JPS58120719A - Manufacture of case hardening b steel - Google Patents

Abstract

PURPOSE:To obtain a case hardening B steel inhibiting the austenitic grains from being made coarse during reheating by heating a steel contg. specified percentages of C, Si, Mn, B, N, SolAl and Ti in iron to a specified temp. and by hot rolling the heated steel. CONSTITUTION:A steel consisting of 0.1-0.3% C, 0.1-0.5% Si, 0.3-2.0% Mn, 0.0005-0.003% B, <=0.0035% N, 0.005-0.1% SolAl, (5XN%+0.02)-0.05% Ti and the balance Fe with impurities or further contg. <=2% Cr and/or <=0.5% Mo is heated to 1,150 deg.C- the A3 point, and the heated steel is hot rolled. The austenitic grains of the resulting case hardening B steel is prevented from being made coarse during carburization or other treatment.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing B-containing case-hardened steel that can suppress coarsening of austenite crystal grains during carburizing and special reheating of B-containing case-hardened steel produced by a low-temperature heating method. It is.

B-containing steel has the advantage of being an inexpensive steel material because its hardenability can be improved by adding a small amount of B. However, in order for B to work effectively, poron must be kept in a 7-li state (precipitated) during austenitization. It is necessary for it to exist in a state where it is not a physical entity (BN). For this reason, it has been conventional practice to add Tl to B-containing steel and fix N with Ti. The addition of T also contributes to the refinement of crystal grains.

By the way, recently as a measure to save energy, steel materials (
billet) heating temperature below 1150°C (previously 120°C)
Low-temperature heating (0 to 1250°C) is becoming increasingly common.

When considering the application of this method to TI-added copper-containing steel, in the case of low-temperature heating, T1 precipitates such as TiN do not melt into the steel and agglomerate, so austenite crystal grains are removed during reheating. It was found that it is easy to become coarse.

The present invention has been made with the aim of solving all the above-mentioned problems in the low-temperature heating method for T+-added B-containing steel, and obtaining a B-containing case-hardened steel that suppresses coarsening of austenite crystal grains during reheating such as carburizing treatment. It is something.

0.005~α1chi, Ti(5XNchi+0.02)~0
．． 05%, and if necessary, Cr 2% or less, M
The austenite grain size during reheating is determined by heating a steel material containing one or two of the following O015 and the balance consisting of iron and impurities to a temperature of 1150°C to A3 point and then hot rolling it. This is a method for producing B-containing case-hardened steel that suppresses coarsening of the steel.

In the present invention, the contents of Ti and N and the relationship between them have important meanings.

That is, it is an element necessary for fixing NO in order to suppress the precipitation of BN which reduces the hardenability effect of TiiζB. Theoretically, N can be fixed if 3.4 times (by weight) of TI exists as N, but when considering Ti in the form of oxides, 5 times as much Ti as N is required.

By the way, two types of B-containing steels (Table 1) containing 5 times more Ti than N or more were hot-rolled at various pre-rolling door heating temperatures, and the resulting steel bars were rolled at 925°C for 6 hours. After carburizing, the coarsening behavior of austenite grains was investigated. The results are shown in FIG.

As is known from Fig. 1, in sample material No. 1, in which the Ti content is about 5 times that of N, when heated at low temperature, the coarse crystal grains with a crystal grain coarsening rate (crystal grain size No. 4) The area ratio occupied by ) increases, while the Ti content increases (N%
In the case of sample materials No. 2, which have an excess of about α03)%, the crystal grain coarsening rate is almost 0 even when heated at low temperature, and the fine crystal grain state is maintained.

As is known from FIG. 1, in order to refine the crystal grains in the low-temperature heating method, it is necessary to contain T1 in excess of five times the amount of N.
The appropriate lower limit of the content is (0.02+5xN%). On the other hand, as can be seen from Figure 3 below, the amount of inclusion-type TI precipitates becomes too large and deteriorates ductility and cold workability, so the upper limit of the Ti content is set at α05.
%.

