature of 550 to 670°C.