Table 1 Chemical Composition (wt) Next, referring to N, N is a harmful element in that it reduces the effect of B, but on the other hand, it can be expected to have a crystal grain refinement effect due to the formation of TiN. Figure 2 shows 0.2%C-0
,25%5t-0,9%Mn-0,03%A4-0
．． Austenite crystal grains when steel bars manufactured by hot rolling 02% or 0.04%Ti-0,0015%B case-hardening steel with varying N content using a low-temperature heating method are reheated to the austenite region. Coarsening temperature (grain size N
o.

The area ratio of coarse particles of 4 or less is 10% (temperature) and N
@Relationship with quantity? FIG. As shown in Figure 2, the Ti content is greater than or equal to the above (N% x 50, al + →), and the N content is 0.003%.
In the following cases, coarsening of austenite grains can be suppressed up to a considerably high temperature. Furthermore, FIG. 3 shows that when the Ti and NO contents are changed for the same B-containing case hardened steel as in FIG. 2, the amount of Ti-based inclusions is extremely reduced.

Next, we will discuss other chemical components. C is a strength-imparting element, and if it is less than α1%, the required strength cannot be obtained, and if α3% is less than
C0.1~0.3% as the ductility deteriorates if it exceeds
It is. Suitable case hardening steels are Cα13 to α27. Si is used as a deoxidizing agent and is required to have an amount of α05 or more, but if it is too large, ductility and cold workability deteriorate, so the upper limit is set at α5.

Mn is contained as a deoxidizing/desulfurizing agent and a hardenability improving element, and is required in an amount of α3 or more, but if it is too large, the structure will become uneven due to segregation and cold workability will deteriorate, so the upper limit should be set at 20%. do. B is an element that improves hardenability when added in a trace amount, and a suitable amount is 00005 to 0.0031. h
(L is used as a deoxidizing agent and is also effective for grain refinement. 0.00 as acid-soluble he (soeAn)
5 to old chi is an appropriate amount.

In the present invention, in addition to the above-mentioned elements, one or both of Cr and Mo can be contained as strength-imparting elements, if necessary. When Cr is 2% or more and MoQ is 5% or more, ductility and cold workability deteriorate.

A steel material (billet) having the above-mentioned chemical composition is multi-hot rolled by a low temperature heating method. The heating temperature needs to be 1150°C or less from the viewpoint of energy saving and to make the structure after rolling fine. Note that the lower limit of the heating temperature is A3 point due to the necessity of complete austenitization.

Next, examples of the present invention will be shown together with comparative examples.

Steel material (billet) having the chemical composition shown in Table 2 'e
A steel bar was produced by heating to a temperature of 1150 to 900°C and hot rolling. Table 3 shows the cleanliness of Ti-based inclusions in these steel bars and the coarsening rate of austenite crystal grains after carburizing at 925°C. As is known from Tables 2 and 3, the symbols D-J, which are the present invention, refer to hot rolling by low temperature heating method for products that satisfy the conditions of Ti and N content stipulated in the present invention. No coarsening of austenite crystal grains occurred during the carburizing process.

Second table production (wt%) * ppm Table 3 *Carburizing treatment at 925℃

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between heating temperature before rolling and coarsening rate of austenite crystal grains, Figure 2 is a diagram showing the relationship between N content and coarsening temperature of austenite grains, and Figure 3 is a diagram showing the relationship between coarsening temperature of austenite crystal grains.
, is a diagram showing the relationship between the N/content and the cleanliness of Ti-based inclusions. λEy and power 8 tables 1 degree (°C) Amount of N in the building (PP')

Claims (1)

[Claims] A steel material comprising (5XN% + 0.02) to 0.05%, the balance being iron and impurities) is heated to a temperature of 1150°C to A3 point and then hot rolled. Manufacturing method of B-containing case hardened steel that suppresses coarsening of austenite crystal grains during reheating
Below, M. Austenite crystals during reheating, characterized in that a steel material containing one or two of the following, with the balance consisting of iron and impurities, is heated to a temperature of 1150"C-As point and then hot rolled. Manufacturing method of B-containing case-hardened steel that suppresses grain coarsening